Conveyor belts have been used since a long time for the transport of the bulk materials and the unit loads. They have proved their utility everywhere since belt conveyor installations can be adapted to meet nearly all local conditions. They are economical as well safe in working.
The demand for ever increasing capacities and ever longer conveying lengths has accelerated the development of the belt conveying technique. In this respect, new materials are being developed, and new conveying systems are being planned and tested especially those which are having regards for the environmental requirements. In addition to this, since every conveying problem is different, it needs careful planning and selection of the right elements of the conveying system in order to achieve the optimum conveying capacity in an economical way.
Further, if excessive emphasis is placed on the safety aspects then the cost of the conveying system increases which sometimes becomes difficult to justify in terms of the performance. On the other hand, if too much emphasis is placed on the economy, then it results into shortening of the life of the conveyor belt resulting into increase in the operational costs.
The conveyor belts play a major part in the whole belt conveying system and have to overcome several and varied stresses. They alone constitutes around 15 % to 50 % of the total cost of the belt conveying system, hence greater attention is needed to be paid for the design and the selection of the conveyor belts, so that they are perfectly suited to their tasks and are as economical as possible during the operation of the belt conveyor system.
Conveyor belt is the most important element of a belt conveyor installation. The conveyor belt fulfills the task of absorbing the stresses developed at the drive start up, transport the load, absorbs the impact energy at the loading point, withstand temperature and chemical effects (heat, oil and acidity etc.), and meet safety requirements (flame resistance and anti-static etc.). The selection of the conveyor belt specification depends on its application. Design considerations of the conveyor belt affect the power demand, belt curves, and transitions etc. The use of synthetic or natural rubbers and the choice of reinforcements for handling different materials, and for meeting different service conditions impact the design as well as the life of the conveyor belt.
The conveyor belt development has taken place in the direction of (i) optimization of the conveyor belt reinforcing materials i.e. for exploiting to their maximum strength limits to achieve optimum working life economy, (ii) optimum dimensioning of installation components e.g. pulley diameters, idlers, bearings and shafts, troughing transitions, drive and take-up systems, (iii) adaptation of cover quality for each duty i.e. the quality of the cover to provide optimum solution at the most economical cost, and (iv) production of the conveyor belts and their installation with the environment in mind.
Until the mid 1970s development and technology of the conveyor belt was concentrated on the search for appropriate materials for the belt and in the solving of the drive problems. In the first instance transmission of traction played a part. As the demand grew for conveyors of larger capacity and longer length, additional requirements affecting the belt had to be considered and studied such as greater work load, elongation, slit resistance, and endless splice jointing.
Selection of the right conveyor belt construction, carcass and cover is based on the specific needs of the particular conveyor system, the material being conveyed, and the conditions under which it is to operate, as well as its cost effectiveness. Some key considerations involved are (i) maximum operating tension (working tension) also known as breaking strength, (ii) minimum pulley diameters, (iii) troughability and transverse rigidity, (iv) load support, (v) transition distance, (vi) impact rating, (vii) covers, and (viii) cost per unit handled.
Conveyor belt has four components namely (i) carcass or carcase which consists of textile plies, steel weave, or steel cord, (ii) covers which are made up of either rubber or PVC (polyvinyl chloride) of different qualities, (ii) additional components of the conveyor belts which are as needed and are normally edge protection, impact protection, and longitudinal slitting prevention etc., and (iv) special construction elements which are the profiles on steep incline belts, cleats, or corrugated edges etc. The schematic cross section of a plied conveyor belt is shown in Fig.1
Fig 1 Schematic construction of a plied conveyor belt
The reinforcement found on the inside of a conveyor belt is normally referred to as the ‘carcass’ or ‘carcase’. In a sense, the carcass is the heart of the conveyor belt since it (i) provides the tensile strength necessary to move the loaded belt, (ii) absorbs the impact of the impinging material being loaded onto the conveyor belt, (iii) provides the bulk and lateral stiffness needed for the load support, and (iv) provides adequate strength for proper bolt holding and / or fastener holding.
