CONVEYOR AND
ELEVATOR BELT
HANDBOOK
© 2011 by the Association for Rubber Products Manufacturers, Inc.
Published in the United States of America
RMA First Edition 1973
RMA Second Edition 1980
RMA Third Edition 1989
ARPM Fourth Edition 2011
INTRODUCTION
PREFACE
Conveyor and elevator belts are made to precise specifi cations and standards to service many useful functions. A better understanding of the complexities involved in manufacturing belting and the standards that are applied to it will be helpful in selecting the proper belt for the intended use and in obtaining good service after installation.
Belting covered in this Handbook includes conveyor belting, used to transport bulk or packaged, boxed and bagged materials, and bucket elevator belting. The belting may be made of natural and synthetic rubbers as well as plastics, such as vinyl, with carcasses of textile fabrics, which are woven, nonwoven, solid woven, or stitched; fabric cords; or of steel cables.
This handbook is intended for the general guidance and reference of persons interested in the selection and use of conveyor and elevator belting, but readers are urged to consult individual manufacturers for specifi c information and recommendations.
ACKNOWLEDGMENT
The Association for Rubber Products Manufacturers is the national trade association of the non-tire rubber manufacturing industry in the United States. ARPM represents manufacturers of fi nished rubber products (excluding tires), and their related suppliers. This publication is provided as a public service, and reference for users of conveyor belt products by U.S.
manufacturers of conveyor belt products, including:
Airboss Compounding Rubber (NC) Fenner Dunlop Americas (Pittsburgh, PA) Garlock Rubber Technologies (Paragould, AR)
Price Rubber Corp. (Montgomery, AL) Veyance Technologies Inc. (Fairlawn, OH)
© 2011 by Association for Rubber Products Manufacturers, Inc. 7321 Shadeland Station Way, Suite 285
Indianapolis, IN 46256 317-863-4072 www.arpminc.org
Published in the United States of America RMA First Edition 1973
RMA Second Edition 1980 RMA Third Edition 1989 ARPM Fourth Edition 2011
TABLE OF CONTENTS
Page
PREFACE...2
ACKNOWLEDGMENT...2
CHAPTER 1 - MATERIALS...4
CHAPTER 2 - ELASTOMER CHARACTERISTICS......11
CHAPTER 3 - TEXTILE BELT TYPES AND MANUFACTURING METHODS...18
CHAPTER 4 - TEXTILE BELT CHARACTERISTICS AND BELT RATINGS...23
CHAPTER 5 - TEXTILE BELT TOLERANCES...35
CHAPTER 6 - TEXTILE BELT TEST METHODS...36
CHAPTER 7 - SPLICING CONVEYOR AND ELEVATOR BELTS...40
CHAPTER 8 - STEEL CORD BELT TYPES AND MANUFACTURING METHODS...51
CHAPTER 9 - STEEL CORD BELT CHARACTERISTICS & BELT RATINGS...53
CHAPTER 10 - STEEL CORD BELT TOLERANCES...56
CHAPTER 11 - STEEL CORD BELT TEST METHODS...58
CHAPTER 12 PART A - SPLICING FABRIC CORD CONVEYOR BELTS...61
CHAPTER 12 PART B - SPLICING STEEL CORD CONVEYOR BELTS...75
CHAPTER 13 - BELT MONITORING...91
CHAPTER 14 - OPERATION AND MAINTENANCE...96
CHAPTER 15 - STORAGE OF BELTING...112
CHAPTER 16 - GLOSSARY OF CONVEYOR BELTING TERMS...113
CHAPTER 17 - USEFUL TABLES...130
CHAPTER 1
MATERIALS
INTRODUCTION
The purpose of this chapter is to present general descriptions of the construction elements of conveyor belts and the materials which are presently available to produce belts for the various materials conveyed with suitable strength for the tensions and other conditions encountered in service. Conveyor belts are sometimes classifi ed as Light and Heavy Weight belts.
Light Weight = RMBT* < 160 PIW Heavy Weight = RMBT > 160 PIW *RMBT = Rated Maximum Belt Tension, in pounds per inch width (PIW)
Light weight belting generally is used in very diverse applications such as food and tobacco products, agricultural products, wood products, baggage and packaging handling, metal stampings, and materials handling in the textile, printing, paper processing, postal, and electronics industries. Heavy weight belting generally conveys heavy and/or coarse abrasive materials like mineral ore, rock, sand, gravel, coal, and cement.
In general, most conveyor belts consist of three elements: a top cover or conveying surface; a carcass; and a bottom cover, or pulley surface. In light weight belting there is a great diversity among the top cover or conveying surfaces used such as smooth or rough covers and raised patterns; whereas heavy weight belting often has smooth top covers. Custom fabrications with light weight belting are also more common, including attaching of cleats or V guides or hole punching, for example.
The elements may also be grouped under several general classifi cations such as: elastomers; fabrics (woven or non-woven); spun;
fi lament, or monofi lament yarn or cord; and steel cords.
A rubber or plastic elastomer is a compounded material that returns rapidly to approximately its initial dimensions and shape after
substantial deformation by a weak stress less than the yield point.
A fi ber is a unit of matter having a length at least 100 times its diameter and which can be spun into a yarn.
A steel cord, when used as the tension member, is usually multiple strands of steel wire twisted together. Yarn is a generic term for continuous strands of textile fi bers or fi laments.
A fabric is a planar textile structure produced by interlacing yarns, fi bers, or fi laments. A fabric may be composed of yarns of cotton,
glass, nylon, polyester, steel or other materials. A fabric may be made from one material or a combination of materials.
RUBBER/PLASTIC ELASTOMERS
Polymers are mixed with various chemicals to obtain reinforcement and develop the physical properties of the resulting elastomer necessary for meeting service conditions. Since it is not the purpose for this Handbook to discuss compounding ingredients or methods of compounding, discussion of polymers will be restricted to the general properties of the basic polymers.
A wide choice of polymers is available. They can also be blended together to obtain many combinations with intermediate properties. Elastomeric compounds are used for the top and bottom covers or surfaces of conveyor belting and for bonding together components of the belt carcass. The elastomeric covering on belts is there to provide protection for the carcass and/or provide a specifi c property. The coverings are applied by several processes, depending on the material (rubber vs. thermoplastic) or thickness of the covering. It is possible to classify elastomers to some extent by the basic polymer used. They are listed in Table 1-1 with a brief description of their general properties.
Table 1-1.
Rubber/Plastic Polymers Used in Belting
Common Name
ASTM Designation D 1418-10
Composition General Properties
Acrylic ABR Acrylate-butadiene Excellent for high temperature oil and air. Poor water resistance. Poor cold fl ow resistance.
Brominated Butyl BIIR Bromo-isobutene-isoprene Similar properties as Butyl except that it can be more readily adhered to or used in combination with other polymers.
Butyl IIR Isobutene- isoprene Excellent resistance to heat. Very good resistance
toozone and aging. Good resistance to abrasion.
Chlorinated Butyl CIIR
Chloro-isobutene-isoprene Similar properties BIIR.
EPDM EPDM
Ethylene-propylene-diene terpolymer
Excellent resistance to heat, ozone, and aging. Very good resistance to abrasion.
Ethylene Propylene EPR Ethylene-propylene Same properties as EPDM.
Hydrin* CO Polychloromethyl-oxirane Excellent oil and ozone resistance. Good resistance and low permeability to gases. Fair fl ame low-temperature properties.
Hydrin* ECO Ethylene oxide and-chloromethyl-oxriane Excellent oil and ozone resistance. Fair and low permeability to gases. Good lowtemperature fl ame resistance properties.
Hypalon* CSM Chloro-sulfonyl-poly-ethylene Excellent ozone, weathering, and acid resistance. Good abrasion and heat resistance. Good oilresistance.
Hytrel* PET PolyethyleneTerephthalate Thermoplastic with excellent abrasion and cutresistance.
Good chemical resistance. Limited temperature range.
Natural Rubber NR Rubber, Natural
Excellent resistance to cutting, gouging, and abrasion. Good elasticity and resiliency. Good low temperature
fl exibility.
