The big idea: A linkage is a set of rigid bars joined by pivots. It changes the direction of a movement, alters the force, or makes a part follow a particular path.
Read one by finding the fixed pivots first — the points attached to the frame, which cannot move. Everything else follows from where they are.
Fixed pivots drawn filled, moving pivots hollow — then the three linkages the guide names.
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| Linkage | What it does | Where you see it |
|---|---|---|
| Reverse motion | One bar on a single fixed pivot: push one end down and the other goes up. Moving the pivot off centre changes the force as well | A bicycle brake caliper, a pop-up card, a toy seesaw |
| Parallel motion | Two equal bars on two fixed pivots joined by a third: the output stays parallel to the input and moves the same way | A toolbox cantilever tray, a desk lamp arm, a tractor loader, a folding step |
| Bell crank | A bar bent through a right angle at a fixed pivot, turning a movement through 90°. Unequal arms also change the force | A bicycle brake rod, a door latch, aircraft controls, a bell pull |
Why a parallel linkage is worth its extra bar: It keeps a platform level through its whole travel. A toolbox tray lifts up and out while staying horizontal, so nothing tips off it.
No single lever can do that: a lever swings its far end through an arc, tilting whatever sits on it.
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| What is needed | How the linkage gives it |
|---|---|
| A movement somewhere awkward | Bars carry a hand movement from a convenient control to a place a hand cannot reach — a brake lever to a wheel rim |
| A change of direction | One fixed pivot reverses the movement; a right-angled bar turns it through 90° without gears or cables |
| More force at the output | Unequal arms about the fixed pivot multiply force exactly as a lever does, because each bar IS a lever |
| A particular path | Bar lengths and pivot positions are chosen so a point traces the path required — level, curved, or straight through a slot |
Every pivot is a cost: Pivots wear, develop free play and add friction, and a linkage with slack in it feels cheap in the hand — which is exactly how a user judges a brake or a latch.
So the design question is always the fewest bars that do the job, with pivots tight enough to stay tight.
How this is tested — identifying parallel, reverse and bell crank linkages and outlining their uses. It comes up two ways:
Paper 1 — multiple choice
- Name the linkage shown in a drawing.
- Identify which pivots in a linkage are fixed.
Paper 2 — analysing a product
- Identify the linkage in a named product and explain its purpose.
- Explain why a parallel linkage was used instead of a single lever.
The trap: Not distinguishing fixed from moving pivots. A linkage drawn with the wrong pivot fixed does something completely different, and the explanation collapses.
A cantilever toolbox has trays that swing up and out when the lid is opened, staying level throughout. Explain the linkage that does this and why a single lever would not.
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