QUICK RETVEN MOTIONS. 299
steam-engines themselves we shall have more to say further on ( 80).
The mechanism ((7' 3 'P- L ) b in the form (C"P)l has found another application in shaping and slotting machines. In this the peculiar motion of the link c, swinging un-uniformly while the crank rotates uniformly, is made use of. The crank a is the driver, during the semi-revolution through 1' it imparts to the block a much smaller mean angular velocity than during the semi-revolution through 1". By connecting the link c with the holder of a cutting tool, as Fig. 219, we can therefore obtain a slow (mean) forward motion of the tool while cutting, and a quick (mean) return. Mechanisms of this kind are known as "quick return" motions. The mechanism (C^P- 1 -)* is known and familiar to the engineer in this and other ways.
2"
FIG. 221.
The mechanism (G" 3 'P^) a . By placing the chain on a we obtain a third mechanism, Fig. 220. The link I which was the coupler now revolves about the axis 2, it has, that is, become a crank ; the crank a becomes the frame. The slide d, driven by the block c, turns about the axis 1. Its rotation, if the crank I turn uniformly, is un-uniform, the latter imparting to it at 3'" a minimum and at 3' a maximum velocity. On account of this
property Whitworth and others have used it in the form (G'^P-^l as a quick return. According to Goodeve* the mechanism is an old one, and was long ago used to represent the motion of the moon relatively to the earth. We shall call it the turning-block (slider-crank).
The mechanism (C" 3 'P-^) C . The fourth and last mechanism is obtained by fixing the block c instead of the link a, Fig. 221. The coupler l> now swings about the fixed axis 3, the slide d moves rectilinearly to and fro in the block c, now become the frame,
- Ooodeve, Elements of Mechanism, p. 63.