Neoprene* CR Chloroprene Good ozone and sun-checking resistance. Goodresistance to petroleum-based oils and to abrasion. Also good fl ame resistance.
Nitrile NBR Nitrile-butadiene Excellent resistance to vegetable, animal and petroleum
oils.
Polybutadiene BR Butadiene
A general purpose synthetic rubber. Generally used inblends with natural or styrene-butadiene rubber. Provides excellent abrasion resistance and
high resiliency. Excellent low temperature fl exibility.
Polyisoprene IR Isoprene, synthetic Same properties as natural rubber.
SBR SBR Styrene-butadiene Excellent abrasion resistance and good resistance
to cutting, gouging, and tearing.
Table 1-1. (continued)
Rubber/Plastic Polymers Used in Belting
TEXTILES
Many types of textiles are used in conveyor and elevator belting. Their use is based on their physical properties, such as strength, elongation, dynamic fatigue resistance, aging resistance, mildew resistance, heat resistance, and other special properties depending on service requirements. For special applications, consult the manufacturer.
Yarns used for belt textile reinforcement are classifi ed as either spun or fi lament depending on whether the base fi ber is in staple (3/4
- 2 1/2 in long single fi ber) or endless fi lament form.
A spun yarn is made by twisting relatively short lengths of staple fi ber together to form a continuous yarn, called a single’s yarn.
When two or more of these single’s yarns are twisted together, the result is a plied yarn. When two a more plied yarns are twisted
together, the result is cable cord. The tensile strength, elongation, and thickness of a yarn of any fi ber type can be changed by varying
twist, size and number of single’s yarns included. Spun yarns may be made from natural or synthetic fi bers.
Spun yarn sizes are designated by the number of “hanks” of yarn it takes to weigh one pound. In the cotton system, one hank is 840
yards (770 m) long. One pound of a 12’s cotton yarn is:
12 x 840 yd (770 m) = 10,080 yd (9217 m) long
A fi lament yarn is produced by extruding synthetic materials through an orifi ce in a continuous process. A single fi lament is called a monofi lament. A number of small “fi laments” are combined to form a multifi lament yarn, which is normally called a fi lament yarn.
Filament yarns are stronger than the same-size spun yarns of the same synthetic material.
Filament yarns are designated by a denier number which is the weight in grams of 9000 meters of yarn, or a decitex number, which is
the weight in grams of 100 meters of yarn.. Thus a 1650 denier yarn will weigh 1650 grams per 9000 meters. Table 1-2 provides information on some of the fi ber yarns used in belting fabrics or cords.
Common Name
ASTM Designation
D 1418-10
Composition
General Comments
Urethane AU PolyesterUrethane Excellent abrasion, cut and tear resistance.Good oil resistance. Urethane EU PolyetherUrethane Excellent abrasion, cut, and tear resistance.Good oil resistance. Vinyl PVC PolyvinylChloride A thermoplastic material which has good resistance to abrasion. Excellent fl ame
resistance. Good resistance to animal and vegetable oils. Limited temperature range. Viton* FKM Fluorocarbon Excellent high temperature and chemical resistance properties.
Tefl on* see manufacturer Polymers
*Trade Names
Common Name
ASTM Designation
D 1418-10
Table 1-2.
Some Materials Used in Belting Reinforcement
TEXTILE REINFORCEMENTS
Textile fabrics are the most commonly used materials for reinforcing plies in conveyor and elevator belting. Textile fabrics are also used for conveyor belt “breakers” plies. Fabric properties are governed by the yarn material and size and by the fabric construction and weave.
Fabric is made of warp yarns, which run lengthwise, and fi lling (weft) yarns, which run crosswise, as the fabric is woven, usually at right angles to each other.
Non-woven fabric is a mat of fi bers bonded together chemically and/or needle-punched, usually to a single-ply of woven scrim. The most common, and least complicated, fabric pattern used for fl at belts is the plain weave, Figure 1-1. In this construction the warp
and fi lling yarns cross each other alternately. A belt with two or more of these plies of fabric is known as a multi-ply belt. Other common constructions used to a lesser degree include broken twill, Figure 1-2 and Leno weave, Figure 1-3, which has an open mesh
and is usually used for a breaker fabric.
Solid woven, Figure 1-4, consists of interwoven multiple layers of warp and fi lling yarns.
Straight warp weave, Figure 1-5, contains basic tension-bearing warp yarns which are essentially straight, that is, without crimp.
Also, binder warp yarns are interwoven with the fi lling yarns to provide mechanical fastener holding strength. Some of the most commonly used belting fabrics known by their major fi ber content are:
Cotton - A fabric with cotton in both the warp and fi lling yarns.
Cotton-Synthetic - A fabric with cotton warp yarns and synthetic fi lling yarns or a fabric with cotton/synthetic blended warp and/or
fi lling yarns. The synthetics most commonly used are nylon, and polyester.
Polyester - A fabric with polyester fi ber warp yarns and fi lling yarns.
Common Name Composition General Comments
Cotton Natural Cellulose High absorption of moisture. Susceptible to mildew Only natural fi ber used to any great extent for belting. attack and loss of strength.
Glass Glass High strength. Very low elongation. Used in hightemperature applications.
Kevlar* Aramid Very low elongation and very high strength. Does not
melt but does decompose at high temperature.
Nomex* Aramid Very high strength, low elongation. Excellent high
temperature properties.
Nylon Polyamide High strength and high elongation, with good resistance to abrasion, fatigue, and impact. Moderate moisture absorption. High resistance to mildew.
Polyester Polyester High strength, low elongation. Good abrasion and fatigue resistance. Low moisture absorption. Excellentresistance to mildew.
Steel Cord Steel Very high strength, very low elongation. Superiortroughing characteristics. Excellent heat resistance. Good fatigue and abrasion resistance. *Trade Name
Polyester-Polyester – A fabric with polyester warp and fi lling yarns. Polyester-Nylon - A fabric with polyester warp and nylon fi lling yarns.
Solid woven fabrics are composed of spun and/or fi lament yarns. The spun yarns commonly used may be either cotton or synthetic or
combinations thereof. The fi lament yarns are usually nylon or polyester.
Figure 1-1. Plain Weave Figure 1-2. (Broken) Twill
Figure 1-4. Solid Woven
Figure 1-7. 7 x 19 Construction Figure 1-6. 7 x 7 Construction
STEEL REINFORCEMENTS
Steel Cord
Steel cord is used in belting where the properties of steel cord reinforcement are better able to satisfy the requirements of the service conditions. Steel cord is used to obtain high strength, excellent length stability, low bending stresses and, in some cases, to provide superior troughing characteristics. The wires, or fi laments, used in conveyor belt steel cords are usually made of high carbon steel and have a surface fi nish to facilitate adhesion to the surrounding rubber, and provide protection against corrosion. Common constructions are 7 x 7, Figure 1-6, and 7 x 19, Figure 1-7, although many other constructions are possible.
Steel cords used in conveyor belts are specially manufactured from high carbon steel to meet the high strength requirements demanded of these belts. The cord is fabricated from strands of wires, or fi laments, twisted together. This gives the cord good fl exibility and fatigue resistance when subjected to cyclic loading and bending around pulleys. Two common constructions are illustrated in Figures 1-6 and 1-7.
In order to protect the steel from corrosion, zinc or brass coatings are applied to the wire before drawing it to the fi nal fi lament size. Zinc is the most commonly used coating. Typically, the minimum zinc coating expressed in grams per square millimeter is 60 times the fi lament size in millimeters.
During belt manufacture, the steel cord is encapsulated in a special core rubber that normally has properties different to the belt covers. It is important during manufacture that the core rubber penetrates right to the center of the steel cord as this stops adjacent
fi laments from contacting one another and fretting during bending and stretching of the cord in service. Once embedded in the core rubber, the cord strength increases by up to 5% and it becomes less likely to suffer from corrosion caused by water penetrating the cord. The effectiveness of the rubber penetration can be determined by a special test (AS 1333) which measures if there is any loss in air pressure along the cord when air is applied to one end of the cord at 14.5 psi (1 bar), and maintained for 1 minute on a 16 in long belt sample. 5% is the maximum acceptable pressure loss.
