466
KINEMATICS OF MACHINERY.
" patent safety " apparatus, we find this train everywhere applied in the motion of the bolt by the key.
The common door-latch, in the first place, shows itself at once to be a free click-train under our definition. Both the common lifting latch and the ordinary spring-bolt or " sneck," form, with the lock-box or frame, the door-frame and the door itself, click- trains, which belong to the class shown in Figs 320 and 321. They differ from these only so far that after the bolt or latch has fallen into gear by the closing of the door, the socket of the bolt
on the one hand, and the frame of the door on the other, prevent any further motion of the door on its hinges, so that the free click has become a fast one.
A bolt moved by a key is al- most invariably a click-rack a of the form P...\\...P Z : , the tumbler is a fixing pawl I, which is made in several pieces in the better
classes of locks for security's sake. The key is the ratchet and lifting tooth d d v the frame of the lock the fixed link c. Besides this the bolt a forms with the lock frame and the door a special
FIG. 334.
FIG. 335.
fast click-train. In those locks in which more than one turn of the key is required to withdraw, or to shoot the bolt, the rack a has more than one ratchet and click-tooth. In order to prevent any unauthorized opening of the lock, the link of the train which
