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Question
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What are covers

Used in belt construction to protect the carcass and if possible extend service life. Cover type, quality and thickness are matched to the service life of the belt involved. The specific cover formula and individual belt construction are determined by the material to be conveyed and the environment in which the belt will operated. Covers do provide the finished belt with a wide variety of desirable properties including:

  • Texture
  • Cleanability
  • Chemical Resistance
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What are skims

The rubber, PVC or urethane between plies. Important contributor to internal belt adhesions, impact resistance, and play a significant role in determining belt “load support” and “troughability”. Improper or marginal skims lead to ply separation and or idler junction failure.

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What is the carcass on a belt

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:

  • Provide the tensile strength
  • Absorb the impact of the impinging material being loaded onto the conveyor belt
  • Provide the bulk and lateral stiffness required for the load support
  • Provide adequate strength for proper splice holding
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Aramid Belt Splices

Aramid-Belt-SplicesWhere Used: Aramid reinforced high tension one and two-ply rubber belts.

Most Aramid rubber conveyor belts are vulcanize-spliced with a finger type splice as shown in Illustration No. 6. A few are spliced with the overlap type splice described in an earlier section. This description applies only to the finger type splice. In general, the best manufacturers insist that all splice jobs for Aramid belts be referred to them for specific instructions and proper materials. The normal procedure involves stripping the belt ends down to the Aramid ply(plies) top and bottom, then cutting long narrow fingers in the Aramid carcass, and then rebuilding the splice area with tie gum, breaker, or reinforcing fabric and cover rubber. The fingers are interlaced but not touching, with a rubber strip separating them. The splices usually are made 90 degree transverse but can be made on an angle. They are cured in vulcanizers large enough to cure entire splices in one heat. As with steel cable belts, it is especially important that the two belt ends are aligned accurately as Aramid belts are aligned accurately as Aramid belts are high modulus and thus don't stretch much to forgive crooked splices.

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Steel Cable Belt Splices

Steel-Cable-Belt-SplicesWhere Used: Steel cable belts. 

Splices in steel cable belts involve basically removing everything except the cables, with a skin of rubber left on them, from the splice area, and then replacing all of the removed materials with new uncured components. The cables are arrayed in the splice area either in an overlapped pattern, a combination overlapped and butted pattern or exclusively a butted pattern. The choice depends on the amount of space between cables in the original belts, and the cable size. The splices are always made on a bias angle. They are cured in portable vulcanizers large enough to cure entire splices in one heat. The splices must be made and cured with the belt ends accurately aligned. This applies to all splices, but is especially important with steel cable belts since they don't stretch as much as other belts, and thus are less forgiving of crookedness.

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Skived Splice

Skived-SpliceWhere Used: Thermoplastic lightweight belts, single and multiple ply, and nylon core transmission belts.

This type of splice can only be made with special skiving tools. It involves skiving each belt end at a very gentle angle, then overlapping the two skived ends with a layer of thermoplastic material between, and heating the splice area in a portable vulcanizer under pressure. The splices may be 90 degrees transverse or at a bias angle. They should be made with the bottom seam leading. The splices must be cooled before removing from the vulcanizer.

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Finger Splice

Finger-SpliceWhere Used: Single ply thermoplastic belts and some lightweight single ply rubber belts without covers or with thin covers. Sometimes used in multiple ply thermoplastic belts when the pulleys are very small.

This type of splice is commonly used in single ply belts that have thin covers that cannot be stripped off, or have no covers. It is the most common non-mechanical splice method for single ply thermoplastic belts. The overlap type splice is not acceptable in these belts either because of its extra thickness or inapplicability. The method then is to cut matching pointed, triangular shaped, fingers. This is done by several methods, including a template, adhesive backed pattern paper, a cutting die and hand layout. The finger patterns may be 90 degree transverse or at a bias angle. Thin belts can be cut with the ends overlapped, ensuring a match. It is important that the edge fingers be on the leading belt end. The splice is made by intermeshing the fingers of the two belt ends, usually with a layer of thermoplastic film or foil on one or both sides, and in some cases with an adhesive poured into the seams between the fingers. The splice is then heated between electric plates under some pressure to flow the thermoplastic material into the finger seams and fuse it together between the mating surfaces. The splices must be cooled before removing the heating plates to allow the softened thermoplastic materials to solidify.

A variation of the finger splice in multiple ply thermoplastic belts is the overlap finger splice. The fingers are split between belt plies so that in the splice the fingers from the two belt ends overlap each other. Special equipment is required for cutting and stripping out the fingers for this type of splice.

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Overlap Splice

Where Used: Single ply rubber belts with thin covers and some Aramid belts.

The overlap splice is used in single ply rubber belts with covers too thin to accommodate splicing fabrics. The ends to be joined are cut at a matching bias angle. The splice configuration normally has the leading belt end over the trailing belt end in the splice area unless the conveyor is a decline unit with a tail holdback. The bottom cover of the leading end and the top cover of the trailing end are removed in the splice area. The reverse would apply for the decline type unit described above. After the covers are removed, the splice areas are cleaned and buffed. Cement and splice rubber are applied, and the leading end is laid over the trailing end, with the belts clamped off and aligned. This yields a splice with the two belt ends exposed. These, and the adjacent belt surfaces, must be buffed, and the notches filled with uncured cover rubber. The assembled splice will be thicker than the original belt, with an abrupt offset at the belt ends in the splice. Curing pads are required to fill the offsets. These splices should be cured in electric vulcanizers large enough to do this in a single heat. The curing time should be based on the splice thickness, not the thickness of the original belt.

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Butt Splice

Butt-SpliceWhere Used: Single ply rubber belts.

This type of splice involves cutting the two belt ends to be joined at a matching bias angle. The belt covers are removed in the splice area. The resulting surfaces are cleaned and buffed. Before assembling the splices, the belt ends must be aligned and clamped down accordingly. The splices are built up using cement, splicing rubber, splicing fabrics and cover rubber. The splice fabrics must be obtained from the manufacturer of the belt involved, and must be applied exactly as directed. The splices are cured in electric vulcanizers, preferably in a single heat.

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Stepped Splice

Stepped-SpliceWhere Used: Multiple ply rubber belts, including rubber type multiple ply lightweight belts, and multiple ply thermoplastic belts.

This splice involves stepping down, ply by ply, the two belt ends to be joined. The dimensions such as step length, cover fill in length, splice angle, etc. will vary depending on the belt ratings and manufacturer. After stepping down, the splice areas are cleaned and buffed to remove irregularities, but without buffing into the fabric surfaces. Thermoplastic belts usually are not buffed. The splices are built up using cements and uncured rubber, or in thermoplastic belts, foils. Accurately fitted splice steps and splice straightness are very important. Assembled rubber belt splices are cured in portable electric vulcanizers, usually with edge irons, and if the belt covers are worn unevenly, with a curing pad. Assembled thermoplastic belt splices are fused using heated plates.

Some high tension two ply rubber belts require the use of special splicing fabric to reinforce the splices. These components must be obtained from the respective belt manufacturers, and must be used exactly as directed to function properly. The splicing fabrics are applied under the cover fill ins in the splices.

Cover fill ins may not be used in vulcanized splices in lightweight multiply belts, either rubber or thermoplastic. Rather, in rubber belts, the covers in the two belt ends are fitted together much like a splice step, with only a bead of new rubber at the seam, or with one cover beveled with an undercut. In thermoplastic belts, the covers in the two belt ends are simply butted at the outside ply seams, with no special bevel cutting. Cover fill ins are not always used in cold vulcanized splices. Rather, the covers in the two belt ends are butted, both having been beveled, one with an undercut.

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Vulcanized Splices

Vulcanized splices provide a method of joining the ends of conveyor belts without interrupting the continuity of the belts, and usually without altering the geometry or dimensions of the belts. Modern conveyor belts, made with synthetic fabrics and normally having high adhesion between components, lend themselves to effective and long lasting vulcanized splices. There are some types of conveyor belts that can only be spliced by vulcanizing. This section of the Conveyor Belting Engineering Handbook is provided to give an overview of vulcanizing. Details are available from belt manufacturers, their authorized agents, and splicing contractors.

