Page:The Kinematics of Machinery.djvu/487

 ESCAPEMENTS. 465

the motion of the pendulum. This, however, is merely an acci- dental feature of the escapement, used to adapt it to particular purposes ; there are many escapements, especially modern ones, in which it does not exist. In some escapements of specially delicate construction, such as the chronometer escapement, a single click only is used, and is lifted and engaged once in each complete vibration (double swing) of the pendulum, allowing one tooth of the escape- wheel to pass it at a time. In Wheatstone's chronoscope, as improved by Hipp, an escapement of this kind, which acts with extraordinary rapidity, is used (Fig. 333). It is so constructed that it can make 1000 complete vibrations per second. In each of these the escape-wheel moves one tooth for- ward and is again arrested at the next. We thus see that in the most delicate machines which have been constructed this click- gear, which at first sight appears so rude an appliance as scarcely to be suited for any approach to machinal exactness of motion, is extensively utilized.

Fig. 334 is an example of a ratchet-train with a fast pawl. If the wheel a is to be moved it is necessary first to raise or disengage the pawl b. This is effected by means of a tooth d v which forms one piece with the revolving ratchet- tooth d, and lifts the pawl by coming in contact with the face l r So soon as this occurs the ratchet d enters one of the spaces of the wheel a, and drives it one tooth - forward. At the end of this motion, however, the pawl again drops, the tooth d l having passed the projecting piece \. As soon therefore as the ratchet motion has occurred the click-train as again fixed. The ratchet dd f may revolve in either direction, so that the wheel may be caused to move either forwards or backwards.

If the radius of the wheel a be made infinite, it becomes, as we know (cf. 69 and 71) a straight rack. We should therefore obtain, if the ratchet d d 1 were suitably formed, a train in which a rack could be moved backwards and forwards by ratchet- gear and held during its pauses by a fast pawl.

Without going here into other forms of this kind of ratchet- gear, I must briefly look at one application of the train which occurs with special frequency. This is its application in locks, where from the common door or box-lock to the most complex K n H