Subject Title
• Command & Communication
• Cordage Fiber Properties
• How Steep is too Steep?
• Component Inspections
• Kootenay Highline System
• Material Strengths
• Pulley Systems
• Purcell Prusik System
• Systems Analysis for RopeRescue
•
The overall objective is for the Team Leader to develop a mental model of the rigging solution that takes into account:
▪ the patient situation and access requirements
▪ the surrounding terrain
▪ the safety considerations Methodology:
1. Safety
• Identify hazards and formulate mitigation measures
• Identify work area boundaries
2. Personnel
• Establish separate group/personal gear caches
• Direct team to don PPEs and organize gear
3. Patient
• Location
• # of patients
• Condition
o Injuries (if determinable) and current status
• Access
o Determine best access for gaining initial contact
• Need for additional resources
o e.g. Helicopter required?
4. Mission Overview
• Determine the type of rescue system to be used
o Pickoff? Highline? Lower/raise?
5. Rigging*
• Establish hot/cold zones
o Hot Zone requires a life safety system
o Typically, Hot Zone is two or more meters back from an edge
• Identify edge transition location
o Avoid loose debris, sharp edges, and obstacles
o Select uniform terrain, near the best pathway for the operation
• Identify rope alignment to the edge
o Alignment should be parallel to the gravitational fall line
o Alignment is typically (but not always) 90° to the edgetransition
• Identify focal points for rope systems along rope alignment
o Look for locations that can assist in elevating the devices off of the ground
o Separate the rope system stations for better operational work space
• Identify focal points for Edge Restraint systems
o Optimally, two systems on either side of the rope alignment path
• Verify anchor points exist to meet focal point objectives
o Set the riggers up for success
• Consider high directional and/or change-of-direction systems, as required
*tip: place visual props at all rigging component locations such as a length of webbing across the Hot/Cold Zone, a rope bag at the rope system focal point(s), a length of cord parallel to the proper rope alignment,etc.
The objective of the team briefing is for the Team Leader to clearly and succinctly articulate the mission parameters, the safety considerations, and their vision for the rigging solution. It is important to paint the picture for the team using a combination of visual props and intuitive descriptions. The goal is to develop a shared mental model
Procedurally, the Team Leader should cover one topic and then engage the group by soliciting questions/critiques prior to continuing (e.g. “The Line 1 focal point will be right here. Any questions about the Line 1 system?”). This format allows team members an opportunity to voice concerns or questions, and achieves overall team consensus.
After the briefing, the Team Leader assigns rigging roles and the team members begin rigging their respective stations. The Team Leader then roams around, checks rigging alignment, fine-tunes, fields questions, and begins focusing on the appropriate operational assignments.
As the rigging gets completed the Team Leader reassigns riggers to help finish other tasks (e.g. rig the stretcher), makes operational assignments, advises on proper operating techniques, and prepares for the next stage of the operation.
Briefing Sequence:
1. Assemble Group
• Position the entire team with a good view of the operational area
2. Mission Overview
• Summarize the patient situation/condition
• Summarize your proposed rescue solution
• A concise overview addressing: Who? Where? How?
3. Safety
• Describe hazards and the mitigation measures
• Clearly identify the Hot/Cold zones
• Articulate expectation for independent and thorough component inspections
4. Rigging
• Edge transition location
• Rope alignment to edge
• Line 1 focal point and anchor system
• Line 2 focal point and anchor system
• Edge Restraint focal point and anchor system
• High directional and/or Change-of-Direction system, if anticipated
5. Assignments
• Edge in-charge and support personnel
• Line 1 in-charge and support personnel
• Line 2 in-charge and support personnel
• High Directional and/or Change-of-Direction system (if inuse)
• Stretcher Assembly (if extra personnel available)
Team Positions:
Team Leader– Has overall responsibility of the operation. Conducts the Scene Size-up and Team Briefing.
Control– Gives the commands to move the load. Operations begin with Roll Call.
Line 1– Manages the Line 1 system.
Line 2– Manages the Line 2system.
Edge– Assists load over edge Acts as a communication relay for the Attendant, asrequired.
Attendant– Manages handling of the patient (with/withoutstretcher).
Once all of the Component Inspections have been completed, it is time to move to the next phase of the operation; the Team Leader positions the load at the Cold/Hot Zone line – and onthe proper alignment - then calls for "quiet on the set, over to Control for Roll Call!"
Ideally, to free up the Team Leader, a separate Control person is used for the actual commands. This allows the Team Leader to maintain a “global perspective” onthe operation and continue thinking about the next phase (e.g.is the airlife helicopter en route?). The Control person should be in a location where the edge transition can be easily seen, yet still clearly communicate with the Line 1 and Line 2 operators. Commands follow a format of ‘who' first, then 'what do' (i.e. namethe station followed by the command: “Line 1, down!”).
Excellent communication relies on agreed-upon terminology. Regardless of the command structure, employ terminology that everyone is familiar with, expects, and understands.
Lowering Operations- There are three steps to a lowering operation:
1. Roll Call: Ensure everyone is physically and mentally ready for operations tocommence.
• This is conducted in the ColdZone.
2. System Check: a final system verification prior to executing the edge transition.
• Conduct a challenge & response toeach station confirming no further changes tothe riggingsince their component inspection (e.g. “All Stations, anychanges since your componentinspection? Line 1? Line 2? Edge?”).
• Visually confirm the proper alignment/orientation of the system components under light tension.
• Conduct a pre-departure briefing and/or rehearsal of theupcoming commands -this helps polish commands and responses for teams that experience infrequent callvolume.
3. Edge Transition: movingfrom the Cold Zone into the Hot Zone and over the edge.
• Position the Attendant/Patient to allow for a smooth transition over the edge. The Edge personnel may need to help the Attendant carry the stretcher to the edge. To facilitate the edge approach process, the Control can call for slack in the rope systems, asrequired.
• Tighten one rope system (e.g. Line1)so that the Attendant has a snug toprope for the edge transition. Confirm that Line 1 is on “full friction” prior to the next step. Note: some edge transitions provide for favorable circumstances for utilizing dual-tension methods of rope management (e.g. presence of a high directional or a rounded, non-abrupt transition).
• Transition the load over the edge. Commands duringthe critical edge transition must be very welltimed so as to maximize the anaerobic ability of the Edge personnel and Attendant - it can be very physical.
