Belt Conveyor. Application

Transcription

1 Belt Conveyor The belt conveyor is an endless belt moving over two end pulleys at fixed positions and used for transporting material horizontally or at an incline up or down. The main components of a belt conveyor are: 1. The belt that forms the moving and supporting surface on which the conveyed material rides. It is the tractive element. The belt should be selected considering the material to be transported. 2. The idlers, which form the supports for the carrying and return stands of the belt. 3. The pulleys that support and move the belt and controls its tension. 4. The drive that imparts power to one or more pulleys to move the belt and its loads. 5. The structure that supports and maintains the alignments of the idlers and pulleys and support the driving machinery. Other components include: 1. Loading chute or feeder chute that organises the flow of material and directs it on the belt conveyor. 2. Take-up-device which is used to maintain the proper tension of the belt for effective power transmission. 3. Belt cleaner that keeps the belt free from materials sticking to the belt. 4. Tramp removal device, which is optionally used in case the conveyed material bears the chance of having tramp iron mixed with it and subsequent handling of the material, demands its removal. 5. Continuous weighing device for constantly measuring the load being carried by the conveyor belt. 6. Discharge chutes to guide the discharged projectile to subsequent conveyor or other receiving point. 7. Surge hopper and feeder, which is essential for supplying material to the conveyor at uniform rate when the supply of material is intermittent. 8. Tripper arrangement to discharge material at different point or to other device. Application Conveyor belts are widely used in mineral industry. Underground mine transport, opencast mine transport and processing plants deploy conveyor belts of different kinds to adopt the specific job requirements. The main advantages of conveyor belt system are: 1. A wider range of material can be handled which pause problems in other transportation means. Belt conveyor can be used for abrasive, wet, dry, sticky or dirty material. The lump size of the transported material is limited by the width of the belt. Belts up to 2500 mm wide are used in mining industry. 1

2 2. Higher capacity can be handled than any other form of conveyor at a considerably lower cost per tonne kilometre. Conveyor belts with capacity of 11000t/h and even higher can be deployed to match with higher capacity mining machinery. 3. Longer distances can be covered more economically than any other transportation system. A single belt conveyor or a series of belt conveyors can do this. Belt conveyors can be adopted for cross-country laying. 4. By the use of many forms of ancillary equipment such as mobile trippers or spreaders bulk material can be distributed and deposited whenever required. 5. Many other functions can be performed with the basic conveying like weighing, sorting, picking, sampling, blending, spraying, cooling, drying etc. 6. Structurally it is one of the lightest forms of conveying machine. It is comparatively cheaper and supporting structures can be used for many otherwise impossible structures such as crossing rivers, streets and valleys. 7. The belt conveyor can be adopted for special purposes (fire resistant, wear resistant, corrosion resistant, high angle negotiation etc.) and can be integrated with other equipment. 8. It can be horizontal, incline or decline or combination of all. 9. Minimum labour is required for the operation and maintenance of belt conveyor system. 10. In underground mine transport, belt conveyor can be used in thin seams as it eliminates the rock works that might otherwise be required to gain haulage height. Moreover, belt conveyor can provide continuous haulage service from pit bottom to the surface. The limitations of conveyor belt are: 1. The loading and transfer points need to be properly designed. 2. Numbers of protective devices have to be incorporated to save the belt from getting damaged by operational problems. 3. The belt needs higher initial tension (40-200% of useful pull). 4. The use of belt is restricted by the lump size. If the maximum diagonal of a irregular lump is X then the belt width (B) is approximately given by: where, B: Belt width, mm B Xa X: Longest diagonal of irregular lump, mm a: Factor to account for grading. a is taken as 2.5 for graded material and 3 for un-graded material. However, for particular material these values must be properly estimated. 5. Conveying of sticky material is associated with problems of cleaning and discharge causing poor productivity. 6. Higher elongation of the belt (4% elongation may take place at the working load). Some of the applications of belt conveyors are shown in Figures below: 2

