An eccentric cam is a disc with its centre of rotation positioned ‘off centre’. This means as the cam rotates the flat follower rises and falls at a constant rate. This type of cam is the easiest to make and yet it is one of the most useful.

The diagrams (1 to 7) seen below show the cam rotating in an anticlockwise direction. As it rotates it pushes the flat follower upwards and then allows it to drop downwards. The movement is smooth and at a constant speed.

A mechanical toy based on a series of eccentric cams is seen below. As the handle is turned the shaft and the cams fixed to it rotate. Placed above the cams are a number of segments representing a ‘snake’. As the cams rotate some of the flat followers are pushed upwards whilst others drop down. This gives the impression that the snake is moving.

The eccentric cam below pushes the 'fish' upwards and allows it to drop downwards at a uniform speed. The overall speed is controlled by the speed the handle is turned.

The T Bolt locking system shown below, is an excellent example of a eccentric cam at work. When the lever is vertical, the system is unlocked and material can be placed in position, or removed from under the drilling area. When the lever is pushed down to a horizontal position, the material is locked in place. It is now safe to drill.

This locking system works because, the circular cam rotates ‘off centre’. Moving the lever into the horizontal position secures the material, by applying pressure, clamping it to the drill table.