• Check in withthe Attendant after the completion of theedge transition, regardingspecific needs such as:
▪ Lowering speed
▪ Edge protection requirements
▪ Distance to the objective
Properties
Degradation:
(properly stored)
Abrasion Resistance: Surface
Thermal Properties:
Chemical Resistance:
Effect of acids
Effect of alkalis
(Type 6)
(Type 66)
Decomposed by strong mineral acids; resistant to weak acids
Resistant to most mineral acids; disintegrate by 95% sulphuric acid Very resistant
Resistant to most weak acids. Strong acids will attack; especially at high concentrations or high temperature Very resistant
Little or none No effect cold; slowly disintegrate by strong alkalis at the boil Very resistant Resistant to most weak alkalis. Strong alkalis will attack; especially at high concentrations or high temperature Very resistant
1 Of the several kinds of nylon used, type 6 and type 66 are the more common. Type 6 is more common for climbing ropes than type 66, as it has slightly better elongation (and therefore shock absorption) . Type 66 has a slightly higher melting and breaking point, less elongation and slightly better resistance to wear than type 6. Type 66 is more common in rescue ropes. However, how the rope is constructed may have a greater overall effect on its behavior than any differences between these two nylons.
2HMPE (extended chain, high modulus polyethylene); Spectra (made by Allied Signal) is one type of HMPE . It is a very slippery material and may require modified knots, bends and hitches for greater security.
Stretcher transport up (or down) steep embankments is an often used rope rescue technique for transporting patients involved in 'over-the-side' vehicle accidents. In mountainous terrain, this technique may also be referred to as a scree evacuation.Usuallytherearethreeorfourstretcherbearers,andthemainlineisattachedtotheheadendofthestretcher. While there are several different ways to rig a stretcher raise/lower on a slope, consideration must be given to the escalating tension thatthe mainline is subjected to as the slope angle increases. If our objective is to operateator above a static systems safety factor (SSSF) of 10:1, then there is a limit on how steep we can go using this technique.
What operating guidelines do we use in determining our maximum slope angle for a given rope type and number of stretcher bearers? While itis impracticalto takea calculator or a set offorce tables to the rescue, some 'rules-of- thumb' can be developed with a basic understanding of the relationship between mass, force, and slope angle.
The graphical use of force vectors provides both a reasonable level of force approximation and a better understanding of, and appreciation for the forces that could be produced within the system. Recognizing that many variables affect the resulting force or tension in the mainline, some assumptions need to be made to simplify it toward the 'rule-ofthumb' level. For this discussion, they are:
1. The first two persons (rescuer and patient) have masses of 100 kilograms each, including equipment, and additional persons have masses of 80 kilograms each,
2. Each stretcher bearer walks with their body positioned perpendicular to the slope,
3. The rope angle is the same as the slope angle, and
4. The path traveled by the stretcher bearers is that of the fall line.
There are basically three forces acting on the system (Figure 1):
mg the mg-force (mass x gravity) due to gravity acting on the combined mass of the stretcher bearers, patient and equipment;
R the resisting force of the ground on which the stretcher bearers walk; and
T the tension in the rope.
Statically, these forces are in equilibrium, in other words, the forces have resolved themselves. Each of these three forces can be represented graphically as force vectors since they have both magnitude and direction. We know the direction in which these three forces are applied, and we also know the magnitude of the mg-force since we know the total mass involved. By selecting a scale for force, such as 1 centimetre (cm) = 1 kiloNewton (kN), we can draw the force vectors to scale, and then determine the T in the rope by physically measuring thatforce vector. Since the system is in equilibrium, the combined effect of the R and T force vectors result in an opposite and equal force to the mg vector, thereby countering its effect.
To draw the vectors, start with the mg force vector, indicating both its magnitude and direction (Figure 2); next draw in the vectors which counter the effect of mg; draw the resistance R vector (Figure 3) in this case you know only its direction and have the tail of the R vector start at the tip of the mg vector. Draw the T vector from the tip of the R vector, and draw it parallel to the slope angle until it intersects the tail of the mg vector (Figure 4). The magnitude of the T vector which is the tension in the rope is determined by measuring its length, and comparing it to the scale to which you drew the mg vector. For example, if you used the above scale, and you drew the mg vector 2 cm long representing 2 kN of force, then if the resultant T vector is 1.2 cm in length, then the corresponding force would be 1.2 kN.
Itbecomesapparentthatastheslopeangleincreases,that the force vector T becomes larger, to the point where T and mg are the same when the angle is 90º (free-hang). Conversely, the tensionT becomes nilwhen the angle of the slope is 0º (level).
Simple guidelines cannow be developed fora 10:1SSSF by comparing the knotted breaking strength of your rope to the resultant tension or T in mainline for
changing levels of mass and/or slope angle. Table 1 shows the resultant force for given slope angles (in degrees) and different sized rescue loads (kg). The nonshaded areas in the table represent acceptable levels of mainline tension (kN) for a 10:1 SSSF using an 11.1 millimetre (mm) or larger nylon kernmantle low-stretch rope, assuming a knotted breaking strength of at least 22 kN1 The lightly shaded region shows acceptable combinations of mass and slope angle for a 12.7 mm mainline, assuming a knotted breaking strength of approximately 33 kN. Force levels thatexceed10:1SSSFforboth11.1 mm and 12.7 mm mainline ropes are shown in the darkest regions of the table. As an example, three stretcher bearers and a patient (i.e., 4 people with a combined mass of 360 kg), can maintain a 10:1 SSSF on slopes to just under 40° using an 11.1 mm mainline. If instead, they are using a 12.7 mm mainline, then the maximum slope angle can be increased (as expected) to 70°.
It is important to note that the non-shaded or shaded regions of the table do not represent “go” or “no go” from an operational standpoint. The different regions on the table simply represent where a 10:1 SSSF does or does not exist given a certain slope angle, rope type, and rescue mass. Under certain conditions a rescue team may choose to deviate from a 10:1 SSSF for very specific reasons. A discussion of those types of conditions and reasons are outside of the scope of this document.
1 PMI Catalog #112, Page 5 states that "...our (11.1 mm) rope breaks at just over (22.2 kN) on a bowline knot."
Inspections are a critical element to conducting reliable operations in technical rope rescue. A proper inspection is your last line of defense between a rigging error and that error being revealed under live load.
A proper inspection should include both a visual and tactile confirmation of the rigging.
Visual: does the anchor point as well as the anchor system meet your mental model? Is it simple, intuitive, and secure?
Tactile: perform a hands-on, critical evaluation of the rigging. Squeeze carabiner gates, tug on ties, and pull on the standing part of the line exiting the device.
Additionally, any changes to a component after the completion of an inspection should then be re-inspected.
Independent- a competent person cross-checking your work
Visual and tactile - meeting your mental model in combination with a hands-on assessment
Methodical- systematically checking each link in the chain
1. 3Ms– an individual anchor point
Microstructure
• Examining the details of the anchoring device (e.g. rock pro piece) and terrain
• Does it have a good surface area contact?
• Is it aligned correctly for the anticipated direction of pull?
• What are the up-close qualities of the terrain/geology?
Macrostructure
• What are the “big picture” qualities of the terrain/geology?
• Are we anchoring to something that is secure? Adequately strong?