The underlying principle in vulcanized splicing is the establishment of adhesion between the components of the two belt ends being joined together in the splices. The goal is to develop adhesion in the splices equal to that in the original belt. There is no intent to physically join the components in the splice, such as stitching together the ends of the fabric plies or joining steel cables with strength-retaining sleeves, etc. The splice lengths, configuration and dimensions are designed to retain continuity of the strength of the belts by the transfer of the tension stresses from one belt end to the other through the adhesion developed between the components mated in the splices.

There are many types of vulcanized splices. Each involves unique procedures, and the materials required depend on the type of belt and the type of vulcanized splice. In the following sections, the more common of the types of vulcanized splices are briefly described along with the types of belt each is used in. No attempt is made herein to provide step-by-step procedures as these vary among belt manufacturers, types of belts, service conditions, etc. The materials required for vulcanized splicing often are unique and specific for the type of belt involved. The procedures and materials or material recommendations must be obtained from the belt manufacturers or their authorized agents.

The term 'vulcanized' may imply a process wherein new materials are used which undergo a chemical change or chemical action as a result of the application of heat and pressure. However, there are commonly used 'cold' vulcanizing processes for rubber belts in which the new splice materials are chemically activated by contact with other chemically active materials, without requiring the use of a vulcanizing press. In general, the splice geometry is the same for hot and cold vulcanized splices, but the materials differ, of course. Materials and procedures for cold vulcanizing usually must be obtained from the cold vulcanizing material manufacturers or their authorized agents, and seldom from the belt manufacturers. In general, cold vulcanized splices can be used in any rubber belt that is step, butt, or overlap spliced, as described in the following sections, but ignoring the references to curing in vulcanizing presses.

Even though this section is titled Vulcanized Splices, the term 'vulcanized' is not correct for thermoplastic belt splices. In thermoplastic belt splices, no chemical change occurs in the materials used. Rather, the thermoplastic materials soften when heated and fuse together with softened material from adjoining components, and then solidify back to their original condition when cooled. Perhaps a better title for this section might be 'Vulcanized or Fused Splices' to reflect thermoplastic as well as rubber belts. 

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Standard Elevator Bucket Punching

Standard-Elevator-Bucket-Punching

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Sidewall Advantages

  • Designed for material movement.  Corrugated sidewall is designed to ensure maximum flexing without fatigue. The profile has excellent vertical stability for load retention and return side support.  Design allows for high compression to ensure smooth inner deflection around small radii.  
  • No transfer points.  Sidewall systems can convey material from the feed hopper to the discharge point due to the ability to turn through any angle up to a vertical line and back to horizontal.  Eliminating the need for multi-drives and prevents product spillage and degradation at transfer points. 
  • No spillage during steep angle conveying.  The material is trapped between the sidewall and prevented from falling back by the cross cleats.  Material remains within the belt’s effective carrying area.
  • Maximum utilization of space.  With a single belt system able to convey material up to 90o vertical, the required ground space for moving material is minimized. 
  • Long Belt Life & Minimum Maintenance.  Compared to mechanical elevators, sidewall belt systems have numerous advantages.  The sidewall belt requires no maintenance.  Sidewall belt systems have fewer moving parts, decreasing costly downtime.
  • Wide range of material handling ability.  A variety of belt sizes and configurations allows the sidewall belt to handle most all materials including; large lumps, free flowing, delicate or fragile substances, highly abrasive material, light weight or heavy loads.   
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Sidewall Specifications

Sidewall-Specifications-1

 

Sidewall-Specifications-2

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Thermoplastic Cleat Sizes

THERMOPLASTIC CLEATS

Style

Size

Height

Width @ Top

Width @ Base

Min. Pulley Diameter

Min.   Centers

T-Cleat

1/2"

1/2"

 

 

2.5”

 

T-Cleat

3/4”

3/4”

 

 

2.5”

 

T-Cleat

1"

1"

 

 

3”

 

T-Cleat

1-1/2"

1-1/2"

 

 

3”

 

T-Cleat

2"

2"

 

 

3”

 

T-Cleat

2-1/2"

2-1/2"

 

 

4”

 

T-Cleat

3"

3"

 

 

5”

 

Scoop

2"

 

 

 

6”

 

Scoop

3”

 

 

 

8”

 

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Rubber Cleat Sizes

RUBBER CLEATS

Style

Size

Height

Width @ Top

Width @ Base

Min. Pulley Diameter

Min.   Centers

T-Cleat

1/2"

1/2"

 

 

4”

3”

T-Cleat

1"

1"

5/16"

7/16"

4”

3”

T-Cleat

MaxClimb

1-1/4"

 

 

 

13.5”

T-Cleat

1-1/2"

1-1/2"

11/32"

1/2"

6”

3”

T-Cleat

2"

2"

3/8"

5/8"

6”

3”

T-Cleat

2-1/2"

2-1/2"

 

 

8”

3”

T-Cleat

3"

3"

1/2"

11/16"

8”

3”

Beefy

1"

1"

1/2"

5/8"

6”

3”

Beefy

2"

2"

5/8"

3/4"

8”

4”

Beefy

3"

3"

13/16"

1-5/16"

10”

4”

Beefy

4"

4"

 

 

 

 

Beefy

4-1/2"

4-1/2"

 

 

 

 

Beefy

6"

6"

 

 

 

 

Scoop

3"

3"

 

 

 

 

Square

1/2" x 1/2"

1/2"

 

 

 

 

Square

1" x 1"

1"

 

 

 

 

Square

2" x 2"

2"

 

 

 

 

Square

3/4" x 2"

 

 

 

 

 

Square

1" x 2"

 

 

 

 

 

Square

Collector

 

 

 

 

 

V-Guide

"A" Section

5/16"

1/2"

5/16"

 

 

V-Guide

"B" Section

3/8"

5/8"

3/8"

 

 

V-Guide

"C" Section

7/16"

7/8"

9/16"

 

 

V-Guide

"D" Section

3/4"

1-1/4"

21/32"

 

 

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Cleat Styles

Cleat-Styles

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Belt Types

belt-types

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Belt Width Requirements for Load Support

Conveyor belts are designed with sufficient carcass transverse rigidity to ensure proper bridging of the idler junction gaps, eliminating the possibility of belt fatigue/failure at these critical areas.  The correct load support design is accomplished with the proper number of belt plies   and maximum width for the weight of the material being conveyed.

Ply / Tension

Light 0 - 40 PCF

Medium 41 - 80 PCF

Heavy 80 - 120 PCF

 

20o

35o

45o

20o

35o

45o

20o

35o

45o

2/150

42”

36”

36”

36”

30”

30”

30”

24”

 

2/220

48”

42”

42”

42”

36”

36”

36”

30”

 

3/330

72”

60”

60”

60”

54”

48”

48”

42”

36”

2/400

72”

66”

60”

60”

54”

48”

48”

42”

36”

4/440

72”

72”

72”

72”

60”

54”

60”

54”

48”

2/600

72”

72”

72”

72”

60”

54”

60”

54”

48”

3/600

72”

72”

72”

72”

60”

60”

60”

54”

48”

4/600

72”

72”

72”

72”

66”

60”

60”

60”

48”

4/800

72”

72”

72”

72”

72”

66”

72”

66”

60”

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Belt Width requirements for Troughing

The minimum belt width for an empty belt to trough correctly during operation is based on the ply and tension rating of the belt. 

Ply / Tension

20o

35o

45o

2/150

14”

18”

18”

2/220

14”

18”

24”

3/330

20”

24”

30”

2/400

24”

24”

30”

4/440

24”

30”

36”

2/600

30”

30”

36”

3/600

30”

30”

36”

4/600

30”

36”

36”

4/800

30”

36”

42”

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Transition Distances

Bulk material conveyors with free flowing products commonly use troughed idler sets along its length, increasing the belt load carrying capacity. Changing the cross belt profile from flat to a wide “U” shape is known as the conveyor transition.  As the conveyor belt changes from a flat profile to a troughed profile in its passage from the tail pulley and the troughed profile to the flat profile at the head pulley possible belt damaging tension and compressive forces can occur.

At the area of profile change, this transition must occur over sufficient conveyor length in order to avoid excessive tension in the belt edges (splice tearing) and avoid belt compression (center buckling). The conveyor system operating tension has a strong influence on the transition length.

Transition distances are measurements from the centerline of a terminal pulley to the first full angle system idler (LI). One or more belt supporting low angle transition idlers may be between the terminal pulley and the first full angle system idler and should not be included in this measurement. 

