Pulleys are made of wood or steel (Figs. 143 and 144). They are measured by their diameter and by the distance across the face or rim.

331 Pulleys And Their Management 258

(a) Taper Cone Pulleys

331 Pulleys And Their Management 259

(6) Step Cone Pulleys

Fig. 143. - Wood Split Pulleys.

As the tendency of the belt on a pulley is to run to its highest portion, the highest part of the face should be in the center of the pulley, towards which the face should taper or crown. It is the practice to crown pulleys with a taper of 3/4 in. per foot or 1 in. per foot.

As shop machines are usually arranged so that they may be disconnected from the power while the shafts are moving, it is very important in starting or stopping them to avoid sudden jars or changes, as such sudden movements are dangerous to the machine. To avoid this danger a device, called the fast and loose pulley (Fig. 145), is used. This consists of two pulleys placed on the shaft, the one being firmly fixed, and the other loose so that it may easily turn while the shaft remains at rest, or vice versa. The belt is made to pass over either pulley by means of a forked guide; if on the fast pulley, the machine moves; if on the loose pulley, the machine remains' at rest.