Mechanism
• How does the device work? Is that being optimized?
• How does the device fail to work? Have we avoided that mode?
• How does the direction of pull influence the quality of the anchor point
2. SAD– the anchor system
Strength
• 20kN at the focal point
Alignment
• A tensioned front-tie of the focal point, as needed, to reduce system elongation
• The anchor points in the anchor system are effectively balanced on both sides of the parallel rope path to the edge
Distribution
• Ensuring that the legs/strands in a multi-point anchor system are appropriately distributing the tension; adjust as required to favor the stronger anchor points in a multipoint anchor system
3. SAR-security and functionality
Security
• Carabiners locked? Carabiner orientation, OK?
• Ties dressed and stressed?
• Device reeved correctly and secured?
•Hands-free stop in place?(e.g. Prusik)
• Rope end terminated?
• Edge padding in place?
Alignment
• Focal point 90° to the edge transition?
• Parallel to the gravitational fall line?
• All anchor system strands load-sharing with proper distribution?
Required system components
• Device matches the system requirements?
4. Inspecting the live load
Bowlines interlocked
• Does the master point of attachment incorporate both operational ropes?
Patient Primary
• Attachment to Interlocking Bowlines?(e.g. the bridle focal point in addition to the bridle legs to the stretcher)
• Patient packaging-appropriate rigging to prevent lateral and head/foot exit?
• Harness secure?
Patient Secondary
• Longtail of the Mainline(aka Line 1) to harness?
• PPE – helmet & eye protection; padding/tarping, as required?
Attendant Primary
• Attachment to Interlocking Bowlines?
• Adjustable attachment with adequate range for the operation?
• Harness secure?
Attendant Secondary
• Longtail of the Belay(aka Line 2) to harness?
• PPE – Helmet, eye protection, gloves?
• Radio? Whistle? Additional rigging tools as required?
5. Other important considerations
Confirmation bias
• Avoid interjecting yourself into an independent cross-check inspection. Let that person do their work without any ‘sales pitch’ or explanation from the rigger whose system is being inspected
• Be wary of the expert halo effect. Don’t let a team member’s perceived expertise affect your inspection methodology
• Seek to reveal errors in your inspection as opposed to confirming security. Security is confirmed when the system is devoid of errors following a thorough and focused inspection by a competent person
Humility
• Don’t sign off on an inspection of a system you do not understand. Advise the team leader that they need to inspect the system themselves
The term 'highline' often inappropriately referred to as a 'Tyrolean' refers to a system that uses a track rope (mainline) to suspend a load over a span. Highlines can be much more practical for rescue work than is commonly assumed. However, many people do not appreciate the high levels of force that can be easily placed on the track rope if some key principles are not taken into account. This has led to highlines being set up with excessively low safety factors through the use of ill- considered rigging concepts. To increase the safety of using highlines, practitioners must first obtain the proper in-depth training in basic physics and rigging principles, and also have access to the appropriate equipment. With this approach, rescuers can rig effective, efficient and safe highlines that can be more practical than other techniques available in certain applications.
In 1985, in the province of British Columbia, serious thought and consideration was given to the rigging and operating principles of highlines. This work led to the development of the key concepts and principles of the Kootenay Highline System. Since then, several practical variations have also been developed, such as the English Reeve, the Norwegian Reeve and the drooping highline variations. In addition, the KHS has been tested in several variations and its concept of belaying with the taglines was demonstrated as early as 1986. This cannot be said for most of the other highline techniques in use today.
Though there are many ways to categorize highlines, for the KHS a distinction is drawn between horizontal highlines, sloping highlines and steep highlines. A horizontal highline is one that has no more than a 10º change in angle between end stations. A sloping highline has more than 10º but no more than 45º angle between end stations; and a steep highline has an angle greater than 45º.
To really understand the KHS is to realize that there is no 'one-correct-way' to rig it. Each KHS is built using the deliberate application of key rigging and operating principles and concepts to the specific rescue situation at hand. While some specialized gear such as Prusik Minding Pulleys and Kootenay Carriages may help the efficiency of operating a KHS, it is by no means essential, and can often be substituted as long as the replacement equipment can meet the same performance and safety criteria.
This handout provides a overview of the primary rigging and operating principles of the KHS up to 100 metres (m) in length. For extreme highlines (i.e. those greater than 100m in length), additional considerations that must be taken into account such as formulas and calculations are not covered in this handout.
Rescue teams wishing to perform safe and effective rescues with this highline technique must have a thorough understanding of all of the KHS principles, the most important of which are:
1. Minimize any loss of breaking strength in the highline's track rope by having no knots or sharp bends in it. Also, anchor the fixed end of the track ropewith ahigh strength tie-off(Figure1) oraKootenayCarriage with the pins inserted.
2. Attach the tensioning pulley system to the track rope with a gently acting slipping clutch to prevent over-tensioning (Figure 2). At the present time, Tandem 8 mm 3 wrap Prusik Hitches are used. Mechanical rope clamps such as Gibbs™ or handled ascenders are unacceptable.
3. Attach the load by hanging it from a device capable of sustaining multiple direction forces, such as a Kootenay Carriage, a solid pear shaped or round rigging ring, or equivalent. There are many ways to accomplish this. Consideration must also be given as to whether or not the load will be suspended from one or two attachment points and if the tagline will be one continuous rope or whether two separate ropes will be used. Some examples of various combinations are shown in Figure 3.
4. Rig the taglines to handle shock forces in case of track rope failure by the use of Prusik by- passes at end knots and managing the taglines with Tandem 8 mm 3 wrap Prusik Hitches as per Prusik Belaying attached to suitable end point anchorage. At the load, the taglines should be attached directly to the device capable of sustaining multiple direction forces, as is shown in the examples in Figure 3, and not to the load itself.
5. The KHS is intended to be operated with a Static System Safety Factor of 10:1 or greater on the track rope. If lack of clearance requires minimal deflection (sag), then the track rope may be doubled or quadrupled in order to maintain the principle of a minimum 10:1 safety factor.
While the first five points describe the most important rigging principles of the KHS, the following principles are also important, but may or may not apply depending on the specific rigging requirements of the highline being set up:
(a) Single Tagline with
(b) Single Tagline with (c) Two Taglines with Pulley & Rigging Ring Kootenay Carriage Kootenay Carriage
d) Two Taglines with two Kootenay Carriages (e) Single Tagline with two Kootenay Carriages
Figure 3: Examples of Methods to attach the load and tagline(s) to central attachment point
6. Consistent with the principle of minimizing any loss of strength in the track rope, any pulleys supporting the track rope must be of a large enough diameter. This is especially true for any track rope support pulleys at the tensioning end of the highline. The track rope tension may be adjusted from time to time, which will result in considerable movement of the track rope over the support pulley. At the fixed end of the track rope, any support which has a smooth, large diameter curved surface would do, since the track rope will not move significantly at this end during any adjustment of track rope tension.