Core rubber to cord adhesion should be adequate to maintain the belt and its splices’ integrity during its normal service life.
Due to the very specialized nature of this cord and the diffi culties in manufacturing cord to achieve these properties, there are only a few manufacturers in the world producing steel cord for conveyor belts.
Other Wire Components
Several other forms of wire are used in belting for special purposes, such as rip resistance and transverse stiffness. A variety of wire structures are used, some of which include: (1) steel fi lling leno weave breakers, (2) straight warp steel fabrics.
CHAPTER 2
ELASTOMER CHARACTERISTICS
HEAVY WEIGHT CONVEYOR BELT
RUBBER COVER CHARACTERISTICS AND CLASSIFICATIONS
Elastomeric covers for general purpose conveyor belts with textile/cord reinforced carcasses will be defi ned as either Grade 1 or Grade 2. The properties, test values and minimum requirements described below can serve as a guideline for acceptable performance in most general purpose applications. It is recognized however that there is no direct correlation between test results and the performance of the belt in service. The test values as outlined are recognized as obtained from new or factory condition belting.
Reference Documents
ASTM D 378 Standard Test Methods for Rubber (Elastomeric) Belting, Flat Type
ASTM D 412 Standard Test Methods for Vulcanized Rubber and Thermoplastic Elastomers -- Tension
CONVEYOR BELT RUBBER COVER GRADES
General Purpose Rubber Covers
ARPM Grade 1- Will consist of natural or synthetic rubber or blends which will be characterized by high cut, gouge, and tear
resistance and very good to excellent abrasion resistance. These covers are recommended for service involving sharp and abrasive materials, and for severe impact loading conditions.
ARPM Grade 2- The elastomeric composition will be similar to that of Grade 1 with good to excellent abrasion resistance in
applications involving the conveying of abrasive materials, but may not provide the degree of cut and gouge resistance of Grade 1 covers.
When covers are tested in accordance with ASTM D 412, the tensile strength, elongation at break shall comply with the requirements of Table 2-1, for the grade of cover, as appropriate.
The tensile strength and elongation at break values are not always suffi cient in themselves to determine the suitability of the belt cover for a particular service.
The values in Table 2-1 should only be specifi ed for conveyors or materials with a known history of performance, and where it is known that compliance with the value will not adversely affect other in-service properties.
Covers for Special Applications
Belt covers may be required to perform in various environments e.g. high heat, exposure to fl uids, abrasive conditions, high ozone concentrations, low temperature exposure and noise generation limits.
Cover and Ply Adhesion
When belting is tested in accordance to ASTM D 378, the adhesion for covers and between adjacent plies should not be less than the values given in Table 2-2. Table 2-2 applies to continuous fi lament carcass.
ABRASION RESISTANCE
As per RMA’s description and classifi cation for both Grade 1 and 2 belt covers; both of these cover types will provide good to excellent abrasion resistance. There are several specifi c tests used by manufacturers to determine the relative abrasion resistance of different cover formulations. The most common is ISO 4649 (DIN 53516).
While there are no specifi c U.S. industry limits, maximum or minimum, for test results from abrasion test for General Purpose (ARPM Grades 1 & 2) Belt Covers; there is enough data to suggest acceptable abrasion values.
A customer preparing to purchase a conveyor belt for abrasion service should, therefore, proceed as follows:
1. Describe as accurately as possible the conditions under which the belt will operate, the nature and composition of the material being carried, the range of particle size, loading conditions, and tons per hour being handled. In those instances where a replacement belt is being ordered, indicate in as complete detail as possible the construction of the belt being replaced and describe the nature of its failure.
2. Point out any condition which might accelerate cover wear, such as excessive heat, moisture, or the presence of oil or other solvents in the installation.
Table 2-1. Properties of Covers
Table 2-2. General Purpose Rubber Cover and Ply Adhesion
COVER THICKNESS
Top Cover Thickness
The major function of a heavy weight belt cover is to protect the strength-bearing carcass from wear or damage during the life of the belt.
In a light weight belt, the cover functions also to provide the required degree of sanitation in food contact applications or the desired friction characteristics, or the required surface characteristics for incline/decline conveying.
The cover thickness required for a specifi c belt is a function of the material conveyed and the handling methods used.
Increased cover thickness is required as the conditions become more severe: e.g. material abrasiveness, maximum lump size of material, material weight, height of material dropped onto the belt, loading angle, belt speed, frequency of loading, etc. The following table shows the suggested minimum belt cover thicknesses for favorable conditions. Wear rates with identical material under adverse loading conditions have been observed to be as much as 6 times the wear rate under favorable conditions.
Grade 1 - Top Cover Thickness
Grade 1 covers should be considered for heavy crushed material over 3 in (75 mm) and when large lumps occur if cut or gouge resistance is the main design criteria. Consult the manufacturer for cover thicknesses.
Grade 2 - Top Cover Thickness (Table 2-3)
Table 2-3. Guide for Minimum Top Cover Thicknesses Under Favorable Conditions for Grade 1 and Grade 2 Belting Note: Cover thicknesses are nominal values subject to manufacturers’ tolerances.
Grade Minimum Tensile Strength (p.s.i.)
Minimum Tensile Strength (MPa)
Minimum Elongation at Break (%)
Maximum Volume Loss (mm3) ISO 4649 Part B
1 2500 p.s.i. 17 MPa 400% 125 mm3
2 2000 p.s.i. 14 MPa 400% 175 mm3
Adhesion between adjacent plies Adhesion between cover & ply
30 lbs/in 5 kN/m
1/32 in (0.8 mm) ≤ Cover Thickness
≤ 1/16” (1.6 mm) Covers greater than 1/16” (1.6 mm)
16 lbs/in 3 kN/m 30 lbs/in 5 kN/m
Class of Material Examples Minimum Thickness
in mm
Package handling Cartons, food products Friction Surface Friction Surface
Light or fi ne, non-abrasive Wood chips, pulp, grain, bituminous coal, potash ore 1/16 1.5
Fine and abrasive Sharp sand, clinker 1/8 3
Heavy, crushed to 3 in (75 mm) Sand, gravel, crushed stone 1/8 3
Heavy, crushed to 8 in (200
mm) ROM coal, rock, ores 3/16 5
Steel Cord Belt Covers
Cover Carcass Dimensions:
To protect the steel cords from impact, abrasion, and water or any other environmental factors, which could cause a loss of strength, during the entire service life of the belt, a minimum thickness of rubber must encapsulate the cords. This cover thickness is usually dictated by the service conditions, but should never be less than 5/32 in (4.0 mm). Failure to respect these limits may lead to uneven, accelerated cover wear or cord damage which would result in reduced belt life. Table 2-4 indicates the minimum thickness “F” above and below the cords that is required for this protection.
Figure 2-1.
A = Protective covering for cords during the entire belt life. (A = 2F + D) B = Amount of top cover used for the service life of the belt.
C = Amount of bottom cover used for the service life of the belt. D = Diameter of the cord.
E = Rubber encapsulating the steel cords and especially compounded for compatibility with the cover rubber and bonding to the steel cords.
F = Thickness of rubber to protect the cords during service. This protective rubber is not part of the top or bottom wear covers used to estimate belt tonnage.
Table 2-4. Guide for Minimum Protective Rubber “F”
* This value has been lowered from the calculated 6.6 mm as a result of favorable fi eld experience. For thickness of covers “B” and “C” consult belt manufacturer.
Note: Minimum thickness of protective rubber “F” should not be less than 3.5 mm or 0.7 times the cord diameter, whichever is greater.For larger diameter cords contact manufacturer.
Cord Diameter Minimum Thickness “F”
(above & below cords)
mm mm in 4.1 5.6 8.3 9.5 3.5 3.9 5.8 5.8* 0.137 0.157 0.228 0.228
Pulley Cover Thickness
The major function of a pulley cover is the same as that of a top cover: to protect the carcass material. In addition, fi eld studies of conveyor power have shown that energy is lost by the pulley cover as it passes over each idler roll. This is called rubber indentation loss and can account for over 60% of the total belt drive power. Special pulley cover rubbers have been developed called “Low Rolling Resistance (LRR)” covers to reduce the amount of power lost. Further details can be obtained from individual belt
manufacturers.. Since a pulley cover is not subjected to the severe conditions imposed upon a conveyor cover, its thickness does not need to be equal to top cover. See below section “Cover Thickness Ratio”.
