Page:The Kinematics of Machinery.djvu/300

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KINEMATICS OF MACHINERY.

sliding-pair, written, according as one or the other element be fixed.

P+ P- or P- P+.

The complete "principle of the inclined plane" gives the condi- tions of equilibrium of the forces in a sliding pair. The common representations show, as in the last case, an incomplete, force- closed pair, which would be written P~P + ,

' /

The Wedge. This arrangement is commonly represented in a very primitive form, and one almost entirely wanting in the strict- ness of machinal motion, namely, as a means for splitting a piece of wood, Tig. 195. In this very rural-looking apparatus the ratio of the driving effort to the lateral resistances against the sides of the wedge is investigated. If we complete the description which

Q

FIG. 195.

FIG. 196.

FIG. 197.

as a rule is so entirely wanting in definiteness sufficiently to make it applicable to a machinal system, we may say shortly, that the two sides of the wedge are imagined to be prismatically paired with surfaces against which they work, and further, that the latter (the halves of the tree stem), having, as they are separated, re- . latively a rectilinear motion, are also imagined to be paired in the same way. The whole represents a mechanism formed of a three- linked prism chain (Fig. 196), of which the formula, taking the links in the order a, I, c, would be :

Pt :... L ... P+P- .. L ...P+P- ... L ... P-.*


 * More strictly, as the chain is force-closed throughout, this should be,