a mechanism that transmits rotational motion from one pulley mounted on a shaft to another by means of a belt. The belt transmits torque from the driving pulley (Figure 1) to the driven pulley by means of the forces of friction that arise between the taut belt and the pulleys. The type of belt used determines whether the mechanism is flat-belt, round-belt, or V-belt. A belt drive with a multiple V-belt, which has several grooves on its inner surface, is now becoming increasingly common. Flat and round belts, as a rule, are used singly in a drive, while several V-belts (usually no more than eight) can be used together.

Figure 1. Diagram of a belt drive and cross sections of the various belts: (a) flat belt, (b) V-belt, (c) round belt, (d) multiple V-belt; (1) driving pulley, (2) belt, and (3) driven pulley

Flat-belt drives are simple and convenient. They permit the use of ordinary pulleys with smooth surfaces, and they can be operated at speeds as high as 40–50 m/sec and more. However, they are bulky in design and low in strength, and their tension ratio usually does not exceed 5. V-belt drives provide improved attachment of the belt to the pulleys, permit shortening of the center distances, and allow a decrease in the size of the drive and an increase in the tension ratio (up to 10-15). Round-belt drives are now rare and are used only in mechanisms of low power, such as those in sewing machines.

The advantages of belt drives are their simplicity of design, relative low cost, capacity to transmit power over significant distances (up to 15 m and more), and smooth and noiseless operation. In addition, the elastic properties of the belt and its ability to slip on the pulleys help prevent overload. The disadvantages include the short lifetime of the belts, relatively large size, heavy stress on the shafts and bearings, and variation in the tension ratio caused by the inevitable slipping of the belt.

Belts made of highly elastic, strong synthetic materials, narrow V-belts, and timing belts are becoming increasingly common. Belt drives are widely used in agricultural machines, electric generators, certain machine tools, and textile machines. They are ordinarily used for transmitting power up to 30–50 kilowatts, but there are machines in which belt drives are used to transmit power of hundreds and even thousands of kilowatts.