CONVEYING Guide 2018/19 Also publishers of “Bulk Handling Today”
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Contents Front Cover Moret Mining Conveyor Equipment Tel: (011) 474-1260 www.moretmining.co.za
Introduction to the Conveyor
How to Buy a Conveyor
Designing a New Conveyor
Safety Around Belt Conveyors
(CMA MS01 Rev04/2016)
Proprietor and Publisher: PROMECH PUBLISHING Tel: (011) 781-1401 Fax: (011) 781-1403 E-mail: firstname.lastname@example.org www.promech.co.za
Types of Conveyors
Managing Editor: Susan Custers
Driving the Conveyor
DTP: Anne Rotteglia
Mass Measurement on Conveyor Belt Scales
Types of Gearboxes
Chutes and Scrapers
Dealing with Dust
Discussion on Idler Standard
Keeping it Clean
Avoiding Pitfalls When Buying a Used Conveyor
Contributor: Simon Curry Disclaimer Neither PROMECH Publishing nor its endorsing bodies will be held responsible for any errors or omissions in this publication and no responsibility will be borne by the publisher for the consequences of any actions based on information so published by Promech Publishing cc. Printed by: Typo Colour Printing Tel: (011) 402-3468 FSC (Forestry Stewardship Accreditation)
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HANDLING Endorsed by: CMA l LEEASA l SAIMechE l SAIMH
T O D A Y
All material published in this guide is copyrighted to Promech Publishing cc. No part of the material may be quoted, photocopied, reproduced or stored electronically without prior written permission.
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Introduction to the Conveyor
Simply put, a conveyor can be described as a moving endless belt on which material is placed and is so transferred from one place to another.
How does a conveyor work?
A typical conveyor consists of two or more pulleys, with a continuous loop of belt â€“ known as the conveyor belt that rotates about them. One of the pulleys is powered, moving the belt and the material on the belt forward. The powered pulley is called the drive pulley while the unpowered pulley at the dead end is called the tail. Todayâ€™s conveyors consist of dozens of other components, each especially designed to cater for different materials being moved, and these will be covered in this guide.
product to point of use in a process plant or loading on to a ship for sale to an overseas user. It covers a wide range of material, sand, stone, coal, iron ore, etc and the disposal of the waste products arising. The amounts which are transported range from a low of 10 TPH to 12 000 TPH and even higher in some cases. The distances that the material is transported could be from a few metres to many kilometres.
A typical conveyor consists of two or more pulleys, fitted with an endless belt
The size and power required to cover this range is very wide and could be from less than 10kW to 10 000kW. Conveyor design is complex and should only be undertaken by persons trained and skilled in this field.
Bulk handling covers the movement of loose material which has been mined or recovered from a mine through to its transportation to a recovery plant, stock piling and reclaiming of concentrated product, disposal of waste, transport of
Every major country has many requirements and specifications which cover the parameters which the design engineer needs to adhere to before permission can be given for a conveyorâ€™s use.
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The South African Institute of Materials Handling
The South African Institute of Materials Handling (SAIMH) is a Non-Proﬁt organisation set up with an objective to encourage the transfer of knowledge associated within the ﬁelds of Bulk Materials Handling (conveyors, stacker, reclaimers, tipplers, train loading systems, etc.) through a combination of seminars and networking sessions. The SAIMH is made up of professional engineers, each practising their trade in a variety of organisations who volunteer their time to execute the SAIMH’s objective. The main aim is thus to promote the advancement of the art, science, theory and practice of the Bulk Materials Handling principles and techniques.
Company Affliates as at May 2017 Afripp Projects cc Brelko Conveyor Products (Pty) Ltd Bulkcon CPM Engineering CT Systems cc Conveyor Watch (Pty) Ltd David Brown Gear lndustries (Pty) Ltd ELB Engineering Services (Pty) Ltd Engicon Systems (Pty) Ltd Facet Engineering cc Flexco (SA) (Pty) Ltd
Hagglunds Drives SA (Pty) Ltd Hansen Transmissions SA (Pty) Ltd Hatch Goba (Pty) Ltd lllustech J & A Engineering Services Kimrae Engineering Prolects Loadtech Load Cells (Pty) Ltd MacsteelVRN Martin Engineering Melco Conveyor Equipment Morris Material Handling SA (Pty) Ltd
Osborn Engineered Products (Pty)Ltd PD Engineering Services cc PH Projects Holdings (Pty) Ltd Rio Carb (Pty) Ltd Sandvik Materials Handling Africa Screw Conveyors & Material Handling Senet SEW Eurodrive Spar Western Cape SSAB South Africa (Pty) Ltd Tenova Zest Electric Motors (Pty) Ltd
6 (011) Conveying Guide 2018/19 Tel: 867-0902 Fax: (011) 867-0036 Email: firstname.lastname@example.org www.saimh.co.za
How to Buy a Conveyor Purchasing a conveyor system can be a daunting task for an inexperienced buyer. You need to provide some basic, yet critical information to get a quote.
he principle that applies to this process is “The questions you ask determines the answers you get.” The more accurate the information, the more accurate the quote will be. Correct information from the outset will simplify the process and lessen the chance for misinformation down the line.
may be required to handle spurts of production, which make the instantaneous or peak rate higher than the average rate. The conveyor has to perform at the maximum rate. The rate of handling should include anticipated increase in production to meet future demands.
Time and again a customer will take the rate of the slowest piece of equipment on a production Be careful not to line and use that as the rate of handling. The The first question is, what is the product to overstate the rate overall rate will never exceed the rate of the be handled? Surprisingly, this simple question dramatically slowest piece of equipment in a production line. is sometimes difficult to answer. In general, Be careful not to overstate the rate dramatically the conveyor system company needs to know because this will raise the cost of the equipment the maximum and minimum dimensions of without giving a return on investment. the products handled. Usually, this information is relayed in terms of composite dimensions, which means providing Environmental considerations largest and smallest dimensions in terms of length, width, What you are conveying and how fast you are conveying it will height, and weight which often do not actually exist in one answer a lot of questions about your conveyor system. There specific product. If possible, it’s best to provide the actual are a few more items to consider. Are there environmental dimensions of all the products conveyed and let the conveyor considerations to be taken into account? In this case, we are supplier discern the important data. looking at the location of the equipment. Temperature and Pay particular attention to the bottom surface of the product moisture will have an effect on the conveyor and should be because that is the actual conveying surface. If there are taken into account during the specification phase. protrusions from the bottom, this information will change the As a customer, the last piece of information required will type of conveyor quoted. Most conveyor suppliers will quote be a result of self-analysis. While the conveyor supplier can with a disclaimer that states that all products conveyed must tell you what type of equipment you will need, he cannot have smooth, flat, conveyable bottoms. It is important to be tell you what your commitment to the equipment will be. aware that even a small protrusion can affect conveyability. Conveyor systems can range from being relatively simple to extremely complicated. Rate of handling Rate of handling is the next piece of the conveyor specificaA conveyor system has to be maintained and requires an tion puzzle. This information can be more difficult to obtain investment of time by the personnel who will operate the than the product data and is often misunderstood. The equipment. This investment of time is proportionate to the rate of handling is the number of units conveyed in a given complexity of the system. The more functions, routing opperiod of time. For the most part, the important statistic for tions, and sophisticated the control sequencing, the more the planning a conveyor system is the “peak” rate of handling. manpower investment will be for the system. Don’t buy the More often than not, this will be defined in “X” number of complex system if you are not going to develop the expertise units per minute. to understand and maintain it. In short, you can make the
What is the product to be handled?
Often customers will state the handling rate based on an average rate over a shift or a day of production. The system
system too complicated for your workforce to operate. Gauge what your commitment will be and purchase accordingly. Conveying Guide
Designing a New Conveyor If you have decided to go the route of buying a new conveyor, make sure you are prepared. Besides knowing how much you want to spend, you will also have to give the designer the following measurements and specifications. The following list is just a guide, highlighting the basics. A reputiable conveyor manufacturer will require far more information from you before he can get started.
nce these figures have been received or estimated, a preliminary design can be done to see what results have been obtained and how they fit into the overall design. They can be adjusted and refined to suit the overall design with the object of trying to standardise as much as possible. At this stage the designer will have to choose which international or national standard to work to. In South Africa, suitable standards are produced by SABS and issued as SANS standards. Other countries even on the African continent require that different standards be worked to, such as ISO or FEM in Europe, BSS in England and CEMA in North America. Actually, most of them are very similar but have to be confirmed and agreed so that at hand-over time, when the QA/ AC documentation is presented, there will be no questions. Different companies will also have their specifications that have been produced in-house and these have to be respected and if necessary, issued with the enquiry documentation. Nowadays most conveyor design is done on standard computer programs which makes the job much simpler but these have to be agreed on before finally receiving approval from the client. Legally, all designs have to be approved and signed off by a Professional Engineer experienced in this particular field of engineering.
Once the preliminary design has been established, the belt can be selected as you will know how long the conveyor is, how much material it has to carry and the power required to do the job. The next decision to be taken is whether to use a fabric belt or a steel cord belt. Generally this will be quite easy to do because each type has certain advantages and disadvantages. Also a decision is needed about the top and bottom rubber covers of the belt, the top cover is important because it protects the carcass against damage. At this stage it is very important to involve belting suppliers as there can be many alternatives. Splicing on a steel cord is more difficult, so if down-time is a critical factor, it may be safer to go with a fabric belt. On very long belts, steel cords have an advantage they do not stretch as much as overfabric belts therefore belt storage and take up can be more easily designed. The factor of safety on load carrying capacity for a fabric belt is 10 whereas for
Angle of repose degrees
Belt top cover
Surcharge angle degrees
Belt bottom cover
Carry idler spacing
Return idler spacing
Wing idler angle
a steel cord belt it is 6.7 therefore the working strength of a steel cord is higher than that for a fabric one. Also, the pulley diameters for steel cord belts are less than for fabric belts. Because of their greater strength, in some cases the steel cord is the only available choice, in other cases it may have to be a financial choice.
All conveyors in a new plant have to be commissioned. This has to be done to a strictly laid out procedure for all aspects of their construction:- civil, structural, electrical, control and instrumental, and mechanical. Records have to be generated to ensure that all aspects have been considered and have been recorded. These will be done under “no-load” conditions initially and when proven to be acceptable they must be performed again under “loaded” conditions to the satisfaction of the contractor and company management.
The conveyors are now formally handed over to the plant. They will be run for an agreed period of time under the control of project and plant personnel to ensure that they meet the required performance. All spares and documents also have to be handed over so that plant personnel can operate them correctly.
CMA MS01 Rev04/2016
Belt conveyors are probably the most efficient means of transporting bulk materials. However, they are considered dangerous due to the sheer size of the installation which prevents clear and unimpeded visibility down the length of the system.
4. SAFETY AROUND BELT CONVEYORS 4.1 Safety Requirements for Maintenance
On a moving conveyor belt, the belt, pulleys and idlers are all in motion, and each idler, chute skirt, belt cleaner or pulley has a potential nip point, depending on its accessibility. The prohibition of work on moving machinery relates to tasks such as belt cleaning, house-keeping and the removal of spillage at localised points. Where build-up of carry-back material occurs on the face of pulleys and idler shells, the removal of this build-up is only permitted when the conveyor system has stopped and been safely locked out.
Belt conveyors are probably the most efficient means of transporting bulk materials. However, they are considered dangerous due to the sheer size of the installation which prevents clear and unimpeded visibility down the length of the system. Conveyors can be one of the most hazardous mine or plant equipment installations if safety regulations are not strictly followed or if the conveyors are not properly maintained. The Mines Health and Safety Act (Act 4 of 1996. Section 21) states that the onus is on the supplier to provide the correct conveyor design taking into consideration the risk to the health and safety of operating personnel. These same conditions are further extended to cover the installation of the conveying system as a whole. The Mines Health and Safety Act (Ibid. Regulation 20.5) stipulate that all exposed machinery, that when in motion, may be dangerous to any person, must be securely fenced off or adequately guarded. Guards must be provided to such part of any machinery that may be a source of danger to any person. Furthermore, the regulations (Chapter 8, Section 98 (1)) as revised in February 2008 need to be considered. See Appendix B.
The purpose of this document is to serve as a minimum specification for the design of safe operating conditions and fulfilment of safety requirements for belt conveyors in accordance with the statutory regulations and Acts pertaining to machinery, particularly those sections applicable to conveyors.