The two most common transition profile types are the full trough depth and ½ trough depth. The full trough is typically found at the tail, with the ½ trough typically found at the head. The full trough terminal pulley may be raised slightly to give a ½ trough configuration. 

Transition Type

Result

Cross belt tension is too great at the edges (more than 130%) and less than zero in the middle of the belt,

Splice edge failures and delaminating in the center of the belt Left unchecked an adhesion breakdown in the center of the belt will occur and thus propagate along the entire belt length.

Proper edge tension without adequate transition distance

Insufficient tension present to keep the bottom of the belt in the trough and avoid some buckling. Long term subsequent problems with the belt and the splices will be encountered.

Cross belt tension is poorly averaged, where the belt edges and the belt center are more than 120% rated tension and the idler junction area of the belt is less than zero

Cs the belt moves onto the pulley the tensions must equalize quickly across the belt width and “belt creep” in the low tension areas can occur. The belt creeps around the pulley face in the low tension areas near the idler junction resulting in skidding, wearing a corresponding channel into the rubber lagging.

Transition length is increased to the belt width

Without sufficient transition distance, too much shearing force across the splice width is encountered. Left unchecked an adhesion breakdown in the idler junction areas of the splice will occur and thus propagate along the entire belt length.

Multiplying the belt width (inches) by the table transition distance factor (below) will give the minimum recommended transition distance (inches).

Idler Angle

% Rated Belt Tension

Full Trough

1/2 Trough

20o

60-90

1.6

0.8

35o

60-90

2.4

1.3

45o

60-90

3.2

1.6

*For  Fabric Belts Only. 

Remember, Transition distance, as defined by NIBA is the length from the center line of the first fully troughed idler roll to the center of the terminal pulley (either the head or tail pulley).

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Belt Repairs

It is generally recognized in bulk materials conveying that optimum belt life has been achieved when belt cover and carcass wear out at about the same time. Obtaining the obviously desirable maximum practical belt life depends on a number of factors, including among others:

  • Belt design, specification and quality
  • Conveyor design and engineering
  • Conveyor accessories and their function
  • Maintenance of the conveyor as a system
  • Scope and quality of repairs performed on belt

It is a relatively rare bulk materials handling belt that is installed and run to optimum wear-out without having some sort of repairs made to it.

General Considerations when deciding if a belt should be repaied or replaced are:

How extensive is the damage? A small rip or tear along an edge or a small puncture, say up to 6” longitudinally, may pose little risk of spillage or snagging or enlargement; such minor damage can usually be left for repair at a scheduled down time. Comparatively widespread injuries, like edge fraying or cover scoring, may occupy hundreds or thousands of feet of belt yet not pose an operations hazard; again this type of injury could be left for repair at a scheduled down time.

Transverse and diagonal rips represent loss of essential strength and the tension forces normally carried by the damaged area are transferred to the adjacent section of the belt. If the width of the injury is great enough, the overstressed balance of the belt will yield producing total failure at that point. A common rule of thumb is that if no more than 25% of the belt width is involved, a repair is practical; when more than 25% of the width is damaged, a full resplice or saddle section insertion is preferable.

In many cases, a rip or tear is ragged or partly hidden under the belt cover. Its extent must be carefully determined when making a repair decision. Relatively localized injuries can be drastically enlarged by snagging of a loose flap or dangling cable in a steel cord belt. Repairing or at least anchoring such damages are imperative before resuming belt operations.

Is replacement of the belt a better option? Belt size is often a determining factor in the repair or replace decision. Replacement obviously means the immediate or near term availability of a spare belt. Changeout time for a smaller (shorter) belt often can be less than needed for a major vulcanized repair. Therefore under such circumstances, the replacement choice is optimum. If a spare belt is not available, the time element in securing one versus the estimated repair time (assuming the present belt can be repaired) must be weighed.

Gross damages to a belt with major areas of ripping and tearing, or fire damage, for example, will obviously demand replacement of the belt or the affected sections.

Another factor when considering replacement of an injured belt is whether the belt is a good candidate for off-the-conveyor repairs. Will pushing the belt to provide a little more production right now reduce its potential repairability? The judgment in such a case is like deciding to drive another mile or two on a flat tire and thereby surely ruining it. Sometimes the age of the belt (or tire) makes the decision easier. If either is nearing the end of its normal useful life, trying to save it in the interest of further repairs just isn’t necessary.

How much time is available? In the majority of cases, serious belt damage means taking corrective action as quickly as possible, not just diverting material flow to the adjacent conveyor.  Time constraints can take many forms, a few of which are: the need to fill a bunker, complete a production run or a work shift, or the loading/unloading of a train or a ship. In each instance, the interruption of operations will bring up the same questions: repair or replace the belt? Depending on specific circumstances, a major rescheduling of operations may become the only choice.

Is temporary repair feasible? One of the most unhappy things imaginable is to make a rushed repair on a broken belt, then restart the conveyor only to see the belt pull apart again. There certainly is a place for improvised or temporary belt repairs--many have proved successful in maintaining at least partial material flow until other arrangements can be made. On the other hand, an ill-conceived repair is just a waste of valuable time.

The most vital question in considering temporary belt repairs is usually whether or not the tensile strength of the belt carcass can be restored or bridged sufficiently at the point of injury to withstand the drive and take-up forces.

If the nature of the damage will not permit this with some assurance of success, the belt should be completely respliced or have a repair section (saddle) spliced in at the point of injury.  Gross carcass damages over extensive areas of the belt would obviously limit the feasibility of temporary repairs and make replacement of all or major sections of the belt a preferable remedy.

Temporary repairs of the type referred to above would include some form of mending with metal fasteners or a scab overlay held in place with fasteners or elevator bolts. Such temporary repairs are usually employed in conjunction with a reduction of the belt feed rate to lessen effective belt tension.

Longitudinal Rips: Carcass rips are often repaired with metal fasteners. Set transverse to the belt width and placed on 6”, 12” or greater spacings, the fasteners will reduce spillage and hold the belt together for days, weeks, or even months of additional service.

Cover Repairs: When a significant area of cover loss occurs and the underlying carcass is still intact, temporary coating or sealing of the exposed carcass fabric or cables is usually worthwhile. Even if no later vulcanized repairs are to be carried out, repeated coatings with various sealers will generally provide some additional useful belt life.  Eli-Flex is a 2-part polyurethane resin formulated to effect quick and easy repairs. 

TYPES OF BELT REPAIR

Vulcanized Repairs are the closest match to the original belt manufacturing procedures employed at the factory. Using the vulcanization process, damaged or missing sections of cover or carcass can be replaced with little sacrifice of belt strength.

Metal Fastener Repairs represent a completely different principle in handling the problem of restoring damaged belting.  Metal fasteners are often the only splicing method used in a wide variety of belt applications. For most bulk materials handling belt repairs the bolted plate type fastener is selected.

Cold Cure Repairs restore the cover and/or by inserting new materials. Instead of heat and pressure to accomplish vulcanization, the cold cure material depends on a chemical cure to achieve adhesion to the virgin belt.

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Loading Conditions

Careful consideration of loading conditions must be considered in conveyor belt selection. If there is a history of belt design and failure, this may be more significant than any guides, tables or calculations.

Improvements in load design and/or conveyor equipment may be required if belting is prematurely failing from material impact. Indications of an impact problem include:

  1. Breaks in belt parallel to belt angle
  2. Star breaks to top cover
  3. Gouges in top cover
  4. Failure of mechanical lacing from physical abuse

To calculate number of plies required, first determine the lump weight factor (Table E1), including calculated variables, and then determine minimum plies to withstand impact from Tables E2 or E3.

Table E1 -- Lump Weight Factor in Pounds

Density

Lump Size

lbs / ft3

2

3

4

5

6

7

8

9

10

12

14

16

18

50

0.4

1.3

3

5.8

10

14

21

30

40

70

100

148

211

75

0.6

1.9

4.5

8.6

15

21

31

44

61

105

149

222

316

100

0.7

2.6

5.9

12

20

28

41

59

81

140

199

296

421

125

0.9

3.2

7.4

14

25

35

52

74

101

175

248

371

527

150

1.1

3.8

9

17

30

42

62

89

121

210

298

444

632

175

1.3

4.5

10.4

20.2

35

49

73

104

142

245

348

518

737

Table E1 may be used to determine lump weight factor that must be adjusted as follows:

If free fall is different than 4 feet, divide Lump Weight Factor by 4 and multiply by free fall of material in feet.

