LEGO Gear Ratios: Work Out Speed and Direction Before You Build
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If a LEGO mechanism turns too quickly, adding more gears won't necessarily slow it down. The tooth counts and the way the gears share axles determine the result. Start by finding the gear you turn, then follow the motion to the part you want to move.
The gear you turn by hand or motor is the driver. The gear it turns is the driven gear. Naming those two parts first prevents a common mix-up: the same pair of gears gives the opposite speed change when you drive it from the other end.
Count teeth, then calculate turns
For a simple pair, divide the driver's tooth count by the driven gear's tooth count to find how many output turns you get for one input turn.
| Driver | Driven gear | Output turns per input turn | Effect |
|---|---|---|---|
| 8 teeth | 24 teeth | 8 ÷ 24 = ⅓ | Slower output |
| 24 teeth | 8 teeth | 24 ÷ 8 = 3 | Faster output |
| 24 teeth | 24 teeth | 24 ÷ 24 = 1 | Same speed |
LEGO Education writes the gear ratio using driven teeth divided by driver teeth. Under that convention, the first example is a 3:1 reduction. Stating both the ratio and the resulting turns makes your meaning clear when another builder uses a different ratio convention.
Slower motion can provide more turning force
A smaller driver turning a larger driven gear reduces speed and increases the available turning force. Reverse that pair and you gain speed while giving up turning force. You can't get both increases from the same gear change.
For a hand-cranked display, decide whether the moving detail needs speed or a controlled, slower movement. A fast-spinning decoration and a slowly opening mechanism have different needs. Choose the gear relationship for that motion, rather than assuming the largest available gear always belongs on the input.
An idler changes direction without adding a reduction
Two meshing spur gears on parallel axles turn in opposite directions when viewed from the same side. Put an idler gear between them and the final gear turns in the same direction as the driver.
In a simple chain, the idler's tooth count doesn't change the speed ratio between the first and last gears. An 8-tooth driver, an intermediate gear and a 24-tooth output still give one output turn for three driver turns. The middle gear helps carry the movement across the space and changes its direction at each mesh.
A second gear on an axle changes the calculation
A compound train is different. Suppose an 8-tooth gear drives a 24-tooth gear. On the 24-tooth gear's axle, a second 8-tooth gear drives another 24-tooth gear. Each stage reduces speed by three, so the combined reduction is 3 × 3 = 9. Nine input turns produce one output turn.
The gears fixed to the shared axle rotate together. That is why the smaller gear on that axle starts a new reduction stage, while a single idler between two gears does not.
Before enclosing the mechanism, mark the input and output positions with a visible pointer or contrasting piece. Turn the input slowly through one full revolution and count what the output does. For a large reduction, count several input turns instead. This proposed check lets you compare the mechanism with your calculation while every connection is still easy to see. LEGO Education's gear lesson provides illustrated examples of simple, idler and compound arrangements.