Fig. 563.
Another plan is to let the back gear move endwise and bush its bearing holes with hardened steel bushes. This possesses the advantage that the gear is sure, if made right, to keep so, but it has some decided disadvantages: first, the pinion a, [Fig. 563], must be enough larger than the smallest cone-step b to give room between b and c for the belt, and this necessitates that d also be larger than otherwise; secondly, the gear-spindle f projects through the bearing at f, and this often comes in the way of the bolt-heads used for chucking work to the face plate. The method of securing the spindle from end motion is as follows: On the back of the head is pivoted at i, a catch g, and on the gear shaft f are two grooves. As shown in the sketch, g is in one of these grooves while h is the other, but when the back gear is in, g would be in h.
Fig. 564.
Sometimes a simple eccentric bush and pin is used as in [Fig. 564], in which a is the spindle journal, b a bush having bearing in the lathe head, and d a taper pin to secure b in its adjusted position.
In large heavy lathes having many changes of speed, there are various other constructions, as will be seen upon the lathes themselves in the various illustrations concerning the methods of throwing the back gear in and out. The eccentric motion shown in [Fig. 573] of the Putnam lathe, is far preferable to any means in which the back-gear spindle moves endways, because, as before stated, the end of the back-gear spindle often comes in the way of the bolts used to fasten work to the large face plate. This occurs mainly in chucked work of the largest diameter within the capacity of the lathe.
Fig. 565.