Meshing teeth transmit motion.

A driving gear pushes on the teeth of a driven gear. For two simple external gears on parallel shafts, they rotate in opposite directions.

The tooth counts set their speed ratio. More teeth on the driven gear mean fewer driven revolutions for each revolution of the driver.

Follow an ideal reduction.

A 3:1 reduction, drawn in proportion.

A 20-tooth driver meshes with a 60-tooth driven gearThe small driver turns clockwise. The larger driven gear turns anticlockwise at one third of its speed. Dashed circles are pitch circles, with diameters in a one-to-three ratio.12
  1. Driver: 20 teeth. 600 revolutions per minute (rpm), with an input torque of 10 N·m.
  2. Driven gear: 60 teeth. 200 rpm in the opposite direction. Ideal output torque is 30 N·m; three driver turns produce one driven turn.
Twenty and sixty teeth are shown. Dashed circles mark the pitch diameters; the tooth profiles are simplified and are not manufacturing geometry. Lower speed and higher torque balance: ideal power is unchanged.
Trading speed for torque

If input torque is 10 N·m, ideal output torque is 30 N·m. Real output torque is lower at that speed because the gear pair has losses.

A gearbox does not multiply power.

Reducing speed can increase torque, but ideal input and output powers remain equal. Real gearboxes lose some power through friction and other effects.

A larger output torque does not mean the gearbox has created energy. The output turns through a smaller angle in the same time.

Different gears suit different layouts.

Spur and helical gears commonly connect parallel shafts. Bevel gears can change the shaft direction. Worm arrangements use a screw-like worm driving a wheel and can provide substantial reduction.

Some lift machines have a gearbox between motor and drive sheave. Gearless machines connect the motor and sheave without that reduction stage.

Teeth need controlled geometry and clearance.

Backlash is clearance between mating teeth. Some clearance allows the design to operate, but excessive wear can affect movement and noise.

Alignment, bearings, lubrication and tooth condition work together. A gearbox should not be assumed to hold a lift merely because its reduction ratio is high.

A closer look

A worm drive may resist being driven backwards under some conditions, but self-locking depends on geometry, friction and operating conditions. It is not a general substitute for a brake.

Check your understanding

An ideal gearbox reduces speed to one third. What happens to torque?

Reveal the explanation

Torque increases to three times the input value. Power remains the same in the ideal model; real losses reduce the output.

Further reading.

Optional reading to explore this lesson in more detail.

  • Open University: gears and gearing

    Section 1.13 uses wheel and gear diagrams to explain speed ratios, tooth counts and direction. Follow Figure 8 before moving to the more complex gear train.