Table 2-5. Suggested Minimum Pulley Cover Thickness for Grades 1 & 2 Belting
* Increased cover thickness helps protect the carcass; however, if impact is severe, the complete system design, including carcass construction, top cover thickness, and impact rolls in the conveyor, must be considered.
Note: Cover thicknesses are nominal values subject to manufacturers’ tolerances. Cover Thickness Ratio
The thickness of the cover on conveyor belting must be selected on the basis of the service conditions to which the belting is to be subjected. The ratio of the thicknesses of the top and bottom covers must also be considered. This factor becomes increasingly important with conveyor belting where the carcass is thinner than those of comparably rated multi-ply conveyor belts.
A large cover thickness ratio, such as greater than 4:1, where one cover, the top, is much thicker than the other, the bottom - may cause a conveyor belt to assume a permanent transverse curl or cup, wherein the edges of the belt curl up on the carrying run and down on the return run. In its more severe state, this curl can adversely affect the training of the belt, especially on the return run. When the curl has progressed to the point that only the edges of the belt contact the return idlers, training of the belt is virtually impossible.
The transverse belt curl that results from a large cover thickness ratio is a result of the shrinkage that occurs in rubber compounds after vulcanizing. With a large cover thickness ratio, the shrinkage force of the thicker cover dominates, causing the belt to curl toward the thicker cover. Multi-ply type belts, with their relatively thick and transversely stiff carcass, tend to resist the curl forces, but thin belt carcasses offer less resistance.
Although transverse curl may occur in any size of conveyor belt, it is most likely to cause operational problems in narrower belts, up to 36 in (900 mm) wide. To a lesser degree, it can cause problems with 48 in (1200 mm) widths. With the wider belts, the belt weight usually forces the center of the belt down into contact with the return idlers, thus allowing normal training action to occur.
Generally, a maximum ratio of 4:1 for multi-ply and 2:1 for single-ply belting is recommended.
Cover thickness ratio specifi cations vary among manufacturers of conveyor belting. Individual belting manufacturers should be consulted for their specifi c recommendations on cover thickness ratios for belting.
POLYVINYL CHLORIDE (PVC) CHARACTERISTICS
PVC is a resin produced from polymerizing vinyl chloride. The term PVC in the belting trade is generally applied to the elastomeric material that results from the resin having been mixed with various liquids and powders and heat treated to change the mixture into a usable elastomeric condition.
The mixture of PVC, liquids, and powders may be used in the form of a liquid plastisol for saturating and top coating fabric or as a fi lm to laminate and top coat fabric.
The PVC elastomer is thermoplastic. It hardens and stiffens with reduced temperature and softens and becomes more fl exible with elevated temperatures.
PVC belting operates well in the range of 20 to 180°F over conventional size pulleys. With special handling, operation down to - 30°F is possible. General purpose PVC belting becomes hard and cracks when subjected to certain hydro-carbons and oils, which cause a softening and swelling action on general purpose rubber. PVC can be compounded to prevent the deleterious effect of those hydrocarbons and oils.
PVC can be compounded to promote good fl exibility at -40ºF and to improve fl ame propagation resistance. PVC elastomers are resistant to acids, alkalies, strong oxidizing agents and strong chlorinated cleaning agents.
Operating Conditions Minimum Thickness
in mm
Slider bed package conveyors bareback or friction surface bareback or friction surface
Abrasive materials 1/32 1
Belting can be designed to operate in various conditions and environments. No one belt type will handle all conditions well. Specifi c environments that require special service belts include: static conductive, fl ame/fi re resistance (MSHA), high (and low) temperature, oil service, high temperature and oil service, high temperature abrasion, etc.
Specifi c test protocols are used to determine the elastomer’s response to these conditions and environments. An abbreviated listing of these tests are offered in Table 2-6 for reference in regards to belt recommendations.
Table 2-6. Test Protocols for Special Service Belt Covers
RECOMMENDATIONS FOR OIL SERVICE BELTING
Various levels of oil service may be required from belt products. These service levels may or may not involve elevated temperatures. The ARPM classifi es belting (or cover formulations) to meet either MOR/VOR (Moderate / Vegetable Oil Resistant). Service requirements or EOR (Extreme Oil Resistance) service requirements based on the following test criteria.
Table 2-7. %Volume Swell (ASTM D 471) 70 hr @ 100°C
MOR / VOR - Belting is designed to resist swelling and deterioration from vegetable based oils as well as light (napthenic / paraffi n / low aromatic) petroleum oils.
EOR / SOR - Belting is designed for use in extremely oily environments, especially where polar aromatic materials are expected to be encoutered. Depending on temperature requirements and manufacturers’ recommendations, this class of belt may be suitable in “Hot Asphalt” applications. Additionally, in coal fi red power generation facilities where the fuel is being enriched with petroleum waste oils or fuel / diesel oils, this may be the belt type required. Consult the manufacturer for recommendations when abnormal conditions are anticipated.
Most of the cover formulations for belting meeting these classifi cations will be comprised of, or contain a certain percentage of, one or more of the following polymers: CR (Polychloroprene / Neoprene), NBR (Nitrile), PVC, Urethane (AU / EU), CPE (Chlorinated
Condition Test Method
Friction (Coeffi cient) ASTM D 1894 -- Standard Test Method for Static and Kinetic Coeffi cients of Friction of Plastic Filmand Sheeting
Flame Resistance ASTM D378 13.1 (MSHA -- 30 CFR: part 14) ASTM D378 13.2 Heat Resistance
Heat Resistance ASTM D 865 -- Standard Test Method for Rubber-Deterioration by Heating in Air (Test Tube Enclosure)
Heat Resistance ISO 4195-1 Conveyor belts Heat resistance Part 1: Test method; ISO 4195-2 Conveyor belts --Heat resistance -- Part 2: Specifi cations
Low Temperature ASTM D 2136 -- Standard Test Method for Coated Fabrics -- Low Temperature Bend Test
Low Temperature ASTM D 2137 -- Standard Test Methods for Rubber Property -- Brittleness Point of Flexible Polymer-sand Coated Fabrics
Oil Service / Chemical ASTM D 471 -- Standard Test Method for Rubber Property -- Effect of Liquids
Ozone ASTM D 1149 -- Standard Test Method for Rubber Deterioration -- Surface Ozone Cracking in a Chamber
Tear Resistance ASTM D 624 -- Standard Test Method for Tear Strength of Conventional Vulcanized Rubber and Thermoplastic Elastomers
Oil Resistance ASTM #1 Oil ASTM #3 or #903 Oil
MOR / VOR
Moderate / Vegetable Oil Resistant 15% Max. 140% Max.
EOR / SOR
HIGH TEMPERATURE SERVICE CLASSIFICATIONS
Belting designed and manufactured to handle elevated temperatures in service will be classifi ed by type depending on the belt cover characteristics when tested to ASTM D 865 at the specifi ed times and temperatures.
Table 2-8. High Temperature Testing (ASTM D 865)
ISO 4195 is referenced as another testing and classifi cation tool. While the classifi cations and value limits are similar between these tests, they differ in both time of exposure (70 hr vs. 168 hr) and method of sample preparation. ISO 4195 calls for the entire belt sample to be exposed, with test specimens to be cut / prepared from the exposed belt samples. ASTM D 865 allows for test specimens to be prepared before exposure. The correlation between these methods has not been determined and differences are expected, since the mass of ASTM sample is small relative to the dimensionally large ISO sample size. Hence the shorter time of exposure per the ASTM / ARPM protocol.
While these tests and classifi cations do not validate product usefulness or acceptability in specifi c environments, they are used as tools by the industry to more narrowly defi ne criteria for applications involving elevated temperatures. It must be noted that temperature alone may not be the overriding / determining factor in product suitability. Certain conveyed materials may degrade various elastomers at test temperatures that the elastomers may be expected to perform based on test conditions. Consult the belt manufacturer for specifi c recommendations.