3 Level or inclined Conveyor receiving material at tail end and discharging at head end. Level Conveyor receiving material at any point through travelling Hopper and discharging at head end. Level and inclined Conveyor receiving material at and near tail end, having vertical curve to incline, and discharging at head end. 3

4 Level and inclined Conveyor with chute. Material received from bins too close to elevated head end to permit use of vertical curve. Inclined and level Conveyor receiving material at tail end and discharging from level section through a movable Tripper. Level Conveyor on raised structure with double-wing Tripper forming storage piles on both sides of Conveyor. 4

5 Inclined Conveyor receiving material at tail end and discharging at several points through a series of fixed Trippers. Level Conveyor with Stacker or Boom Conveyor mounted on revolving turntable so as to discharge to both sides of Conveyor. INFORMATION REQUIRED TO DESIGN A BELT CONVEYOR 1. Length of conveyor from centre to centre of end pulleys. 2. Inclination-level or inclined. Either degree of inclination, or distance to be lifted or lowered. 3. Average capacity per hour. 4. Maximum capacity per hour. 5. Material to be conveyed, and weight per cubic foot. 6. Average size of material. 7. Size of largest pieces and percentage in feed. 8. (a) Nature of material -dry or wet (moisture content). (b) Abrasive or corrosive? 9. How material is to be fed to the belt and particulars of feed point or points. 10. How material is to be discharged from the belt i.e. overhead pulley or by trippers, and particulars of discharge points. 11. General indication of supporting structure. 5

6 12. Power available for driving. If by A.C. electric motor, state voltage, phase and frequency. If D.C. motor state voltage. Items 1 and 2 determine the suitability of belt conveyors, since inclination is a limiting factor. Items 1-7 determine the speed and width of the conveyor belt, the power needed for the drive, the type of drive, the number of belt plies, size of pulleys, shafts and spacing of idlers. Items 8-10 determine the quality and thickness of the rubber cover on the belt. Suitability of a Belt Conveyor Table I shows the maximum safe inclination for troughed belt conveyors handling various bulk materials and for convenience, chart A shows the rise in feet corresponding to the horizontal distances at various angles of inclination. Material Angle Rise in m per 100 m Material Angle Rise in m per 100 m Cement-Loose Coal-Bit Slack Clay-Fine Dry Coal-Anthracite Clay-Wet Coke oven run Coal-Mine Run Coke-Sized Coal-Sized Coke-Breeze Concrete-Wet Ore-Sized Earth-Loose Packages-Paper Wrap. Glass-Batch Rock-Fine Crushed Grain Rock-Mixed Gravel-Bank Run Rock-Sized Gravel-Screened Salt Gypsum-Powdered Sand-Dry lime-powdered Sand-Damp limestone Sand-Tempered Foundry Ore-Fine Sulphur- Powdered

7 Ore-Crushed Wood-Chips 2r G Should the horizontal distance be more than sufficient for the required rise, it may be more economical to run the first section of the conveyor horizontally, and incline it upwards at the discharge end, rather than use a smaller angle of inclination for the entire length. In such a case, the transition from the horizontal to the inclined sections must be made by a vertical curve, to ensure that the belt does not lift off the idlers. Belt Conveyor Types Permanent: This type of conveyors is installed for the life of the mine. They are used in main line, slope, long overland installation, preparation plants and stockpiles. Portable: Shiftable: These are characterised by relative ease of assembling and disassembling to facilitate advances and recovery in development and retreat operations in underground mining. Portable conveyors mounted on crawler or wheels are also used in mechanised quarries and surface mines. Used in continuous surface mining this type of conveyor is mounted on skid or supporting structures aligned together and the whole can be shifted transversely to follow the advancing working face. High Angle Conveyor: These are special type of conveyor belt arrangement used for negotiating steeper angle of inclination. Such belts can work in slope up to Sandwich belt conveyor is a type of such belt conveyor. Cable Belt Conveyor: Where the belt is carried on moving wire ropes and the tractive force is applied through the rope to the belt is known as cable belt conveyor. NALCO (in collaboration with France) is using such system for long distance conveying of bauxite in their mines in Orissa. Description of Components Belt The belt consists of a carcass covered from all sides with a filler material like PVC and neoprene as shown in Figure 1. Top Cover (Carrying Side) End Cover End Cover Carcass Bottom Cover (Non-Carrying Side) 7

