scope:

The fascination of the automotive differential has led to the idea to build a second differential unit around a first center unit. Both units have the same axes around which they rotate with different speeds.

The potential of double differentials as ultrahigh reduction speed reducers is significant. Only the tooth-count of the gears in the outer differential unit must be changed to achieve ratios between 5 and 80 without a noticeable change of the transmission size.

Double differentials are well suited for high input speeds. The fact that the carrier rotates with about half of the input speed reduces the relative motion and with it the sliding velocity to 50% of the value of two conventionally meshing bevel gears which roll with the same input speed.

Ground spiral bevel gears are recommended for the double differential application. Due to the load sharing of the two opposite planets, the torque of each gear is only 50% compared to a conventional bevel gear mesh. This effect results in very high-power density of this already very compact unit.

Also, the efficiency of the double differential is high in contrast to the fact that always two pairs of gears are transmitting the rotation and torque. Double differentials show good efficiency results, which qualifies this new transmission type very well for the speed reduction and transmission in electric vehicles and hybrids.

In addition to electric vehicles and hybrid cars, there are many other applications in the industry which require high ratios. Double differentials could be used in helicopters, wind turbines, agricultural equipment, and many other industrial applications.

The objective of the paper is to compare a transmission concept, which is based on bevel gear technology, with cylindrical gear-based solutions, for designers and manufacturers of high reduction transmissions to have an additional technology available for their different applications.