CROJFE - Volume 28, Issue 2

28

Issue 2

2007

2

Original scientific paper – Izvorni znanstveni rad

Improvement of wheel skidder tractive performance by tire inflation pressure and tire chains Stanimir Stoilov Abstract – Nacrtak The motion resistance ratio, gross traction ratio and net traction ratio of a wheel cable skidder were determined and mathematical models derived for three tire inflation pressure values with or without tire chains on forest road in mountainous conditions.

The motion resistance ratio increases with the increase of tire inflation pressure. On the contrary, the gross traction ratio increases with the decrease of tire inflation pressure.

However, when the tires are equipped with tire chains the skidder motion resistance is higher. The motion resistance ratio with tire chains also increases with the decrease of tire inflation pressure due to windage between the tires and chains, which results in lower tractive performance of tire chains.

Therefore, by using tire chains, an increase of gross tractive ratio could be achieved, as well as the increase of motion resistance ratio compared to tires without chains.

The results of the study show that it is strongly recommended to use tire chains as tightly as possible.

Keywords: wheel skidder, tire inflation pressure, tire chains, tractive performance, motion resistance

1. Introduction – Uvod The wheel cable skidder is a self-propelled machine designed to transport trees or parts of trees by trailing or dragging them with one end on the ground.

Cable skidders use a main winch cable and cable chokers to assemble and hold the load (Stokes et al. 1989).

Worldwide the wheel skidders are one of the basic machines in mountain logging on 10–20° steep slopes during thinnings, selection silvicultural systems and shelterwood regeneration.

The increasing demand for more efficient and productive logging technologies in mountain conditions requires improvement of mobility and tractive performance of forest machines. One of possible solutions is to equip skidders with special high-flotation tires with low inflation pressure controlled manually or by Central Tire Inflation System (CTIS), which allows the increase of tire contact area, thus Croatian Journal of Forest Engineering 28(2007)2

preventing slip, soil compaction and sinkage, and improving traction. Tire chains are often in use to maximize traction, reduce slipping and fuel consumption, and providing minimal brake distance from the wheel skidder (Fig. 1). Sometimes the advantages of four-wheel or six-wheel drive skidder transmission cannot replace the need for tire chains during all seasons. In extreme muddy and deep snow conditions tire chains will enhance the mobility and improve tractive performance, as well as protect the tread and tire side wall from excessive damages.

Greater tractive force along with minimum slip provided by tire chains means bigger tree load and higher speed, and therefore, greater productivity and efficiency of wheel skidder.

The tractive performance, as one of the main characteristics of wheel skidders, determines their mobility, productivity, and environmental damage under different terrain conditions of forest areas.

Tractive performance is in close dependence on soil-

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Improvement of wheel skidder tractive performance by tire inflation pressure and tire chains (137–144)

mgt = T / (r · W) = Fper / W

(1)

Þ motion resistance ratio mr describes the resistance to movement of a vehicle provided by both the surface on which it moves and internal friction of its tires, and the energy losses in the wheels divided by the dynamic load of the skidder: mr = Fr / W

(2)

Þ net traction ratio mnt demonstrates tractive force divided by the dynamic load of the skidder: mnt = Ft / W = mgt – mr

(3)

where: W – dynamic load of the skidder T – torque delivered to the wheels (input torque) r – rolling radius The force equilibrium for wheel skidder is as follows:

Fig. 1 Skidder wheel equipped with tire chains Slika 1. Skiderski kota~ s postavljenim lancima wheel interaction. The milestones of terramechanics are the studies of M. G. Bekker (1956, 1969) and J. Y. Wong (1978, 1989). There are numerous papers that explored soil-wheel interaction, but investigations into use of tire chains have not been given sufficient attention.

The subject of this paper is the research of motion resistance ratio, gross traction ratio and net traction ratio of wheel skidder with or without tire chains.

2. Earlier research – Prija{nja istra`ivanja The following five dimensionless ratios are used to describe tractive performance (Standards of ISTVS 1977): Þ travel reduction ratio, commonly called »slip«, is an indication of how the speed of the wheels differs from the forward speed of the vehicle; Þ tractive efficiency presents the ratio of tractive (drawbar) power divided by corresponding power at driven wheels (Wong 1998); Þ gross traction ratio mgt shows the torque delivered to the wheels divided by the rolling radius of the wheels and divided by the dynamic load of the skidder:

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Fper = Fr ± Fs ± Fa + Fw + Ft where: Fper – Fr – Fs – Fa – Fw – Ft –

(4)

peripheral force of skidder wheels motion resistance force slope resistance force resistance of inertia aerodynamic resistance force tractive force (drawbar pull parallel to ground surface)

In mountainous logging the travel speed of loaded wheel cable skidder is not higher than 6–8 km/h.

Therefore aerodynamic and inertial effect could be ignored due to low ground speed (i.e. Fa = 0 and Fw = 0).

Thus tractive force equation (4) is simplified as follows: Ft = W · (mgt – mr) = W · mnt

(5)

For a four-wheel-drive skidder with rigidly coupled front and rear drive axles, the tractive performance under a hard surface condition can be achieved if theoretical speed of the front and rear wheels is equal. On forest terrain, where sinkage takes place, the theoretical speed of the front wheels should be faster (i.e. forerunning) than that of the rear because of different tire deflection, soil compaction and rut depth beneath front and rear wheels. To prevent transmission mechanical damage and to reduce tires wear, it is necessary to provide a kinematic discrepancy between the theoretical speeds of the front and rear wheels on deformable forest terrain to be used (Dimitrov and Stoilov 2005).

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Improvement of wheel skidder tractive performance by tire inflation pressure and tire chains (137–144)

There are several studies of tractive performance of wheel skidders in field conditions (Hassan and Sirois 1984, Hassan and Sirois 1985, Sever 1989, Dimitrov and Stoilov 2005, Stoilov 2005) and laboratory conditions (Vechinski et al. 1998, Vechinski et al. 1999). However, only the last two investigations are focused on the effect of different tire inflation pressures and chains on motion resistance and traction of skidder tires in laboratory conditions.

The research of skidders equipped with tire chains operating in logging conditions will be of great importance and results will provide better understanding of vehicle-terrain interaction as well as ways to increase productivity and overall efficiency of wheeled machines during skidding. All this could be achieved by proper field testing and interpreting the experimental results.

The main objective of this paper is to present the effects of tire inflation pressure and use of tire chains on the motion resistance and tractive force of a four-

S. STOILOV

-wheel-drive cable skidder, respectively shown by motion resistance ratio, gross traction ratio and net traction ratio.

3. Materials and method – Materijal i metode 3.1 Technical data of the studied skidder – Tehni~ki podaci o ispitivanom skideru The tests were conducted with two four-wheel-drive LKT–81T cable skidders, owned by the Petrohan Training and Experimental Forestry at University of Forestry. Skidder technical data are shown in Table 1.

3.2 Site and stand conditions – Stani{ni i sastojinski uvjeti The tests were conducted on a flat forest road in a 160-year beech stand, located in Petrohan Training

Table 1 Technical data of LKT–81T articulated four-wheel-drive cable skidder Tablica 1. Tehni~ki podaci zglobnoga skidera LKT–81T opremljenoga {umskim vitlom formule pogona 4 x 4 Manufacturer – Proizvo|a~ Diesel engine – Dizelski motor Manufacturer – Proizvo|a~ Type – Tip Rated power – Nazivna snaga Engine cooling – Hla|enje motora Transmission – Sustav prijenosa snage Gear box – Mjenja~ Transfer box – Razdjelnik pogona Axes – Osovine Tires – Gume Type – Tip Tire size – Veli~ina Tire inflation pressure – Tlak u gumama Skidder mass – Masa skidera Weight distribution (unloaded skidder) Raspored mase neoptere}enoga skidera Winch – Vitlo Max. line pull – Nazivna vu~na sila Max. cable length – Najve}a duljina u`eta Drive – Pogon Control – Upravljanje Tire chains – Lanci

Martimex–Alfa A. S. (Slovak Republic – Slova~ka) 4-stroke, direct injection, turbocharger – 4 cilindra, izravno ubrizgavanje, turbopunja~ Martimex–Alfa A. S. (Slovak Republic – Slova~ka) Martin Diesel (Zetor)-8022.138 72.25 + 3 % kW @ 2200 min–1 water type – hla|enje vodom mechanical – mehani~ki 5-speed synchromesh – 5 brzina, sinkronizirani 2-speed – 2 brzine rigid, mechanical differential lock with electropneumatic control, with final drive kruta sa zavr{nim reduktorima, mehani~ka blokada diferencijala s elektropneumatskim upravljanjem Barum Continental s.r.o.

TL–1 Log Skidder PR 12 16.9–30’’ max. 240 kPa, min. 80 kPa 7145 kg front axe – prednja osovina (61.5 %) rear axe – stra`nja osovina (38.5 %) double drum, free unwinded cable dvobubanjsko, slobodno odmotavanje u`eta 70 kN (bare drum – prazan bubanj) 77 m (Ø14 mm) hydrostatic – hidrostatski hydroelectric – hidroelektri~no conventional type with lug rings uobi~ajeni tip s oslanjaju}im prstenovima

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Improvement of wheel skidder tractive performance by tire inflation pressure and tire chains (137–144)

and Experimental Forestry at University of Forestry, situated in Western Balkan Mountain.

Undisturbed soil properties of the site: brown forest soil, average moisture content: w = 20.3%, average cone index: 385 kPa. Atmosphere conditions: air temperature: 22°C, air moisture content: 61%, and air pressure: 940 hPa.

3.3 Measuring equipment – Mjerna oprema The test skidder was instrumented with the following transducers for measuring tractive force (line pull) Ft (Fig. 2): Þ a 100 kN load cell for measuring tractive force (line pull) (Dept. of Strength of Materials at Technical University of Sofia); Þ a frequency amplifier with 3 ranges: 20, 40 and 100 kN (KWS-3073, Hottinger Baldwin Messetechnik GmbH); Þ a recorder (TSS-101, RFT Robotron).

The measuring equipment devices were powered by an autonomous source – two 12 V accumulator batteries.

3.4 Study layout – Postavka istra`ivanja The two-way tests carried out to eliminate possible effects of test area grade (Lta) 30 m in length of the road. Tire inflation pressure was set at different combinations and the above test sequence was repeated.

Equal tire inflation pressure was used in front and rear wheels so as to reduce the effects of kinematic discrepancy between the theoretical speeds of the front and rear wheels on deformable forest terrain.

3.5 Motion resistance measurements – Mjerenje otpora kotrljanja The motion resistance force Fr was determined by the following method (Fig. 3). An auxiliary cable skidder towed the test skidder with a load cell (LC) connected between them. The gear box of the test skidder was placed in neutral position with tire inflation pressure set at one of the combinations shown in Table 2. At speed lower then 15–20 m/s the motion resistance ratio is assumed to be constant. The measured horizontal force is motion resistance force Fr.

Fig. 2 Measuring equipment Slika 2. Mjerna oprema 140

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Improvement of wheel skidder tractive performance by tire inflation pressure and tire chains (137–144)

S. STOILOV

The gross traction ratio mgt is defined as the ratio of sum of the motion resistance force Fr and maximum tractive force Ft(max) to the skidder weight W: mgt = (Ft(max) + Fr) / W

(7)

4. Research results – Rezultati istra`ivanja Fig. 3 Scheme of motion resistance test Slika 3. Shema ispitivanja otpora kotrljanja In this case the motion resistance ratio mr is the ratio of tractive force Ft = Fr to the skidder weight W: mr = Fr / W = Ft / W

Motion resistance ratio mr, gross traction ratio mgt and net traction ratio mnt are calculated from experimental data using equations (6), (7) and (3), respectively. The average calculation results of the three tire inflation pressure combinations are shown in Table 2 and Figures 5, 6, 7.

(6)

3.6 Traction measurements – Vu~ni pokusi The net traction force was determined by the following method (Fig. 4). The test skidder, operated with the throttle open wide, towed the auxiliary cable skidder with a load cell (LC) connected between them. The auxiliary skidder created increasing resistance force by transmission, breaks, and stacking blade until test skidder stopped due to 100% slip.

The measured horizontal resistance force is maximum tractive force Ft(max).

Fig. 5 Motion resistance ratio vs. tire inflation pressure Slika 5. Ovisnost faktora otpora kotrljanja o tlaku u gumama

Fig. 4 Scheme of traction test

Table 2 Calculated results Table 2. Rezultati prora~una Tire inflation pressure Tlak u gumama

Motion resistance ratio Faktor otpora kotrljanja

Gross traction ratio Faktor bruto vu~e

Net traction ratio Faktor neto vu~e

pi, kPa

mr

mgt

mnt

Front Prednjim

Rear Stra`njim

without chains bez lanaca

chains s lancima

without chains bez lanaca

chains s lancima

without chains bez lanaca

chains s lancima

230

230

0.0728

0.0798

0.4579

0.5048

0.3851

0.425

210

210

0.0672

0.0800

0.4628

0.5023

0.4021

0.4223

190

190

0.0607

0.0841

0.4695

0.5012

0.4113

0.4171

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Improvement of wheel skidder tractive performance by tire inflation pressure and tire chains (137–144)

The best fitted models describing mr, mgt and mnt with or without chains are polynomial models shown in Figures 5, 6, 7.

5. Discussion – Diskusija

Fig. 6 Gross traction ratio vs. tire inflation pressure Slika 6. Ovisnost faktora bruto vu~e o tlaku u gumama

Fig. 7 Net traction ratio vs. tire inflation pressure Slika 7. Ovisnost faktora neto vu~e o tlaku u gumama The rut depth after the passes was 50–80 mm. The rut depth difference after front and rear wheels was not significant due to macadam bottoming of the forest road.

After calculating these data, statistical processing of mathematical models was carried out. The linear, polynomial, logarithmic and exponential models were used for describing the relationship between tire inflation pressure and motion resistance ratio, and gross traction ratio and net traction ratio.

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In case of skidder wheels without tire chains the motion resistance ratio decreases while tire inflation pressure is reduced (Fig. 5). Reduction of tire inflation pressure causes the decrease in motion resistance ratio (7.69%) and increase in gross tractive ratio (2.5%) (Fig. 6) and net tractive ratio (6.8%) (Fig. 7) without tire chains on deformable forest road at pi = 190 kPa in comparison with pi = 230 kPa. These improvements in tractive performance are on account of increased wheel contact area and lower sinkage depth. All these reduce the motion resistance component (»bulldozing resistance«, Wong 1989) of soil deformation resistance and improve traction forces.

For wheels equipped with tire chains the decrease of the tire inflation pressure leads to the increase of the motion resistance ratio (5.4%) at pi = 190 kPa in comparison with pi = 230 kPa (Fig. 5). The combination of tire chains and tire inflation pressure reduction caused the decrease of gross traction ratio (0,7%) (Fig. 6) and net tractive ratio (1.9%) (Fig. 7) at pi = 190 kPa in comparison with pi = 230 kPa. Reduction of tire inflation pressure leads to the decrease of the rolling radii of the wheels and windage between tires and chains.

Generally, motion resistance ratio of the wheel skidder was higher with chains than without chains due to windage between tires and chains caused by the reduction of tire inflation pressure.

Use of tire chains improves the gross tractive ratio because of growth of the traction. Without tire chains, the decrease of tire inflation pressure gives rise to the gross tractive ratio due to the increase of the tire-soil contact area and adhesion forces, and lower sinkage.

Net traction ratio is the difference between gross tractive ratio and motion resistance ratio. Therefore, on deformable forest road the decrease of tire inflation pressure has a favorable impact on the increase of the net tractive ratio and tractive performance of the skidder with wheels without tire chains. Use of chains and reduction of tire inflation pressure cause a slight decrease of net tractive ratio due to higher motion resistance ratio and enlarged windage between the tires and chains.

6. Conclusions – Zaklju~ci Along with all-drive transmission of wheel skidder and operating factors, the reduction of tire inflation Croatian Journal of Forest Engineering 28(2007)2

Improvement of wheel skidder tractive performance by tire inflation pressure and tire chains (137–144)

pressure has a noticeable effect on the increase of tire contact area and improvement of traction, and hence its mobility and productivity.

The results of field tests confirm theoretical findings that reduction of tire inflation pressure causes the decrease of motion resistance ratio and increase of gross tractive ratio and net tractive ratio without tire chains on deformable forest road.

The use of tire chains could cause an increase in gross tractive ratio, net tractive ratio, as well as motion resistance ratio in comparison with the case without tire chains. Experimental evidence shows that the use of tire chains leads to the decrease of gross tractive ratio and net tractive ratio at pi = 190 kPa in comparison with pi = 230 kPa, but also to the increase of motion resistance ratio due to the rise of windage between the wheels and tire chains.

The results of the study indicate that the use of tire chains applied as tightly as possible is strongly recommended, and that tire inflation pressure must be kept in appropriate range during operation to avoid the increase of windage and ensure the optimum performance of wheel skidder.

Improved mobility and tractive performance of wheel skidders equipped with tire chains could decrease the amount and costs of forest road maintenance.

S. STOILOV

tive performance). Selskostopanska tekhnika (Journal of Agricultural Engineering) 3: 8–13.

Hassan, A. E., Sirois, D. L., 1984: Rolling Resistance of a Cable Skidder – Effect of Inflation Pressure and Tire Sizes.

ASAE Meeting Paper No. 84: 10–50.

Hassan, A. E., Sirois, D. L., 1985: Traction and Rolling Resistance of a Dual-Tired Skidder on Wetland. Transactions of ASAE 28(4): 1083–1042.

Sever, S., 1990: Skidder Traction Factors. International Journal of Forest Engineering 1(2): 15–23.

Standards of International Society of Terrain-Vehicle Systems, 1977: Journal of Terramechanics 14(3): 153–182.

Stoilov, S., 2005: Izsledvane na teglitelno-scepnite svoystva na kolesniya traktor za darvodobiva (Research of wheel skidder tractive performance). Dissertation, University of Forestry, Sofia, Bulgaria.

Stokes, B. J., et al., 1989: Glossary of terms used in timber harvesting and forest engineering. Gen. Tech. Rep. SO-73.

New Orleans, LA, USDA, Forest Service, Southern forest experimental station.

Vechinski, C. R., Johnson, C. E., Raper, R. L., 1998: Evaluation of an empirical traction equation for forestry tires.

Journal of Terramechanics 35(1): 55–67.

Vechinski, C. R., Johnson, C. E., Raper, R. L., McDonald, T. P., 1999: Forestry Tire Tractive Performance: New, Worn, and with Chains. Appllied Engineering in Agriculture 15(4): 263–266.

7. References – Literatura

Wong, J. Y., 1978: Theory of Ground Vehicles. John Wiley, N.Y.

Bekker, M. G., 1956: Theory of Land Locomotion. The University of Michigan Press. 1–499.

Wong, J. Y., 1989: Terramechanics and Off-Road Vehicles.

Elsevier, Amsterdam.

Bekker, M. G., 1969: Introduction to Terrain-Vehicle Systems. The University of Michigan Press. 1–520.

Dimitrov, J., Stoilov, S., 2005: Vliyanie na transmisiyata varhu teglitelno-scepnite svoystva na kolesniya traktor za darvodobiva (Effect of transmission on wheel skidder trac-

Wong, J. Y., et al., 1998: Optimization of the Tractive Performance of Four-Wheel-Drive Tractors: Theoretical Analysis and Experimental Substantiation. Proceedings of the Institution of Mechanical Engineers, Part D, Journal of Automobile Engineering 212(D4): 285–297.

Sa`etak

Pobolj{anje vu~ne zna~ajke skidera promjenom tlaka u gumama i primjenom lanaca na kota~ima Potreba za ve}om djelotvorno{}u privla~enja drva u planinskim podru~jima zahtijeva pobolj{anje kretnosti i vu~nih zna~ajki {umskih vozila. Kota~ni je skider naj~e{}e {umsko vozilo koje se upotrebljava pri privla~enju drva u planinskim podru~jima. Opremanje skidera {irim gumama te manjim tlakom u gumama omogu}uje pove}anje dodirne povr{ine kota~a i tla, smanjenje klizanja kota~a, zbijanja tla, nastanka kolotraga te pobolj{ava vu~nu zna~ajku vozila. Opremanje kota~a skidera lancima (slika 1) tako|er pove}ava vu~nu zna~ajku skidera uz smanjenje klizanja kota~a, potro{nje goriva i puta zaustavljanja. U uvjetima slabe nosivosti tla ili visokoga snje`noga pokriva~a uporaba lanaca na kota~ima omogu}it }e ve}u kretnost skidera i vu~nu silu te manje klizanje kota~a, {to }e se o~itovati u ve}im obujmima tovara i brzinama kretanja zbog ~ega }e se pove}ati proizvodnost skidera.

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S. STOILOV

Improvement of wheel skidder tractive performance by tire inflation pressure and tire chains (137–144)

Cilj je rada istra`ivanje faktora otpora kotrljanja, bruto i neto vu~e skidera ovisno o uporabi lanaca na gumama kota~a i promjeni tlaka zraka u gumama.

Faktor se otpora kotrljanja opisuje kao otpor pokretanja vozila zbog stanja podloge, unutra{njega trenja gume i energetskoga gubitka kota~a u odnosu na dinami~ko optere}enje kota~a. Zbog djelovanja zakretnoga momenta na kota~u javlja se obodna sila (Fper) koja slu`i za svladavanje otpora kotrljanja vozila (Fr), a ostali dio sile (Ft) za vu~u tereta, svladavanje nagiba (Fs), otpora inercije (Fa) i otpora zraka (Fw). Faktor bruto vu~e opisuje se odnosom obodne sile i optere}enja kota~a, a faktor neto vu~e odnosom vu~ne sile i optere}enja kota~a.

Istra`ivanje je provedeno na ravnom dijelu {umske ceste, pa nije bila potrebna sila za svladavanje nagiba, a zbog malih brzina kretanja optere}enoga skidera izostali su otpor inercije i otpor zraka.

U istra`ivanju se koristio skider LKT 81 T ~iji su tehni~ki podaci prikazani u tablici 1. Tlak se zraka u gumama tijekom istra`ivanja postavljao na vrijednosti 190 kPa, 210 kPa i 230 kPa. U pojedinom je vu~nom pokusu tlak bio jednak u prednjim i stra`njim gumama kota~a.

Otpor se kotrljanja mjerio povla~enjem ispitivanoga skidera u praznom hodu ili neutralnim polo`ajem transmisije te dinamometrom u~vr{}enim izme|u skidera i vozila koje ga vu~e (slika 3). Odnos zabilje`ene sile na dinamometru i te`ine vozila pokazuje vrijednost faktora kotrljanja.

