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 [