A differential consists of a housing with three bevel gears inside, two which are set on two axles that enter it from opposite side. The third gear is connected to the housing only. The first two gear are called spider gears or side gear, and the gear fixed to the housing is called a planet gear.
When a differential is used to couple motors, any difference between inputs will be equalized by the system of the differential's inner gears. The output will be driven by the sum of the inputs' torques and the average of their speeds. Figures 18-3 through 18-6 show some examples of two motors coupled with an adder in various ways. The biggest distinction among these variations is the use of different differential. The motors inputs are blue and green, and the adder's output is red