The four gear types are spur gears, bevel gears, rack-and-pinion gears, and worm gears. They differ mainly in the relative positions of their axes, the type or direction of motion transmitted, and the mechanical advantage produced.
A gear is a toothed machine element that transmits motion and torque through direct contact with another toothed component. For two meshing circular gears, the driven gear rotates in the opposite direction to the driver, and the gear ratio is:
| Gear type | Mechanism and motion | Typical application |
|---|---|---|
| Spur | Cylindrical gears with straight teeth operate on parallel axes. A larger driven gear decreases rotational speed but increases torque. | Clocks, gearboxes and machinery |
| Bevel | Conical gears transmit rotary motion between intersecting axes, commonly through . They can change speed, torque and the axis of rotation. | Hand drills and vehicle differentials |
| Rack and pinion | A circular pinion meshes with a straight toothed rack, converting rotary motion into linear motion or linear motion into rotary motion. | Steering systems and sliding gates |
| Worm | A screw-shaped worm drives a worm wheel on a non-parallel, non-intersecting axis, usually at . It provides a large speed reduction and corresponding torque increase. | Lifts, hoists and tuning mechanisms |
For example, a 10-tooth driver turning a 40-tooth spur gear gives a gear ratio of . The driven gear rotates at one-quarter of the driver's speed, ideally with four times the torque.
A common misconception is that every worm gear is automatically self-locking. Reverse motion may be prevented in some designs, but this depends on the worm's lead angle, friction, materials and efficiency.
Exam technique: For A3.3 HL questions, identify the gear from its geometry, state the input and output motion, and explain any change in axis, speed, direction or torque. When given tooth numbers, show the gear-ratio calculation and identify which component is the driver.