Vu~na se sila mjerila povla~enjem pomo}noga vozila pomo}u ispitivanoga skidera s u~vr{}enim dinamometrom na vu~nom u`etu vitla. Pomo}nim se vozilom pove}avala sila otpora ko~enjem, transmisijom i spu{tanjem prednje odrivne daske sve do zaustavljanja ispitivanoga skidera zbog 100 % klizanja kota~a. Odnos zabilje`ene sile na dinamometru i te`ine vozila pokazuje vrijednost faktora bruto vu~e. Faktor neto vu~e za svaki se vu~ni pokus odredio kao razlika izme|u faktora neto vu~e i faktora otpora kotrljanja.

Srednje vrijednosti faktora vu~e pri razli~itim tlakovima u gumama kota~a s lancima ili bez lanaca prikazani su u tablici 2. Ovisnosti pojednih faktora vu~e o tlaku u gumama odre|ene su regresijskom analizom podataka (slike 5, 6 i 7).

Manji tlak u gumama bez lanaca na kota~ima uzrokuje smanjenje faktora otpora kotrljanja i pove}anje faktora bruto i neto vu~e zbog pove}anja dodirne povr{ine kota~a i tla i manjega propadanja kota~a u tlo. Kod kota~a s postavljenim lancima smanjenje tlaka u gumama dovodi do pove}anja faktora otpora kotrljanja i smanjenja faktora bruto i neto vu~e zbog slabijega prijanjanja izme|u guma i lanaca te smanjenja polumjera kota~a.

Op}enito skider s postavljenim lancima na kota~ima ima ve}i otpor kotrljanja od skidera bez primjene lanaca.

Ve}e vrijednosti faktora bruto i neto vu~e u primjeni lanaca na kota~ima posljedica su boljih vu~nih mogu}nosti skidera zbog smanjenja klizanja kota~a.

Rezultati istra`ivanja pokazuju zna~ajan utjecaj smanjenja tlaka u gumama na pove}anje dodirne povr{ine kota~a i tla i pobolj{anje vu~ne zna~ajke skidera te time na pove}anje proizvodnosti. Na osnovi rezultata preporu~uje se postavljanje lanaca na kota~ima {to je mogu}e ~vr{}e uz gumu uz odr`avanje tlaka u gumama u potrebnom rasponu kako bi se omogu}ilo bolje prijanjanje lanaca i kota~a te osigurala optimalna vu~na zna~ajka skidera.

Rad pridonosi boljemu razumijevanju odnosa izme|u zna~ajki vozila i zna~ajki tla te slu`i kao smjernica za pove}anje proizvodnosti skidera pri privla~enju drva.

Klju~ne rije~i: kota~ni skider, tlak u gumama, lanci na kota~ima, vu~na zna~ajka, otpor kotrljanja

Author’s address – Autorova adresa:

Received (Primljeno): October 19, 2007 Accepted (Prihva}eno): December 6, 2007

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Asst. Prof. Stanimir Stoilov, PhD. e-mail: stoilovs@ltu.bg University of Forestry Faculty of Forestry Department of Technologies and Mechanization of Forestry 10 Kliment Ohridski Blvd.

1756 Sofia BULGARIA Croatian Journal of Forest Engineering 28(2007)2

Orginal scientific paper – Izvorni znanstveni rad

Evaluation of two harvesting systems for the supply of wood-chips in Norway spruce forests affected by bark beetles Tobias Cremer, Borja Velazquez-Marti Abstract – Nacrtak For sanitary reasons, spruce trees affected by bark beetles (Ips typographus L.) should be removed out of the stand as soon as possible, to avoid the propagation of the beetles to healthy trees. One possibility, to utilize the accruing crown material in a reasonable way (instead of burning it) could be, to use it as wood-chips for biomass heating plants. The aim of this project was therefore to determine the productivity of two harvesting and processing systems for wood-chips as a joint-product of round wood in Norway spruce (Picea abies L.) forests affected by bark beetles. Two systems with different sorting criteria were studied: processing of sawlogs, pulpwood and wood-chips (System A) in comparison to the processing of only sawlogs and wood-chips (System B). In System A, the energy wood was chipped with a chipper mounted on a forwarder that was working directly in the stand. In System B, the material to be chipped was previously concentrated along the forest road with a forwarder, and a chipper mounted on a truck was used for chipping.

In System A, 0.18 t of dried chips could be harvested per m3 of round wood, and in System B 0.26 t of dried chips per m3 of round wood. The cost of chipping in the stand was 4.74 /m3 of chips and the cost of chipping along the forest road after transporting the chipping material by a forwarder was 5.63 /m3 of chips. Therewith, a cost-covering supply of wood-chips may be obtained out of such stands. Concerning the ratio of energy input to energy output it can be said that the systems required 1.5% and 2% of energy output that was obtained using the respective system.

Keywords: biomass, wood-chips, Picea abies, bark beetle

1. Introduction – Uvod In the last years, governments of EU member countries have promoted the use of renewable energy sources. One of the main sources for renewable energy is the combustion of biomass, which is nearly neutral in the cycle of CO2. Therefore, many biomass-heating plants have been constructed. Actually, most of these plants are supplied with residues from the wood industry (Heller et al. 2004). As this raw material is limited, new resources have to be tapped. For example up to now the biomass produced in agricultural and forestry systems has not been fully mobilized and used for energetic purposes, due to still unsolved technical problems, high costs or missing information about the potential and quality of such biomass (FAO 1997, FAO 2003, AnCroatian Journal of Forest Engineering 28(2007)2

dersen et al. 2005). Therefore, it is necessary to evaluate the potentials of biomass and its quality coming from forestry and agriculture, and especially to examine the technology available for harvesting and processing it.

One possibility to obtain woody biomass for energetic purposes is the utilization of trees that have to be felled and removed due to attacks of bark beetles (Ips typographus L.). These operations show special characteristics. Typically, they are small clear cuts with an area ranging between 0.3 and 1.0 ha. This is due to the fact that the trees, surrounding the infected trees, are often affected by bark-beetles, too, although they do not show yet any visible signs of attack on the surface. A further spread of the beetle should be hindered by cutting the neighbouring trees.

Contrary to conventional harvesting operations, all

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Table 1 Main characteristics of the stands Tablica 1. Osnovne zna~ajke sastojina Stand – Sastojina Harvesting system – Sustav pridobivanja drva Species – Vrsta drve}a Area – Povr{ina, ha Medium DBH – Srednji prsni promjer, cm No. of trees per hectare – Broj stabala po ha Terrain slope – Nagib terena, % Skidding distance – Udaljenost privla~enja, m

Fig. 1 Processing systems studied (d – the top-diameter of the stem-parts) Slika 1. Istra`ivani sustavi izradbe drva (d – promjer na tanjem kraju dijela debla) the crown material has to be taken out of the stand, to deprive the bark beetles’ breeding material. Typically, the material is then burnt, which means very high labour costs and no income at all. Furthermore, the felling of affected trees is more difficult than the felling of trees that are not affected, as their crowns have less weight. Therefore the trees do not fall as easy as trees with green, living crowns. So far only very few studies have been made on the harvest of trees affected by bark-beetles (example KWF 2004).

Although many foresters have to deal with these problems in their day to day work, hardly any recommendations can be found as to how to best deal with them.

Therefore, the objective of the present work was to compare different approaches for harvesting and

1 2 A B Picea abies (L.) 0.46 0.56 49 44 124 264 2 5 350 500

processing trees in Norway spruce (Picea abies L.) stands affected by bark beetle (Ips typographus L.).

Hence, the goal was to compare the profitability of processing sawlogs, pulpwood and wood-chips (System A) with the profitability of processing only sawlogs and wood-chips (including the chipping of the traditional pulpwood-assortments (System B).

The sorting criteria of both systems can be seen in Figure 1.

Additionally, two chipping systems (chipping directly in the stand and chipping along the forest road) were analyzed in order to prove techniques that could be suitable for certain stands. In both systems the following parameters were evaluated: productivity, costs and energy balance of the whole supply chain and the volume of wood-chips that could be obtained with the respective system.

2. Material and Methods – Materijal i metode The main characteristics of the stands to be harvested can be seen in Table 1. In both systems conventional (motor-manual) chainsaw felling and processing and skidder log extraction have been carried

Table 2 Main characteristics of the machines Tablica 2. Osnovne zna~ajke strojeva Machines – Strojevi

Manufacturer and model – Proizvo|a~ i model

Chainsaw – Motorna pila

Husqvarna 394XP – Husqvarna 357XP

Skidder – Skider

Mercedes Benz Trac 800

Forwarder (System B) – Forvarder (sustav B)

Gremo 950R

Chipper mounted on a forwarder (System A) Ivera~ postavljen na forvarder (sustav A)

ERJO 7/65 RC (»ERJOFANT«) Power – Snaga: 272 kW Opening – Ulazni otvor: 40 x 67 cm, Chip reservoir – Obujam spremnika iverja: 10 m3

Chipper mounted on a truck (System B) Ivera~ postavljen na kamion (sustav B)

Man Truck, Wüestling 600 CV Power – Snaga: 442 kW; Opening – Ulazni otvor: 70 x 120 cm

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Fig. 2 Mobile chipper working in the stand Slika 2. Rad mobilnoga ivera~a u sastojini

Fig. 3 Chipper mounted on a truck, working along the forest road Slika 3. Ivera~ postavljen na kamion pri radu na {umskoj cesti out. The team was formed by two workers. One of them only cut and processed the trees; the other worker drove the skidder and occasionally supported the felling. In System A, the energy wood was chipped directly in the stand using a mobile chipper mounted on a forwarder (Figure 2). In System B, the Croatian Journal of Forest Engineering 28(2007)2

energy wood was concentrated with a forwarder in piles along the forest road and then chipped using a chipper mounted on a truck (Figure 3). The characteristics of the machines used are shown in Table 2.

To evaluate the productivity, all operations were supported by time studies. The time of effective work

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and the total working time were recorded for each worker and machine. All times were defined according to the REFA-guidelines (1991). At the same time, the volume was measured of all the logs produced during the operation, as well as the volume of each container filled with chips.

In addition to this, the following parameters were determined for every container filled with chips: Þ Moisture content of chips (%) (determined according to prCEN/TS 14774-2) Þ Ratio of different size fractions (%) (determined according to CEN/TC 335/EG 4) Þ Calorific value at different moisture contents and separated for different size fractions (determined with a calorimeter (IKA 2000)) Þ Coefficient of wood-chips potential, calculated by the following equation: li =

Vi chips V

Where li is the gravimetric coefficient of the potential biomass for energetic utilization in a system of i-characteristics; this coefficient is defined as tons of dry chips (Vi chips) that can be obtained as a byproduct by recovering the residues generated from the harvest of 1 m3 of conventional roundwood – sawlogs and/or pulpwood (V).

3. Results and Discussion – Rezultati s diskusijom 3.1 Processed timber products – Izra|eni drvni proizvodi The products obtained in both systems are shown in Table 3: a remarkably lower volume of sawlogs and wood-chips was harvested in System A. This is due to the fact, that in System A some trees (especially fir – Abies alba Mill.) could be left in the stand, whereas in System B all trees had to be taken out. As in System B a bigger part of the trees’ biomass is used for energetic purposes instead of producing pulpwood as in System A, the coefficient for the wood-chips potential in System B is 0.26 and therewith 44.4% higher in comparison to System B, where the coefficient is 0.18.

3.2 Productivity for felling and processing of sawlogs and pulpwood – Proizvodnost sje~e i izrade pilanskih trupaca i celuloznoga drva The distribution of the effective work time and the productivity of forest workers in both systems are shown in Figure 4. It can be noticed that forest workers had a higher productivity in System B, although the average diameter of trees in System A was 5 cm bigger than the average diameter of trees in System B, and hence a higher volume of wood as-

Table 3 Processed timber products Tablica 3. Izra|eni drvni proizvodi Stand – Sastojina

1

2

Harvesting system – Sustav pridobivanja drva

A

B

Number of trees – Broj stabala

57

148

Sawlogs (diameter > 48 cm, length 5 m), solid Pilanski trupci (promjer > 48 cm, duljina 5 m), m3oblovine

24.6

16.7

Sawlogs (diameter > 15 and < 48 cm, length max. 19 m), m3solid Pilanski trupci (promjer od 15 cm do 48 cm, najve}a duljina 19 m), m3oblovine

70.6

189.4

Pulpwood (diameter > 15 cm, length 5 m), m3solid Celulozno drvo (promjer > 15 cm, duljina 5 m), m3oblovine

11.8

–

Total volume of sawlogs and pulpwood, m3solid Ukupni obujam pilanskih trupaca i celuloznoga drva, m3oblovine

107

206.1

Total volume of wood-chips, m3loose Ukupni obujam drvnoga iverja, m3nasipni

88

243

19,530

53,940

0.18

0.26

m3

Total mass of wood-chips (oven dry), kg Ukupna masa drvnoga iverja (suhe tvari), kg Coefficient lj, (tons of wood-chips per m3 of sawlogs and pulpwood) Koeficijent lj, (tona drvnoga iverja po m3 pilanskih trupaca i celuloznoga drva)

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T. CREMER and B. VELAZQUEZ-MARTI

sortments was obtained. This is mainly due to the fact, that in System B no pulpwood was produced.

Therefore, less time was needed for debranching, cross-cutting and measuring the assortments. Consequently, the processing ratio in System A is 53% of effective work, whereas it is only 45% in System B. The evaluation of working times and the productivity of the skidder are depicted in Figure 5. The effective work-time distribution of the skidder does not differ much between the two systems. In System A, less time is needed for driving into the stand and driving back to the piling site, which can be explained with a shorter skidding distance (System A: 350 m, System B: 500 m). Nevertheless, more time is needed for unloading the skidder in this system. This is due to the fact, that, by processing pulpwood, three instead of two assortments had to be transported, which means less productivity due to a lower volume per piece and more time for sorting at the piling site.

Fig. 4 Effective work-time distribution and productivity of forest workers – Motor-manual felling and processing Slika 4. Raspodjela efektivnoga vremena rada i proizvodnosti radnika sjeka~a pri strojno-ru~noj sje~i i izradi

Fig. 5 Effective work-time distribution and productivity of the skidder Slika 5. Raspodjela efektivnoga vremena rada i proizvodnosti skidera Croatian Journal of Forest Engineering 28(2007)2

3.3 Productivity of the forwarder (only System B) and the chipper – Proizvodnost forvardera (samo u sustavu B) i ivera~a The forwarder, required in System B for concentrating the chipping material along the forest road, had a productivity of 23.7 m3loose/h. This rather high productivity that was fostered by a comparably low skidding distance (<100 m) is remarkably higher compared to e.g. a study conducted by the KWF (2004) in comparable stands, where a forwarder reached a productivity of only 17 m3loose/h.

The following chipper mounted on a truck in this system reached a productivity of 69.8 m3loose/h. This is slightly higher in comparison to other studies done for example by Asikainen and Pulkkinen (1998) or Basse et al. (2002) for chippers with similar characteristics. Asikainen and Pulkkinen (1998) determined a productivity of 55 m3loose/h, whereas Basse et al. (2002) calculated a productivity of 40–60 m3loose/h, depending on the average volume of the trees chipped. In studies by Deutschländer-Wolff (2006) or Schuler (2007) comparable chipping systems reached (only) similar productivities, although the chipping conditions were more favourable in comparison to the present study, due to a much higher pre-concentration of the chipping material.

In System A, the productivity of the mobile chipper working directly in the stand is 36.4 m3loose/h.

Therewith, the chippers’ productivity is higher in comparison to other studies: Lechner et al. (2007), calculated for a comparable chipper in beech-stands that are ready for thinning a productivity of only 22.5 m3loose/h and in a study of Thor (1996), a similar

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chipper reaches a productivity of approx. 30 m3loose/h in spruce-beech-stands.

When only looking at the productivity of the chipper, it can be said that the productivity of a chipper working at the forest road is nearly 50 % higher in comparison to a chipper working directly in the stand. The higher productivity resulted from the fact that the material was very well concentrated in piles along the forest-road and the chipper therewith could work more continuously. Additionally, the assortment that could have been used for pulpwood was chipped, too, which obviously increases productivity, too. Another reason – of course – is the higher engine power of the chipper working at the forest road (442 kW in comparison to 272 kW of the chipper working directly in the stand), which also highly influences productivity.

In Figure 6, the differences between the two chipping systems can clearly be seen: the time for manipulating the wood is 34.1% when looking at the mobile chipper working directly in the stand and therewith remarkably higher in comparison to the chipper working at the forest road with only 4.9%.

On the other hand, the ratio of the time for the chipping itself is significantly higher when looking at the chipper working at the forest road (58.6%) in comparison to the mobile chipper working in the

Fig. 6 Effective work-time distribution of the chipper Slika 6. Raspodjela efektivnoga vremena rada ivera~a 150

stand (38.8%). The working time for driving in and out the stand, that do not exist in System B, are relatively low (8.8% and 6.3%), due to a rather short skidding distance of less than 100 m. In the study by Lechner et al. (2007) that was already mentioned above, slightly higher ratios occur, as a consequence of a longer skidding distance. As the transport of the chips was well organised when working with the mobile chipper in the stand, no waiting times for empty containers need to be noted.

On the other hand the chipper in System B was waiting about 40% of the working time for new, empty containers (Figure 6), which is a very high quota, and still – according to Wittkopf (2005) – this is a rather usual proportion in practice. The goal of chipping operations should always be to minimize standing-times of the chipper and there is still a rather high potential for optimising logistics. If the logistic system were organised in a better way, and if waiting-times could be reduced to 10%, the productivity of the chipper working at the forest road would be close to 100 m3loose/h. Therewith, it can clearly be seen that an optimization of logistics in chipping operations (continuous transport of chips and delivery of new, empty containers) is crucial.

3.4 Costs for the supply of wood-chips – Tro{kovi dobave drvnoga iverja The costs for tree felling are related to different assortments of round wood obtained (saw logs and pulpwood). Therewith, no costs accrue for the harvest of the chipping material and only the costs for forwarding the material out of the stand and the costs for chipping have to be considered. In the System A, the calculated cost per working hour of the mobile chipper were 150 /h (including 15% additional times for delays and rests). In System B, the underlying costs were 65 /h for the forwarder and 150 /h for the chipper mounted on a truck (also including 15% additional times for delays and rests). The total costs of both systems are shown in Figure 7.

The costs of both systems are comparable: 4.74 /m3loose in System A and 5.63 /m3loose in System B (including forwarder and chipper). Therewith, assuming a revenue for wood-chips of 12 /m3loose at forest road, a net revenue between 6.37 and 7.26 /m3loose can be gained when producing wood-chips with the presented systems. Of course, the costs only for chipping of the material are remarkably higher in System A. However, as the costs for forwarding the chipping material to the forest road are obligatory in System B, System A is a more favourable solution, when taking into consideration overall costs for the supply of wood-chips. This is even truer, when looking at the assortments produced in both systems. It Croatian Journal of Forest Engineering 28(2007)2

Evaluation of two harvesting systems for the supply of wood-chips ... (145–155)

Fig. 7 Costs for the supply of wood-chips Slika 7. Tro{kovi dobave drvnoga iverja can be assumed that the mobile chipper in System A would have a significantly higher productivity if the pulpwood-assortments were chipped, too, as in System B. In reverse, the productivity of the forwarder and chipper in System B would both reach a lower productivity, if the pulpwood assortments were chipped. As costs per m3loose differ only by 1 /m3loose, it can be assumed that the mobile chipper working directly in the stand is a more favourable system. On the other side, if costs per working hour of the chippers rise up to 250 /h, costs per m3 chips approximate and differences become only marginal. Still, it should be taken into consideration that when using a mobile chipper working directly in the stand, fewer machines are needed and the organizational efforts, etc. are lower.

These results are significant insofar, as the conventional wisdom is refuted that chipping in the stand is more expensive and not profitable (Wittkopf et al. 2003). On the other hand, the results gained by Lechner et al. (2007) are confirmed. On a cautionary note, however, it has to be said that skidding distance is a factor that highly affects the productivity of a mobile chipper working directly in the stand: in this study, skidding distance was less than 100 m.

From a certain distance onwards, a shuttle-forwarder for transporting the chips to the forest road has to be used. This again leads to higher costs and fosters therewith a chipping of the traditional pulpwood assortment together with the remaining crown-material. As shown in Figure 4 and 5, the producCroatian Journal of Forest Engineering 28(2007)2

T. CREMER and B. VELAZQUEZ-MARTI

tivity of forest workers and skidder is lower when producing stem wood and pulpwood instead of producing only stem wood and avoiding the processing of pulpwood. The same is true for the chipper: its productivity is remarkably higher, when chipping crown material and the traditional pulpwood assortments. Therewith, costs for felling, skidding and chipping rise when pulpwood is processed as a separate assortment. Consequently, it can be estimated, that with a motor-manual supply of pulpwood, a cost recovery for this assortment can scarcely be reached. This conclusion is confirmed through a study made by Köberle (2007), who also states lower costs for the motor-manual felling and processing of trees, when only stem wood in combination with woodchips is produced and no pulpwood is processed.

However, it has to be clearly pointed out that this conclusion is not true for fully mechanized harvest, when a harvester is used, as the additional working time for processing pulpwood is only marginal for a harvester! In this situation, the processing of pulpwood is – assuming current revenues for pulpwood – a more favourable solution (see also Cremer 2007, Lechner 2007).

3.5 Wood energy characteristics – Energetske zna~ajke drva The chips of Norway spruce (Picea abies L.), produced in this study, had a moisture content of 34.7% in respect to wet weight (Mh), and 56.2% in respect to dry weight (Ms). This rather low moisture content is not surprising, as the trees were standing dead in the forest for several months before they were felled and chipped. At this moisture content, chips density was 217.14 kg/m3loose and the obtained calorific value was 12.35 MJ/kg. Both values are comparable to the values determined by Golser (2004). After oven-drying the chips, the density was 141.8 kg/m3 and the calorific value increased up to 19.33 MJ/kg.