8 Essential Properties Figure 1 Construction of conveyor belt. The belt works as a tractive element as well as load-carrying element. It may be used for different kind of material transportation at a higher speed ( 6-8 m/s). For this purpose the belt need to have the following essential properties: 1. Flexibility 2. Transverse rigidity 3. Low mass per unit length 4. High strength 5. Simplicity and inexpensive 6. Longer life 7. Should not stretch under normal working stresses,i.e., low relative elongation. 8. Wear resistant 9. Fire resistant Recommended maximum belt speeds are given as shown in Table 1 Speed (m/sec) Speed (fpm) Width (mm) Width (inches) Coal, damp clay, soft ore, fine crushed stone, Over burden and earth Heavy, hard, sharp edged ore. Coarse crushed stone over over 36 The carcass can absorb the longitudinal tensile stresses and make the belt rigid transversely. Filler protects carcass against moisture, mechanical damage, abrasion and combine the carcass into an integral body forming top cover and bottom cover. The filler can be synthetic rubber or polymer like PVC with adequate additives for getting particular expected characteristics. The carcass can be a fabric type with plies of fabric or steel chord type. A fabric ply has a longitudinal thread called warp and a transverse thread called weft. The fabric plies are made of capron, nylon or lavsan etc. The longitudinal member of the carcass transfers the tension. Both the warp and weft carry the load via the cover. The cover while protecting the carcass from external damage bear the impact and abrasion at the feeding points and wear at the cleaning points. Figure 2 shows the cross section of the types of conveyor belt. 8

9 Top Cover Bottom Cover Rubber Interply Figure 2 Cross section of a multi ply conveyor belt. The belt ends are joined together by mechanical means or by vulcanising. The belt needs to be protected from damage of its sides that may occur due to sway of the belt. In case of belt carrying wet sticky material fixtures for cleaning the belt should be properly selected. Materials coming on to the return side of the belt needs to be arrested from coming under the end pulley by adequate scrapers. As shown in the Figure 4 belt cleaners are used to clean the return side of the conveyor belt. These cleaning belt can be of V shape as well. Figure 3 Belt scrapers As shown in the Figure 3(a), the pow blade's 20 leading edge "spirals" debris off the return-side belt surface. There are specially designed belt cleaners for the tail pulley, they are engineered to clean the inside of the belt so rocks, lumps and other debris never get the chance to damage the pulley, belt, splices or lagging. Normally plows are easy to install on virtually any conveyor structure, and come with universal mounting brackets. Though there are suppliers of such scrapers, these can be fabricated at the mine s workshop. The diagonal deflector plow (Figure 3a) is installed on a 45 angle across the belt to discharge debris to one side of the belt. In such cleaners the blade is installed in a fixed position, eliminating bouncing problems associated with some floating style plows. Twin-bladed plow ( Figure 3b) is designed to clean belt in both directions of travel. This cleaner discharges material to either side of the belt. It is installed securely to conveyor structure and can work on conveyors with severe belt vibration. CONVEYOR BELT FASTENERS One of the important components that require attention in maintaining services of belt conveyor is the conveyor belt fasteners. Selection of fasteners should consider required performance, expected life and ease of installation and maintenance so that higher availability of the belt conveyor is ensured. 9

