How do ratchets, clutches and intermittent mechanisms control and direct motion?
Ratchet and pawl, clutches and brakes, the Geneva mechanism and intermittent motion, and screw threads for converting rotary to linear motion.
A CCEA A-Level Technology and Design answer on advanced mechanisms - the ratchet and pawl for one-way motion, clutches and brakes for engaging and stopping drives, the Geneva mechanism for intermittent motion, and screw threads converting rotary to linear motion.
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What this dot point is asking
CCEA expects you to describe advanced mechanisms: the ratchet and pawl (one-way motion), clutches and brakes (engaging/stopping a drive), the Geneva mechanism (intermittent motion), and screw threads (rotary to linear with large force). These extend the AS mechanisms into control and motion direction.
The answer
Ratchet and pawl
Clutches and brakes
Geneva mechanism and screw threads
Worked example: choosing mechanisms for an indexing clamp
Examples in context
Example 1. Film projector / indexing table. A Geneva mechanism advances film or a tooling table one frame/step at a time, holding still in between, the classic use of intermittent motion.
Example 2. Car drivetrain. A clutch lets the driver disconnect the engine to change gear, and the brakes turn the car's kinetic energy into heat to stop it, both mechanisms controlling motion rather than changing its type.
Try this
Q1. What motion does a ratchet and pawl allow and prevent? [2 marks]
- Cue. It allows motion in one direction only and prevents (locks against) motion in the reverse direction.
Q2. What type of motion does a Geneva mechanism produce at its output? [1 mark]
- Cue. Intermittent (stepwise/indexed) rotary motion, with dwell between steps.
Q3. State the two effects of a screw thread. [2 marks]
- Cue. It converts rotary motion to linear motion (one pitch per turn) and provides a large mechanical advantage (large axial force from a small effort).
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 20206 marksDescribe how a ratchet and pawl works and give two uses. Explain what is meant by intermittent motion and name a mechanism that produces it.Show worked answer →
A ratchet and pawl consists of a toothed wheel (ratchet) and a spring-loaded lever (pawl) that engages the teeth. The shape of the teeth lets the ratchet wheel turn freely in one direction (the pawl slides over the sloped face) but locks against the steep face if it tries to turn the other way, so it allows motion in one direction only and prevents reverse motion.
Two uses: a socket wrench/ratchet spanner (lets you turn a bolt one way and re-position without removing the spanner), a winch or hoist (holds the load and stops it unwinding), or a clock/winding mechanism.
Intermittent motion is motion that stops and starts repeatedly - the output moves for part of the cycle and dwells (stays still) for the rest, rather than moving continuously. A mechanism that produces it is the Geneva mechanism (Geneva wheel), which converts continuous rotary motion of a driver into stepwise (indexed) rotation of the driven wheel.
Markers reward the toothed-wheel-and-pawl one-way action (free one way, locks the other), two valid uses, the stop-start definition of intermittent motion, and the Geneva mechanism as an example.
CCEA 20214 marksExplain the function of a clutch, and describe how a screw thread converts one type of motion into another.Show worked answer →
A clutch is a mechanism that engages and disengages the drive between a power source and the output, allowing the transmission of rotary motion to be connected or interrupted without stopping the engine/motor. When engaged, friction plates lock together and drive is transmitted; when disengaged, they separate and the output is free (for example to change gear or stop the wheels turning while the engine runs).
A screw thread converts rotary motion into linear motion: as the screw (or nut) is rotated, the helical thread advances it along its axis by one pitch per revolution, so turning produces straight-line movement. It also gives a large mechanical advantage (a small turning effort produces a large axial force), as in a vice, a G-clamp, a screw jack or a lathe leadscrew.
Markers want the clutch as a means of engaging/disengaging drive (connect or interrupt transmission), and the screw thread converting rotary to linear motion (one pitch per turn, large force), with a valid example.
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Sources & how we know this
- CCEA GCE Technology and Design specification — CCEA (2016)