Limited Slip Additive Testing and Development: New Products with Improved Thermal Stability 2007-01-1988

Limited slip differentials, developed over 40 years ago to counter drive wheel slippage when different traction conditions exist on either side of an axle, are still widely employed by the automotive industry to improve driving control. In a limited slip differential (LSD) frictional couplings connect the axle shafts to the differential and provide the means of transmitting power to the wheels. The friction plates in the coupling may contain a variety of friction materials including metal, paper, sintered bronze, and carbon. Each one of these materials has very different frictional and wear characteristics and each one requires a different response from the gear additive package. Each plate must be durable over the course of the vehicle lifetime irrespective of the material used. As the demands on rear axles increases with the application of greater horsepower and the increasing requirements of aerodynamic engineers, the lubrication of these friction plates remains an ongoing challenge.

Lubricant frictional characteristics are very important in determining the quiet and smooth operation of LSD's. Fully formulated API GL-5 J 2360 gear lubricants are unable to fulfill all the lubrication requirements of LSD's. Special additives have therefore been developed to improve the frictional response in the coupling to eliminate noise, vibration, and stick slip problems.

There is a requirement to provide limited slip additives that give excellent friction plate lubrication. The goal of lubricant formulators is to develop new products that retain frictional performance with minimal effect on the thermal stability of the gear lubricant. Test methodology has been developed that correlates friction characteristics with known field performance and testing has been performed on a variety of friction plate materials. The results of testing with current commercially available limited slip additives and new additives with much improved thermal characteristics in different gear lubricants and with different hardware configurations are discussed. This paper adds to previous work in this area and brings to a conclusion the development, design and experimentation associated with this extensive program.