10 There are different types of fasteners. Mechanical fasteners are commonly used in underground coal mining. The required rating of the mechanincal fasteners is calculated based on the required belt tension and it may be upto 263 kn/m of belt width. One such fastener is Flexco SR Scalloped Edge RAR8 belt fastener, designed for use on mainline and panel belts with mechanical fastener ratings up to 1,500 PIW (263kN/m). The 8-rivet pattern, along with the scalloped edge design, ensures quality performance in high-tension applications. Figure 4 Mechanical Belt joint Fasteners are available with self-setting rivets which allow to install quickly from the topside of the belt. Most important in belt fastening is to take care that the plates must be accurately positioned to get perfect edges and stress distribution should be uniform along the joints. Effectiveness of maintenance will be enhanced by provision of adequate and improved tools for carrying out the job. Management should encourage innovation of workers for making work tools or improvement of work environment and safety. Idlers In a conveyor belt installation different types of idlers or roller supports are used. The idlers are required for proper support and protection of the belt and proper support of the load being conveyed.idlers are designed with different diameters and are provided with antifriction bearings and seals, and are mounted on shafts. Frictional resistance of idlers influences the belt tension and consequently the power requirement. Idlers are mounted on a support frame, which can be shiftable or permanent. The carrying side of the belt is supported on the carrier rollers sets. A set of three rollers are arranged to form a trough for the troughed belt conveyor. The return side of the belt is supported on straight return idlers. The spacing of the idlers is determined based on the belt sag between the idlers. The sag depends on the belt tension, belt width, belt properties and the pay-load per meter of the belt. The idlers are specified by its length and diameter. These parameters are selected based on the required belt speed for the particular width of the belt. 10

11 Figure 5 Different types of roller supports Idlers are classified as shown in Table 2 Table 2 Idler Classification. Figure 6 Self aligning idler 11

12 Classification Diameter (inches) Belt Width (Inch) Description B Light duty B Light Duty C Medium Duty C Medium Duty C Medium Duty D Medium Duty D Medium Duty E Heavy Duty E Heavy Duty Closely spaced impact idlers with rubber cushions are used the support the belt at the loading point, below the skirt board. Figure 7. Figure 7 Components of belt conveyor Self aligning idler set is used infront of the loading point. This set of idlers can rotate on a horizontal plan depending on the belt sway and restores true running of the belt. Belt training idlers should be spaced 100 to 150 feet apart and at least one such idlers should be used on conveyors less thqan 100 feet long. Such idlers are not used in the areas of belt transition. Fixed guide rolls placed perpendicular to the edge of the conveyor belt are not generally recommended as they cause edge wear and reduce belt life. Offcentre running of the belt occurs when the belt loading is not proper. Garland type idlers assist true running of the belt under difficult loading conditions. In this type of idlers the idlers are connected to form a string. The connections are through universal 12

13 type coupling that allows each idlers to rotate about its own axis independently. Garland idlers are available as two roll, three roll or five roll units. Normally two-roll units are used as return idlers and three-rolls or five-rolls units are used in carrying idlers. Belt weight, material weight, idler load rating, belt sag, idler life, belt rating, belt tension and radius in vertical curves determine the spacing of idlers. Some suggested normal spacing are shown in Table 3. Table 3 Idler spacing Belt Width (inches) Troughed Idler Spacing Weigh of Material Handled (lbs/cu ft) Return Idler Pulley A conveyor belt system uses different types of pulleys like end pulley, snub pulley, bend pulley etc. as shown in the Figure 3. The end pulleys are used for driving and sometimes for making tensioning arrangements. Snub pulleys increase the angle of wrap thereby increasing the effective tension in the belt. The pulley diameter depends on the belt width and belt speed. Pulleys are used for providing the drive to the belt as well as for maintaining the proper tension to the belt. Minimum transition distance as shown in Table 4 should be followed while placing idlers in front of pulleys. 13

14 Table 4 Recommended minimum transition distance Troughing angle % Rated Tension Fabric Belt (for half trough depth). Fabric Belt (for trough depth) full Steel Cord Belt (for half trough depth). Steel Cord Belt (for ful trough depth) 20 0 >90 0.9b 2.0b b 1.6b 1.6b 3.2b <60 0.6b 1.2b 1.ob >90 1.6b 3.2b 3.4b 6.8b b 2.4b 2.6b 5.2b <60 1.0b 1.8b 1.8b 3.6b 45 0 >90 2.0b b b 8.0b b 3.2b 3.2b 6.4b <60 1.3b 2.4b 2.3b 4.4b Snub pulleys may be fitted in as shown in the following figures: Loose Snub Tight Snub Figure 8 The angle of wrap is increased by using tandem drive as shown in Figure below: Figure 9 Tandem drive 14

