The most commonly used type of carrying idlers used for handling bulk load consist of three in line idler rolls of equal length. The three equal length roll troughing idlers form the belt into the best troughed shape to carry a maximum load cross section.For handling unit load or for handling nominal bulk load, or for supporting belt in return side, straight idlers are used, positioned between brackets attached directly to the conveyor frame. Another type of carrying idlers is used at the loading points where the lump size and the weight of the material ma seriously damage the belt if the belt were rigidly supported. Such idlers are called impact idlers. The most frequently used type of impact idlers consist of a three roll assembly, each roll being made of spaced resilient discs. These idlers are also known as cushion idlers.
Troughed Belt Conveyor & Flat Belt Conveyor
Troughed belt conveyor is that in which the belt forms a trough on the carrying side while running over idler rollers which are either in set of 5 rolls, 3 rolls or 2rolls. The troughing angle adopted are: 15°, 20° , 25° , 30° , 35° , 40° , 45° . Return idlers are usually straight roller type. Transverse flexibility or rigidity of the belt is another significant consideration. It is important that the belt trough properly. The empty conveyor belt must make sufficient contact with the center roll in order to track properly.
Application OF BELT CONVEYOR 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: 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. 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. 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. By the use of many forms of ancillary equipment such as mobile trippers or spreaders bulk material can be distributed and deposited whenever required. Many other functions can be performed with the basic conveying like weighing, sorting, picking, sampling, blending, spraying, cooling, drying etc. 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. 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. It can be horizontal, incline or decline or combination of all. Minimum labour is required for the operation and maintenance of belt conveyor system. 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.
Suitability of a Belt Conveyor(TABLE-I) Rise in m per 100 Material AngleMaterial Angle m Rise in m per 100 mCement-Loose 22° 40.4 Coal-Bit Slack 23° 42.4Clay-Fine Dry 23° 42.4 Coal-Anthracite 16° 28.6Clay-Wet 18° 32.5 Coke oven run 18° 32.5Coal-Mine Run 16° 28.6 Coke-Sized 16° 28.6Coal-Sized 16° 28.6 Coke-Breeze 18° 32.5Concrete-Wet 15° 26.8 Ore-Sized 18° 32.5Earth-Loose 20° 36.4 Packages-Paper 16° 28.6 Wrap.Glass-Batch 21° 38.4 Rock-Fine Crushed 22° 40.4Grain 16° 28.6 Rock-Mixed 18° 32.5
Suitability of a Belt Conveyor(Contd..)Material Angle Rise in m per 100 Material Angle Rise in m per 100 m mGravel-Bank Run 18° 32.5 Rock-Sized 18° 32.5Gravel-Screened 15° 26.8 Salt 20° 36.4Gypsum-Powdered 23° 42.4 Sand-Dry 15° 26.8lime-Powdered. 23° 42.4 Sand-Damp 20° 36.4limestone 18° 32.5 Sand-Tempered 24° 44.5 FoundryOre-Fine 20° 36.4 Sulphur-Powdered 23° 42.4
Maximum safe inclination of BELTCONVEYOR: THE 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. 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: 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. Shiftable: 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 70-800. 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.
Troughed belt conveyors are used for higher capacity, higher speed requirement, and for handling bulk material oflarge lump size. It suitable for inclined or declined type conveyors.Transition distance is defined as the distance from the center line of the first fully troughed idler roll to the centerline of either the head or tail pulley. The distance from the pulley to the top of the wing idler is certainly greaterthan the distance from the pulley to the center roll of the troughing set. If the transition distance is too short, theedge of the belt can be over stretched. This will adversely affect the load support and belt life.
Flat belt conveyor is that in which the belt runs flat on the carrying side overstraight roller type idler or a set of idlers. Flat belt conveyors are suitable forlower capacity, low speed requirement and for handling unit goods or bulkmaterial of small lump size and higher angle of repose. Declination isundesirable.
