Page:The Kinematics of Machinery.djvu/569

 DIFFERENTIAL SCREWS.

�647

�known " differential screws/' whose invention has been ascribed both to Prony and to White. Our previous investigations have put the reader in a position from, which he will recognise in Hunter's press,* the differential screw vice f and so on, only such alterations of ($ 2 'P') C as are due to the reversal of pairs or to external differences in constructive form. The train ( a 'P')* does not appear to have been hitherto applied.

If we make the pairs 2 and 3 = ($), and pair 1=($) =, we obtain the chain shown in Fig. 384, which gives the two mechan-

��FIG. 384.

isms (S 2 'C") = (S^Cy and (S 2 'G") b . The latter appears new, the first has been more than once applied, as for instance, very happily by Skinner J in his steering-gear, which, however, is a compound train.

If the pair 1 be made=(>S r )==(7, the pair 3 = = (P), the pair 2 alone remaining (S), we get the chain (S'P'C"), Fig. 385, which

��FIG. 385.

has already several times come under our notice. Of the three mechanisms which it gives us ($'P'(7) C especially has found, as we know, numerous applications. (Cf. 43 and 107.)

The synthesis has therefore given us here three classes of chains furnishing seven mechanisms, and three of the latter appear new. Let us now apply our method to the cases in which one of the rigid elements is replaced by a Sectional one.

�* Moseley, Mechanical Principles, &c., vol. i. t Ibid, also Weisbach, Mechanik, iii., p. 288. $ The Engineer, 1868, vol. xvi., p. 182.

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