15 Drive Belt drive is provided normally at the discharge ends, however, it may be provided through the head end or through intermediate pulley by coupling the pulley shaft to the reducing gear s output shaft. The coupling is selected based on the load characteristics and applications. Flexible coupling or fluid couplings are often used. Various drive arrangements are shown in the Figures below: Take-up The purposes of take-up are: 1. To allow for stretch and shrinkage of the belt. 2. To ensure that the minimum tension in the belt is sufficient to prevent undue sag between idlers. 3. To ensure that the tension in the belt in the rear of the drive pulley is sufficient to permit such pulley to transmit the load. There are different types of take-up systems as shown in the Figures below: Figure 10 Screw take-up 15

16 16

17 Conveyor Support Figure 11 Gravity Take up The support of conveyor is normally a structural frame. Depending on the situation the structure can be mounted on floor or on skid. The main job of the support is to let the belt run without getting skewed. Depending on situations the support can be made moving type. In such cases idler a wheel mounted or crawler mounted platform keeps the necessary provision to support the idlers on which the conveyor runs. BELT CONVEYOR MAINTENANCE Belt conveyor system can serve economically provided it is maintained well. Maintenance of this system involves number of routine works. Inspection of the various components and initiating timely repair or servicing of these components in case any default is noticed is of premier importance. To maintain a belt conveyor system following items must be carefully looked into: Belt cleaners, Belt cleaners are required for both the carrying side and the non carrying side of the belt. Material falling on the non-carrying side of the belt needs to be arrested before it get traped in the tail pulley to cause damage to the lagging and sway of the belt. The carrying side needs cleaning when carried material is sticky in nature. To deflect the material coming on the non-carrying side deflector belt plows are used. Such plows can be selected from number of available sources or can be locally made considering that the rubber used in it should be softer than the surface of the belt conveyor. It must be easy to install and maintain. 17

18 Belt trainers Belt trainer is a device that helps true running of belt conveyor. It prevents off-center loading and belt mistracking. The belt trainers are also called self aligning idlers designed with a pivot and tilt action. They can immediately centre and control the misalignment problems of a running belt. Skirt clamps, Skirt clamps are used for eliminating loading point spillage at the conveyor loading and transfer points. The skirt rubber used for this should be selected considering its hardness with respect to the top cover of the belt. As it will be requiring adjustments after wear so the fixing arrangement should be simple, easily accessible and less time consuming. Pulley lagging Pulley laggings are necessary for better traction and to eliminate belt slippage. CONVEYOR BELT MAINTENANCE TOOLS To save time and money, and to simplify routine belt maintenance procedures, proper service and maintenance tools should be procured. Some examples are: MULTIPLE RIVET DRIVING TOOLS This tool is used for mechanical belt fastening. ( Flexco produces MSRT AND MSRT8 ).This easy-to-use, portable tool makes multiple rivet driving simple and fast. Some tools have drive rods that can drive five rivets simultaneously. BELT CUTTER Belt cutters are designed to enhance operator safety. Some belt cutters are chaindriven, double-edged safety blade and are shielded from operator contact. Such blade can be driven from either end of the cutter and cuts in both directions. Belt Protection Controls There are number of devices provided for protecting conveyor belt system. These are: 1. Belt alignement control: Belt should be 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 at the discharge and at the loading zones, however can be distributed over the length of the conveyor. Switches consists of Roller Switches, Limit Switches, Proximity Switches, Photoelectric Switches, etc. When the edge of the belt trips the alignement switch for a timed period, the conveyor is halted. In steel cord belts edge tracking is done by using alignment sensors. This helps to get signature of tension distribution within the carcass which in turn does the condition monitoring. 2. Belt Overload: The electric drive motor has overload protection. The motor overload can be a simple bi-metallic or melting eutectic alloy or a complex computer based motor-thermal model. Sometimes belt weigh scales are also used for overload protection. 18

