Key Idea: Four motions: linear (straight, one way), rotary (a full circle), oscillating (an arc that reverses), reciprocating (a straight line that reverses). A mechanical system is input → process → output, where the input and output are motions and the process is the mechanism. A process changes one of four things: the type of motion, its speed, its direction, or the force. MA = load ÷ effort, and force is always bought with distance. Five families: gears (speed and force, no slip), belts (speed across a gap, quietly, with slip), cams (a timed rise and fall), levers (force), linkages (direction and path). Real products chain mechanisms, each changing one thing — and the efficiencies multiply.
Paper 1 — multiple choice
- Name the motion a labelled part undergoes
- Identify a gear, cam, lever or linkage from a drawing
- Work out a gear or pulley ratio
- Classify a lever by what is in the middle
Paper 2 — analysing a product
- Follow the mechanism chain through a named product
- Explain what each mechanism changes, and why
- Justify a belt against gears, or one cam against another
- Recommend a change and name the arm or ratio it alters
Carried into the design project
- Criterion D — a mechanism claim must be modelled and tested
- Prototype free play and friction are evidence: record them
- Fewest pivots that do the job is a real design target
Every mechanisms question opens the same way. Write the motion going in and the motion coming out, and the mechanism between them is usually the only one that could have done it.
| Motion in → out | The mechanism | A product |
|---|---|---|
| Rotary → rotary, slower | Gears, or a belt and pulleys | A drill, a washing machine, a food mixer |
| Rotary → rotary, round a corner | Bevel gears, or a worm and wheel | A hand drill head, a wiper motor, a hoist |
| Rotary → linear | A rack and pinion, or a screw thread | Car steering, a pillar drill feed, a garage door |
| Rotary → reciprocating | A crank and connecting rod | An engine piston, a sewing-machine needle |
| Rotary → a timed rise and fall | A cam and follower | An engine valve, an automaton, a mechanical timer |
| Hand movement → force elsewhere | A lever, or a linkage | A nutcracker, a bicycle brake, a toolbox tray |
Name the motion by the path it traces, not by the example you remember.
🔒 Interactive diagram
Explore the labelled diagram, charts and maps for this topic in study mode.
Important: Oscillating vs reciprocating — both reverse. One swings on an ARC about a pivot, the other travels along a STRAIGHT line. Ratio the wrong way up — it is always driven ÷ driver. Small driving large is slower and stronger. An idler changing the ratio — it does not. Only the first and last gears set it; an idler restores the direction.
| Family | What it changes | Chosen when |
|---|---|---|
| Gears | Speed and force, with no slip | Shafts must stay exactly in step, the space is small, or the torque is high |
| Belts and pulleys | Speed, across a gap | The shafts are far apart or not aligned, noise or cost matter, or slip is wanted as overload protection |
| Cams | Rotary into a timed rise and fall | Something must move at a chosen point in the cycle — and a dwell is needed |
| Levers | Force, using distance from a fulcrum | A hand cannot produce enough force, or needs more speed and reach than it has |
| Linkages | Direction, force and path | A movement must reach somewhere awkward, reverse, turn a corner, or stay level |
Identify the family from the drawing, then say what it is there to change.
🔒 Interactive diagram
Explore the labelled diagram, charts and maps for this topic in study mode.
Ratio = driven ÷ driver — teeth for gears, diameters for belts. MA = effort arm ÷ load arm, both measured from the fulcrum. F, L, E in class order: fulcrum in the middle is a first-class lever, load in the middle second, effort in the middle third.
A hand-operated garlic press is being replaced by a battery-powered one. Analyse the mechanisms in each and what the change costs the user.
🔒 Model answer plan
See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.
A design student specifies a plain V-belt to drive the printhead of a small 3D printer. Explain why this is the wrong choice and what should be used instead.
🔒 Model answer plan
See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.
A stapler is pressed down to drive a staple through paper. Identify the lever class and the consequences for the user.
🔒 Model answer plan
See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.
How do you tell oscillating from reciprocating motion?
What is paid for a mechanical advantage of 10?
What does an idler gear actually do?
What is a dwell, and which cam has one?
Why does a toolbox tray use a parallel linkage rather than a lever?
Why does a chain of mechanisms lose so much?
Exam tips
- Open every mechanisms answer by naming the motion in and the motion out. The mechanism then names itself.
- Say what each mechanism changes — type, speed, direction or force. Naming the family alone scores nothing.
- Quote ratio = driven ÷ driver and MA = effort arm ÷ load arm, and say which way the speed or force went.
- Classify a lever by what is in the middle, never by the picture you remember.
- Treat slip as a benefit where overload protection matters, and as a failure where timing matters.
- For justify, name what the rejected mechanism would have cost.