Changing the direction of a turn
Gears do two jobs and they are worth keeping apart. One is the trade between turns and force — that is Gearing up and gearing down. The other is geometry: which way the far end turns, and about which axis.
Meshed gears turn opposite ways
This follows from what a tooth actually does. Where two gears touch, one tooth is pushing the other sideways — so they cannot possibly be going the same way round. Every mesh in a train flips the direction again.
- Two gears — one mesh — the output turns backwards.
- Three gears — two meshes — the output turns the same way as the input.
- An odd number of gears comes out the same way; an even number comes out reversed.
The idler gear
A gear dropped between two others changes the direction and nothing else. Its own size cancels out — it takes exactly as much as it gives — so the ratio is still first gear ÷ last gear however big or small the middle one is.
That makes an idler the normal, sensible way to fix a direction. It costs one gear and one hole in a beam, and it is a far better answer than rewiring a motor or writing counterclockwise in half your program and forgetting it in the other half.
Watch the red marks, not the gears. Two gears meshed together can never turn the same way — everything else follows from that one fact.
Turning the corner: 90° drives
Ordinary spur gears keep the two axles parallel. Some jobs need the output at right angles to the input — a motor lying flat in the chassis driving an upright turntable, or a horizontal axle driving a vertical drill.
- Crown gear (the one with its teeth standing up on the rim) meshed with an ordinary spur gear. Simple, common in the EV3 set, and fine at modest loads.
- Bevel gears, cut at an angle and used in pairs. Stronger and quieter than a crown pair, and the right choice if the corner is carrying real force.
- Worm gear driving a spur gear. Also turns the corner, gears down enormously in one step, and cannot be back-driven — the output cannot turn the input. That last property makes it the standard choice for a lifting arm that must hold its position when the motor stops.
The teeth still set the ratio in all of these. Changing the axis and changing the speed are separate decisions that happen to be made by the same pair of parts.
Direct connection — no gears at all
Two parts on the same axle turn together, at the same speed, in the same direction, with no losses. It is the simplest answer and worth reaching for first: if the motor can already do the job at its own speed and force, gears add friction, backlash and parts for nothing.
Gears earn their place when you need a different speed, a different force, a different direction, or the drive to come out somewhere the motor cannot reach.
Why it matters
A car’s differential is a set of bevel gears turning the drive shaft’s rotation through 90° to the wheels. A hand drill turns your horizontal cranking into vertical drilling with the same trick. Once you can see the corner, you start seeing it everywhere.
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.
- 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.
- Oscillatory motion — Turning a rotation into a back-and-forth, and what sets the rhythm.
- 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.