The carcass of a conveyor belt can be made from various materials and in different construction. The most commonly used are textile ply carcasses and steel cord. Textile ply carcasses are with one or more textile plies. The maximum numbers of plies can be upto 6 numbers. In mono-ply belts, a PVC impregnated textile carcass is used. Depending on tensile strength and duty, the carcass fibres are in polyester, polyamide, or aramid.
Each belt is rated as to its strength which is the amount of pulling force that it can withstand. The strength of a belt (or more accurately, the tension it is able to withstand) is rated in the USA in ‘pounds per inch of width’, commonly abbreviated as PIW. In other parts of the world, the belt is rated in ultimate breaking strength in the metric units of newtons per millimetre (N/mm) or kilo-newtons per meter (kN/m). The strength rating is a function of the reinforcement included in the carcass of the belt and the number of, and type of, material in the fabric plies, or, if it is a steel-cord belt, the size of the cords. The top and bottom covers of a belt provide very little of the strength or tension rating of the belt.
Each conveyor structure needs a belt with a specific tension rating. The strength of the belt, either carcass tension rating or ultimate breaking-strength, represents the amount of force which can be applied to the belt. Putting greater demands in the form of material load, take-up weight, and incline gravity against this belt cause severe problems, including the possibility of breaking of the belt. The higher the rated tension of the belt, the more critical becomes the compatibility of the belt with the structure and rolling components.
The carcass is normally rated by the belt manufacturer in terms of the ‘maximum recommended breaking strength’ permissible in kilo-newtons per metre (kN/m). Similarly, the manufacturer rates the finished belt in terms of the ‘maximum recommended breaking strength’ per metre of width (which is the total of the preceding, multiplied by the number of plies in the belt construction). Thus there is a relationship between the recommended maximum breaking strength per metre of width of the belt and the ultimate tensile strength (breaking strength) of the belt.
The conveyor belt manufacturer determines the maximum recommended breaking strength per metre of width with considerations given to (i) stretch characteristics of the belt, (ii) fastener / bolt holding capability, (iii) load characteristics, (iv) stiffness, and (v) impact resistance of the belt. The normal carcass design is made up of layers or plies of woven fabrics bonded together. This ‘conventional plied’ belt construction, normally uses a plain weave or twill weave carcass which is built up into as many layers as is needed to provide the necessary belt strength, normally bound together with rubber.
In the carcass of steel weave type, the transmission of force is by means of the longitudinal steel cords laid to one another in the same plane. Above this carcass is a transverse layer also of steel which is held in place by a polyamide binder cord. In carcass with steel cord, the belt force is transmitted through steel cords of the appropriate strength. Steel cords are transversely bound together by an intermediate layer of rubber only. Transverse elements serve to prevent impact damage or longitudinal slitting.
In the plain weave, the warp yarns (lengthwise yarns) and the fill yarns (crosswise yarns) pass over and under each other. This means that both members are crimped (essentially, each assumes a sine-wave-like configuration). This fact, plus the basic characteristics of the fibre used give the belt its stretch characteristics.
Conventional plied carcass belts are being used for a long time. Hence, it is the most common belt design used today. When cotton and similar materials were widely used as carcass components in plied belts, a breaker strip, an additional layer of open weave fabric, was added between the carcass and the top cover for heavy abuse constructions, helping absorb the loading impact. The switch to modern synthetic carcass materials (like polyester and nylon) has essentially eliminated the need for the breaker strip. Today, breaker strips are seldom used in plied belt constructions except in extreme impact applications.
The five common types of belt carcasses are namely (i) mono-ply belt, (ii) multiple-ply belt, (iii) reduced-ply belt, (iv) solid woven belt, and (v) steel cord belt.