The ratio of different fractions and its characteristics that were obtained after sifting the chips is shown in Table 4. It can be noted that the smaller fractions have lower calorific values. This is surprising: often, the smaller fractions contain a high proportion of needles and bark (Suadicani and Gamborg 1999), which have – due to a high content of extractives – a significantly higher calorific value in comparison to stem wood (Nurmi 1993), which is mainly found in the coarser fractions. On the other side, the trees in this study were dead for several months before they were felled and most of the needles and high portions of the bark were already fallen down. Therewith, the calorific value of the smallest fraction obviously decreased (Suadicani and Gamborg 1999). Additionally, a high mineral

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Table 4 Characteristics of different chip-fractions Tablica 4. Zna~ajke razli~itih ~estica drvnoga iverja Fraction diameter Veli~ina ~estice

Ratio Udjel

Density Gusto}a

Calorific value* Ogrjevna vrijednost*

mm

%

g/cm3

MJ/kg

63

3.2

0.1821

19.52

45

32.9

0.2391

19.24

16

31.2

0.1996

18.95

5

21.0

0.1929

18.31

3.15

11.7

0.2270

17.32

to System A. This is due to the fact that the chip production in System B needs two machines (forwarder and chipper) whereas in System A only one machine (the chipper) is needed. Still, the energetic input in both systems is generally low: as it can be seen in Table 5, in System A, 1.5% of the energy that is obtained has to be put into the system to produce wood-chips. In System B, slightly more energy has to be used to obtain the same energetic output. Still, these results strongly support an application of both systems for the production of wood-chips from trees affected by bark-beetles.

4. Conclusions – Zaklju~ci

* oven dried – suha tvar

content is often found in the smallest fraction due to contamination with mainly mineral soil, which again decreases the calorific value.

According to Table 4, it can be said that it is useful to sift the chips. Therewith it is possible to eliminate the smallest fractions of the chips and the energy that can be obtained increases by 6%. Hence, the uniformity and quality of the chips increases and the chips can be sold at a better price.

3.6 Energy balance – Energetska bilanca As the calorific value of diesel is approx. 47 MJ/kg and its density is 680 kg/m3, the calorific value per litre of diesel is 31.96 MJ/L. In System A, the calculated diesel consumption per effective working hour of the mobile chipper resulted in approx. 40 L/h.

In System B, the average diesel consumption of the forwarder was approx. 10 L/h and diesel consumption of the chipper mounted on the truck was 68 L/h in average. In Table 5, the energetic balance is carried out. It can be noted that System B requires an energetic input that is 34.7% higher in comparison

Summarizing the results, it can be said that a cost-effective supply of wood-chips out of stands affected by bark beetles is possible. Consequently, a reasonable utilization is given of the crown material that was burnt so far. When looking at suitable supply chains, the differences in costs are less than 1 /m3loose between the two chipping systems and therewith surprisingly small.

Regarding the optimal sorting, it can be said that avoiding the processing of pulpwood (and producing only stem wood assortments and wood-chips) seems to be a favourable alternative when harvest is done motor-manually. As a consequence, forest workers as well as the skidder and chipper reach a higher productivity and therewith cost per piece for stem wood and for wood-chips decreases.

The calorific value of wood-chips did not differ significantly from other studies. Sifting of chips can be useful to eliminate the smallest fractions and thereby to increase the energy output. Additionally, the chips can be sold at a better price. In both systems, the energy output of the chips is by far higher in comparison to the energetic input that has to be invested to produce the chips.

Table 5 Energetic balance of the production of wood-chips Tablica 5. Energetska bilanca pridobivanja drvnoga iverja System – Sustav

A

B

Fuel consumed for the supply of wood-chips, L/m3loose Utro{ak goriva pri dobavi drvnoga iverja, L/m3nasipni

1.13

1.74

36.11

55.61

2681.68

2681.68

Energetic input per m3 of chips at natural moisture content, MJ/m3loose Ulo`ena energetska vrijednost po m3 iverja pri prirodnom sadr`aju vlage, MJ/m3 nasipni Energetic output per m3 of chips at natural moisture content, MJ/m3loose Dobivena energetska vrijednost po m3 iverja pri prirodnom sadr`aju vlage, MJ/m3 nasipni Energetic balance (ratio energetic input / output), % Energetska bilanca (odnos ulo`ene i dobivene energetske vrijednosti), %

152

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Acknowledgements – Zahvala This work has been carried out by the Institute of Forest Utilization and Work Science of the AlbertLudwigs-University of Freiburg (Germany) and was funded by the Deutsche Bundesstiftung Umwelt (DBU). The participation of Dr. Borja Velázquez-Martí in this project was enabled through a postdoctoral grant from the Ministry of Education and Science of the Spanish government.

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ferent systems for the supply of wood-chips from crown material with respect to profitability and quality of the wood-chips). Diplomarbeit an der Hochschule für Forstwirtschaft Rottenburg.

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Suadicani, K., Gamborg, C., 1999: Fuel quality of wholetree chips from freshly felled and summer dried Norway spruce on a poor sandy soil and a rich loamy soil. Biomass & Bioenergy 17, 199–208.

Thor, M., 1996: Chipset 536 C stickvägsgaende flisare – tidstudie ach systemanalys (Chipset 536C stickvägsgaende flisare – time study and system analysis) Skog Forsk, Uppsala, Sweden, 1–16.

Wittkopf, S., Hömer, U., Feller, S., 2003: Bereitstellungsverfahren für Waldhackschnitzel: Leistungen, Kosten, Rahmenbedingungen (Supply-chains for chips out of the forest: productivity, costs and general conditions). Bericht aus der Bayerischen Landesanstalt für Wald und Forstwirtschaft, Munich, Germany, 1–82.

Wittkopf, S., 2005: Bereitstellung von Hackgut zur thermischen Verwertung durch Forstbetriebe in Bayern (Supply chains for thermic utilization by forest companies in Bavaria). Dissertation an der Technischen Universität München, Fakultät Wissenschaftszentrum Weihenstephan für Ernährung, Landnutzung und Umwelt., 1–209.

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Sa`etak

Ocjena dvaju sustava pridobivanja drvnoga iverja iz smrekovih {uma o{te}enih pojavom potkornjaka Smrekova je stabla o{te}ena napadom potkornjaka potrebno izvaditi iz sastojine {to je prije mogu}e kako bi se sprije~ilo {irenje {tetnika. Pri tome se na malim povr{inama izvodi ~ista sje~a o{te}enih stabala i susjednih stabala na kojima jo{ nisu vidljivi znakovi o{te}enja. Za razliku od uobi~ajenih postupaka pridobivanja drva potrebno je sve drvo (oblo drvo i granjevinu) iznijeti iz sastojine. Dosada{nje spaljivanje drva imalo je visoke tro{kove rada bez financijske isplativosti. Stoga se preporu~uje iskoristiti drvo iz sanitarne sje~e za pridobivanje iverja. Cilj je rada usporediti dva razli~ita sustava pridobivanja drvnoga iverja iz smrekovih sastojina o{te}enih pojavom potkornjaka.

Sustav A razumijeva izradu pilanskih trupaca, celuloznoga drva i drvnoga iverja iz preostaloga drva. Sustav B razumijeva izradu jedino pilanskih trupaca iz debla, dok se celulozno drvo pridodaje drvu za iveranje. Kriterij je razvrstavanja dijelova stabala u navedene drvne proizvode u sustavima A i B prikazan na slici 1. Tako|er su analizirana dva sustava iveranja (iveranje u sastojini i iveranje na {umskoj cesti) radi odre|ivanja prikladnije metode u uvjetima sanitarne sje~e smrekovih stabala. Pri tome su promatrani ovi parametri: proizvodnost, tro{kovi, energetska bilanca sustava i obujam drvnoga iverja koje se mo`e proizvesti primjenom odre|enoga sustava pridobivanja.

Osnovne su zna~ajke ispitivanih sastojina prikazane u tablici 1. U oba se sustava sje~a i izrada stabala obavljala motornom pilom, dok su se pilanski trupci i celulozno drvo privla~ili skiderima. U sustavu A drvo se iveralo u sastojini mobilnim ivera~em postavljenim na forvarder (slika 2), dok se u sustavu B drvo za iveranje (uklju~uju}i celulozno drvo) prikupljalo forvarderom i slagalo u slo`ajeve pored {umske ceste gdje se iveralo ivera~em postavljenim na kamion (slika 3). Osnovne su zna~ajke kori{tenih strojeva prikazane u tablici 2. Za odre|ivanje proizvodnosti proveden je studij rada i vremena za svakoga radnika i stroj u sustavu te izmjereni obujmi izra|enih drvnih sortimenata i obujmi kontejnera napunjenih drvnim iverjem. Iz uzorka je svakoga kontejnera odre|en sadr`aj vlage iverja, raspodjela iverja po veli~ini ~estica, ogrjevna vrijednost i koeficijent iverja odre|en odnosom mase suhe tvari koja se mo`e pridobiti od drvnoga obujma obloga drva.

U sustavu A zna~ajno je manji obujam izra|enih drvnih sortimenata (pilanskih trupaca i celuloznoga drva) jer su pojedina jelova stabla ostala na povr{ini sje~ine, dok su u sustavu B sva stabla posje~ena na povr{ini sje~ine. U sustavu B ve}i se dio stabala koristi za pridobivanje drvnoga iverja te je zna~ajno ve}i ukupni obujam dobivenoga drvnoga iverja (tablica 3).

Utro{ci su efektivnoga vremena rada i prozvodnosti radnika sjeka~a i skidera prikazani na slikama 4 i 5. Radnik sjeka~ u sustavu B ima ve}u proizvodnost, iako je u sustavu A ve}i srednji promjer stabala i ukupni obujam izra|enih drvnih sortimenata. Razlog le`i u ~injenici da se u sustavu B iz debla izra|uju jedino pilanski trupci te je potrebno manje vremena za kresanje grana, trupljenje i preuzimanje. Efektivno se vrijeme rada skidera ne razlikuje izme|u sustava A i B. U sustavu A udaljenost je privla~enja manja, ali je ve}i utro{ak vremena na pomo}nom stovari{tu zbog razvrstavanja drvnih sortimenata iz tovara na pilanske trupce i celulozno drvo.

U sustavu A proizvodnost mobilnoga ivera~a iznosi 36,4 m3nasipni/h. U sustavu B proizvodnost forvardera pri izvo`enju energijskoga drva iznosi 23,7 m3nasipni/h, a ivera~a postavljenoga na kamion 69,8 m3nasipni/h. Ve}a je proizvodnost ivera~a na kamionu posljedica ve}e snage pogonskoga motora te skupljanja drva za iveranje u slo`ajeve uz {umsku cestu, ~ime se omogu}uje neprekidan rada ivera~a (slika 6).

Ukupni su tro{kovi po jedinici izra|enoga drvnoga iverja (slika 7) izra~unati na osnovi proizvodnosti i odre|enoga tro{ka radnoga sata, koji iznosi 150 EUR/h za mobilni ivera~ i ivera~ na kamionu te 65 EUR/h za forvarder (uz dodatno vrijeme od 15 % za sve strojeve). Jedini~ni je tro{ak iveranja manji u sustavu B, ali je ukupni jedini~ni tro{ak ve}i nego u sustavu A zbog tro{ka forvardera pri izvo`enju drva za iveranje.

Pri sje~i i izradi stabala preporu~uje se izrada jedino tehni~ke oblovine iz debla te iveranje celuloznoga drva uz preostalo drvo, {to }e za posljedicu imati ve}u proizvodnost radnika sjeka~a, skidera i ivera~a. Mobilni ivera~ u sustavu A mo`e posti}i zna~ajno ve}u proizvodnost ako se za pridobivanje drvnoga iverja koristi i celulozno drvo.

Prednost sustava A tako|er se ogleda u primjeni manje sredstava rada te time u jednostavnijoj organizaciji rada.

Na osnovi se rezultata zaklju~uje da je pridobivanje drvnoga iverja metodom iveranja u sastojini povoljniji sustav.

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T. CREMER and B. VELAZQUEZ-MARTI

Sadr`aj je vlage u iverju vrlo nizak (34,7 % u odnosu na masu svje`e tvari, tj. 56,2 % u odnosu na masu suhe tvari) jer je pridobiven iz suhih smrekovih stabala zbog napada potkornjaka. U tablici 4 vidljivo je da manje ~estice drvnoga iverja imaju manju ogrjevnu vrijednost, {to je posljedica velikoga udjela iglica i kore. Ve}e se ~estice iverja pridobivaju ponajprije iveranjem drva debla te stoga imaju ve}u ogrjevnu vrijednost. Razdvajanje drvnoga iverja po veli~ini ~estica mo`e biti opravdano radi pove}anja dobivene energetske vrijednosti i postizanja ve}e cijene drvnoga iverja.

Za prora~un se energetske bilance (tablica 5) u sustavu A uzela potro{nja goriva ivera~a na forvarderu od 40 L/h, odnosno u sustavu B potro{nja goriva ivera~a na kamionu od 68 L/h i forvardera od 10 L/h. Sustav B tro{i 34,7 % vi{e energije od sustava A jer zahtijeva uporabu dvaju strojeva – forvardera i ivera~a. No, op}enito je u oba sustava malen utro{ak energije s obzirom na energetsku vrijednost dobivenoga drvnoga iverja.

Klju~ne rije~i: biomasa, drvno iverje, Picea abies, potkornjak

Authors’ addresses – Adresa autorâ: Tobias Cremer, MSc. e-mail: tobias.cremer@fobawi.uni-freiburg.de Albert-Ludwigs University Freiburg Institute of Forest Utilization and Work Science Werthmannstraße 6 79085 Freiburg i. Br. GERMANY

Received (Primljeno): September 28, 2007 Accepted (Prihva}eno): November 19, 2007 Croatian Journal of Forest Engineering 28(2007)2

Borja Velazquez-Marti, PhD. e-mail: borvemar@dmta.upv.es Polytechnic University of Valencia Department of Mechanization and Agrarian Technology Camino de Vera 14 46022 Valencia SPAIN

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Original scientific paper – Izvorni znanstveni rad

Influence of working conditions on overlapping of cutting and ground skidding in group work Anton Poje, Igor Poto~nik Abstract – Nacrtak This research deals with the influence of working conditions on the duration of cutters participation in group work skidding. Two main theoretical assumptions were considered.

First, working conditions that decrease the skidding efficiency increase the time of cutters assistance, because the group as a whole strives to improve efficiency, and second, the duration of assistance to the working group is specific regardless of working conditions. The research included 5 places of research (compartments) in Slovenian state forests, with 3 working groups consisting of two cutters and a cable skidder driver. All working groups used the cable skidder IWAFUJI–T41. The model included 100 cycles of downhill timber skidding. We have established that the duration of cutters assistance is reciprocal to the skidding distance, average bunching distance and number of logs in the load. The model accounts for 40.7% of the whole variability. After excluding the influence of working conditions between the groups, there are no typical differences in the duration of cutters assistance. Results imply that the number of working group members should be adjusted to working conditions. On the other hand, insignificant differences of cutters assistance duration between the groups show that the mere group daily standard is a motivational factor big enough for the rationalization of group work of workers who are not additionally educated and qualified for group work.

Keywords: group work, cutting, skidding, working conditions, multivariate analysis

1. Introduction – Uvod Regardless of new technologies, such as cut-tolength logging, the traditional way of logging (use of chainsaw and cable skidder) will remain an important working technology in future, especially in the forests with difficult terrain conditions, in smaller private forests where personal work represents the essential profit for the owner, and in forests with small private patches of land. The fundamental characteristic of group work, as one of the organizational forms of traditional logging, is the group work with at least two members, who simultaneously do the cutting and ground skidding, and also assist each other. We estimate that this kind of method represents the prevailing organizational form, according to the amount of work and quantity of timber cut in Slovenian state forests.

Croatian Journal of Forest Engineering 28(2007)2

In practice the size of working group depends especially on the capacity of skidding means. Therefore, the working groups are usually bigger with the use of cable skidders as opposed to the use of adjusted farm tractors. The reason lies in the relatively high price of skidding means. For the purpose of increasing the utilization of skidding means, the workers in a group have a group daily standard (expected daily efficiency) and receive piece-rate payments. Thus, the workers, as circumstances require, are »forced« to participate in a working phase that represents the bottleneck for the optimal group work efficiency. Therefore, the working time structure and overlapping of cutting and skidding processes change from group to group (Klun and Poje 2000).

With this research we have tried to add some new knowledge to relatively poorly studied relations between cutting and ground skidding in group work, and also highlight the problem of group work form

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Influence of working conditions on overlapping of cutting .... (157–167)

in altering working conditions in the forest, with the purpose of including the ergonomic and economic work organization as additional starting points.

2. Previous research – Dosada{nja istra`ivanja In Slovenian forestry, the group work has been introduced to cutting and ground skidding process in the early 1970s. With complete mechanization of timber skidding that occurred around 1970, the tractor work efficiencies started to decrease, especially with cable skidders. Thus, the precondition for forming the new working processes was made, suggesting cutting and skidding group work. The group work demands a highly skilled worker, who is able to perform any kind of work in the group. This requires additional and constant education, and also employment stability for workers. This can only be achieved with better evaluation of forest work (Krivec 1979).

Since 1979, different authors have theoretically and practically considered group work from different perspectives. Kri` (1984) describes the reasons for introducing group work and group work experience in the company Posestvo Sne`nik. Organizational forms of group work have changed from I+2 (1 tractor driver, 2 cutters), I+3, to II+6, and in extreme cases up to II+12. The reasons for introducing group work were the increased timber cut stocks in forests and unused co-worker in skidding process; whereas the reasons for increase of the later group were timber volume composition at the roadside landing and additional but unused group leader.

Kerne`a (1999) in his later research in the same company, states that the efficiency of cutting and skidding with adapted farm tractors in smaller groups has increased with respect to larger groups, whereas the trend is quite the opposite in the skidding process with cable skidders.

Cimper{ek (1987) emphasizes that the reliability of working means is the most important production factor and also adds that the workers’ premium pay based on quantity and disregarding the quality, economy and damage decrease has an adverse effect on the worker. The work performance in this case is a routine one. Only after the change of remuneration mode the process of mass innovation can begin.

Korbar (1988) theoretically discusses the form and conditions of group work organization, and advantages and disadvantages of this kind of work. As essential advantages he highlights humanization, synchronization that enables the implementation of forest management plans and shortens the time from the cut to the timber sale. The work gets intellectual-

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ly more demanding and enables workers’ creativity and innovation, strengthens the sense of responsibility, increases performance and improves the worker’s relationship to forest and working means. The conditions for group work are proper working means, quick flow of material and information, physically and psychically fit workers with adequate knowledge.

By comparing individual and group cutting and skidding work, Kruh (1989) states the rationalization of chainsaw working time, unproductive time, time for wood bunching from the point of view of the cable skidder, and also the whole time required for cutting and skidding. He defines the decrease of unproductive time as the disadvantage of group work, because it decreases the time for breaks and other worker’s needs.

Winkler (1990) sees the optimal form of working group as the complex working group that performs all works on the permanent, smaller forest surface, with strict cutting and skidding time synchronization with silviculture performed only in specific time periods.

When determining the standard times for the organizational form I+2 and skidding timber with cable skidder IWAFUJI T–41, Klun and Poje (2000a, 2000b) establish that the efficiency of wood bunching and work on roadside landing is lower in comparison with traditional organizational form I+1 (one driver and one helper – hooker). The reason lies in the assistance of tractor driver to cutters and additional time required for timber classifying at the roadside landing. Indirectly, they also establish as follows: cutters participate in ¾ of all recorded cycles of the skidding process; according to working operations where the cutters participate in the skidding process, each group has a specific working method; the time cutters spend participating in a bunching process during the timber skidding at the same time depends on the number of logs in the load, the average piece volume and the distance in wood bunching process, and separately on the skidding distance.

Authors state that the need for introducing group work has changed through time. While in the 1970s there were tendencies for the humanization of work, today the aim is to decrease expenses and increase flexibility, and thus enable the increase of innovation flow in companies and working processes. Therefore, the group work seems ideal for raising productivity and quality on one hand and for decreasing expenses on the other hand. By participation of forest workers in planning, organization and control, possibilities arise for the utilization of unused innovation and talent potentials (Lewark et al. 1996).

With greater participation of workers in company’s Croatian Journal of Forest Engineering 28(2007)2

Influence of working conditions on overlapping of cutting .... (157–167)

activities, partial responsibility shifted to workers, and additional education in German ThüringenForst, the company has managed to improve working environment and joy at work, decrease delays and increase workers’ self control in highly mechanized cutting and skidding processes. In four years the positive effects of group work were seen in greater harvester (44%) and forwarder (49%) utilization (Findeisen 2002).

Utilization of working means influences the optimal size of working groups. Ze~i} and Maren~e (2005) have developed a mathematical model for optimal group size, conducting cutting, skidding processes and simultaneous timber manipulation on roadside landings. They state that with skidding distance from 150 m to 650 m the optimal working group consists of 6 cutters (5.3), 5–7 tractor drivers (5.1–6.7) and 1 cutter – group leader.

3. Theoretical basis and research aims – Teorijske osnove i cilj istra`ivanja Cutting and skidding group work consists of two working processes that are, unlike the individual work, partly overlapping. Apart from safe and ergonomically designed work, one of the main aims of group work is also to maximize the effect. It means that the sum of cutting time (tc), skidding time (ts), and the time when these two phases overlap (tc–s) has to be minimal (Fig. 1). All times depend on working conditions and working technology. However, only a part of skidding time is irrespective of organizational form of group work (or the group size), because it is mostly connected to the capacity of skidding means (unloaded drive, loaded drive, winching).

When working technology, organizational form and skidding means are unchanged, the time of both processes overlapping depends on cutting and skidding working conditions. Disadvantageous cutting working conditions (smaller trees, dense stands) in

A. POJE and I. POTO^NIK

general decrease the cutters involvement time in skidding process and increase tractor drivers’ involvement time in cutting process, and vice versa when the skidding conditions are disadvantageous (long skidding and bunching distances, low timber concentrations, steep terrain and skid trails). Utilization rate of skidding means is thus greater in disadvantageous skidding conditions, because it enables cutters’ help to tractor driver.

The assumptions mentioned above are true, if groups consider only working conditions and if their specific work realization or work method has no influence on the rationality of work. The existing research (Klun and Poje 2000) and the fact that groups were not additionally educated for group work, show that the differences could exist between groups regarding cutter’s involvement in skidding process (and also the tractor driver in cutting process), being more a consequence of self-taught working method than of rational approach.

According to the enumerated theoretical assumptions considering group work and data adequacy, we limited our research only to analysis of cutters’ involvement in skidding process and set two basic hypotheses: Þ Working conditions factors (skidding distance, bunching distance, skidder short movements distance during formation of full load, the number of logs in the load and the load volume) increase the duration of cutters’ assistance to the tractor driver. Þ The duration of cutters’ assistance is different according to different working groups, even if the influence of working conditions is included in the comparisons.

