Further diff-essentials

Deliberately introducing friction into the equation

- MARK ORTIZ

In RE V30N5 we considered the various types of differentials, in their ‘open’ form. We noted a differential is a gear mechanism that splits the power from one input shaft between two output shafts, providing a fixed torque split while allowing free variation of the output shaft speeds with respect to each other.

This property can create a problem when the distribution of available traction at individual wheels is different to the distribution of torque provided by the differential. In such situations, one or more wheels will spin prematurely and the traction of others will not be fully utilised.

Methods of modifying the differential’s characteristics to reduce this tendency generally involve introducing some form of friction, usually within the differential itself, though sometimes through the use of a brake. It is also possible in some applications to drive two output shafts from a single input shaft with devices that are not properly differentials.

Friction within the differential can be provided by clutches, by gear design, by some form of viscous coupling or by a combination of these. The friction can be constant, torque dependent, speed-dependent or controlled electronically, either by a computer or by a manually-operated control.

Clutch packs

The most popular approach is to use either one or two multi-disc clutch packs within the differential unit. These introduce friction between the carrier and one or both output shafts. It is sufficient to have this on only one output shaft, because the mean rotational velocity of the two shafts has to equal the input velocity, and therefore restricting the speed difference between one output shaft and the carrier also restricts the speed difference between the other output shaft and the carrier, and restricts the speed difference between the two output shafts.