Mono-ply belt – Mono-ply belt has single ply carcass. The one-ply carcass provides over 2000 kN/m tension. It has low elongation which allows short take-up strokes. Single layer carcass structure provides optimum troughing as well as provides high bendability hence, allowing small-diameter pulleys. Besides high bendability, the belt has impact-resistance. The thin carcass allows thick cover rubber layers, extending belt life span. Fig 2 shows mono-ply carcass.
Fig 2 Multi-ply and mono-ply belt carcass
Multiple-ply belt – Multiple-ply belt is normally made up of two or more plies, or layers, of woven cotton, rayon, or a combination of these fabrics, bonded together by an elastomer compound. Belt strength and load support characteristics vary according to the number of plies and the fabric used. The multiple-ply conveyor belt was the most widely used belt through the mid 1960s, but today it has been replaced by reduced-ply belt. Fig 2 shows multi-ply carcass.
Reduced-ply belt – Reduced-ply belts consist of carcasses with either fewer plies than comparable multiple-ply belts or special weaves. The features of reduced-ply belt include (i) straight warp ply carcass structure which allows designs to provide higher tension than ordinary multi-ply belts, (ii) provides high bendability and impact resistance, and (iii) belts can be easily joined.
The straight warp carcass design yields a carcass construction wherein the basic lengthwise (warp) yarns are essentially uncrimped. These are the main load-carrying tension yarns. Fill yarns are then laid transversely and alternately, above and below the main tension yarns. This construction gives greater dimensional stability to the belt, and does employ a ‘beam’ effect for better load support and transverse rigidity.
The yarns used are much thicker than yarns in the conventional fabrics. Further, they are locked together by means of another series of lengthwise yarns, known as the binder warp system. The binder warp system locks the tension and fills cords tightly together creating a belt which is unusually tough and which has exceptional tear and impact resistance, as well as good fastener and bolt holding ability.
In majority of the cases, the reduced-ply belt depends on the use of higher-strength synthetic textile fibres concentrated in a carcass of fewer plies to provide higher unit strength than in a comparable multiple-ply belt. The technical data available from belt manufacturers normally indicate that reduced-ply belt can be used for the full range of applications specified for multiple-ply belt. Fig 3 shows reduced ply conveyor belt. Fig 3 shows reduced ply conveyor belt and the straight warp.
Fig 3 Reduced ply conveyor belt
Solid-woven belt – This type of belt consists of a single ply of solid-woven fabric, normally impregnated and covered with PVC with relatively thin top and bottom covers. The surface of PVC belts is frequently rough on purpose to aid in conveying on inclines, but the rough surface makes belt cleaning more difficult. The abrasion resistance of PVC is lower than rubber, so some solid-woven belts are made with a combination of PVC core and rubber covers.
Steel-cord belt – Steel-cord conveyor belts are made with a single layer of parallel steel cords completely embedded in the rubber as the tension element. The carcass of steel-cord belt is available in two types of construction. The all-gum construction uses only the steel cords and rubber while the fabric-reinforced construction has one or more plies of fabric above and / or below the cords but separated from the cords by the cord rubber. Both types have appropriate top and bottom covers.
Steel-cord belt utilizes a single layer of uniformly tensioned steel cords as tension members, encased in rubber. Steel-cord belts are normally found in high tension and / or long distance and large capacity applications and / or where extremely low stretch is a necessity. These tensions typically range from ST500 kilo-newton per meter (kN/m) to ST5400 kN/m and higher. Typical elongation for steel cord conveyor belt is less than 0.25 %. Low elongation allows short take-up strokes. Small diameter pulleys can be used in steel-cord belt construction.
Steel cord construction with breaker(reinforcing fabric) (i) prevents length-wise tearing by foreign substances or sharp objects, (ii) prevents broken steel cord from protruding through cover rubber, (iii) provides high withdrawal strength (in stationary tests), and (iv) reduces risks for belt breakage by impact.