3. ABBREVIATIONS USED IN THIS DOCUMENT MHSA
Mine Health and Safety Act (Act No. 29 of 1996) PPE Personal Protection Equipment MCC Motor Control Centre CMEE Chief Mechanical and Electrical Engineer SANS South African National Standard
In instances where work needs to be carried out on the conveyor while the belt is running, such as belt training or the adjustment of material stream deflectors, it is important that this be performed by competent teams, in accordance with approved risk assessments and safe working procedures pertaining to the task being performed. While undertaking the necessary task, it is important for operators to be on the alert and to stop the conveyor by activating a pull key or an emergency stop button which must be readily accessible. In all cases, except for those mentioned in the previous paragraph, pull keys and 3-phase isolation must be locked out and tagged prior to the commencement of any maintenance, construction or repairs. 4.2 Stored Energy When maintenance is required on a conveyor system, it is important to remember the danger presented by residual energy stored within the system and to address this adequately. Thus it is necessary to isolate the stored energy from the work area or to completely release all stored energy from the system, so that work can be performed in a safe environment. This can be done by applying clamps to isolate this energy from the work area or releasing the energy applied by the take-up system. The system tensions may also be relieved by the controlled lifting of the counterweight, or the controlled pay-out of the take-up winch system. Where belt clamps are utilised, these must be securely anchored to the structure. This applies to both permanent clamps and temporary belt pulling clamps. Belt clamps must be inspected and tested before attachment to ensure that they are able to withstand the belt tensions in the localised area. It is vital that a competent engineer designs the belt clamps, followed by verification by a Professional Engineer. The clamps can be designed in accordance with the CMA Specification Number MC01 (2005) and other relevant, safe engineering standards. 4.3 Lock out Systems When any work is carried out on the conveyor, whether to the belting, components, or to the structure, the responsible person must ensure that the system is properly locked out, following the prescribed lock out procedures, safety regulations, Code of Practices and risk assessments. Where more than one team is required to work on the conveyor, the conveyor is to Conveying Guide
CONVEYOR MANUFACTURERS ASSOCIATION
Safety Around Belt Conveyors
CONVEYOR MANUFACTURERS ASSOCIATION
isolate the conveyor and request that it be repaired. 4.6 Basic Check List Prior to Re-starting a Conveyor Ensure that: • nobody is working on the belt; • g uards have been re-fitted and that all the safety interlocks are operational; • the area is clean and clear of equipment and/or debris or spillages; •a ll the fire fighting and fire suppression devices and equipment are in place and operational; • all clamps are removed or released; • all other spragging devices have been removed; • the take-up system is operational.
5. CONVEYOR SYSTEM PROTECTION DEVICES
be locked out for one task only at any given point in time. On completion of the task, the conveyor is unlocked. Only then can the conveyor be locked out by another team to conduct an additional task. 4.4 Personnel Training in Safe Working and Operating Procedures It is mandatory that all maintenance operations have prescribed safe working procedures and policies which must be adhered to. It is important that operating staff be regularly reminded of the necessity to adhere to these safe working procedures. All new staff, whether temporary or permanent, must be formally instructed in the safe work procedures for a particular task, and records of training must be maintained. Regular training of the work force is a priority. 4.5 Safe Operating Procedures •E nsure that all personnel are equipped with the correct Personal Protection Equipment (PPE) relevant to the task and work area. Using PPE shall be strictly monitored by the appropriate safety officer. •E nsure that all STOP/START and emergency controls are clearly marked and that maintenance staff are familiar with the location of these safety systems. •K eep the area around the belt clean and tidy and apply good housekeeping practices to minimise potential hazards. •L ock out, isolate and tag all areas before working on any part of the conveyor. • Do not climb on, over or crawl under any conveyor. •D o not ride on any conveyor unless the conveyor is approved and licensed for man-riding purposes. •T he only actions that can be undertaken with the belt in motion are tracking of the belt and cleaning by means of high water pressure in accordance with the necessary Risk Assessment. There may be no mechanical or manual intervention. • Ensure that pre-start alarm is working correctly and if not,
The belt conveyor shall be provided with various devices and systems for protecting the system. These devices are used as run-permissive input commands to the general belt control system. The devices must be seen as safety-critical items and for that reason, deserve a high degree of attention and maintenance. The safety of personnel and the integrity of the conveyor system are largely dependent on the correct specification, installation and operation of these devices. 5.1 Belt Control Belt control normally consists of the net sum of the belt permissive, the operator start/stop stations, the start warning system, interlock sequencing of individual conveyors and other process controls. Belt control initiates a run command to the drive controller. Sometimes the belt control issues a running reference speed to the drive controller. For stopping, the belt control simply removes the positive run signal to the drive controller or initiates a ramped stop command. Stop/Start A belt conveyor system is usually provided with one or more control stations for operators. Start stations normally require a momentary operator input to initiate a start sequence. Stop stations monitor a maintained input for a run permissive. Many conveyors are started and stopped from a central control room. Complex belts have many operator stations distributed at various physical locations. A stop/start station is a control device and should not be considered a lock out of the conveyor power source. Pre-Start Warning Where the unexpected moving of machinery or any part of machinery could pose a significant risk to any person, appropriate pre-start warning devices, such as audible warning devices, the delay time must be determined by risk assessment with a minimum of a ten second delay, are fitted to such machinery and used to warn persons that such machinery is about to be set in motion. It is further recommended that this audible warning remains in operation until the entire conveyor is in motion. Interlock Classically, interlock is the run permissive for the conveyor to any other unit’s run status. It is the control relationship between adjacent material transferring and interdependent machines. Interlock normally proceeds through a system in the reverse order of material flow. For example, a belt conveyor numbered 01 transfers material to another belt conveyor numbered 02.
for large steel cord belts is the continuous measurement of edge displacement, termed â€˜edge trackingâ€™.
Running a belt out of interlock or in bypass are common terms for operation of a conveyor with the interlock system disabled or defeated.
Edge tracking in steel cable belts provides an indication of tension distribution within the carcass among the support cables. Upon installation, each steel cable belt exhibits an edge-tracking signature for a belt revolution.
Interlock can be performed by physically wiring the conveyor control systems together, by computer coding interlock, by providing a motion sensing switch on the tail of the receiving conveyor and sensing that signal as a run permissive for the feeding conveyor. An alternative is to signal by telemetry from one conveyor to the next over a distance. A conveyor may be interlocked to other machinery and devices such as screens, breakers, crushers, magnets or as the process requires. Telemetry Telemetry is the distribution of belt control and informational signals over significant distances. Since conveyors transport material over wide areas, some belts require signal telemetry. Signal telemetry can be simply multi-conductor cables with DC digital on/off controls or can involve a multiplex of more than one signal over a single wire path. Today, telemetry may involve the conversion of electrical signals to computer-based serial transmission of data, to light signals run over fibre optics, or to wireless radio transmission. Control, remote operator interface and conveyor monitoring can be geographically located a distance away from the physical conveyor location and controlled using commercial telephone networks and modern technology. Lock out Lock out of a belt conveyor is the physical lock out of all motive power sources to the conveyor so that people may access the conveyor equipment for service, inspection, clean up or maintenance. Lock out implies security supervision of the lock out elements and involves all sources of power including electrical, hydraulic and pneumatic. Each drive and conveyor system requires an assessment of lock out requirements which includes any equipment or apparatus that is compliant with owner practices and policies, manufacturerâ€™s recommendations, and regulatory requirements. The lock out system must be interfaced with the belt control system. A permit system is necessary to monitor the maintenance crew and record exactly what work is carried out. After the system/conveyor is locked out, the system must be tested by attempting to start it to confirm the lock out. SPECIAL ATTENTION MUST BE PAID TO THE LOCK OUT PROCEDURE OF RING FEED SYSTEMS ON CONVEYOR DRIVES. 5.2 Belt Alignment It is important that the belt stays aligned with the drive pulleys and the carrying and return idlers. Belt alignment sensors are typically positioned along the edges of the conveyor fabric. They are usually located at the discharge and at the loading areas of the conveyor, but can be distributed along the conveyor at intervals, depending on the conveyor route and the requirement. Belt alignment switches are often located on the unsupported section of belting in a horizontal take-up system in order to minimise the damage that misalignment can do in this area. Switches consist of roller switches, limit switches, whisker switches, proximity switches or photoelectric switches. When the edge of the belt trips the alignment switch for a timed period, power to the conveyor is interrupted and the system halts immediately. An adaptation of alignment sensors
A deviation in the edge tracking displacement at a later time would suggest a problem in the belt cable tension distribution. However, these systems are relatively sophisticated and are usually installed only on extremely strategically sensitive conveyor systems. 5.3 Belt Overload The belt conveyor system is protected from overload via the overload of the electric drive motors. The motor overload indicator can be a simple bi-metallic or melting eutectic alloy or a complex computer-based motor thermal model. Alternatively, the motor current can be monitored and any significant deviation from the standard operating signature for a pre-determined time will cause a power interruption. A belt loading sail or paddle switch senses a belt overload at a specific point. However, such units must be designed to cater for the largest lump likely to be encountered in order to minimise spurious stops. On the other hand, if a lump is large enough to activate the paddle switch, it makes operating sense to investigate the lump before it causes consequential damage downstream. Complex belts are sometimes protected from overload by belt weigh scales that measure the belt loading at a given point. Alternatively, a non-contact belt profile sensor, such as an ultrasonic, radar, laser or video device is used to measure the belt loading depth. Based on an assumed material density, the loading tonnes per hour can be projected. The actions regarding a single large lump apply in these cases as well. Weigh meter controls are usually coupled to the belt-feeding device, such as a belt, apron or vibrating feeder. The overload sensing signal is then relayed to the feeder controller and the feeder rate is reduced to comply with the requirements of the system. Of course, unscrupulous operators can bridge, for example, any control and continuous spillage occurrences, despite any other protective measures that are in place. There is often evidence of such bridging or over-riding control of controls found during routine inspections. Other methods of overload control are fusible plugs on fluid couplings and shear pins on flexible couplings. Electronic sensing has largely overtaken the use of mechanical devices and is less easily tampered with. 5.4 Belt Slip Protection Belt slip is the loss in transmission of tension from the drive pulley(s) to the belt cover and can destroy a belt or drive pulley, causing a fire hazard. With the modern high-friction ceramic lagging of drive pulleys, the lagging itself may be destroyed depending on its type, or the belt cover completely stripped in localised areas. Belt slip protection includes a belt drive speed sensor that compares the measured belt speed with the belt signature or specified design speed. Large conveyors with long ramp times require comparative slip detection during ramping similar to the slip protection applied to variable speed conveyors. For constant speed belts this normally consists of a belt speed Conveying Guide
CONVEYOR MANUFACTURERS ASSOCIATION
Conveyor 01 is interlocked to 02. If conveyor 02 shuts down, 01 must shut down. Interlock then flows from 02 to 01.
CONVEYOR MANUFACTURERS ASSOCIATION
switch with a set point that trips the conveyor drive when the belt speed is below 80 percent of full speed. In order to prevent controller confusion, the belt slip switch is bypassed during starting and stopping and this is usually incorporated in the MCC.
A popular system is to use a mercury switch unit that interrupts the power in the event of a tilt beyond 15Â° to the vertical.
Belt slip in variable speed conveyors consists of a speed sensor that measures belt speed and compares it with the speed reference sent to the drive system. When the belt speed drops below 80 percent of the set speed, the drive is tripped. This type of belt slip is active during starting, running, and stopping.
5.7 Bin Level
In multiple pulley adjacent drives, tachometers are provided for each drive motor. The tachometer signals are compared to the normalised belt speed and sense slippage on any one of the multiple drive pulleys.
These can consist of simple hanging tilt switches or analogue measurement devices such as ultrasonic, radar or laser.
A method to adjust and test belt slip is normally an integral part of the belt control system. Slip detectors are often installed at other locations along the line of the belt, particularly at the tail pulley. In the event of the belt breaking for any reason, the tail pulley is usually the first to stop rotating. 5.5 Take-up Over-travel Over-travel limit switches can be placed at the far extremes of the counterweight or take-up device travel. In a gravity counterweight take-up, the top-over travel switch trip may suggest a jammed conveyor fabric condition. A bottom over-travel switch may indicate belt stretch, or a broken belt fabric flight. Excessive take-up motion during starting and stopping indicates an inadequate or malfunctioning drive control. Alternatively, excessive travel could indicate that one or more splices are failing or have failed. 5.6 Transfer Chute Plug or Blocked Chute A plugged chute or blocked chute device provides belt protection at the discharge end of the conveyor into a transfer chute. Blocked flow can result in damage to the moving conveyor. A blocked chute can also cause severe damage to the belt being fed, particularly in the case of a single large lump stuck in the feeding boot and slitting the belt. Plugged chute switches are used in many configurations depending on the application. Actuation of the plugged chute switch with time delay normally results in the tripping of the conveyor drive. Typical devices used are laser, ultrasonic, pressure diaphragm or simple overflow detection.
