Law of gearing
In working more than one pair of teeth are always in sliding contact and the rotation brings successive pairs into contact. And the pressure between the two sliding surfaces in normal to their common tangent. This maintains the ratio of angular velocities of the two gears to certain constant. This is the Law of Gearing. Full satisfaction of the Law of Gearing ensures the motion to be smooth, quiet and free of vibration. If not satisfied, the gears will shake and vibrate as they rotate, there will be loss in power. The tooth profiles of gears that satisfy the Law of Gravity is called conjugate.

Kinetics
Number of teeth per gear and the arrangement of teeth on the gear surface determine the torque and direction. The gear that transmits motion or directs motion is known as driver, and the other is known as driven.

If two gears have the same number of teeth then same amount of rotation is transferred to the driven gear, as one turn of driver gear causes the driven gear to turn once. Difference in number of teeth of the two gears in contact brings change in amount of rotation in the driven gear. If the driver gear is smaller or has less number of teeth, the driven gear will receive less rotations but amplified torque.

For a single pair of gears where the output shaft rotates at a different speed than that of the input shaft, the torque applied on the output shaft drives the load. The size of the teeth on the pinion should be the same as the size of the teeth on the wheel. This module must be common to both the gears. Pitch circles contact one another at the pitch point and the pinion's pitch line velocity must be identical to the wheels pitch line velocity. And at the pitch point develops a tangential component of action-reaction due to contact between the gears.

The only way that the input and output shafts of a gear pair can be made to rotate in the same sense is by interposition of an odd number of intermediate gears. These do not affect the speed ratio between input and output shafts. Such a gear train is called a simple train. If there is no power flow through the shaft of an intermediate gear then it is an idler gear.