Page:The Kinematics of Machinery.djvu/479

 FREE AND FAST CLICK-TRAINS. 457

A second property of the gear which must be indicated by our notation is the single action of the click-wheel. We may show this by substituting a semi -col on for the comma between G' z and Z. With the addition of the sign for force-closure the pair will

7+ Z~

therefore be written C z ; ^ or C z ;. The point may be taken to

J J

denote the immoveability of the chain in one direction, the comma

showing that it is moveable in the other.

Placing the chain on c, its complete formula will be therefore

a ~b c

0+~./il ... C, ; Z ... |[ ... C+ C- ... Ji ... C-.

The form symbol for Z has been here omitted in order to make the expression more general. The sign for forceclosure is also omitted; it can usually be dispensed with the unusual nature of the pair being sufficiently pointed out by the semicolon. The latter, indeed, makes it possible for us to use a single element symbol only for the pair C z ; Z, for we shall indicate it quite sufficiently if we take G z ; Z, = (C z ;). This contracted form is also justified by the analogy of (C z ) for the spur-wheel pair (7 Z, C x for we may consider the pawl C ... || ... Z essentially as a piece of a spur-wheel, carrying a single tooth.

The rack click-gear of Fig. 322, with fixed frame, would have for its extended formula :

P+ ... || ... P z ; Z+ ... || ... C+ <?-... JL...P-

for which the contracted form would be (C P P 2 ;) c.

There is another class of click-gear which differs in one very important particular from that which we have been considering ; an example of it is shown in Fig. 323. Here we have click-wheel, pawl and frame exactly as above, but here the pawl so grips the teeth of the wheel as to make its motion in either direction impossible. The click I is therefore, as it were, a combination of those of Figs. 320 and 321, for it acts as a pressure-click against motion in the one direction and as a tension-click against motion in the other. While, therefore, the click-trains just considered were single-acting, the one now before us is double-acting; we may call them free and fast click-trains respectively.