7. The pulley from which the load is supported, also needs to be of large enough diameter or proper configuration to minimize the loss of any track rope breaking strength.
8. The track rope and taglines should have their own independent anchor systems with a breaking strength greater thanor equal to the breaking strength of the ropes. For example, an 11.1 mm track rope with a breaking strength of 30 kiloNewtons (kN) must have an anchor system strength of 30+ kN. If the track rope is doubled (i.e. two 11.1 mm ropes), then the minimum anchor system breaking strength must be 60+ kN.
KHS-1 © Rigging for Rescue® 1997 P.O. Box 745 Ouray, CO 81427 U.S.A Graphics created by Earl Fröm
9. Both the track rope and taglines must be able to run freely through pulleys or rollers without contacting the ground, cliff, or other obstacles.
10. On highline spans greater than 30m or so, tagline hangers should be employed to support the taglines thereby minimizing excessive tagline slack. A taglinehanger can be easily be constructed using a sling of accessory cord, Girth Hitch it around the tagline and clip it to the track rope with a non-locking carabiner at regular intervals as required (see Figure 4).
11. The attendant (if used) and patient are connected to the carriage(s) with a back-up in addition to their primary support. If the attendant's primary support is an attachment directly to their harness, then the harness connection should be such that it would keep the attendant upright and maintain their position relative to the patient in the event of a track rope failure.
12. The tension in the track rope should never exceed one tenth of the track rope's breaking strength to maintain a minimum 10:1 Static System Safety Factor. Some general guidelines have been developed for the proper tensioning of the KHS. These 'general' guidelines are based on the knowledge gained from a series of trials to see how much force can be applied to a track rope with various combinations of pullers and pulley system mechanical advantages. With the following suggested guidelines, 'average' rescuers should not be able to over-tension the track rope (i.e., to more than one tenth its breaking strength). Variation between people's pulling ability must be taken into account and corrected for. The guidelines for 11 mm (30 kN breaking strength) track ropes are as follows:
→ It is important to distinguish between tensioning the track rope with and without a load on it. With no load placed on the system, have only one person pre-tension the system by pulling on a 2:1 pulley system. Pulling on the pulley system assumes 'normal' pulling; the rope is not wrapped around any part of the puller's body, and once the puller reaches his/her maximum pulling ability, no additional attempts are made to 'heave-ho' more tension into the system.
→ With the load at center-span, the maximum force that can be applied to the track rope is a multiple of 12 (i.e., the number of pullers times themechanical advantage is less than or equal to 12). This means, for example, that 6 people can pull on a 2:1, 3 people can pull on a 4:1, or even 2 people can pull ona 6:1. The same guidelines apply to single, double, and quadruple track rope tensioning.
If the track rope is 12.5 mm in diameter (40+ kN), the sample principles apply except that a multiple of 18 can be used instead of the multiple of 12 described above. Therefore, 9 people can pull on a 2:1 (though usually impractical), 6 people can pull on a 3:1, 3 people can pull on a 6:1 or 2 people can pull on a 9:1.
It is important to recognize, understand and take into account that there will be differences in the capabilities between people's pulling ability, as well as differences in actual versus ideal mechanical advantage between pulley systems.
13. When double or quadruple track ropes are used to either reduce sag or enable larger loads to be transported, it is important to ensure thatthere is equal tension in each track rope. This can be accomplished with the use of pulley systems-in- series pulling on all track ropes. Figure 5 is an example of a compound 6:1 pulling on each track rope of a double trackropehighline. Ifthese were 11 mm trackropes, then a maximum of 2 people would pull on this pulley system as per the criteria outlined in #12 above. Note the connector strap between the pulleys connecting the Tandem Prusiks to each track rope. This is used so that the integrity of the pulley system will be maintained, even if one of the track ropes were to fail. Additional anchorage of the track ropes with Tandem Prusiks is
Figure 5: Example of Pulley System Set-up to Apply Equal Tension to each Track Rope in a Double Track Rope System
optional, and mostly used for drooping highlines or those highlines that require the tensioning system to be reset during the operation.
14. Oncetheloadhaspassedmid-span andbegins to travel towards the destination station, a pulley system on the tagline may be required to help pull the tagline and load into the station. This can be easily done by making a simple 3:1 pulley system out of the tagline (Figure 6). Note that the Tandem Prusiks are still in place to maintain the belay capability of the tagline.
15. For sloping highlines, the speed of descent out of the top station must be controlled (Figure 7). A brakerack can be used at this station for additional control. Tandem Prusiks with an LR Hitch are still placed on the tagline in front of the brakerack so that the tagline can still act as a belay in the event of a mainline failure.
There are many ways to set up a highline. Therefore, any examples in this handout are not to be construed as 'the way' of how to set up a KHS. Many factors must be taken into consideration, and rescuers should practice various ways of setting up highlines so as not to resort to just one method. The key variables that determine how a KHS will be set up are: the span, the angle between end stations, length of ropes, available equipment, access limitations to either side, the terrain, number of people, the size and location of the load, and communication considerations.
Often the greatest difficulty in setting up a highline is getting the first line across. Pre-planning which equipment needs to be at each end station, and once the first line is across, prioritizing which ropes and equipment will be ferried back and forth is the key to efficiency. If the span is very short, then the track rope and taglines themselves may be tossed to the other station. However, if this can not be done, then either a messenger cord or a pilot line may need to be put in place first. A messenger cord is a small diameter cord capable of hauling the ropes across, whereas a pilot line is an even smaller diameter cord like that used for a line gun which is used to pull a messenger cord in place. There are various ways in which the first line (or rope) can be placed across the span: it may be carried or walked over; a cord may be lowered from each end station and then tied together; it may be thrown across; shot across with a line gun, compound bow, slingshot, or solid or air fueled rocket, or it may be transported across with a water craft in river rescue. These are just a few examples and there are many more options other than just these listed here.
KHS-1 © Rigging for Rescue® 1997 P.O. Box 745 Ouray, CO 81427 U.S.A.
Again, there are numerous ways to set up a highline. The following example is by no means comprehensive and does not do justice to some of the logistical priorities, operational movements for smoothness and communication requirements which lend to efficiency and expediency. This is best learned through competent instruction and practice in setting up highlines a variety of ways. The following example assumes that this is a single track rope, two taglinehighline; therescuers have preplanned which equipment will go to which end station. For clarity, the end stations are called Station One and Station Two:
1. From Station One, establish a messenger cord between end stations that is twice the length of the span. Tie a 'floating middle tie' (e.g. butterfly knot) in the middle of the messenger cord.
