HISTORY OF CONVEYOR BELTS
Primitive conveyor belts were used since the 19th century. In 1892, Thomas Robins began a series of inventions which led to the development of a conveyor belt used for carrying coal, ores and other products. In 1901, Sandvik invented and started the production of steel conveyor belts. In 1905, Richard Sutcliffe invented the first conveyor belts for use in coal mines which revolutionized the mining industry.
At the time, its design was governed mainly by the requirements of the mining industry. The mining industry proved to be a trendsetter once again. Particularly, in the field of underground mining where stringent requirements gave rise to a high standard of belting. Also the new ignite surface mines were responsible for bringing about major improvements in the design of heavy-duty conveyor belts. These changes were subsequently adopted by other industries. Over the years, the belt materials and their properties have undergone considerable changes. The need for increased tensile properties, abrasion resistance and higher economy has encouraged belt users to join with belting manufacturers as well as plane engineers to develop tougher and more economical conveyor belting.
The belt reinforcement was originally in the form of cotton plies. These were later replaced by plies such as rayon, polyester, polyamide and especially steel cords, which were to carry the tension loads of the belt. Steel cords not only proved to have outstanding flexibility, but also able to carry the highest belt tensions. The belt plies were originally embedded in natural rubber. In the course of time, the rubber compounds were considerably improved to meet the stringent safety requirements for underground use, and inflammable natural rubber was supersede by non inflammable or self-extinguishing rubber, or even plastics such as PVC. Extremely wear-resistant belts were developed for use in surface mining. In 1913, Henry Ford introduced conveyor-belt assembly lines at Ford Motor Company’s Rouge factory, in Dearborn Michigan. In 1972, the French society REI created in New Caledonia the then longest straight-belt conveyor in the world, at a length of 13.8 km.
BASIC PROPERTIES REQUIRED INCLUDES
Conveyor belts have to meet rather different requirements, depending on the particular application.
- High strength and flexibility
- Low extension in service
- Resistance to abrasion
- Impact and tearing resistance
- Resistance to moisture, oils and chemicals
- Temperature resistant
- Fire resistant
In addition to the above properties, conveyor belt material should also exhibit-
- Harmless as regards hygiene
- Good adhesion of the constituent plies and good adhesion to the driving drums
- Minimal tendency to electrostatic charging and last but not least,
- Should be reasonably priced.
The importance of these individual factors may considerably vary in different products.
The two main components in conveyors belts are the rubber and the reinforcing member that carries the load.
The rubber has two primary functions;
- first to act as a binding element and
- second to protect the tension member.
The performance level of conveyor belt, of course, posed on reinforcement materials as regards both fibre properties and fabric construction. The traditional reinforcement materials are woven textile fabric and steel cord. The selection is determined by the working tension in the system.
CONVEYOR BELT CONSTRUCTION
Conveyor belts generally are composed of three main components:
3. Covers (carry cover and pulley cover)
The reinforcement usually found on the inside of a conveyor belt is normally referred to as the “carcass.” In a sense, the carcass is the conveyor belt since it must:
a) Provide the tensile strength necessary to move the loaded belt.
b) Absorb the impact of the impinging material being loaded onto the conveyor belt.
c) Provide the bulk and lateral stiffness required for the load support.
d) Provide adequate strength for proper bolt holding and/or fastener holding.
The carcass is normally rated by the manufacturer in terms of “maximum recommended operating tension” permissible (pounds per inch i.e., ppi).
PRIMARY BELT MATERIAL
Aramid: Aramid fibers exhibit good impact resistance, toughness, low elongation, and resistance to damage by penetration of powders or siIt.
Cotton / Canvas: Any weave of cotton, cotton blends, or heavy-duty canvas.
EPDM: Good resistance to sunlight, weathering and ozone. It has poor resistance to petroleum oils and fuel. Good heat and compression set resistance. Suggested operating temperature was -70° to 275° F. Trade names include Nordel® (Dupont Dow Elastomers), Vistalon® (Exxon Mobil Chemical), Epsyn® (DSM Elastomers), Royalene® (Uniroyal Chemical), and Epcar® (B.F. Goodrich).
Hydrin®: Good gas impermeability and maintains physical properties over a wide temperature while maintaining resistance to petroleum oils. Ozone, oxidation, weathering, and sunlight resistances are other qualities. Suggested operating temperatures (_600 to 3000 F). Hydrin® is a registered trademark of Zeon Chemicals L.P.
