Page:The Kinematics of Machinery.djvu/208

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Dead Points in Mechanism, their Passage by Means of Sensible Forces.

THE incompleteness possessed by certain pairs of elements, which we have now found means for rendering harmless, occurs also in many kinematic chains ; and there also similar means can be employed to neutralise it. The common weighing-machine furnishes to a certain extent an illustration of this (see Fig. 122), for it must, so far at least as concerns the mere connections of its links, be regarded as an incomplete kinematic chain. Incompleteness of this sort, however, is nothing more than the incompleteness of a pair of elements ; and we already know how it can be rectified. The real question rather concerns the incompleteness of chains which consist wholly of closed pairs, or in which, by any suitable means, the closure of every single pair has been completed.

The application of a driving-force to one link of a mechanism in such a way that that link may have the motion especially be- longing to it, and the whole mechanism be moved, is not sufficient of itself to insure that continuous motion shall be possible. In the mechanism shown in Fig. 145, for example, a force always normal