MISCELLANEOUS MECHANISMS 399 
_ restrains the downward motion of the follower, regulating the velocity of 
fall. In Fig. 636 the acting edge of the cam is made up of straight 
lines, and it is obvious that the follower will move through equal 
distances, in rising and falling, for equal movements of the cam. Also, 
saad will be periods of rest for the follower at the bottom and top of its 
tra 
Fig. 637 shows how the same kind of cam is designed to give the 
same kind of motion to the follower, except that the motion of the 
follower is angular instead of rectilinear, that is to say, in both examples 
the follower moves through equal distances, in rising and falling, for 
equal movements of the cam, and there are the same periods of rest. 
In Figs. 636 and 637 the outline of the cam is obtained by assuming 
that the cam is fixed and that the follower moves towards the right 
through equal distances A to 1, 1 to 2, 2 to 3, ete., and at the same time 
rises through the distances A to 1, 1 to 2, 2 to 3, etc., shown by the 
. divisions on AB. 
In Figs. 636 and 637 the lower end of the follower is wedge-shaped, 
the edge of the wedge being-in contact with the cam. Greater durability 
is obtained by replacing the wedge end by a pin, or by a pin and roller, 
Fig. 638. Fie. 639. 
the axis of the pin taking the place of the edge of the wedge. With the 
pin and roller there is less friction than with the pin alone. In designing 
a cam to work against a pin or roller, the acting surface of the cam is 
first determined as for contact with a wedge ; this acting surface is called 
the pitch surface of the cam, and the trace of the pitch surface on a 
surface normal to it is called a pitch line. The axis of the pin or roller is 
then supposed to travel so as to generate the pitch surface, and the proper 
acting surface of the cam is the envelope of the moving pin or roller, as 
shown in elevation in Figs. 638 and 639. In Fig. 639 the complete 
envelope is used, and becomes a slot in the cam plate ; such a cam will 
move the follower positively in both directions. In Fig. 638 only one 
side of the envelope is used, and this cam requires that the follower be 
pushed against the cam during the downward stroke. A common defect 
due to the use of a roller is referred to in Art. 339. 
336. Plane Rotating Cams.—The method to be adopted in designing 
plane rotating cams is similar to that already described for plane sliding 
cams. Figs. 640, 641, and 642 show plane rotating cams for working on 
wedge-ended followers. The followers for the cams shown in Figs. 640 
and 641 have rectilinear motion, and would have the form shown in Fig. 
627. In Fig. 640, AB, the path of the end of the follower in contact 
with the cam, when produced, passes through C, the axis of rotation of 
the cam, while in Fig. 641, AB produced does not pass through C. The 
