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Flat Belts--Are They Still Around?

From the earliest application of electric motors to industrial loads, belt drives have been a common means of reducing motor speed to that required by the driven machinery. Flat leather belts were most often used. Power was transmitted through friction between belts and the surfaces of driving and driven pulleys. Commencing about 75 years ago, elastomeric V-belts began replacing most flat belts, largely because of much greater friction created by the wedging action of V-belts in grooved pulleys. Also, V-belts were quieter; they could readily be added or removed to suit varying power requirements; and belt tension was easy to recheck periodically.

However, flat belts (usually made now with a polymer core for high strength, faced with rubber or leather for oil resistance) remain useful for drives in which motor pulley diameters are too small for V-belts, or in which centrifugal force due to high speed might cause V-belts to fly out of their grooves. Also, the pulley groove wear that leads to belt damage is not present in a flat belt drive.

The shaft stress and bearing loading imposed on a motor by a V-belt drive is easily calculated. Also, drive design is simplified by the existence of only a few standard belt cross sections, as well as motor standards governing allowable driving pulley diameters. With flat belts, a multiplicity of belt thicknesses and materials makes drive design and load calculation more complex, although basic formulae relating belt tension and friction force were developed a century ago.

Motor manufacturers in the 1920s standardized on certain calculation procedures, and pulley diameters were agreed upon by a leather belting trade association. However, those standards have either lapsed, or were never extended to motor sizes of several hundred hp at speeds of 1200 rpm and below. Typical calculations indicate that a flat belt drive for a modern large motor may involve much higher shaft stress and bearing loading that would result from the use of V-belts. Close coordination between motor manufacturer and drive designer is necessary to ensure a trouble-free application.

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