472 KINEMATICS OF MACHINERY.
124.
Recapitulation of the Methods used for Stopping and Setting in Motion.
We have seen that disengagement and engagement may be used either in a pair or in a link of a kinematic chain. Its object is in each case to bring to rest, or to set in motion again, some portion of the mechanism. Eemembering at the same time that click-trains and brakes often serve the same purpose in connection with the whole mechanism, it will be useful for us to recapitulate here the principles upon which the different methods used for stopping and setting in motion mechanisms or parts of them are founded.
We have already noticed that click and brake act upon the ele- ments of a pair in such a way as to unite them into one body. Such a union of an element with its partner we may call the fix- ing of a pair. Eemembering this, we see that the stoppage of a mechanism or of a portion of one is effected
(a) by fixing a pair of elements in the kinematic chain (as in click-gear or brakes) ;
(6) by disuniting a pair of elements in the kinematic chain (as in disengaging toothed wheels, throwing off belts, lifting a " gab," etc.) ;
(c) by dividing a link in the kinematic chain (as in claw- couplings, in throwing off pump-rods by removing a key, etc.),
while the original motion again becomes possible if the chain be restored to its normal constrained condition. We shall see imme- diately that this classification applies equally to pressure-organs ; it covers therefore the whole ground which we are examining. A general examination of the ways in which it is possible to make a kinematic chain inlmoveable and moveable at will, without destroy- ing any of its parts, makes it evident that in the classification given above we have exhausted all the means available for this purpose.
