Page:The Kinematics of Machinery.djvu/478

 456 KINEMATICS OF MACHINERY.

exhaustively, but must here content ourselves by looking at a few of the more important cases which occur.

Among the numerous forms in which click-gear is used the most common is that of a toothed wheel provided with a click or pawl, Figs. 320 and 321. The train consists, in both the cases shown, of three links viz. the wheel a = C...\ ...C z, the click I Z...\\...C, and the frame c = <7...||...G'; we shall suppose this

FIG. (520. FIG. a^l.

last to be the fixed link. The tooth Z, the working end of the pawl, lies force-closed (as we have already pointed but, p. 180) in the spaces of the wheel a, the catch being held down either by a spring or by its own weight. It must also be remarked that b is kinematically paired with a only for one direction of rotation, left-handed rotation in Fig. 320 and right-handed in Fig. 321. If any turning commence in the opposite direction the wheel is at once held fast by the click, so that the whole mechanism becomes equivalent to a single piece.

At first sight it might appear that the difference, between the two trains was constructive only, the pawl being formed to resist pressure in the first case and tension in the second. If, however, the direction of the arrows shown upon the figures be noticed, it will be seen that in the first case the turning of the wheel and click, if the former be set in motion, takes place in opposite directions, while in the second case it occurs in similar directions. Between the pressure or push-click and the wheel there is thus the same relation as between externally toothed wheels, between the tension or pull-click and the wheel the same as between an annular-wheel and a spur-wheel. We may therefore use the symbol Z + for the tooth of the pressure-click, and Z~ for that of the tension-click.