A cam and follower mechanism is a machine element that converts rotary motion into a planned back and forth or up and down motion. It is used when a machine needs a part to move at the right distance, speed, and time during each rotation. Cams are important in engines, pumps, textile machines, packaging equipment, and automatic tools.
The shape of the cam is the main control feature, because it decides how the follower moves.
Understanding Engineering: Cam and Follower Mechanisms
A cam works because its edge acts like a moving track. As the shaft turns, the follower stays in contact with that track through gravity, a spring, or a positive mechanical link. A radial cam has its profile arranged around the rotating shaft.
A translating follower moves in a straight line, while an oscillating follower pivots about a fixed point. Roller followers reduce sliding friction because a small wheel rolls on the cam surface. Flat faced followers can carry heavy loads, but their contact conditions need careful design.
The required motion is planned before the cam shape is drawn. Designers split one rotation into angular intervals for rise, dwell, and return. They choose the lift, meaning the total follower travel, then decide how gently that travel begins and ends.
A simple constant velocity motion has abrupt changes at the start and finish. These changes can cause impact and vibration. Better motion laws, such as simple harmonic motion or cycloidal motion, smooth the change in speed.
The most important quantity is often jerk, which is the rate at which acceleration changes. High jerk makes a machine noisy, shakes parts loose, and increases wear at high speed.
For a given lift, less time means greater average velocity. In words, average velocity equals lift divided by rise time. Yet average velocity does not show the whole story.
A follower can have the same average velocity under two profiles while reaching very different peak speeds and accelerations. This matters in a car engine. The camshaft must open and close valves at precise times, but a valve cannot safely follow a profile that demands more acceleration than the spring and moving parts can handle.
If contact is lost, the follower can strike the cam when it returns. This is called follower jump, and it can damage the mechanism.
Geometry creates further limits. The pressure angle describes how much the cam pushes sideways on the follower rather than directly along its path. A large pressure angle produces higher side forces, friction, and guide wear.
Increasing the cam size or changing the follower offset can reduce this problem. Students should sketch a displacement diagram before attempting a cam profile. The horizontal axis represents cam rotation and the vertical axis represents follower position.
Check that the curve joins smoothly at every change of motion. Then consider velocity, acceleration, contact force, material strength, lubrication, and manufacturing accuracy. A profile that looks correct on paper may fail if its curved surface is hard to machine or if clearance between parts is too large.
Key Facts
- One full cam rotation is 360 degrees of timing.
- Follower displacement s depends on cam angle theta, so s = f(theta).
- Rise is the part of the cycle where follower displacement increases.
- Dwell is the part of the cycle where follower displacement stays constant.
- Return is the part of the cycle where the follower moves back toward its starting position.
- Average follower velocity during a rise is v_avg = h / t, where h is lift and t is rise time.
Vocabulary
- Cam
- A cam is a rotating shaped part that pushes or guides a follower to create a specific motion pattern.
- Follower
- A follower is the moving part that stays in contact with the cam and responds to the cam profile.
- Cam profile
- The cam profile is the outer shape or surface of the cam that determines the follower displacement at each angle.
- Lift
- Lift is the maximum distance the follower moves from its lowest position during a cam cycle.
- Dwell
- Dwell is an interval of cam rotation during which the follower remains at the same displacement.
Common Mistakes to Avoid
- Confusing cam rotation with follower motion, because the cam turns continuously while the follower may rise, dwell, and return in separate parts of the cycle.
- Assuming a circular cam always produces follower motion, because a centered circular cam with no offset can rotate without changing follower displacement.
- Ignoring dwell angles, because many real mechanisms need the follower to pause so actions like valve opening or part holding occur at the correct time.
- Treating lift and velocity as the same quantity, because lift is displacement while velocity depends on how quickly that displacement occurs.
Practice Questions
- 1 A cam rotates at 600 rpm. How many complete cam cycles occur each second?
- 2 A cam gives a follower a lift of 12 mm during a rise that lasts 90 degrees of cam rotation. If the cam rotates at 300 rpm, find the rise time and the average follower velocity during the rise.
- 3 An engine valve must stay fully open for part of a cycle before closing. Explain which part of the cam motion diagram represents this requirement and why the cam profile must include it.