Page:The Kinematics of Machinery.djvu/335

 DOUBLE SLIDER-CRANK. 313

Tn general the expansion of elements occasions, as we have seen, extraordinary alterations in the form of a mechanism, alterations which on the one hand tend very much to conceal its original and real nature, and on the other hand frequently offer great constructive advantages. This is true also for other mechanisms besides those we have been considering. Many familiar arrangements appear in a new and unexpected light if we replace slots and sectors by the complete cylindric forms, C ... || ... C, which they represent ; others again, by the reversed process, can be put into a form which allows of their use in practice where otherwise this would be impossible.*

72. The Normal Double Slider-Crank Chain. (

We have already, in 68, considered the limiting case of the substitution of the pair (A) for a swinging (C) pair, in taking the lever c of infinite length. If we apply the method there used to the coupler 6 of the slider-crank chain, which appears already in Fig. 240 as a sector working in a slot, we can make it also infinite. The slot of the block c will then be straight and at right angles to the line 1, 3, the coupler becomes a prismatic slide with a cylinder normal to it, as Fig. 243 shows. If we write the new chain in full, beginning with the crank a, we have :

abed '(!+ ...1| ... C+ C- ... I ... P+ P- ... I ... /'+ P- ... JL ... C-.

The block c has become a pair of prisms at right angles to each other, one of them (as in Fig. 243,) or both (as in Fig. 248) being open, or in the form of slots. We shall call it a cross-block, or in particular a normal cross-block, and the whole chain (as it now contains two sli ding-pairs) a normal double slider-chain. The crank a remains as before, the coupler, however, has assumed the form G ... _L ... P.

expansion of elements, but I have not there been able to analyse it causally, for this, as we have seen, is a matter which requires a somewhat lengthy investiga- tion. I do not wonder therefore that it has remained greatly misunderstood, and has been sometimes pronounced unimportant, and even superfluous./?.
 * In the Constructeur I have for a long time made use of the method of