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A gear is a rotating machine component 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 because force is being "transmitted" from one source to another. This union can produce a mechanical advantage through a gear ratio. Geared devices can change the speed, magnitude, and direction of a power source.
A gear can also connect with a non-rotating toothed part, called a rack, thereby producing lateral movement instead of rotation. (Illustrated on the right) This particular style of gear is commonly used in a refrigerator to open and close an air passage between the refrigerator and freezer compartments.
The other main advantage in the use of a gear besides the potential mechanical advantage that can be produced, is a gears ability to prevent slipping. This characteristic guarantees that none of the force or movement being transferred is lost and can be exactly calculated. This is why gears are used in machines that require calculated precision such as clocks and appliances.
Gears are generally used for one of four following reasons:
1. To reverse the direction of rotation
2. To increase or decrease the speed of rotation
3. To move rotational motion to a different axis
4. To keep the rotation of two axes synchronized
A mechanical advantage is created when two gears with unequal number of teeth are combined. Through this simple relationship both the rotational speeds and the torque of the two connected gears will differ, this difference is described as the gear ratio.
In this animation the smaller gear has a total of 10 teeth while the larger gear has 28 teeth the gear ratio would be described as 28/10 or 2.8/1 or 2.8:1 meaning that the smaller gear must rotate 2.8 times for every single revolution of the larger gear.