Viscous Couplings in 4WD Vehicles: Application of Computational Modelling

With the increasing popularity of all wheel drive (AWD) vehicles, the use of viscous couplings (VCs) in drive systems is becoming more common. The torque-speed characteristic of the VC is used to limit the slip between the driven axles and provide effective traction under adverse conditions. Typically, the torque is transmitted through the shearing action of a viscous fluid between alternating sets of rotating plates. The VC can be designed to achieve a self energized torque amplification called “humping” under certain temperature and speed conditions. During “hump”, the plates come into mechanical contact and Coulomb friction is thought to enhance the torque transfer. Existing theories to explain the axial pressure gradient and the resulting movement of the plates necessary for mechanical contact are found to be inadequate.

The present work analyzes the shear and hump characteristics of the VC. The influence of the plate geometry and fluid characteristics on “humping” are studied. The earlier theories proposed by Peschke [1], Taureg & Horst [3] and Takemura & Niikura [6] are assessed in the light of experimental results. An outline of a new theory being developed is given. This theory is based on analysis of the creeping flow between the rotating plates and the flow through the plate perforations. Preliminary experimental results are presented to support this theory. The insights gained have been used to optimize the design and testing of the VC in the NV249 transfer case of the new Jeep® ZJ sports utility vehicle.