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More about cams
A cam is usually a non-circular wheel, often egg shaped. As the cam turns it causes a follower to move up and down. On the circular part of the cam the follower stays still, on the oval part it rises smoothly as the cam pushes the follower up and then falls smoothly as the cam lowers it down. In this case the follower rises and falls just once for each complete revolution of the cam.
If the cam is completely oval shaped i.e. it has no circular part, then the follower will rise and fall twice for each revolution of the cam.
Note that the amount of movement of the follower is determined by the extent to which the shape of the cam (sometimes called the profile) departs from the circular. It is possible to use a circular wheel with added 'bumps' (made from round headed drawing pins pushed into the edge of the wheel) to cause a follower to 'jiggle' up and down. It is possible to use a wheel as a cam if the shaft is off centre. This is called an eccentric. The amount of movement of the follower is determined by the amount by which the shaft is off centre.