Blocked chute sensors require careful maintenance because they are required to operate in extremely harsh conditions, often in the flow of material and in relatively inaccessible locations. When conveyors discharge into bins or hoppers, bin level sensors provide protection to the belt in that they shut down the conveyor if the predetermined level is exceeded.
5.8 Pull-cord Stations Pull-cord stations are distributed stop switches with latching attachments. Pull-cord or pull-wire switches are required on all conveyors. Where conveyors are able to be accessed from both sides, the pull-switches must be located on both sides of the conveyor. Ingenious crossover systems have been developed to allow the use of pull-cord switches on both sides of the conveyor while utilising only one control system. Pull-switches are located along the conveyor at intervals not exceeding the dimensions as specified by the pull key manufacturer in conjunction with the risk assessments findings as legislated. The units are interconnected with a pull-wire and pull wires shall terminate at a live end. An operator activates the switch by pulling the pull-cord until the switch trips, interrupting power to the conveyor and usually raising a visual indicator flag. The switch remains tripped until reset manually at the switch location. The belt does not restart on reset of the pull-cord for safety reasons. Tripping of the pull-cord is a controlled stop, and shall not be considered a lock out of the conveyor power source, unless the units are specifically Safety Around Belt Conveyors CMA MS01 Rev04/2016 11 so designed. It is important to note that pull-wires are not substitutes for guards. Pull-wires must be installed in such a way that they are clearly visible and readily accessible from all areas that provide access to the conveyor. 5.9 Rip Detectors Rip detectors indicate rips or tears in the belt fabric, allowing quick action to be taken to protect the belt from further damage. Simple rip detectors are usually spill switches located below the centre of the belt near the point of belt loading. Note that particularly with steel cord conveyors, a central rip is often undetectable with the naked eye, due to the high closing forces of the troughed belt. For this reason, mechanical systems tend to be unreliable. Complex belt rip detectors on larger belts involve the embedding of antennae into the belt construction, generally in the bottom cover, about 50 metres apart. The antennae usually consist of looped copper wire, and the sensor on the opposite side of the belt detects, by induction, the transmission of a pulsed signal from the sensor on the other side of the belt. If a rip cuts an antenna, signal sources and detectors located along the edge of the belt detect a broken antenna and shut down the belt. Complex belt rip detection systems require periodic maintenance, especially to their controllers and sensors. If a rip has been repaired, the sensors are programmed to skip the broken antenna, thereby preventing erroneous trips.
Other electronic systems rely on the ultrasonic transmission of pulses transversely through the conveyor carcass. When the belt is ripped, the signal will change and trip the conveyor. The major cause of belt loss on overland systems is belt rip. In most cases, simple items such as liner plates coming loose or jumper bars or rods in the material stream passing over the feeding conveyor terminal cause the damage. In most cases, minimal maintenance carried out on a regular basis prevents the rips occurring. 5.10 Fire Detection Some belts carrying combustible materials are fitted with fire detection protection systems. The belt material of construction can, however, also burn and give off noxious gasses and is protected in the same way. These systems include point or distributed thermal trip switches located above the belt fabric, smoke sensors, carbon monoxide sensors, or fibre optic temperature sensors. The fire detection systems may be incorporated in the pull-wire switch systems, or may be installed as standalone systems. 5.11 Lightning Protection Underground conveyors are earthed and electrically supplied from cables normally installed in the shaft or through boreholes allowing an electrical lightning path to the underground conveyor. Lightning within the operational area needs to be monitored such that systems can be shut down in the event of danger levels reaching predetermined limits. 5.12 Dust Suppression Belts transporting dusty material are equipped with water or chemical based dust suppression systems. These systems spray the belt material at selected transfer and belt loading points. In some instances, dust suppression systems are coupled to ultrasonic spray nozzles. Systems spray a constant amount of dust suppression per unit of time whilst the belt is running. The dust sprays are turned off when the belt is idle or unloaded to prevent puddling, waste and slippage. The way in which dust suppression mechanisms work is to reduce the size of the water droplets, making them smaller than the dust particles. This enables the dust particles to break the water surface tension, adhering to the water droplet and forming larger drops. In the case of coal dust, wetting agents are required, since coal dust and water are immiscible under normal conditions.
6. BASICS OF CONVEYOR GUARD DESIGN 6.1 Guards and Fences
A guard or fence is only effective if it is constructed to prevent a person from reaching the danger or nip point. A person is capable of reaching upwards, over, into, around or through a guard or fence, and all these aspects must be taken into account when considering the effectiveness of a guard or fence. For belt conveyor installations the so-called ‘nip guard’, examples of which are shown in the sketch below, extend over the whole width of the pulley and are regarded as a reasonable solution to prevent access to the danger points. Installation of
this type of guard is strongly recommended but unfortunately it is impossible to install it in such a way that a person is completely prevented from reaching around it. A nip guard alone cannot therefore be regarded as sufficient protection and it is essential that pulleys are further guarded or fenced off to meet the requirements of the regulations. The following may be provisionally accepted as safe in the absence of facts to the contrary: Upwards Any pulley or idler, which is 3,5 metres or more in height and therefore beyond an upward reach, may be regarded as being positionally safe and need not be guarded. The possible reduction of this safe clearance by a build-up of spillage or discharge of material shall, however, be borne in mind. Over Head and tail pulleys must be guarded on at least the two sides and the top unless the guards or fences on the sides are extended to a height that makes it impossible to reach over and contact the nip point. If side guards only are attached with a very small clearance between the edge of the belt and the side guard, this may perhaps be regarded as adequate to prevent reach over the guard to the nip point, but will not necessarily prevent tools or clothing from being caught in the nip point. If a top guard is attached, it must be high enough above the belt to ensure that the load on the belt will not damage it. Into The distance that the guard or fence is placed from the side of the belt determines the distance that these extend away from the nip point along the length of the belt. An acceptable distance is at least 0,85 metres away from the nip point, preferably from the position of the nip guard. Around This is similar to ‘into’ so far as the conveyor pulley guard is concerned, but may also be applied to determine the length of the top section of the guard. The same minimum distance of 0,85 metres applies. When a V-belt or chain drive is associated with the conveyor installation, a common point of error is that while the V-belts or chains are perfectly guarded around the perimeter and on one side, the guard is installed in such a way that the nip points can easily be touched by reaching around the section forming the perimeter guard. Through The protection afforded against injury by reaching through the guard is determined by the shape and size of openings in the material used for construction of the guard or fence. Square Openings: It may be assumed that there is no reach through an opening of 10 mm x 10 mm or less, as it is too small for fingers. If the opening is such that it will admit one, two or three fingers, the reach is restricted by the roots of the fingers, a distance normally not exceeding 100 mm. When the opening is sufficient to admit the whole arm and a small portion of the shoulder, the reasonable safe distance is based on the distance from the fingertips to the armpit, which is assumed to be 0,85 metres. Screening materials with openings in excess of 80 mm x 80 mm shall not be used in the construction of guards or fences. Preference shall be given to materials with openings not exceeding 25 mm x 25 mm. Elongated Openings (openings with parallel sides): Openings up to 6 mm wide are of no consequence. The guard or fence so constructed may virtually be regarded as a sheet, and a working clearance of approximately 25 mm is all that is required. Openings greater than 6 mm but less than 13 mm will admit part of a finger and require at least 50 mm clearance from danger points. Openings in excess of 13 mm but not greater than 80 mm are subject to the following formula: X = 10Y where: X = reasonable safe distance from danger point in millimetres. Conveying Guide
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However, in the event of several adjacent broken antennae, there is, in this example, a potential that 150 metres of belting is, essentially, unprotected. It is possible to have the antennae removed by local skiving of the belt covers, with the broken antennae being replaced and re-vulcanised in-situ.
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Y = width of opening in millimetres.
Note: The tail pulley guard shall be closed at the rear.
6.2 Nip Points and Nip Guards
Accessible (with regard to Nip Points) A nip point is regarded as accessible if there is a reasonable chance that a person will be able to reach the nip point when the normal peripheral guards are removed.
6.2.2 Guarding of Nip Points
Research in the industry has shown the need to assess human factors associated with the operation, use and maintenance of conveyor systems.
Nip Guard (Nip Point Guard) An effective guard or barrier installed across the width of the belt protecting any part of the body from reaching the Nip point Nip Point A nip point is the dangerous pinch zone which occurs at the line of contact between the rotating drum or roller and the moving conveyor belt on the in-running side of the drum or roller.
Some take the view that peripheral guarding alone is adequate to control the risks from nip points and that conveyors should not be allowed to operate unless these guards are fitted. Unfortunately, accident experience in the mining industry clearly shows that peripheral guard removal is often necessary to allow cleaning and maintenance. This results in the exposure of the nip point during the cleaning and maintenance operations and during normal operation of the conveyor, as guards are often left off at locations where they have to be frequently removed. Previous initiatives have involved improved peripheral guard fixing arrangements and/or emergency stop cables interlinked with the guards. However, none have proved as successful as nip guards fitted directly at the nip points. They offer the most reliable form of continuous protection for dangerous nip points. 6.2.3 Nip points and associated risks All Nip Points that are accessible are dangerous and should be guarded. Industry statistics of injuries caused at Nip Points highlight the fact that there are definitely higher and lower risk Nip Points. Each installation should be assessed on risk before the guarding principles of a specific installation is decided on. Risk Guideline – Nip points on a typical installation
Notes 1 2 2
Note 1: Statistics indicate that the vast majority of fatal accidents on conveyors happened at the tail ends. Factors that contribute to this may be the following: • Accessibility of the tail ends by operating and maintenance personnel. • The need for cleaning at the tail ends due to spillage on the return belts that is carried back to the tail pulleys. • Unauthorised cleaning operations at the tail ends (when some guards may be removed) while the conveyors are not locked out. • Any person or material that may be on the return belt will be carried to the tail end, should the belt be running. All tail pulleys that are accessible should be fitted with nip guards. As part of the risk assessment process, the following additional measures should be considered to reduce the risk of injuries at tail pulleys: • Installation of ploughs outside the guarded areas that will scrape off spillage on the return belts. • Installation of a barrier on the return belt that will prevent persons or large material (that may pass over the plough) to be carried into the danger area at the tail end. • Review loading point and transfer arrangement design to prevent spillage. • Minimising of mud rush conditions that cause spillage. Note 2: The head pulley area is normally where there is relatively more activity due to maintenance of scrapers, bearings and chutes. For this reason, there are normally access platforms at the head pulleys. If the nip point of the head pulley is accessible the danger of injury at head end is regarded to be high and nip guards should be fitted. Note 3: At pulleys where the nip points are on the underside of the belt, the risk of injury is regarded as medium, but they should still require nip guards if they are accessible.
Categorisation of danger levels of pulleys under normal operating conditions:
Note 4: At pulleys where the nip points are at the upper side of the belt, any person or material that may be on the belts will be carried to the nip points, should the belts be running. These nip points are regarded as high risk and should be fitted with nip guards if they are accessible.
PULLEY DIA 8 0 x 8 0 x 1 0 120x120x10 RSA 8 0 x 8 0 x 1 0 120x120x10 RSA (mm) RSA RSA 450
Note 6: At many conveyor installations there is no access to the nip points at the second LT bend pulleys. In such cases the risk of injury is low and the outcome of a risk assessment may indicate that nip guards are not required. If the nip points are accessible the risk will be medium and these points should have nip guards.
6.2.3 Nip Points at Inaccessible positions
6.2.4 Nip Guard Installation Nip Guard Gap • Traditionally over the past ±25 years a document has been used in South Africa which was released by the CMEE of Rand Coal. This document called for a Nip Gap of 8mm. Conveyor designers adopted this as a standard and Project Houses designed and manufactured the conveyors accordingly. This gap was however not documented or Gazetted into law until 19 December 2014 and was never strictly applied by end users. • Maintaining an 8 mm gap taking cognisance of pulley crowning, pulley lagging wear, belt tensions and spillage has proved to be problematic and extremely difficult to maintain. Review of 8 mm gap previously recommended
Nip Point at head pulleys where access to the nip point is obscured by steelwork. Refer to position A on the above picture. The installation and adjustment of nip guards, inside head chutes, can be problematic and dangerous to execute. If the nip guard is largely concealed behind the plate work and structures, it will be difficult to inspect and verify the gaps to the pulley and belt. In such cases where access to the nip point is made impossible by the chute plate work and structures after removal of normal guards (refer to Table C4), a risk assessment should be done. Depending on the outcome, it may be advisable not to fit a nip guard. Nip Points at bend pulleys where access to the nip point is highly unlikely. Refer to position D on the above picture. At certain conveyor installations access to nip point D is highly unlikely (impossible to reach from any walkway or platform). The risk personnel are exposed to during installation, inspection and adjustment of such nip guard may be higher than the advantages of having a nip guard in that position. In such cases a risk assessment should be done. Depending on the outcome, it may be advisable not to fit a nip guard.
