Cams

Imagine turning the handle of a mechanical toy. The handle rotates a cam beneath a guided rod carrying a figure. A part of the cam that extends further from the shaft pushes the rod upwards; a part closer to the shaft allows it to move downwards. Gravity or a spring keeps the follower against the cam as the profile falls away.

The cam's profile, meaning its outline, determines the sequence of movement. Designers therefore choose a cam shape to suit what the product needs to do, rather than simply to make something move.

Lift, fall, dwell and stroke

A follower's upward movement is called lift, and its downward movement is called fall. A dwell is a pause in the follower's movement even though the cam keeps rotating. This happens when the part passing beneath the follower stays the same distance from the shaft axis.

The stroke is the total distance between the follower's lowest and highest positions. Changing the cam's profile can change both the stroke and the way that movement is spread through each revolution. A product might need a smooth continuous movement, a movement followed by a pause, or a slow lift followed by sudden release.

Pear-shaped cams: movement followed by a pause

A pear-shaped cam has a rounded base and a projecting lobe. As the lobe passes beneath the follower, it causes the follower to rise and then fall. The circular base portion, centred on the shaft, provides a dwell before the movement repeats.

This makes the pear-shaped cam useful when a part must move and then remain still for part of the cycle. For example, a mechanical toy figure could rise, lower and then wait before repeating its action. The pause gives the toy a different character from one that continually bobs up and down.

Cams also control the opening and closing of engine valves. The important design connection is that the profile controls when movement happens and when the valve remains in position; different profiles can provide different timing. A pear-shaped profile demonstrates this combination of lift, fall and dwell.

Eccentric cams: continuous smooth rise and fall

An eccentric cam is circular, but its shaft is positioned off-centre. Although the outline is a circle, its edge is therefore at different distances from the shaft axis as it turns. This produces a smooth rise and fall with no dwell.

In a mechanical toy, an eccentric cam can keep a figure bobbing continuously. It suits a repeated flowing movement, but not a design that needs the figure to pause between movements.

The off-centre mounting is essential. A circular disc rotating about its exact centre would keep its edge at a constant distance from the shaft and would not produce the required rise and fall.

Drop or snail cams: gradual lift and sudden release

A drop cam, also called a snail cam, has a profile that gradually increases in distance from the shaft and then ends in a sharp step. Turning it in the intended direction lifts the follower gradually. When the follower reaches the step, it drops suddenly back to its lowest position.

This action is useful in a trip hammer: the cam gradually lifts the hammer, then releases it to deliver a blow. It can also operate an alarm-clock striker. The sudden release is the useful action, rather than an unwanted interruption to smooth movement.

Rotation direction matters. The cam must turn so that the follower travels up the gradual slope before reaching the drop.

Choosing a cam for a product

The three profiles solve different movement problems. A pear-shaped cam provides movement with a pause; an eccentric cam provides continuous smooth movement; a snail cam provides gradual lift followed by sudden release. For a toy that should bob gently without stopping, an eccentric cam is more suitable than a snail cam. For lifting and releasing a hammer, the snail cam's abrupt drop is an advantage.

Reliable operation also depends on keeping the follower in contact with the cam and reducing wear where the parts touch. Lubrication reduces friction and wear, helping the mechanism maintain its intended movement over time.