The big idea: Every mechanism in this course belongs to one of five families: gear-driven, belt-driven, cam, lever and linkage.
Identify the family first, because each one exists to change a different thing.
The five families drawn as they appear inside a product, each with the thing it is there to change.
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| Family | What you can see | What it is there to change |
|---|---|---|
| Gear-driven | Toothed wheels meshing directly with each other | Speed and force, with no slip and exact timing between the shafts |
| Belt-driven | Two grooved wheels with a belt running between them | Speed, across a gap, quietly — and it slips rather than breaking |
| Cam | A shaped disc on a shaft with a follower resting on its edge | Rotary motion into a timed pattern of rise and fall |
| Lever | A rigid beam turning about a single pivot | Force, using the distance from the pivot |
| Linkage | Two or more rigid bars joined by pivots | The direction of a movement, or the path a part follows |
The question that identifies the family: Ask what has to change.
Force only — a lever. Speed with no slip — gears. Speed across a gap, quietly — a belt. A timed rise and fall — a cam. A direction or a path — a linkage.
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Slower is stronger: For gears, ratio = teeth on the driven gear ÷ teeth on the driver. For belts it is the diameters instead, and the rule is the same.
Small driving large means slower and stronger. Large driving small means faster and weaker. You never get both, because power in equals power out less friction.
| Product | The change it needs | The mechanism chosen |
|---|---|---|
| Electric screwdriver | A fast, weak motor made slow and strong | A compound gear train — a large reduction in a small housing |
| Washing machine | Motor speed reduced, across a gap, quietly | A belt from a small motor pulley to a large drum pulley |
| Car steering | Rotary at the wheel turned into linear at the track rods | A rack and pinion |
| Bicycle brake | A hand movement taken round a corner to the rim | A cable, then a bell-crank linkage at the caliper |
How this is tested — identifying gear-driven, belt-driven, cam, lever and linkage systems. It comes up two ways:
Paper 1 — multiple choice
- Name the mechanism family shown in a drawing.
- Choose the mechanism that suits a stated change of motion.
Paper 2 — analysing a product
- Identify the mechanisms in a named product and say what each changes.
- Justify the choice of one family over another for a given product.
The trap: Naming the family without saying what it CHANGES. "It has gears" scores nothing; "gears reduce the motor speed and raise the torque" is the answer.
A washing machine uses a belt between the motor and the drum, but the same manufacturer's food mixer uses gears. Justify each choice.
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