Steel-cord belts are to be manufactured to width. There are mainly two different types of cords. One is a 7×7 cord for low to mid tension and the other is a 7×19 cord for mid to high tension. These two types of cord enable rubber to fully penetrate to the centre core. If rubber does not penetrate to the core, the cord can corrode from small cuts or at the belt ends.
Steel-cord belt is produced using a broad range of cord diameters and spacing, depending primarily on the desired belt strength. Steel-cord belt is frequently used in applications needing breaking strengths beyond the range of fabric belts. Another application is on conveyors where, due to limitations in the distance the take-up system can travel, the belt cannot be allowed to stretch significantly. Fig 4 gives conveyor belt with steel cod construction.
Fig 4 Conveyor belt with steel cord construction
The rubber between plies is called a skim. Skims (Fig 2) are important contributors to internal belt adhesions, impact resistance, and play a significant role in determining belt load support and troughability. Improper or marginal skims can adversely affect belt performance in general and can lead to ply separation and / or idler junction failure.
Materials for carcass
Carcass materials used in belt manufacture in recent years are given below. The information includes common name, composition, and some general comments about each material.
Cotton – It has natural cellulose composition. It is the only natural fibre used to a great extent in belt manufacture. It increases in strength when wet. It has high moisture absorption and hence, poor dimensional stability. It is susceptible to mildew attack. At one time it represented 80 % of the raw fibre input into the belt manufacture. Presently its use is less than 5 %.
Rayon – It has regenerated cellulose composition. It is slightly stronger than cotton, but tensile strength is lowered by water. Chemical resistance is similar to cotton. It also has high moisture absorption and hence, poor dimensional stability. It is susceptible to mildew attack. It is practically non-existent in conveyor belt today
Glass – Glass has very high strength compared to rayon. It has low elongation. It is mainly used in high temperature applications. It has poor flex life. It has limited use in belt manufacture presently.
Nylon – Nylon is polyamide which has high strength, high elongation, and good resistance to abrasion, fatigue and impact. While moisture absorption not as high as cotton, it absorbs upto 10 % of its own weight in moisture, and hence, it has poor dimensional stability.
Polyester – Polyester has high strength, exceptionally good abrasion and fatigue resistance. It has extremely low moisture absorption and hence has good dimensional stability. It is unaffected by mildew.
Steel – Steel is used where high strength and extremely low stretch are a necessity. A small amount of woven steel carcass is being used. However, more steel is used in steel cord-like belt constructions.
Aramid – Aramid has twice the strength of steel, with stretch characteristics roughly halfway between steel and polyester. It is considerably lower in weight than steel and does not rust. Presently cost and availability of materials limit the use of Aramid in belt carcass construction.
While comparing different types of carcasses, the wholly synthetic plies have proved to give better service. Polyester plies are better in the longitudinal direction (warp) and polyamide plies are better in the transverse direction (weft). Carcasses with aramid plies are high in tensile strength and low in elongation. Steel cord belts have a very low elongation and are used predominantly on long haul installation. Evaluation of different types of plies are summarized in Tab. 1
|Tab 1 Evaluation of plies|
|Characteristics||Cotton||Polyamide||Polyester||Polyester-polyamide||Aramid||Steel weave||Steel cord|
|•• bad, • medium, * good, ** very good, *** excellent|
Conventional plied belting constructions, employing all synthetic carcasses and elastomeric covers are appropriate for the end use in (i) hard rock mining, (ii) aggregate, sand and ore, (iii) normal applications, (iv) forest products, and (v) soft minerals such as coal, potash, phosphates, and grain.
As a general rule, present fabrics in use are designated by the working tension or breaking strength of the fabric. When dealing with carcass fabrics, two separate strength measurements are used. The first is the ‘maximum working tension or breaking strength of the belt. This is the highest tension occurring in any portion of the belt on the conveyor system under normal operating conditions. This is the strength measurement used to determine the proper belt for the system. The second measurement is the ultimate tensile strength of the belt. The ultimate tensile strength of a belt is the point at which the belt ruptures and fails due to excessive tension.