2. Clip the track rope to the floating middle tie and have Station Two pull it over by pulling in the messenger cord. Once the Station Two receives the track rope, they can quickly attach a Prusik Hitch a few metres from the end of the track rope and clip it to an anchor (e.g. a sling in a tree where the high strength tie-off will go). Station Two then clips their tagline to the floating middle tie to be pulled back across by Station One. If the span requires it, tagline hangers are put on at regular intervals; feeding out enough tagline to allow it to hang below the track rope.
3. Station One now receives Station Two's tagline, and connects it to Station One's tagline and carriage unit which was set up while they were waiting for Station Two's tagline to arrive. (Note: the messenger cord should be left across the span for the duration of the operation though it can be placed out of the way on either side).
4. Station Two now proceeds with tying a high strength tie-off with the track rope and should advise Station One when this has been completed. They can also set up a tagline/belay anchor system, as well as any backties required (for both the track rope and tagline anchor systems).
5. Station One completes any rigging required while Station Two is working on their end. This may include setting up the tensioning system, tagline and track rope anchor systems, and placing support pulleys. Once Station Two has completed the high-strength tie-off, Station One can complete the pre-tensioning of the track rope and get the attendant/patient ready.
6. Once all system components are checked, the patient and/or attendant can be attached to the carriage unit and communication set-up to start sending them across the highline.
An important element of rigging is being able to quantify the overall breaking strength of a given system. However, it is common to have a number of unknowns in the system that cannot easily or accurately be quantified. Examples include: a large rock mass, a parked vehicle on a dirt road, or a 30 cm diameter tree. How much strength do we assign to these anchors? It is at best a guess. Practically, you’ll continue to add anchor points to your overall anchor system until you are satisfied that the rig is suitably strong for the given task.
However, where we can quantify the breaking strengths of the component parts of a given anchoring system, it behooves us to do so. For example, calculations addressing the Static System Safety Factor (SSSF) of an anchor system involves detailed knowledge of the breaking strengths of the individual component parts. In order to make these calculations, a proficient rigger needs at their disposal the requisite knowledge of the various material strengths that comprise the overall system, as well as how knots de-rate those materials.
The table below offers some typical breaking strength values (kN) for a variety of materials and material configurations common to rescue rigging. Naturally, there will be some deviation from these values based on specific make/model/manufacturer differences. In the end, it is a good practice to be familiar with the breaking strengths of the materials your team uses in its rigging applications.
The kN values of the fiber materials listed above are based on manufacturer’s MBS. However, practical application of those materials in a rigging system typically includes a tie (i.e. knot, bend or hitch). The rope/ cord/webbing (constructed with Nylon or Polyester) typically results in a 1/3 de-rate from MBS. For example, a 30kN rope with a knotted end would have a breaking strength of approximately 20kN at the knot. Dyneema/Spectra sewn runners - as well as other high modulus fibers such as Kevlar and Technora –typically exhibit a knotted de-rate of 50-55% off of MBS.
MS-1 ©2007 Rigging for Rescue® P.O. Box 745 Ouray, Colorado 81427 U.S.A.
The ability to raise loads with a rope is increased when the rope is used in conjunction with a pulley or pulleys. Combinations of fixed and moving pulleys create systems that multiply the force that rescuers are able to apply - making use of mechanical advantage to reduce required strength, at a trade-off of increased endurance. Stated differently, mechanical advantage enables a rescuer to lift a load applying less force than the load itself, but over a longer distance.
Pulley Systems are just one method of obtaining mechanical advantage, and they are not new. As early as 350 BC Aristotle had left arecord of a "sophisticated" pulley arrangement, showing how half the effort was required to lift the load1. Sometimes also referred to as "block and tackles", pulley systems are common to other fields such as seamanship, construction and even auto mechanics; most cranes and hoists use some kind of a block and tackle system.
The simplest example of a pulley system is shown in Figure 1. Here the fixed pulley provides no mechanical advantage (MA) but rather only changes the direction of pull. The force required to lift the load is approximately the same as the load itself and the same amount of rope is brought in for the distance that the load is raised.
The MA changes, however, if the same pulley is moved to a different location so that it is now a traveling pulley; one that moves toward the anchor as the load is raised (Figure 2a). Now only half as much effort is required, but over twice the distance. Even if a second pulley is added so that the direction of pull is reversed (Figure 2b), the fixed pulley does not affect the Ideal Mechanical Advantage (IMA), though in some cases it may make it more practical for the haulers to pull.
Figure 1: Simple 1:1 with a change of direction
Mechanical Advantage is gained from the moving pulleys in exchange of effort for distance. If the last pulley in the system is fixed, it does not affect the IMA but may improve the practical use of the pulley system by changing the direction of pull (e.g. downhill instead of uphill). In both Figures 2a and 2b, twice as much rope is required on the pull side to raise the load a certain distance, although only half the force is required. This is referred to as a system with a MA of 2, or a "2:1" system.
(a) Simple 2:1
(b) Simple 2:1 with a change of direction
Figure 2:
A pulley system is just one of many ways to achieve MA (or, possibly mechanical disadvantage depending on howit isrigged). Thekeytolearningpulleysystemsisunderstandingthebasicconcepts and principles that distinguish one system from another. All too often, rescuers are shown just one or two pulley systems and end up force fitting them to all rescue situations. Learning a few pulley systems by heart does not provide rescuers with the knowledge and flexibility to make the
1The How and Why of Mechanical Movements, Harry Walton 1968, p.45
best use of people and equipment. To be efficient, flexible and effective, rescuers need to have the ability to quickly decide which system is most appropriate for the given working conditions such as the amount of tension the load places in the mainline, the availability of equipment including rope, working (setup) room, friction points, and the number of haulers.
Pulley systems can be divided into three categories: simple, compound and complex. In addition, most pulley systems can be rigged either by using the mainline itself or using a separate rope, often referred to as 'acting on the mainline' (Figure 3). While most pulley systems used in rope rescue will be either simple or compound, rescuers still need to be able to recognize and understand the advantages and disadvantages of all types of pulley systems.
(a) Pulley System using Mainline
(b) Pulley System acting on the Mainline
Figure 3:
There are some components that can be added to a pulley system to make its operation practical during a rescue. The first is a ‘self-minding ratchet’. Such a device enables the haulers to maintain lift distance gained without having to hold onto the rope at all times. One such device is the use of a Prusik and a Prusik Minding Pulley (Figure 4). Devices that act as ratchets (allow one way movement) also enable
resets of the pulley system as they maintain the tension in the mainline while the pulley system is slackened and reset. This can also be accomplished by having a rescuer tend a device that will grab the rope when the pulley system needs a reset.
Simple Pulley Systems are characterized by having one continuous rope flowing back and forth alternately between the pulleys on load and the anchor (or the anchor and the load), and all pulleys at the load side (referred to as traveling pulleys) travel towards the anchor at the same speed (Figure 5). All pulleys at the anchor side of the system remain stationary. The tension in the rope remains the same throughout the pulley system.