Kevlar®: Kevlar® is a man-made organic fiber developed by DuPont. The general features of Kevlar are high tensile strength at low weight, structural rigidity and durability, high chemical, flame and cutting resistance, and low electrical conductivity. It is used in many safety and heavy-duty industrial applications.
Leather: The characteristics of leather include flexibility, toughness and resistance to abrasion. It is composed of high strength interlocking fibers. There are two main advantages to using leather as a sealing material, its ability to absorb and retain lubricants and its effectiveness sealing against rough surfaces.
Mylar®: Mylar® is a polyester film and laminating substrate, designed by DuPont. Its primary characteristics include excellent flexibility, machinability, and puncture resistance while providing a barrier against gas and water vapor. It also retains its form and function within very high or low temperatures. Its uses are extremely varied, from food service; to seals, barriers and personal protection; to heavy duty industrial application.
Neoprene: A synthetic rubber that resists degradation from sun, ozone, and weather. It performs well in contact with oils and many chemicals. Neoprene remains useful over a wide temperature range, displays outstanding physical toughness, and resists burning inherently better than exclusively hydrocarbon rubbers. Neoprene also offers resistance to damage caused by flexing and twisting. Suggested operating temperature (-45° to 230° F). Trade names include Neoprene (DuPont Dow), Baypren® (Mobay), and Butachlor® (Ditsugil).
Nitrile: Good resistance to petroleum hydrocarbons and fuels. Widely used with most oils, hydraulic fluids, and alcohol. Many compound variations are available for specific applications. Suggested operating temperature (-30° to 275° F). Trade names include Breon® (BP Chemicals), Chemigum® (Goodyear), Hycar® (B F Goodrich), Krynac® (Polysar Ltd.), Nipol® (Zeon Chemicals), Nysyn® NBR, (DSM Elastomers), Paracril® (Uniroyal Chemical), and Perbunan® (Mobay).
Nylon: Nylon, comprising several grades of polyamides, is a general purpose material in wide use; it is tough and resistant and has good pressure ratings.
Polyester: Polyethylene Terephthalate, also called polyester fiber, refers to anyone of a large family of synthetic polymers composed of at least 85% by weight of an ester of a substituted aromatic carboxylic acid. General characteristics of this family include high tensile strength; chemical, wrinkle, and abrasion resistance; and ease of drying and washability. Industrial uses include belting, hoses, cords and threads; essentially any application where the fiber must be highly flexible, yet durable.
Polyurethane / Urethane: Polyurethane is a diverse class of materials exhibiting good elongation, recovery and toughness properties. They are flexible and have good abrasion resistance. (NOTE: The urethanes of the plastics industry are so named because the repeating units of their structures resemble the chemical urethane.) Trade names include Texin® (Bayer), Adiprene® and Vibrathane® (Uniroyal Chemical), Estane® (B F Goodrich), Genthane® (General Tire and Rubber), Millathane®, and Peliethane® (Dow Chemical).
PVC: PolyVinyl Chloride is a widely used material that has good flexibility, smooth surface, and nontoxic qualities. Some grades are used in food and chemical processes due to the inert nature of PVC. Brand names include: ACP® and Dural® (Alpha Gary), Geon® (Geon), Benvic® (Solvay), Flexalloy® (Teknor Apex).
Rubber: Natural compounds such as gum rubber (polyisoprene) and latex.
Silicone: Silicones are polymers in which organics groups, such as methyl and phenyl groups are bonded to the silicone atoms in chains of inorganic siloxanes (-Si-O-Si-). Their properties include heat, cold and weather resistance, electrical insulation, release, water repellency and defoaming.
Steel Belt: Includes all grades of carbon, mild and other non-stainless steels.
Steel Belt – Stainless: Includes all grades of stainless steel belting.
Steel Cord: “Belt” consists primarily of steel cord for high tensile strength; usually contained within a binding matrix.
Tape – All Materials: Very thin section most frequently made of steel or other metal; usually for light-duty tracking and control operations.
Teflon®: Teflon® refers to a class of fluoropolymer resins used for a wide variety of commercial applications. They are highly-resistant to temperature, chemical reaction, corrosion, and stress-cracking. Teflon is a registered trademark of DuPont Dow Elastomers.
Wire Mesh or Weave: Meshed or woven wire can exhibit high temperature stability and material toughness. Gaps or openings in the mesh can be desirable for certain types of food and material processing.
The rubber, PVC or urethane between plies is called a “skim.” Skims are important contributors to internal belt adhesions, impact resistance, and play a significant role in determining belt “load support” and “trough ability.” Improper or marginal “skims” can adversely affect belt performance in general and can lead to ply separation and/or idler junction failure.
