Page:The Kinematics of Machinery.djvu/195

 TENSION-ORGANS.

�173

�41.

Flectional Kinematic Elements.

We have hitherto supposed the capacity for resistance, which we recognised as an essential for those bodies from which a machine could be constructed, to be attained by giving to the elements complete rigidity, molecular immovability. We' assumed that the material and dimensions of the element had been suitably chosen for this purpose, this being the function of machine design. The admissibility of force-closure, however, shows us that bodies may serve for the formation of elements which cannot be considered as sensibly rigid. If, namely, we choose such bodies as, while not resistant in all directions, can maintain approximate molecular immovability under the action of sensible force in at least one direction, and employ these bodies under such force-closure as

��FIG. 124,

���FIG. 126.

�FIG. 125.

�corresponds to their special capability of resistance, they will act exactly as if they were completely resistant.

As bodies possessing these peculiarities we have ordinary cords or ropes, woven or leathern bands and belts, bands or ropes of metal or wire, every kind of chain, all those organs in short which, offering no sensible resistance except to tension, can yet be made sufficiently rigid under the action of tensile forces of any magnitude. We may include them all under the name of tensio n- organs.

On account of their want of rigidity in other directions, the

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