A wide variety of fl ame tests for conveyor belts exists throughout the world. The standard used in a particular country is usually dictated by a national or local governing body. For general fl ame resistant conveyor belting, the selection of the most suitable quality may be made by the ARPM-FR class designations.
ARPM-FR Class I
Based on the December 31, 2008 U.S. Mine Safety & Health Administration’s (MSHA), CFR Title 30 Section 14, “Requirements for the Approval of Flame-Resistant Conveyor Belts”, also known as the Belt Evaluation Laboratory Test or “BELT” test, this new ARPM-FR standard provides a fl ame resistance quality that is currently mandated by MSHA in the USA for underground coal mines. This belt quality is appropriate for belts that require fl ame resistance and which are included in the December 31, 2008 CFR, Title 30, Mineral Resources, Section 14, which primarily applies to conveyor belts used in underground coal mines.
The test procedure is described in ASTM D 378 Section 13.1 and employs 60 in x 9 in sized belt test samples. Following the original MSHA guidelines, the acceptance criteria for three belt samples tested to this ARPM-FR Class I standard is each tested sample must exhibit an undamaged portion across its entire width.
FLAME RESISTANCE SERVICE CLASSIFICATIONS
WARNING: All belting will burn when adequately ignited
Time 70 hr - Test Temperature Retained Tensile-from original
Retained Elongation
from original Hardness pt. change
ARPM-HR Class 1 212°F (100°C) -25% (max.) -50% (max.) +20 (max.)
ARPM-HR Class 2 257°F (125°C) -30% (max.) -50% (max.) +20 (max.)
ARPM-HR Class 3 302°F (150°C) -40% (max.) -60% (max.) +20 (max.)
Belt Designation
Sample Size (Qty.)
Method (Time)
Pass Criteria
ARPM-HR Class 1
60” x 9” (3)
1524 mm x 229mm
ASTM D378 13:1
Burner (5 min)
Some undamage belt in
each sample
ARPM-HR Class 2
6” x 0.5” (4)
152mm x 13mm
ASTM D378 13:2
Bunsen Burner (1 min)
Flame out average < 1 min
No afterglow after 3 min
Table 2-9. Flame Resistance Testing (ASTM D865)
Effective December 31, 2008, the United States changed the minimum standard for fl ame performance of underground coal mine conveyor testing. Until December 31, 2009 conveyor belts placed in service in underground coal mines shall be either approved under Part 14; or accepted under Part 18. Part 18, is an old MSHA standard, “Code of Federal Regulations, Title 30, Mineral Resources, Section 18.65, Flame Testing of Conveyor Belting and Hose.” Part 18 is commonly known as “2G”. Effective December 31, 2009 conveyor belts placed in service in underground coal mines shall be approved under Part 14. If MSHA determines that Part 14 approved belt is not available, the Agency will consider an extension of the effective date. Effective December 31, 2018 all conveyor belts used in underground coal mines shall be approved under Part 14.
Effective December 2, 2005, in Canada, the CAN/CSA M422 M87 “Fire Performance and Antistatic Requirements for Conveyor Belting” standard was withdrawn. Formerly this standard was the minimum standard for fl ame performance and electrostatic conductance for underground belting which was tested in accordance with the CAN/CSA M422 M87 “Fire Performance and Antistatic Requirements for Conveyor Belting” by Energy, Mines and General
When it is the user’s opinion there is a potential fi re hazard, he should consult the belt manufacturer and consider whether belting manufactured to the above specifi cations is suitable for the application. In each installation, consideration should also be given to the following:
a. Fire detection systems
b. Automatic fi re suppression systems c. Slip and sequence interlock systems
d. Sprinklers at transfer points to reduce fl ammable dust e. Belt lateral alignment controls
f. Elimination of combustible materials near the conveyor belt
g. Conductive paths to ground for static electricity including conductive grease in bearings h. Chute probe or level indicators at transfer points
i. Fire retardant, static electricity conducting drum lagging, skirts, scrapers, and chute lining j. Heat sensors for conveying pulley bearings.
ARPM-FR Class II
Based on the pre - December 31, 2008 U.S. Mine Safety & Health Administration’s (MSHA), CFR Title 30 Section 18.65, “Requirements for the Approval of Flame-Resistant Conveyor Belts”, also known as the “2G” test, this new ARPM-FR standard provides a basic fl ame resistance quality that was formerly mandated by MSHA and was used successfully in the USA for many years. This belt quality is appropriate for belts, such as above ground belts, that require fl ame resistance and which are not included in the December 31, 2008 Code of Federal Regulations, Title 30, Mineral resources, Section 14, which primarily applies to conveyor belts used in underground coal mines.
The test procedure is described in ASTM D 378 Section 13.2 and employs 6 in x 0.5 in sized belt test samples.Following the original MSHA guidelines, the acceptance criteria for belt samples tested to this ARPM-FR Grade II standard is defi ned as the tests of four specimens cut from any belt sample shall not result in, either duration of fl ame exceeding an average of 1 minute after removal of the applied fl ame, or the continuation of visible glowing of a specimen after fl aming has ceased (afterglow) exceeding an average of 3 minutes duration.
ARPM-FR Class Test Responsibility
Each belt manufacturer is responsible to ensure tests are conducted to the appropriate ARPM-FR class specifi cation on each belt order claiming the ARPM-FR class quality. Tests may be witnessed at any time by the customer or his representative to ensure compliance to the test standard.
Marking
A ARPM-FR class conveyor belt must be permanently and legibly marked with the appropriate ARPM-FR class designation (and/or MSHA approval number for ARPM-FR Class I) for the service life of the product. The marking must be at least 0.5 in (1.27 cm) high and placed at intervals not to exceed 60 ft (18.3 m) repeated once every foot (.3 m) across the width of the belt. Records of the initial sale of each belt order having the ARPM-FR class marking and actual test conditions and test results must be retained for at least 5 years.
POLYURETHANE (PU) CHARACTERISTICS
Polyurethane is generally characterized as a cut and abrasion resistant polymer with excellent mechanical properties in the range of about -65 to 212°F (-54 to 100°C). There are both thermosetting and thermoplastic grades used in belting, and polymer back bones that enhance oil resistance or water resistance. The thermoplastic grades are easily spliced in belt constructions, and food contact polyurethane compounds are available.
CHAPTER 3
TEXTILE BELT TYPES AND MANUFACTURING METHODS
INTRODUCTION
This chapter describes types of textile belting in terms of carcass types and of edge protection. This will be followed by a description of belt cover designs and textile belt manufacturing methods.
BELT CARCASS TYPES
The belt carcass primarily provides resistance to tension forces that build up in the conveyor system. Also it provides strength to resist belt tear and loading impact and for load support, troughing, mechanical fastener holding ability, and resistance to wrinkling or edge cupping.
Textile Fabric Carcass - (See Figure 3-1)
The textile fabric carcass may have one or more plies of fabric bonded by elastomeric compounds to both themselves and to the belt cover. Belt strength and load support characteristics depend on the fabric construction and the number of plies used. Flexibility/ stiffness are functions of the fabric construction and number of plies of fabric, and skim and cover thicknesses and their elastomeric properties. The elastomeric compounds in heavy weight belting are often thermosetting.
Light weight belting is reinforced in some constructions by one or more plies of fabric like the heavy weight belting or in other con-structions by solid woven or interwoven fabric or by non-woven fabric which generally has a woven scrim component. The individual plies in light weight belting often have monofi laments in the weft to impart transverse stiffness, and the elastomeric materials in the plied constructions are predominantly thermoplastic.
Textile Fabric -- Multi-Ply Belt Shown with Three Fabric Plies and Cut Edges*
* Refer to Glossary for defi nition of cut and slit edges..
Solid Woven Carcass
Solid woven belting consists of a single ply carcass made up of multiple layers of warp and fi lling yarns interwoven. The carcass is usually impregnated and/or coated with thermoplastic compounds.