Covers are used in conveyor belt constructions in order to protect the base conveyor belt carcass and, if possible, to extend its service life. In addition, covers do provide the finished belt with a wide variety of desirable properties, including the following:
- To increase friction
- To increase inclination
- To control product
C. e. A specific coefficient of friction
D. A specific color
E. Cut resistance
F. Enhanced impact resistance, etc.
H. Fire Resistance, Oil & Chemical Resistance
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 will operate.
CONVEYOR BELTS PRODUCED FROM TEXTILE MATERIAL AND STEEL CORDS AS REINFORCING MATERIAL
Steel Cord Belts
Steel cord belts consist in principle of parallel steel cords which are embedded lengthwise in rubber. The steel cords act as reinforcement, while the rubber protects them and forms a surface for conveying the material. The steel cords are purpose-made and extremely flexible. They are specially coated to make them corrosion-resistant.
The rubber is of a high quality synthetic type such as SBR (styrene butadiene rubber) which has excellent wear resistance. The belt incorporates a special layer of rubber around the steel cords designed to bond to the coated surfaces and wires and to fill in the spaces between them. The bond is extremely important.
To facilitate transport, belts are manufactured in sections which are spliced together by overlapping the steel cords, and by vulcanizing new cover rubber both above and below the overlapping area with the aid of special mobile vulcanizing presses.
This splice has the same tensile strength and service life as the belt as a whole. This is particularly important for the belt performance, as a weak splice could result in costly downtime, and may even be a safety hazard, especially in the case of steep conveyor systems.
This conveyor belt design has proved to be a reliable and economical means of handling high capacities. The surface of the individual wire is galvanized to avoid rapid body corrosion when the protective rubber coating is damaged.
Textile Reinforced Conveyor Belt
The performance level of the conveyor belt mainly depends upon the type of textile material, i.e. fibre used and textile material construction. The product construction includes that of the basic threads used in the warp and weft, including the twist level, as well as that of the woven fabric, including the interlacing technique used.
Fibres for Conveyor Belt
The individual types of reinforcement materials have different shares of the market. In general, cotton fabrics are taken into consideration in certain application areas only when strength of up to some 100 N/mm is adequate. In applications requiring a higher strength level, viscose rayon fibres are used but, of course, these suffer from strength loss in a wet environment. Number of plies in yarn used for conveyor belt cannot be increased at will. Usually 3 to 6 plies are considered convenient. In fact, an increased number of plies mean a higher, production cost and a poorer utilization of fabric strength, i.e., the ratio of the sum of the strengths of all plies to the strength of the conveyor belt body. In conveyor belts designed for heavy-duty application, nylon, polyester, glass and Kevlar, PTFE and PEEK fibres are used. Although rarely, but Jute and leaf fibre are also used as conveyor cords. They have very good adhesion with rubber.
There are limitations to optimal utilization of each of the materials mentioned, and also different technical problems determine the forms of application of these materials. One way to obtain optimal properties of such a fabric is to combine two different fibres and thus utilize the most convenient properties of the two. Currently used are the viscose rayon/ nylon and the nylon/cotton combinations, but by far the most popular is the combination polyester/nylon (with the polyester fibres in the warp). With these blend fabrics, no adhesions problems are encountered as with the case with the all-polyester woven fabrics.
The other advantage includes the following:
1. High strength and hence the possibility of construction transport conveyors of great lengths;
2. Excellent resistance to moisture, as a result, neither separation of individual plies nor a strength loss can occur.
3. High impact resistance;
4. High resistance to dynamic stresses enabling driving drums of smaller diameter to be used;
5. Complete resistance to extension in service makes it possible to use very long transport conveyors;
6. High resistance to chemicals enables conveyor belt filters to be used in the chemical industry;
7. Outstanding resistance to elevated temperatures.
Amongst the other textile fibre Kevlar is the best fibre for the reinforcement of conveyor belts. Kevlar has very good physical and chemical properties.
These physical and chemical properties of Kevlar fibres allow using it as the reinforcement material for conveyor belts. By using this it is possible to design light and linger conveyor belts. This design feature is a major advantage in applications that require a high modulus, light weight belt as a replacement to steel cords, thus providing for the use of narrow, fast belts of high strength which can be employed efficiently over long distances. The use of these type of belt led to lower manufacturing and installation costs, reduced energy consumption, no sparking and non-flammability giving improved safety, better corrosion, better impact resistance and longer life.