19 3. Belt Slip: Belt slip is the loss in traction of the drive pulley(s) to the belt cover. Belt slip can destroy a belt or drive pulley. Belt slip protection provides a belt drive speed sensor that compares the present belt speed with the belt intended speed. For constant speed belts belt slip monitoring is done by a slip switch with a setpoint that trips the conveyor drive when the belt speed is below 80% of full speed. The belt slip switch is bypassed during starting and stopping. For variable speed conveyor belt slip monitoring consists of a speed sensor that measures the belt speed and compares with the speed reference sent to the drive system. When the belt speed is below 80% of the intended speed the conveyor drive is tripped. A method to adjust and test belt slip is normally provided in the belt control system. 4. Take-up Overtravel : Overtravel 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 overtravel switch trip may suggest a jammed conveyor belt condition. A bottom overtravel switch may indicate belt stretch, or a broken belt fabric flight. Excessive take-up motion during starting and stopping indicate that the type of drive control is either inadequate or that is not working properly. 5. Transfer Chute Plug: If the conveyor transfers material through a transfer chute, the transfer chute plug monitors blocked flow as blocked flow may damage the conveyor belt. Under blocked condition the transfer chute plug switch trips the conveyor drive. 6. Bin Level Control: If the conveyor belt is used to fill bins or stock piles over dumping may damage the belt. Bin level sensors protect the belt from probable beltdamage. Simple hanging tilt switch or analogue devices like ultrasonic, radar or laser devices may be used. 7. Pull-chord Stop Switches: These are emergency switches that can be actuated by pulling a chord provided along the belt conveyor. 8. Rip Detectors: Rip detectors provide belt protection in case of a rip or tear of the belt. Simple rip detector is spill switch located below the conveyor near the loading point. There are complex rip switches available, which require periodic maintenance. 9. Fire Detection: Thermal trip switches are provided with smoke sensors, CO sensors or fibre optic temperature sensors. 10. Dust Sprays: Water or chemical based dust suppressing system can eliminate or reduce dust problems in certain special conveyor belt installations Belt Controls The belt control involves the following: Stop/start: At single or multiple stations. Start warning: Audible or visual signal for pre-start warning Interlock: The sequence control between the master and slave conveyors. Telemetry: Distribution of belt control and information over significant distance. Lockout: Physical lockout of the all motive power sources. 19

20 Regulatory Provisions For use of belt conveyors in special applications like under ground coal mining state imposed regulations must be followed. DGMS circulars and CMR should be considered during installation, running and maintenance of belt conveyor in underground coal mines. Belt Conveyor Trouble Shooting (The number refers to the list below the table.) Complaint Cause Complaint Cause In order of probable occurrences In order of probable occurrences Belt runs off at tail pulley Entire belt runs off at all points of the line One belt section runs off at all points of the line Excessive wear, including rips, gouges, ruptures, and tears 26, 17, 15, 21, 4, 16 Excessive bottom cover wear 2, 11, 1 Excessive edge wear, broken edges 12, 25, 17, 21, 8, 5 21, 14, 5, 19, 20, 22 26, 4, 17, 8, 1, 21 Belt runs off at head pulley 15, 22, 21, 16 Belt hardens or crack 8, 23, 22, 18 Belt slip 19, 7, 21, 14, 22 Covers become checked or brittle Belt slip on starting 19, 7, 22, 10 Longitudinal grooving or cracking of top cover Excessive belt stretch 13, 10, 21, 6, 9 Longitudinal grooving or cracking of bottom cover 8, 18 14, 21, 22 14, 21, 22 Belt breaks at or behind fasteners; fasteners tear loose 2, 23, 13, 22, 20, 10 Fabric decay, carcass cracks, ruptures, soft spots in belt(gouges) 12, 20, 5, 10, 8, 24 Vulcanised separation splice 13, 23, 10, 20, 2, 9 Ply separation 13, 23, 11, 8, 3 1. Belt bowed: Avoid telescoping belt rolls or storing them in damp locations. 2. Belt improperly spliced or wrong fasteners: Use correct fasteners. Retighten after running for a short while. If temporarily spliced remove belt splice and make new splice. Set up regular inspetion schedule. 3. Belt speed too fast: Reduce belt speed 4. Belt strained on one side: Allow time for new belt to break in. If belt does not break in properly or is not new, remove strained section and splice in a new piece. 20