Principles:
• if the tied end of the rope is at the anchor, the IMA will be an even number (2:1, 4:1, 6:1, etc.). If the tied end of the rope is toward the load, the IMA will be an odd number (1:1, 3:1, 5,1 etc.).
• if the last pulley in the system (one closest to
Pulley System Figure 5:
the haulers) is at the anchor (fixed), it does not add MA, but just changes the direction of pull.
• the IMA of a simple system is determined by counting the number of ropes under tension at the load side of the pulley system.
• the number of pulleys required for a simple system (without a change of direction) is always the IMA minus one.
• to incorporate a ‘self-minding ratchet’ located at the anchor – while also allowing the system to be reset for another raise- the IMA of the simple pulley system must be an odd number.
Compound Pulley Systems are characterized as one simple pulley system pulling on another simple pulley system; the traveling pulleys travel towards the anchor at different speeds. Compound Pulley systems are useful because they can provide greater MA than simple systems for the same number of pulleys, thereby reducing overall loss due to friction for the same IMA.
Simple 4:1
(h) Simple 5:1
(i) Simple 3:1 acting on the Mainline
Figure 6: Simple Pulley Systems Examples cont…
• the IMA of a compound pulley system is determined by multiplying the IMA of each simple pulley system together. For example, a simple 3:1 pulling on a simple 2:1 becomes a compound 6:1 as 3 x 2 = 6; also note, however, thata simple 2:1 pulling on a simple 3:1 is also a compound 6:1 as 2 x 3 = 6 (see Figures 7b and 7c).
• if it is important to get the load upwith the least number of resets and you are using a compound pulley system comprised of two dissimilar MA simple pulley systems, have the higher MA system pull on the lower MA system (e.g. have the 3:1 pull on the 2:1 in a compound 6:1). Recall, however, from the Simple Pulley System Principles, if you want to use a ‘self-minding ratchet’, you need an odd-numbered pulley system. You will have to decide which factor is more important or change your compound pulley system to another combination that can meet both those needs (e.g., a compound 9:1 or a simple 5:1).
• longer throw distances per reset can be achieved by positioning the anchor pulley(s) of the last (closest to the hauler) simple pulley system far enough back to allow each simple pulley system to collapse at the same time. For example, for both simple 3:1's in a compound 9:1 to collapse at the same time, the last 3:1 must have three times the reset distance of the first simple 3:1. This is due to the fact that three times more rope will be pulled through the last simple 3:1 than the first simple 3:1.
• the highest MA with the least number of pulleys is achieved by repeatedly compounding a simple 2:1 on a simple 2:1.
4:1 (Simple 2:1 pulling on a Simple 2:1)
6:1 (Simple 3:1 pulling on a Simple 2:1)
(c) Compound 6:1 with a ratchet (Simple 2:1 pulling on a Simple 3:1)
9:1 (Simple 3:1 pulling on a Simple 3:1)
• when constructing a compound pulley system, think of all the possible combinations that when multiplied together will equal your desired MA; then consider the advantages and disadvantages of each and determine which combination will best meet your needs given your available equipment and working constraints.
Pulley Systems are characterized by being neither simple nor compound. There is no one definition that characterizes all complex systems due to their great diversity. With only four pulleys, over 100 combinations of pulley systems can be made, most of them being complex pulley systems. With the exception of a few common complex pulley systems such as the "Spanish Burton" shown in Figure 8, complex pulley systems are not often seen being used in rescue work. Typically, similar objectives can be met using Simple or Compound Pulley Systems that are easier for rescuers to recognize and are more flexible for modifications as required.
Complex 3:1 / Spanish Burton
Figure 8:
The IMA of a complex pulley system can be determined using either of two methods. While impractical in the field, thefirst method is to pull in a known length of rope and compare that distance to how far the load was raised. The second, more practical method, is the "T-method" which can be used to determine the IMA of any pulley system, and is covered in a separate handout.
Theoretical Mechanical Advantage (TMA) is the estimated Actual Mechanical Advantage (AMA) that you calculate after taking into consideration factors that would affect IMA. These include several factors, the largest component of which is friction. Calculating the TMA is covered in a separate handout, but it is important to note that with pulleys and/or carabiners within the pulley system, the TMA and AMA will always be less than the IMA.
While most terms are defined at the end of this handout, there are a few special notes with respect to the use of terminology during rescues and practices. First, whenreferring to theconstruction of Pulley Systems, the IMA is most often used - "build a simple 3:1 or build a compound 6:1 with a change of direction and a self-minding ratchet". Practically, the team is aware that due to friction and other considerations, the AMA and the TMA will be less than the IMA.
In addition, better communication and understanding can be achieved by using terms and definitions that are more descriptive of the actual pulley system, such as, "simple 3:1" rather than "z-rig." The latter has led to a lack of understanding as it has been used to describe everything from a simple 2:1 with a change of direction, a simple 3:1, or even a simple 4:1. Being as descriptive as possible when describing pulley systems means that you are more likely to get the pulley system you want and not something else.
Pulley System Graphics created by Earl Fröm
Conditions can change through the course of raising a load; steepness of terrain may change, more haulers may become available, more rope becomes available as the load is raised, or a knot may have to be passed. Efficiency is achieved by having the knowledge and skill to be flexible enough to recognize what practical and simple changes can be made during an operation when conditions change. Knowledge provides understanding, skill provides ability, practice provides proficiency, and from these, together with experience, comes judgment.
These notes provide an overview of three categories of pulley systems: simple, compound and complex, as well as four different categories of MA: IMA, PMA, TMA and AMA. A separate handout covers the application of the T-method for calculating the IMA of any pulley system. Neither of these summaries covers more advanced types of pulley systems such as pulley systems in series or parallel or the use of equalizing or differential pulleys. Refer to your notes from the Rigging for Rescue Seminar and additional handout materials for more information.
MECHANICAL ADVANTAGE (MA): The ratio of the load to the pull required to lift the load. For example if 1 kN of force is required to raise 2kN, the mechanical advantage said to be "2 to 1" or 2:1. Mechanical Advantage is gained at the expense of endurance. Even though less force is required, it is required over a longer distance.
IDEAL MECHANICAL ADVANTAGE (IMA): The MA of a pulley system without taking into account friction and other factors.
ACTUAL MECHANICAL ADVANTAGE (AMA OR PMA): The actual observed and/or measured MA when the pulley system is being pulled on.
THEORETICAL MECHANICAL ADVANTAGE (TMA): The estimated actual (or Practical) MA that is calculated when friction losses are taken into account.
PULLEY SYSTEM: Combination of fixed and travelling pulleys and rope used to create MA.
TRAVELING (MOVING) PULLEYS: Those pulleys in a pulley system that move when the pulley system is pulled on.