The difference between the maximum breaking strength and the ultimate tensile strength of the belt is frequently referred to as the service factor. For the top quality conveyor belts, this service factor is 8 -10 to 1 (Fig 5). Belts utilizing nylon constructions normally have a service factor of more than 10 to 1. This higher service factor is necessary to overcome some of the inherent properties of nylon, such as excessive elongation. Majority of conveyor belt fabrics are produced today with polyester warps (lengthwise yarns) and nylon fills (crosswise yarns). This combines the best properties of both textiles, offering high strength / low stretch conveyor belt with excellent impact resistance, troughability, load support, and fastener and / or bolt-holding ability.
Fig 5 Tensile strength versus elongation of conveyor belts
Covers protect the carcass of the belt from load abrasion and any other conditions which can contribute to belt deterioration. The top and bottom covers of the conveyor belt provide very little, if any, structural strength to the belt. The purpose of the top cover is to protect the carcass from impact damage and wear. The bottom cover provides a friction surface for driving and tracking the belt. Normally, the top cover is thicker than the bottom cover and more durable for abrasion, impact damage, and wear, due to its increased potential for damage. Abrasion and cutting can be so severe that a top cover as thick as 18 millimetres or more can be sometimes needed. In any case, the goal of cover selection is to provide sufficient thickness to protect the carcass to the practical limit of carcass-life.
Covers also provide the finished belt with a wide variety of desirable properties which include (i) textures for increasing the inclination and for controlling the product, (ii) cleanability, (iii) resistance to cut, (iv) improved impact resistance, (v) wear resistance, (vi) fire retardance, (vii) oil and chemical use, (viii) use of hot and cold materials.
Cover type, quality and thickness are matched to the service life of the belt involved. A specific cover formulation used in an individual belt construction is determined by the material to be carried and the environment in which the belt is to operate. Historic belt constructions were highly susceptible to moisture and chemical attack because of their cotton carcass components. Accordingly, it was common to extend the belt covers over the edges of the belt in what is known as the moulded edge construction. This type of manufacturing was expensive because of the additional manhour and machine time involved.
Present day belt constructions, with their high adhesion levels and synthetic carcasses, are considerably less susceptible to moisture and chemical attack and do not need edge protection. They make possible the slit-edge belt distribution programs presently used in the conveyor belt industry.
The aspect ratio of a conveyor belt is the thickness of the top cover in relation to the thickness of the bottom cover. The carrying side cover is not to be more than 3 times thicker than the running side cover, which means that the cover thickness ratio (carrying side: running side) is 3:1. The covers can be made of a number of elastomers, including natural and synthetic rubbers, PVC, and materials specially formulated to meet special application requirements such as resistance to oil, fire, or abrasion.
The users of the belts are sometimes tempted to turn a belt over when the carrying side has become worn. Normally, it is better to avoid inverting the belt after deep wear on the top side. Turning the belt over presents an irregular surface to the pulley, resulting in poor lateral distribution of tension, and can lead to the wandering of the belt. Another issue is that there can be material fines embedded into what was formerly the carrying surface of the belt. When the belt is turned over, this material is now placed in abrasive contact with pulley lagging, idlers, and other belt support systems. In addition, after years of being troughed in one direction, the belt tends to take a ‘set’ (a pre-disposition to a direction) and resists the necessary reversing of trough needed to invert the belt. Sometimes this can take weeks to overcome and can lead to belt-tracking problems.
Some manufacturers of the conveyor belts have the practice of stamping their logo into the carrying surface of the belt. Even when near the belt edge, this recessed area becomes a trap for the materials being conveyed, and the roughness of the area can abuse the belt-cleaning and sealing which the embossed area passes. Hence, it is desirable that the supplier logos are positioned on the return, non-carrying side of the belting.
