Page:The Kinematics of Machinery.djvu/485

 DOUBLE-ACTING RATCHET-TRAINS. 463

one with that of the retaining-gear, and which moves relatively to the fixed frame c. When c l is moved downwards the rack a tfj is held by the retaining-gear, while the pawl \ of the ratchet- train slips over the tops of its teeth ; when it is lifted, on the other hand, b v c v and a^ behave as a single piece, while b allows the rack to move upwards.

If, retaining the same relative motion of c and c x as before, we make half of it into absolute motion, we obtain the double-acting ratchet- train which is represented by Fig. 330. Here the two racks a and a^ are formed upon opposite sides of the same rod, and guided by an internal prism pair, The pawl rods, c and c v are moved by couplers from an equal armed lever above them, in the same way as before. (In practice, where the double-acting ratchet- train occurs not unfrequently, the pawls are generally placed directly upon the working lever, so as to dispense with the couplers and rods.) No retaining-gear is now used, the two sets of ratchet-gear acting alternately. It is to be noticed that the pawls here, although they have only half the stroke of those in Fig. 329, pass over the same distance on the rack in each down- ward motion as in the former case, supposing the whole travel of the rack to be the same, per period, as in the single-acting ratchet-train.

The "levers of Lagarousse " (Fig. 331) form another double- acting ratchet-train. Here one block carries both a pull- and a push-click, and these act alternately on the ratchet-wheel a. For each upward or downward motion of c, the number of teeth over which the one pawl slips corresponds to twice the distance through which the other (acting) pawl moves the wheel forward. This peculiarity of the motion should be remembered, as we shall have to return to it again.

We notice here that free click-gear is very suitable for use in ratchet work. This is specially the case in one important class of ratchet-trains, the escapements of clocks or watches. These act generally in the manner described in the foregoing section, by the alternate engagement and disengagement of a click with a click-wheel, the latter being continuously driven by some external force in one direction. The engagement and disengage- ment are caused to take place at intervals of time as nearly uniform as possible, so that the escapement may regulate the