BELT EDGE PROTECTION - MOLDED EDGES
Molded (Capped) Edge Belting
Historically all conveyor belting was made with molded (capped) edges (Figure 3-2). Molded edges were necessary to protect the cotton fi ber in the carcass against mildew or chemical action. Thus the carcass, in addition to being covered, was encapsulated around the edge with the elastomeric compound of the covers, and molded into a square capped edge. It must be recognized this was only a temporary expedient; since, when the covers were cut, gouged or worn to the fabric and the molded edges were torn or worn off, the absorption of water and chemicals would occur. With the availability of nylon and polyester fabrics, cut edge belting is now commonly used.
In light weight belt applications capped edges are used in applications where improved edge protection is required. For example: food processing to eliminate edge fraying and subsequent absorption of fl uids.
Figure 3-2.
Multi-Ply Belt Shown with Four Plies of Reinforcing Fabric, A Breaker Ply, Covers and Rubber-Capped Edges Cut/Slit Edge Belting
The general use of nylon and polyester yarn for conveyor belt carcasses has eliminated the concern for protecting the belt carcass with molded edges. The nylon and polyester fi bers are resistant to mildew attack and the polyester to most chemicals. Thus most belting is now supplied with slit edges.
CARCASS PROTECTION
Breaker
Before the use of nylon and polyester carcass fabric, breaker plies of open texture leno weave cotton or nylon yarn were frequently used between the carrying cover and the belt carcass. It was believed a breaker ply improved the adhesion of the cover. The breaker ply next to the carcass improved cover cut and gouge resistance and provided material loading impact resistance. Breaker plies are used where severe impact conditions exist. Sometimes a breaker fabric in a molded edge is wrapped around the fabric edge to provide edge protection.
BELT COVER DESIGNS
For most applications, conveyor belts have a smooth top and/or bottom cover made of elastomeric compound suitable for the material to be conveyed. There are, however, some special purpose belt surface fi nishes described in the following.
Bareback Surface
The outer surface of the top or bottom of the fabric of a bareback belt has neither an elastomeric compound cover nor is it impregnated with an elastomeric compound. A bare fabric surface provides a low coeffi cient of friction. A slider bed package conveyor with the bareback surface down against the slider bed or the bareback surface up in connection with a diverter bar are examples of bareback surface applications.
Friction Surface
The outer top and/or bottom surface of the fabric of a friction surface belt has a light impregnation of elastomeric compound.
Brushback Surface
Certain friction compounds may be buffed to further reduce the coeffi cient of friction while retaining the elastomeric compound in the interstices of the fabric.
Bareback, brushback and friction surface belts can be provided with a cover on one side of the belt.
Impression (Rough Top) Surface
Impression belts have an embossed profi le in a cover made by curing the elastomeric cover against a mold, fabric, or stamped metal or by embossing a thermoplastic cover. Impression belts are often used to convey material on inclines and declines where slippage may occur.
Cleated, Flanged (Sidewall) or Ribbed Top Surface
Cleats, fl anges or ribs in transverse, longitudinal, continuous or intermittent, and of angular, straight or curved design may be molded onto or affi xed to the cover. They improve the ability to carry coarse material on incline and decline applications. The height and spacing of the cleats, fl anges or ribs depend on the size of the material to be conveyed.
BELT MANUFACTURING METHODS
SINGLE AND MULTI-PLY BELTS
Drying
Cotton and spun synthetic yarn fabrics must be heated before they are frictioned so the friction rubber can be properly impregnated into
the interstices of the fabric. Also the cotton, rayon, and nylon spun yarn fabrics must be thoroughly dried to remove moisture which in the belt curing operation could cause blisters between the plies of fabric or under the covers.
Textile Fabric Treatment
Generally, most multi-fi lament textiles (nylon, polyester, etc.) require an RFL treatment to ensure adequate adhesion in service. RFL is an industry term designating a treatment mixture of resorcinol formaldehyde latex (RFL), whereby the woven textile is dipped in the emulsion and dried under specifi c temperature and tension conditions. This process is used for most rubber based belting (Natural, SBR, NBR, CR, EPDM, etc.). For thermoplastic type belt, the treatment can involve acylics, polyurethane, PVC or other treatment for the respective textile reinforcements.
Rubberizing (Skimming, Bank Coating, and Frictioning)
The fabric is impregnated with a suitable elastomer by “frictioning” and/or “skim coating” on 3-roll or 4-roll calenders. Frictioning forces the pre-softened rubber compound into the interstices of the fabric by the wiping action of two calender rolls running at different surface speeds.
In skim coating the calender roll speeds are essentially the same, and a thin layer of rubber compound is laid on the fabric. During the calendering operations, uniform tensions are maintained on the fabric to prevent undesirable distortion.
Carcass Building
Calendered plies of fabric are laminated and consolidated by squeezing between two rolls of a building unit. Depending on equipment design, from two to fi ve plies can be laminated in a single pass through the unit. Uniform tension is maintained on each ply to ensure maximum effi ciency during service.
Longitudinal seams (ply splices) result when it is necessary to use more than one strip of fabric to make the full ply width. The seam is made by bringing the two edges together and, if necessary, placing a rubber cord over the joint so that a void does not occur when vulcanizing the fi nished belt. Longitudinal seams are generally made during the laminating pass through the building unit. Seams shall be at least 4 in from the edge, separated by 12 in within the ply and be removed from the idler junction area. Number of seams are limited by belt width.
Tranverse seams (ply splices) result when the fabric length is less than the full length of the fi nished belt. The ends of the two or more pieces are prepared by cutting on a 20° to 45° bias angle. The ends are then butted against each other and if necessary, a strip of rubber compound is placed over the joint to prevent a void from forming during subsequent manufacturing operations. The preparation of the bias cut ends is done during the actual laminating operation at the carcass building machine, which results in a good matching of the two ends being joined. Transverse seams shall be at an angle between 26.5° and 70°, shall be separated by at least 50 ft, and be at least 50 ft from the end of the belt. No transverse seams are allowed in the outer plies.
Belt Covers
The elastomeric covering on belts is there to provide protection for the carcass, and/or provide a specifi c property. These coverings are applied by several processes, depending on the material (rubber vs. thermoplastic) or thickness of the covering.
For rubber belting covers are either extruded or calendered. Extruded rubber sheets of specifi c widths and thickness are then laminated or press plied onto the carcass. similarly, thermoplastic covers can also be extruded and laminated.
For most thin belt covers (i.e. pulley “side” covers), less than 1/8 in (3.2 mm), application is performed at a calender unit where the elastomeric compound is “skimmed” onto the textile. This process can accommodate some thermoplastic materials as well as rubber. PVC covers are also applied with roll or knife coating processes.
Release Coating
After applying the last cover, a light coat of release agent, is applied to one or both surfaces of the belt. This is done to prevent the unvulcanized belt from sticking in the roll before cure and to help in stripping the belt from the press surface after cure.
After release coating and before curing, the cover is usually perforated with fi ne pricker needles to help release gases that may be present withing the body of the belt. These holes are completely sealed during the vulcanization operation.
Curing
The belt is vulcanized in either a fl at platen press (index cure) or a rotary press (continuous cure). In either case, curing is done at a temperature in the range of 280-320°F (140-160°C) while under pressure. Edge irons or rings are set at the desired belt width to retain and/or mold the rubber covered edges.
Since it is essential that a small excess of material be present to create proper pressure during cure, a small overfl ow of cover occurs at the side retaining irons. This is removed by trimming or buffi ng as the cured belt comes from the press. Slab belts which are slit to width have the entire edge cut away during a subsequent operation.
Tension is generally applied to the belt during cure so that the elongation of the fi nished product is within acceptable limits. Branding of the belt with the manufacturer’s name, grade or type of belt, and date of manufacture is generally accomplished by placing a metal stencil on the uncured belt at regular intervals. This produces an embossed label cured onto the surface.
Slitting
Slab belting is slit to the fi nal width after it is cured. Full-width rubberized fabric is used to build the carcass.
SOLID-WOVEN RUBBER BELTS
Carcass
The woven fabric is generally treated with a special bonding adhesive which is applied by passing the fabric through a bath containing the adhesive. (See “Dipping.” under Single and Multi-ply Belts above.)
Rubberizing
The dried carcass is then impregnated by frictioning and/or coating on a calender. (See “Rubberizing” under Single and Multi-ply Belts above.)
Covering, Dusting, and Curing
These steps are essentially the same as for Single and Multi-ply Belts above.
