dose not stretch or slip and hence transmits power at a constant angular velocity ratio. This permits timing belts to be employed for many applications requiring precise speed radio, such as a driving an engine camshaft form the crankshaft. Toothed belts also allow the use of small pulleys and small ares of contact. They are relatively lightweight and can efficiently operate at speeds up to at least 16,000 fpm. A timing belt fits into the grooves cut on the periphery of wheels or sprockets, as shown in Figure 13.3. The sprockets come in sizes form 0.60-in. Diameter to 35.8 in. And with teeth numbers ranging form 10 to 120. the efficiently of a toothed belt drive ranges form about 97 to 99%
figure 13.4 illustrates a portion of the timing-belt drive. The teeth are coated with nylon fabric. The tension member, usually steel wire, of timing-bel is positioned at the belt pitch line. The pitch length therefore is the same regardless of the backing thickness. Note that, as in the case of gears. The circular pitch p is the distance, measured on the pitch circle, from a point on the tooth to a corresponding point on an adjacent tooth. Since timing-belt, no initial tension is necessary and a fixed center drive may be used at any slow or fast speed. The disadvantages are the cost of the belt, the necessity of grooving the sprocket, and the dynamic fluctuations generated at the belt-tooth meshing frequency.