• When an angle section is used as a nip guard, the opening through which any part of the body can reach the Nip point can be seen as an opening (slot) with parallel sides. This implies applying the requirement of X=10Y where X = reasonable safe distance from danger point in millimetres. Y = width of opening in millimetres. Relationship between nip guard gap and nip guard design Relationship between nip guard gap and nip guard design • The sketch below illustrates how the safe distance from the nip point can be maintained by different designs and positioning of nip guards. • A 15 mm gap requires a safe distance of 150 mm from the nip point (X=10Y) and this also complies with the required safety distances given in APPENDIX C – Table C4 (REACHING IN AND THROUGH REGULAR OPENINGS) Recommended maximum nip guard gap Carry Idlers Although it has been stated that it is essential that the head, tail and snub pulleys of belt conveyor installations which are ‘within reach’ shall be guarded, accidents have happened on carrying idlers. The outcome has frequently been serious particularly where the amount the belt that can lift off the idlers is restricted. The danger at idlers is more serious when fixed hoppers or skirt plates under which the hand can be trapped are fitted directly above the idlers. If this is the case, the danger points must be very carefully guarded or completely enclosed. This also applies, even more so, to belts on which hand sorting is performed. Return Idlers On belt conveyors, the return belt or idlers may also present a hazard especially if specific places exist where persons regularly pass underneath the belt. At such places, it is recommended that the underside shall be guarded and crossing at other places shall be discouraged or prevented, even if only two or three strands of eight-gauge galvanised wire is used along the outside of the supporting framework to achieve this purpose. Conveying Guide
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Note 5: At vertical take-up pulleys, where cone canopies are fitted over the pulleys, the canopies will prevent persons or material reaching the nip points. In such cases the nip points are not accessible and the risk of injury at the nip points is regarded as low. Nip guards may then not be required. If the nip points are accessible the risk will be high and these points should have nip guards.
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veyors, no adequate hand and knee rails are installed on the outer sides. This presents a danger, as there is often a large opening between the inside of the walkway and conveyor stringer section at knee height. These areas shall be guarded off with knee rails.
NIP GUARD Nip guards must be ﬁxed to the pulley supports in such a way that the distance between the guards and the pulley do not vary, even when the belt is tensioned.
Safety at belt conveyor installations may be further enhanced by creating the optimum working environment including not only adequate ventilation, illumination and absence of undue noise, but also sufficient clearance around the installation and along walkways. Walkways shall have an even, non-slip surface, be properly drained and free from obstructions.
6.4 Ergonomics (Human - Machine Interface). Drive Units Driving belts, chains and couplings between driving motors and gear boxes or drive pulleys must be effectively guarded. Experience has shown that even when transmissions are apparently inaccessible they can still be a hazard. If the driving mechanism or any other part is fenced off completely in such a way that access thereto can only be gained through a gate or door forming part of the fencing, then this gate or door shall be interlocked so that the conveyor stops when the gate or door is opened. Trip Wires When faults, accidents or blockages occur, it is necessary to bring the conveyor to an immediate halt. If pulled, a continuous ‘tripwire’ stretching the whole length of the conveyor shall be set to actuate the conveyor’s stop switch. This is an effective and essential safety device. With such a facility available, the operator will be less tempted to try to rectify faults while the conveyor is running. The tripwire must, however, extend to the full length of the conveyor, even as far as the inside of the guarded sections. If the belt conveyor is installed in such a way that people can walk along the conveyor avoiding the wire, then a tripwire must be installed on both sides of the conveyor. It is also recommended that a ‘lock out’ facility be provided on this trip wire arrangement. Interlocked Guards In some applications guards are fitted in conjunction with limit switches interlocked with the safety system such that if a guard is removed, power to the conveyor is immediately cut and the conveyor will coast to a stop. Equally so, the conveyor will not start up if the guard has not been replaced or re-fitted correctly after maintenance has been done. 6.3 Maintenance and Access Lateral movement of the belt is usually caused by a build-up of material on the head and tail pulleys, the carrying idlers or snub pulleys. The manual removal of build-up is slow and complicated, and frequently dangerous. To keep the pulleys and rollers clean, suitable mechanical devices must be installed. The manual removal of build-up shall not be permitted whilst the belt conveyor is in motion. It is often necessary for an attendant to cross a conveyor at various points. It is dangerous to climb onto the moving belt. Where it is impossible to establish safe passageways underneath the belt, crossover bridges with handrails must be provided. The position of these bridges will depend on conditions at the belt conveyor installation, but unless a sufficient number are installed, they will not always be used. The crossover bridge must be accessed via stairs equipped with handrails and a ‘toe-board’ as well as an intermediate or knee rail. Avoid vertical ladders. In many cases where walkways are fitted on elevated con-
To prevent accidents on conveyors it is vital to take engineering safety measures. It is possible to increase safety in existing installations at a very low cost. This document suggests ways of solving safety problems. Good engineering safety measures and an optimum working environment are not the only factors conducive to combating the high annual casualty rate associated with belt conveyors. One of the principal keys to success is an understanding of the human element. Even a properly guarded belt conveyor installation is not in itself inherently safe but with adequate training and proper awareness of dangers, an operator may use it with perfect confidence. Operator training is usually the personal responsibility of the staff member in charge of the correct operation and running of the machinery. Awareness of the fact that familiarity with the machine on his part and an over-estimation of the operators’ skills and knowledge does not result in an under-estimation of the amount of instruction and degree of supervision necessary for the safe execution of tasks. Comprehensive training schemes to ensure that operators have the required knowledge and skills to run the relevant equipment, including compulsory re-training opportunities are essential.
Disclaimer This CMA “Safety Guideline” was drawn up by a committee of people drawn from member companies of the Conveyor Manufacturers Association of South Africa NPC (CMA). The purpose of the guideline is to provide information that would enhance the safe operation and maintenance of belt conveyor systems. Many man hours were expended to gather information and document local and international practices that were considered by the committee to be safe and practical. Every effort has been made to ensure that the information provided is accurate. In all cases the applicable National legislation, local procedures, duly documented and approved risk assessments and safe working practices shall take precedence over anything else contained in this “Safety Guideline”. The CMA, its corporate members, directors, committee members or any individual associated with the generation of this “safety guideline”, or any individual committee member is not responsible for any consequences, legal or financial or otherwise, arising from the use of this guideline. The entire CMA “safety guideline” is applied and used solely at the discretion of the user.
It should come as no surprise that there are many types of conveyor systems available. Each one of these systems is especially suited for the job at hand. Here is a brief outline of the major conveyor systems available.
Bucket elevators are used for conveying material vertically. In principle they consist of a steel vertical enclosure that contains a head drive pulley and a tail pulley. Over these, there runs a belt or a chain to which the buckets are attached.
Fixed boom conveyors are fixed and as such cannot be moved. They are ideal for loading docks.
In the lower part of the steel casing called the boot, material is scooped up by the buckets that then convey it up to the upper pulley around which the conveyor runs and the material is discharged onto the following conveyor (often screw conveyors). Bucket conveyors are excellent for transporting material from one floor to higher floors and require a very small footprint. They are often found in food producing operations as they protect the material from contamination by means of their enclosed nature.
Horizontally curved For some time now belt conveyors which in the past could only travel in a straight line from feed point to discharge have been built which can curve horizontally, in other words go around corners. They were developed primarily to convey material over long distances navigating permanent obstacles such as cliffs and buildings
Cable Belt Cable belts are used where normal belts would be unable to accommodate the required tension imposed on them. In the edges of a rubber belt conveyor, a steel cable is bonded or introduced. This cable is capable of accepting the high tension required to operate. The cable also supports the belt between suspension supports which eliminates the need for idlers.
In the past the only solution would have been to split the conveyor into 2 or more separate lengths requiring various drive stations along the conveyor run. This is very costly and requires high maintenance. At first, the design of these curved conveyors was restricted to a few companies which had developed them, nowadays they are more commonly available and are even being used for shorter in plant-conveyors.
Because the tension is so high, there is very little sag between the supports.
TYPES OF CONVEYORS
Types of Conveyors
TYPES OF CONVEYORS
These are used for loading trucks as they can be easily maneuvered into position behind the truck. The boom allows the operator to extend the conveyor to the exact position above the truckâ€™s load deck.
Every pneumatic system makes use of pipes or ducts called transportation lines that carry a mixture of materials and a stream of air. These materials, such as dry pulverised or free flowing or light powdery materials like cement, fly ash etc, can be transported conveniently to various destinations by means of a stream of high velocity air through pipe lines.
In high capacity materials handling plants, a boom conveyor is mounted on a stacker or reclaimer and is used to deposit ore onto stockpiles or to receive reclaimed ore.
Three basic systems are used to generate high velocity air stream: 1. Suction or vacuum systems: utilising a vacuum created in the pipeline to draw the material with the surrounding air.The system operates at a low pressure, which is practically 0.4 to 0.5 atm (standard atmosphere) below atmosphere, and is utilised mainly in conveying light free flowing materials.
Pipe Pipe conveyors are more expensive types of flat belt conveyors but have a number of advantages. As their conveying element they use as special flat belt but it is only flat at the feed point and the discharge point. After the feed point it is formed into a tube by bending up the edges and holding the belt closed by a number of idlers placed all around the belt. This pipe arrangement makes it very flexible and it can be run up or down or left or right.
2. Pressure type systems: in which a positive pressure is used to push material from one point to the next. The system is ideal for conveying material from one loading point to a number of unloading points. 3. Combination systems: in which a suction system is used to convey material from a number of loading points and a pressure system is employed to deliver it to a number of unloading points.
Another advantage is that it protects its contents making sure that no unwanted contaminants can enter the material stream. A disadvantage is that it is limited in cross-sectional areas and therefore in capacity. The structure required to support it is also more complex than flat-belt conveyors making it more expensive.
Shuttle This conveyor is a flat conveyor, not usually very long that can move along a track. It is fed by means of a pulley at a fixed point, thus it can travel up and down and feed into many different locations reducing the need for the height required by transfer chutes. It can be of fairly high capacity and is reliable. The shuttle conveyor could also be reversed so that it can feed units to the left or right of it. It is powered by means of festoon cables that are generally more reliable than rails. An adaption is the shuttle head which can be moved a short distance to feed on the two following conveyors. The advantage is that it considerably reduces the height
required for a 2way splitter chute and thereby reduces power requirements and running costs.
This is a conveyor that can be operated in either direction and so from a central feed point, can discharge to two locations alternatively.
These conveyors are all metal with scraper blades attached to a chain. They are used for handling hot material such as burnt line discharged from a kiln. They can be horizontal or inclined and run fairly slowly to allow the conveyed material to cool off.
Sandwich Sandwich conveyors are used when a normal conveyor is required to rise up at its discharge end in a steep slope. Usually steeper than 20o. At the bottom of the lift, a separate upper conveyor is placed in contact with the main conveyor stand carrying the burden and encloses it like two slices of bread enclose a sandwich. This allows the material to rise at an angle that would otherwise not be possible if it was lying free on the lower main carrying belt. This solution is not often used but can be beneficial in certain cases.
Screw This is a device that uses an inclined screw to move material. The spiral of the screw conveyor is stretched out over the distance to be traversed and is either positioned in a trough or fully enclosed in a cylinder. One end of the conveyor screw is in contact with a liquid, semi-solid, or granulated substance. When the spiral is rotated, it forces the substance to move along the trough or cylinder to the discharge. Other names for the device include auger conveyor, spiral conveyor, and helix conveyor.
Scraper This is a material transportation device utilising a continuous, driven chain circuit equipped with regularly spaced cross members. The chain travels through a channel or trough which prevents the transported materials from falling off the conveyor while the cross members drag or â€œscrapeâ€? them forward to the discharge point. Scraper conveyors are typically used for coarse, heavy materials such as raw coal, iron ore, cast metal parts, and metal scrap. The design is ideal for slower transportation speeds over short distances, on moderate inclines, or even in submerged applications. Conveying Guide
TYPES OF CONVEYORS
TYPES OF CONVEYORS
Sidewall belt eliminates spillage at transfer points in a vertical plane. There can, however, be problems cleaning the belt, because of the profiles which are often fitted in inclined conveying applications.
