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:
- simple gears
- bevel gears
- worm gears
- rack and pinion
Simple gears
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".
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Simple gears |
Bevel gears
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.
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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.
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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. |  |
