special-belts-technology center

Power transmission belts

  • 8:44 am
  • 01 07 19

Power transmission belts has been used for more than 200 years. The first belts were flat and ran on flat pulleys. Later, cotton or hemp rope was used with V-groove pulleys to reduce belt tension. This led to the development of the vulcanized rubber V-belt in 1917. The need to eliminate speed variations led to the development of synchronous or toothed belts about 1950 and the later development of fabric-reinforced elastomer materials.

Today, flat, V, and synchronous belting is still being used in power transmission. When compared to other forms of power transmission, belts provide a good combination of flexibility, low cost, simple installation and maintenance, and minimal space requirements.

Belt-driven equipment uses readily available components. Replacement parts can be easily obtained from local distributors. This availability reduces downtime and inventory. Sheaves and pulleys are usually less expensive than chain drive sprockets and have little wear over long periods of operation.

Belt types

All power transmission belts are either friction drive or positive drive. Friction drive belts rely on the friction between the belt and pulley to transmit power. They require tension to maintain the right amount of friction. Flat belts are the purest form of friction drive while V-belts have a friction multiplying effect because of wedging action on the pulley.

Positive drive or synchronous belts rely on the engagement of teeth on the belt with grooves on the pulley. There is no slip with this belt except for ratcheting or tooth jumping.

Flat belts

Modern flat belts are made with reinforced, rubberized fabric that provides strength and high friction levels with the pulley. This eliminates the need for high tension, lowering shaft and bearing loads. Flat belts can transmit up to 150 hp/in. at speeds exceeding 20,000 fpm.

Power transmission belts

A significant advantage of flat belts is efficiency of nearly 99%, about 2.5-3% better than V-belts. Good efficiency is due to lower bending losses from a thin cross-section, low creep because of friction covers and high modulus of elasticity traction layers, and no wedging action into pulleys.

Pulley alignment is important to flat belts. Belt tracking is improved by crowning at least one pulley, usually the larger one. Flat belts are forgiving of misalignment; however, proper alignment improves belt life.

Different flat belt surface patterns serve various transmission requirements. In high-horsepower applications and outdoor installations, longitudinal grooves in the belt surface reduce the air cushion flat belts generate. The air cushion reduces friction between the pulley and belt. The grooves nearly eliminate the effects of dirt, dust, oil, and grease and help reduce the noise level.

Flat belts operate most efficiently on drives with speeds above 3000 fpm. Continuous, smooth-running applications are preferred. Speed ratios usually should not exceed 6:1. At higher ratios, longer center distances or idlers placed on the slack side of the belt create more wrap around the smaller pulley to transmit the required load.

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V-belts

V-belts are commonly used in industrial applications because of their relative low cost, ease of installation, and wide range of sizes. The V-shape makes it easier to keep fast-moving belts in sheave grooves than it is to keep a flat belt on a pulley. The biggest operational advantage of a V-belt is the wedging action into the sheave groove. This geometry multiplies the low tensioning force to increase friction force on the pulley sidewalls. Classical V-belts are frequently used individually, particularly in A and B sizes. The larger C, D, and E sizes generally are not used in single-belt drives because of cost penalties and inefficiencies. Multiple A or B belts are economical alternatives to using single-belt C, D, or E sections.

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Narrow V-belts, for a given width, offer higher power ratings than conventional V-belts. They have a greater depth-to-width ratio, placing more of the sheave under the reinforcing cord. These belts are suited for severe duty applications, including shock and high starting loads.

Banded V-belts solve problems conventional multiple V-belt drives have with pulsating loads. The intermittent forces can induce a whipping action in multiple-belt systems, sometimes causing belts to turn over. The joined configuration avoids the need to order multiple belts as matched sets.

Banded V-belts should not be mounted on deep-groove sheaves, which are used to avoid turnover in standard V-belts. Such sheaves have the potential for cutting the band of joined belts. Extremely worn sheaves produce the same result.

V-ribbed belts combine some of the best features of flat belts and V-belts. The thin belt operates efficiently and can run at high speeds. Tensioning requirements are about 20% higher than V-belts. The ribs ensure the belt tracks properly, making alignment less critical than it is for flat belts.

