Fig. 235.
Fig. 236.
Fig. 237.
We may now consider the return motion, and in [Fig. 233] we find that the order of things is reversed, for the small roller has contact at o, while the large one has contact at p; hence the small one leads and gives the most rapid motion, which it continues to do, as is shown in [Figs. 234], [235], and [236], and we may plot out the two motions as in [Fig. 237]—that for the large roller being above and that for the small one below the line a a. First we set a pair of compasses to the radius from the axis of the large and small roller when in the position shown in [Fig. 231] (which corresponds to the same radius in [Fig. 228]), and mark two centres, m and n, as we did in [Fig. 232]. Of these n is the centre for plotting the motion of the large roller and m the centre for plotting the motion of the small one. We set a pair of compasses to the radius from the axis of the cam and that of the large roller in [Fig. 231], and then resting the compasses at n we mark arc 5 above the line a a, [Fig. 237]. The compasses are then set from the cam to the roller axis in [Fig. 233], and arc 6 is marked above line a a. From [Figs. 234], [235], and [236] we get the radii to mark arcs 7, 8, 9 above a a, and the motion of the large roller is plotted. We proceed in the same way for the small one, but use the centre m, [Fig. 237], to mark the arcs 5, 6, 7, 8, and 9 below the line a a, and find that the small roller has moved quickest throughout. It appears, then, that the larger the roller the quicker the forward motion and the slower the return one, which is advantageous, because the object is to move the roller out quickly and close it slowly, so that under a quick speed the cam shall not run away from the roller as it is apt to do in the absence of a return or backing cam, which consists of a separate cam for moving the roller on its return stroke, thus dispensing with the use of springs or weights to keep the roller upon the cam and making the motion positive.