If material momentum includes a chute preceding free fall, add to the above the value determined by multiplying the distance of drop in chute area times the size of the chute angle squared.

[Additional factor = h (sine Angle in degrees)2]

If large lumps exceed 10% of material, add one more ply to minimum table rating (E2).

If impact idlers are not used, multiply the lump factor by 2 for belts 4-ply and less, by 1.25 for 5-ply belts, and 1.15 for 6-ply and over belts.

For multiple-ply belts, use calculated lump factor to determine minimum number of plies to resist impact from Table E2. For reduced-ply belts, use Table E3.

NOTE: Individual belting manufacturers should be consulted for recommendations on proper reduced-ply belting impacted resistance

Table E2 - Maximum Impact Rating Multiple-Ply Belts

MP Fabric Identification

Number of Plies

35

43

50

60 & 70

90 240

3

8

16

20

38

48

4

16

28

38

62

80

5

40

60

75

175

320

6

120

160

210

475

700

7

240

320

410

775

1060

8

-

520

660

1050

1440

 Table E3 - Maximum Impact Rating Reduced-Ply Belts

Number of Plies

35

43

50

60 & 70

90 240

3

8

16

20

38

48

4

16

28

38

62

80

5

40

60

75

175

320

6

120

160

210

475

700

7

240

320

410

775

1060

8

-

520

660

1050

1440

+

Gauge – Belt Thickness Tolerances

There are no RMA thickness tolerances established for conveyor belting because they are often of no consequence.  The following standards are for guides if thickness is determined to be important and declared at time of order for make-up conditions. These values are generally reasonable, but tighter tolerances may be negotiated. NOTE: Thickness tolerances must be specified on purchase orders if they are to be considered binding.

Gauge Tolerance Guide

Overall Belt Thickness

Tolerance

Zero - 0.094" (3/32")

± 0.015" (1/64")

0.095" - 0.156" (5/32")

± 0.020"

0.157" - 0.344" (11/32")

± 0.031" (1/32")

0.345" - 0.500" (1/2")

± 0.047" (3/64")

0.501" and Over

± 10%

+

Length Tolerance

Endless-Net Endless Length (NEL)

Loop Belt

± 1%

Vulcanized

± 1/2%

Specified Length - Cut Ends

 

± 1/2%

Specified Length - Laced with Mechanical Fasteners (measured Pin to Pin)

 

± 1/2%

Roll - Bulk Quantities

 

± 10%

 * On Specified Lengths the manufacturer will often cut belt 2% longer than specified to avoid shipping short.

**Belts may be shipped in one, two, or three pieces - none less than 50 feet unless agreed to by customer.

+

Width Tolerance

Belt Width

(inches)

Molded Width Tolerance

Maximum Width

Variation In Any one Belt

Cut Width

Tolerance

24 or less

± 1/4"

± 1/4"

± 1/8"

25 to 36

± 3/8"

± 3/8"

± 3/16"

37 to 48

± 1/2"

± 1/2"

± 1/4"

49 to 53

± 7/32"

± 7/32"

± 7/64"

54 to 59

± 19/32"

± 19/32"

± 19/64"

60 to 71

± 23/32"

± 23/32"

± 23/64"

72 to 78

± 25/32"

± 25/32"

± 25/64"

79 to 84

± 27/32"

± 27/32"

± 27/64"

85 to 90

± 29/32"

± 29/32"

± 29/64"

91 and over

± 1%

± 1%

± 1/2%

+

Cleat Spacing Tolerances

Cleat Spacing Center-To-Center

Tolerance

2" to 8"

± 1/4"

9" to 12"

± 5/16"

14" to 24"

± 7/16"

25" to 60"

± 1/2"

+

Cleat height tolerances

Standard Height

3/8"

1/2"

3/4"

1""

1-1/2"

2"

3"

4"

Tolerance

± .020"

± .020"

± .031"

± .062"

± .062"

± .062"

± .062"

± .062"

+

Sidewall Horizontal Carrying Capacity

Belt Width

Sidewall Height

2-1/2"

3"

4"

5"

6"

18''

3280

4200

5300

7620

9450

20''

3910

4960

5890

8470

10525

24''

5280

6600

7695

10160

12360

30''

7280

9350

10630

14030

17450

36''

9280

12070

13860

18290

19260

42''

 

14810

17400

22970

24190

48''

 

17550

20950

23050

29450

Cubic Feet at 200 FPM

+

Sidewall Incline Carrying Capacity

Degree of Incline

Cleat Spacing

3"

T-cleat

3" Scoop

3-1/2" T-cleat

3-1/2" S-cleat

4"

T-cleat

4-1/2" S-cleat

5-1/2" S-cleat

30o

6''

132

148

166

176

208

244

324

9''

89

113

122

133

159

219

301

12''

67

85

91

100

119

165

262

40o

6''

89

123

122

138

158

213

284

9''

59

84

81

93

106

155

240

12''

45

63

61

70

79

117

184

50o

6''

62

99

85

102

111

175

253

9''

42

66

57

68

74

118

184

12''

31

49

43

51

56

89

139

60o

6''

 

80

 

76

 

135

209

9''

 

53

 

51

 

91

142

12''

 

40

 

39

 

69

107

70o

6''

 

65

 

57

 

103

164

9''

 

44

 

38

 

70

110

12''

 

33

 

29

 

53

83

80o

6''

 

53

 

39

 

77

118

9''

 

35

 

27

 

53

81

12''

 

26

 

20

 

40

62

Cubic Feet per Hour per Inch Length of cleat between sidewalls at 100 FPM

+

The belt transports sticky material, resulting in material buildup on the snub pulley and return idlers.

  • Install a belt cleaner at the head pulley. Scrapers that have a continuous, even contact to the belt work best. Apply rubber lagging to the head and snub pulleys. Cover the return idlers with rubber or plastic sleeves to repel buildup.
+

Excessive Cover Wear

  • Improper loading
  • Material trapped in the system
  • Incorrect skirt board materials/installation
  • Improperly installed belt cleaners
  • Tail transitions too short
+

Improper Loading

  • Ideally, material being introduced onto the belt should be flowing in the same direction as the belt travel and at roughly the same speed as the belt is running.
  • The product conveyed should be loaded in the center of the belt.
  • In transfer areas, the take-away belt should be running at least 15% faster than the loading belt and be capable of handling slightly more material.
  • Transfer areas should be free of obstructions to allow the product to flow smoothly.
+

Be Careful With Skirtboard

  • Never use conveyor belt as skirtboard
  • Only use skirtboard that is softer than your belt covers
  • Adjust skirtboard near covers not into covers
  • If more rigidity is required in skirtboard use thicker material not harder material
  • Insure proper clearance of frame from belt
+

The belt is shrinking on a slider bed system.

  • Usually occurs when the system is handling a product that either is, or produces, very fine particles.  The particles work their way into the fabric yarns, between the yarn fibers.  This causes the yarns to increase in diameter thus causing the belt to shrink.  This often times can cause the belt to shrink with enough force to break pulleys and/or pulley shafts.  
  • A chemical reaction is taking place due to the material being handled.  Especially when handling fertilizers.
+

Carcass fatigue at the junction of the individual idlers on a carrying idler set.

  • Insufficient transverse belt stiffness.  Replace belt with one recommended for the weight of load and belt width.  Decrease load.
  • Inadequate transition distance from the pulley to the troughing idlers.  Increase transition length.  Rule of thumb, transition is equal to 2.5 times belt width minimum.
  • A condition of tilted or over tilted troughing idlers exists.  Idler frames should be located per manufacturers recommendations, usually at 90 degrees to the belt and conveyor frame.  Align idlers per manufacturers recommendations.
  • The spacing between the individual idler rollers is too great.  Replace idlers utilizing a system with reduced roller gap.
  • Too much distance between the idler sets causing the belt to be forced between the individual idler rollers.  Decrease the distance between idler sets, increase tension if the belt is under tensioned.
+

Ply separation of the belt

  • Insufficient ply adhesions in the belt to start with.  Check with the manufacturer to make sure the correct belt is being used.
  • Insufficient transverse belt stiffness.  Replace belt with one recommended for the weight of load and belt width.  Decrease load.
  • One or more of the system pulleys are below the acceptable diameter.  Replace pulleys with diameters acceptable to belt requirements.
  • Incorrect fastener for the belt/application.  Improper fastener installation.  Check fastener application specifications, make sure it’s the right fastener for the job.  Make sure fastener is installed per manufacturer’s instructions.
+

Belt breaks just behind the mechanical fastener, or the mechanical fastener pulls out.