21 5. Breaker strip missing or inadequate: When service is lost, install proper breaker strip ( Breakers are woven fabric of nylon &/or polyster placed above the carcass to dissipate impact energy and to prevent belt puncture) 6. Counterweight too heavy: Recalculate weight required and adjust counterweight accordingly. Reduce take-up tension to point of slip then lighten slightly. 7. Counterweight too light: Recalculate and adjust or screw take-up accordingly. 8. Damage by abrasive, acid, chemicals, heat etc: Use belt designed for specific purpose. Repair cuts. Use enclosure where needed. 9. Differential speed on dual pulleys: Make necessary adjustments 10. Drive underbelted: Recalculate maximum belt tensions and select correcxt belt. Determine if extension is feasible or another belt to be installed. Check the carcass is rigid enough for load, replace if service is lost. 11. Edge worn or broken: Repair edge. If the edge is out of square or badly damage replace the worn portion of the belt. 12. Excessive impact of material on belt or fasteners: Use correctly designed chutes and baffles. Make vulcanised splices. Instal impact idlers. Where possible load fines first. Where material is trapped under skirts, adjust skirtboards to minimum clearance or install cushioning idlers to hold belt against skirts. 13. Excessive tension: Recalculate and adjust tension. Use vulcanised splice within recommended limits. 14. Frozen Idlers: Free idlers. Lubricate. Improve maintenance. Don t over lubricate. 15. Idlers or pulleys out of square with centre line of the conveyor: Realign. Install limit switches for greater safety. 16. Idlers improperly placed: Relocate idlers or insert insert additional idlers spaced to support belts. 17. Improper loading, spillage: Feed should be in direction of belt travel and at belt speed, centred on the belt. Control flow with feeders, chutes and skirtboard. 18. Improper storage or handling: Refer to the manufacturer s instructions for storage and handling. 19. Insufficient traction between belt and pulley: Increase wrap angle with snub pulley. Lag drive pulley. In wet condition use grooved lagging. Install correct cleaning devices for safety. 20. Material between belt and pulley: Use skirtboard properly. Remove accumulation. Maintain properly. 21. Material build-up: Remove accumulation. Install cleaning devices, scrapers and inverted V decking. Improve housekeeping. 22. Pulley lagging worn: Replace worn pulley lagging. Use grooved lagging for wet conditions. Tighten loose and protruding bolts. 21

22 23. Pulleys too small: Use large diameter pulleys. 24. Radius of convex vertical curve too small: Increase radius by vertical realignment of idlers to reduce excessive edge tension. 25. Relative loading velocity too high or too low: Adjust chute or correct belt velocities. Use impact idlers. 26. Side loading: Load in the direction of belt run and at the centre of the belt. 27. Skirts improperly Placed: Use adjustable skirtboards and check that it does not rub the belt. Armoured Face Conveyers for Underground Coal Transportation Underground coal conveyers widely employed in collieries relate to the group of conveyers with low flights, but differ substantially from the conveyers used in other industries. They perform many various operations in collieries and accordingly have a large number of design versions. Figure 12 Armoured face conveyor An underground coal conveyer has one (two or three) hauling chain with low flights attached to it. The chain along the bottom of an open trough draws the flights. The chain bends around terminal (drive and take-up) sprockets at the ends of the conveyer and is moved by a drive. The conveyer is mounted directly on the ground of a mine drift. One of the principal requirements to underground coal conveyers is that their height, and especially that of the carrying run, must be as low as possible in order to minimize the loading height. Depending on the scheme of arrangement of the tractive element (of the carrying and return run), it is distinguished between vertically closed conveyers ( a to d in the above figure), horizontally closed (e), and those with a combined path. In the last 22

