plan. The bolts and nuts must be shown in each view. The holes for the bolts are square, and the bolts have square necks. Draw this example half full size.

Pillow Block. One form of pillow block is shown in Fig. 162. A is the block proper, B the sole-plate through which pass the holding down bolts. C is the cap. Between the block and the cap, is the brass bushing which is in halves.

In the block illustrated the journal is lubricated by a needle lubricator, this consists of an inverted glass bottle fitted with a wood stopper, through a hole in which passes a piece of wire, which has one end in the oil within the bottle, and the other resting on the journal of the shaft. The wire or needle does not fill the hole in the stopper, but if the needle is kept from vibrating the oil does not escape owing to capillary attraction. When, however, the shaft rotates, the needle begins to vibrate, and the oil runs down slowly on to the journal, oil is therefore only used when the shaft is running.

Example 7. Pillow Block for a Four-inch Shaft. Draw the views shown of this block in Fig. 162. Scale 6 inches to 1 foot.

Proportions of Pillow Blocks. The following rules may be used for proportioning pillow blocks for shafts up to 8 inches diameter. It should be remembered that the proportions used by different

The nave or boss of the loose pulley is here fitted with a brass bushing, which may be renewed when it becomes too much worn. Draw the elevations shown, completing the left-hand one. Scale 6 inches to 1 foot.

By the above arrangement of pulleys a machine may be stopped or set in motion at pleasure. When the driving belt is on the loose pulley the machine is at rest, and when it is on the tight pulley the machine is in motion. The driving belt is shifted from the one pulley to the other by pressing on that side of the belt which is advancing towards the pulleys.

Gear Wheels. Let two smooth rollers be placed in contact with their axes parallel, and let one of them rotate about its axis, then if there is no slipping the other roller will rotate in the opposite direction with the same surface velocity, and if D1, D2 be the diameters of the rollers, and N1, N2 their speeds in revolutions per minute, it follows as in belt gearing that

the same as that of the smooth rollers, the surfaces of the latter are called the pitch surfaces of the former.

Pitch Circle. A section of the pitch surface of a toothed wheel by a plane perpendicular to its axis is a circle, and is called a pitch circle. We may also say that the pitch circle is the edge of the pitch surface. The pitch circle is generally traced on the side of a toothed wheel, and is rather nearer the points of the teeth than the roots.

Pitch of Teeth. The distance from the center of one tooth to the center of the next, or from the front of one to the front of the next, measured at the pitch circle, is called the pitch of the teeth. If D be the diameter of the pitch circle of a wheel, n the number of teeth, and p the pitch of the teeth, then DX3.1416=nXp.

By the diameter of a wheel is meant the diameter of its pitch circle.

Form and Proportions of Teeth. The ordinary form of wheel teeth is shown in Fig. 167. The curves of the teeth should be cycloidal curves, although they are generally drawn in as arcs of circles. It does not fall within the scope of this work to discuss the correct forms of gear teeth.

Example 12. Spur Gear. Fig. 167 shows the elevation and sectional plan of a portion of a cast-iron spur gear. The diameter of the pitch circle is 23% inches, and the pitch of the teeth is 12 inches, so that there will be 50 teeth in the