three forms which we have found as distinct from each other, although they might all be considered as modifications of the screw, and we have thus three closed pairs to distinguish. These are, to recapitulate in a few words:
S-
1. The common screw and nut (twisting pair);
2. The solid and hollow solid of revolution, which for the sake
of brevity we shall call full and open re volutes respect- ively (turning pair);
3. The full and open prism, (sliding pair);
^
all three having normal profiles which prevent what we have called cross-motion; they are adapted for the production of
Fig. 42. FIG. 43.
three kinds of constrained motion, viz. motion (a) in helical paths; (ft) in circular paths; and (c) in rectilinear paths. All three are well known in machine construction, the screw- pair both in fastenings and in moving pieces; the pair of revolutes in journals, bearings, &c. and the prism-pair in guides of all sorts. The use of such normal profiles as prevent any motion of a pair except in the one required direction, is also very familiar. The rings, collars, or flanges of the bearings of shafts and spindles, for instance, carry these profiles. If it be desired to use a cylindrical shaft, which for convenience has been made without collars, as one element of a turning-pair, the well-known " loose ring" (Fig. 42) is used to close the pair. The ease with which cylinders can be formed in the lathe causes them often to be employed in places where they have to become elements in sliding-pairs. To make a cylinder into a prism for this purpose is the object of the common arrangement of feather and groove