Specification of covers can include one or more of the special qualities such as flame resistant, anti static, flame resistant along with anti static, heat resistant, low temperature resistant, oil and grease resistant, food grade and chemical products grade etc. The carrying side surface depends on the load, the inclination of the installation or depending on the use of the belt, smooth, profiled, cleated and with the corrugated edges. The thickness of the carrying side cover depends upon the nature of the load and loading conditions (type of load, gradient and height of fall etc.). Based on the conditions the cover thickness on the carrying side can vary from 2 mm to 18 mm and that on the pulley side from 2 mm to 5 mm.
Basic materials for cover are (i) natural rubber, (ii) styrene-butadiene rubber, (iii) nitrile rubber, (iv) butyl rubber, (v) ethylene – propylene- diene rubber, (vi) chloroprene rubber, and (vii) nitrile chloroprene rubber.
Natural rubber – It is a good material for belt cover rubbers since it has a very good tensile strength and elongation, high heat resistance and elasticity, high shear and tearing strength, and good abrasion resistance characteristics. It is stable within the temperature range of -30 deg C to + 80 deg C. With special rubber compounding a widening of this range can be achieved from -40 deg C to + 100 deg C. Natural rubber is resistant to water, alcohol, acetone, dilute acids, and alkalis. It has limited resistance to concentrated acids and alkalis where compounding and service temperatures are major consideration. With special compounding, natural rubber based mixes can be made antistatic and flame resistant. By adding anti-ozonants a substantial protection against harsh temperature effects, sunlight, and ambient weather conditions can be achieved.
Styrene-butadiene rubber – It is a synthetic polymerization product consisting of styrene and butadiene whose characteristics are similar to natural rubber. Tensile and cut resistance are good. Abrasion, heat and ozone resistance are better than natural rubber.
Nitrile rubber – It is a copolymer of butadiene and acrylo-nitrile. It is not resistant to ketones, esters, aromatics, and hydrocarbons. The physical properties are slightly lower than those of natural rubber. The temperature operating range can be controlled between -40 deg C to +120 deg C. It is normally abrasion resistant, resistant to ageing and used for oil and grease resistant belt covers.
Butyl rubber – Butyl rubber is a polymerization product of isobutylene and isoprene (IIR). It has a very good ozone and temperature resistance. It has a very good resistance to ageing. It is able to withstand temperatures of – 30 deg C to + 150 deg C. It has limited resistance to acids and alkalis. It is mainly used for heat resistant conveyor belting.
Ethylene – propylene – diene rubber – It is known as EPDM (ethylene propylene diene monomer) rubber. The temperature resistance of this rubber is similar to butyl rubber but with a considerable higher resistance to wear and tear. It has also got a better ozone resistance.
Chloroprene rubber – It is a synthetic polymerization product of chloro-butadiene. The mechanical properties are similar to natural rubber. The rubber is considerably better in respect of ozone and oil resistance. Because of chlorine, chloroprene rubber has a high degree of flame resistance. The working temperature range is – 30 deg C to + 80 deg C. Ageing resistance and oil resistance is better with this rubber.
Nitrile chloroprene rubber – The use of nitrile in chloroprene rubber enhances the dynamic properties. It is used for cover rubber which needs high oil and fat resistance whether it be animal, vegetable or mineral and also requiring superior mechanical properties. Nitrile chloroprene rubber is better than chloroprene rubber.
Conveyor belts for specific applications
In addition to the five types of belts namely (i) mono-ply belt, (ii) multiple-ply belt, (iii) reduced-ply belt, (iv) solid woven belt, and (v) steel cord belt described earlier there are some types of conveyor belts which are suitable for specific applications. These belts are described below.
