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Gears are mechanical parts with cut teeth designed to fit with teeth on another part so as to transmit or receive force and motion. Gears are also sometimes called toothed wheels or cogged wheels or cogs. The cut teeth are also sometimes called cogs.
Numerous nonferrous alloys, cast irons, powder-metallurgy and even plastics are used in the manufacture of gears. However steels are most commonly used because of their high strength to weight ratio and high cost.
One of the very useful things about gears is their ability to change the rate at which things spin. You can make something spin faster with less power, or slower with more power. This is done by coupling gears with more or less gear teeth.
The basic gear ration formula is T1*S1=S2*T2. T1 is the number of teeth on the drive gear and T2 is the number of teeth on the gear you want to change. S1 is the rotational speed of the driver gear and S2 is the rotational speed of the gear being driven.
For example, suppose you have a drive gear moving at 100 RPM (revolutions per minute) with 50 teeth, and you want another gear to turn much slower at 10 RPM. How many teeth should it have? Using the gear formula, you just need to solve for T2, T2 = T1*S1/S2. T2 = 50*100/10. Reducing gives 500 teeth on the gear being driven.
Gears are used in many applications that include clocks, engines, power transmission, raising and lowering very heavy objects, detecting astronomy events, and much more. It is one of the biggest achievements in engineering that was invented 1000's of years ago.