SOLID-WOVEN PVC BELTS
Textile Fabric Treatment
The single- and multi-ply fabric is impregnated with PVC plastisol during and/or following weaving.
Covering and Fusing
The carcass is fi rst passed through a plastisol dip tank for impregnation and cover application and then into a heated oven where plastisols are fused to the consistency required to meet service conditions. The PVC compound can alternatively be calendered into a fi lm or sheet which can then be applied to the carcass. If smooth cover surfaces are required, fusing may be accomplished in fl at or rotary presses. If rough top, cleated or ribbed top cover surfaces are required, embossing of the cover may be done immediately following the fusion process.
SINGLE AND MULTI-PLY THERMOPLASTIC BELTS
Single and multi-ply PVC belts may be produced by dipping and/or top coating the carcass fabric with PVC plastisols, which provide the elastomeric binding layer between plies and the top cover surface. Fusion of the PVC compounds is done by heating to
temperatures of approximately 350-380°F (177-193°C), and the surfaces of the belting are smoothed or embossed to provide the required textures and fi nish.
Single and multi-ply polyurethane belts may be produced by coating the carcass with a fi lm or sheet of compound from a hot melt coater or extruder or spraying operation and then cooling or by building a laminated construction using fi lms or sheets of compound that are later heated in a fl at or rotary press. Pre-treating the carcass is done to enhance adhesion of compounds.
INTRODUCTION
The tension rating for a belt is the recommended maximum safe working stress that can be applied to the belt.
Belt tension is commonly referred to as the force applied to the belt per unit of belt width, such as Pounds per Inch width (PIW), or Kilo Newtons per Meter width (kN/m). Textile fabrics are frequently rated for their maximum safe working stress which is expressed as the force applied per ply of fabric per unit width of the belt.
There is variation among manufacturers about the following information that relates to number of plies of fabric, belt carcass thickness, minimum pulley diameter, troughability, etc., to the belt maximum safe working stress because of differences in materials and manufacturing methods.
Some key differences which exist are:
1.The fi ber, polyester and/or nylon, used for the fabric. 2.Recommended safe working strength for the fabric used.
3.Ratio of belt breaking strength to belt maximum safe working stress (safety factor).
These factors also affect the belt carcass thickness, belt weight, minimum pulley diameter, troughability, load support with different angle idlers, transition distance, impact resistance, etc. Thus, it is essential to confer with the belt manufacturer about the belt proposed for each application.
CONVEYOR BELT AND SYSTEM TENSION CALCULATIONS
Conveyor systems will take on a variety of confi gurations relative to drive location, elevation or descent of the load, idler and pulley type and condition, and other factors too numerous to detail in this handbook. Belt manufacturers or conveyor engineering
companies should be consulted for belt (system) recommendations. The Conveyor Equipment Manufacturers Association (CEMA) provides a Handbook for in-depth system analysis and tension calculations. International Standard ISO 5048 and the German standard DIN 22101 also provide detailed methods for system tension calculations.
The tables below provide an example of the basic information on multi- and single-ply fabric belt tension ratings. This information is for illustrative purposes only. Information on a specifi c belt construction can be provided by the belt manufacturer.
The data in the following tables apply if the following service conditions occur:
Mechanical Fastener Splice
1.Pulley diamters recommended by the belt manufacturer and fastener manufacturer are used.
2.No abnormal conditions, such as heat or chemicals, are exposed to the belt that will reduce the belt fabric strength or change the fl exibility of the belt fabric.
3.Recommended fasteners are properly applied.
4.Across the line starting tension is limited to not greater than 150% of the splice rating. Step phase or soft starting is preferred.
Vulcanized Splice
1.Pulley diameters recommended by the belt manufacturer are used. 2.Automatic take-up with adequate take-up travel.
3.Splices are made strictly in accordance with the belt manufacturer’s specifi cations.
Where an adverse environmental condition or some special belt application exists, it is critical that the belt fabric ply tension rating be reduced by some factor recommended by the belt manufacturer. Some of the special conditions are:
1.Continuous excessive ambient temperature. 2.Exposure to deleterious chemicals.
Elevator Belt Tension Recommendations
Elevator tension ratings may require modifi cation under certain adverse environmental conditions. In such cases the rating in the following tables should be multiplied by an environmental factor of 0.75.
Adverse environmental factors for elevator belts include:
1.Elevated temperatures in the belt reinforcing fabric due either to high ambient temperatures or to conveying hot materials. 2.Abrasion of surface plies which are not protected by an elastomeric cover, such as friction surface belting in abrasive service. 3.Chemical service detrimental to the carcass fi ber.
Safety Factors
Conveyor belt operating tensions are chosen as a small percentage of the belt’s breaking strength. This provides spare strength for (1) temporary higher transient loads such as during starting and stopping, (2) handling unusual system loads such as misalignments or frozen idlers, and (3) loss of strength due to materials’ aging and other degradation factors. The ratio of original belt strength to operating tension is called the belt’s Safety Factor. Traditionally, the conveyor industry has used safety factors around 10:1 for fabric belts and around 6.7:1 for steel cord belts, however, higher and lower factors are common. It is recommended to contact the belt manufacturer for a safety factor recommendation for a specifi c application.
In recent years, studies have linked a belt’s safety factor to its dynamic splice strength and tests have been developed to measure the dynamic strength of the splice. There are now international standards, such as DIN 22110, that defi ne how the dynamic splice strength can be measured. There are also standards, such as DIN 22101, that provide a method to calculate the safety factor for a belt. A general guideline is that fabric belt splices have a dynamic splice effi ciency of 35% of the belt’s breaking strength and steel cord belt have 45%. In practice, many conveyor belts deteriorate due to abuse or accidental damage and historical data should always be considered when selecting a safety factor. Other factors that should be considered when selecting a belt’s safety factor include the effects of a catastrophic belt break. For example, personnel safety, loss of production, clean up cost, repair time, accessibility of the belt for repair, and availability of repair labor and materials. There are examples where a critical conveyor belt has broken due to loss of strength from accidental damage combined with a high peak transient load. Such events can cost millions of dollars of lost production. The recent availability of cord monitoring systems for conveyor belts offers improved capability of accidental damage surveillance in steel cord belts. When used correctly, such systems offer additional safeguards for the operation of belts with lower safety factors.
Table 4-1. Typical Ratings CONVEYOR
Working Strength (PIW) 220 250 330 375 400 440 500 600 750 800 1000 1000 1200
Number of Plies 2 2 3 3 2 4 4 3 3 4 4 5 6
Approximate Carcass Thickness
(in) .12 .146 .16 .20 .182 .220 .254 .258 .27 .265 .28 .335 .38
Approximate Carcass Weight
(lb/sq. ft) .70 .81 .94 1.12 1.02 1.3 1.39 1.42 .135 .125 .141 .165 .188
Minimum Pulley Diameter (in) (% of rated max. belt tension)
81-100% 16 16 18 18 16 24 24 24 22 30 30 36 42
61-80% 14 14 16 16 14 20 20 20 24 20 24 24 30
41-60% 10 10 12 14 12 18 18 18 20 16 20 20 24
To 40% 10 10 12 12 10 16 16 16 18 14 18 18 20
TROUGHABILITY
Idler Troughing Minimum Belt Width (in) for Empty Troughing
Angle˚
20˚ 14 18 18 24 18 24 30 24 36 36 42 48 NR
35˚ 18 24 24 30 24 30 36 30 36 30 36 42 42
45˚ 24 30 30 36 30 36 42 36 42 36 42 48 48
Maximum Belt Width (in) for Empty Troughing
Material Weight (lb/cu. ft)
20˚ 0-40 48 54 60 72 60 72 84 84 84 72 84 84 84 41-80 48 48 60 60 54 66 72 72 72 72 72 84 84 81-120 42 42 54 54 48 60 72 72 72 72 72 84 84 Over 120 36 36 48 48 42 54 60 60 60 60 60 72 84 35˚ 0-40 42 48 54 60 54 60 72 72 84 72 72 84 84 41-80 36 42 48 60 48 60 60 60 72 60 66 72 84 81-120 36 42 48 54 48 54 60 60 72 60 60 72 84 Over 120 30 30 42 42 36 48 54 54 60 54 54 60 72 45˚ 0-40 36 48 48 60 54 54 72 72 72 72 72 84 84 41-80 36 36 42 48 42 48 54 54 60 54 72 72 84 81-120 30 30 42 48 42 48 54 54 60 54 72 72 84 Over 120 NR NR 36 36 30 42 48 48 54 48 54 54 72 ELEVATOR
Minimum Pulley Diameter
81-100% tension 18 18 20 20 18 30 30 30 36 22 30 36 42
61-80% 16 16 18 18 16 24 24 24 30 20 24 30 36
Up to 60% 12 12 14 14 14 20 20 20 18 20 24 30 36
Maximum Pulley Projection
Spaced Industrial 100 lb/cu. ft 6 7 7 8 9 10 11 10 11 9 10 11 12
Table 4-2. Typical Ratings - Straight Warp Conveyor or Elevator Rubber Belting
Note 1:These are typical values only, please consult your belt manufacturer for specifi c product values. Note 2: Table 4.2 includes expanded product ratings.