The positive answer on the first hypothesis would enable us to optimally form the working group size according to effectiveness and difficulty of work.

The confirmation of second hypothesis would advise us on the need of workers’ education for group work and relevance of income or group daily efficiency as a motivational factor.

4. Research places – Mjesta istra`ivanja

Fig. 1 Cutting and skidding group work scheme Slika 1. Skica sje~e i privla~enja drva u skupnom radu Croatian Journal of Forest Engineering 28(2007)2

The research was limited to 9 places (compartments) in Slovenian state forests situated on the altitude from 470 m to 1070 m, where the characteristics of high Karst prevail (Table 1). In lower parts, communities Querco-, Blechno- and Hacquetio- Fagetum prevailed, whereas in higher parts sub-association of forest communities Abieti-Fagetum prevailed. The average volume of tree selected to cut ranged from 0.63 to 2.03 m3/tree, and from 1.71 to 4.10 m3/tree in

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Influence of working conditions on overlapping of cutting .... (157–167)

Table 1 Compartment characteristics Tablica 1. Zna~ajke istra`ivanih odjela Place Mjesto

Compartment – Forest management unit Odjel – Gospodarska jedinica

Surface area, ha Povr{ina odjela, ha

Altitude, m Nadmorska visina, m

Mean cut tree, m3 Srednje sje~no stablo, m3

Group Skupina

1

61 – Grintovec

32.10

450–510

1.05

4

2

121 – Grintovec

43.30

780–870

2.03

1

3

49 – Mozelj

43.74

470–620

0.63

2

4

74 – Mozelj

40.59

520–600

0.86

3

5

143 – Mozelj

40.86

530–620

0.73

3

6

48 – Stojna

56.28

900–1070

1.02

2

7

90 – Gr~arice

65.81

740–830

1.22

6

8

100 – Gr~arice

33.90

800–900

4.10

6

working operations, i.e. empty and fully loaded drive and winching logs, and also the majority of remaining work operations, all by himself. The cutters assisted mainly in wood bunching process. The work particularity is the additional cutting and timber assortment done at the roadside landing by the tractor driver. The workers had to meet the group productivity expressed in daily timber quantity on the roadside landing (by the forest road).

5. Methods of research – Metode istra`ivanja Fig. 2 Group work with skidder IWAFUJI–T41 Slika 2. Rad skupine sa skiderom IWAFUJI-T41 compartments with sanitation cut (subjects 8 and 9).

The compartment surface ranged from 32 ha to 66 ha; however the research comprised only a part of these surfaces.

In the selected compartments we monitored the work of 6 working groups with organizational form I+2, meaning that the tractor driver and 2 cutters formed a group. For skidding, all groups used cable skidder IWAFUJI-T41 without remotely controlled winch. The maximal average age and working experience difference within groups were 8.7 and 13.5 years, 7 respectively.

In general, the cutting process proceeded in such a way, that the majority of work was done by the cutters. Only in some specific and more difficult operations, clearing the lodged tree or activities to meet the forest regulations (pile up branches) for example, tractor driver assisted them. Regarding skidding process, the tractor driver managed all

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The time studying was done in 29 days, in summer months of 1996 and 1997. During that period, on the skidding distance of 1000 m, we recorded 218 timber skidding cycles, of which 78 under uphill skidding, 114 downhill skidding, and 26 level skidding categories (Table 2). The work was done by two researchers, one recorded the duration of individual working operations by using the snap back timing, whereas the other measured the volume of loads, the controlling time, and evaluated the bunching, stand movement and movement of skidder on roadside landing distances. The time study was made with 4/1000 of minute accuracy, whereas the error between recording and controlling time did not exceed 5%.

Due to considerable difference between the number of recorded cycles by working groups, especially by individual skidding categories (Table 2), our analysis, out of all recorded timber skidding cycles, included 100 downhill cycles recorded in groups 2, 3, and 4 on the subjects 1, 3, 4, 5, and 6. The categories of skidding were calculated as weighted average incline in the direction of skidding with skidding Croatian Journal of Forest Engineering 28(2007)2

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Table 2 Structure of recorded cycles regarding skidding direction Tablica 2. Struktura snimljenih turnusa s obzirom na smjer privla~enja Group Skupina

Number of cycles – Broj turnusa All Svih

Uphill Uzbrdo

Level Bez nagiba

Downhill Nizbrdo

1

25

4

12

9

2

33

–

–

33

3

56

17

5

34

4

42

–

9

33

5

10

10

–

–

6

52

47

–

5

Sum – Ukupno

218

78

26

114

distance as weight (uphill >5%, –5% £ level ³ +5%, downhill <–5%).

The overlap of cutting and skidding in group work was the reason for adding two new operations to »traditional« skidding operations, naming them assistance to cutters and cutters assistance. Due to the fact, that during recording the tractor driver was the priority, the cutters assistance was noted only in case of his inactivity or where one or both cutters assisted in timber skidding. In theory, the time values of cutters assistance should change according to working conditions or working methods within the group. These values served us as a dependant variable in our further analysis.

For the independent variables we used the factors influencing the cutters participation in timber skidding process i.e. the factors from the first hypothesis. The skidding distance was measured as the distance between the spot where the full load was made and the spot for timber unhooking (roadside

A. POJE and I. POTO^NIK

landing). The average bunching distance was calculated as the average of distances required for bunching of individual logs in the load. Short movements distance equals the sum of all skidder movements in the stand during the formation of full load and depends on the timber concentration. The number of logs in the load and the load volume are calculated as the sums of individual logs.

6. Results – Rezultati The comparison of average duration of cutters assistance among groups (Table 3) showed that there are differences between the groups (p <0.000). The Welch test was used for testing the equality of means because the Levene test of homogeneity of variances showed that the differences among the variances are significant (p = 0.021). By applying the posterior difference tests among the means (Tukey HSD), we found out that the cutters assistance differences are only characteristic between the groups 2 and 3, and 2 and 4, whereas there are no statistically confirmed differences between groups 3 and 4.

With average values of working condition factors we can infer that the working conditions differ by working groups and that they are generally lower for the group 3, which has also lower cutters assistance value in comparison to other two groups.

It is necessary to stress here that the average values can also indicate the danger of stratified data, not only by working group but also by the individual factor. In such case we cannot establish precisely whether the differences of cutters assistance are the consequence of studied factor or the specific group work method. In our case this happened with the skidding distance, which is the consequence of the sampling method (Fig. 3). Primarily, we used the data for the standard time calculation and had the

Table 3 Average values and correlation between cutters assistance and working condition factors Tablica 3. Prosje~ne vrijednosti i korelacija izme|u pomo}i sjeka~a te ~imbenika radnih uvjeta Cutters assistance Skidding distance Bunching distance Pomo} sjeka~a Udaljenost privla~enja Udaljenost skupljanja min m m 2 9.31 694 11.5 3 4.47 189 8.6 4 6.18 313 14.4 Pearson correlation – r 0.332 (**) 0.222 (*) Pearsonova korelacija – r (Cutters assistance – Pomo} sjeka~a) Group Skupina

Skidder moving No. of logs in the load Premje{tanje skidera Broj trupaca u tovaru m pcs. – kom.

12.3 9.0 15.8 7.6 31.4 7.8 0.086

0.564(**)

Load volume Obujam tovara m3 3.64 2.57 3.83 0.096

** Correlation is significant at the 0,01 level (2-tailed) – Korelacija je statisti~ki zna~ajna uz razinu zna~ajnosti 0,01 (dvosmjeran test) * Correlation is significant at the 0,05 level (2-tailed) – Korelacija je statisti~ki zna~ajna uz razinu zna~ajnosti 0,05 (dvosmjeran test)

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Fig. 3 Cutters assistance according to skidding distance by groups Slika 3. Ovisnost pomo}i sjeka~a o udaljenosti privla~enja po skupinama

only goal to record some cycles every 100 m of skidding distance.

Figure 3 shows that the data on cutters assistance by groups is within different skidding intervals, meaning that we can conclude that the cutters assistance increases with the increase of skidding distance. Taking into consideration the theoretical basis and research projects of other authors (Ze~i} and Maren~e 2005), we can state with strong likelihood that the shown cutters assistance differences depend also on the skidding distance and not just on the working method or organization of individual groups.

Studying the linear dependence among individual factors and cutters assistance (Table 3), we found out that the cutters assistance increases with the increase of all working condition factors, according

to positive value of correlation coefficient (r). Correlations are statistically confirmed with skidding and bunching distance, and with number of logs in the load. The values of correlation coefficients are relatively low, thus indicating the weak correlation of cutters assistance with working condition factors.

Correlation values among working condition factors were 0.306 at the most, enabling the use of all factors in multivariate regression. By applying linear multivariate regression, we tried to establish the influence of individual working condition factor on cutters assistance with the constant value of other factors.

By applying the stepwise method of multivariate regression, we established that the skidding distance, average bunching distance, and the number of logs in the load (Table 4) simultaneously influence

Table 4 Coefficients and correlations of multiple linear regression Tablica 4. Koeficijenti i korelacije multivarjantne linearne regresije Coefficients – Koeficijenti Factors Faktori

t

Significant Zna~ajnost

Partial correlation Parcijalna korelacija

B

Std. Error St. pogre{ka

Beta

–3.038

1.272

–

–2.388

0.019

–

Skidding distance – Udaljenost privla~enja drva

0.004

0.002

0.233

2.924

0.004

0.286

Bunching distance – Udaljenost skupljanja drva

0.138

0.056

0.193

2.452

0.016

0.243

Constant – Konstanta

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cutters assistance. With every meter of timber skidding, the cutters assistance increases by 0.004 minute, with every meter of bunching distance by 0.139 minute and with every additional log in the load by 0.780 minute. Small differences between partial regression coefficients, showing the correlation between cutters assistance and working condition factor with constant value of other two factors, and Pearson correlation coefficients (Table 3) indicate that each factor directly influences cutters assistance. The number of logs in the load accounts for 25% of total variability, the skidding distance 5% and the bunching distance 4% (the square of part correlations).

Multivariate model accounts for 40.7% of cutters assistance variability (p <0.000).

Since we wanted to find out whether the working method within the groups had any influence on cutters assistance, we excluded the influence of working conditions (skidding and bunching distance, and the number of logs in the load) from cutters assistance. By comparing the means of the residues by groups after regression, we established that there are no statistically characteristic differences between the groups in cutters assistance (Welch, p = 0.998), meaning that we explained all the differences in cutters assistance, established at the beginning of this chapter, with the differences in working conditions.

7. Discussion – Rasprava With this research we proved that working conditions determined by skidding distance, bunching distance and number of logs in the load influence the cutters participation in timber skidding in group work. There are no differences in work methods between work groups that would reflect themselves in absolute values of cutters assistance. The result does not negate differences in the group work methods, established by Klun and Poje (2000a, 2000b) with the comparison of cutters assistance frequency within individual operation, because the comparison of absolute values provides us with a general answer to the assistance increase with changing of working conditions and other conditions, while the frequency distribution of assistance within groups answers to the differences of the method for achieving maximal group effect.

The research results can be applied to logging organization, especially in work arrangement and education of workers, and directly in measurement and defining the standards for group work. When defining organizational form for cutting and skidding in group work we also need to take into consideration working conditions, i.e. skidding distance, bunching distance and the number of logs in the Croatian Journal of Forest Engineering 28(2007)2

A. POJE and I. POTO^NIK

load. By increasing the value of the above factors, the timber skidding becomes a bottleneck for total effect of the group, leading to greater need for additional skidding capacities or inversely, decreasing the need for additional cutters. Also Ze~i} and Krpan (2004), and Ze~i} and Maren~e (2005) came to similar conclusions, but mainly in regard to skidding distance.

When determining the group size, that has the common standard determined on the basis of daily timber quantity on the roadside landing, the basic principle of maximal means utilization for timber skidding has to be followed. On the other hand, it means that a cutter or two cutters should also participate in skidding process in good working conditions for timber skidding, which is frequently impossible, due to great skidding effectiveness (not utilization). With better working conditions for timber skidding, the cutters cut the pre-determined quantity of timber with great difficulty. For adjustment of cutting and skidding processes in practice, the sum of cutting daily standard times in all working conditions has to be lower than with timber skidding process. For increasing group efficiency and productivity, it is necessary to provide detailed planning of working sites and adjustable size of working groups. Beside positive effects, these arrangements can also have negative consequences, especially in the increased effect control and more precise work planning. For successful introduction of the changed working method, it is necessary to achieve positive attitude of workers to planned changes.

Since cutters assistance between groups according to working conditions is not different, cutters within groups participate in timber skidding process in the same way, although they are not additionally educated for group work. It is obvious that the motive for bigger salaries is by itself a motivation big enough to demand rational behavior of workers according to working conditions. However, the workers should be informed about the risks related to group work. The greater number of workers on the same site and intertwining processes of cutting and skidding demands the increased and constant work and coworker movement control, and also the need for worker qualification for cutting and skidding.

Special attention should also be paid to a decrease of work difficulty, especially in extreme working conditions in skidding process (i.e. good and bad working conditions). Both instances can cause »burnout« of workers because of cutting and skidding non-adjustment.

Classic division of cutting and skidding can lead us to false and misleading conclusions when studying group work efficiency. Due to overlap of cutting and skidding, apart from traditional working operations

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the new arise, whereas synergetic effects prevent the simple sum of standard times. The same holds true for the division of productive and unproductive time within working processes. For example, from the cutters standpoint the participation in the skidding process is regarded as unproductive time, or delay because of organization, whereas from the group work standpoint, it is regarded as productive time, necessary for effect increase of group work.

The ergonomic and economic development of group work with existing »traditional« work technology still has its potential. Apart from flexible work groups adjustable to working conditions according to size (Ze~i} and Maren~e 2005) and capability of various forest work performance (Winkler 1990), it is reasonable to at least try to follow the recommendations for workers participation in planning process and decision making, and constant education (Krivec 1979, Lewark et al. 1996). This brings better relationships between group members and between group and company, and also increases the information flow and innovation possibilities.

8. Conclusion – Zaklju~ak Group work is still one of the most perspective organizational forms in forest work. This is not only true for »traditional« form where chainsaws and different kinds of tractors are used, but also for highly mechanized technologies with harvesters and forwarders. The aim of group work is to increase work efficiency by simultaneously decreasing, or at least preserving the level of work difficulty.

The simultaneous cutting and skidding processes, and the common group daily standard cause cutting and skidding overlap. On one hand this depends on cutters and capacity of skidding means, and on the other hand on working and environmental conditions and specific method of group work.

In this research of group work we focused on the influence of working conditions on cutters participation in timber skidding process.

The research took place in decreased range on 5 places of research in Slovenian state forests, where we conducted time studies of timber skidding with IWAFUJI–T41 tractor in 3 working groups, with two cutters and one tractor driver. In our analysis we included 100 cycles of downhill timber skidding.

With multivariate analyses we confirmed the hypothesis that three working condition factors simultaneously influence the duration of cutters participation in timber skidding process. Thus with every meter of timber skidding, the cutters assistance is increased by 0.004 minute, with every meter of bunching distance for 0.139 minute and with every

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additional log in the load by 0.780 minute. The number of logs in the load has the greatest influence on the assistance duration. Multivariate model accounts for 40.7% of variability. After eliminating the working condition influence on cutters assistance, we disproved the hypothesis that the groups have a duration-specific work method, as other research projects have indirectly indicated.

The results can be applied to define the working group size that depends on all three factors at the same time and not only on the skidding distance as the other researchers suggested. By increasing all three factors, the skidding means becomes a bottleneck for the whole group effectiveness, which basically means that we have to increase the timber skidding capacity, or decrease the number of cutters.

The size flexibility of the working group brings the need for more detailed work organization. Insignificant differences between cutters assistance duration in different groups mean that the group daily efficiency is a motivational factor big enough for the rational work. The result does not exclude education and qualification of workers for safe group work and also the need for decreasing work difficulty.

9. References – Literatura Cimper{ek, M., 1987: Skupinsko delo v gozdni proizvodnji (Group work in forestry production). Gozdarski vestnik 45(6): 277–284.

Findeisen, E., 2002: ThüringenForst-Erfahrung zur teilautonomen Gruppenarbeit in der hochmechanisierten Holzernte (ThuringenForst – experiences with partial autonomous group work for highly mechanized timber harvesting). Forstechnische Informationen 4: 37–43.

Kerne`a, J., 1999: U~inki se~nje in spravila lesa pri Sne`nik d.d. po letu 1980 (Wood cutting and timber logging productivity in joint stock company Sne`nik by the year 1980).

BSc. Thesis, University of Ljubljana – Biotechnical Faculty, Department of Forestry and Forest Resources, p. 30.

Klun, J., Poje A., 2000a: Spravilo lesa z zgibnim traktorjem IWAFUJI T–41 in po{kodbe pri se~nji in spravilu (Timber skidding with IWAFUJI T–41 skidder and stand damage due to cutting and logging operations). BSc. Thesis, University of Ljubljana – Biotechnical Faculty, Department of Forestry and Forest Resources, p. 150.

Klun, J., Poje A., 2000b: Spravilo lesa z zgibnim traktorjem IWAFUJI T–41 (Timber skidding with IWAFUJI T–41 skidder). Gozdarski vestnik 58(7–8): 291–303.

Korbar, U., 1988: Skupinsko delo v gozdni proizvodnji (Group work in forestry production). BSc. Thesis, University of Ljubljana – Biotechnical Faculty, Department of Forestry and Forest Resources, p. 49.

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Krivec, A., 1979: U~inkovitost in oblikovanje novih organizacijskih postopkov pri spravljanju lesa s traktorji (Efficiency and modeling of new organizational methods by wood extraction with tractors). Gozdarski vestnik 37(7–8): 315–323.

Lewark, S., Strömquist, L., Kastenholz, E., Meier, D., 1996: Mit teilautonomen Gruppen zu höherer Effizienz und Qualität der Waldarbeit? (Semi-autonomous groups for higher efficiency and quality in forest work). Forstechnische Informationen 11: 109–115.

Kri`, A., 1984: Izku{nje skupinskega dela v gozdarstvu na posestvu Sne`nik, Ko~evska Reka (Experience with work group in forestry in company Posestvo Sne`nik, Ko~evska Reka). Gozdarski vestnik 42(6): 266–270.

Winkler, I., 1990: Skupinsko delo v gozdni proizvodnji (Group work in forestry production). Zbornik gozdarstva in lesarstva 35: 69–82.

Kruh, A., 1989: Posami~no in skupinsko delo v gozdni proizvodnji (Individual and group work in forestry production). BSc. Thesis, University of Ljubljana – Biotechnical Faculty, Department of Forestry and Forest Resources, p. 35.

Ze~i}, @., Krpan, P. B., 2004: Efficiency of group work in harvesting mountainous broadleaf thinning stand. Zbornik gozdarstva in lesarstva 74: 41–58.

Ze~i}, @., Maren~e, J., 2005: Mathematical models for optimization of group work in harvesting operation. Croatian Journal of Forest Engineering 26(1): 29–37.

Sa`etak

Utjecaj radnih uvjeta na preklapanje sje~e i privla~enja drva pri skupnom radu Klasi~no pridobivanje drva u kojem se sje~a i sortimentnom metodom izradba drva obavljaju ru~no-strojnim radom uz primjenu motornih pila lan~anica, a privla~enje drva skiderom, zadr`at }e, kao na~in rada, svoju va`nost i u budu}nosti, posebno u {umama koje se odlikuju te{kim terenskim radnim uvjetima te u manjim {umama privatnih {umoposjednika. Osnovne zna~ajke skupnoga rada, kao jednoga od organizacijskoga oblika pridobivanja drva, jesu rad u skupini najmanje dvaju radnika koji istodobno obavljaju sje~u i izradbu te privla~enje drva te pri tome poma`u jedan drugomu. Opisani je na~in rada prete`it organizacijski oblik pridobivanja drva u slovenskim dr`avnim {umama.

Veli~ina radne skupine ovisi ponajvi{e o kapacitetu stroja koji se koristi pri privla~enju drva.

Radnici u radnoj skupini imaju jedinstven (o~ekivan) dnevni u~inak te svi primaju jednaku naknadu za rad zasnovanu na ostvarenim rezultatima. Na taj su na~in radnici motivirani sudjelovati u onim fazama rada koje se poka`u kao »usko grlo« pri ostvarivanju optimalnoga dnevnoga u~inka.

U slovenskom je {umarstvu skupni rad u proces pridobivanja drva uveden ranih 70-ih godina pro{loga stolje}a.

Skupni rad zahtijeva svestranoga radnika koji je sposoban obaviti bilo koji posao u svojoj radnoj skupini. Od 1979. godine mnogi su se autori, u Sloveniji i u drugim europskim dr`avama bavili, bilo na teoretskim postavkama bilo prakti~nim istra`ivanjima, skupnim radom (Kri` 1984, Cimper{ek 1987, Korbar 1988, Kruh 1989, Winkler 1990, Lewark i dr. 1996, Kerne`a 1999, Klun i Poje 2000a, Klun i Poje 2000b, Findeisen 2002, Ze~i} i Maren~e 2005 i drugi).

Pri skupnom radu, kao organizacijskom obliku pridobivanja drva, za razliku od individualnoga rada, preklapaju se sje~a i izradba s privla~enjem drva. Uz zahtjev sigurnosti i ergonomske orijentiranosti, za skupni je rad posebno va`no postizanje najve}e mogu}e u~inkovitosti. To zna~i (slika 1) da je zbroj vremena sje~e i izradbe drva, privla~enja drva i vremena kada dolazi do preklapanja tih dviju faza rada minimalan. Navedena vremena ovise o radnim uvjetima u sje~ini te primijenjenom postupku rada (samo je dio vremena privla~enja drva: vo`nja neoptere}enoga zglobnoga traktora, vo`nja optere}enoga zglobnoga traktora i privitlavanje, neovisan o organizacijskom obliku skupnoga rada, a ponajvi{e pod utjecajem tehni~kih zna~ajki primijenjenoga zglobnoga traktora).

Kada su na~in rada, organizacija rada i tip zglobnoga traktora odre|eni i stalni, tada na vrijeme preklapanja obaju radnih procesa utje~u radni uvjeti koji se javljaju pri sje~i i izradbi te pri privla~enju drva. Lo{i radni uvjeti pri sje~i i izradbi drva (tanka stabla, gusta sastojina) op}enito smanjuju udio sjeka~a pri privla~enju drva te pove}avaju udio traktorista u sje~i i izradbi drva; vrijedi i obrnuto kada su uvjeti privla~enja drva lo{i (velika udaljenost privla~enja, mala sje~na gusto}a, strm teren, veliki uzdu`ni nagib traktorskih putova).

