Cams are devices for transforming rotary motion into linear motion. Conventional cams consist of an eccentric projection on a shaft and have sloping sides, or ramps, on either side of the position of maximum projection, or lift, so that the motion imparted to a cam follower by the rotation of the shaft is controlled in both directions and is continuous.

Snail cams, on the other hand, have a working surface which terminates abruptly at, or slightly beyond the position of maximum lift so that the motion of a cam follower is discontinuous with a smooth rise to the point of maximum lift and a virtually instantaneous return to the starting position, if this position is passed.

It is an object of the present invention to provide an improved snail cam.

According to the present invention, there is provided a snail cam wherein the working surface terminates in a stop positioned at or beyond the position of maximum lift of the cam.

Preferably, the stop comprises a re-entrant projection adapted to receive a cam follower peg so as to provide a stable final position for the cam follower peg.

The working surface may commence from a re-entrant portion of the snail cam thereby to provide a stable initial position for a peg type cam follower.

The invention will now be described, by way of example, with reference to the accompanying drawing which shows the profile of a snail cam embodying the invention.

Referring to the drawing, a snail cam 100 embodying the invention has a working surface 101 which commences at a position such that the centre of a cam follower peg 102 of radius Q is in alignment with the centre of rotation of the snail cam 100 and at a distance of 4Q from it. The cam follower peg 102 forms a projection on a body (not shown) to which linear motion is to be imparted by the snail cam 100. The radius of curvature of the working surface 101 of the snail cam at 90 degrees stations is indicated and it varies continuously between these positions.

The working surface 101 of the snail cam 100 extends some 30 degrees beyond the top dead centre position relative to the centre of rotation of the snail cam 100 and terminates in a re-entrant stop 103 which is adapted to receive the cam follower 102 so as to provide a stable final position for it. The angular distance past top dead centre of the re- entrant stop 103 is not critical but needs only to be sufficient to ensure that, under load, there is no tendency for the cam follower peg 102 to drive the snail cam in reverse.

The snail cam 100 shown in the drawing is profiled for clockwise rotation, but an anti- clockwise rotation can be provided for, if so desired.

The profile of the working surface 101 of the snail cam 100 also is a matter of choice.