Fig. 28. Equalizing Mechanism.
Fig. 29. Resistance in Equalization.
While differential gears are very simple structurally, it is not an easy matter to explain the principle on which a faster motion is transmitted to one wheel than another, and under conditions where the speed is constantly changing.
Fig. 30. Equalizer and Differential Movements.
For instance, in Fig. 30, a cord A, over a pulley B, has weights C, D, at its ends. If the pivot or fulcrum E, of the wheel, is stationary, as in sketch 1, and the wheel is turned, say a quarter of the way around, one weight will move down below the line X the same distance that the other weight moves above it, as shown in 2.
Thus far we have an equalizer, pure and simple. But a differential requires something more. It is necessary, under certain conditions, for the weight D to move a greater distance in the same time than C, or the reverse. Or, as sometimes happens, one of the weights, as for instance, in 3, remains fixed while the other moves.
In this case, with the pivot pin E fixed, such a thing would be impossible, hence, in order to make such a relative movement between the two weights, the pin must move, and this motion is shown in 3, where it moves down from the line F. That movement, or change of position of the pivot E, is what takes place in the small intermediate gears in a train of differential gearing.