Externally controlled limited slip differentials, with wet-plate clutches, are increasingly being used in order to improve the traction and handling of vehicles. Such devices require an actuating mechanism which offers sufficient clutch actuating force together with rapid and accurate operation. In addition, a compact design, with relatively low power requirements, and excellent controllability of the clutch torque, should be offered-all within the lowest possible cost. In practice, most of these requirements can be realised by using an hydraulic actuator, with the unfortunate penalty of relatively high cost. An actuating mechanism has been developed which offers similar properties but at considerably lower cost. With this mechanism, the actuation of the clutch pack takes places via an expander mechanism which uses balls working on inclined tracks in the annular faces of opposing discs. Through the rotation of one of the expander discs, relative to the other, a similar function to that of a hydraulic actuator is achieved mechanically. The expander disc is rotated, via a two stage gear set, by means of an electric motor. The actuating force, and thereby the clutch torque, can be controlled steplessly, and with a very high degree of accuracy, by means of electronic control of the electric motor. The POWRLOK actuating mechanism, together with a wet-plate clutch pack, is suitable for almost all possible vehicle enhanced traction requirements: as limited slip differential, POWRLOK 2 is currently under test in both front and rear wheel drive vehicles. For four wheel drive vehicles, POWRLOK 4 has been developed as a limited slip device for centre differentials. In addition, a relatively low cost version of POWRLOK 4 is available as a 'hang-on' four wheel drive system.