# Gear

A gear is a rotating machine part having cut teeth, or cogs, which mesh with another toothed part in order to transmit torque. Two or more gears working in tandem are called a transmission and can produce a mechanical advantage through a gear ratio and thus may be considered a simple machine. Geared devices can change the speed, torque, and direction of a power source. The most common situation is for a gear to mesh with another gear, however a gear can also mesh with a non-rotating toothed part, called a rack, thereby producing translation instead of rotation.

The gears in a transmission are analogous to the wheels in a pulley. An advantage of gears is that the teeth of a gear prevent slipping.

When two gears of unequal number of teeth are combined a mechanical advantage is produced, with both the rotational speeds and the torques of the two gears differing in a simple relationship.

In transmissions which offer multiple gear ratios, such as bicycles and cars, the term gear, as in first gear, refers to a gear ratio rather than an actual physical gear. The term is used to describe similar devices even when gear ratio is continuous rather than discrete, or when the device does not actually contain any gears, as in a continuously variable transmission.

The earliest known reference to gears was circa A.D. 50 by Hero of Alexandria, but they can be traced back to the Greek mechanics of the Alexandrian school in the 3rd century B.C. and were greatly developed by the Greek polymath Archimedes (287–212 B.C.). The Antikythera mechanism is an example of a very early and intricate geared device, designed to calculate astronomical positions. Its time of construction is now estimated between 150 and 100 BC.

### Other articles related to "gear ratio, gear, gears, ratio":

U Engine - Variations - Gear Ratio
... In the Sulzer LDA engine, a gear wheel on each crankshaft meshed with a slightly smaller gear wheel on the central output shaft ...
Overdrive (mechanics) - Description - Background
... speed is not the same as the engine RPM at that power, a transmission is used with a gear ratio to convert one to the other ... this condition calls for a different gear ratio ... but instead sets the "maximum speed" to that of highway cruising, the output is a higher gear ratio that provides ideal gas milage ...
Epicyclic Gearing - Gear Ratio - Compound Planetary Gears
... Compound planetary gear" is a general concept and it refers to any planetary gears involving one or more of the following three types of structures meshed-planet (there are at least two more planets in mesh with ... Some designs use "stepped-planet" which have two differently-sized gears on either end of a common casting ... This may be necessary to achieve smaller step changes in gear ratio when the overall package size is limited ...
Toyota Celica - Fifth Generation / T180 Series (1989–1993)
... The E150F gearbox with 4.285 final gear ratio was installed in the JDM and All-trac ... Sainz/Group A models, came with the E151F gearbox with 3.933 ratio ... confused with the Australian GT-Four Grp A Rallye model), has the E152F gearbox with close ratio on the 1st through 4th gear and 4.285 final ratio ...

### Famous quotes containing the words ratio and/or gear:

People are lucky and unlucky not according to what they get absolutely, but according to the ratio between what they get and what they have been led to expect.
Samuel Butler (1835–1902)

But Lord, remember me and mine
Wi’ mercies temporal and divine!
That I for grace and gear may shine,
Excelled by nane!
And a’ the glory shall be thine!
Amen! Amen!
Robert Burns (1759–1796)