The term differential refers to a mechanical device that receives energy from a power source and converts the direction of that energy at right angles, left and right, with limited loss of power. While the term, these days, is generally considered a key automotive part, mechanical differentials can be used in any situation where such directional conversion of power is desirable. In automotive uses, wheels are attached at the end of the left and right driven arms extending out of the directional power converter mechanism.
In addition to converting the application of power, this directional power converter has the ability to operate with the left and right converted power arms, called axles, rotating at different speeds. In automotive applications this allows a transference of power even while the two converted power arms, and the attached wheels, rotate at different speeds. Meaning the vehicle is turning. This is accomplished by the use of slip gears that mechanically ensure, even though only one physical power force enters the power conversion mechanism, the operation of the various sized gears results in a mechanical equivalent of two power inputs and two powered outputs. This arrangement of gears is therefore a mechanical equivalent of the mathematical equation that is a proof of this direction of power conversion system.
In early automotive differentials this caused a loss of traction at the point where the wheels met the road. This, depending on the vehicle weight and amount of applied power, could be a problem. The result was development of a differential that had a limited amount of slip or loss of power to the left and right angle powered arms exiting from the case containing the differential mechanism. In automotive applications the left and right arms are called axles. Since the use of this mechanical principle for automotive purposes the term axle is used for the left and right power converted arms to even non-automotive mechanisms.
Early Forms of Mechanical Differentials
The ability to covert the direction of applied power is actually not a modern invention. Ancient devices have been found that also used this manner of converting applied power from one direction and applied that power in one or more different directions.
In the first quarter of the eighteenth century this mechanical principle was applied to clocks. By the first half of the twentieth century analog computers were using this principle to do simple arithmetic calculations.