23 type, the terminal sprockets are arranged at an angle of 45 0 to the horizontal. The flights are attached to the chain at an angle of 45 0 and are oriented in the horizontal plane in the carrying run and in the vertical plane, in the return run. Vertically closed conveyers are made (with one, two or three parallel hauling chains (a, b, and c respectively). A three-chain conveyer may have a greater length and higher reliability, since breakage of. a chain does not disturb the stability of flights and the operating capability of the conveyer. Their application is however limited by their higher cost and large mass. In single-chain conveyers, the flights may have symmetrical (the chain at the centre of flight) or unsymmetrical and rigidly fixed or hinged attachment to the hauling chain. The path of a vertically closed conveyer for coal transportation in curved drifts may be either straight (horizontal, inclined or horizontal-inclined) or bending in the horizontal plane (spatial). The tractive element is in the form of one, two or three detachable chains of elevated strength with the pitch 80 mm or round-link chains with the chain pitch of 50 or 64 mm and rod links. of a diametet of 14 or 18 mm. The chains are made of steel of specified grade and heat-treated. The flights are made of steel and have a height roughly equal to the height of chain. The pitch of flights is 480,640 or 1024 mm, and the linear mass of a chain with flights is kg/m on the average. The trough is stamped from steel sheets (steel grades 35 or 30f) in sections of a length of m; the working portion (bottom) is hardened. The drive is electrical, pneumatic or electro-hydraulic in an explosion-safe embodiment. Drives with one or two motors at the front end and one motor at the rear end of a conveyer have found wide application. The transmission mechanism is of the reducer type; the electric motor is connected with the reducer gear through a hydraulic coupling. Topics for discussion in the classroom 1. Steep Angle conveying and their applicability in underground use. 2. Belt tension and power calculation 3. Conveyor accessories 4. Practical problems and innovative solutions 5. Information System for Bulk Solid Handling 23

24 Conveyor Calculation Initial Data: 1. Characteristic of material like lumpiness, density, moistue content, angle of repose, particle flowability, abrasiveness, particle strength, slumping (losing of mobility on long storage, e.g. lime, cement etc.), stickyness etc. 2. The average and maximum load carrying capacity 3. Diagram of the designed conveyor with its principal dimensions 4. Data on the exceptional working conditions 5. Arrangements of the operation of the conveyors. Determining Belt Width The belt width depends on maximum rated capacity and lump size. The cross sectional profile of the bulk material on the conveyor belt is determined by the profile of the load carried on the belt. On straight roller this area is : 1 A = bh 2 h = 0. 5btanφ A = 0. 25b 2 tanφ where, b: width of the base of material on the belt h:height of material on the belt φ: Angle of repose A: area of cross section of the material On an inclined conveyor this area will be reduced depending on the angle of inclination of the belt, mobility or particles and angle of repose. For a belt running over the troughed idler the cross sectional area of the material is the sum of the trapezium and the circular sector as shown in Figure: 24

25 The area is given as: l l A b = m sin β 2 A s πr α = r sin 2α 2 where, the symbols are as illustrated in the figure. Total area is the sum of A b and A s. Capacity of Conveyor in te/hr Q = 3600.A.v.ρ (A) If belt width is B and area of cross section of material is A, it is often taken that: 2 B A = (B) 10 Thus from equation (A) and (B) the belt width can be calculated. To test the calculated belt width for suitability with lump size the following relation is used: B > Xa

26 where X= coefficient of lump size= 3.5 for graded material =2.5 for ungraded material a= maximum linear size of the represntative lump B is selected from the available width as: 400, 500,650, 800, 1000,1250, 1400, 1600, 2000, 2500, 3000 mm. Idler Spacing The spacing of idlers depends on belt width and bulk density of material. This is calculated from : 2 S W H = 8T where, H: sag, in feet S: Spacing in feet T: Tension in pound W: Weight of the loaded belt in lb/ft. Idler spacing in the mid portion of the conveyor in meter for different material is given in the following table. Belt Width mm Density t/m 3 <0.8 Density t/m Density t/m Density t/m Density t/m 3 > The return idlers are spaced at 2 to 2.5 times the spacing of carryng idler subject to the condition that it is not more than 3.5 m. 26