Fabric Construction for Conveyor Belt
There are four basic types of construction for reinforcing
i. High strength belts- cord fabrics,
ii. Straight warp fabrics,
iii. Solid woven fabric and
iv. Cabled cords.
1) Cord Fabrics
Filaments of Kevlar yarns have only 1.5 denier linear density. The cord constructions use Kevlar yarns as the strength member laid in warp direction, usually with a light weft of nylon or polyester to hold the cords together.
2) Straight Warp Fabric:
In the preferred straight-warp the Kevlar warp cords are place by a series of binder V\ (of nylon) and weft yarns. This ensures that the straight-warp weave has minimum crimp and, hence, obtains 90% of the strength of the fibres in warp direction.
Using a “stacked” or double warp construction the elongation goes maximum 5%. Using straight-warp weaves; it is possible to achieve breaking strength of up to 150 kN/m. This construction offers the cost-saving benefits of a single ply plain woven fabric and, also, provides high transverse impact and penetration resistance, thus eliminating the need for breakers.
3) Solid Woven Construction
Kevlar fibres have disadvantage of being brittle, and they can be damaged by lateral pressure and friction. This demerit of Kevlar fibre can be overcome by solid woven construction. The solid woven construction causes the Kevlar warp to go through crimp, resulting in a higher elongation break. Because of the heavier construction, belt breaking strengths of up to 4000 kN/m are achievable. These constructions have high impact and tear resistance. Belt joining is achieved with metal fasteners or with standard finger splices. This type of construction is limited to medium or high tension belts
4) Cabled Cord Construction
The cable cord construction is used in ultra-high tension belting ranging up to 5 400 k N / m breaking strength. Adhesion to rubber is built into the cable of Kevlar, which allows direct replacement of steel cables within the steel belt manufacturing process.
Another advantage with this construction could be gained with splices designed similar to those used with steel cable cord belts. Unfortunately, this type of belt construction has some of the disadvantages of the steel cable reinforced belts, such as being expensive to manufacture and having poor tear resistance along the belt and between the cable reinforcement.
COATING OF CONVEYOR BELT
Problem associated with the use of Kevlar in conveyor belt is to obtain a good bond between the fibres and the rubber matrix through interface that will transfer the belt loads from the matrix to the fibres. To avoid this problem special dip systems have been developed for the aramid fibres, consisting of an aqueous solution of an epoxy for a pre-dip which is dried and baked on to the fibres. This is followed by a topcoat combination of resorcinol formaldehyde-latex (containing vinyl pyridine), which gives a high level of adhesion of the fibres to the rubber. Carbon black is dispersed in the RFL to increase the modulus of the dip film to a level intermediate between that of the fibre and the rubber stock. A typical aramid pre-dip (sub coat) comprises water soluble epoxide (2%), water (97.9%) and wetting agent (0.1 %), with a mixture pH value of 11.5 (NaOH). The subcoat, which requires high temperature treatment to achieve optimum adhesion, is heat set to 245°C, and the topcoat (RFL) is heat set to 220°C. Instauration in the cord adhesive rubber is essential for maximum adhesion to the rubber compounds of the matrix; otherwise adhesion to the rubber is substantially reduced.
Problems Associated with Textile Reinforced Conveyor Belt
Challenges faced by the fabric conveyor belt are:
- Conveyor Belt Width
- Conveyor Speed
There are two solutions to the challenge, and the first one is partially effective due to property trade-off. Firstly, by increasing the number of plies in the belt, the tensile strength will increase but at the same time the resulting fabric composite will be stiffer in both the traverse and longitudinal axes. The belt will likely to have inadequate troughabilty translating into belt tracking problems, spillage and edge damage when the belt runs into the structure. Also, the increased number of plies will intensify the bending stresses and the inner ply shear forces when the belt and the splices go around the pulley. Larger stresses require larger pulleys.
The second approach is to develop a new textile-reinforcing member that will carry far more load width of belt. When built into a carcass, the resulting product will trough on the idlers will bend easier on the extra benefit of vulcanized or mechanical splicing. For better performance hybrid cords are also used. Hybrid cords with Kevlar and nylon are preferred for best fatigue resistance and compatibility of adhesive dips. Kevlar and polyester hybrid cords have higher initial modulus and lower growth than corresponding nylon and may be useful for application where dimensional stability is critical.