Belt Conveyor(Details) A belt conveyor system consists of an endless belt of resilient material connected between two flat pulleys and moved by rotating one of the pulleys by electric motor. Normally material is fed on the belt near the other end pulley. The moving belt carrying the material towards the driver pulley is likely to sag between the two end pulleys due to its self weight and pay load. That is why the belt is supported both on the carrying side and the return side by a number of rollers, called the idlers. As the belt is always under tension, it is prone to elongation resulting in slackness over the pulleys and loss of tension and power. That is why some kind of device for tensioning the belt is incorporated in the system. This tensioning device is known as take up arrangement. The system is inherently very simple. Belt conveyors have attained a dominant position in transporting bulk materials due to a number of inherent advantages like the economy and safety of operation, reliability and practically numerous processing functions while supporting a continuous flow of material between operations. Low labor and low energy requirements are fundamental with belt conveyors as compared with other means of transportation. The reliability of belt conveyors has been proved over decades and in every industry. Belt conveyors are environmentally more acceptable than other means of transport. They neither pollutes the air nor deafens the ears. Conveying of a wide variety of materials is possible with belt conveyor, and belt conveyors are capable of transporting at a hourly rate much in excess of any practical requirement. Belt conveyors can deliver materials at a great distance from the loading point covering all odd terrain.
Application of Belt Conveyor: Belt conveyors are employed to convey a great variety of bulk materials and also unit loads along a horizontal or gently inclined paths. Such duties can be performed within the factory shed or in outdoor work. Within the factory shed belt conveyors are also employed to carry articles of light weight in line production from one operation to another. Belt conveyors are used to mechanize material handling operations in foundries to distribute moulding sand, mould cores and cast articles. Belt conveyors are used for carrying coal, ores, and minerals in power plants, mining industries, and metallurgical process plants. For handling food grains and building materials belt conveyors are frequently employed. Capacities of a belt conveyor system may range from 500 to 5000 m3 / Hr. and more. The distance covered for a belt conveyor system may be as small as 50m (for indoor work) to 5 Km for supplying raw material from mine area.
Belt Conveyor Drive Arrangement | BeltConveyor Design Calculation
Drive arrangement: In belt conveyors the driving power is transmitted to the belt by the driving pulley which is rotated by an electric motor. The basic mechanism of transmission of power from the pulley to the belt is based on the theory of friction drive. The fundamental equation for a belt conveyor drive is given by: (The Euler’s equation) T1 ≤ T2.eµα Where, T1 and T2 are the tight side and slack side tensions of the belt at the driving pulley α = wrap angle of the belt in radian e = Naperian base µ = Friction factor The peripheral effective pull TE from a driving pulley, neglecting losses on the driving pulley due to belt stiffness is determined from the following reaction: Te = T1 – T2 T2min > T E Max (1 / (eµα -1)) Where, T E Max is the maximum effective peripheral pull in N, which often occurs when starting up or when braking the completely loaded conveyor. In other conditions Te is the average effective pull. Maximum effective pull is usually 20% to 50% more than the average effective pull, depending on the type of motor starter and coupling.
Essential components of a beltconveyor: Belt conveyors consists of a driving pulley at the head end, and take up pulley at the tail end, both being accommodated in a steel frame. An endless belt connects the two pulleys on both runs and supported by suitable roller type supports, called the idlers. At the upper side of the conveyor also called the carrying side, because the pay load is carried on this side, the belt is supported by idler. At the other side also called the return side because usually there is no pay load, the belt is supported by idlers. In some designs the belt is supported by a stationery runway instead of idlers. In some special installations both the upper and lower sides transport loads (usually unit load) simultaneously in opposite directions. The driving pulley receives its rotation from drive unit. The material to be conveyed is loaded on the belt by one or more feed hoppers mounted over the conveyor belt line normally placed at the tail end side. The load conveyed by the moving belt is discharged over the driving pulley into discharge spout. However discharge can be arranged at any point along the conveying run by means of special discharge devices. At the tail end side a device called the take up arrangement is attached with the take up pulley to keep the belt under a minimum tension so that the belt does not slip over the pulleys due to extension of its length. Belt cleaner removes material adhering to the outer belt face. It is usually located near the driving pulley.
