six points be taken upon the edges of the end surfaces (Fig. 90), and suitable inclinations be given to the restraining surfaces, these six points will also restrain the end-long sliding of the
cylinder, and so will suffice for the whole required restraint.
Eestraint in the Twisting- pair. In order to restrain a screw spindle (Fig. 91) with the minimum number of points, we must restrain FlG> 89> two longitudinal sections of it,
passing through its axis and at
right angles to each other, against both sliding and turn- ing. If this be done the spindle will be as completely
Fig. 90.
restrained as if it were enclosed by a nut. For this purpose each section (see Fig. 87) requires four points of restraint, so that here again eight points of re- straint in all are required. Two pairs of these, however, can be re- duced to one by a method similar to that shown in Fig. 88, so that the actual minimum number of points of restraint is once more six.
We find therefore that for all three closed pairs, eight points of restraint are sufficient, and that by the use of double points six may suffice, to hold the moving element in the same position as that which it would have were it restrained by the infinite number of points of its partner element in a closed pair.
Fig. 91.
