In Fig. 81, the case is identical with that of Fig. 79, except that the shape of the follower has been changed to the more practical form of a roll, which can turn about a pin, thus relieving the crowding, grinding action characteristic of the pointed follower hitherto discussed. The path of follower roll F is divided as before into six equal parts.
The original radius CO, the radius CX, limiting the arc of rise, the radius CB, limiting the arcs of rest and fall, are all drawn precisely as before, and the subdivisions of the arcs are in nowise changed from the preceding cases.
The follower is now rotated about the cam, giving the intersections R1, R2, R3, R4, R5, and R6 for the period of rise; the corresponding intersections for the period of fall are F6, F5, F4, F3, F2, and F1. With each of these intersections as a center, and a radius equal to the radius of the follower roll, an arc is struck, which represents the follower in its rotated position.
A common tangent line is now drawn to the several positions of the rotated follower, giving the outline of the cam as a smooth curve. In order to give the follower its full period of rest from R6 to F6, the portion of the cam lying between the radii CX and CB must be a true arc of a circle struck from center C. Special attention must be paid to this point, because, if the true arc is not maintained between these radii, the full period of rest will not be secured.
The testing of the cam is accomplished in the same way as previously described, by the tracing-cloth method. The several positions of the follower in its path should be drawn; and as the cam is rotated into its several positions, if the work has been accurately done, the cam will be perfectly tangent to each position of the follower.
By a well-known principle of mechanics, when two bodies are in contact, the line of pressure between them is always perpendicular to the surfaces at their point of contact; in other words, the line of pressure is the common normal at the point of contact. The follower being a true circle, the perpendicular at any point of its surface must pass through the center of the roll; therefore, if we draw a line from the center of each position of the roll to the point of contact between it and the cam, this line will be the line of pressure between the cam and the roll. This has been done in Fig. 81, and the arrows indicate the direction of the pressure of the cam against the roll. In order to group these lines of pressure so that the action may be clear as the follower moves over its path, the lines of pressure as drawn are rotated back to the corresponding points in the path of the follower. By this it is readily seen that during the period of rise the lines of pressure are all slightly inclined towards the left of the line of travel, while during the period of fall the lines of pressure are all slightly inclined towards the right. The cam as shown in Fig. 81 is a very good cam, so far as the lines of pressure are concerned. The ideal condition would be to have the lines of pressure all coincident with the line of travel. This is impossible, because the only shapes which would give a common normal along the line of travel would be two circles, revolving about their centers, and such a cam could give no travel to the follower. The fact that the lines of pressure are at such a slight angle to the line of travel indicates that there is very little side pressure on the follower and that, therefore, the cam will be an easy working cam.
Fig. 81. Diagram of Cam with Roll Follower on Center Line.