A tripper conveyor is one which usually has a mobile structure moving along a flat conveyor. The mobile structure which carries the tripper raises the belt from the flat conveyer and discharges the product over a head pulley into one or two leg chutes that feed it into bins or onto a storage pile.
Sidewall conveyors are used for carrying material up steep slopes.
A tripper is usually a travelling unit that is powered along a track over the abovementioned bins. This can spread the ore over a number of receptacles or along a longitudinal stock pile to increase the storage capacity.
Troughed The idea behind a troughed conveyor is to transport the material on a belt and to reduce spillage as much as possible. It is for this reason that the sides of the belt are slightly raised to form a trough. They are raised with the help of wing idlers on either side of the belt. The actual belt is normally made of SBR rubber with a synthetic fabric or steel cable core to provide strength.
The wing idlers can be angled up from 20° to 45° (or more) to suit the conditions.
Safety Tips Keep clothing, fingers, hair, and other parts of the body away from conveyor, at all times. Don’t even attempt to fix a snag or something while the conveyor is operational.
Don’t climb, step, sit or ride on conveyor at any time.
Don’t load the conveyor outside of the design limits.
When you first take a look at a conveyor system, the most obvious element will be the belt, whether it be steel, rubber or a fabric belt. However, endless components keep the belt moving from A to B including idlers, motors, couplings and belt cleaners. In this section we take a look at the various components involved. A drive for a conveyor will typically include an electric motor, a high-speed coupling, a gearbox and a low-speed flexible coupling attached to the head shaft of the conveyor. The location of the drive for a conveyor is very important. Whenever possible it should be at the head end, but on more heavily loaded rising conveyors it may be more suitably located in the middle of the conveyor run at a lower level.
n a few rare cases it may be necessary to place the drive at the tail end. If the conveyor is a reversing one then the drive will be at the head in one operating direction and at the tail in the other operating direction.
The location and type of drive affects the angle of the wrap of the belt around the pulley. The greater the angle of wrap, the lower the return belt tension can be which has an effect on the total tension and could mean that a lower class of belt can be used. This is because the total tension in the belt is the sum of the belt load tension plus the sum of the return belt tension plus the sum of all friction loads due to idler, pulley, chutes, scrapers and other resistances. Access to the drive must be considered and it has to be located where it is possible to perform maintenance and servicing on it. To be able to transmit the driving force required the slack side tension has to be sufficient to prevent the belt from slipping on the pulleys so the slack belt pull has to be high enough. This is dependent on the friction between the pulley or pulleys and the angle of wrap. On a single pulley drive this is limited to about 2000 max, but on a multiple pulley drive itâ€™s the sum of both pulleys angle of wrap which could be 2000+2000, thus reducing the slack side tension. In the design of a number of conveyors for a plant, a wide variation of power requirements will be seen. In such a case it may be better on the larger drives to use 2 or more drive units with lower powered motors and speed reducers, to standardise on equipment. This will make handling, replacement, servicing and maintenance less costly. It may also provide an opportunity to install multiple drives as described above and reduce the belt strength to bring it in line with other belts.
This must be heavily built to support the components so as to ensure that movement between them cannot take place. All equipment mounting pads have to be machined so that the units can be correctly aligned. Provision for adjustment of the motor and gearbox must be provided so that alignment tolerances between the motor and gearbox can be achieved. It is necessary that this fabricated component be substantial so that after being put into use, it will still retain its alignment accuracy.
Driving the Conveyor Motor
The drive motor is usually selected to have a power rating higher than the calculated power required. It is generally selected to run at 4 and 6 pole speeds, and power rating on multiple conveyors are grouped so that standardisation of the drive units can be arranged. Thus if calculations show that 3 conveyors require 60 or 65 or 70 kW, they will all be fitted with 75 kW drive units. On heavily loaded conveyors it is necessary to limit the start up power on a variable frequency application. This can be controlled by adjusting the voltage to the motor, which allows the motor power to increase gradually ie, soft start. If this is not done the power drawn by the motor can overload all the mechanical components of the drive unit, or even trip the motor. Another method of limiting start up power is to use a hydraulic coupling on the high speed input shaft. By filling the hydraulic coupling with a measured amount of oil in accordance with the suppliersâ€™ recommendation, a gradual start can be obtained.
Variable speed drive
For lightly loaded conveyors, the start-up can be D.O.L. (Direct on Line) which applies the full torque of the electric motor through the drive gearbox to the drive pulley. This may cause overloading of some of the drive component systems and must be carefully checked to make sure that premature failure cannot occur. If there is any doubt as to the torque capacity of the drive line, depending on its specification this could be from 1.5 to 2 times the normal running torque, then some method of limiting the power must be implemented. Generally, this is a choice between fluid couplings or electrical methods. A fluid coupling comprises an impeller and a housing having vanes in it. The whole unit is enclosed and sealed so that the driving element, the oil, cannot escape. The size of the coupling and the amount of oil it contains is carefully selected to give the correct start up time by limiting the torque transmitted. Scoop couplings are also available, which introduce a moving scoop into the bath oil. This bleeds off a certain amount of oil to make the torque transmitted even more variable. Properly selected fluid couplings work well but do require attention and maintenance. If the coupling is subjected to a number of attempts at start up which do not get the conveyor going, a fusible plug set into the casing, blows and oil comes out making a mess and requiring maintenance effort.
The variable speed drive is an electrical system. In many ways it is mechanically more fool-proof. It works by applying a variable voltage to the motor which then produces a variable torque output to the drive unit and conveyor. The latest systems are easy to adjust and coupled to speed sensors on the conveyor will give a smooth safe start-up until the conveyor reaches full load. Conveying Guide
TYPES OF COUPLINGS
Couplings Fluid couplings These are used to ensure a soft start-up and shut down of the conveyor belt. This minimises belt stretch and belt lift-off. They also reduce the electrical draw on startup. Where more than one motor is used in a conveyor system, couplings can be used to start the motors in succession – ensuring a smooth belt startup. Fluid coupling operation is based on a hydrokinetic principle with the power transmitted through hydraulic fluid. Inside the coupling housing are two elements: an impeller acting as a centrifugal pump and a runner acting as a hydraulic turbine. There is no mechanical connection between driven and driving shafts. When the motor is started, the driven impeller begins to pump fluid within the coupling to the stationary runner. As the force on the runner increases, the runner starts rotating and begins to accelerate the driven load. Starting torque is gradually increased and controlled as power is transmitted smoothly to the driven load. High speed coupling
Low speed coupling
These couplings must be able to handle shaft misalignment and be able to dampen shock loading from the motor to the reducer. To do this the power transmission can be through rubber boots, rubber discs or steel “springs”. This also allows for slight misalignment between motor and reducer steel.
Generally, the drive unit is on a separate base to the pulley to which it is coupled, particularly on a high power drive.
The high-speed coupling can also be a hydraulic fluid coupling where the input has a turbine disc and drives a turbine disc mounted on the output, by means of hydraulic oil. The amount of oil can be modified to suit the start up and running torque
In order to allow for a certain misalignments between box and pulley, the low speed coupling has flexible elements. These elements could be rubber or they could be steel springs which allow for slight movement between the conveyor drive and the conveyor pulley. This movement could be due to different thermal expansions or to movement between the conveyor structure and the drive base, in either case it has to be accommodated, otherwise loading will be imposed on either of the 2 elements which will eventually destroy either or both.
Gearing Up enerally, two types of gearboxes are used, either worm or spur.
Worm boxes can be used for high reduction ratios but suffer from low efficiency, ie, a lot of power is lost in the gearbox which requires a motor larger than that needed to drive the pulley. Worm boxes are also not made which can handle the high power required by large conveyors. Generally, gearboxes used on large conveyors are of the right angled horizontal bevel shaft or the parallel shaft type. They can be provided to handle powers up to 2000kW. The right angled construction gives them the ability to be provided or converted to right or left handed drives.
Smaller gearboxes are often of the shaft mounted type where the gearbox fits directly onto the extension of the drive pulley shaft. These are often combined with the V-belt drives to their input shaft with the motor, electric, or sometimes hydraulic, mounted on the gearbox. This arrangement is very compact but is limited in the power which it can transmit. These boxes can be of the bevel helical type or they can be of the worm gear type. Worm gear units are also limited to the power they can transmit, in the order of 100kw but their main drawback is that they are inefficient, generally running in the order
of 50 to 70 %. This is far less than the bevel helical units which are capable of efficiencies in the order of 95 to 97%. In these days of attaining high efficiencies, the saving using bevel helical or straight helical boxes is evident.
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Mass Measurement on Conveyor Belt Scales
eighing of bulk materials on a moving conveyor belt is not as complex as it seems. In fact it is reasonably simple. The scale in itself only has three components, namely a weigh-frame (including loadcells) a speed sensor and an electronic integrator. Most scales will be accurate within 0,1 % when tested on the factory floor, that is of course if sound engineering practices are used in the design of the scale. The complications come in when integrating the belt scale with the conveyor itself. Any conveyor design engineer will be quick to tell you that each conveyor has its own “personality” and is uniquely different. Let us say a belt scale is a simple creature for ease of reference. Let’s call it Male and a conveyor is of the fairer sex (Female). When we put the two together the Male will have to be installed in the Female's life in such a way as to allow for the thirty three intricacies that the Female possesses not to affect the performance of the Male.
Reliable and accurate
Yes there are thirty three integral design traits within a conveyor that determine if your belt scale will function properly or indeed at all. Reputable scale companies do not have a range of Belt Scales that vary between type and configuration for the sake of it. Each application must be engineered to allow for the belt scale to function both reliably and accurately.
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Some of the issues that have to be dealt with are Belt Tension, Belt type, Conveyor structural integrity, Deflection, Tracking, Loading, Speed etc. Choosing the correct scale for your application becomes a art more than a science where experience is as important as science.
This is the reason that many scale installations do not provide the accuracy that is shown on the glossy brochure spec sheets. In conclusion, treat each application as totally unique and get the supplier to guarantee accuracy of that specific application not of the scale itself as shown on the data sheet.
All the described gearboxes are utilised in the materials handling industry. Selection is often based on commercial implications as well as personal preferences from past experiences. Various options for driving any specified equipment should be considered before final selection is made. It should be mentioned that generally all spur and helical units can be back driven provided that no holdback is fitted. With wormgear units the general rule is that units up to 30:1 can be back driven and those above 30:1 cannot.
Types of Gearboxes Shaft-Mounted Helical Units Single helical gears with single reduction (5:1) and double reduction (13:1 - 20:1 and 25:1). Generally used with a primary Vee-Belt Drive and supplied with a torque arm. Internal Holdbacks can be fitted. These gearboxes are suitable for a 0.25 to 240 kW power range.
Bevel and Helical Gear Units This is the most popular gearbox used on conveyor drives. Available as single, double, triple and quadruple reduction. Parallel helical or right angled bevel-helical designs either with solid or hollow output shafts with ratios up to 1500:1 Holdbacks can be fitted internally and the power range is very high. Normal standard ranges can go up to 5500 kW.
Wormgears Single reduction with nominal ratios from 5:1 to 70:1 and double reduction with ratios from 105:1 to 4900:1. The low ratios are reasonably efficient (95% for 5:1), but the higher the ratio the less efficient they become. At 4900:1 the efficiency can be as low as 30 %. Helical/Worm combinations can be obtained with ratios of from around 7:1 up to 310:1 These gearboxes are suitable for a 0.08 to 1200kW power range. These units can be fitted with internal holdbacks.
Helical Gear Units Available in single, double and triple reductions with either a flange mounted motor or as a complete gear unit. Relatively low power capacity - 60 kW.
These units are available with solid output shafts or hollow shafts for direct mounting.
Planetary Gearboxes Planetary gearboxes are becoming increasingly popular. They offer certain advantages over parallel shaft gearboxes, including higher reduction ratios, higher transmissible torques and higher radial loads on output shafts.
Put simply, a pulley is an iron pipe that can usually be found at either end of the conveyor belt. The one at the beginning of the conveyor is called the drive pulley and is usually larger than the belt so as to provide a bigger surface to drive the belt. It is also larger in the centre to help keep the belt in place. As the name suggests, the drive pulley is responsible for driving the belt. A tail pulley is found at the end of the conveyor belt but is usually free flowing.
Drive pulley The drive pulley is connected to a motor via a chain, and in some cases employs a speed reducer as the motor can spin at speeds of up to 1 500 rpm. Collectively, the motor, speedreducer and drive pulley are known as the conveyor drive.