Heat resistant conveyor belt – It is more economical to use a thermo-stable conveyor belt when transporting materials with temperatures of 60 deg C and higher. Damage to cover rubber varies depending on the temperature or shape of transported materials and it is critical to choose suitable belt materials depending on the use conditions. The relationship between the temperatures of the material and the belt surface is especially noteworthy. This is since cooling is mainly achieved on the return trip as the temperature of the belt surface is different from that of the material, although it varies depending on the material shape, belt length, speed, operation environment, and operating hours. Tab 2 gives the grades and features of heat resistant conveyor belts.
|Tab 2 Grades and features of heat resistant conveyor belts|
|HRS||Being a belt made of heat-hardening styrene-butadiene rubber materials, it has good abrasion resistance though some cracks are generated as cover rubber gets hardened by heat if used for a long time.||High abrasion resistance, medium temperature|
|HRB||Being a belt made of heat-softening isobutylene and isoprene rubber (IIR) materials; it has good resistance against cracks though its cover rubber gets softened by heat if used for a long time. It is suitable to transport cement or clinker with little abrasion.||Crack resistance, medium high temperature|
|HRP||Being a belt made of EPR (ethylene propylene rubber) materials, it demonstrates outstanding performance in transporting hot materials as it has high resistance against abrasion and cracks.||High abrasion resistance, crack resistance, high temperature|
|HRP (plus)||Being a new concept HRP product whose crack resistance is reinforced, one can experience a longer life span.||High abrasion resistance, crack resistance, high temperature|
Belt surface temperature varies depending on material types, shapes, sizes, belt cycle time, and trough angle. The heat-resistant belts are to be selected based on their surface temperature as their life span depends on the belt surface temperature.
Fire resistant conveyor belt – This belt is used to prevent line loss while reducing damage to the belt from flames by suppressing ignition. It is mainly used to transport grains, fertilizer, or coal.
Oil resistant conveyor belt – Ordinary grade belts are damaged quickly by imbibition of oil into cover rubber, peeling of cover rubber, and reverse troughing of the belt if oily materials are transported. Hence, oil resistant belts are to be used to transport oily materials.
Chemical resistant conveyor belt – This belt is used for transporting chemicals, pulp, ceramic, foodstuffs, fertilizer, and materials with chemicals attached. It is necessary to select cover rubber which is resistant to acid or alkali depending on the types of transport materials or chemicals attached to the materials.
Anti static conveyor belt – This belt is made of cover rubber especially mixed to prevent static electricity. Anti-static belt is essential to transport fabrics which stick on the belt surface with static electricity or electronic products which can explode or ignite by electrification. Cover rubber electric resistance (ISO-284) is to be 300 millions ohms or less
Colour conveyor belt – This type of belt is made of white, green, yellow or any other colour of the cover rubber to match the colour in workplace, or to protect eyesight of the worker or to provide safety. It stimulates easy sorting of transport materials and improves the safety of the workers.
Bucket elevator conveyor belt – This belt has bolt holes to fix buckets along the entire belt length, making it apt to be torn easily and receiving pulled force with the weight of the bucket. It also has a withdrawal force different from that of the ordinary belts in bending in connection with pulleys. Hence, its carcass mainly uses polyester fabric which can address to such problems. Steel cord is used when a highly powerful belt is needed. Fig 6 gives the structure of bucket elevator conveyor belt.
Fig 6 Bucket elevator and chevron conveyor belts
Chevron conveyor belt – This belt is are highly effective for transporting sand, coal powder, grain, or other powder or granular materials or materials packed in bags or boxes on incline though they can transport coal or mineral ore just like ordinary conveyor belts. They can transport powder or granular materials on 17 degrees to around 28 degrees inclines and materials packed in bags or boxes on 30 degrees to around 35 degrees inclines. Fig 6 shows typical protrusions of chevron conveyor belt.
Rough top conveyor belt – The uneven cover rubber surface of this conveyor belt prevents slippage while preventing vibration of, or absorbing and reducing impact to transport materials by providing a cushion effect. This belt is ideal for transporting plywood or packed items. Fig 7 shows belt structure of rough top conveyor belt.
Fig 7 Structure of different types of conveyor belts
Rip protection steel cord conveyor belt – This type of belt provides excellent rip protection and impact resistance, minimizes damage to belt carcass from sharp objects or strong impact, and prevents the belt from being torn lengthwise by sharp objects inserted between the belt and other equipment. Fig 7 shows belt structure for rip protection steel cord conveyor belt.
