Wedges, wheels and axles, gears and cams — every complex machine is really just a combination of simple mechanisms. Learn how they multiply force, change speed, or change direction of movement.
A well-designed machine gives us mechanical advantage (MA) — it lets a smaller effort force move or overcome a larger load. A machine doesn't create energy from nothing; it simply changes the size or direction of a force, usually by trading force for distance (or the other way around). Every complex machine — a car engine, a robot arm, a bicycle — is built up from combinations of just a handful of simple mechanisms.
A wedge is really two inclined planes (ramps) joined back to back, tapering to a thin edge. Pushing a wedge a long distance sideways forces objects apart with a much larger force over a short distance — that's mechanical advantage at work.
A wheel and axle is a mechanism where a large wheel is fixed to a smaller axle (or shaft) so they rotate together. Turning the wheel with a small effort at its rim produces a much larger turning force at the axle — the bigger the wheel is compared to the axle, the greater the mechanical advantage.
A gear is simply a wheel with teeth cut into its rim — and each tooth is a small wedge. When two spur gears (gears with straight teeth cut parallel to the axle) mesh together, their teeth push against each other.
Sometimes we want the driven gear to turn in the same direction as the driver. To do this, we place a third gear — an idler gear — between the driver and the driven gear. The idler doesn't change the speed relationship between the driver and driven gear; it only reverses the direction a second time, so the driven gear ends up turning the same way as the driver (synchronised rotation).
When two gears of different sizes mesh, the size difference changes both the velocity ratio (how fast one turns compared to the other) and the force ratio (how much the turning force is multiplied) — and these two ratios always change in opposite directions.
| Driver → Driven | Effect on speed | Effect on force |
|---|---|---|
| Small gear drives a large gear | Driven gear turns slower | Turning force increases (MA > 1) |
| Large gear drives a small gear | Driven gear turns faster | Turning force decreases (MA < 1) |
| Equal-sized gears | Same speed | Same force (MA = 1) |
Some mechanisms are used specifically to convert rotary motion (continuous circular movement, like a turning shaft) into reciprocating motion (repeated back-and-forth movement, like a sewing machine needle).
The cam: a cam is a specially shaped rotating part that pushes a follower up and down (or side to side) as it turns.
| Cam type | Shape | Motion produced |
|---|---|---|
| Eccentric wheel | A circular disc mounted off-centre on its shaft | Smooth, gradual rise and fall of the follower |
| Snail cam | A spiral, snail-shell shaped profile | Slow gradual rise, then a sudden drop back to the start |
The crank: a crank is an adaptation of a second-class lever, bent into a right-angled handle attached to a rotating shaft. Just like a cam, a crank converts rotary motion into reciprocating motion (or vice versa) — this is exactly how the up-and-down motion of a piston in a car engine is converted into the rotary motion that turns the wheels.
Choose the size of the driver and driven gears, and toggle the idler gear on or off. Watch the direction of rotation and read the live velocity ratio.