  • The fastener plates used in the mechanical fastener are too long.  Replace with a smaller fastener if tensions will permit it, or increase the pulley diameters.
  • Incorrect fastener for the belt/application.  Improper fastener installation.  Check fastener application specifications, make sure it’s the right fastener for the job.  Make sure fastener is installed per manufacturer’s instructions.
  • The tension is too great for the fastener.  Upgrade to a higher rated fastener if possible.  Switch to a vulcanized slice.
  • One or more of the system pulleys are below the acceptable diameter.  Replace pulleys with diameters acceptable to belt requirements.
  • Belt scrapers are not correctly adjusted or installed.  Check belt scrapers, readjust or replace if necessary.
  • The carcass of the belt is too light or of the wrong material.  Upgrade to a heavier belt carcass or one that incorporates a nylon fill yarn, or a monofilament fill yarn.
+

Longitudinal grooves or cracks in the bottom cover.

  • Frozen idlers.  Replace or repair frozen idlers.  Improve maintenance of idlers (lubrication, cleaning, alignment).
  • Material build-up on the pulley face or conveyor structure.  Clean system, improve material containment, install cleaners, check skirting, install material plows in front of pulley.
  • Insufficient traction between belt and drive pulley.  Make sure drive pulley is free of buildup.  Lag Drive pulley.  Increase belt wrap on drive pulley.  Increase belt tension.
  • Pulley lagging is worn.  Replace with new or correct pulley lagging.
  • Too much distance between the idlers causing excessive material movement as the load travels up and over the idlers, forcing the belt into the idler junctions.  Decrease the distance between idlers, increase tension if the belt is under tensioned.
  • Insufficient transverse belt stiffness.  Replace belt with one recommended for the weight of load and belt width.  Decrease load.
+

The belt is getting narrower

  • The belt is stretching causing the belt to “neck down” due to excess tension.  The system is under belted or over tensioned.  Replace belt with a higher tension belt.
+

You are experiencing excessive belt stretch.

  • The operating and/or start up tension is too great for the belt.  Decrease the load, increase the speed utilizing the same drive motor, decrease the angle of incline, increase tension rating and number of plies in the belt.  Apply or adjust soft start drive motor controls.
  • The belt is not adequate for the generated tensions.  Upgrade to a higher tension belt.
  • The counter weight is too heavy or the screw take-up is over tensioned.  Reduce weight in counter weight or reduce tension at screw take-up.
+

The top cover is grooved, gouged, or the top cover is stripped off.

  • Improper skirt board, improper skirt board adjustment, belt to skirting holding structure.  Make sure to use proper skirt board, not old belt!  Make sure load point is in proper adjustment, i.e. belt clearance.  Adjust skirt board to reduce pressure on the belt top cover.
  • Material is lodging in the chute.  Open up the chute, reduce size of lumps in material, restrict feed rate.
  • Material hanging up at the load point.  Either in the chute or under the chute.  Increase distance from belt to chute.  Widen the chute.  Install baffles to spread the material at the load point.
  • Too great of impact on belt at the load point.  Install impact idlers or impact bed.  Redesign chute with deflection (slow down) bars to reduce drop.  Upgrade top cover compound.
  • Sharp edges on material, foreign material such as tramp iron in the product.  Remove foreign material, i.e.. magnet, improve load point drop, upgrade top cover compound.
  • Improper top cover compound for the application.  Check with manufacturer for proper compound.
+

Belt runs fine when it’s empty but won’t track right when it’s loaded.

  • The belt is not making good contact with all of the idlers.  Re-adjust the idlers for proper belt contact.
  • Off center loading or improper loading of the belt.  Make sure load chute places load in the center of the belt.  Make sure the direction of the material down the chute is in the direction of the belt travel.
  • Variations in the formation and nature of the load at the load point.  Use a deflector/notched chute to keep the load peak as close to the center of the belt as possible.
+

Belt is erratic – does not follow a pattern.

  • Belt too stiff to train. Use self-aligning idlers. Increase tension/conforms to crowns. 
  • Use more flexible belt on replacement.
  • Tilt troughing idlers forward, but not over 2 degrees.
+

The belt is curling or cupping down, towards the bottom cover.

  • Oils in the product are causing the top cover to swell or expand.  Replace belt with one that is rated for higher oil resistance.
+

The belt is curling up on the edges, cupping. The belt was fine when installed but over time it cupped.

Heat or chemical damage to the belt.  Make sure to use the correct belt carcass and compounds for the application.  There may be cases where this is the norm in handling extremely dry products that pull the plasticizers out of the cover compound.  In these cases using a belt with a heavier bottom cover will help off-set the problem.  Or, try a belt with balanced covers which would allow you to turn the belt over several times in the life of the belt to counteract the problem.

+

Vulcanize splice failure

  • Incorrect belt splice type, or incorrect implementation of the splice procedure.  Re-splice belt following manufacturer’s recommended splice materials and procedures.
  • One or more of the system pulleys are below the acceptable diameter.  Replace pulleys with diameters acceptable to belt requirements.
  • The operating and/or start up tension is too great for the belt.  Decrease the load, increase the speed utilizing the same drive motor, decrease the angle of incline, increase tension rating and/or number of plies in the next belt.
  • Material is getting trapped between belt and pulley.  Improve containment at load point.  Improve containment along the conveyor.  Install plows or scrapers in front of tail pulley.  Practice good housekeeping.
  • Inadequate transition distance from the pulley to the troughing idlers.  Increase transition length.  Rule of thumb, transition is equal to 2.5 times belt width minimum.
  • Contrary to popular belief, the original belt has less to do with splice integrity than the splice material does.  You can take a belt with exceptional adhesions and produce a splice with little to no adhesions.  Conversely, you can take a belt with very poor adhesions, and utilizing the proper splice materials, produce a splice with exceptional adhesions.  Also, the RFL (Resorcinol Formaldehyde Latex) treatment of fabrics is 90% activated during the initial curing of the belt, and does very little to nothing for the splice adhesions.
+

Transverse breaks in belt at the edge

  • Inadequate transition distance from the pulley to the toughing idlers.  Increase transition length.  Rule of thumb, transition is equal to 2.5 times belt width minimum.
  • The belt edges are folding over on the system or folding up on the structure.  Re-track belt, center load point, check the belt as it comes into and through the load point/skirting, check to make sure the belt isn’t coming into contact with the conveyor structure.
+

Star breaks in belt carcass, or short breaks in belt carcass parallel to belt edge

  • Material is getting trapped between belt and pulley.  Improve containment at load point.  Improve containment along the conveyor.  Install plows or scrapers in front of tail pulley.  Practice good housekeeping.
  • Too great of an impact on the belt at the load point.  Install impact idlers or impact bed.  Redesign chute with slow down bars or reduce drop.
  • Upgrade to a carcass with a nylon fill yarn, or an all nylon carcass.  (But in the long run fixing the problem will be less expensive.)
+

The belt runs to one side only in a certain section or portion of the conveyor

  • Frozen idlers.  Replace or repair frozen idlers.  Improve maintenance of idlers (lubrication, cleaning, alignment).
  • The pulleys and/or the idlers are out of square with the belt centerline.  Re-adjust pulleys and or idlers.
  • Buildup of material on idler rollers.  Clean and maintain.  Install scrapers, brushes, and other cleaning devices.
  • The conveyor frame is not square or the system is not properly supported.  Straighten the frame in the affected area and check to make sure support in that area is correct.
  • The idler stands are not centered to the belt.  Readjust the stands for proper alignment.
  • Make sure the conveyor structures is level from side to side.  Level structure.
  • One or more idlers immediately preceding trouble point not at right angles to the direction of belt travel.
  • Belt runs off terminal pulley.  Check terminal pulley alignment.  Check alignments of idlers approaching terminal pulley.
+

The cover/s are swelling and/or getting soft in certain areas

  • The belt is contaminated with spilled oil or grease.  Improve housekeeping, avoid spilling of hydrocarbon based lubricants on the belt.  Don’t over grease bearings.  Check grease seals on bearings.
+

A certain section of the belt runs to one side regardless of the location on the conveyor