STATIONARY (FIXED) PULLEYS: Those pulleys in a pulley systems that do not move when the pulley system is pulled on (e.g. pulleys at the anchors or used as a change of direction).
EFFICIENCY: The measure of friction loss calculated as the input force over the output force, expressed as a percent. For example if 90 N is required on 1 side of a pulley to hold a 100 N load on the other side, the efficiency of the pulley is said to be 90% or 90/100. In pulley systems, a 0.9 efficiency value is typical of well constructed pulleys.
RESET: As a pulley system is pulled on, it collapses to the point where one or more of the travelling pulleys meet the stationary pulleys. At this point, the load cannot be pulled up any further. The term ‘reset’ describes the act of re-expanding the pulley system to its original dimensions so that pulling may continue.
RESET OR THROW DISTANCE: the distance that a pulley system collapses between resets.
HAUL PRUSIK: the Prusik in the pulley system that is closest to the load that attaches the pulley system to the mainline (i.e. the rope grab initially extended to maximum throw distance).
RATCHET PRUSIK: A Prusik used to hold the mainline while the haulers reset the pulley system, so that progress is not lost .
‘SELF-MINDING RATCHET’: The use of a Prusik Minding Pulley to mind the ratchet Prusik and therefore eliminate the need for a rescuer to mind it.
For self-reliance, safety and flexibility, a rescuer should always have the ability to either descend or ascend a rope. Therefore, while rappelling, being lowered or raised, or working an edge, rescuers should always have their ascending system with them and know how to use it competently. A rescuer should have a separate, untensioned belay rope as a back-up in case something happens to him/her, or if the main rope, anchors, or ascending system fails. This handout does not cover single rope technique considerations.
There are many types of ascending systems. Some have been highly refined for special applications, such as long free-hanging ascents in caves. In rope rescue, however, there are compelling arguments for equipment that has multi-purpose capabilities to increase efficiency, minimize equipment requirements and reduce cost.
The Purcell Prusik System is an ascending system that was developed by members of the Columbia Mountain Rescue Group in British Columbia. It evolved from a need to combine equipment that would allow rescuers to ascend in either a free-hang or sloping environment, tie into an anchor system or edge/safety line, or have an adjustable tie-in link for litter work. Several other uses have come about since it’s introduction in the early nineteen eighties.
The Purcell Prusik System shown in Figure 1 incorporates the use of three Prusiks: two foot Prusiks and oneharnessPrusik.Thetwofoot Prusiks allow easier movement in non-free-hanging terrain (e.g. an icey or lichen-covered slab of rock). Also, if one foot Prusik is being used as an adjustable tie-in (e.g. attendant tie-in), then the other can be used to ascend a short distance, if required.
The three Prusiks are different lengths: short, medium and long. With the long foot Prusik tightened over a boot, the top of the figure-of-eight on a bight should reach the chest/nipple height of the rescuer, and the medium foot Prusik should reach the height of the rescuer’s iliac crest (top of the hip bone). The short harness Prusik should reachfrom the chest/nipple area to a point about one handwidth above the top of the helmet (Fig. 1). The reason for the different foot Prusik lengths is to allow enough room to comfortably advance them up the rope without having them bump into
each other. The short harness Prusik length is long enough to enable the rescuer to bypass a descent device (e.g. brake rack) if changing over from a rappel, but not so long that it is out of arm’s reach.
Three-wrap Prusik hitches are used to attach the foot Prusiks and short harness Prusik to the mainline as shown in Figure 2. In the past, two-wrap Prusik hitches were advocated for the foot Prusiks. They are easier to slide up the rope than three-wrap versions, but were also more susceptible to slipping. A gentle loosening of the back of the three-wrap Prusik hitch before advancing up the rope will make this operation go easier.
From top down, the general order in which the Purcell Prusiks are placed on the rope are: short, long, medium as shown in Figure 2. The acronym “SLM” or “slim” helps to remember this order. While this is the final order which the Prusiks should be on the mainline, it is recommended that they be put on from the bottom up medium first, then long, then short. This way, the placing of each Prusik hitch on the rope is not being hampered by any Prusiks dangling from above. While the medium and long Prusiks are used as foot Prusiks, the short Prusik is clipped to a connector strap between the sit and chest harness of the rescuer or to the top chest D-ring of an NFPA Class III full body harness. The proper tying of the harness connector strap is not covered in this handout.
Generally, there are two types of terrain in which a rescuer may have to ascend a rope. The first is a complete free-hang wherein no contact is made with the cliff or building face by the rescuer. The second type is on terrain which is less than vertical, wherein the rescuer will have contact with the cliff or building face.
Thefree-hangtechniqueresemblesthatofaninchworm.Thelong andmediumfootPrusiksaremoved up the rope to the point where both feet are the same elevation. The short harness Prusik is then advanced as the person smoothly stands up on the foot Prusiks. This process is repeated to ascend up the rope in a free-hang.
In less than vertical terrain (“slab” in climber’s lingo), the technique used is referred to as the "toe- in technique," which more closely resembles the movements of climbing up a ladder. The body is kept vertical, and the long and medium foot Prusiks are advanced alternately between advancement of the short harness Prusik.
Competent instruction should be sought in both the free-hang and toe-in techniques, as well as in techniques to pass knots, ascend over an edge, and/or change over from rappelling to ascending, or ascending to rappelling.
Start with a 10 metre (m) length of good quality nylon kernmantle accessory cord that is 6 or 7 millimetres (mm) in diameter, having a manufacturer’s rated breaking strength of at least 7.5 kiloNewtons. This length will be sufficient to make all three Prusiks for people up to 2m tall. To minimize waste, all three Prusiks can be tied before any cut is made to the cord.
Begin with the long foot Prusik:
1. Tie a figure-of-eight on a bight at one end of the 10m length of cord as shown in Figure 3. For rescuers using 11mm host rope, a 25cm bight is recommended. For rescuers using a 12.5mm host rope, a 30cm bight is recommended.
2. While standing, position the top of the figure-of-eight on a bight at the chest/nipple height. From there, run the cord down to the ground and make one loop around your foot as shown in Figure 4. Locate the point on the cord approximately 10cm pastwheretheloop crossesitself around your boot. Pinch the cord at this point. Be careful not to lose this point on the cord as you unwrap it from your foot. This point on the cord will become the back, or bridge, of the Prusik hitch upon itself.
3. To make a Prusik hitch upon itself, follow the steps in Figure 5.
a.Lay the cord across the back of your handwith thepinched point at the center of your hand. Wrap one end around your thumb three times and the other around your pinky three times as shown.
b.Close your thumb and pinky tightly together.
c. Lift the loop of cord off the back of your hand and over the top as shown forming a Prusik around your thumb.
d.Open up the Prusik and feed the figure-of-eight on a bight and the running end of the cord back through the Prusik.
e. Dress the Prusik down neatly.