CONVEYOR
Single-Ply Straight Warp Fabric Double-Ply Straight Warp Fabric
Working Strength (PIW) 190 220 275 390 385 440 440 550 660 800 1000 1250 1500
Approx. Carcass Gauge (in) .078 .103 .125 .131 .157 .165 .195 .234 .250 .281 .320 .328 .359
Approx. Carcass Weight
(PIW) Factor (lb) .038 .049 .056 .064 .067 .072 .105 .114 .120 .134 .148 .165 .172
Minimum Pulley Diameter (in) Depending on Fastner Splice Selected
Working Tension
81-100% 16 16 18 20 24 24 24 30 30 36 42 42 42
61-80% 14 14 16 18 20 20 20 24 24 30 36 36 36
Up to 60% 12 12 14 16 18 18 18 20 20 24 30 30 30
TROUGHABILITY
Troughing Angle Minimum Belt Width (in) for Empty Troughing
20˚ 12 14 14 18 18 18 24 24 24 30 30 30 30
35˚ 14 20 20 24 24 24 30 30 30 36 36 36 36
45˚ 16 24 24 24 24 24 36 36 36 42 42 42 42
Maximum Belt Width (in) for Load Support
Material Weight (lb/cu. ft)
20˚ 41-80 lb/cu. ft 36 42 54 60 60 66 84 84 84 84 84 84 84 81-120 lb/cu. ft 30 36 42 48 48 54 84 84 84 84 84 84 84 Over 120 lb/cu. ft 30 36 42 48 48 54 60 66 84 84 84 84 84 35˚ 41-80 lb/cu. ft 30 36 42 54 54 60 84 84 84 84 84 84 84 81-120 lb/cu. ft 24 30 36 42 48 48 84 84 84 84 84 84 84 Over 120 lb/cu. ft 24 30 36 42 42 48 54 60 66 84 84 84 84 45˚ 41-80 lb/cu. ft 30 36 42 48 48 54 60 66 84 84 84 84 84 81-120 lb/cu. ft 24 30 36 42 42 48 54 60 84 84 84 84 84 Over 120 lb/cu. ft 18 24 30 36 36 42 48 54 60 66 84 84 84 ELEVATOR
Minimum Pulley Diameter Working Tension
81-100% 16 16 18 20 24 24 30 30 30 36 42 42 42
61-80% 14 14 16 18 20 20 24 24 24 30 36 36 36
Up to 60% 12 12 14 16 18 18 20 20 20 24 30 30 30
Maximum Bucket Protection
Space Industrial 6 7 8 9 9 9 11 11 13 13 14 15 16
Belt operating tension is not the only belt characteristic to be considered when selecting belt design for an application. Other important items exist that effect how the belt will perform on a given system. The importance of these characteristics is presented below.
ELONGATION
Most new conveyor belts will exhibit some permanent stretch very early in their service life, as a result of the normal cyclic tensile forces exerted by the conveyor system on the belt. This length change will vary among belt constructions, but it is generally much less than one percent of the original relaxed length of the belt. The conveyor take-up system must compensate for this length change as well as the normal belt elongations which are proportional to belt tensions in the elastic region of the stress strain curve.
Table 3-3. Recommended Minimum Take-Up Travel
(percentage of the distance between centers of the convenor*)
*For belts installed at average empty running, take-up position 90% of the travel, and drive location at or near the high tension end of
the conveyor.
**Only short endless feeder belts and the like would normally be vulcanized on conveyors with a manual take-up.
TROUGHABILITY AND LOAD SUPPORT
In order to achieve the desired carrying capacities of bulk materials without spillage over the edges, most conveyor belts are operated in a troughed confi guration where the trough is usually formed by a 3-roll idler system as indicated by Figure 4-1 below. The angle of the troughing rolls will usually range from 20° to 45°.
Figure 4-1. Belt Troughing In-Line Idler
When the belt is running empty, it must have suffi cient lateral fl exibility to retain contact with the center roll. Failure to do so will usually cause the belt to wander from side to side, and considerable edge damage may result.
Conversely, when the belt is running fully loaded, it must have suffi cient lateral stiffness to support the load and bridge the gap between the center and troughing rolls. If the belt is too fl exible in this regard, it will tend to crease into the idler gap and fail prematurely at that point. This potential problem can be reduced by using offset troughing idlers. With offset idler systems the load support may be liberalized (consult belt manufacturer).
Type of Take-Up and Carcass Material (warp)
Percent of Rated Tension
100% 75% 50% or less Manual Take-Up** Nylon 4.00% 3.00% 2.00% Polyester 2.50% 2.00% 1.50% Automatic Take-Up Nylon 3.00% 2.50% 1.50% Polyester 0.00% 1.00% 1.00%
LIMIT OF IDLER GAP BETWEEN CARRYING IDLERS FOR TROUGHED BELT CONVEYORS
The Association for Rubber Products Manufacturers has established the following limit for gap between carrying idlers for troughed conveyors.(ARPM IP-1-2) The limits provided serve as a guideline for acceptable performance of conveyor belts in the idler junction area, preventing junction failure.
Reference Document: ISO 1537 -- Continuous mechanical handling equipment for loose bulk materials -- Troughed belt conveyors (other than portable conveyors) -- Idlers
Limit for gap between in-line positioned troughed carrying idlers:
The maximum gap between the carrying idlers will be 3/8 in (10 mm).
Figure 4-2.
Overlap and Offset Dimension for staggered (or off-set) troughed carrying idlers:
A minimum overlap between the carrying idlers will be 3/8 in (10 mm).
Figure 4-3. End View
A maximum offset dimension of the idler in the running direction will be: Idler diameter plus 3/16 in (5 mm).
From the foregoing it is apparent that there are two extremes of lateral belt fl exibility to be considered in making a belt selection, and these are generally referred to as minimum and maximum ply design. Reference to manufacturers’ published tables is recommended, especially when the belt selection will be at or near either the minimum or maximum ply extreme, because of belt design variations and the fact that there are often two or more fabrics available with differing trough characteristics.
The ability of the belt to trough may be measured by using a standard test method (ASTM D 378). In this test, the troughability of the belt is defi ned as the ratio F/L where F is the natural drop height at the center of a 6 ft (1.8 m) long belt sample freely suspended at its edges and L is the belt sample width. Table 4-4 provides a guideline for the minimum values of F/L required to ensure that a belt will trough correctly in the listed troughing idlers.
Table 4-4*. Three Identical Idler Rollers -- Minimum Required Values of the Ratio of Defl ection (F) to the Belt Width (L)
Several belt constructions made from two or more plies of synthetic fabrics are widely used and are generally referred to as
multi-ply constructions. Because of the wide variety of fabric strengths, constructions, and other factors offered in these types of belt, it is necessary to consult the various manufacturers for specifi c data. Tables showing typical belt selection data are in Chapter 5.
TRANSITION DISTANCE ON THREE EQUAL LENGTH IDLER ROLLS FOR TEXTILE BELTS
A. General
In changing the troughed belt to a fl at section at the head pulley or the fl at belt to a troughed section at