Idlers:Conveyor belts are usually supported on idler rollers. In some cases they are supported by solid wood or runway ofsteel sheet or a combination support comprising of the two types of supports placed alternatively.There are two basic types of idlers:Straight, Carrying idlers & Return idlers are used for supporting active side of the Belt for a flat belt Conveyor andalso for supporting the return belt in flat orientation in both flat or troughed belt conveyors.
Parts of Belt Conveyors A) CONVEYOR BELT:Belt which supports and convey the load , is the essential and most important component. Most common type of Belt Conveyor belting is rubber , Plastic –the internal caracass of woven fabric gives longitudinal strength of loaded pulling belt & transverse strength for supporting the load and the cover of the belt/or plastics protect the caracass from damage. Belt Construction: Cotton fabric ply constructed rubber covered belting is the mostly used belt for flat and troughed conveyor. The caracass consists of one or more plies of woven fabric or of solid woven fabric impregnated with a rubber or plastic mix, which after vulcanization binds the plies together. The carasass is covered by special wear and impact resisting rubber compounds/plastics. A layer or layers of open- mesh or cord fabric , termed as breaker may be placed between between the cover and Caracass.
Parts of Belt Conveyors(Contd..) Belt Designation: As per IS 1891(Part I) 1994,belts are designated by IS no., Grade of the cover, the type of belting defined by the full thickness breaking strength, in KN/m and number of plies. Belt Width: unless otherwise agreed between the manufacturer and buyer, the standard widths of belting as per IS specification are: 300, 400,500, 600, 650, 800, 1000, 1200, etc with a tolerances of +/- 5mm/1mm. Belt Splicing: Two ends of a belt may be joined either by metallic belt fasteners or by Vulcanization.(for Troughed belt)
Basic Power Requirements The horse power, hp, required at the drive of a belt conveyor, is derived from the pounds of the effective tension, Te, required at the drive pulley to propel or restrain the loaded conveyor at the design velocity of the belt V, in fpm: hp=Te . V/33, 000 To determine the effective tension, Te, it is necessary to identify and evaluate each of the individual forces acting on the conveyor belt and contributing to the tension required to drive the belt at the driving pulley. Te is the final summarization of the belt tensions produced by forces such as:
1.The gravitational load to lift or lower the material being transported. 2.The frictional resistance of the conveyor components, drive, and all accessories while operating at design capacity. 3.The frictional resistance of the material as it is being conveyed. 4.The force required to accelerate the material continuously as it is fed onto the conveyor by a chute or a feeder.
Parts of Belt Conveyors(Contd..) B) IDLER: the rollers used at certain spacing for supporting the active as well as return side of the belt are called idlers. Accurately made, rigidly installed and well maintained idlers are vital for smooth and efficient running of belt conveyor. While designing a idler please note about IDLER CONSTRUCTION , IDLER DIMENSIONS & IDLER SPACINGS. C) CONVEYOR PULLEYS: 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 should be followed while placing idlers in front of pulleys. D)DRIVES FOR BELT CONVEYORS E) TAKE UPS OR BELT TENSIONING DEVICES-endless Belt conveyor after being threaded through the entire length of the conveyor needs to be tightened so that sufficient friction force is developed between the drive pulley and the belt to make the belt move.
Parts of Belt Conveyors(Contd..) F) LOADING & UNLOADING DEVICES G) 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 trapped 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. H) TRAINING IDLERS I) CONVEYOR STRUCTURE J) TRANSFER TERMINALS
Aspects of Belt Conveyor Design The major points in selection and design of a belt conveyor are: Checking/determining capacity of a Conveyor. Calculating Maximum belt Tension required to convey the Load and selection of belt. Selection of Driving pulley Determination of motor Power. Selection of Idler and its Spacings.