  • Belt is cambered.  If the camber is less than 1/2%, the belt will train and run fine.  If the camber is greater than 1/2%, the belt needs to be replace.
  • If the section is at the splice then the belt splice is crooked.  Remove splice and rejoin belt ends insuring belt ends are square in accordance with belt/fastener manufacturer.
+

Belt slips while running

  • Insufficient traction between belt and drive pulley.  Make sure drive pulley is free of buildup.  Lag Drive pulley.  Increase belt wrap on drive pulley.  Increase belt tension.
  • Inadequate belt tension.  Increase weight on gravity take up.  Increase tension at screw take-up.
  • Pulley lagging is worn or not adequate to produce sufficient traction.  Replace with new or correct pulley lagging.
  • Frozen idlers.  Replace or repair frozen idlers.  Improve maintenance of idlers (lubrication, cleaning, alignment).
  • Material build-up on the pulley face or conveyor structure.  Clean system, improve material containment, install cleaners, check skirting, install material plows and scrapers.
  • One or more of the system pulleys are below the acceptable diameter.  Replace pulleys with diameters acceptable to belt requirements.
+

The covers are hardening and/or cracking

  • Heat or chemical damage to the belt.  Make sure to use the correct belt carcass and compounds for the application.
  • Compound degradation due to ozone and ultraviolet light during long term storage.  Store inside, out of direct sunlight and weather.  Utilize spare belts sooner.
  • It is a natural tendency for rubber to get harder as it ages.  This is due to the drying out of the plasticizers in the compound.  As belts age, and the rubber dries out, the cover wear will be accelerated. 
+

The belt is stalling or jerking

  • An improper slack side tension problem exists.  Adjust the take up position.  Lag the drive pulley or replace the current drive pulley lagging.  Add a snub pulley behind the drive pulley to increase the wrap of the belt on the drive pulley.
+

Belt slips when conveyor is started

  • Insufficient traction between belt and drive pulley.  Make sure drive pulley is free of buildup.  Lag Drive pulley.  Increase belt wrap on drive pulley.  Increase belt tension.
  • Inadequate belt tension is possible, recalculate tension requirements.  If necessary increase weight on gravity take up or increase tension at screw take-up.
  • Pulley lagging is worn or not adequate to produce sufficient traction.  Replace with new or correct pulley lagging.
  • Counter weight hitting bottom, not enough belt tension.  Shorten belt and re-splice.
  • One or more of the system pulleys are below the acceptable diameter.  Replace pulleys with diameters acceptable to belt requirements.
  • Conveyor is overpowered.  Reduce HP or consider a soft start if applicable.
+

Belt runs to one side for a considerable distance, or the entire conveyor

  • The belt is running off center as it comes around the tail pulley and/or through the load point.  Re-track belt, install training idlers on the return prior to the tail pulley.
  • Buildup of material on idler rollers.  Clean and maintain.  Install scrapers, brushes, and other cleaning devices.
  • Off center loading or improper loading of the belt.  Make sure load chute places load in the center of the belt.  Make sure the direction of the material down the chute is in the direction of the belt travel.
  • The pulleys and or the idlers are out of square with the belt center-line.  Re-adjust pulleys and or idlers.
  • Idler stands are not centered to the belt.  Re-adjust the stands for proper alignment.
  • The conveyor frame is not square or the system is not properly supported.  Straighten the frame in the affected area and check to make sure support in that area is correct.
+

Belt runs off at tail pulley

  • The belt is running off center as it comes around the tail pulley and/or through the load point.  Re-track belt, install training idlers on the return prior to the tail pulley.
  • Material build-up on the pulley face or conveyor structure.  Clean system, improve material containment, install cleaners, check skirting, install material plows in front of pulley.
  • The pulleys and or the idlers are out of square with the belt center-line.  Re-adjust pulleys and or idlers.
  • Frozen, dirty, or misaligned return idlers.  Clean rollers, properly align rollers, install cleaning devices at head pulley, use self-cleaning rollers, improve maintenance (alignment, lubrication, and cleaning).
  • Inadequate belt tension is possible, recalculate tension requirements.  If necessary increase weight on gravity take up or increase tension at screw take-up.
+

Belt runs off at the head pulley

  • Material build-up on the pulley face or conveyor structure.  Clean system, improve material containment, install cleaners, check skirting, install material plows at head pulley.
  • The pulleys and or the idlers are out of square with the belt centerline.  Readjust pulleys and or idlers.
  • Pulley lagging is worn or not adequate to produce sufficient traction.  Replace with new or correct pulley lagging.
  • Idler stands are not centered to the belt.  Re-adjust the stands for proper alignment.
+

Excessive edge wear

  • The belt is misaligned.  Track and train belt.
  • Belt is coming in contact with the conveyor frame and or hardware.  Make sure belt is tracking properly.  Make sure there is proper clearance between belt and any hardware.  Make sure load is centered on belt.  Clear any jammed material.  Install training idlers on carry and return side of belt.
  • Off center loading or improper loading of the belt.  Make sure the load chute places the load in the center of the belt.  Make sure the direction of the material down the chute is in the direction of the belt travel.
  • Material build-up on the pulley face or conveyor structure.  Clean system, improve material containment, install cleaners, check skirting, install material plows in front of pulley.
  • Belt is cambered.  If the camber is less than 1/2%, the belt will train and run fine.  If the camber is greater than 1/2%, the belt may need to be replaced.
  • If the edge wear occurs in the splice area, the splice may have been installed crooked.  Re-splice belt.
+

Excessive pulley cover wear

  • Frozen idlers.  Replace or repair frozen idlers.  Improve maintenance of idlers (lubrication, cleaning, alignment).
  • Insufficient traction between belt and drive pulley.  Make sure drive pulley is free of build up.  Lag Drive pulley.  Increase belt wrap on drive pulley.  Increase belt tension if the belt is under tensioned.
  • Material build-up on the pulley face or conveyor structure.  Clean system, improve material containment, install cleaners, check skirting, install material plows in front of pulleys.
  • Material is getting trapped between belt and pulleys.  Improve containment at load point.  Improve containment along the conveyor.  Install plows or scrapers in front of tail pulley.  Practice good house keeping.
  • Bolt heads from pulley lagging, or from slider bed material hold down on bare backs, are sticking up and catching the belt.  Inspect and replace or tighten as required.
  • A condition of tilted or over tilted throwing idlers exists.  Idler frames should be located per manufacturers recommendations, usually at 90 degrees to the belt and conveyor frame.  Align idlers per manufacturer’s recommendations.
+

Excessive top cover wear over entire top surface or in load carrying area

  • The top cover quality is not adequate for the system/material being conveyed.  Upgrade to a heavier top cover.  Upgrade to a better cover compound.
  • Off center loading or improper loading of the belt.  Make sure load chute places the load in the center of the belt.  Make sure the direction of the material down the chute is in the direction of the belt travel.
  • Material build-up on the pulley faces of the return idlers, or on the conveyor structure. Clean system, improve material containment, install cleaners, check skirting, install material plows after the head pulley.
  • Frozen, dirty, or misaligned return idlers.  Clean rollers, properly align rollers, install cleaning devices at head pulley, use self cleaning rollers, improve maintenance (alignment, lubrication, and cleaning).
  • Too much distance between the idlers causing excessive material movement as the load travels up and over the idlers.  Decrease the distance between idlers, increase tension if the belt is under tensioned.
+

Conveyor Belt Storage

  • PROTECT STORED BELTS FROM DIRECT SUNLIGHT AND OZONE. These elements can cause surface hardening and cracking. Ozone is associated with electric motors, generators, and arc welders. Susceptibility to these conditions varies widely among the various grades of belts, depending on intended use. Consult your belt manufacturer for help in this area.
  • COVER AND ELEVATE BELTS STORED OUTSIDE. Set the rolls of belt on a skid or platform to prevent direct exposure to surface moisture and standing water. The rolls should be covered with a rain and sunlight restricting material, most commonly black plastic film, perforated to allow ventilation and trapped water drainage.
  • AVOID ROLLING BELTS. This usually leads to loosening of the belt wraps, and may result in “telescoping” of the rolls. If necessary, roll belts only in the direction they are wound.
  • AVOID DRAGGING BELTS. Do not pick up one end of belt and drag during material movement. This can result in damage to the cover on the belt.  If dragging is necessary, place a pad under the unsupported end to prevent damage.
  • USE CARE WHEN HANDLING WITH FORKLIFT. Forks can puncture or damage belt covers. Take care when positioning forks and moving material to ensure damage does not occur.  Be sure to use a forklift that is rated for the material weight.
+

What is the difference between belting grades?

