Torque-Sensing Differentials

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As four-wheel drive systems have evolved over the years, many different variants have surfaced to do essentially the same job: distribute power to all four wheels. How this is achieved through various systems affects vehicle dynamics on- and off-road. Last month we explained how electronically-controlled clutches automatically distribute power, a system typically used by All Wheel Drive (AWD) soft roaders, which are not equipped with low-range gearing.

In this article we explore how the mechanical Torsen (TORque SENsing) differential works, what its benefits are and its application – typically on large SUV-type 4x4s with longitudinal drivetrains, low-range gearing and permanent four-wheel drive (4WD).

The history

Having a fail-safe centre differential is at the core of permanent 4WD technology; one which allows the wheels on the front and rear axles to follow a different path to prevent transmission wind-up, but which can also lock front and rear when traction is variable.

An invention patented by American gear boffin, Vernon Gleasman, in 1958 provided a solution. His unique combination of worm wheels and worm gears allowed for axle speed differences just like an open differential, but locked up automatically when there was inter-axle slip. It was a purely mechanical system, acted immediately, and was very robust.

Gleason’s ‘Dual-Drive Differential’ was initially manufactured by Gleason Power Systems, and then by Torsen Traction, which became a subsidiary of JTEKT over the course of mergers and acquisitions (via Zexel Corporation of Japan, then Bosch Automotive Systems and Toyoda Machine Works, followed by a merger with Koyo Corporation to form JTEKT Corporation in 2006). Torsen Traction now supplies helical gears to customers in Europe, Asia and North America, including Toyota, Audi, Ford, FCA, GM and BMW.