Page:The Kinematics of Machinery.djvu/484

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KINEMATICS OF MACHINERY.

A piece of a machine is said to receive a ratchet motion if it be moved always in the same direction but with an intermittent instead of a continuous motion. The special mechanism by which this motion is given is called a ratchet-train. Such a train requires that the link receiving the intermittent forward motion should be prevented during the longer or shorter pauses that occur from moving backwards, and for this purpose click-gear is very often employed. A complete ratchet-train is therefore frequently, although not always, a combination of ratchet-gear and click-gear.

FIG. 329.

FIG. 330.

The click or other gear thus coming into use we shall call retaining-gear.

A form of ratchet-train which often occurs is sketched in Fig. 329, the object here being the lifting of the rod a a r The rack click-train (GPP S *), which we have already examined, is here used as retaining-gear ; and the ratchet work consists of an exactly similar chain placed upon a, whose rack a^ is made