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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).

There is variation among manufacturers about the information in the following paragraphs that relates to certain system design items such as the minimum pulley diameter, troughability, and maximum safe belt working stress to mention a few because of differences in materials and manufacturing methods. As a direct result other belt parameters are effected such as the number of cords in a belt, belt weight, minimum pulley diameter, troughability, belt modulus, 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. ISO 5048 and DIN 22101 also provide detailed methods for system tension calculations.

The tables below provide an example of the basic information on steel cord reinforced belt tension ratings. This information is for il- lustrative 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 are met:

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 tension rating be reviewed with the belt manufacturer. Some of the special conditions are:

1. Continuous excessive ambient temperature. 2. Exposure to deleterious chemicals.

3. Reduced safety factors.

Table 9-1. Steel Cord Belt Standard Specifi cations

Belt Tension Rating

Minimum Ultimate Tension Operating Tension Cord Diameter (Nominal) Cord Pitch (Approximate) Belt Modulus

PIW kN/m PIW kN/m in mm in mm PIW kN/m

ST800 4568 800 686 120 0.142 3.61 0.688 17.48 329000 57617 ST1000 5710 1000 856 150 0.142 3.61 0.547 13.89 411000 71977 ST1250 7138 1250 1070 187 0.205 5.21 0.855 21.72 514000 90015 ST1600 9136 1600 1370 240 0.205 5.21 0.666 16.92 657000 115058 ST2000 11420 2000 1712 300 0.205 5.21 0.533 13.54 822000 143954 ST2500 14275 2500 2140 375 0.205 5.21 0.450 11.43 1030000 180381 ST3150 17987 3150 2697 472 0.315 8.00 0.768 19.51 1290000 225914 ST3500 19985 3500 2996 525 0.315 8.00 0.690 17.53 1440000 252183 ST4000 22840 4000 3424 600 0.362 9.19 0.792 20.12 1640000 287208 ST4500 25695 4500 3852 675 0.394 10.01 0.805 20.45 1850000 323985 ST5000 28550 5000 4280 750 0.433 11.00 1.098 27.89 2050000 359010

Table 9-2. Steel Cord Belt Thickness

Table 9-3. Steel Cord Belt Weight

Table 9-4.

Approximate Belt Weight = Carcass Weight + Cover Weight Minimum pulley cover requirement 5/32 in

Table 9-5. Steel Cord Belt Standard Classifi cations

Snubs are defi ned as having 6 in or less belt contact and tension less than 50% of belt rating. Pulley sizes for belts are determined by face pressure on the pulley and/or the pulley-to-cord diamteter ratio. All pulleys must be fl at as crowned pulleys will cause excessive center tension in the high modulus steel cord product. Contact belt manufacturer for belt tensions higher than 4623 PIW.

Cover Compound ARPM 1 ARPM 2

Table 9-6. Recommended Transition Distances, Minimum Transition Distance: One-Half Trough Depth

Table 9-7. Recommended Transition Distances, Minimum Transition Distance: Full Trough Depth

Belt operating tension is not the only belt characteristic to be considered when selecting a 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 are presented below.

ELONGATION

Most new conveyor belts will exhibit 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.

The initial take-up position is that which the take-up fi nds after the clamps have been removed and the belt run empty a few belt revolutions to produce a natural tension distribution.

Table 9-8. Recommended Initial Take-Up Position

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.

Idler % of Rated Transition Length W = Belt Width

20 More than 90 60 to 90 Less than 60 2.0W 1.6W 1.0W

35 More than 90 60 to 90 Less than 60 3.4W 2.6W 1.8W

45 More than 90 60 to 90 Less than 60 4.0W 3.2W 2.2W

Idler % of Rated Transition Length W = Belt Width

20 More than 90 60 to 90 Less than 60 4.0W 3.2W 2.8W

35 More than 90 60 to 90 Less than 60 6.8W 5.2W 3.6W

45 More than 90 60 to 90 Less than 60 8.0W 6.4W 4.4W

Belt Type Percent available for length increase Percent available for length decrease