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Synchronous belts

Synchronous belts have a toothed profile that mates with corresponding grooves in the pulleys, providing the same positive engagement as gears or chains. They are used in applications where indexing, positioning, or a constant speed ratio is required.

The first tooth profile used on synchronous belts was the trapezoidal shape. It is still recognized as standard. Recent modifications to tooth profiles have improved on the original shape. The full-rounded profile distributes tooth loads better to the belt tension members. It also provides greater tooth shear strength for improved load capacity.

A modified curvilinear tooth design has a different pressure angle, tooth depth, and materials for improved load/life capacity and no ratcheting resistance.

Synchronous belts can wear rapidly if pulleys are not aligned properly, especially in long-center-distance drives, where belts tend to rub against pulley flanges. To prevent the belt from riding off the pulleys, one of them is usually flanged. A recent development has produced a belt and pulley that use a V-shaped, instead of straight, tooth shape. It runs quieter than the other shapes and doesn’t require pulley flanges.

Under tensioning causes performance problems. The drive may be noisy because belt teeth do not mate properly with pulley grooves or the belt may prematurely wear from ratcheting. High forces generated during belt ratcheting are transmitted directly to shafts and bearings and can cause damage.

Link belts

Link-type V-belts consist of removable links that are joined to adjacent links by shaped ends twisted through the next link (Fig. 5). With this design, belts can be made up of any length, reducing inventory. The belts are available in 3L, A/4L, B, C, and D widths in lengths from 5 to 100 ft.

Power transmission belts

These belts can transmit the same horsepower as classic V-belts. The links are made of plies of polyester fabric and polyurethane that resist heat, oil, water, and many chemicals.

Advantages of link belts include quickly making up matched sets, fast installation because machinery doesn’t have to be disassembled, and vibration dampening.

Disadvantages include cost and the possible generation of static charges. The belt should be grounded when used in high-dust applications.

Alignment

Misalignment is one of the most common causes of premature belt failure. The problem gradually reduces belt performance by increasing wear and fatigue. Depending on severity, misalignment can destroy a belt in a matter of hours. Sheave misalignment on V-belt drives should not exceed 1/2 deg. or 1/10 -in. of center distance. For synchronous belts it should not exceed 1/4 deg. or 1/16-in. of center distance.

Angular misalignment results in accelerated belt/sheave wear and potential stability problems with individual V-belts. A related problem, uneven belt and cord loading, results in unequal load sharing with multiple belt drives and leads to premature failure.

Angular misalignment has a severe effect on synchronous belt drives. Symptoms such as high belt tracking forces, uneven tooth/land wear, edge wear, high noise levels, and potential failure due to uneven cord loading are possible. Wide belts are more sensitive to angular misalignment than narrow belts.

Parallel misalignment also results in accelerated belt/sheave wear and potential stability problems with individual belts. Uneven belt and cord loading is not as significant a concern as with angular misalignment.

Parallel misalignment is typically more of a concern with V-belts. They run in fixed grooves and cannot free float between flanges to a limited degree as synchronous belts can. Parallel misalignment is generally not a critical concern with synchronous belts as long as the belt is not trapped or pinched between opposite sprocket flanges and tracks completely on both sprockets.

Tension

Total tension required in a belt drive depends on the type of belt, the design horsepower, and the drive rpm. Since running tensions cannot be measured, it is necessary to tension a drive statically.

The force/deflection method is most often used. Once a calculated force is applied to the center of a belt span to obtain a known deflection, the recommended static tension is established. Most design catalogs provide force and deflection formulas.

With too little tension in a V-belt drive, slippage can occur and lead to spin burns, cover wear, overheating of the belt, and possibly overheating of bearings. Not enough tension in a synchronous belt causes premature tooth wear or possible ratcheting that will destroy the belt and could break a shaft.

When installing a new belt, installation tension should be set higher. Generally 1.4-1.5 times the normal static tension. This is necessary because drive tension drops rapidly during the seating-in process. This extra initial tension does not affect bearings because it decays rapidly.

Reference

Basics of belt drives