Ovim je istra`ivanjem dan prilog dosada relativno slabo istra`enim odnosima izme|u sje~e, izradbe i privla~enja drva pri skupnom radu, osvijetljen je problem promjenjivih radnih uvjeta u {umi (od sje~ine do sje~ine) uz uva-

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`avanje ergonomskih, ekonomskih i organizacijskih ~imbenika. Istra`ivanja su ograni~ena na udio sjeka~a u privla~enju drva. Postavljene su dvije hipoteze: Þ ^imbenici radnih uvjeta (udaljenost privla~enja, udaljenost privitlavanja, premje{tanje zglobnoga traktora pri oblikovanju tovara, broj komada trupaca u tovaru i obujam tovara) pove}avaju vrijeme preklapanja sje~e i izradbe s privla~enjem drva (pomo} sjeka~a traktoristu). Þ Vrijeme trajanja pomo}i sjeka~a traktoristu razli~ito je od radne skupine do radne skupine (bez obzira na to {to su razli~iti radni uvjeti uzeti u obra~un).

Potvrda prve hipoteze omogu}ila je optimizaciju radne skupine sukladno u~inkovitosti rada i te`ini radnih uvjeta. Pozitivan odgovor na drugu hipotezu upu}uje na potrebu izobrazbe radnika u skupni te isti~e va`nost dnevnoga u~inka skupine kao motivacijskoga ~imbenika.

Istra`ivanja su provedena u devet odjela u slovenskim dr`avnim {umama na nadmorskim visinama od 470 do 1070 m. Osnovni su podaci o istra`ivanim odsjecima prikazani u tablici 1. U odjelima 1 – 7 provedena je redovita, a u odjelima 8 i 9 sanitarna sje~a stabala. Pra}en je rad {est skupina radnika, a svaka se radna skupina sastojala od jednoga traktorista i dvaju sjeka~a (1 + 2). Za privla~enje je kori{ten zglobni traktor IWAFUJI–T41 bez daljinski upravljanoga vitla. Najve}a prosje~na dobna razlika izme|u radnih skupina bila je 8,7 godina, a najve}e prosje~no radno iskustvo izme|u radnih skupina 13,5 godina.

U pravilu su glavninu sje~e i izradbe obavljali sami sjeka~i, tek kod specifi~nih i najzahtjevnijih radnih operacija ili radnih operacija propisanih zakonom (npr. uhrpavanje grana), traktorist je pomagao sjeka~ima. Ve}inu je radnih operacija privla~enja drva obavljao traktorist, sjeka~i su mu pomagali pri oblikovanju tovara. Povremeno je dodatno prerezivao trupce traktorist na pomo}nom stovari{tu. Dnevni u~inak skupine predstavlja izra|eno drvo na pomo}nom stovari{tu.

Terenska su mjerenja obavljana 29 dana tijekom ljetnih mjeseci 1996. i 1997. godine. Na udaljenosti je privla~enja od 1000 m snimljeno 218 turnusa privla~enja, od ~ega 78 uzbrdo, 114 nizbrdo te 26 na terenu bez utjecaja nagiba (tablica 2). Prosje~an nagib traktorskoga puta odre|en je kao aritmeti~ka sredina pojedinih nagiba na tom traktorskom putu, a duljine pojedinoga nagiba uzete su kao te`ine. Traktorski put prosje~noga uzdu`noga nagiba ve}ega od +5 % predstavljao je privla~enje uzbrdo, traktorski put prosje~noga uzdu`noga nagiba manjega od –5 % predstavljao je privla~enje nizbrdo, a traktorski put prosje~noga uzdu`noga nagiba izme|u navedenih vrijednosti odredio je privla~enje drva po terenu bez utjecaja nagiba.

Zbog zna~ajnih razlika izme|u snimljenoga broja turnusa privla~enja drva izme|u radnih skupina (posebno s obzirom na smjer privla~enja drva: uzbrdo, nizbrdo, ravno) daljnja je analiza uklju~ila 100 turnusa privla~enja drva nizbrdo, snimljenih u radnim skupinama 2, 3, 4 na mjestima istra`ivanja 1, 3, 4, 5 i 6. Udaljenost privla~enja drva mjerena je od mjesta kona~noga formiranja tovara do mjesta odvezivanja tovara (pomo}no stovari{te).

Prosje~na udaljenost privitlavanja izra~unata je kao prosjek udaljenosti privitlavanja pojedinoga trupca u tovaru.

Premje{tanje zglobnoga traktora pri formiranju tovara predstavlja zbroj svih pomicanja stroja u sastojini do formiranja punoga tovara. Obujam je tovara zbroj obujma svih trupaca u tovaru.

U tablici 3 prikazane su prosje~ne vrijednosti i korelacija izme|u trajanja vremena pomo}i sjeka~a i ~imbenika koji odre|uju radne uvjete. Usporedbom prosje~noga trajanja vremena pomo}i sjeka~a izme|u radnih skupina uo~avamo razlike (p <0,000), a primjenom razlikovnoga testa sredina (Tukey HSD) dokazano je kako je prosje~no vrijeme trajanja pomo}i sjeka~a statisti~ki zna~ajno izme|u radnih skupina 2 i 3 te 2 i 4, dok nema statisti~ke potvrde razlike izme|u radnih skupina 3 i 4. Prema prikazu na slici 2 mo`e se zaklju~iti da vrijeme pomo}i sjeka~a traktoristu raste s pove}anjem udaljenosti privla~enja drva, odnosno s velikom vjerojatno{}u mo`emo tvrditi da na vrijeme pomo}i sjeka~a traktoristu osim metode rada i organizacije pojedine skupine utje~e i duljina udaljenosti privla~enja, {to se podudara s teoretskim postavkama i rezultatima istra`ivanja drugih autora (npr. Ze~i} i Maren~e 2005).

Prou~avanjem linearne ovisnosti izme|u individualnih utjecajnih ~imbenika radnih uvjeta i trajanja vremena pomo}i sjeka~a otkrili smo da duljina vremena pomo}i sjeka~a raste s porastom svih utjecajnih ~imbenika, a sukladno pozitivnoj vrijednosti koeficijenta korelacije (r). Korelacija je statisti~ki ispitana za udaljenost privla~enja, udaljenost privitlavanja i broj komada trupaca u tovaru. Vrijednosti koeficijenata korelacije relativno su niske, {to indicira slabu korelaciju izme|u trajanja vremena pomo}i sjeka~a i individualnih utjecajnih ~imbenika radnih uvjeta. Primjenom linearne multivarjantne regresije (tablica 4) `elio se ispitati individualni utjecaj pojedinoga ~imbenika radnih uvjeta (uz stalne vrijednosti ostalih ~imbenika radnih uvjeta) na trajanje vremena pomo}i sjeka~a. Utvr|eno je da udaljenost privla~enja, udaljenost privitlavanja i broj komada trupaca u tovaru istodobno utje~u na pomo} sjeka~a.

Sa svakim metrom privla~enja trajanje vremena pomo}i sjeka~a raste 0,004 min., sa svakim metrom privitlavanja trajanje vremena pomo}i sjeka~a raste 0,139 min., a sa svakim dodatnim trupcem u tovaru za 0,780 min.

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Broj komada trupaca u tovaru pokazuje varijabilnost od 25 %, udaljenost privla~enja 5 %, a udaljenost privitlavanja 4 %. Multivarjantni model ima varijabilnost trajanja vremena pomo}i sjeka~a od 40 do 7 %.

Statisti~ki je dokazano (Welchovim testom, p = 0,998) da na trajanje vremena pomo}i sjeka~a ne utje~e radna metoda unutar pojedine skupine ve} samo radni uvjeti.

Rezultati provedenoga istra`ivanja primjenjivi su pri organizaciji skupnoga rada u pridobivanju drva, u izobrazbi radnika te za mjerenje i odre|ivanje u~inkovitosti skupnoga rada. Pri organizaciji skupnoga rada posebnu pa`nju treba usmjeriti odre|ivanju radnih uvjeta u sje~ini imaju}i na umu ove ~imbenike: udaljenost privla~enja, udaljenost privitlavanja i broj komada trupaca u tovaru. Pove}avanjem vrijednosti nabrojenih ~imbenika privla~enje drva postaje »usko grlo« pri ostvaraju propisanih dnevnih u~inaka skupine. Navedeni je problem mogu}e rije{iti pove}anjem kapaciteta privla~enja drva ili suprotno, smanjenjem broja sjeka~a. Do sli~nih su zaklju~aka, ali prete`no u svezi s udaljeno{}u privla~enja drva, do{li Ze~i} i Krpan (2004) i Ze~i} i Maren~e (2005).

Pri optimizaciji radne skupine radi ostvarivanja njezina propisanoga dnevnoga u~inka nastoji se posti}i najve}a mogu}a u~inkovitost zglobnoga traktora. Za pove}anje u~inkovitosti radne skupine nu`no je detaljno planiranje radova. Skupni rad sa stajali{ta sigurnosti pri {umskom radu nosi odre|ene negativnosti i opasnosti.

Radnici moraju biti osposobljeni za obje faze rada: sje~u i izradbu te privla~enje drva, preklapanje sje~e i izradbe s privla~enjem drva tra`i pove}an i stalan rad, potreban je poseban oprez zbog ve}ega broja radnika i stroja/strojeva koji se kre}u na malom podru~ju i dr.

Treba povesti ra~una o ergonomskom i ekonomskom razvoju skupnoga rada. Preporu~uje se uklju~ivanje radnika u postupak planiranja i dono{enja odluka te njihova stalna izobrazba (Krivec 1979, Lewark 1996). Tako }e odnosi izme|u radnika u radnoj skupini, ali i odnosi izme|u radnika i poslodavca postati bolji i kvalitetniji, ubrzat }e se protok informacija i pove}ati mogu}nost inovacija i unapre|enja.

Skupni je rad, uistinu, jedna od najperspektivnijih organizacijskih oblika {umskoga rada, i to ne samo za »klasi~no« pridobivanje drva gdje se sje~a i izradba obavlja ru~no-strojnim radom uz primjenu motornih pila lan~anica, a privla~enje drva razli~itim tipovima zglobnih traktora, ve} i za visokomehanizirano pridobivanje drva uz uporabu harvestera i forvardera.

Klju~ne rije~i: skupni rad, sje~a, privla~enje, radni uvjeti, multivarjantna analiza

Authors’ address – Adresa autorâ:

Received (Primljeno): November 15, 2007 Accepted (Prihva}eno): December 6, 2007 Croatian Journal of Forest Engineering 28(2007)2

Anton Poje, MSc. e-mail: anton.poje@bf.uni-lj.si Assoc. Prof. Igor Poto~nik, Ph.D. e-mail: igor.potocnik@bf.uni-lj.si University of Ljubljana, Biotechnical Faculty Department of Forestry and Forest Resources Ve~na pot 83 1000 Ljubljana SLOVENIA

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Original scientific paper â&#x20AC;&#x201C; Izvorni znanstveni rad

Forwarding productivity in Southern Austria Mohammad Reza Ghaffarian, Karl Stampfer, John Sessions Abstract â&#x20AC;&#x201C; Nacrtak Forwarders are common machines in the cut to length harvesting system. A general regression model for predicting the time of forwarding was developed using 82 working cycles of two kinds of machines in downhill timber extraction. Using stepwise regression, variables such as forwarding distance, slope, type of forwarder and piece volume were significantly entered to the model. Increasing forwarding distance increases the forwarding time. However, if piece volume and slope of the trail increase, the forwarding time decreases.

The average forwarding production was estimated to about 17.9 m3/PSH0 (Productive System Hours) while mean load per trip was 10.04 m3 and average forwarding distance was 97 m.

Keyword: forwarder, productivity, cost, model, regression

1. Introduction â&#x20AC;&#x201C; Uvod Cut-to-length systems generally comprise two machines: a harvester and a forwarder. In Austria, 17% of the timber is extracted by forwarders (www.lebensministerum.at). Many factors can affect the productivity of forwarders. Kellogg and Bettinger (1994) in Oregon (USA) developed a productivity model for forwarding, which included variables such as assortments (pulpwood, sawlog or mixed loads), volume per load, travel distance of unloaded forwarder, travel distance during loading and travel distance of loaded forwarder to landing.

The productivity of forwarder is strongly correlated to stand type, average extraction distance, timber volume density at the strip road and load volume of the forwarder (Tufts and Brinkeer 1993, Kuitto et al. 1994).

In other studies carried out in central Finland for different cutting sites, harvesting density on strip roads, the average extraction distance, forwarder load capacity, timber assortment and bunching of assortments on the strip road had an effect on the haulage system (Nurminen et al. 2006).

The working method can also affect the productivity of harwarders as well as variables such as load Croatian Journal of Forest Engineering 28(2007)2

volume, tree size and extraction distance (Andersson and Eliasson 2004).

A study of short-wood forwarding carried out in Northern Spain resulted in a productivity of 6 to 15 t/SMH (Spinelli et al. 2003). Akay and Sessions (2001) reported the production rates in USA for small, medium and large size of forwarder as 0.51, 0.6 and 0.69 m3/h, respectively.

Other productivity studies resulted in the range of 8 to over 20 t/SMH, depending on the model and working conditions (UK Forestry Commission 1995, Gullberg 1997, Martin dos Santos et al. 1995, Saunders 1996, Goglia et al. 1999, Horvat et al. 1990).

Cordeo et al. (2006) used GPS technology to monitor cut-to-length (CTL) system in Chile. In this method, by capturing the position of machine, it was possible to generate surface progress grids, which when combined with inventory grids, result in yield information by surface and time unit. The hourly production rate was 35.8 m3 in clear cut sites.

Due to increasing forwarding system in the forests of central Europe, it is necessary to study the productivity and cost of this system to give planners a useful tool for harvest planning. There is no general time predicting model available for forwarders in Austria. Therefor this study was carried out to de-

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By the Dual concept of a combined harvester-forwarder system, known as the harwarder, trees are felled, delimbed, topped, and bunched using the harvesting head of the harwarder in pre-planned machine trails. The operator then fits the load bunk for forwarding the logs to the roadside landing. The grapple head is used to load the logs onto the bunk of the machine. For the machine specifications of the harwarder refer to Table 1.

Table 2 Specifications for the Ponsse Buffalo Dual Harwarder Tablica 2. Tehni~ke zna~ajke harvardera Ponsse Buffalo Dual Vehicle technical data – Tehni~ki podaci vozila

Fig. 1 Ponsse Buffalo Dual Harwarder Slika 1. Harvarder Ponsse Buffalo Dual

Engine – Motor

Mercedes-Benz OM906LA

Power – Snaga

180 kW 900 Nm @ 1400 min-1

Torque – Moment velop such a model using time study databases on two types of forwarders. Verification of the model was done. The effect of variables on the time of forwarding is also presented in this paper.

2. Method of study – Metoda istra`ivanja 2.1 Site of study – Mjesto istra`ivanja The first study was carried out in Weiz in Steiermark in southern Austria. In this area a Ponsse Buffalo Dual forwarder was used in a mixed stand of spruce, fir, larch and pine. The terrain slope was moderate (11%).

Table 1 Description of study sites Tablica 1. Opis mjesta istra`ivanja Stand – Sastojina

100 cm3

Pump capacity – Obujam pumpe Fuel tank volume Obujam spremnika goriva

130 L

Length – harvester Duljina kad se koristi kao harvester

8850–9150 mm

Length – forwarder Duljina kad se koristi kao forvarder

9400–9950 mm

Width (600/700 tires) [irina (gume 600/700)

2670–2810 mm

Ground clearance – Klirens

690 mm

Tare mass of harvester Masa praznoga harvestera

15700 kg

Tare mass of forwarder Masa praznoga forvardera

16400 kg

Load capacity – Nosivost

14000 kg

Length of loading space Duljina tovarnoga prostora

4040–4590 mm

1

2

Area, ha – Povr{ina, ha

2.27

1.83

Slope, % – Nagib terena, %

11

39

70–130

90

No. of trees per hectare – Broj stabala po ha

1089

729

Harvesting head – Harvesterska glava

Growing stock, m3/ha – Drvna zaliha, m3/ha

510.4

646

Power – Snaga

45 kW

No. of harvested trees – Br. posje~enih stabala

1073

470

Length – Duljina

64 cm

Harvested volume, m3 – Posje~eno drvo, m3

331.8

513

Felling diameter – Sje~ni promjer

52 cm

Tree volume, m3 – Srednji obujam stabla, m3

0.31

0.7

Feed force – Posmi~na sila

18 kN

Harvesting intensity, % – Intenzitet sje~e, %

28.7

45

Feed speed – Posmi~na brzina

4 m/s

Number of trails – Broj traktorskih vlaka

15

5

Number of delimbing knives Broj no`eva za kresanje grana

5

Stand age, years – Dob sastojine, godine

Length of trails, m – Duljina vlaka, m Time of harvesting – Vrijeme sje~e

170

40–200 190–235 Spring – Prolje}e

Extension of loading space Produljenje tovarnoga prostora

0–700 mm

Harvesting head – Sje~na glava

Measuring system – Mjerni sustav

Ponsse H53

Ponsse Opti 4G

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Forwarding productivity in Southern Austria (169–175)

Table 3 Specification of Gremo 950R Forwarder Tablica 3. Tehni~ke zna~ajke forvardera Gremo 950R Engine – Motor

DEUTZ Type BF4M 1013 EC

Power – Snaga

111 kW

Pump capacity – Obujam pumpe

100 cm3

Hydraulic tank volume – Obujam spremnika ulja

100 L

Fuel tank volume – Obujam spremnika goriva

110 L

Tyre dimension – Dimenzije guma Load capacity – Nosivost

600/50–22.5 10000 kg

Boom – Dizalica

Loglift type 51F

Range – Doseg

6.5 m

Lifting capacity – Podizni moment

59 kNm

Length – Duljina

7895 mm

Width – [irina

2600 mm

Height – Visina

3445 mm

Turn radius – Polumjer okretanja

12.7 m

Ground Clearance – Klirens

580 mm

Tare mass of forwarder – Masa praznoga forvardera

11185 kg

The second site of study was near to Muerzzuschlag in Steiermark. The mixed species stand consisted of spruce, larch and fir. A Gremo 950R forwarder was used to extract logs that had been processed and piled by a harvester. The terrain slope was 39%. In steep terrain, this forwarder would use a cable fixed to a standing tree to allow safe operation on steep trails (Wratschko 2006).

The general stand and terrain characteristics on both sites are presented in Table 1. According to information from the forest office in Steirmark, the harvesting density was 100 m3/ha with a mean DBH of 25 cm.

2.2 Data collection – Prikupljanje podataka A continuous time study method was used in both production studies using an electronic EG20 timer. A typical work cycle included loading, travel loaded, unloading and travel empty.

Loading element included the time used to load the logs for one trip. Travel loaded was defined as the time to move a loaded machine to the landing.

Unloading included the time spent to unload the logs on the landing and travel empty consisted in the time to move empty from the landing to loading site.

The delays of more or less 15 minutes were recorded as well as miscellaneous delays in each working cycles.

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M. REZA GHAFFARIAN et al.

The same variables were used at both study sites.

Forwarding distance, piece volume, total load volume and slope were recorded during the data collection. Data for 82 working cycles were collected.

It was assumed that forwarding time was a function of forwarding distance, piece volume, slope of trail and type of forwarder. Two time databases were used to develop the time prediction model using stepwise multiple regression.

3. Research results – Rezultati istra`ivanja 3.1 Productivity – Proizvodnost The observed productivity was 17.9 m3/PSH0 and the average load per cycle was 10.04 m3. The average loaded and empty travel speeds of the forwarder were 35.9 m/min (0.6 m/s) and 26.3 m/min (0.45 m/s), respectively. Loaded travel was in a downhill direction which resulted in a higher speed than travel empty.

Using the system cost for the Buffalo Dual Harwarder of 120 EUR/h (Affenzeller 2005), the average forwarding cost was estimated to 6.72 EUR/m3.

3.2 Delays – Prekidi The percentage delay times and time for fixing and opening the cable, relative to total time of working for each forwarder type is presented in Table 5.

The total number of delays was greater for the Gremo 950R forwarder than for Ponsse Buffalo Dual Harwarder. The Gremo 950R forwarder had greater number of delays exceeding 15 minutes, while Ponsse Buffalo Dual Harwarder had greater number of short delays.

3.3 Model – Model SPSS software was used for processing the stepwise multiple regression. Stepwise multiple regres-

Table 4 Worktime delays Tablica 4. Prekidi rada Forwarder – Forvarder Delays – Prekidi (<15min) Delays – Prekidi (>15min) Miscellaneous delays – Slu~ajni prekidi Total delays – Prekidi ukupno Fixing the cable – Postavljanje u`eta Opening the cable – Odvezivanje u`eta

Gremo 5.87 % 11.87 % – 17.74 % 0.25 % 0.85 %

Ponsse 10.6 % 5.49 % 0.51 % 16.6 % – –

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Table 5 ANOVA table – Overall goodness of fit Tablica 5. Analiza varijance – Testiranje zna~ajnosti modela Sum of Squares Zbroj kvadrata

Degree of freedom Br. stupnjeva slobode

Mean Square Varijanca

F

Significance Zna~ajnost

Regression model – Regresijski model

2805.529

4

701.382

8.817

.000

Residual – Ostatak

5966.085

75

79.548

Total – Ukupno

8771.614

79

sion assumes that if any variable has a significant effect on the Residual Mean Squares (RMS) of the model, it will be included in the model. Forwarding distance, piece volume, type of machine and slope were all significant variables at a = 0.05.

The following forwarding model was determined on the basis of 82 recorded working cycles: t = 81.293 – 47.886 × V – 46.795 × F + + 0.076 × L – 1.189 × I where: t – forwarding time, min/cycle V – piece volume, m3/pcs.