For power to be transmitted to a conveyor belt, there has to be tight contact between the drive pulley and the belt. This means that there must be tension on the slack at non-driven side. To provide this, a force has to be applied to the slack side of the belt and this is done by the take up. The simplest form of take-up is the screw T.U. The tail pulley is mounted on a frame and the frame is pulled back by means of long screws which are tightened so that the drive pulley does not slip when operating. A similar take up is one where the screws are replaced by one or two hydraulic cylinders which apply a constant pull on the take-up frame. Its limitation is that its travel is restricted and can therefore only be used on conveyor with short centres. A take-up suitable for long conveyors is the counterweight type where a weight in a tower transmits its force by rope and pulleys to a trolley carrying the take-up pulley. This system is very adaptable as the length of travel can be very variable as can its force, it is adopted for many conveyors. The take up trolley can travel vertically or horizontally according to the requirements. Some times, for very long conveyors, say above 1km the counterweight take-up is used in conjunction with a winch take-up. This has to be done because on fabric belts, the belts stretch in use and this permanent stretch has to be taken care of before the counterweight take-up can operate properly.
The snub pulley Many conveyor belts make use of a snub pulley. This idler is located very close to the drive pulley and is designed to push the belt onto the pulley, effectively creating more traction between the belt and the drive pulley. Snub pulleys are located on either side of the conveyor and can be independently adjusted â€“ they also act as guides to keep the belt centred on the pulleys.
This phenomenon is not found on steel cord belts because the amount of stretch is very much less.
Vertical gravity take-up
Tail pulley Besides the drive and tail pulley, many conveyor belts make use of return idlers. These smaller rollers are situated on the underside of the conveyor and are designed to stop the belt from â€˜hanging downâ€™. Horizontal gravity take-up
There are basically 2 types of pulleys on a conveyor: driven and non-driven. There are basically 2 types of manufacture or design for pulleys. â€˘ Pulley drum keyed to shaft â€˘ Pulley drum attached to shaft by shrink disc elements
he design will be considered first as it has a major effect on the life of the equipment. This is often specified as 60 000 hours to 100 000 hours of operation. This and other factors can be applied in the design specification to give the dimension of the pulley components. The shaft is the most likely to fail so design details here must be carefully checked. Changes in diameter must not be too large and the corner has to have the largest radius possible and the surface finish has to be smooth. A design using shrink disk elements is much safer as the shaft and drum side plate or boss are not slotted or cut into in any way. The design of the shrink element is carefully controlled and all stress raising factors must be considered. The drive pulley differs from other pulleys in that it resists drive torque as well as bending moments which makes its design and manufacture more critical. As shaft deflection is very critical, medium strength steel is often specified for this application. The design and manufacture of the drum must also be carefully considered as the weld between the drum and discs has to be free of defect as this is where cracking can start.
Ceramic tiles can be adhered to the face of the drive pulleysâ€™ when the pulleys are expected to operate under wet conditions. The coefficient of friction between the drive pulley and the belt varies with the conditions under which it operates. A safe figure to start off with for a new plant would be 0.3. This friction factor determines the belt loading as it has to be provided by the take-ups. Depending on the conveyor belt selected, the pulleys will have to be sized for the correct face width and diameter, for instance a 1000mm wide belt which is 2 ply fabric would need a high tension pulley 1150 wide x 800mm diameter whereas a 1 000mm belt 5 ply would need a pulley 1150 wide x 1 000mm dia. These required dimensions can be obtained from belting supplier for fabric and steel cord belts.
Rail-Veyors The electrically powered Rail-Veyor system operates remotely through a control centre and incorporates a light rail track with a series of interconnected two-wheel cars capable of continuous movement. The Rail-Veyor cars travel at speeds of up to 10m/s or 32km/h as they climb grades of twenty percent (20%) and negotiate complex turns within a 30 metres radius. The open trough formed by all the rail cars can load, unload and transport any type of industrial material including mineral ores, aggregate, coal, wood chips and more. The cars are connected to allow for articulated movement along curves and for continuous reloading.
Although it is not necessary to lag non drive pulleys, it is commonly done so nowadays. If this is required then plain elastomer/rubber lagging can be used for all non drive pulleys and should be used for drive pulleys chevron or diamond pattern grooved depending on the direction of pulley rotation.
CHUTES AND SCRAPERS
Chutes and Scrapers All conveyors have to have a product fed to them which they convey to a discharge point. This is done by chutes.
eed chutes receive product from either other conveyors, feed bins or processing units such as screens or crushers, etc. This product could be very large such as minus 300mm or very small such as minus 60mesh. In almost every case it will be abrasive to a lesser or greater degree and if the chute is not protected from wear it will be soon wear through. To prevent this from ocurring, wear bin plates can be used which may be of hardened steel, cast nickel chrome steel or other special steel or they could be ceramic or elastomer. The choice is great and the suitable type to use has been determined by practice. Also to prevent wear dead, boxes can be used where the material being handled is allowed to settle against a ledge until new feed falls upon it so there is no contact with the steel plate forming the body of the chute. To reduce dust,
With the use of a chute, conveyor life can be significantly increased. This is because the chute controls the rate at which the load falls onto the belt. A load that â€˜free-fallsâ€™ onto a belt can cause it to stretch which, in turn,will cause slippage on the rollers.
Vibrators Vibrators are sometimes used in chutes to promote material flow and prevent material bridging. They are used to promote the steady flow of granular or powdered materials.
Skirtboards In order to keep material from falling off the conveyor after it has left the loading chute, skirtboards are used. In effect they are an extension of the chuteâ€™s sides and run parallel with one another for some distance down the belt. The skirtings are installed some distance above the actual belt, and the gaps are sealed with rubber strips which are normally clamped on.
water sprays are turned on to help settle the fine dust but if too much water is used this can cause trouble by allowing the material to flush down or stick in the chutes. Feed chutes have to be designed to have the correct angle of the back or sliding plate to direct the material at the best speed in the correct direction onto the receiving conveyor or into a bin or a processing unit. Head chutes receive material from the conveyor and discharge it onto a following conveyor feed chute or onto a processing unit or into a bin. In practice, this feed chute performs the function of directing the material to where it is required. Often in high speed conveyors running at 5 plus metres per second, the head chute has fitted to it a bash plate to accept the force of the material and direct it in the way required. It will also contain any water jets or dust extraction units. An important feature of a head chute is that it is usually fitted with a scraper or series of scrapers which remove the fine particles adhering to the belt to prevent this fine material from falling off the belt on its return to the tail. The material scraped off can be directed back to the main chute to discharge onto the following conveyor. The design of scrapers has gone through many stages but is usually a rubber blade bearing on the belt and held in contact by means of a weighted lever system. It could be a number of rubber blades or a series of primary scrapers followed by secondary scrapers. It could include a rapper which is a tube with rods welded to it which rap against the belt as it moves past. Chutes come in many forms and types, cascade, spiral etc which are especially suited to certain conditions.
Dealing with Dust In many regions, dust pollution is not an option. But choosing the correct solution to cater for your needs is sometimes a little daunting as the solutions abound.
It all boils down to dust education. You need to know what your requirements are and what the different dust control techniques are available. Wet spray technology. This is probably one of the most popular dust control methods in the conveying industry. Although it can in some cases be more expensive than other systems, it is more reliable and effective.
The return idlers, due to carry-back of fine dust on the return belt The head end, where material is discharged.
Most dust suppression systems use dual spray bars that apply water directly to the material while it is in the belt. By strategically wetting the material as it moves, the potential to create dust is eliminated. In cases where very light loads are being moved, two spray bars can be used. Should this be the case, a spray bar is usually located at the bottom to wet the material from underneath while one is located above the material to suppress any dust.
Belt sag must be eliminated
There must be a good seal between the conveyor belt and the skirt system
The belt loading must be central and the length of the loading point must be adequate
A typical dust suppression system consists of skirting boards and belt support idlers to ensure no gaps open up when the conveyor is loaded. Front and rear support beams are also installed on which a top plate is placed to achieve complete dust-free conveying. Belt conveyors emit dust from the following four points: ►►
The tail end, where material is received
The conveyor skirting
Where wetting the material is not an option, but dust and spillage still needs to be kept to a minimum, dust encapsulation systems need to be used. To create a good dust encapsulation system, the following needs to be kept in mind:
Safety Tips Operate the conveyor only if you know what you are doing and are properly trained. Always make sure that the areas surrounding the belt are clear of all obstacles. Should you see any of your fellow employees fooling around near the conveyor, report them to your foreman or their supervisor.
Keeping Control If a conveyor rises from a low level to a higher level and for some reason power is interrupted, it will tend to run back unless it is prevented from doing so.
The usual manner is to fit a hold back on either the head pulley shaft or on the gearbox high speed shaft. A hold back is a braking system which only operates in one direction. When the conveyor is running normally, the shafts are quite free to rotate but if they try to turn in the opposite direction, the hold back locks on the shaft and prevents it from doing so. Without a provision such as this, the conveyor would run in reverse, and deposit its load at the tail end. When the conveyor is started up again, the hold back releases its hold on the shaft and allows the belt to move in the correct direction. On large powerful conveyors, the hold back is usually mounted on the head pulley shaft on the opposite side to the drive.
Running conveyors often discharge onto other running conveyors or into a process system which has a limited holding capacity. If the conveyor being fed stops suddenly for longer than the feeding conveyors, a lot of material will be discharged which cannot be accepted. This would produce a hold up which can cause a shut down for a long time.
Runaway belt A brake is an important component on a conveyor system â€“ especially an inclined one. Should a malfunction occur anywhere on the conveyor, a runaway belt can be dangerous â€“ even deadly as many of the components are not designed to cope with the speed at which the belt is running.
Therefore a brake is fitted to the feeding conveyor which is sent a signal to shut down. The braking force is carefully set to prevent a sudden stop that could over-stress the entire drive system but which is adjusted to correspond to the length of time that the conveyor being fed will take to stop. Brake operation is usually to power the brake off so that when power is cut, the brake is applied by springs. Brakes could be drum type, generally external drum, or disc type. Both types have certain advantages and have to be selected to suit the condition.
Providing Support All conveyors require that the mechanical components be supported on a structure which leads them from the receiving point to the discharge point.
n the simplest arrangement this will consist of legs onto which conveyor stringers are attached and which carry the idlers and pulley.
theft of valuable product such as diamonds. These gantries have at least one walkway at the side of the conveyor so that maintenance and observations can be carried out.
In long overland conveyors, this simple structure can run for many kilometres and can also provide conveyor protection by means of a housing over the conveyor belt to protect it from rain and wind. There are many in plant conveyors which transport material around from point to point which will require complex housing. These may be required to provide for raising the material to a receiving bin to discharge to another conveyor, to cross a road or a railway line, many different requirements.
On wide conveyors, say more than 1 050mm, it is often necessary to provide walkways on both sides so that the much heavier components can be handled. For personnel protection, trip wires are required whenever contact with the moving belt is possible.
To do this, the conveyors are supported in steel gantries which are supported on towers or legs. Very often these gantries are completely sheeted on all sides to protect them against the weather and, in some cases, to protect them against
Another design requirement on every conveyor is for belt maintenance. It must be possible to access them with a coil of new belting at a reasonable level so that the destroyed belt can be cut out and the new belt be spliced in. Depending on the type of belt and splice being done, it may require that the splicing station be supplied with electrical power to head the splicing platens.
Idlers Idlers carry the conveyor belt, they bear the weight of the belt and burden on the carrying side of the conveyor and the belt only on the return side, but in certain cases material can also be carried on the return strand.
arrying idlers can be flat but these are not normally used in bulk handling, or they can be 2 roll where only a light load is being transported. Normally idlers are 3 roll or 5 roll. The wing angle, which is the angle of the outer rolls, can be anywhere from 20° to 45° depending on the material density and load capacity required. The most common type is a 3 roll idler with a wing angle of 20° to 35°. It provides good support to the load and has a good load cross section. Also used a lot are 5 roll idlers which have a greater load carrying capacity for heavy loads such as iron ore. Their cross-sectional area is slightly greater than 3 roll idlers. The spacing of the idlers is determined by the allowable sag between idler sets and is usually chosen to suit the material being transported. Starting from the tail, as the belt leaves the tail pulley, the first idler it meets will have a wing angle less than that of the main load carrying section, then this follows an idler with a slightly greater wing angle and so progresses until the final wing angle is reached.
occasionally even more. The most common being 100mm dia and 150mm dia. The larger diameter allows for heavier construction for belts handling heavier loads and also for a reduction in rotational speed. Return idlers which carry the belt back from the head pulley to the tail pulley can be flat or V shaped or a mixture of the two.
