How do cams convert rotary motion into a controlled reciprocating or oscillating output?
Cams and followers, cam profiles (pear, circular/eccentric, heart-shaped/snail), dwell, rise and fall, and crank-and-slider mechanisms.
A CCEA A-Level Technology and Design answer on cams and followers, common cam profiles (pear, circular/eccentric, heart-shaped), the meaning of dwell, rise and fall, follower types, and the crank-and-slider mechanism for rotary-to-reciprocating conversion.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
Jump to a section
What this dot point is asking
CCEA expects you to explain how a cam and follower convert rotary motion into reciprocating or oscillating motion, to recognise common cam profiles (pear, circular/eccentric, heart-shaped/snail) and follower types, to define rise, fall and dwell, and to describe the crank-and-slider mechanism. Reading a cam's output from its profile is a common task.
The answer
Cam and follower
Cam profiles
Rise, fall and dwell
Worked example: reading a cam profile
Examples in context
Example 1. Engine camshaft. Pear-shaped cams open the valves at set points and hold them shut (dwell) for the rest of the cycle, the textbook use of cam dwell to time an action.
Example 2. Sewing machine. A crank and slider drives the needle up and down (rotary to reciprocating) while cams time other actions, showing both mechanisms working together in one product.
Try this
Q1. What output motion does a rotating cam usually give its follower? [1 mark]
- Cue. Reciprocating (up and down) or oscillating motion.
Q2. Define dwell in the motion of a cam follower. [1 mark]
- Cue. The part of the rotation where the follower stays still because the cam radius is constant.
Q3. State the input and output motions of a crank-and-slider driven by a rotating crank. [2 marks]
- Cue. Rotary input (the crank), reciprocating output (the slider).
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA 20196 marksDescribe how a cam and follower work, and explain the terms rise, fall and dwell. State the output produced by a pear-shaped cam.Show worked answer →
A cam is a specially shaped piece (usually rotating) whose profile pushes a follower that rests against it. As the cam turns, the changing radius of its profile moves the follower up and down. The cam usually has rotary input and the follower has a reciprocating (up and down) or oscillating output, so the mechanism converts rotary motion into reciprocating/oscillating motion.
The three terms describe the follower's movement during one rotation:
- Rise: the follower is lifted (moving up) as the cam radius increases.
- Fall: the follower drops (moving down) as the cam radius decreases.
- Dwell: the follower stays still while the cam radius is constant (a flat or circular-arc section of the profile).
A pear-shaped cam gives roughly half a turn of dwell (the follower stays still for about half the rotation, on the circular part) and a smooth rise and fall for the other half. It is used where the follower must pause for part of the cycle (for example operating valves or contacts at set points).
Markers reward the cam-pushes-follower description, rotary-to-reciprocating conversion, correct definitions of rise, fall and dwell, and the pear cam's long dwell.
CCEA 20214 marksA crank and slider is driven by a rotating crank. State the input and output motions, and give one product that uses this mechanism.Show worked answer →
A crank and slider converts between rotary and reciprocating motion. Driven by a rotating crank, the input is rotary motion and the output is reciprocating motion (the slider moves back and forth in a straight line as the crank turns).
(It also works in reverse: a reciprocating input at the slider produces rotary motion at the crank, as in a car engine where the pistons drive the crankshaft.)
A product using it: a piston engine/pump, a sewing machine (needle drive), or a reciprocating compressor.
Markers want rotary input, reciprocating output (for a crank-driven slider), and a valid example.
Related dot points
- The four types of motion (linear, rotary, reciprocating, oscillating) and the role of mechanisms in changing the type, direction or magnitude of motion and force.
A CCEA A-Level Technology and Design answer on the four types of motion - linear, rotary, reciprocating and oscillating - and how mechanisms change the type, direction or magnitude of motion and force.
- The three classes of lever, the principle of moments, mechanical advantage, and linkages (reverse-motion, bell-crank, parallel).
A CCEA A-Level Technology and Design answer on the three classes of lever, the principle of moments, calculating mechanical advantage and velocity ratio, and common linkages such as reverse-motion, bell-crank and parallel linkages.
- Spur, idler, compound, bevel and worm gears, the gear ratio, and the trade-off between speed and torque.
A CCEA A-Level Technology and Design answer on gear types - spur, idler, compound, bevel and worm - calculating the gear ratio from tooth numbers, and the trade-off between output speed and torque.
- Belt and pulley drives, the velocity ratio from pulley diameters, chain and sprocket drives, and pulley systems for lifting.
A CCEA A-Level Technology and Design answer on belt and pulley drives, calculating the velocity ratio from pulley diameters, chain and sprocket drives, and the mechanical advantage of pulley lifting systems.
- Pneumatic components: single- and double-acting cylinders, control valves (3/2 and 5/2), and calculating the thrust of a cylinder.
A CCEA A-Level Technology and Design answer on pneumatic systems, single- and double-acting cylinders, 3/2 and 5/2 control valves, and calculating the output force (thrust) of a cylinder from air pressure and piston area.
Sources & how we know this
- CCEA GCE Technology and Design specification — CCEA (2016)