F – type of machine L – forwarding distance, m I – slope (inclination) of skid trail, % The value for the Ponsse Buffalo Dual Harwarder is 1 and the value of 0 is considered for Gremo 950R forwarder. The multiple correlation coefficient (R2) of 0.32 is interpreted as 32% of total variability, which

Fig. 3 Cycle time vs. skid trail slope Slika 3. Ovisnost vremena turnusa o nagibu vlake

Fig. 2 Cycle time vs. forwarding distance Slika 2. Ovisnost vremena turnusa o udaljenosti izvo`enja

Fig. 4 Cycle time vs. piece volume Slika 4. Ovisnost vremena turnusa o obujmu komada

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Fig. 5 Cycle time vs. type of machine Slika 5. Ovisnost vremena turnusa o tipu vozila is explained by the regression equation. The significant level of ANOVA (0.000) shows that the model is significant at a = 0.05 (Table 5).

The effect of each variable on forwarding time was studied by changing one variable while holding the other variables constant at their mean value.

Forwarding time includes travel empty, loading, travel loaded, and unloading.

Figures 2, 3, 4 and 5 show the effect of forwarding distance, slope of skid trail, piece volume and type of machine on forwarding time respectively within the recorded range of variables. In Figure 6, the percent

M. REZA GHAFFARIAN et al.

Fig. 6 Percentage of each work component in the cycle time Slika 6. Udjeli pojedinih sastavnica rada u vremenu turnusa of time spent on each element of the forwarding cycle is shown. The most time is spent loading the logs. Table 6 presents the summary statistics.

4. Conclusions – Zaklju~ci The variables such as forwarding distance, piece volume, type of machine and slope of the skid trail were entered into the general model for predicting forwarding time as significant variables, which can be applied in logging planning. Increasing forwarding distance will increase forwarding time, but if piece volume and downhill slope increase, the forwarding time decreases.

Table 6 Summary statistics of parameters Tablica 6. Statisti~ki podaci istra`ivanih parametara Parameter Parametar

Maximum Najve}e opa`anje

Mean Aritmeti~ka sredina

Minimum Najmanje opa`anje

Travel Empty, min/cycle – Neoptere}eno kretanje vozila, min/tura

18.67

5.76

0.4

Loading, min/cycle – Utovar drva, min/tura

42.24

17.23

2.78

Travel Loaded, min/cycle – Vo`nja optere}enoga forvardera, min/tura

10.72

4.22

0.35

Unloading, min/cycle – Istovar drva, min/tura

15.31

6.5

0.97

Cycle time, min/cycle – Vrijeme turnusa, min/tura

57.68

33.72

8.9

Forwarding distance, m – Udaljenost izvo`enja, m

280

96.64

4

Slope of skid trail, % – Nagib traktorske vlake, %

40

21.62

5

Load volume, m3/cycle – Obujam tovara, m3/tura

18.7

10.04

1.37

0.49

0.14

0.04

Piece volume,

m3/pcs.

– Obujam komada,

m3/kom.

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Follow-up research should examine the productivity of uphill forwarding operations on a variety of sites.

5. References – Literatura Affenzeller, G., 2005: Integrierte Harvester-Forwarder-Konzepte Harwarder. MSc. thesis, Institute of Forest Engineering, University of Natural Resources and Applied Life Sciences Vienna.

Akay, A., Sessions, J., 2001: Minimizing road construction plus forwarding costs under a maximum soil disturbance constraint. The International Mountain Logging and 11th Pacific Northwest Skyline Symposium December 10–12, Seattle, Washington, USA, p. 268–279.

Andersson, J., Eliasson, L., 2004: Effect of three harvesting work methods on Harwarder productivity in final felling.

Silva Fennica 38(2): 195–202.

Cordero, R., Mardones, O., Marticorena, M., 2006: Evaluation of forestry machinery performance in harvesting operations using GPS technology. Proceedings of IUFRO Precision Forestry Symposium, Stellenbosch, 2006, p. 163– 173.

Goglia, V., Horvat, D., Sever, S., 1999: Technical characteristics and test of the forwarder Valmet 860 equipped with a Cranab 1200 crane. Forestry Faculty of Zagreb University, Internal Report, p. 23.

Gulberg, T., 1997: Time consumption model of off-road extraction of shortwood. Institutionen foer Skogsteknik, Sveriges Lantbruksuniversitet, Uppsatser och Resultat 297, p. 29.

Holzeinschlagmeldung, 2004: < http:// www.lebensministerium.at > Horvat, D., Goglia, V., Sever, S., 1999: Technical characteristics and test of the forwarder Timberjack 1410 and

Timberjack 1710. Forestry Faculty of Zagreb University, p. 32.

Huggard, E. R., 1978: Optimum road spacing. Quarterly journal of forestry 72(4): 207–210.

Kellogg, D. L., Bettinger, P., 1994: Thinning productivity and cost for a mechanized cut-to-length system in the Northwest Pacific coast region of the USA. J. For. Eng. 5(2): 43–54.

Kuitto, P. J., Keskinen, S., Lindroos, J., Oijiala, T., Rajamaeki, J., Rasanen, T., Terava, J., 1994: Mechanized cutting and forest haulage. Mestaeteho Report 410, p. 38.

Martin dos Santos, S., Machado, C., Leite, H., 1995: Technoeconomical analysis of eucalyptus extraction with forwarder in flat terrain. Revista Arvore, Vicosa 19(2): 213–227.

Nurminen, T., Heikki, K., Uusitalo, J., 2006: Time consumption analysis of the mechanized cut-to-length harvesting system. Silva Fennica 40(2): 335–363.

Saunders, C., 1996: West Argyll Valmet 890 forwarder trial 1996. Forestry Commission Research Division – Technical Development Branch, Internet Project Information Note 7/96: p. 9.

Spinelli, R., Owende, P., Ward, S., Torneo, M., 2003: Comparison of short-wood forwarding systems used in Iberia.

Silvia Fennica 38(1): 85–94.

Tufts, R. A., Brinker, R. W., 1993: Productivity of Scandinavian cut-to-length system while second thinning pine plantations. Forest Product Journal 43 (11–12): 24–32.

UK Forestry Commision, 1995. Terrain classification. Technical Note 16/95: p. 5 Wratschko, B., 2006: Production of cable forwarder. MSc. Thesis, Institute of Forest Engineering, University of Natural Resources and Applied Life Sciences Vienna.

Sa`etak

Proizvodnost izvo`enja drva u ju`noj Austriji Forvarderi se uobi~ajeno koriste pri sortimentnoj metodi izradbe drva. U Austriji se 17 % izra|enih drvnih sortimenata izvozi forvarderima. Mnogi ~imbenici utje~u na proizvodnost forvardera: vrsta drvnih sortimenata, obujam tovara, udaljenost izvo`enja, sje~na gusto}a. Stoga su potrebna istra`ivanja proizvodnosti i tro{kova sustava izvo`enja drva koja bi bila upotrebljiva pri planiranju i organizaciji radova.

Istra`ivanje je provedeno na dvije sje~ine na osnovi studija vremena pri radu dvaju razli~itih tipova strojeva: forvardera i harvardera. Cilj je istra`ivanja razvoj modela proizvodnosti i tro{kova izvo`enja drva s obzirom na utjecajne ~imbenike.

Na prvoj je sje~ini kori{ten harvarder Ponsse Buffalo Dual pri sje~i stabala i izvo`enju drvnih sortimenata iz mje{ovitih sastojina smreke, jele, ari{a i bora. Na drugoj je sje~ini kori{ten forvarder Gremo 950R za izvo`enje drvnih sortimenta, dok se za sje~u i izradu koristio harvester. Osnovne su zna~ajke sje~ina prikazane u tablici 1, a

174

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M. REZA GHAFFARIAN et al.

{umskih vozila u tablicama 2 i 3. Prosje~ni je nagib terena na prvoj sje~ini iznosio 11 %, a na drugoj 39 %. Pri radu na strmom terenu forvarder je bio opremljen vitlom te se kretao od pomo}noga stovari{ta do mjesta utovara u smjeru uzdu`noga nagiba terena namatanjem u`eta koje je bilo vezano na dube}e stablo.

Ukupno su snimljena 82 radna turnusa proto~nom metodom studija vremena. Radni turnus forvardera uklju~uje neoptere}eno kretanje vozila, utovar drva, optere}eno kretanje vozila te istovar drva. Prekidi su rada zabilje`eni u svakom turnusu. Tako|er su mjereni utjecajni ~imbenici izvo`enja drva: udaljenost izvo`enja, obujam tovara, obujam pojedinoga drvnoga sortimenta te nagib traktorske vlake.

Ostvarena je prosje~na proizvodnost iznosila 17,9 m3 po pogonskom satu rada. Optere}eno se kretanje vozila odvijalo niz nagib te su ostvarene ve}e brzine kretanja nego pri neoptere}enom kretanju vozila. Prosje~ni je jedini~ni tro{ak izvo`enja drva iznosio 6,72 EUR/m3.

Linearnom multivarjantnom su se regresijom odredile statisti~ki zna~ajne varijable (udaljenost izvo`enja, obujam komada, tip forvardera i nagib terena) te iskazao model utro{ka vremena pri izvo`enju drva. U modelu varijabla F iznosi 1 za Ponsse Buffalo Dual harvarder, odnosno 0 za Gremo 950R forvarder.

Utjecaj pojedinih ~imbenika na utro{ak vremena izvo`enja drva prikazan je na slikama 2, 3, 4 i 5 u opsegu izmjerenih vrijednosti. Slika 6 prikazuje postotni udio utro{ka vremena pojedine sastavnice radnoga turnusa, a tablica 6 zbirne statisti~ke podatke mjerenih vrijednosti.

Rezultati istra`ivanja pokazuju da se pove}anjem udaljenosti izvo`enja pove}ava utro{ak vremena rada forvardera. Utro{ak se vremena izvo`enja drva forvarderima smanjuje s pove}anjem obujma komada te pove}enjem nagiba vlake pri izvo`enju drva nizbrdo. Daljnjim je istra`ivanjima potrebno odrediti proizvodnost forvardera pri izvo`enju drva uz nagib pri razli~itim uvjetima rada.

Klju~ne rije~i: forvarder, proizvodnost, tro{ak, model, regresija

Authors’ addresses – Adresa autorâ: Mohammad Reza Ghaffarian, MSc. e-mail: ghafari901@yahoo.com Assoc. Prof. Karl Stampfer, PhD. karl.stampfer@boku.ac.at University of Natural Resources and Applied Life Sciences Vienna Department of Forest and Soil Sciences Institute of Forest Engineering Peter Jordan Strasse 82 1190 Vienna AUSTRIA

Received (Primljeno): December 21, 2006 Accepted (Prihva}eno): November 19, 2007 Croatian Journal of Forest Engineering 28(2007)2

Prof. John Sessions, PhD. e-mail: john.sessions@oregonstate.edu Oregon State University College of Forestry Department of Forest Engineering 204 Peavy Hall Corvallis, OR 97331-5706 USA

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Original scientific paper – Izvorni znanstveni rad

Costs and efficiency of timber harvesting

by NIAB 5–15 processor mounted on a farm tractor Janusz Sowa, Dariusz Kulak, Grzegorz Szewczyk Abstract – Nacrtak The present research deals with costs and economic effectiveness of timber harvesting technology with the use of the NIAB 5–15 processor mounted on a farm tractor, which method is used in Poland. Measurements were conducted in pine, fir and spruce stands, which underwent early and late thinnings. During harvesting, a time study was performed using the continuous reading method. On completion of felling works, the volume of timber harvested was measured.

The efficiency and unit costs of timber harvesting were calculated in the operational working time for the chainsaw operator and processor operator.

In all analysed stands, significantly higher efficiency was observed in late thinnings than in early ones. This resulted in higher economic effectiveness of this technology in the thinnings of older age classes. The approximation of regression functions allowed for the prediction of fixed unit costs and efficiency depending on the average volume of trees being removed. The low share of fixed costs in the costs of exploitation by processor proves that a longer shift only affects to a small degree the economic effectiveness of the analysed technology.

Keywords: timber harvesting, thinnings, processor, costs, productivity

1. Introduction – Uvod Considering the fierce competition in the Polish market of forest services, timber harvesting companies can achieve the greatest advantage by using work methods of low unit costs. In Poland, the majority of companies are small, with limited financial capacity (Kocel 2003) and lack of highly efficient and very expensive multi-operational machinery. The degree of harvesting mechanization may be increased by means of machines accessible to middle-size forest service companies thanks to the use of farm tractor-mounted processors (Karlsson 1988, Walczyk 1997).

The use of such machines makes the working environment relatively less strenuous (Giefing 1994b), which is another argument for their widespread introduction. However, in the free market economy, one of the basic criteria of assessment of technological processes of timber harvesting is their economic effectiveness. That is why it is necessary to analyse the Croatian Journal of Forest Engineering 28(2007)2

processor exploitation costs in detail before making a decision to purchase it. At present there are no results of such analyses conducted in Poland (Giefing 1994a), which may constitute an obstacle to the introduction of processors in forest work.

The aim of the present research is to determine the efficiency and timber harvesting costs by the NIAB 5–15 processor mounted on a farm tractor. The scope of the research is limited to early and late thinnings, performed in pine, fir and spruce stands.

2. Research area – Podru~je istra`ivanja The present research was located in southern Poland. Measurements were taken on square sample plots with the surface of 0.5 ha each. The longer side of each plot ran along a skid trail. The basic features of the researched stands and the characteristics of the measures taken in them are presented in Table 1.

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Costs and efficiency of timber harvesting by NIAB 5–15 processor mounted on a farm tractor (177–184)

Table 1 Characteristics of researched stands Tablica 1. Zna~ajke istra`ivanih sastojina Stand number Broj sastojine

1

2

3

4

5

6

7

8

No. of sample plots Broj pokusnih ploha

3

3

3

3

1

2

2

1

Category of utilisation Vrsta prihoda

Early thinning Late thinning Early thinning Late thinning Rana proreda Kasna proreda Rana proreda Kasna proreda

Species composition, % Omjer smjese, %

Pinus – 100

Pinus – 90 Betula – 10

Abies – 70 Fagus – 30

Abies – 70 Fagus – 30

Picea – 70 Abies – 30

Picea – 50 Abies – 50

Picea – 80 Abies – 20

Picea – 90 Abies – 10

Age, years Dob, godine

25

45

37

52

35

40

80

60

Stand density index Obrast

0.9

0.7

0.9

0.6

0.8

1.1

1.0

1.2

full potpun

moderate djelomi~an

full potpun

moderate djelomi~an

moderate djelomi~an

full potpun

moderate djelomi~an

moderate djelomi~an

Ia

Ia I

I

I

I

I

I

I

Breast height diameter, cm Prsni promjer, cm

Pinus – 13

Pinus – 22 Betula – 29

Abies – 13 Fagus – 14

Abies – 24 Fagus – 20

Picea – 14 Abies – 10

Picea – 13 Abies – 10

Picea – 35 Abies – 30

Picea – 24 Abies – 19

Height, m Visina, m

Pinus – 12

Pinus – 20 Betula – 22

Abies – 12 Fagus – 12

Abies – 18 Fagus – 17

Picea – 14 Abies – 10

Picea – 16 Abies – 13

Picea – 30 Abies – 23

Picea – 23 Abies – 18

Growing stock, m3/ha Drvna zaliha, m3/ha

140

228

164

311

90

320

668

637

Mean cutting tree, m3 Srednje sje~no stablo, m3

0.06

0.32

0.07

0.38

0.09

0.09

0.56

0.47

Stand density Sklop sastojine Stand quality class Bonitetni razred

Table 2 Technical data of the NIAB 5–15 processor Tablica 2. Tehni~ki podaci procesora NIAB 5–15 Technical parameters – Tehni~ki podaci

Values – Vrijednosti 1030 kg

Length – Duljina

2000 mm

Width – [irina

2450 mm

Height – Visina

2300 mm

Engine power – Snaga motora

30 kW

Maximum cutting diameter – Najve}i sje~ivi promjer

500 mm

Length of winch cable – Duljina u`eta vitla

50 m

Cable winch pulling power – Vu~na sila vitla

25 kN

Working pressure – Radni tlak

20 MPa

Oil tank capacity – Obujam spremnika ulja

178

Late thinning Kasna proreda

3. Used machines and harvesting technology – Istra`ivani strojevi i na~ini pridobivanja drva

Mass – Masa

Pump capacity – Protok ulja

Early thinning Rana proreda

60 dm3/min 60 dm3

The basic parameters of the NIAB 5–15 processor mounted on a MTZ 100 farm tractor, used in the research, are presented in Table 2.

Harvesting was conducted according to the following schedule. Using the chain saw, the operator felled the trees marked by the staff of the State Forest Administration. The felling was performed in the direction opposite to the skid trail. If a tree remained suspended, the operator cut only the hinge. The processor operator stretched the cable and attached it to the butt end of the tree. Then, he performed winching operating the winch via radio, and sometimes preceded by removing the suspension. At the skid trail, the operator detached the load and performed timber debranching and cross-cutting using the processor (Fig. 1). The timber was cut into 1.25-m Croatian Journal of Forest Engineering 28(2007)2

Costs and efficiency of timber harvesting by NIAB 5–15 processor mounted on a farm tractor (177–184)

Fig. 1 Timber debranching and cross-cutting Slika 1. Kresanje grana i trupljenje long pieces (Fig. 2). After the winching and timber processing was completed within the range of the cable, a new working point of the processor was found on the skid trail.

4. Methods of measurements and calculations – Metode mjerenja i obrade podataka A time study of harvesting was performed using data loggers of the PSION type, equipped with appropriate software. The precision of time measurements was 1 s. The time study was conducted separately for the chainsaw operator and for the processor operator. The amount of the fuel used was measured during each tank filling. After the completion of harvesting, the volume of the timber harvested was determined on the basis of log pieces diameters.

It was assumed that the economic effectiveness of the analysed timber harvesting variants, performed during thinnings, would be best characterized by their unit costs, expressed in /m3. Considering the methods of cost calculation provided in the literature (Suwa³a and Rzadkowski 2001, Suwa³a 1998, Zychowicz 1998), the following equipment harvesting costs (C) were taken into account: C = Ca + Ci + Cfl + Cr [EUR/h] where: Þ harvesting costs C [EUR/h] P é EUR ù T × H ëê h ûú P p × EUR ù Þ interest of capital Ci = 2 100 é T × H êë h úû Þ amortization Ca =

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Fig. 2 Processed timber on the skid trail Slika 2. Izra|eno drvo na traktorskoj vlaci Þ fuel and lubrication cost Cfl [EUR/h] k P× é EUR ù 100 Þ repair cost Cr = T × A êë h úû Þ machine purchase price P [EUR] Þ machine economic life T [years] Þ scheduled operating time H [h/year] Þ capital interest rate p [%] Þ repair cost index n [%] The calculations disregard the cost of wages for machine operators. According to other authors’ work (Jod³owski 2000, Maciak and Skar¿yñski 1995, Porter 1998), all calculations were performed for the effective work time. This allowed for the elimination of contingency in the assessment of efficiency, caused by e.g. the number of machine failures or the length of breaks during work (Giefing and Gackowski 2001). Both the processor and the tractor under analysis were in their post-amortization period. However, for the sake of the calculations, it was assumed that the machines were new (Tab. 3).

The cost of fuel and lubricants (Cfl) was determined on the basis of their use in field conditions.

Unit costs c0 were calculated for each machine separately using the following formula: c0 =

C é EUR ù , V0 êë m 3 úû

where:

Þ V0 – productivity in the effective time (m3/h).

Unit costs for the whole technology were determined by summing the unit costs of felling by chainsaw and the unit costs of winching, debranching and cross-cutting by processor.

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Table 3 Assumed cost parameters for machine rate calculation Tablica 3. Pretpostavljeni tro{kovni parametri za kalkulaciju strojnoga rada Cost calculation inputs Ulazni podaci kalkulacije Machine purchase price – Nabavna vrijednost stroja P [ ] Machine economic life T [years] – Normalno vrijeme uporabe stroja T [godine] Scheduled operating time H [h/year] – Iskori{tenost stroja H [h/godina] Capital interest rate – Kamatna stopa p [%] Fuel price – Cijena goriva [ /dm3] Oil price – Cijena maziva [ /dm3] Repair cost index – Indeks tro{kova popravka n [%]

Chain saw Motorna pila 658 2 1800 13.2 1.11 1.8 30

NIAB 5–15 processor Procesor NIAB 5–15 21,053 8 1800 13.2 – 1.8 30

Farm tractor Poljoprivredni traktor 26,316 8 1800 13.2 1.03 – 10

5. Results and discussion – Rezultati i rasprava On the basis of the above assumptions, the costs of exploitation of each machine used in the analysed technological process were calculated as follows: the chain saw: C = 2.28 /h, the farm tractor (carrier): C = 6.75 /h, the processor: C = 2.45 /h, which means that the costs of exploitation of the processor with the tractor equal 9.20 /h. A detailed cost structure related to each machine is presented in Table 4.

The cost structure of the processor is dominated by fixed costs because it is not fuelled but driven by tractor. The fixed costs constitute over 60% of the total cost. This percentage is less than 40% for the whole assembly because the tractor, whose exploitation costs dominate in the assembly, has a share of 29% in fixed costs. The conclusion is that by prolonging the working period of the whole assembly, the costs of its exploitation will not be considerably reduced. For example, work during 1.5 shifts (12 hours/day) will reduce the costs of exploitation by 15%, i.e. bring them down to 7.82 /h.

Effectiveness on each work-stand, calculated on the basis of the field time study, is presented in Figures 3 and 4.

Fig. 3 Productivity of the chainsaw operator in effective time Slika 3. Proizvodnost radnika s motornom pilom u efektivnom vremenu In the analysed technology, the chainsaw operator’s work consists exclusively of tree felling; that is why the effectiveness of his job was very high. In

Table 4 Cost structure per hour of machine Tablica 4. Struktura tro{kova po satu rada stroja Cost item Sastavnice tro{ka Amortization – Amortizacija [ /h] Cost of interest – Tro{ak kamata [ /h] Fixed costs – Fiksni tro{kovi [ /h] Fuel and lubrication – Gorivo i mazivo [ /h] Repairs – Popravci [ /h] Variable costs – Varijabilni tro{kovi [ /h]

180

Chain saw Motorna pila 0.18 0.01 0.19 2.03 0.05 2.08

NIAB 5–15 processor NIAB 5–15 procesor 1.46 0.10 1.56 0.46 0.44 0.90

Farm tractor Poljoprivredni traktor 1.83 0.12 1.95 4.62 0.18 4.80

Harvesting system Sustav pridobivanja drva 3.29 0.22 3.51 5.08 0.62 5.70

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early thinnings, the effectiveness of felling was not very differentiated and remained in the range from 4.3 to 5.0 m3/h (Fig. 3). In late thinnings, the effectiveness was higher than in the early ones, which was related to a greater volume of a single tree. The effectiveness of the processor was more differentiated in early thinnings (Fig. 4). The highest effectiveness, i.e. almost 2 m3/h, was observed in fir forest while in

spruce and pine forests it was by about 0.7 m3/h lower. In late thinnings the effectiveness of the processor was very balanced and amounted to about 3 m3/h. The results obtained are close to the ones observed in Germany and Sweden, where, depending on the stand composition and age, the effectiveness was between 1.5 and 3.8 m3/h (Giefing 1994a, Marntell and Marntell 1988, Walczyk 1997).

