Page:The Kinematics of Machinery.djvu/480

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KINEMATICS OF MACHINERY.

We must find a new symbol of relation to enable us to express this double action of the fast click. The tooth may, in the first place, be indicated by Z, and further, following out the same reason for which we chose the semicolon above, we may here use the colon for the sign of pairing. We shall therefore indicate the pair of elements consisting of a click- wheel and double acting pawl by C z : Z, or by the contracted symbol ( 6 T Z :). The fast click train of Fig. 323 will therefore be (CJC,:)'.

This train differs very greatly from the free click train (C^;). In the latter nothing whatever pre- vents the free turning of the wheel a in the proper direction; the pawl is lifted by the motion of the wheel itself, and drops again immediately by force- closure, If it be desired that turning should take place in the opposite direction, some special means must be provided for lifting the pawl b, and so throwing the whole train "out of gear." With (C'^C Z ^ C on the other hand, motion cannot oc- cur in either direction unless the click be first thrown out of gear. If any fast click be thrown out of gear, and then, motion being com- menced, be again brought under the action of the closing force (Fig. 324), the rotation lasts only until the next space comes under the tooth of the pawl. The latter then falls instantaneously, and wheel, pawl, and frame become equivalent to a single piece only.

With the free click-train (Fig. 325), on the other hand, the pawl falls gradually under the same circumstances (the wheel, Fig. 325, turning to the right), and reaches the bottom of the space even before the moment of closure. It in- tercepts the wheel teeth therefore with greater safety than in the other case.

If after any loaded click train a train,

that is, whose click-piece a is subjected to the action of some con- tinued forward force be set in motion, its click does not come under the action of the closing force, the wheel a will continue to turn, and will turn the quicker the greater the load be. This motion

FIG. 322.

FIG.