408 APPLIED MECHANICS 
342. Oldham’s Coupling.—When the axes of two shafts are parallel, 
and the distance between them is small and variable, the shafts may be 
coupled, so that one will 
drive the other at the same 
speed by means of Oldham’s 
coupling, which is shown in 
Fig. 661. KL and MN are 
the axes of the shafts. A 
and B are flanges secured or 
forged to the shafts E and F 
respectively. C is an inter- Fic. 661. 
mediate piece. In one with 
C on its opposite faces are prismatic pieces a and b at right angles to one 
another. These pieces fit into grooves formed in A and B, as shown. It 
is evident that whatever angle A turns through C must turn through the 
same angle, and whatever angle C turns through B must turn through 
the same angle; hence A, C, and B must, at every instant, have the same 
angular velocity. 
343. Ratchets.—The principal parts of a ratchet mechanism are, a 
wheel or sector or rack having teeth, and a ratchet, click, or pawl, which 
engages with the teeth. In general the ratchet mechanism is used either 
to give intermittent motion in one direction, or to permit of motion in one 
direction and prevent it in the opposite direction. 
In Fig. 662 A is a ratchet wheel, and B a pawl carried on a pin 
attached to a lever C. The lever has an oscillating motion, in this case 
Fig. 662. Fig. 663. Fig. 664. 
about the axis of A. When the lever is moving in the direction of the 
arrow, the pawl B engages with a tooth on the wheel, and the lever and 
wheel move as one piece. When the motion of the lever is reversed the 
pawl B rides over the teeth of the wheel, which remains at rest, either 
because of some resistance, ‘such as friction, or because of the action of the 
pawl or catch or detent D, which is mounted on a fixed pin. 
In the arrangement shown in Fig. 663 an almost continuous rotation 
of the wheel A in the direction of the arrow is obtained by the use of two 
pawls B, and B, mounted on pins attached to arms on the lever C, 
which oscillates on a fixed pin E. In Figs. 662 and 663 the teeth of the 
wheel exert.a thrust on the pawls when the latter are in action,.but the 