Calculated on the basis of exploitation costs per hour and the effectiveness obtained, the unit costs of tree felling by chain saw are presented in Figure 5.

Low costs of the chain saw harvesting (Tab. 4) and its high effectiveness, connected with the use of this equipment for only one technological operation (i.e. felling), resulted in very low unit costs. In early thinnings, they amounted to 0.48 /m3 on average and in late thinnings to 0.28 /m3.

The work of the assembly with the processor constitutes the chief item in the analysed technology costs (Fig. 6).

The application of the processor in late thinnings resulted in the costs of 2.53 /m3 in the operational time; and in early thinnings the costs were almost twice higher: 4.74 /m3. A larger differentiation of unit costs, obtained in early thinnings carried out in stands composed of different species, results from a higher effectiveness of the processor in fir stands.

Summary unit costs in the operational time for the whole technology are as follows: felling, winching, debranching and cross-cutting (Fig. 7) do not exceed 6.44 /m3 in younger stands (early thinnings) and

Fig. 5 Unit costs of tree felling (chainsaw operator) in effective time Slika 5. Jedini~ni tro{kovi sje~e stabala (radnik s motornom pilom) u efektivnom vremenu

Fig. 6 Unit costs of work of the processor NIAB 5–15 in effective time Slika 6. Jedini~ni tro{kovi rada procesora NIAB 5–15 u efektivnom vremenu

Fig. 4 Productivity of the processor in effective time Slika 4. Proizvodnost procesora u efektivnom vremenu

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Fig. 7 Unit costs of analysed technology (chainsaw operator + processor NIAB 5–15) in effective time Slika 7. Jedini~ni tro{kovi istra`ivane tehnologije (radnik s motornom pilom + procesor NIAB 5–15) u efektivnom vremenu

Fig. 8 Unit costs and productivity in effective time vs. the mean tree volume Slika 8. Ovisnost jedini~noga tro{ka i proizvodnosti u efektivnom vremenu o srednjem obujmu stabla

2.95 /m3 in older stands (late thinnings). The lack of processors mounted on farm tractors in the Polish market of forest services (Kocel 2003) and, hence, the lack of economic analyses of such equipment makes it impossible to directly compare the results obtained in the present research with those by other authors.

Comparisons with the economic effectiveness of harvesting of such equipment achieved abroad are not reliable due to considerable differences in the general economic situation, which affects the level of unit costs. For example, according to Giefing (1994a) the unit costs of timber harvesting in Germany, in conditions very similar to stand No. 1 (Tab. 1), amounted to 23 DM/m3.

A simulation of total unit costs, obtained for the whole shift, was performed for the analysed technology. It was assumed that the monthly costs per person for the chainsaw operator and the driveroperator (gross wages with surcharge) are 6.58 . In accordance with the results, the coefficient of the shift utilisation was established on the level of 0.75.

For such conditions, the average total unit costs are: 10.67 /m3 in early thinnings and 7.49 /m3 in late thinnings.

The present research results concerning the exploitation and effectiveness of the processor indicate the existence of direct relations between them and the average volume of harvested trees. These relations are presented in Figure 8. The regression equations shown there turned out to be statistically sig-

nificant. In the case of effectiveness, the significance level, calculated by means of the t-Student test, was 0.00 (t = 4.11) and in the case of unit costs 0.00 (t = –3.78). The strength of the relation between the average volume of harvested trees and effectiveness Vo, measured by the value of the Pearson coefficient of linear correlation R, amounted to 0.72. In the case of unit costs co, the correlation coefficient was –0.69.

The straight lines of regression (Fig. 8) allow for predicting the value of unit costs and effectiveness depending on the average volume of harvested trees exclusively based on the range of the volume of analysed trees. This is due to two facts: firstly, the processor can debranch trees whose maximum diameter is 50 cm; secondly, using the processor to harvest trees with small mass, e.g. in late thinnings, would require additional research to be performed in such conditions because manual bunching of logs would have to be considered as well as a lot of work for harvesting of timber of low market value.

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6. Conclusions – Zaklju~ci On the basis of research results, the following conclusions should be pointed out: Þ Hourly costs of harvesting of the processor assembly amounted to about 9.2 . Þ Different values of effectiveness obtained in early and late thinnings resulted in different levels of unit costs in these groups of stands.

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Þ The costs of harvesting of the tractor are the dominant item in the costs of harvesting of the processor-tractor assembly. Þ Due to a small share of fixed costs in the costs of harvesting of the harvesting system, prolonging the shift length from 8 to 12 h lowered the unit costs only by 15%. Þ Considering the relations between the average volume of processed trees and effectiveness (and, hence, unit costs), as shown in the present research, it is more profitable to use the processor in late thinnings.

7. References – Literatura Giefing, D. F., 1994a: Badania eksploatacyjne procesora HYPRO. Przegl¹d Techniki Rolniczej i Leœnej 12: 20–22.

Giefing, D. F., 1994b: Ci¹gniki rolnicze w procesie pozyskiwania drewna. Przegl¹d Techniki Rolniczej i Leœnej 10: 22–23.

Giefing, D. F., Gackowski, M., 2001: Ekonomiczna efektywnoœ} pozyskiwania drewna krótkiego w drzewostanach III kl. wieku w zale¿noœci od zastosowanych urz¹dzeñ zrywkowych. Polska Akademia Umiejêtnoœci, Prace Komisji Nauk Rolniczych 3, p. 17–26.

Jod³owski, K., (2000): Technologia i technika pozyskiwania drewna we wczesnych trzebie¿ach w drzewostanach sosnowych. In: Stan i perspektywy badañ z zakresu u¿yt-

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kowania lasu. Instytut Badawczy Leœnictwa, Warszawa, p.

97–108.

Kocel, J., 2003: Sektor us³ug leœnych – stopieñ i zakres przemian, instrumenty wspierania oraz rola sektora w ³agodzeniu bezrobocia. Mat. IBL, Warszawa.

Karlsson, T., 1988: System Skonsam – studie av ett småskaligt mekaniserat gallringssystem. Sveriges lantbruksuniversitet, Institutionen for skogstechnik 119, p. 1–100.

Maciak, A., Skar¿yñski, J., 1995: Efekty pracy ci¹gnika URSUS 2812 przy zrywce drewna krótkiego. Przegl¹d Techniki Rolniczej i Leœnej 10, p. 27–29.

Marntell, A., Marntell, T., 1988: Vimek G30 i praktisk drift.

Vimek G30 in operation. Sveriges lantbruksuniversitet, Institutionen for skogstechnik, p. 1–45.

Porter, B., 1998: Analiza wybranych metod pozyskiwania drewna w ciêciach jednostkowych w drzewostanach œwierkowych. Przegl¹d Techniki Rolniczej i Leœnej 5: 10–12.

Suwa³a, M., 1998: Koszty pracy wybranych œrodków do pozyskiwania drewna. Sylwan 142(11): 27–36.

Suwa³a, M., Rzadkowski, S., 2001: Wydajnoœ} pracy, koszty i uszkodzenia drzew przy pozyskiwaniu drewna w trzebie¿ach drzewostanów górskich. Prace IBL, Seria A, 1 (911), p. 85–111.

Walczyk, J., 1997: Procesory tworzone na bazie ci¹gników rolniczych. Przegl¹d Techniki Rolniczej i Leœnej 3: 18–21.

Zychowicz, W., 1998: Metoda obliczania kosztów eksploatacji maszyn leœnych. In: Mat. Symp. Efekty stosowania maszyn o du¿ej wydajnoœci, przyjaznych dla œrodowiska w lasach polskich, Warszawa, p. 5–12.

Sa`etak

Djelotvornost pridobivanja drva procesorom NIAB 5–15 postavljenim na poljoprivrednom traktoru Rad prikazuje istra`ivanje djelotvornosti pridobivanja drva procesorom NIAB 5–15 postavljenim na poljoprivrednom traktoru. U Poljskoj postoji velik broj poduze}a koja se bave {umskim radovima. Ve}inom su to mala poduze}a ograni~enih financijskih mogu}nosti te s nedostatkom visokou~inkovitih, ali vrlo skupih {umskih strojeva.

Pove}anje stupnja mehaniziranosti sje~e i izrade stabala omogu}eno je primjenom procesora postavljenoga na poljoprivrednom traktoru. Na slobodnom tr`i{tu jedan je od osnovnih kriterija primjene odre|enoga postupka pridobivanja drva ekonomska isplativost. Stoga je potrebno detaljno analizirati tro{kove sustava rada, {to je ujedno i cilj ovoga istra`ivanja.

Istra`ivanje je ograni~eno na rane i kasne prorede borovih, jelovih i smrekovih sastojina. Mjerenja su provedena na pokusnim plohama veli~ine 0,5 ha uz traktorsku vlaku. Osnovne zna~ajke istra`ivanih sastojina prikazane su u tablici 1, a tehni~ki podaci o ispitivanom procesoru u tablici 2.

Postupak pridobivanja drva podrazumijeva usmjerenu sje~u dozna~enih stabala motornom pilom okomito prema smjeru {umske vlake, vezanje tovara i privitlavanje vitlom, kresanje grana i trupljenje debla procesorom.

Drvni su se sortimenti izra|ivali u duljinama od 1,25 m.

Studij rada i vremena proveden je proto~nom metodom zasebno za radnika s motornom pilom i radnika na procesoru. Izra|enim drvnim sortimentima odre|en je obujam na osnovi mjerenja srednjega promjera. Tako|er je mjerena potro{nja goriva i maziva motorne pile i traktora.

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Costs and efficiency of timber harvesting by NIAB 5–15 processor mounted on a farm tractor (177–184)

Tro{kovi su izra~unati na osnovi efektivnoga vremena rada. Za potrebe istra`ivanja pretpostavljeno je da su strojevi novi te da su u izra~unu uzete vrijednosti prikazane u tablici 3.

Satni tro{ak pojedinoga stroja pri ispitivanom postupku iznosi: motorna pila 2,28 EUR/h, poljoprivredni traktor 6,75 EUR/h, procesor 2,45 EUR/h, {to daje ukupni tro{ak od 9,20 EUR/h. Detaljna je struktura tro{kova prikazana u tablici 4. U strukturi tro{kova procesora prevladavaju fiksni tro{kovi (preko 60 %), jer procesor ne tro{i gorivo ve} ga pogoni traktor. Za cijeli sustav pridobivanja drva fiksni tro{kovi iznose manje od 40 % jer traktor, ~iji su tro{kovi najve}i, ima samo 29 % fiksnih tro{kova. Zaklju~ak je da produljenje radnoga vremena ne}e bitno utjecati na smanjenje tro{kova (npr. radnim danom od 12 sati mogu se smanjiti tro{kovi za samo 15 %).

Proizvodnost radnika s motornom pilom i radnika na procesoru izra~unata je na osnovi provedenoga studija rada i vremena. Radnik s motornom pilom radi isklju~ivo na sje~i stabala te je stoga njegova proizvodnost velika i kre}e se od 4,3 m3/h do 5 m3/h u ranim proredama. U kasnim proredama proizvodnost radnika s motornom pilom jo{ je ve}a zbog ve}ega obujma stabala (slika 3). Proizvodnost radnika na procesoru razlikuje se kod ranih proreda ovisno o vrsti drve}a, a u kasnim proredama iznosi pribli`no 3 m3/h u uvjetima rada obuhva}enim istra`ivanjem (slika 4).

Jedini~ni su tro{kovi izra~unati na osnovi satnoga tro{ka rada pojedinoga sredstva i ostvarene proizvodnosti.

Rad motornom pilom ostvario je male jedini~ne tro{kove (0,48 EUR/m3 u ranim proredama, odnosno 0,28 EUR/m3 u kasnim proredama) zbog niskoga satnoga tro{ka i velike prizvodnosti (slika 5). Najve}i su jedini~ni tro{kovi zabilje`eni u radu na procesoru – 4,74 EUR/m3 u ranim proredama i 2,53 EUR/m3 u kasnim proredama (slika 6).

Ukupni jedini~ni tro{kovi sustava pridobivanja drva ne iznose vi{e od 6,44 EUR/m3 u ranim proredama, odnosno 2,95 EUR/m3 u kasnim proredama (slika 7).

Provedeno je istra`ivanje pokazalo ovisnost proizvodnosti procesora te jedini~nih tro{kova o obujmu posje~enoga stabla. Stoga je provedena regresijska analiza navedenih parova podataka te se stati~kim ispitivanjem utvrdila zna~ajna povezanost podataka. Rezultati regresijske analize omogu}uju predvi|anje vrijednosti jedini~nih tro{kova i proizvodnosti rada procesora ovisno o srednjem obujmu posje~enoga stabla. Na osnovi promatranih odnosa zaklju~uje se da je ve}a isplativosti rada procesora u kasnim proredama.

Klju~ne rije~i: pridobivanje drva, prorede, procesor, tro{kovi, proizvodnost

Authors’ address – Adresa autorâ:

Received (Primljeno): December 19, 2006 Accepted (Prihva}eno): November 19, 2007

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Prof. Janusz Michal Sowa, PhD. e-mail: rlsowa@cyf-kr.edu.pl Dariusz Kulak, PhD. e-mail: rlkulak@cyf-kr.edu.pl Grzegorz Szewczyk, MSc. e-mail: rlszewcz@cyf-kr.edu.pl Agricultural University of Cracow Faculty of Forestry Department of Forest and Wood Utilization Al. 29 Listopada 46 31–425 Kraków POLAND Croatian Journal of Forest Engineering 28(2007)2

Preliminary note – Prethodno priop}enje

Optimization of an existing forest road network using Network 2000 Mohammad Reza Ghaffarian, Hooshang Sobhani Abstract – Nacrtak Optimization of forest road network is an important part of logging planning. Matthews (1942) was the first to introduce a method for optimization of road spacing based on minimization of roading and skidding cost. The goal of this paper is to find the best road network for a district harvested by skidder. The skidding model developed by stepwise regression model was used to predict the cost of skidding per cubic meter for the 39 nodes, which were planned in the district map. The harvesting volume and roading cost per each node were computed.

The data were entered into NETWORK 2000 and shortest path algorithm; simulated annealing and great deluge algorithms were run to find the best solution to optimize logging cost of the district. The results showed which roads can be eliminated from the existing forest road network.

Keywords: road spacing, optimal road density, skidder, model of network analysis, Iran

1. Introduction – Uvod Forest road network planning is an important task of forest engineers. Matthews (1942) was the first to present a model for defining optimum road spacing based on minimization of skidding and roading costs from a landowner’s point of view. Optimal road spacing can also be influenced by other factors having impact on optimal road network such as logging method, price of products, taxation policies, landing costs, overhead costs, equipment opportunity costs, road width and size of landing, skidding pattern, profit of logging contractor, slope and topography and soil disturbance (Segebaden 1964, Sundberg 1976, Peters 1978, Bryer 1983, Wenger 1984, Sessions 1986, Yeap and Sessions 1988, Thompson 1992, Liu and Corcoran 1993, Heinimann 1997, Thompson 1998, Akay and Sessions 2001, Sessions and Boston 2006).

A couple of studies on optimal road density (ORD) have been reported by several researchers. Pi~man and Pentek (1998) calculated ORD of 14.7 m/ha for ground based skidding system using farm tractors in Croatia. In the Northern forests of Iran, the case studies on selection cuttings and skidding operations showed that optimal road density ranged from 9 to 28 m/ha for different areas (Mostafanejad 1995, Eghtesadi 2000, Lotfalian 2001, Naghdi 2004). Most Croatian Journal of Forest Engineering 28(2007)2

of the above studies used minimization of total cost of roading and skidding. Optimal road spacing is only a value that provides a guide for locating roads and cost target but does not suggest where the roads should be actually placed (Tan 1999). In the past years, the mixed integer mathematical programming and heuristic algorithms such as: TIMBRI (Sullivan 1974), NETWORK (Sessions 1978), TRANSHIP (Kirbey et al. 1981), MINCOST (Wong 1981), NETCOST (Weintraub 1990) and NETWORK 2000 (Sessions and Chung 2003) have been used to find the appropriate solution for certain fixed and variable cost problem.

Sessions (1992) introduced the method of using network analysis for road and harvesting planning, which is applied in this study. Gullison and Hardner (1993) described a rule-based simulation model designed to examine options for reducing the total length of forest roads. Clarck et al. 2000) also used a heuristic for access road development where roads are defined a priori. Akay et al. (2005) described commonly used modern heuristic techniques (Simulated Annealing, Genetic Algorithm, Tabu Search, and Shortest Path Algorithm). Simulated Annealing (SA) was used to guide the search for the best vertical alignment that minimizes the total costs of construction, transportation, and maintenance costs for a single forest road. Ichihara et al. (1996) proposed Genetic Algorithm (GA) model integrating two optimization

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Optimization of an existing forest road network using Network 2000 (185–193)

techniques to optimize forest road profiles. Tabu Search and GA were compared to manually designed forest road profile by Aruga et al. (2005). Shortest path algorithms have been implemented by Sessions (1991) for secondary harvest transport. Anderson and Nelson (2004) used Dijkrstra’s shortest path algorithm to project a road link that minimized the distance between a landing and the current road network.

Tan (1999) developed the spatial and heuristic procedure to locate forest roads. He reported that the improved procedure proved to be beneficial in helping forest road planning managers evaluate alternatives and hence select the optimal location for a road network.

Stueckelberger et al. (2006) considered roading cost, ecological effects and suitability for cable yarding landings in their automatic road-network planning using multi-objective optimization in Switzerland.

The current paper evaluates an existing forest road network in a district of research forest in Northern Iran using Network analysis. The goal is to find the best road network which minimizes total cost of skidding and roading in this district. The results of network procedure can show which road segments can be eliminated.

2. Method of study – Metode istra`ivanja 2.1 Site of study – Mjesto istra`ivanja The study site is located in Northern forests of Iran in Nowshahr. The research was carried out on road network of Namkhaneh research district and training forest centre of Tehran University. The management method is mixed un-even aged high forest with single and group selective cutting regime. The district covers the area of 1,080.9 ha with the growing stock of 434 m3/ha. The broadleaves stands mostly consist of Fagus sp., Quercus sp., Carpinus sp. The cutting volume of the district is 5,850 m3 per year which means 5.41 m3/ha. The slope ranges from 15 to 60%. The felling is done motor-manually using chainsaws. The felled trees are then delimbed and bucked to the assortments. The sawmill logs are skidded by wheeled cable skidders or tracked skidders to the roadside landings. The fuel woods are extracted by mules. Also, in steep terrain that can not be harvested by skidders, logs are processed to small lumber so as to be extracted by mules. Skidding group includes operator and chokerman.

2.2 Data collection – Prikupljanje podataka Jour Gholami (2005) studied cost production of TAF 1004P and Timberjack 450C skidders in this

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area. He used the continuous time study method in both production studies. A typical work cycle included: travel empty, releasing the winch, choker setting, winching, travel loaded, unhooking and dacking. During this time study technical, personal and operational delays were recorded. The same variables were used in both data collections. Variables included skidding distance, piece volume, load volume, number of pieces per turn and slope of trail. In this time study forty-four cycles for Timberjack 450C and forty-six for TAF 1004P were collected. Using the stepwise regression method, the time predicting model was developed for each cycle.

The system cost of the skidder was about 46.91 /h (Jour Gholami 2005). The developed regression models to predict the time of skidding were used to study the road spacing.

The roading cost in this forestry centre included the costs of planning (384.6 /km), construction of subbase (6,837.6 /km), subgrade construction (11,794.8 /km), culverts and ditches (4,273.5 /km), and maintenance and repair cost as 15% of the roading cost in the 10 years period (349.3 /km). The road the construction cost was 23,639.8 /km or 23.64 Euro/m.

The interest of investment would be 2.19 /m considering 18.5% interest rate in Iran. If the life time of the forest road were 50 years, the deprecation cost would be 0.47 /m. The total annual cost of roading includes the sum of the interest and deprecation costs of 2.66 /m.

The mean harvesting volume of Namkhaneh district is about 5.41 m3/ha/year. The existing road network has a density of 28.16 m/ha. The compartments 201, 202, 203, 204 and 205 are protected because of steep slopes (Fig. 1).

Network 2000 was developed to optimize large fixed and variable cost problems related to transportation. It provides three different algorithms: short path algorithm, simulated annealing (SA) and great deluge (GD). The first algorithm solves the network problem using a heuristic method that prorates the fixed costs in an iterative mode. The algorithm can solve very quickly a large fixed and variable cost problem related to transportation. However, the best solution found by this algorithm might not be optimal, but it is a »good« solution. If the solution is not optimal, it is possible to search for a better solution with continues iterations in this program.

To solve the problems of shortest path length, a Dutch computer scientist named Dijkstra, developed the shortest path algorithm in 1959. The basic premise of this algorithm is to find the length of the shortest path between the starting vertex and the first vertex; then the length of the shortest path between the staring vertex and the second vertex; continuing Croatian Journal of Forest Engineering 28(2007)2

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Fig. 1 Existing road network of Namkhaneh district Slika1. Postoje}a mre`a {umskih cesta podru~ja Namkhaneh until the length of the shortest path between the starting vertex and ending vertex is found.

Simulated annealing (SA) is a search technique which exploits an analogy between the way in which a metal cools and freezes into a minimum energy crystalline structure (the annealing process) and the search for a minimum in a more general system; it forms the basis of an optimization technique for combinatorial problems, etc. It was developed in 1983 to deal with highly nonlinear problems. SA’s major advantage over other methods is an ability to avoid becoming trapped in local minima. The algorithm employs a random search, which not only accepts changes that decrease the objective function f (assuming a minimization problem), but also some changes that increase it.

The great deluge algorithm (GDA) is a recently developed variant of simulated annealing. It is similar to SA in that only a single change is considered as a »current« solution, the resulting temporary solution is evaluated, and a decision is made whether or not to convert the temporary solution to the current solution (Bettinger et al. 2002).

