Simple gears Bevel gears Worm gears Rack and pinion

Gears are typically wheels with teeth or cogs. They are used to transmit effort, change torque (turning force), change direction, or change speed. There are several basic types of gears including:

  1. simple gears
  2. bevel gears
  3. worm gears
  4. rack and pinion

Simple gears
Click for larger image The simplest form is two gear wheels with the teeth meshing. In all gear systems one gear will be powered. This is called the drive gear and the other gear is called the driven gear. In a series of gears, each pair of gears can be considered to consist of a drive gear and a driven gear. If the number of teeth is the same on each gear, then the driven gear rotates at the same rotational speed as the driving gear, but in the opposite direction.

If the driven gear has more teeth than the driving gear, the driven gear rotates at a slower rotational speed.

If the driven gear has less teeth than the driving gear, the driven gear rotates at a faster rotational speed.

The mechanical advantage of a pair of gears is the number of driven gear teeth divided by the number of driving gear teeth.

As an example, if the driving gear has 30 teeth and the driven gear 15 teeth, the mechanical advantage is 15/30 = 0.5. Conversely, if the driven gear has 30 teeth and the driving gear 15 teeth the mechanical advantage is 30/15 = 2.

Bikes use gears to increase the efficiency of the rider, depending on the surface and slope being tackled! Bikes are designed to have a mechanical advantage of less than one. That is, the input force of the rider acts over a short distance on the crank wheel gear, pushing the larger rear wheel over a longer distance. As a rider drops down through the gears on riding up a hill, the crank wheel gear is made larger and the rear gear wheel smaller until a MA of 1 is approached on very steep hills. Various mechanical advantages can be achieved by changing the rear and crank gear sizes, commonly know as "changing gears".

Simple gears

Bevel gears
Click for larger image Bevel gears have teeth set at a 45° angle to the direction of turning. The same rules apply as for standard gears, but the two gear shafts are at right angles. This system of gears is useful to transmit force at a 90° angle. Other angles are possible by varying the bevel on the gears. Common examples include a hand driven kitchen beater, and the differential in a car.

Worm gears
Worm gears are also valuable in transmitting torque from one shaft to another at a 90° angle. They also reduce the speed of the rotating gear wheel. A worm gear train consists of a standard gear wheel on a drive shaft, with an endless screw that meshes with the gear wheel as it is turned. This type of gear is useful for fine adjustments, as a large number of rotations of the "worm" results in many fewer rotations of the gear. Its uses include fine tuning of radio receivers where precision is important.

Worm gears

Rack and pinion
This form of gearing is used to transmit rotational motion of a gear wheel to a straight bar with teeth. The rack and pinion was once common in car steering systems. There are stops at the end of the straight section to limit movement.

Rack and pinion gears

In a different application, railway locomotives pulling loads up steep inclines sometimes have a pinion wheel under the locomotive, engaging with a rack in the centre of the rail tracks. The Apt railway in Tasmania is a fine example of this type of gearing.

Copyright owned by the State of Victoria (Department of Education and Early Childhood Development). Used with Permission.

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