Hybrid cords have very good fatigue resistance. Some special designs are now used for manufacturing of conveyor which prevents failure and improve the performance of conveyor belts. Fabric performance can be improved by cross wise reinforcement. Cross wise reinforcement increase the impact resistance of the fabric. It also increases the longitudinal tearing or initial tearing resistance of the conveyor belt.
SPLICING OF THE CONVEYOR BELT
The splicing is very important in conveyor belt because it is the weakest point of conveyor belt. Most of the failure takes place in the splicing zone. Normally V-butt splice is used for splicing. But the problem with this type of splice is failure under dynamic conditions, with cracking beginning at the top surface of the belt and then propagating along the fingers. The overall efficiency of this kind of splice is about 0.32 times that of the belt. To improve splice strength, a new design concept is proposed, known as single lap joint splice, as shown in the diagram. It can be seen from figure that the rubber cover beneath the splice is retained to support the fabric attached. The splice geometry then resolves into a lap joint, with a full width layer of identical fabric weave forming a sheet of lap material placed on the top side of the butt joint. A small gap in the rubber beneath the fabric butt shown below is filled with raw rubber strip, and a raw rubber sheet is laid on top of the splice to fill in the 3 mm space left above the lap joint. The whole lap joint area is then vulcanized to form the original shape of the belt cross section.
By their very nature, conveyor belts have a finite life. While it is difficult to define this life, the modes of failure are known. These failures could be classified as gradual failure or sudden failure. Gradual failures mean that the development of a defect takes place slowly; implying that a certain defect or failure development time is involved.
Some factors which govern life of splice are-
- Environmental conditions – moisture, humidity, dust, handling (cleanliness) and temperature of the splice area
- Materials – age, shelf life, size of tie gum strips for the splice stage (step), type of solvents, compatibility of covers to bonder material, new to old belt splicing issues
- Mechanical – splice step design, cord spacing for the particular chosen step and cord diameter, splice length, length of butt gaps on the splice, matching belts from different manufacturers, splice curing time, layup quality, splice straightness
- Electromechanical- vulcanize calibration, temperature sensor monitors, platten
- Temperature distribution, pressure control, temperature control.
By understanding the three primary mechanisms of failure, one should be able to identify the factors that maximize belt life. The three primary failure mechanisms for conveyor belts are: Yield, Wear, and Fatigue.
Fatigue is the growth of microscopic cracks in a material caused by repeated loading and unloading. Fatigue failures are often confused with the belt becoming brittle. Most failures are caused by yield, which occurs when a belt is permanently deformed (bent). These failures are usually due to accidents or misuse. Wear occurs as the joints hinge, or the belt rubs against other components. This results in a slight loss of material and weakening of the effected zone.
There are different types of conveying belts available for unique applications. The construction of conveyor belts and materials used are often application-specific. For example:
- General-purpose belting: This is for general conveying or power transmission.
- Agricultural belting: This is for agricultural applications like silage transfer, farm equipment belts, etc.
- Retail belts: They used in checkout counters or inventory transfer for various commercial purposes.
- Construction belting: They are used for materials such as roofing shingles or plywood. They are heavy-duty belts designed for use on construction equipment.
- Elevator conveyor belts: These kinds of belts are typically used in vertical applications where there are additional safety factors for bucket attachments as well as holes for meshing with drive and tracking components.
- Food and beverage processing belts: They are designed to be used in food or beverage processing applications.
- Forest conveyor belts: These are designed for use in the logging, sawmill, tree farm, and
- Mining and quarrying belts: They are heavy-duty belts used for applications in mines and quarries. They can transport materials such as ore, stone, tailings, gravel, aggregate, etc.
- High temperature belting: They are used for materials that can safely tolerate extreme temperatures. They can transfer high temperature materials.
- Inclined conveyor belts: They are typically designed for use in conveyance of material up an incline or down an incline.
- Manufacturing and fabrication belting: They are used in factory production lines. They include belts designed for specific fabrication applications like semiconductor chip manufacturing.
- Belting for packaging operations: These belts are abrasion resistant for use with cardboard, package paper, etc.
- Power transmission belting: They are used in power transmission applications like engine belts, belting for power takeoffs, industrial machinery etc.
- Belting is used in the pulp and paper industry to transfer pulp, paper or paper products.
- Custom conveyor belts: They can be designed for special applications.
Fabric material is better than steel reinforcement in the conveyor belts. The kind of fibre and the type of fabric weave used as the carcass material depends upon the requirement of the application. Conveyor belts are the boon in the transportation system and textiles are making it much flexible, energy efficient, corrosion resistant, high strength, light weight as all compared to steel and conveyor belts are finding application in any areas.