Cams and Displacement Graphs

Cams and Displacement Graphs

A cam converts rotation into a precisely controlled linear motion. It turns; a follower rides on its edge and rises and falls according to the cam's profile. Engine valves are the familiar example.

The displacement graph comes first. It plots follower displacement (vertical) against cam rotation in degrees (horizontal), and it is the specification — the cam profile is derived from it, never the other way round.

Terms:

• Rise — the follower moves out.
• Dwell — the follower stays still while the cam continues to turn. On the graph this is a horizontal line.
• Fall (return) — the follower moves back.
• Base circle — the smallest radius of the cam, from which all displacements are measured.

Constructing the profile. Divide the rotation into equal angular steps, usually 30°. For each step, read the displacement off the graph, and set that distance outward from the base circle along that radial. Join the points smoothly.

Worked example. A cam specified as "rise 20 mm over 0–120°, dwell 120–180°, fall over 180–300°, dwell 300–360°" is fully described by its graph. Two draughtspeople working from that graph produce identical cams; working from a sketched profile they would not.

Core checkpoint: Displacements are measured from the base circle, radially outward, not from the cam's centre.