The cams at present universally used in stamp mills lift the tappets with an involute form of curve, to which the surface of the tappet is always tangent; moreover, the line of contact between tappet and cam, if produced, would pass through the center of the stem. This is no doubt a desirable feature, but the writer has long believed that it would be much more desirable if the cam were to pick up the stamp without shock, and gradually increase the upward velocity of the stamp throughout its upward movement. The involute form of cam attempts to impart instantly a considerable upward velocity to the stamp, starting it from a state of rest, and the result is a destructive blow, a great deal of noise, and much wear and vibration.
The writer has developed a form of cam which will lift the stamp pith a motion similar to that of the piston of an engine between the end and middle of its stroke; in other words, harmonic motion, from the woint of zero to maximum velocity. The curve will give this ideal motion only when the stamp is set for the exact drop for which the cam was designed, but the improved cam will not be as bad as the involute cam until the drop is reduced one-half. In other words, if a cam is designed for a 6-in. drop, it will be some improvement over the usual form of cam until the drop is only 3 in.
Any cam, whatever its form, must of course lift the stamp in the same number of degrees of revolution; therefore, with this new form, since the stamp starts more slowly on its upward course, it must end up by going faster, the average speed being the same as with the involute cam. The stamp, after it is no longer in contact with the cam, keeps on going up a certain distance, which is easily calculated. It is therefore possible to hang up a stamp without a cam stick, if the fingers are not more than 1/8 in. longer than necessary to hold the tappet above, the cam.
With this design of cam the surface of the tappet is not tangent to the surface of the cam throughout the lift. The possible consequences of this were studied with a full-size model, and did not seem serious, as the engagement between the surfaces was an easy sliding motion, instead of a blow. If the drop is shortened, the blow becomes more and more pronounced, but the surfaces also become more and more nearly tangent.
Five cams of this new design have been running in one of the mills of the North Star Mines Co. now for over a month fulfilling every expectation of the writer. Holding the hand on the tappet it is impossible to feel the cam strike the tappet, although the mill is running 107 drops per minute.
Fig. 1 shows the method of laying off the curve of the cam which will give approximately harmonic motion to the stamp. Spaces s a1, a1 b1, b1 c1, etc., represent the distances traversed by the stem in equal intervals of time.