Conveyor Calculation Initial Data: Characteristic of material like lumpiness, density, moisture content, angle of repose, particle flow ability, abrasiveness, particle strength, slumping (losing of mobility on long storage, e.g. lime, cement etc.), stickiness etc. The average and maximum load carrying capacity Diagram of the designed conveyor with its principal dimensions Data on the exceptional working conditions 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 : where, b: width of the base of material on the belt h:height of material on the belt f: 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.
The area is given as: where, the symbols are as illustrated in the figure. Total area is the sum of Ab and As. Capacity of Conveyor in te/hr (A) If belt width is B and area of cross section of material is A, it is often taken that: (B) 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: where X= coefficient of lump size= 3.5 for graded material =2.5 for ungraded material a= maximum linear size of the representative 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 : where, H: sag, in feet S: Spacing in feet T: Tension in pound
The return idlers which carry the weight of the empty belt in the return side orthe lower side of the conveyor are mostly single roller straight idler. The maindimensions of the idler are the diameter and length. The diameters inmillimeters of carrying and return idlers shall be selected from the following:63.5, 76.1, 88.9, 101.6, 108, 114.3, 127, 133, 139.7, 152.4, 168.3, 193.7.
Belt Conveyor Trouble ShootingComplaint Cause Complaint Cause In order of probable In order of probable occurrences occurrencesBelt runs off at tail pulley 7 15 14 17 21 Excessive wear, including 12, 25, 17, 21, 8, 5 rips, gouges, ruptures, and tearsEntire belt runs off at all 26, 17, 15, 21, 4, 16 Excessive bottom cover 21, 14, 5, 19, 20, 22points of the line wearOne belt section runs off 2, 11, 1 Excessive edge wear, 26, 4, 17, 8, 1, 21at all points of the line broken edgesBelt runs off at head 15, 22, 21, 16 Belt hardens or crack 8, 23, 22, 18pulleyBelt slip 19, 7, 21, 14, 22 Covers become checked 8, 18 or brittleBelt slip on starting 19, 7, 22, 10 Longitudinal grooving or 14, 21, 22 cracking of top coverExcessive belt stretch 13, 10, 21, 6, 9 Longitudinal grooving or 14, 21, 22 cracking of bottom coverBelt breaks at or behind 2, 23, 13, 22, 20, 10 Fabric decay, carcass 12, 20, 5, 10, 8, 24fasteners; fasteners tear cracks, ruptures, soft spotsloose in belt(gouges)Vulcanised 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 inspection 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. 5)Breaker strip missing or inadequate: When service is lost, install proper breaker strip ( Breakers are woven fabric of nylon &/or polyester 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 under belted: Recalculate maximum belt tensions and select correct 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.
Regulatory ProvisionsFor use of belt conveyors in special applications like under ground coal miningstate imposed regulations must be followed. DGMS circulars and CMR shouldbe considered during installation, running and maintenance of belt conveyor inunderground coal mines 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. Install impact idlers. Where possible load fines first. Where material is trapped under skirts , adjust skirt boards 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 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 skirt board. 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 skirt board 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. 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 skirt boards and check that it does not rub the belt.
Belt Protection Controls There are number of devices provided for protecting conveyor belt system. These are: 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. 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. 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.
Belt Protection Controls(contd) 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 belt condition. A bottom over travel 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. 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. 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 belt damage. Simple hanging tilt switch or analogue devices like ultrasonic, radar or laser devices may be used. Pull-chord Stop Switches: These are emergency switches that can be actuated by pulling a chord provided along the belt conveyor. 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.
Belt Protection Controls(contd.) Fire Detection: Thermal trip switches are provided with smoke sensors, CO sensors or fibre optic temperature sensors. Dust Sprays: Water or chemical based dust suppressing system can eliminate or reduce dust problems in certain special conveyor belt installations 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.