The gears are part of a rotating engine that is useful for transmitting power. The gears have teeth tangent to the teeth of the other gears. Two or more gears that intersect and work together are referred to as gear transmissions, and can produce a mechanical gain through the ratio of the number of teeth. The gears are able to change the rotation speed, torque, and direction of power to the power source. Not all gears are connected to another gear; one of the cases is a pair of gears and pinions that are sourced from or produce a translational force, not a rotational force.
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From @andesboned