Fig. 1117. A method of engaging, disengaging, and reversing the upright shaft at the left. The belt is shown on the middle one of the 3 pulleys on the lower shafts a, b, which pulley is loose, and consequently no movement is communicated to the said shafts. When the belt is traversed on the left-hand pulley, which is fast on the hollow shaft b, carrying the bevel-gear B, motion is communicated in one direction to the upright shaft; and on its being traversed on to the right-hand pulley, motion is transmitted through the gear A, fast on the shaft a, which runs inside of b, and the direction of the upright shaft is reversed.

Fig. 1118. Spur-gears.

Fig. 1119. The wheel to the right is termed a "crown-wheel"; that gearing with it is a spur-gear. These wheels are not much used, and are only available for light work, as the teeth of the crown-wheel must necessarily be thin.

Fig. 1120. Multiple-gearing - a recent invention. The smaller triangular wheel drives the larger one by the movement of its attached friction-rollers in the radial grooves.

Fig. 1121. These are sometimes called brush-wheels. The relative speeds can be varied by changing the distance of the upper wheel from the centre of the lower one. The one drives the other by the friction or adhesion, and this may be increased by facing the lower one with indiarubber.

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Fig. 1122. Transmission of rotary motion from one shaft at right angles to another. The spiral thread of the disc-wheel drives the spur-gear, moving it the distance of one tooth at every revolution.

Fig. 1123. Worm or endless screw and a worm-wheel. This effects the same result as Fig. 1122; and as it is more easily constructed, it is oftener used.

Fig. 1124. Friction-wheels. The surfaces of these wheels are made rough, so as to "bite" as much as possible; one is sometimes faced with leather, or, better, with vulcanized indiarubber.

Fig. 1125. Elliptical spur-gears. These are used where a rotary motion of varying speed is required, and the variation of speed is determined by the relation between the lengths of the major and minor axes of the ellipses.

Fig. 1126. An internally-toothed spur-gear and pinion. With ordinary spur-gears the direction of rotation is opposite; but with the internally-toothed gear, the two rotate in the same direction; and with the same strength of tooth the gears are capable of transmitting greater force, because more teeth are engaged.

Fig. 1127. Variable rotary motion produced by uniform rotary motion. The small spur-pinion works in a slot cut in the bar, which turns loosely upon the shaft of the elliptical gear. The bearing of the pinion-shaft has applied to it a spring, which keeps it engaged; the slot in the bar is to allow for the variation of length of radius of the elliptical gear.

Fig. 1128. Uniform into variable rotary motion. The bevel-wheel or pinion to the left has teeth cut through the whole width of its face. Its teeth work with a spirally-arranged series of studs on a conical wheel.

Fig. 1129. A means of converting rotary motion, by which the speed is made uniform during a part, and varied during another part, of the revolution.

Fig. 1130. Sun-and-planet motion. The spur-gear to the right, called the planet-gear, is tied to the centre of the other, or sun-gear, by an arm which preserves a constant distance between their centres. This was used as a substitute for the crank in a steam engine by James Watt, after the use of the crank had been patented by another party. Each revolution of the planet-gear, which is rigidly attached to the connecting rod, gives two to the sun-gear, which is keyed to the fly-wheel shaft.