182 KINEMATICS OF MACHINERY.
used, is obviously the contra-positive of the old bell-rope ; the (single-acting) " water-rods " used in mines, of the iron tension- rods, and partly also the hydraulic crane of the rope or chain crane.
The water-wheel (Fig. 141) gives us a further illustration of a force-closed mechanism. The enclosure of the water in the chan- nel c is again two-fold. It is due in the first place to the action of latent forces in the channel walls, and then further to the force of gravity, which prevents the water moving upwards. In the bent portion of the channel the water is paired both with the floats, virtually a toothed rim of the wheel and also with the
channel; the wheel itself, on the other hand, is again pair- closed with the channel through its shaft and bearings. The kinematic chain has three links, the wheel a, with its shaft, the water b, and the channel c with the shaft bear- ings. The pairing of the flec- FIG. 141. tional element b with a is
effected by the chain ; the
force-closure produces at the same time the envelopment (here enclosure) of the floats with the flectional element 5, and the confine- ment of the latter within its channel, which we must look upon as a vessel only partially closed.
There are many cases in which the axoid rolling (37 and 40) takes place, or at least is rendered possible, by chain-closure instead of direct force-closure. The Fell railway is a good example of this. Here the driving-wheels, instead of being held on the rails, as usual, by the weight above them, are made horizontal and held against a central by an arrangement of springs forming a kinematic chain. In this and many similar cases the spring can be used with great advan- tage, for as an element at once flectional and elastic it admits of advantageous employment under varying forces. In this special arrangement, however, there is often a certain indeterminateness of motion ; the possibility occurs of a sort of overbalancing of the force- closure.
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