  • The RMA (Rubber Manufacturers Association) established specifications for Conveyor Belting grades several decades ago. Though many early members of the association are no longer manufacturing conveyor belting, the RMA updated the standards to accommodate the new raw materials utilized in belt manufacturing.

The RMA only grades rubber belting for the most commonly used industrial belts (generally SBR, natural or a combination). The carcass fabrics are essentially the same when covered by specialty rubbers such as Neoprene, Nitrile or Chlorobutal.

We list the original standards for the three grades of general purpose belts below.

RMA Original Specifications

RMA Grade #1

Cover tensiles of 3500 to 4000 lbs. Adhesion between components of 20 to 24 lbs.

RMA Grade #2

Cover tensiles of 2500 to 3000 lbs. Adhesion between components of 16 to 19 lbs.

RMA Grade #3

Cover tensiles of 800 to 1000 lbs. Adhesion between components of 12 to 15 lbs.

Because cotton was the basic carcass material when the RMA established these standards, adhesions were more difficult to achieve. Today, with synthetic fabrics and RFL treatments, average adhesions are in the 60 to 70 lb. range on most general purpose belts.

Though clouded by the introduction of more modern rubbers such as EBR (Polybutadiene), there is little justification for relaxing the cover tensiles first prescribed -- unless manufacturers want the freedom to use lesser rubbers. We list the new specifications below.

RMA 1994 Specifications

RMA Grade #1

Cover tensiles of 2500 lbs. 400% elongation.

RMA Grade #2

Cover tensiles of 2000 lbs. 400% elongation..

RMA Grade #3

No specifications.

Rubber & Plastics, Inc. continues to recognize the original conveyor belting specifications and provides complete details of all rubber belting components. We show the comparitive grades of belts common to the industry on the Conveyor Belt Cross Reference Chart, listing the current trade names of various belts in each category.

+

What is Belt Camber?

  • If unbalanced warp tensions exist in a conveyor belt, that belt will usually assume a "crescent" or "banana" shape when laid flat upon a horizontal surface. This deviation from a straight line is hereby defined as "camber."

To measure belt camber, it is recommended that the belt be unrolled on a flat surface like the warehouse floor, a flat horizontal driveway, etc. Next, one end of the belt should be grasped (and one end only), then the belt dragged in a perfectly straight line for some ten to twenty feet. If the belt is too heavy for one person to move, that one end should be clamped to a fork lift and the same procedure performed. At this point, the belt should lie flat. Unequal and unresolved warp tensions in the belt will cause it to assume a "crescent" or "banana" shape.

(It is extremely important that the preceding procedure be followed to the letter. It is very difficult to have both edges of the belt at the same thickness--particularly wide belts. Accordingly, if we simply unroll the belt on a flat surface, that belt will always unroll in a banana shape--due to geometry, not unbalanced warp tensions. "Dragging" one end of the belt for some ten to twenty feet eliminates this geometrical consideration and does tell us whether we indeed do have a cambered.

FAQ Belt Camber

Compute percent camber as follows:

% Camber =(Maximum Deviation (inches))/(Length of Taut Line (inches))x 100

Currently, there is no accepted Camber standard in the U.S. market. For a maximum acceptable camber relative to a given conveyor belt construction, please contact your belt manufacturer.

Camber can be instilled into a belt during the slitting operation if one of the slitting knives is dull. A dull slitting knife will tear the fill yarns (crosswise yarns) rather than cut them. (While the belt is in roll form, the side of the belt which has gone through the dull knife will exhibit a "fuzzy" appearance due to the torn fill yarn.) Usually, this type of camber will be less than 1/2 of 1% and can be pulled out handily when the belt is properly tensioned and tracked.

Camber, quite frequently, is instilled into a belt during improper storage. For a guide to "proper storage," please refer to NIBA Technote #2.

Skew (Bow)

The fill yarns (horizontal yarns) in the belt carcass will usually lie along the perpendicular to the belt center line. Any deviation from this perpendicular by the fill yarn is hereby defined as "skew" or "bow."

A skewed pick in a plain, or square, weave is cause for concern since it is generally indicative of unbalanced warp tensions and will usually go hand-in-hand with a significant camber.

In a straight warp or solid woven carcass, skew is of little significance. It is usually a cosmetic defect and is not indicative of a cambered belt.

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What can I do about belt stretch?

  • Discuss your specific belt with your belting manufacturer. He is knowledgeable and will be able to tell you what you can expect. If the particular belt you are working with has a polyester warp system, you can expect to achieve maximum stretch within the first 24 hours that the belt is operating under full load. At this point, the belt will usually stabilize. Make your final installation tension adjustment and any tracking adjustments indicated.

Maintenance, from this point forward, will usually be minimal. Although "good maintenance practices" dictate frequent, periodic inspections.

If, on the other hand, your belt construction employs a nylon or a cotton warp system (lengthwise yarns), you will encounter two to three times as much stretch in that first 24 hour period (and even more). You should now re-tension the belt as above.

Unfortunately, however, nylon will creep. Cotton and nylon will absorb and/or lose moisture and change dimension. This means that they will not stabilize after the initial 24 hour "break-in" period, but will continue to stretch and/or contract. Accordingly, belt retensioning must be a routine maintenance procedure in these cases.

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Do I have to re-tension the belt?

  • Possibly. When you originally tensioned the belt, you were working with a clean belt and clean pulley lagging. This gave you a relative coefficient of friction such that, when coupled with the minimum belt tension you provided, any tendency to slip was overcome. Now, however, if a contaminant has been interjected between the conveyor belt surface and the pulley lagging or pulley surface, that relative coefficient of friction may change. Should this occur, it is important that you clean both surfaces in order to re-establish the original relative coefficient of friction. If, on the other hand, it is impractical to do this because of environmental factors, you may need to compensate with increased tension.

Further, you will encounter belt stretch. Know what to expect and how to handle it.

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What is involved in Belt Tensioning?

  • Essentially, all you have to do is make sure the belt is conforming to the crowns on the pulleys and that you have tensioned the belt until slippage stops under the most adverse conditions this system will experience.

The force required to drive a belt conveyor or belt elevator usually is transmitted from the drive pulley to the belt by means of friction between the pulley surface and the belt surface.

The force required to restrain a down-hill regenerative conveyor is transmitted in exactly the same manner. In order to transmit the right amount of power to drive the conveyor or elevator system at rated speed and under full load, there will be a difference in the tension in the belt as it approaches and leaves the drive pulley. This difference in tension is supplied by the driving power source and is known as effective tension ( TE ).

For convenience of discussion, let us call the tight side tension at the pulley T1, and the slack side tension at the pulley T2. Therefore: TE = TO - TS.

It can be shown that the slack side tension, T2, is equal to kTe where this so-called "k" factor is equal to the following expression:

k = 1/ e fQ -1 ( e = base of naperian logarithm = 2.718 )

(When computing belt tensions, you would typically refer to a "k" factor table such as can be found in most major belting product line brochures.) To determine the "k" factor, you would need to know the following:

  1. Belt wrap at drive
  2. Bare steel pulley or lagged pulley, and
  3. "Screw" take-up or "gravity" take-up.

The 'k' factor, as can be seen from the expression

k = 1/ e fQ -1

is dependent upon the coefficient of friction ( f ) between the pulley surface and the belt surface and the wrap ( Q ) of the belt around the pulley with Q measured in radians.

With the conveyor/elevator system designed and built, the installation or maintenance man is faced with the problem of making the entire system function correctly. Essentially, the only variables over which he has some control are the coefficient of friction and belt tension. He can "control" coefficient of friction by making sure that the pulley face or pulley lagging and the belt surface are both clean. If a contaminant is introduced between those surfaces, the belt can slip relative to the drive pulley unless they are cleaned or belt tension is increased.

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How Can I tell How tight the belt really is?

  • Trying to determine belt tension by subjective evaluation is extremely difficult. Even the most experienced development and maintenance engineers can not do it, except by accident. If you feel you really need to know the exact tension of your belt, use the belt as a stress/strain gauge. Start with the belt in a totally relaxed condition. Draw a line perpendicular to the belt center line; then draw another, perpendicular to the belt center line, exactly 100" away. At this point, you should contact your belt manufacturer. He will be happy to tell you the stress/strain characteristics of the specific belt you are working with.