Fig. 3: Figure-of-Eight on a Bight
Fig. 4: Locating Back of Prusik
4. Re-trace the remaining length of cord through the figure-of-eight, as shown in Figure 6. After completely tracing this length of cord through the figure-of-eight knot and tightening it, trim back any excess to leave an appropriate tail, which should match the length of the other tail exiting the knot (~5+cm).
5. If you did these steps correctly, then when you slip your foot into the foot loop and cinch it down, the distal end of the figure-of-eight on a bight should reach the level of your chest/nipple line.
Now construct the medium foot Prusik by following the same steps that you did for the long foot Prusik, only this time measure the length up to your iliac crest (top of hip bone) instead of your chest/nipple line.
Finally, after finishing the two foot Prusiks, construct the short harness Prusik as shown in Figure 2 by tying the ends of a length of accessory cord with a double fisherman bend with ~5+cm tails. The finished loop should be approximately the distance from your chest/nipple line to a hand-width above your helmet (Fig.1).
Once you have ascended a rope with your Purcell Prusik System using both the free-hang and toe-in techniques, you can make any minor adjustments in length or fine tuning you deem necessary.
A thorough examination of any technique involves a rigorous Systems Analysis. A Systems Analysis used by Rigging for Rescue includes the following:
1. Whiteboard Analysis
a.Drawing the system out in great detail and critically analyzing key criteria:
i. Static System Safety Factor (e.g. 10:1)
ii.Critical Point Test (i.e. is there one failure point that would cause the live load to go to the ground? Note: easily avoided in any 2-rope system)
iii. Whistle Test (i.e. does the system include a self-actuating quality?)
iv. Other important factors (e.g. commonality of equipment)
2. Comparative Analysis
i. Developing a pro vs. con list
i. Comparing the pros/cons of the new technique/device to your current practices
ii. Assessing training requirements for adoption
iii. A complete cost/benefit analysis
ii.Field Trials
i. Looking for step-change improvements vs. your current practices
ii.Be wary of adopting new equipment or techniques just for the sake of change (i.e. what difference does a difference make if all it makes is a difference?)
iii. Ensuring that there is no inherent safety issue with the new technique (Note: this is very difficult to assess with minimal field trials and is better suited to a welldesigned Failure Analysis)
3. Failure Analysis
a.Destructive Testing
i. Do the backups work as intended?
ii.Does the test methodology replicate a credible event? (i.e. can it be reproduced in the field of use?)
iii.Ideally, including human factor tests as well. Rope rescue systems require human beings to operate. Therefore, testing/data that is absent human trials is limited in scope
A convincing argument for a given technique can often be made based upon a well- delivered (albeit partial) Comparative Analysis. A long list of pros vs. cons can sway our decision making towards a given approach. However, does the Comparative Analysis include a robust number of field trials? Or is it limited in scope to merely an abstract approach? Additionally, a proposed system change needs to also defend itself in a Failure Analysis – ideally, including trials with human operators.
A pulleysystem's Ideal Mechanical Advantage1 (IMA) is expressed as aratioof theamount of output force to the amount of input force (e.g. 6:1 or "6 to 1"). The input force is the tension you apply to the system, and it is always expressed as one. One method of calculating the IMA of any pulley system in the world is often referred to as the Tension Method, or T-Method.
Some basic physics principles need to be understood and applied to knowing how tension is distributed through a pulley system. Mechanical advantage in pulley systems is gained by increasing the number of times your initial one unit of tension is applied to the load. Recognize that there are many ways that this can be accomplished, or rigged, using simple, compound or complex pulley systems.
By assigning one unit of tension (called "T" in subsequent diagrams) to where you pull on the pulley system, then following the path of the rope through the pulley system to the load itself, the IMA can be determined by keeping track of howthat initial unit of tension is distributed throughout the system. Simply compare the amount of tension the load receives to the initial input unit of tension.
The key to understanding the T-method is in recognizing what happens to the tension in the rope as it flows through the pulley system. Whenever there is a 'junction' in the ropes of the pulley system where either more than one rope acts on another rope, or one rope acts on more than one rope, then the tension on one side of the junction must be equal to the tension on the other side of the junction, and for each side of the junction, the tension must be distributed appropriately (not always equally) to each rope. For example, if a rope having one unit of tension makes a 180º change of direction through a pulley (a junction), then whatever that pulley is connected to receives two units of tension (Fig 1). In essence, two ropes each having a tension of one (for a total of two units of tension) are acting on (and being opposed by) what the pulley is connected to. Below are some illustrations of tension distribution in ropes at junctions:
1Ideal Mechanical Advantage assumes that there are no losses in pulley system mechanical advantage due to factors such as pulley friction, or ropes rubbing, bending or unbending. Pulley System Graphics created by Earl Fröm
Summary of how to apply and use the T-Method to Calculate the IMA of any Pulley System:
1. Assign one unit of tension to where you pull on the pulley system.
2. Follow the rope through the pulley system and when you encounter a junction, apply the principles of tension distribution. Keep track of all units of tension through to the load.
3. Total all units of tension that reach the load; the Ideal Mechanical Advantage is the ratio between this total and the initial one unit of tension.
Examples of using the T-Method to Calculate the IMA of pulley systems:
Simple Pulley System:
The TMA is the estimated Actual Mechanical Advantage (AMA) calculated after taking into account factors that affect IMA; the largest component of which is friction. The greatest friction losses occur as the rope comes into contact with the pulleys. Sometimes carabiners are used in place of pulleys which results in an even greater friction loss.
To calculate the losses due to friction, one must first know the efficiency of the pulleys and/or carabiners being used. Efficiency is the measure of friction loss calculated as the input force over the output force, expressed as a percent. For example if 90 N is required on 1 side of a pulley to hold a 100 N load on the other side, the efficiency of the pulley is said to be 90% or 90/100.
With efficiency information, the friction loss through the system can be calculated. Figure 7 shows the calculations for a pulley system with pulleys that have an efficiency of 0.90.
Assuming that the pullers pull at the end of the pulley system with 1 unit of Tension (1T), only 0.90 T will be transferred past the first pulley. When that 0.9 T reaches the 2nd pulley, only 0.81T will be transferred on (0.9 * 0.9 = 0.81) as the friction loss is now compounded over two pulleys. Follow this process all the way through the pulley system. When you are finished, use the T-method to determine the final TMA, which in this example is 2.71:1.
If higher efficiencies pulleys are used (i.e. 0.95 efficiency), the TMA is increased to 2.85:1, which is closer to the IMA of 3:1. Also important to note, is that if you are using pulleys of different efficiencies, less losses occur if the most efficient pulley is placed closest to the pullers. This is because the loss at the 1st pulley is compounded throughout the system.