A cam and follower is a mechanical system that converts the rotation of a shaped cam into a controlled back and forth or up and down motion of a follower. In robotics and automation, this is useful when a machine must repeat a precise motion at a precise time. The cam profile acts like a physical program that tells the follower when to rise, pause, return, or dwell.
This makes cams important in grippers, feeders, valves, packaging machines, and timing mechanisms.
As the cam rotates, different radii of the cam surface push the follower to different heights. A displacement graph shows follower position versus cam angle, so the graph directly represents the motion created by the cam shape. Steep changes in the graph mean rapid motion, while flat sections mean dwell where the follower stays still.
Engineers choose the cam profile, rotation speed, and follower type to control timing, force, smoothness, and wear.
Understanding Robotics: Cam and Follower
The important part of a cam mechanism is the contact between the two pieces. The follower must remain pressed against the cam surface throughout the cycle. A spring often supplies this contact force.
In heavier machines, gravity, a groove, or a second shaped surface can keep the follower in place. If contact is lost at high speed, the follower can jump away from the cam.
It may strike the surface when it returns, causing noise, inaccurate motion, and damage. Designers therefore consider the moving mass of the follower, the spring strength, and the speed of rotation together.
The shape of the profile controls more than position. It controls velocity and acceleration during each part of the turn. A follower that starts moving suddenly must accelerate suddenly.
This creates a large force because force equals mass times acceleration. It can shake the machine even when the follower is light. Sudden changes in acceleration are called jerk.
High jerk makes motion feel sharp or violent. Smooth profiles spread acceleration over more of the cycle.
Common choices include simple harmonic motion and cycloidal motion. These profiles are useful where a part must move quickly but settle gently, such as a pick and place arm placing a delicate item.
Geometry affects whether the mechanism works safely. The pressure angle is the angle between the direction the follower should move and the force pushing it from the cam. A large pressure angle creates a strong sideways push.
That sideways force increases friction in guides and can make the follower stick. A larger cam or a different profile can reduce this problem. Roller followers reduce sliding friction because a small wheel rolls on the cam surface.
Flat faced followers can handle some profiles that rollers cannot. Every choice has a tradeoff involving size, cost, wear, and the force the mechanism must carry. Lubrication matters because repeated surface contact can remove material over time.
Students can read a displacement graph as a set of instructions for drawing a profile. First divide one turn into angular sections for the required motion stages. Then mark the follower height at each section.
A rise that takes a small angle must happen in less time when the cam rotates at a steady speed. Its graph will be steeper, so its velocity and acceleration are usually greater. Flat graph sections are useful when another machine part needs time to act while the follower is held still.
When studying a design, pay attention to the maximum lift, the angles assigned to each stage, the smoothness near transitions, and the likely contact forces. These details explain why two cams with the same overall lift can behave very differently.
Key Facts
- Follower displacement is the distance the follower moves from its lowest position.
- One full cam rotation is 360 degrees, so cam timing is often described by angle.
- Angular speed is omega = 2 pi f, where f is rotations per second.
- For constant cam speed, time for a motion segment is t = theta / omega, with theta in radians.
- Follower velocity can be found from v = ds/dt, where s is displacement.
- Dwell occurs when the cam radius is constant over a range of angles, so follower displacement stays constant.
Vocabulary
- Cam
- A rotating shaped part that drives another part by its changing surface radius.
- Follower
- The moving part that stays in contact with the cam and follows its profile.
- Displacement
- The change in position of the follower measured from a chosen reference point.
- Dwell
- A part of the cam rotation during which the follower remains at the same position.
- Cam profile
- The outline or shape of the cam that determines the follower motion.
Common Mistakes to Avoid
- Confusing cam angle with time. Cam angle only becomes time when the rotation speed is known.
- Assuming a bigger cam always creates more lift. Lift depends on the change in cam radius, not the overall size alone.
- Ignoring dwell sections on the displacement graph. A flat graph segment means the follower is not moving even though the cam is still rotating.
- Using degrees directly in formulas that require radians. Convert with theta radians = theta degrees times pi / 180 before using t = theta / omega.
Practice Questions
- 1 A cam rotates at 120 rpm. How long does one full rotation take, and how long does a 90 degree rise segment take?
- 2 A follower rises 18 mm during a 60 degree cam rotation. If the cam speed is 2 revolutions per second and the rise is approximately uniform, what is the follower's average upward speed?
- 3 A displacement graph has a flat section from 150 degrees to 230 degrees, followed by a steep downward slope. Explain what the follower is doing during these two parts and why this might be useful in a robot mechanism.