Page:The Kinematics of Machinery.djvu/502

 480 KINEMATICS OF MACHINERY.

127. Springs as Constructive Elements.

We have already examined the function of springs in kinematic chains. We found ( 42) that they were flectional kinematic elements, and might be arranged so as to work under every kind of force- closure, while the tension- and pressure-organs could be used with one force-closure only. Along with the parts attached to or connected with them springs become kinematic links. It follows unquestionably from our earlier examination of the matter that they should be reckoned among the constructive elements.

128. General Conclusions from the Foregoing Analysis.

The foregoing analysis of the constructive elements of machines has given us some not unimportant results. It has shown us, in the first place, that the parts generally included under this common designation are kinematically of very various descriptions. In part they are really kinematic elements (pins, bearing-blocks, tubes, pistons, stuffing-boxes, cords, belts, chains, springs), in part links of kinematic chains (shafts, axles, frames, levers, cranks, connecting-rods, cross-heads, steam-cylinders, &c.), in part complete pairs of elements (friction-wheels, toothed-wheels) ; some too are portions of kinematic chains (belt-gear, click-gear, brakes, moveable couplings and disengaging-gear, valves, &c.), and a few complete kinematic chains (screwed and keyed joints). Look- ing at them as a whole we may draw the general conclusion that the "constructive elements" are really those pairs of elements and kinematic links which are most frequently used. For some of those which are complete chains in themselves, such as the screwed and keyed joints, are not used constructively to obtain the motions of the chains which they represent, but simply as fastenings, that is, for link-formation ; and the belt and cord-gear occurs as a part of a chain, simply because flectional elements can only be used in closed chains under chain-closure. Some moveable