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How do gears work?

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Quick answer

Gears are toothed wheels that interlock so one turns the next. The ratio of their tooth counts decides the result: a small gear driving a big one turns it slower but with more force, while a big gear driving a small one gives more speed and less force. Each meshing pair also reverses rotation direction.

Gears work by locking toothed wheels together so that turning one forces the other to turn, passing motion and force from shaft to shaft. Changing the number of teeth on each wheel trades speed for turning force, and meshing two gears reverses the direction of rotation.

Step by step

  1. A motor, pedal or hand turns the first gear, called the driver, on its shaft.
  2. Teeth on the driver press against the teeth of a second meshed gear, pushing it around in the opposite direction.
  3. Because each tooth passes one tooth, the driven gear turns fewer times per minute if it has more teeth.
  4. That slower rotation carries greater torque, so the output shaft can move a heavier load than the input could alone.
  5. Further gears added in a train multiply the ratio, change direction again, or shift the motion to a new angle.

Key things to understand

  1. A gear is a wheel with evenly spaced teeth cut around its rim or along a straight bar.
  2. When two gears mesh, each tooth of the driving gear pushes a tooth of the driven gear forward.
  3. The gear ratio equals the driven gear's tooth count divided by the driving gear's tooth count.
  4. A small gear turning a larger one makes the output slower but increases its turning force, or torque.
  5. Different tooth shapes such as bevel, worm and rack-and-pinion gears let motion change angle or become straight-line movement.

Labelled diagram

  1. Driver gear

    The small input gear turned by a motor, pedal or hand crank.

  2. Meshing teeth

    Each driver tooth pushes one tooth of the next gear at the contact point.

  3. Driven gear

    The larger gear turns slower, in the opposite direction, with more torque.

  4. Output shaft

    Carries the slower, stronger rotation to wheels, hands or blades.

Two meshed gears trading speed for torque

Real-world examples

  • A bicycle rider shifts to a larger rear sprocket so each pedal turn delivers more torque for climbing a hill.
  • A mechanical wristwatch uses a train of tiny gears to make the hour hand turn twelve times slower than the minute hand.
  • A car's steering box uses a rack-and-pinion gear to turn the steering wheel's rotation into sideways wheel movement.
  • A ceiling fan or kitchen mixer uses gears to reduce a fast motor's speed to a useful, stronger turning motion.

Common mistake

✗ Myth: A bigger gear always makes a machine faster.

✓ Fact: It depends on which gear drives which. A large gear driving a small one increases speed, but a small gear driving a large one slows the output and boosts torque instead.

People also ask

How do gears increase torque?
When a small gear drives a larger one, the larger gear turns fewer times but each tooth on it is pushed farther from its centre. That longer lever arm multiplies the turning force. Energy is conserved, so the torque gain is paid for with a matching loss of rotational speed, minus small friction losses.
What are the different types of gears?
Spur gears have straight teeth on parallel shafts, while helical gears have angled teeth that run quieter. Bevel gears are cone-shaped to turn motion through a corner, worm gears give very large speed reductions, and a rack and pinion converts rotation into straight-line motion, as in car steering.
Why do bikes have gears?
Bike gears let a rider keep pedalling at a comfortable rate whatever the road. A small front chainring with a large rear sprocket gives low gear, trading speed for torque on climbs. A large chainring with a small sprocket gives high gear, so each pedal turn moves the bike farther on flat roads.
Why do meshing gears turn in opposite directions?
The teeth touch at a single point between the two wheels, so a tooth moving down on one gear pushes the neighbouring tooth up on the other. Adding a third idler gear between them restores the original direction.
Do gears create extra energy?
No. Gears only trade speed against torque; the power coming out is always slightly less than the power going in because friction between teeth turns some energy into heat and noise.
What is a gear ratio of 3:1?
It means the driving gear turns three times for every single turn of the driven gear. For example, a 12-tooth gear driving a 36-tooth gear gives 3:1, cutting output speed to a third and roughly tripling torque.
Why are gear teeth curved instead of square?
Most gear teeth follow a curve called an involute, which keeps the push between teeth pointing in a constant direction as they roll together. That gives smooth, steady motion with less vibration and wear than straight-sided teeth.

Part of the guide: Everyday Machines →