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How does a gearbox work?

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Quick answer

A gearbox is a set of gears that changes the speed and torque from a power source. By meshing gears of different sizes, it can trade speed for turning force or vice versa. This lets an engine run efficiently while the wheels turn at the right speed for the road.

A gearbox uses different-sized toothed wheels to change the speed and turning force (torque) coming from a power source. By engaging different pairs of gears, it can make an output turn faster with less force or slower with more force, while keeping the same power.

Step by step

  1. The power source, such as an engine or motor, turns an input shaft connected to a gear.
  2. That gear meshes with another gear on a parallel shaft, transferring motion and force.
  3. The number of teeth on each gear sets the ratio: a small gear driving a large one slows the output but increases torque.
  4. A selector mechanism moves gears or locking collars to engage different pairs, changing the ratio.
  5. The output shaft turns at the selected speed and torque, driving the wheels or other machinery.

Key things to understand

  1. Gears are toothed wheels that mesh together, so when one turns, it turns the other without slipping.
  2. The ratio of the number of teeth on two meshing gears determines how their speeds and torques relate.
  3. A small gear driving a large gear reduces speed but increases torque, and vice versa.
  4. A gearbox contains several gears on parallel shafts, and a selector mechanism chooses which pair to engage.
  5. By selecting different gear pairs, the gearbox matches the engine's narrow efficient speed range to varied road speeds and loads.

Labelled diagram

  1. Input shaft

    Receives power from engine or motor and spins at a certain speed.

  2. Small gear

    Has few teeth; turns faster but with less torque.

  3. Large gear

    Has many teeth; meshes with small gear, turns slower but with more torque.

  4. Output shaft

    Delivers the changed speed and torque to the wheels or machine.

How a simple gearbox changes speed and torque

Real-world examples

  • In a car, first gear uses a small gear to drive a large one, giving high torque to start from rest.
  • A bicycle derailleur moves the chain between different-sized sprockets, acting like a gearbox to ease pedaling uphill.
  • A wind turbine gearbox speeds up the slow rotation of the blades to the high speed needed by the generator.
  • A drill gearbox lets the user switch between high speed for drilling and high torque for driving screws.

Common mistake

✗ Myth: A gearbox creates energy or power.

✓ Fact: A gearbox only changes the balance between speed and torque; it cannot increase total power. In fact, friction inside the gearbox means the output power is always slightly less than the input power.

People also ask

What is a gear ratio?
A gear ratio is the number of teeth on the driven gear divided by the number on the driving gear. It tells you how many times the driving gear must turn to make the driven gear turn once. A ratio greater than one means slower output speed but higher torque; less than one means faster output speed but lower torque.
How does a differential work with a gearbox?
A differential is a set of gears that allows the two driven wheels to turn at different speeds while still receiving power from the gearbox. When a car turns, the outer wheel travels a longer path and must spin faster. The differential splits the torque and adjusts speeds accordingly, preventing wheel slip and tire wear.
Why do gears whine?
Gears whine because the teeth mesh and unmesh under load, causing tiny vibrations that travel through the metal and create sound. The pitch of the whine depends on how fast the gears turn and how many teeth they have. Worn or poorly lubricated gears can whine louder as friction increases.
Why do cars need multiple gears?
Engines produce useful power only within a limited range of rotational speeds. At low speeds, they lack torque to move a heavy car; at high speeds, they become inefficient. Multiple gears let the engine stay in its efficient range while the car's wheels turn at the speed demanded by the road.
What is the difference between manual and automatic gearboxes?
A manual gearbox uses a clutch and a lever to let the driver select gears. An automatic gearbox uses fluid or electronic controls to shift gears based on speed and load, without driver input. Both achieve the same goal: matching engine speed to driving conditions.
How does a gearbox change torque?
When a small gear drives a larger gear, the larger gear turns more slowly but with greater turning force, or torque. The gear ratio—the number of teeth on the driven gear divided by the number on the driving gear—determines the exact change. Power remains roughly constant, minus friction losses.
What does a gearbox do in an electric car?
Most electric cars use a single-speed gearbox because electric motors produce high torque from zero speed and can spin very fast. The gearbox simply reduces the motor's high speed to the wheel speed and multiplies torque, so no multiple gears are needed.

Part of the guide: Everyday Machines →