Machine Design

Flat Belts

Flat rubber belts were developed around the turn of the century primarily as replacements for leather belts. With the advent of V-belts, fewer machines were designed to use flat belts, and their production became largely a matter of supplying replacement parts.

Recent developments in flat-belt technology have overcome their previous drawbacks of high tension and mistracking. New designs and advances in materials have made both low and high-power transmission practical and cost efficient, and at speeds that usually exceed other belt designs.

Small woven endless belts. This type of flat belt is used where minimum vibration is required at the driven pulley. Maximum damping requires tuning the spring rate of these belts. In addition, semielastic belts have many features of unsupported stretch belts while maintaining a fabric substrate for support.

Semielastic belts do not require pulley take-up or adjustment, easily work in sets, work well over a wider length tolerance, and are relatively inexpensive in high volume. Because of these features, many OEMs are able to simplify and compact otherwise complicated designs. Over the past few years, a high volume of semielastic belts have gone into machines for the banking industry, copiers, computer disk drives and peripherals, and office equipment. Most of these designs use a wide variety of small multipulley precision drives.

Nonstretch endless flat belts also deserve consideration, especially in applications where small pulleys, high speeds (10,000 to 15,000 fpm), or both are involved. Because endless flat belts can be made very thin, 0.015 to 0.062 in., they are not as susceptible to centrifugal loads at higher speeds. Flat belts also are less costly than gears when teamed with small high-speed motors, especially when a compact package is important. Typical applications generally involve motors of 1 hp or less and speeds to 10,000 fpm.

Higher power flat belts. Developments here include sticky yet abrasion-resistant rubber compounds that eliminate the need for high tension to grip pulleys. These materials also allow lower shaft and bearing loads to transmit significant power. The strongest flat belts now transmit over 100 hp/in. of belt width.

Different flat belt surface patterns serve different transmission requirements. For example, in high-power applications and outdoor installations, longitudinal grooves in the belt surface reduce the air cushion that flat belts generate when they run at speed onto a pulley. An air cushion reduces friction between pulley and belt. In addition, the longitudinal profile nearly eliminates the effect of dirt, dust, oil, or grease. Furthermore, the grooves reduce the noise level of an already quiet power transmission design even more.

Perhaps the most significant advantage of flat belts is their high efficiency -- nearly 99%; about 2.5 to 3% better than V-belts. Three factors account for the good efficiency: lower bending losses due to the thin cross section, low creep because of special friction covers and high modulus of elasticity traction layers, and no wedging into pulleys like V-belts. Without the wedging action as in V-belts, flat-belt and pulley wear is minimal.

Flat belts offer greater design freedom than standardized designs because they are available in most any width and length, in increments of 1/16 in. This means drives can be sized closer to optimum rather than the next size larger.

Pulley alignment is equally important to flat belts as it is to other styles. Crowning at least one pulley, usually the larger one, improves belt tracking. Flat belts are forgiving of misalignment; however, proper alignment improves belt life for any drive.

Low-horsepower drives with small pulleys are usually more expensive than comparable V-belt drives. But once the larger pulley diameter reaches 30 in., flat-belt drives become less expensive. Despite initial cost of smaller flat-belt drives, their efficiency gain makes up for the cost differential within a few years.

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