438 KINEMATICS OF MACHINERY.
Pins, Fly-wheels,
Shafts, axles and spindles, Levers,
Couplings, Cranks,
Framing, bed-plates, brackets Connecting-rods, couplers,
&c. Crossheads and guides,
Belts, cords and ropes, Click and ratchet-wheels and Chains and their connec- gear,
tions, Brake-wheels and gear,
Friction-wheels, Pipes and their connections,
Belt pullies and gear, Steam and pump cylinders,
Rope pullies and gear, Valves,
Toothed-wheels, Pistons and stuffing boxes,
Chain-wheels, Springs.
In addition to these parts, all of which have very numerous applications, there are others which come into use only in single classes of machinery, spinning and weaving machines, machines for working in metal, etc., but are still employed often enough to have been sometimes included with those above mentioned. By a dis- tinction which appears quite justifiable they have occasionally been called " special " parts as distinct from the above " general " ones. Without giving any illustrations of this second class of construc- tive elements we shall proceed to consider the first in order. We shall endeavour first to ascertain precisely the kinematic meaning of each, and shall afterwards see how far we can find any general kinematic connection between the whole.
107. Screws and Screwed Joints.
In the common screw and nut, Fig. 288, we at once recognise the twisting pair ($) or S+S~, and we can do the same in some other applications of the screw where, as in the screw-joint, Fig. 289, the nut and screw are themselves parts of the two elements to be united.
The case of the common screw-joint, of which Fig. 290 gives a familiar illustration, is, however, a different one. Here we have a combination of four pieces, a, b, ^ and c, the object of the whole