Impact / cut resistant conveyor belt – This type of belt minimizes damages to belt carcass from sharp objects or strong impact, and prevents belts from being cut or broken by damage. Fig 7 gives belt structure for impact / cut resistant conveyor belt.
Hybrid belts – These belts combine merits of the flexibility of nylon belt and low elongation of polyester belt. Satisfactory improvement is confirmed on those lines where belts are lengthened since sufficient take-up stroke is not secured.
Clean face conveyor belt – With this belt, overall clean environment of lines is enhanced by minimizing debris from transport materials remaining on the return trip with a maximized belt cleaner performance. Belt life span is extended with abrasion resistance twice that of the abrasion resistant grade.
Energy saving conveyor belt – Energy saving conveyor belt applies the cover rubber to minimize the transformation of rubber by idler. This conveyor belt can improve in reducing the electrical power needed operate the conveyor belt by decreasing the loss of energy.
Corrugated side wall conveyor belt – Conveyor belt equipped with special waves or cleats called corrugated sidewall on the belt provides the features such as (i) corrugated sidewall design allowing for greater transport volume with increased sectional loading area, enabling a narrower conveyor design, (ii) reducing the foot space as steep incline or vertical transport is possible with cleats attached belt width wise, (iii) no skirt board is needed as transport materials spillage is prevented by high wave like raised strip, (iv) incline angle can be easily adjusted with specially strong belts and press rollers, and (v) flat rollers can be used to reduce the equipment cost.
Handling and storage of the conveyor belt roll
The belt packaging of the manufacturer is designed to protect the conveyor belt during normal shipping and handling. When a belt arrives at the stores, careful unloading of the belt is needed. The belt roll is not to be dropped nor handled roughly. This can break the packaging and cause the belt to telescope. Once a belt telescopes, it is almost impossible to re-roll. The rolling of the belt is not to be tried but if it is necessary then the rolling is to be done in the direction the belt is wound. Rolling a belt in the opposite direction can cause it to loosen and telescope.
The best way to move a belt is to slip a sturdy hoisting bar through the centre core. Then, it is lifted with a sling or with strong hoisting cables. Care is to be taken so that these hoisting cables do not damage the outer wraps at the belt edges. The edges are to be protected with special ‘spreader bars,’ or short wooden planks. A sling around the circumference of a roll of the conveyor belt is not to be used since it is not safe. Alternative method for moving a belt safely is by laying the roll flat on a skid and hoisting the skid with a forklift. However, it is to be ensured that the lift forks do not come in contact with the belt itself.
When storing a new conveyor belt, it is to be left hoisted or it is to stand in upright condition, preferably on a dry surface (the roll is not to be laid on its side). A wooden skid is the best. The belt roll is to be blocked safely so that it cannot roll accidentally. Extreme temperature variations can have an adverse affect on a belt over long periods of time. The ideal storage temperature is around 25 deg C. Long exposure at low temperatures can harden or stiffen the compounds. If installed on a conveyor in this stiffened state, the belt does not train well until it adjusts or ‘warms up’ to the system. Temperatures over 30 deg C have an adverse effect as well, and are to be avoided. Further, conveyor belts are not to be stored near flammable substances such as oils, gasoline, or paint etc. Belts are not to be stored in excessively wet places.
Used belt is to be thoroughly cleaned and dried prior to storage. A dry place out of direct sunlight is preferred for storage with excessive temperature variations or extremes are to be avoided. Sunlight and ozone can also deteriorate any exposed rubber over time. Conveyor belt is to be stored out of the direct sunlight wherever possible. Electrical generators or arc welders can sometimes generate ozone. It is best to store the belt some distance away from these types of equipments. In general, it is wise to keep any unused belt stored in its protective packaging of the manufacturer until it is ready for installation.