Oscillatory motion
A motor does exactly one thing: it goes round. Everything that wipes, saws, pumps, nods or waves is a mechanism converting that rotation into oscillation — a movement that goes one way, stops, and comes back.
Crank and slider
Put a pin near the edge of a turning wheel and connect a rod from the pin to something that can only slide in a straight line. The wheel keeps turning one way; the slider goes out, stops, comes back and stops, once per revolution.
- How far it travels is twice the crank radius — how far the pin is from the centre. Nothing else changes it.
- How often is one there-and-back per turn of the wheel, so the motor speed sets the rhythm directly.
- How it moves is the interesting part: fastest in the middle, motionless at each end. That is what stops a crank mechanism from jolting when it turns around, and it happens without anything slowing the motor down.
A crank with the rod attached to a pivoting arm rather than a slider gives a rocking motion instead of a sliding one. Same mechanism, and it is how a windscreen wiper works.
The wheel turns at a steady rate and the slider does not. Watch the two ends: it stops dead at both of them without the motor ever stopping.
Pendulums — when the mechanism sets the rhythm
A pendulum swings at a rhythm of its own, and that rhythm is set by exactly one thing you can control: its length. Longer means slower. Specifically the period goes as the square root of the length, so making it four times as long makes each swing twice as slow.
Two things students expect to matter and which do not:
- The weight on the end. A heavy bob and a light one on the same string swing at the same rate.
- How far you pull it back. Within reason, a wide swing takes the same time as a narrow one — it just travels further, faster.
This is worth meeting because it is the first mechanism in the course whose timing is not yours to program. If a nodding or swinging model has to keep a particular beat, you change its geometry, not its code.
Driven, or free
| Rhythm set by | To change it | |
|---|---|---|
| Crank mechanism | the motor | change the motor speed — see Motor speed |
| Pendulum | the mechanism | change its length |
Push a free-swinging thing at its own rhythm and the swings grow — that is resonance, and it is how a child on a swing goes higher without anybody pushing harder. Push at the wrong rhythm and you fight it. A model that has to swing widely is far easier to build by matching the push to the mechanism than by pushing harder.
Why it matters
Engines run this backwards: the up-and-down of a piston becomes the round-and-round of a crankshaft, using exactly the linkage above. Sewing machines, pumps, saws and metronomes are all in the same family, and a clock is a pendulum used as a ruler for time.
More mechanics tutorials
- Mechanisms without motors — How to investigate a build with no electronics in it, and where each idea lives.
- Gearing up and gearing down — Trade turns for force, or force for turns — you cannot have both.
- Changing the direction of a turn — Reverse a turn, restore it with an idler, or send it round a 90° corner.
- Pulley systems — Fixed, movable and combined — how rope pulled trades against force needed.
- Levers — effort, load and pivot — Where the pivot sits decides the force you need and the distance you get.
- Elastic and stored energy — Stretch a band, store energy, let it go — and find where more stops helping.
- Scissor mechanisms — Crossed links that extend and retract, and why the last bit is the hardest.
- Centre of gravity — Why a robot tips, and how to build one that does not.
- Biomimetic mechanisms — Linkages that copy how animals move — turning a rotation into a step.