The spacing of the return idlers can be much greater than the carrying idlers as the load on them is only the mass of the belt. A rule of thumb is about 3 times the carrying idler spacing. V shaped return idlers have 2 rolls with wing angles of about 10° and are used in long belts to assist in belt training. On some conveyors, a system of normal V return idlers passes over a flat idler and then an inverted V idler and back again to force the conveyor belt to run centrally.
For example the first idler wing angle, 10° , second 20°, third 30° and final 35°. It is usual to make this wing angle adjustable to give good support to the belt without over stressing the belt edges.
After the transition section, the load section is found where the burden is deposited on to the belt. It is common for this area to have impact resistant idlers, which are closely spaced to restrict spillage. These idler rolls may be made of elastomers or rubber to absorb the impact of large pieces of ore striking the belt. After the load area, the idlers will be selected for the main run of the belt until the head is reached where once again the wing angle is reduced to allow the belt to flatten out as it passes over the head pulley.
Idler roll diameter can vary from 75mm to 150mm and
Getting the belt to run straight is often very difficult so spaced along the carrying (top) side there will be training/ self aligning idlers of various designs which ensure that the conveyor belt tracks centrally, ie, runs in the centre of the carrying idlers. If the material at the feed end is deposited slightly off centre due to varying feed rates or poor chute design, the belt tends to drift to one side. If this drift is great enough, the edge of the belt will strike a belt drift switch and stop the conveyor, this means that all the systems prior to this belt will have to shut down. Conveying Guide
From The Chairmanâ€™s Desk
would like to describe some interesting history on our National Idler Standards, and for this we have been delving back into the records from the earliest days of the CMA. I hope it will be interesting reading for many of our newer members! A sub-committee was set
up as early as 1972 to study the need for Idler dimensional standards, the scope of which was for conveyor idlers manufactured in South Africa, covering leading dimensions, conversions from inches to millimeters and allowable tolerances, quality of materials, and load carrying capacity. A first draft of an Idler dimensional standard was drawn up by CMA members using their own manufacturing specifications, ISO and British Standards, and Eskom and Anglo American tendering specifications and standards.
The draft was registered with SABS in December 1973 as a private standard with the intention that it would later become a National Standard. Minor technical changes were made to the CMA document by the technical committee in April 1975 and in July 1975 SABS agreed to set up a committee to start the process for publishing the National Standard. Moving forward to October 1976, the CMA drafted an Idler Standards Brochure that members could send to end users outlining the provisions of the CMA Idler standard.
Wing roll about to fail
At the end of March 1977, SABS held a technical committee meeting to record changes to the technical document but major hurdles still had to be overcome on SABS proposal to standardise individual idler rolls, which manufacturers thought impractical, and some of the limitations of ISO should thus be adopted instead.
Comment and voting
In March 1978 the CMA Uniform Dimensions for Idlers was published. During the period August 1978 to November 1979 a SABS document originally developed without the assistance of the CMA was reviewed and changed by our technical committee, until finally SABS had a document that could be sent to countries around the world for comment and voting prior to publication, a process that was expected to take six months.
CONVEYOR MANUFACTURERS ASSOCIATION
Damage due to seized roll
Subsequently, in 1980 the National Standard which we know as SANS 1313-1 was published. Over the years since, Parts 2 and 3 were added and improvements to the original document continued to be made with the close co-operation of SABS and the CMA. The current version of SANS 1313 was published in 2012, but regrettably several errors appear to have crept in during the re-drafting process. The CMA has identified the errors and requested that the Standards be amended by SABS, but these things take time as can be seen from the history described herein.
To limit potential manufacturing errors, a list of these potential errors is being made available to the industry in the interim period before the standards are amended Intellectual property rights
Solely as a service to industry, and in the interests of safety and to limit potential manufacturing errors, a list of these potential errors is being made available to the industry in the interim period before the standards are amended. Anyone interested, but especially users of SANS 1313 of 2012, may download the list from the website www.cmasa. co.za. This list is made available subject to the intellectual property rights of SANS 1313:2012 remaining with the SABS and is used and applied solely at the discretion of the user. We hope you find it useful. Jay Pillay, Chairman
Belting The fundamental purpose of a conveyor belt is to carry the loaded material along the length of the conveyor structure, from the feed point at the tail of the conveyor, to the discharge point at the head of the conveyor.
elting has been developed over many years to fulfill this very purpose in the most cost-effective manner and with the highest practical degree of reliability.
Some 40 to 50 years ago, conveyor designers and belting manufacturers recognized the need to diversify into producing 'special' types of belting which were different from the main-stream, standard type of belting. This need was borne out of the fact that at that time conveyors were being recognised as a serious method for transporting products in bulk over longer and longer distances, at ever-increasing rates. Belting was therefore 'tailored' to suit different applications such as larger particle sizes which would introduce greater impact loads into the belt; higher material temperature resistance to convey warm products in process plants; oilresistant belting suitable for transporting oil-contaminated products including food-stuffs. The longer conveyors were made and the greater the load transported, the higher the tensions became and so fabric belt carcasses were strengthened by introducing additional plies and using different raw materials as a basis for manufacture. The demand for even longer and higher duty conveyors finally gave rise to the introduction of the steel cord belt which is so prevalent today.
Many belts in general material handling have two layers. An under layer of material to provide linear strength and shape called a carcass and an over layer called the cover. The carcass is often a cotton or plastic web or mesh. The cover is often various rubber or plastic compounds specified by use of the belt. Covers can be made from more exotic materials for unusual applications such as silicone for heat or gum rubber when traction is essential. There are two basic categories into which all troughed conveyor belting falls namely, fabric belting and steel cord belting. Externally both types of belt appear to be identical however,
the difference is in the internal structure of the belt. The internal structure or carcass of a belt dictates the tensile strength of the belt. In fabric belts the carcass comprises 'plies' or mats of reinforced fabric separated by cushioning layers. Steel cord belts on the other hand have a series of steel cables embedded into the belt, separated by rubber. When tension is applied to the belt, the carcass absorbs the force. The greater the required tensile force to move the transported material, the greater the required strength of the belts' carcass. In both fabric and steel cord belts, the carcass is covered by rubberized covers to protect the carcass or cables.
Fabric plied belting
Fabric plied belt consists of a single or multi-layered series of synthetic fabric layers interlaced between rubber based shock absorbent layers. The "top" and "bottom" sides of the belt consist of hard wearing, abrasion and cut resistant, rubber covers. These covers protect the belt from damage, especially at the loading points of the conveyor.
Steel cable belting
Steel cable belting consists of steel and rubber only. Sometimes there are fabric plies involved. Steel cable belts consist of steel cables manufactured from high tensile steel wire. These steel cables are surrounded by a layer of high grade rubber to facilitate adhesion to the outer covers and to improve lateral tear resistance. As with fabric belting, the ‘top’ and ‘bottom’ sides of the belt consist of hard wearing, abrasion and cut resistant, rubber covers.
Troughed belt selection criteria
The procedure for selecting a conveyor belt for any given application involves the evaluation of a number of factors pertinent to the installation. The designer must bear in mind that there is a variety of 'standard' belts to choose from and that the properties of
each belt have been incorporated to suit a number of basic criteria such as hot material, large or small lump size, etc.
The following fundamental criteria must be considered for each belt selection:
• The reinforcement or a carcases which provides the tensile strength of the belt.
• An elastometric cover which protects the carcases against damage from the material being conveyed and provides a satisfactory surface for transmitting the drive power.
All conveyor belts have been rated according to the operating tensions they will be able to withstand. Usually this tension rating is expressed in kN/m of belt width. These ratings have been standardised within the industry and normally include a generous factor of safety, ie, 6.7 : 1 and 10 : 1 for steel cord and fabric belting respectively. The conveyor designer will calculate the maximum tension generated within any given conveyor system and will ensure that the belt selected satisfies this maximum calculated tension.
Load support and number of plies (fabric belting only)
The selection of the conveyor belt must also ensure that the full load of material for which the conveyor has been designed can be supported on the belt, as the belt spans between two idler sets. Belting manufacturers supply tables for fabric belting which reflect the number of plies recommended for a belt, based on the various types and grades of material to be transported on the belt.
A conveyor belt comprises of two main components :
In selecting the most suitable belt for a particular application, several factors have to be considered namely; • The tensile strength of the belt carcases must be adequate to transmit the power required in conveying the material over the distance involved. • The belt carcases selected must have the characteristics necessary to - provide load support for the duty. - conform to the contour of the troughing idlers when empty. - flex satisfactorily around the various pulleys used on the conveyor installation. • The quality and gauge of belt cover material must be suitable to withstand the physical and chemical effects of the material conveyed.
In addition to the selection of a belt based on the minimum number of plies, the stiffness of a fabric belt across its width is affected by the number of plies in the belt, ie, more plies result in a stiffer belt. If the belt is too stiff, it will not seat correctly in the troughed idler sets in an empty condition. This often results in misalignment of the belt relative to the conveyor structure.
Minimum pulley diameter
The diameter of pulleys utilised within a conveyor system must be selected giving consideration to the flexibility of the belt as the belt navigates the pulley circumference. Where a pulleys' diameter is too small, the belting is wrapped too tightly and is flexed (forced) around the pulley circumference generating excessive internal stresses which progressively damage the belts' integrity and result in premature belt failure.
Belting manufacturers supply tables reflecting suggested minimum pulley diameters for different belt classes. These tables assume that the belt classes reflect the maximum tensions that the belt will be subjected to as it passes around the various types of pulleys.
Type of covers
Depending on the application and material to be conveyed, the designer should select the type of belt covers. There are a number of standard types or 'grades' of belt covers available in the market which have been developed for different applications.
Belt cover thickness
In addition to the selection of an appropriate belt cover 'grade', the thickness of the belt covers in both fabric and steel cord belting must be of sufficient thickness to suit the application. Belting manufacturers have developed standard tables reflecting suggested minimum top (carry side) cover thickness based on an ‘impact exposure frequency factor’ as well as characteristics of materials being conveyed.
Belt thickness and mass
The overall thickness and mass of a conveyor belt is a function of the standard carcass thickness for the selected belt plus the thicknesses of the top and bottom covers. Conveying Guide
KEEPING IT CLEAN
Keeping it Clean Over the years, the cleaning of conveyor belts has been a major problem in most plants. Many attempts have been made to develop conveyor belt cleaners to solve the problem of carry-back once and for all.
Pressure jet washing system No matter how sophisticated a belt cleaner may be, there are certain instances where a pressure jet washing system should be used.
A belt brush is a suitable method for cleaning rare belts, as scrapers do not work well with this type of belt. A belt brush can be mounted anywhere on the return part of the conveyor, but the most suitable place is immediately behind the drive pulley. The bristles should not be pressed against the belt, but only gently touching for the best result and wear life. A belt brush works best on conveyors transporting relatively dry material.
Return belt cleaner Material that gets pinched between the belt and the tail pulley can cause the carcass of the belt to break and has to be removed before the belt reaches the tail pulley. Using a plough-cleaner achieves this best. Rubber or polyurethane strips are suitable as the wearing part on a plough-cleaner.
Pre-cleaner A pre-cleaner is normally positioned against the drive pulley immediately below the flow of material. A rule of thumb says that the lower the belt speed, the lower the cleaner should be mounted and the higher the belt speed, the higher the cleaner can be mounted. This is to ensure that the cleaner isnâ€™t hit by the material flow. The pre-cleaner has a tough task. It removes most of the material that adheres to the belt after unloading. The pre-cleaner is especially suitable when coarse or moist and sticky material is transported.
Return cleaner A return cleaner is in most cases installed underneath the conveyor. Its job is to remove any carry-back that may be stuck to the belt on the way back to the head pulley. If the carry-back material is not removed or does not fall into the chute, it will build up until it touches the belt and could cause a fire.
T O D A Y
Scraper blade Scraper blades are used at the head. They need to be installed to remove any material sticking to the belt
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Secondary cleaner Secondary cleaners are available in a number of different designs. They are installed immediately behind the drive pulley. Two commonly used models are the T-cleaner and the arm-cleaner. The T-cleaner is pressed straight up against the belt, while the arm-cleaner has a twisting motion. Secondary cleaners are normally used in combination with a pre-cleaner for the highest cleaning efficiency, but they can also be used as stand-alone if the material transported is dry and fine.
