468 KINEMATICS OF MACHINERY.
that in the case of the brake the two elements are combined by making the motion of the pair gradually more and more difficult, the union of the two elements occurring when this difficulty be- comes a maximum.
Brakes have the further resemblance to click-trains that one class of them act equally well for motion in either direction, as in the fast click-train, while those of another class, like the free click-trains, are either single-acting, or act differently in the different directions, as e.g., in the case of the band-brake in Fig. 336. In the following section we shall be able to ^investigate more generally these points of similarity.
123. Engaging and Disengaging Gear.
Among the constructive elements which we have considered there have been several specially arranged and used so as to stop the action of a part of the machine when required, or to set it again free to move. Such arrangements are known as engaging and disengaging gear. It is obviously important that we should