Fig. 72. RACKS AND PINIONS.--The object of this form of mechanism is to provide a reciprocating, or back-and-forth motion, from a shaft which turns continually in one direction. A is the rack and B a mutilated gear. When the gear turns it moves the rack in one direction, because the teeth of the gear engage the lower rack teeth, and when the rack has moved to the end its teeth engage the teeth of the upper rack, thus reversing the movement of the rack.
Fig. 73. MUTILATED GEARS.--These are made in so many forms, and adapted for such a variety of purposes, that we merely give a few samples to show what is meant by the term.
[Illustration: _Fig. 69. Trammel_ _Fig. 70. Escapement_ _Fig. 71. Device for Holding Wheel_ _Fig. 72. Rack and Pinion_ _Fig. 73. Mutilated Gears_ _Fig. 74. Shaft Coupling_]
Fig. 74. SIMPLE SHAFT COUPLING.--Prepare two similarly formed discs (A, B), which are provided with hubs so they may be keyed to the ends of the respective shafts. One disc has four or more projecting pins (C), and the other disc suitable holes (D) to receive the pins.
Fig. 75. CLUTCHES.--This is a piece of mechanism which is required in so many kinds of machinery, that we show several of the most approved types.
Fig. 76. BALL AND SOCKET JOINTS.--The most practical form of ball and socket joints is simply a head in which is a bowl-shaped cavity the depth of one-half of the ball. A plate with a central opening small enough to hold in the ball, and still large enough at the neck to permit the arm carrying the ball to swing a limited distance, is secured by threads, or by bolts, to the head. The first figure shows this.
Fig. 77 illustrates a simple manner of tightening the ball so as to hold the standard in any desired position.
Fig. 78. TRIPPING DEVICES.--These are usually in the form of hooks, so arranged that a slight pull on the tripping lever will cause the suspended articles to drop.
Fig. 79. ANCHOR BOLT.--These are used in brick or cement walls. The bolt itself screws into a sleeve which is split, and draws a wedge nut up to the split end of the sleeve. As a result the split sleeve opens or spreads out and binds against the wall sufficiently to prevent the bolt from being withdrawn.