Suppose now, you are working with a given belt and Sales/Service has told you that the belt will stretch 1% at full rated load. You then go through the above procedure so as to arrive at proper minimum belt tension. Now that you are satisfied that you will not encounter any slippage at worst conditions, stop the conveyor and measure the two base lines again. You will find that your 100" dimension has grown. If it has grown to perhaps 100.25" this means the belt has been stretched .25% which is 1/4 of the 1% figure given you by Sales/Service. (Since we are operating so low on the stress/strain curve, we can assume a straight line relationship and be fairly accurate.)

Therefore, .25% means that you have achieved 1/4 of the rated belt tension. Careful: you will notice that your two lines are no longer parallel to one another. They will assume a "parenthesis-like" shape, reflecting the kind of pulley crown or crowns employed in the system. If positive crowns are used, you will note that the center of the belt has been stretched a good bit more than the edges of the belt. To arrive at an average belt tension, you need to choose the points at which you make your measurement.

At this point, you have determined average "belt at rest" tension. This is not identical to "slack side" tension -- it is usually greater.

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How can I achieve proper minimum belt tension?

  • Start by shutting off the conveyor and tensioning the belt until there is no "play" between the edge of the belt and the crowned pulley. Push (with your finger) against the belt to find out if there is any play. If more than one pulley has a crown, check the edge of the belt on each of the pulleys. If there is any play, tighten the belt until play is eliminated. At this point, you are very close to a proper minimum tension, as the belt is conforming to the crown on the pulley (or pulleys) and, therefore, will respond to them.

Now operate the belt conveyor or belt elevator system under the most adverse conditions it will encounter, and observe closely to see if there is any slippage between the drive pulley and the belt. If there is, more tension is required.

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What is Proper Minimum Belt Tension?

  • Proper minimum belt tension is the tension required so that a given belt conveyor or belt elevator system will operate properly in its environment.

This means minimum belt tension is great enough so that the belt conforms to the crown on any crowned pulley or pulleys. After all, how can the crown do its job if the belt is not in contact with it? It also means that minimum belt tension is great enough so that the belt does not slip relative to the drive pulley under the most demanding conditions which can be expected.

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Why do Adhesions Matter?

  • Low adhesion will result in ply delamination, cleat separation and sidewall failure.
  • Adhesion is firm or steady contact between two surfaces. In rubber, it generally applies to the "grab" of rubber to rubber or rubber to metal, fabric, plastics, or the other components of a finished product.

Because most synthetic fabrics are slick and will not stick to most rubber compounds, they must be treated with a "go-between." The most commonly used treatment for rubber to fabric adhesions is Resorcinol-Formaldehyde-Latex (RFL). However, sometimes you must use other coatings such as special resins or isocyanates. This RFL coating is applied at the fabric mill, where they dry and heat set the fabric at the same time.

Mills heat set the fabric between 325°-375°F while under tension. Such heating can shrink nylon and polyesters as much as 15% from their woven dimensions, both width and length. The mill must keep this shrinkage in mind throughout the manufacturing process.

The Scott Tester is widely used to measure and record the force necessary to separate two surfaces, usually in pounds per inch of width. This information is critical for deciding the quality of a product, and the expected life in actual use. In conveyor belting, common Styrene Butadiene Rubbers (SBR) are compounded to yield an adhesion between the covers and plies in the 60 to 70 lb. range. Nitrites, EPDM, coal stocks, and Chlorobutyl will measure somewhat less.

Natural rubber compounds, high grade truck tread stocks, and specially treated fabrics can produce test figures as high as 150 to 200 lbs. per inch of width on the Scott Tester. Industrial hoses and OEM tires are usually made with lower adhesions, where tests as low as 24 lbs. per inch of width may be considered adequate. As a rule, thicker rubber compounds will produce higher adhesions, both between the rubbers and between rubber and metal, fabric, or plastic. Thin rubber layers do not test as well.

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Why does Safety Factor matter?

  • Safety factory is a measurement of the strength of a belt at its breaking point as a reflection of the strength of the fabric. 
  • The safety factor is a figure given by the belt manufacturer for a particular belt. It is expressed in a ratio of break strength to working strength of the fabrics in the belt and thus the belt's overall strength.  The safety factor is based only on the fabric in the belt and the cover compounds of the belt have no direct influence on the safety factor. 

For a belt to have a 10:1 safety factor, the manufacturer must source the fabric material based on the PIW of the belt they are building, for example; 2 ply 220 lb belt.  Each of the plies of fabric will be 110lb fabric, generating the 220lb belt rating once combined.  In order for the finished belt to have a 10:1 safety factor a single ply of  the fabric must exceed 1100 lbs of strength, resulting in a combined strength of 2200 lbs, 10x that of the PIW rating for the belt.  The same belt with an 8:1 safety factor would break at 1760 lbs.

Belts with a safety rating between 8 and 10 are generally accepted in the industry. In most applications a belt with an 8:1 safety factor is acceptable, however, applications under extreme tension, heavy impact or with hard start-ups warrant a 10:1 safety factor. 

When safety factors drop below 8:1, belts can start to see excessive stretching, fastener retention failure and belt failure. The lighter weight fabric can’t support a troughed load as effectively, letting the belt fall in between the idlers and causing excessive wear and eventually failure.  The mechanical fasteners can pull right through the end of the belt when tension exceeds the strength of the fabric to hold them in.

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What is Belt Modulus, Belt Elongation or Belt Elasticity?

  •  Belt modulus is a measure of a belt's resistance to stretching, the higher the belt modulus value, the lower the belt elongation or stretch will be at any given tension.  The modulus value of conveyor belting is primarily dependent upon the fabric or fiber used in the belt's running direction and the weave pattern.  Belt manufacturing techniques and the specific tension range over which the value is determined also have an effect on the belt modulus.

An expression referred to as the belt modulus of elasticity, or elastic belt modulus, is the tool used for many belt and conveyor calculations. The modulus of elasticity in tension is defined as the ratio of the increment of unit stress to increment of unit deformation within the elastic limit. This is also stated as the change in stress divided by the change in strain.

Elasticity is the ability of a material to return to its original dimensions after the removal of stresses. This can be likened to stretching a rubber band and watching it return to its original shape as you let go.

Permanent elongation is described as the belt stretch that is somewhat inelastic or unable to be easily recovered simply by removing the tension. Permanent elongation will give insight as to the amount of initial stretch that will be encountered upon routine startup of the belt on any given system.

To use the elastic modulus value in determination of how much belt stretch might be expected, it is common to divide the belt modulus into the conveyor operating tension. For example, a belt with a modulus of 5000 piw operating at 200 piw would be expected to elongate by 4.0 percent........ 200 / 5000 = .04

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Why do Belts Stretch?

  • Conveyor belts are constructed of elastic materials. Therefore, these materials change dimension when some force is applied upon them.
  • Belts stretch may be caused by the materials used in construction, environmental factors or system issues.  The fabric used in the belt has an effect on the stretch of the belt, for instance, Nylon belts will generally be more stretchy than polyester belts.
  • Environmental factors such as acids, oils or other chemicals attacking the belt can cause a loss of fabric and rubber stability, allowing more stretching than the belt would typically see.  System issues such as excessive tensioning can break the fabric weave and cause the belt to stretch. 
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Is Hot Vulcanization the same as Hot Bonding?

  • No!  Hot bonding is a fancy way to say that heat is used during the curing process of a chemical bond.
  • Vulcanization is a chemical process for converting rubber into more durable materials via the addition of "curatives" or "accelerators". These additives modify the polymer by forming crosslinks (bridges) between individual polymer chains.  Vulcanized materials are less sticky and have superior mechanical properties.
  • Hot Bonding is a process of applying chemical adhesives that uses heat to accelerate the bonding process.   The addition of heat to the chemical bond adds no additional adhesion value.   
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Is Hot Vulcanization really better than Chemical Bonding?

  • Hot vulcanization provides a 10x greater adhesion than either hot or cold chemical bonding.
  • Hot Vulcanization provides a rubber to rubber adhesion of 80 to 100 PIW, compared to 35 to 45 PIW found in Chemical bonds.  RPI Vulcanized sidewalls have a less than 1% failure rate, as compared to the industry average of 20% failure on chemically bonded sidewalls.