The GDA was introduced by Dueck (1993) and proved superior to similar Monte-Carlo based algorithms in solving a 442-city and 532-city Traveling Salesman Problem. The form of the GDA as presented by Dueck (1993) consisted of using a single parameter in the determining of whether or not to Croatian Journal of Forest Engineering 28(2007)2

convert the temporary solution to the current solution (and perhaps change to an inferior solution).

The use of one parameter rather than two, as in a simulated annealing algorithm, is believed to desensitize the algorithm thus leading to equally good results even when parameter estimation and formulation is poor.

In order to use Network 2000 program, the map of compartments was used including the existing forest road in the district (Fig. 1). Each compartment was used to define the nodes for the network approach. It was assumed that 2/3 of the logs in the compartments are skidded downhill and the rest are skidded uphill. Based on this assumption, the nodes were determined using Arc Map (Fig. 2). The area of the nodes was calculated, and then the logging volume for the nodes was determined based on its area and logging volume of the management area. The logs are transported from all the nodes to 3 mills which are marked in Fig. 2.

In each node the mean skidding distance was computed based on measuring 10 samples on the map. The skidding cost per cubic meter (Variable cost) was computed for each node using the mean skidding distance, hourly cost and time predicting model.

The road length for each node was measured on the map. The roading cost per node was computed by multiplying the road length and the roading cost per unit length. In Network 2000 program, the Link

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Optimization of an existing forest road network using Network 2000 (185–193)

Fig. 2 Nodes and mills of network analysis Slika 2. ^vori{ta i pilane u mre`noj analizi file provides the variable cost and upper and lower volume bounds from node to node.

The Sale file includes entry nodes, destination node, harvesting volume, harvest year and discount rate.

The objective is to minimize skidding and roading cost. The network model can be expressed mathematically as: i = 39

Minimize z =

i = 39

å Sc V + å Rc V i =0

i

i

i =0

i

i

Subject to: Vi > 0 where: Sci – Skidding cost per node per m3 Rci – Roading cost per segment Vi – Harvesting volume per node 0 < i < 120.

3. Results – Rezultati 3.1 Production – Proizvodnost The time study resulted in a net production rate of 8.22 m3/h (based on free delay hours). The gross production was 8.88 m3/h (including delay times).

The skidding cost based on the net production rate was 5.69 /m3 (Jour Gholami 2005).

3.2 Time predicting model for skidding – Model procjene vremena privla~enja Jour Gholami (2005) used stepwise regression method using variables such as skidding distance, piece

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volume, load volume, number of pieces per turn and slope of the skid trail. The model was developed by SPSS. t = 4.142 + 1.988 × N + 0.0176 × L + + 1.093 × V (R2 = 0.786, n = 43) where: t – skidding time, min/cycle N – number of pieces per cycle L – skidding distance, m V – volume of load, m3/cycle The effect of each variable on skidding time was studied by changing one variable while holding the other variables constant at their mean value. Figures 3, 4 and 5 show the effect of skidding distance, number of pieces per turn and load volume on skidding time, respectively.

Increasing skidding distance, number of pieces per turn and load volume will increase skidding time.

3.3 Optimum road network – Optimalna mre`a prometnica The variable cost (skidding cost per m3) and fixed cost (road construction cost per segment) were entered for all the nodes in the Link file. For the Sale file, the entry node, destination node and harvesting volume per node were entered. The same harvest year was assumed for all nodes.

Shortest path algorithm was run based on the data and the best solution was found at 45 iteration Croatian Journal of Forest Engineering 28(2007)2

Optimization of an existing forest road network using Network 2000 (185â&#x20AC;&#x201C;193)

Fig. 3 Cycle time vs. skidding distance Slika 3. Ovisnost vremena turnusa o udaljenosti privla~enja

M. R. GHAFFARIAN and H. SOBHANI

Fig. 5 Cycle time vs. load volume Slika 5. Ovisnost vremena turnusa o obujmu tovara

27.19 /m3, respectively. SA and GDA were also run to see if they can find better solution, but these algorithms could not find better solution than the shortest path algorithm.

The network analysis suggested eliminating the road from node 1 to 2 and the road from compartment 28 to 38.

4. Discussion â&#x20AC;&#x201C; Rasprava

Fig. 4 Cycle time vs. number of pieces per turn Slika 4. Ovisnost vremena turnusa o broju komada u tovaru

of the shortest path algorithm. Figure 6 shows the links to be used based on the result of running the algorithm. Using these results, the proposed links were marked on the map and the best solution was obtained as shown in Figure 7.

Total variable cost, fixed cost and sum of variable and fixed cost was 8.30 /m3, 18.89 /m3 and Croatian Journal of Forest Engineering 28(2007)2

The results of work and time study indicated that the variables such as number of pieces per turn, load volume and skidding distance have significant effect on RMS of the model. If these variables increase, the cycle time and cost of skidding will increase.

The solution found by running the shortest path algorithm helps logging planners decide which road segments can be used and which segments should be eliminated to achieve the minimum total cost of roading and skidding.

The solution suggested eliminating the road segments of nodes 1, 2, 28 and 38. If these roads are closed, the forest company not only can avoid their maintenance cost but can also decrease total cost of skidding and roading of the district. The logs in nodes 1 and 2 can be extracted to nodes 0 and 3 using the longer skidding distance. The felled trees in nodes 28, 29 and 38 should be skidded to node 16 and 27.

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Optimization of an existing forest road network using Network 2000 (185â&#x20AC;&#x201C;193)

Fig. 6 Links in network analysis Slika 6. Poveznice u mre`noj analizi

Fig. 7 Best solution of Network Analysis for the road network of Namkhaneh district Slika 7. Najpovoljnije rje{enje mre`e {umskih cesta u podru~ju Namkhaneh dobiveno mre`nom analizom 190

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Optimization of an existing forest road network using Network 2000 (185–193)

5. Conclusions – Zaklju~ci The results of time study and the model developed for predicting skidding time give the forest engineers a tool for logging planning.

Network analysis using the shortest path algorithm, GD or SA, is a useful method to optimize transportation problems. The results of running Network 2000 based on an existing forest road network harvested by cable skidder, showed that the best solution (Fig. 7) can be found with the total cost of 27.19 /m3. The solution showed which links can be used to achieve the lowest cost of skidding and roading (Fig. 6). Using the proposed links, it was established which road segments can be eliminated.

Acknowledgement – Zahvala

M. R. GHAFFARIAN and H. SOBHANI

Gullison, R. E., Hardner, J. J., 1993: The effect of road design and harvest intensity on forest damage caused by selective logging: Empirical results and a simulation model from the Bosque Chimanes, Bolivia. Forest Ecology and Mangement 59: 1–14.

Eghtesadi, A., 2000: Study of transportation network and machinery in Vaz Forest Area. PhD. Thesis, Azad University.

Heinimann, H. R., 1997: A computer model to differentiate skidder and cable-yarder based road network concepts on steep slopes. Journal of Forest Research (Japan) 3(1): 1–9.

Ichihara, K., Tanaka, T., Sawaguchi, I., Umeda, S., Toyokawa, K., 1996: The method for the profile of forest roads supported by genetic algorithm. The Japanese Forestry Society, Journal of Forest Research 1: 45–49.

Jour Gholami, M., 2005: Study of efficiency, production and cost of the large and small skidders (Case study of Taf and Timberjack 450C). MSc. Thesis, Faculty of Natural Resources, Tehran University.

The authors wish to thank Prof. John Sessions from Oregon State University for his help in applying Network Analysis in this paper. The authors would also like to thank Mr. Martin Kuehmaier for his support.

Kirby, M., Wong, P., Hager, W., 1981: Guide to Tranship model. USDA For. Serv. Pac. Southwest For. Range Exp. Stn. Berkeley.

6. References – Literatura

Lotfalian, M. 2001: Study of the factors influencing optimum road density in Sangdeh-Mazandaran. PhD. Thesis, Tehran University, Faculty of Natural Resources.

Akay, A., Boston, K., Sessions, J., 2005: The evolution of computer-aided road design systems. International Journal of Forest Engineering 16(2): 73–79.

Akay, A., Sessions, J., 2001: Minimizing road construction plus forwarding costs under a maximum soil disturbance constraint. The International Mountain Logging and 11th Pacific Northwest Skyline Symposium December 10–12, Seattle, Washington, USA, p. 268–279.

Anderson, A. E., Sessions, J., 2004: Projecting vector based road networks with a shortest path algorithm. Can. J. For. Res. 34(7): 1444–1457.

Aruga, K., Sessions, J., Akay, A., 2005: Heuristics techniques applied to forest road profile. Japanese Forestry Society, Journal of Forest Research 10(2): 83–92.

Bettinger, P., Graetz, D., Boston, K., Sessions, J., Chung, W., 2002: Eight heuristic planning techniques applied to three increasingly difficult wildlife planning problems. Silva Fennica 36(2): 561–584.

Bryer, J. B., 1983: The effects of a Geometric Redefinition of the Classical Road and Landing Spacing Model Through Shifting. Journal of Forest Science 29(3): 670-674.

Liu, S., Corcoran, T. J., 1993: Road and landing spacing under the consideration of surface dimension of road and landings. Journal of Forest Engineering 5(1): 49–53.

Matthews, D. M., 1942: Cost control in the logging industry.

McGraw-Hill, New York, 374 p.

Mostafanejad, A., 1995: Study of cost production of skidder Timberjack 450C and optimal length of skid trails. Master Thesis, Tehran University, Faculty of Natural Resources.

Naghdi, R., 2004: Study of optimum road density in tree length and cut to length system, PhD. Thesis, University of Tarbiat Modarres, Faculty of Natural Resources.

Pi~man, D., Pentek, T., 1998: The influence of forest roads building and maintenance costs on their optimum density in low lying forests of Croatia. Proceedings of the Seminar on Environmentally Sound Forest Roads and Wood Transport in Sinaia, Romania, FAO Rome, 87–102.

Segebaden, G. V., 1964: Studies of cross-country transportation distances and road net extension. Studia Forestalia Suecica, Nr. 18 Sessions, J., 1986: Can income tax rules affect management strategies for forest roads. Western Journal of Applied Forestry 1(1): 26–28.

Clark, M. M., Meller, R. D., McDonald, T. P., 2000: A threestage heuristic for harvest scheduling with access road network development. Forest Science 46: 204–218.

Sessions, J., 1978: A Heuristic Algorithm for the Solution of the Variable and Fixed Cost Transportation Problem. In proceeding: The 1985 Symposium on Microcomputers.

Journal of Forest Engineering 10(2): 91–100.

Dueck, G., 1993: New optimization heuristics: The great deluge algorithm and the record-to-record travel. Journal of Computational Physics 104: 86–92.

Thompson, M. A., 1992: Considering overhead costs in road and landing spacing models. Journal of Forest Engineering 3(2): 13–19.

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Thompson, M. A., 1988: Optimizing spur road spacing on the basis of profit potential. Forest Product Journal 38(5): 53–57.

Weintraub, A., 1986: NETCOST, a heuristic approach for roading and forest management planning. COOP Agreement Rep., Berkeley, Univ. California.

Peters, P. A., 1978: Spacing of roads and landings to minimize timber harvest cost. Journal of Forest Science 24(2): 209–217.

Wenger, K., 1984: Cost control formulas for logging operations. p. 560–563 in Forestry Handbook, 2nd Edition. Society of American Foresters. John Wiley and Sons. New York. 1335 p.

Yeap, Y. H., Sessions, J., 1988: Optimizing spacing and standards of logging roads on uniform terrain. Journal of Tropical Forest Sceince 1(3): 215–228.

Wong, P., 1981: An empirical evaluation of the proration option of the MINCOST network program. USDA Forest Service Engineering Field Notes. Washington D.C. pp.

15–22.

Sa`etak

Optimizacija postoje}e mre`e {umskih cesta aplikacijom Network 2000 Planiranje mre`e {umskih cesta, odnosno njihova optimizacija jedan je od vrlo va`nih zadataka in`enjera {umarstva. Mathews je (1942) prvi autor koji je predstavio model optimizacije mre`e {umskih cesta (optimalnoga razmaka izme|u {umskih cesta) zasnovan na najmanjim ukupnim tro{kovima (s jedne strane tro{kova privla~enja drva, a s druge strane svih tro{kova povezanih s uspostavom {umskih cesta na terenu).

Mnogi su autori (Sundberg 1976, Peters 1978, Bryer 1983, Wenger 1984, Sessions 1986, Yeap i Sessions 1988, Thompson 1992, Liu i Corcoran 1993, Segebaden 1964, Heinimann 1997, Thompson 1998, Akay i Sessions 2001, Sessions i Boston 2006) odredili osnovne ~imbenike koji utje~u na optimalan razmak {umskih cesta. To su: metoda sje~e i izradbe, tehnologija pridobivanja drva, cijena drvnih proizvoda, politika oporezivanja, tro{kovi skladi{tenja trupaca, op}i tro{kovi, oblik sekundarne mre`e {umskih prometnica, dobit izvoditelja radova pridobivanja drva, {irina tijela {umskih cesta, veli~ina stovari{ta, nagib terena, konfiguracija terena i mogu}a o{te}enja tla.

U prebornim je {umama sjevernoga Irana, u razli~itim reljefnim podru~jima, izra|eno vi{e studija (Mostafanejad 1995, Eghtesadi 2000, Lotfalian 2001, Naghdi 2004) kojima je utvr|ena optimalna gusto}a mre`e {umskih cesta u rasponu od 9 do 28 m/ha. Pri tome je primijenjena metoda najmanjih ukupnih tro{kova pridobivanja drva.

Optimalan razmak izme|u {umskih cesta, s jedne strane, predstavlja broj~anu vrijednost koja je putokaz i vodilja {umarskim stru~njacima pri provedbi primarnoga otvaranja {uma, ali s druge strane ne kazuje ni{ta o stvarnim trasama {umskih cesta na terenu (Tan 1999). Radi optimizacije mre`e {umskih cesta i odre|ivanja optimalnoga polo`aja svake pojedine trase {umske ceste razvijen je ve}i broj matemati~kih programa i algoritama: TIMBRI – Sullivan 1974, TRANSHIP – Kirbey i dr. 1981, MINCOST – Wong 1981, NETCOST – Weintraub 1990, NETWORK – Sessions 1978 te NETWORK 2000 – Sessions i Chung 2003.

U ovom je radu, za mre`nu analizu postoje}ega primarnoga sustava {umskih cesta, primijenjen ra~unalni programski paket NETWORK 2000 koji je dizajniran radi optimizacije fiksnih i varijabilnih tro{kova koji se javljaju pri transportu drva, a sastoji se od triju razli~itih algoritama. Definiran je i cilj istra`ivanja – prona}i, u okviru postoje}e primarne {umske prometne infrastrukture, najbolju mre`u {umskih cesta sa stajali{ta najmanjih ukupnih tro{kova pridobivanja drva (tro{kova privla~enja drva i tro{kova povezanih sa {umskim cestama).

Podru~je istra`ivanja – gospodarska jedinica Namkhaneh (1080,90 ha) smje{tena je u mje{ovitim prebornim {umama sjevernoga Irana. To je nastavno-pokusni {umski objekt Sveu~ili{ta u Teheranu. Prosje~na drvna zaliha iznosi 434 m3/ha, a godi{nji je etat 5,41 m3/ha (oko 5850 m3 na ~itavom istra`ivanom podru~ju). Sje~a i izradba obavlja su motornim pilama lan~anicama uz primjenu. Pilanska se oblovina privla~i do pomo}nih stovari{ta kota~nim i gusjeni~nim skiderima. Jednometarsko ogrjevno drvo iznosi se mulama kao i sve izra|eno drvo na strmim terenima koji su nedostupni skiderima (tada se od pilanske oblovine na mjestu sje~e izra|uju piljenice). Nagibi se terena kre}u od 15 do 60 %. Postoje}a primarna otvorenost {uma iznosi 28,16 m/ha uz napomenu da su odjeli 201, 202, 203, 204 i 205 za{titni zbog velikoga nagiba terena te nisu obuhva}eni daljnjim analizama.

U ovom istra`ivanju kori{teni su rezultati prija{njih istra`ivanja. Jour Gholami (2005) do{ao je do ovih rezultata: u~inak zglobnoga traktora Timberjack 450C iznosi 8,22 m3/h, tro{ak strojnoga rada 46,91 EUR/h, a jedini~ni tro{ak 5,69 EUR/m3. Isti je autor razvio i regresijski model za odre|ivanje utro{ka vremena pojedinoga turnusa privla~enja drva, ~iji su ulazni parametri: udaljenost privla~enja, nagib traktorskoga puta (vlake), obujam pojedi-

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noga trupca, obujam tovara i broj komada trupaca u tovaru. Utjecaj udaljenosti privla~enja, broja komada trupaca u tovaru i obujma tovara na trajanje turnusa privla~enja drva prikazan je na slikama 3, 4 i 5. Smanjenje srednje udaljenosti privla~enja, manji broj komada trupaca u tovaru i manji obujam tovara utje~u na smanjenje turnusa privla~enja drva.

Tro{kovi povezani sa {umskim cestama uklju~uju: projektiranje (384,6 EUR/km), izgradnju donjega stroja (6837,6 EUR/km), izvedbu gornjega stroja (11 794,8 EUR/km), izradu objekata odvodnje – cijevnih propusta i odvodnih jaraka (4273,5 EUR/km) i odr`avanje (349,3 EUR/km). Ukupni je tro{ak povezan sa {umskim cestama i iznosi 23 639,8 EUR/km. Uz razdoblje amortizacije od 50 godina i va`e}e financijske pokazatelje u Iranu godi{nji je tro{ak povezan sa {umskim cestama 2,66 EUR/m.

Svaki odjel istra`ivane gospodarske jedinice razdijeljen je, pomo}u programa ArcMap, na odre|eni broj gravitacijskih podru~ja, uz pretpostavku privla~enja 2/3 etata smjerom nizbrdo te 1/3 smjerom uzbrdo. Te`i{te pojedinoga gravitacijskoga podru~ja predstavljalo je ~vor (nod) mre`e, poslije kori{ten pri mre`noj analizi.

Sva je izra|ena pilanska oblovina transportirana do jedne od triju pilana ~iji je polo`aj ozna~en na slici 2. Srednja je udaljenost privla~enja odre|ena izmjerom na digitalnom zemljovidu. Na zemljovidu je odre|ena i duljina {umskih cesta pojedinoga ~vora. Prema navedenim modelima i cijenama izra~unati su, primjenom matemati~koga oblika mre`noga modela, tro{kovi privla~enja drva i tro{kovi povezani sa {umskim cestama. Cilj je minimalizirati ukupne tro{kove pridobivanja drva.

Unosom i obradom podataka za svaki ~vor te pokretanjem algoritma za tra`enje najkra}ega puta (shortest path algorithm) dobivena je najbolja povezanost prikazana na slici 6. Kori{tenjem navedenoga rje{enja preporu~ena je optimalna mre`a primarnih {umskih prometnica (slika 7).

Ukupni su varijabilni tro{kovi (tro{kovi privla~enja drva) 8,30 EUR/m3, varijabilni tro{kovi (tro{kovi povezani sa {umskim cestama) 18,89 EUR/m3, odnosno ukupni tro{kovi pridobivanja drva 27,19 EUR/m3. Radi provjere dobivenih rezultata pokrenuti su i algoritmi SA (simulated annealing algorithm) i GDA (great deluge algorithm) u okviru programskoga paketa NETWORK 2000, ali oni nisu prona{li bolje rje{enje za primarno otvaranje {uma od onoga koje je ponudio algoritam za tra`enje najkra}ega puta.

Mre`nom analizom sugerirana je eliminacija {umskih cesta od ~vora 1 do ~vora 2 te od ~vora 28 do ~vora 38.

Privla~enje izra|enih trupaca iz ~vora 1 i ~vora 2 treba obavljati prema pomo}nim stovari{tima u ~vorovima 0 i 3, dok trupce iz ~vorova 28 i 29 treba privla~iti prema stovari{tima u ~vorovima 16 i 27. Na taj su na~in ukupni tro{kovi pridobivanja drva za istra`ivano podru~je minimalizirani.

Klju~ne rije~i: raspored {umskih cesta, optimalna gusto}a {umskih prometnica, skider, model mre`ne analize, Iran

Authors’ addresses – Adresa autorâ: Mohammad Reza Ghaffarian, MSc. e-mail: ghafari901@yahoo.com University of Natural Resources and Applied Life Sciences Vienna Department of Forest and Soil Sciences Institute of Forest Engineering Peter Jordan Strasse 82 1190 Vienna AUSTRIA

Received (Primljeno): October 16, 2007 Accepted (Prihva}eno): December 6, 2007 Croatian Journal of Forest Engineering 28(2007)2

Assoc. Prof. Hooshang Sobhani, PhD. e-mail: sobhani@nrf.ut.ac.ir Tehran University College of Natural Resources Department of Forestry P.O. Box 31585–4314 Karaj IRAN

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Original scientific papers – Izvorni znanstveni radovi STANIMIR STOILOV Improvement of wheel skidder tractive performance by tire inflation pressure and tire chains ................................................................... 137 Pobolj{anje vu~ne zna~ajke skidera promjenom tlaka u gumama i primjenom lanaca na kota~ima TOBIAS CREMER, BORJA VELAZQUEZ-MARTI Evaluation of two harvesting systems for the supply of wood-chips in Norway spruce forests affected by bark beetles ........................................... 145 Ocjena dvaju sustava pridobivanja drvnoga iverja iz smrekovih {uma o{te}enih pojavom potkornjaka ANTON POJE, IGOR POTO^NIK Influence of working conditions on overlapping of cutting and ground skidding in group work ............................................................................ 157 Utjecaj radnih uvjeta na preklapanje sje~e i privla~enja drva pri skupnom radu MOHAMMAD REZA GHAFFARIAN, KARL STAMPFER, JOHN SESSIONS Forwarding productivity in Southern Austria ............................................... 169 Proizvodnost izvo`enja drva u ju`noj Austriji JANUSZ SOWA, DARIUSZ KULAK, GRZEGORZ SZEWCZYK Costs and efficiency of timber harvesting by NIAB 5–15 processor mounted on a farm tractor ................................................................. 177 Djelotvornost pridobivanja drva procesorom NIAB 5–15 postavljenim na poljoprivrednom traktoru

Preliminary note – Prethodno priop}enje MOHAMMAD REZA GHAFFARIAN, HOOSHANG SOBHANI Optimization of an existing forest road network using Network 2000 ........................ 185 Optimizacija postoje}e mre`e {umskih cesta aplikacijom Network 2000

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