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Maintenance Conveyor belt maintenance not only includes proper care of the belt itself but also includes care and maintenance of the frame and accessories. It is advisable to compile an inspection form before you begin.
his inspection form should take into account the various components of the conveyor, when they were inspected, and what, if any work was done to them.
Ideally you should inspect the conveyor in three states. When it is shut down and empty â€“ or when there is no load on it. When it is running, but empty and when it is running and carrying materials.
First inspection: belt shut down and empty
The first step is to inspect the conveyor belt when the system is shut down and empty. This allows the opportunity to check for any damage to the belt or splice. The conveyor should be locked while making this inspection. Rubber belt damage should be repaired using the hot vulcanised repair method or the cold repair method. Belt fabrics that are exposed to the weather or to product contamination should be properly cleaned, dried, then covered with new rubber. These repairs are critical to prevent moisture from penetrating the belt and breaking down the cover adhesions, and to prevent product contamination from abrading the carcass. The splice can also be inspected and if damage in the splice is visible it is suggested that the splice be repaired or replaced. This is also a good time to walk the conveyor and check the following components:
Second inspection: belt running empty
The conveyor should be turned on and run empty. The purpose of this is to walk the conveyor, while running empty, to check for any tracking problems. Before any adjustments are made to correct a tracking problem, the system will need to be inspected under running conditions when loaded, because empty belts and loaded belts do not necessarily track the same way.
Third inspection: belt running loaded
The next step in the inspection process is to run the belt in a loaded condition and check the following: 1. Tail pulley: It must be turning freely without bearing noise, product build up or carryback; the belt tracking must also be checked. 2. Load area for any spillage 3. Carry side idlers should be turning freely
4. Carry side self-trainers must be working 5. The tripper area needs to be checked for tracking and spillage 6. The head pulley and or drive pulley need to be running smoothly and there must be no slippage when starting or running. The conveyorâ€™s belt cleaners must also be functioning correctly. The belt tracking also needs to be checked. 7. Check the head pulley, the snub needs to be turning freely without bearing noise and clean 8. Return idlers have to be clean and turning freely 9. Bend pulleys must also be turning freely without bearing noise and must be clean 10. Take-up pulleys have to turn freely without bearing noise, must be clean and move freely in the frame 11. Return side self trainers must be working 12. Check the general belt tracking 13. The plow or scraper in front of the tail pulley must be clean and working.
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Belt tracking Always check the general belt tracking
Impact beds Check the impact bed for any damage
Conveyor splices Conveyor splices must be checked and if there is any damage, need to be replaced.
Skirtings Skirtings along the belt need to be clear of the belt, but must be close enough to stop any spillage. They should also be cleared of any debris.
Head and tail pulleys Both head and tail pulleys need to be running smoothly and there must be no slippage between the belt and either pulley.
1. Tail pulley free from build-up and trapped material
Idlers All idlers along the belt need to be clean and rotating freely
2. Tail pulley damage 3. Skirting in the loading area 4. Impact bed or impact idler damage 5. Slider bed clean and smooth 6. Carrying side idler damage 7. Carrying side self trainers - operational and not tied off
Splicing Situations The durability of a conveyor belt is largely dependent on the splices made in the belt. These splices are in most cases weaker than the actual belt and are frequently the reason for conveyor disruptions.
he strength of a splice depends on numerous factors including:
The linkage layer
The splicing method
The materials used to make the splice
The environment in which the splice was made
A mechanical splice is created by using metal hinges or plates. This method requires a mechanical fastener system and a hammer or electric rivet driver of some sort to install the fasteners. Mechanical splicing is a versatile solution because it can be done in many different environments and on many types of belts. It is typically used in applications in which belts go through lots of wear and tear or need to be continuously expanded, or on belts in dirty, high-moisture environments or cramped spaces. These applications include mining, quarry and other heavy-duty applications.
Vulcanisation Vulcanisation creates a splice by using heat and/or chemicals. This mode of splicing is more involved and requires special tools, expertise, and a clean, temperature- and moisture-controlled environment. If done correctly, however, this type of splice is smoother and typically more durable than a mechanical splice. There are two types of vulcanised splices: hot and cold.
Hot vulcanisation creates splices by using heat and pressure through use of a vulcanising press. Cold vulcanisation creates a splice through use of chemicals that bond two pieces of belt.
Which is best?
Both types of vulcanisation require more time and preparation than mechanical splices. Also, only certain belts, used in certain types of environments, lend themselves to vulcanisation. Because the belt must be taken apart and removed from the conveyer system, vulcanisation is more of a long-term splicing solution for applications that are light-duty and don’t require constant repairs and extensions of the belt.
Hot or cold vulcanised splices are used on more sophisticated conveyors. Steel cord belts have to be spliced.
Mechanical splices are usually used where portable conveyors are required which have to shift from location to location, or on low-cost installations. Also used on temperorary repairs to a snapped conveyor to get it going until it can be properly repaired.
There are many variables to consider when choosing between mechanical or vulcanised splices:
Safety Tips Know the location and function of all stop/start controls. Conveyor belt hazards need to be identified and appropriate safeguards need to be implemented to limit the chance of injury.
Used conveyor systems can obviously save your business a lot of money when compared with the expense of new equipment, but there are many factors to take into consideration, and a few pitfalls you will need to avoid.
ou need to be sure of what you want before you start looking at used equipment. No system, no matter how cheap, is a bargain if it is not completely suitable for the role you are buying it for. If your conveyor knowledge is lacking, do some research on the Internet as a matter of top priority.
onveyors which receive material directly from mines, open cast pits, diggings etc sometimes have metal objects mixed in with the feed product. These objects have found their way there because they have been lost or broken off some mining equipment such as front end loaders, bull dozers or drilling equipment as well as broken chute liners or both which have loosened.
Mostly custom made
Assuming you have a basic awareness of the type of used conveyor equipment you are looking for, you will need to think hard about whether this equipment is better bought used or new. Some types of conveyor are hardly ever bought as off the shelf solutions, but are usually bought custom made. The chances of someone else’s custom needs being exactly the same as yours are extremely slim. Of course, there is never any harm in looking, and if someone else’s old system does happen to fit your needs exactly, there is no reason not to make an offer on it. You are more likely to find used equipment, though, if you are wanting one of the more common roller based or belt driven models. If it is a used conveyor belt driven model you are after, be aware that these are more difficult to service and to maintain. This also means they are harder to check out for potential faults and worn parts. If you don’t have extensive knowledge of these models, it will be best to seek the opinion of someone who does.
Check for spares
A used roller conveyor is far more likely to be in good condition, as it is much easier to maintain and keep serviced in good condition. If it comes from within a large organisation which can afford to employ people directly to service and maintain equipment, this will be even more true. In any case, you can easily carry out your own research on parts which are open and available to everyone. Used conveyors can be found in many different places, so make sure you are aware of the different rights you have in each venue. If you are buying at auction, for example, you will nearly always be buying under the understanding that what you see is what you get. If something turns out to have been tampered with or covered up, you will have no rights against the seller. Buying a used conveyor through a retailer will give you more rights. Sourced from www.hostelaleph.com
If allowed to carry through to the following conveyors or into the processing plant they could cause considerable damage. As a first line of defense, strong electro magnets are suspended over the conveyor belt. These could be placed either at the start of the conveyor belt or at the head end as the material is discharged off the head pulley. The location selected depends on the experience of the plant operation and the size and type of product. The magnets could be either a large circular design which covers the entire width of the conveyor and is suspended from a rail which allows it to be moved from its position over the conveyor to the side of the conveyor. The power is then switched off and any steel object which has been attracted to the magnet drops off into a hopper. The magnet is then returned over the conveyor belt and the power is switched on. The other type of overland magnet looks like a short conveyor belt. Above the lower strand of this short conveyor is placed an electro magnet. Any steel object which is picked up is held to the under side of the short belt which transports it to the edge of the main conveyor and out of the magnetic field of the overland magnet and it drops off.
Avoiding Pitfalls when Buying Overhead Magnets a Used Conveyor
Managing Spil age Material spillage on bulk conveyors accounts for plant inefficiencies, increased overheads and occupational hazards. After all it is more difficult to clean up after a mess than to prevent it from happening in the first place.
owever, by careful evaluation and procurement of well designed spillage control devices for the conveyor system, the inherent investment will subsequently increase productivity, profitability and provide timely return on capital. According to research, many material handling companies donâ€™t actually know how much spillage is costing them. It is only after analysis that they realise that a properly installed spillage prevention plan will save them thousands. These companies would have been able to save even more if they had built the conveyor system with spillage in mind than adding spillage prevention techniques afterwards.
Spillage control techniques and devices
While spillage varies according to the type of materials being transported, the proper use of chutes, rails and skirtings will ensure you keep this to a minimum.
Many conveyors are troughed in that the walls of the belt are slightly raised, this in conjunction with suitable skirtings will help you decrease your spillage as much as possible.
With properly installed chutes at either end of a conveyor belt, spillage of material while loading and unloading will be minimised. Many chutes also have a slow release system that allows for large quantities of material such as ore to be deposited evenly over a running belt. This also decreases the wear on the belt as the load is lighter and constant. Often a chute is also used when moving material from one conveyor to the other. The main design objectives of chutes are to guide material onto a conveyor belt, at the speed of the belt and in the
direction that the belt is travelling. They are also there to eliminate material spillage as well as to enclose material dribbles. Chutes also prevent wastage in the form of dust. The containment of conveyed materials can be considered in three stages, namely containment at the loading point, slowing the escape of materials while on the conveyor and the management of the discharge of the materials at their destination.
The inherent investment will subsequently increase productivity, profitability and provide timely return on capital
At the loading point
It is most important to create a tight seal at the loading and transfer points as this is where the bulk of the spillage will occur. At a loading point, this seal can be created with snug fitting skirtings starting at the chute and running the length of the belt. Once installed though, routine maintenance needs to be carried out to ensure these seals are in fact sealing properly. It is also recommended that the belt is properly supported at the loading point with the use of impact bars. When materials are loaded on the belt, it will sag under the weight. This beaks the seal which will allow spillage to occur and will also cause the belt to stretch overtime, resulting in slippage and many other problems.
All along the conveyor
Controlling spillage while it is on the belt can be done by making sure the belt runs over the rollers and idlers as smoothly as possible. A belt that jerks up and down will not only throw the material off it, but will also cause dust. It is also advisable to make sure the belt runs straight from start to end and for long belts you may want to make use of belt rotating devices. This also aids in keeping the belt clean. If you are conveying very light materials, it is advisable to use dust-encapsulating covers to protect your materials from the wind. Once on the conveyor, materials being blown away is a huge problem. It has even been said that in some cases entire belts have lifted, wasting huge amounts of material. To minimise the effects of wind, the use of wind deflectors and conveyor covers can be used. Conveying Guide
Devices that can be implemented to lessen the chance of injury: Belt rip switches
Belt tear switches are found at set intervals along the length of the conveyor belt. As their name implies, their sole purpose is to detect any tears in the belt as it passes through the sensors. Should a tear be found, the switch automatically stops the belt before any additional damage is done or before someone is injured due to flying belt debris.
Kill switches need to be installed at certain intervals along the conveyor belt. These switches need to be clearly visible and easy to access but not easy enough to accidentally activate them. Once the kill-switch has been activated it must shut of all power to all components of the conveyor system and sound the alarm.
Interlocking devices These are devices connected between the drive pulley and the feeding device. Should the belt stop, the feeding device trips, to prevent an overload on the belt. An interlocking device is also used where one conveyor feeds another. Should one stop, the trip device will automatically stop the other.
Corrosion Protection Mostly all of the components making up a conveyor are made of steel which, if not properly protected, will corrode. The corrosion protection systems developed are very good and if properly applied and maintained will give excellent protection for many years. The most important step is to ensure that all surfaces are clean down to metal, if this is done and the correct primers applied it will ensure protection. Any outer coat will of course help, mainly by protecting the primer coat from mechanical wear. Damage will occur particularly during construction. If this is the case, the surface has to be shot blasted back to the clean metal and the approved coats applied.
Sirens and lights
Pull key switches
These are used to alert other employees that the conveyor is about to start.
These must be situated along the entire length of the conveyor belt that can be accessed by an operator and should be in easy reach should the operator find himself in trouble. Once the wire is pulled, the entire system needs to shut-down immediately.
Most operating companies and mines nowadays are aware of the importance of the corrosion protection function and have issued specifications which cover their requirements and must be worked to. Finally all surfaces must be checked by proper inspection and certificates of compliance then issued.
Did you know? Over 50 percent of all injuries reported in terms of the OHS Act are as a result of incorrect lifting procedures. Over 50 percent of all fatalities reported are as a result of workers falling from height
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