Uniform Motion and Its Problem

Uniform Motion and Its Problem

Uniform motion means the follower rises at a constant rate: equal displacement for equal rotation. On the displacement graph it is a straight sloping line, and it is the motion CAPS Grade 11 requires.

It is the simplest to construct — divide both the rotation and the total rise into the same number of equal parts and pair them.

But uniform motion has a real mechanical problem. At the start of the rise the follower goes from stationary to full speed instantly. That is an infinite acceleration, which means a theoretically infinite force. In practice the follower hammers the cam, the mechanism is noisy, and both wear rapidly.

This is why real high-speed cams use modified uniform motion, with the sharp corners of the graph rounded off, or the smoother motions studied in Grade 12: simple harmonic motion and uniform acceleration/retardation.

Worked example. A 40 mm rise over 180° in uniform motion rises 40 ÷ 6 ≈ 6.67 mm per 30° step. Every step is identical — that constancy is exactly what makes the graph straight, and exactly what makes the start and end corners abrupt.

Core checkpoint: Uniform motion gives equal steps and a straight graph. Its weakness is at the corners, where velocity changes instantaneously.