Viscous couplings normally operate in the “viscous mode”, where torque is generated by viscous shear. The main influences involved in calculating the viscous characteristics are briefly explained below.
Under certain conditions, the coupling function changes from the viscous mode into the so called “hump” (induced torque amplification), characterized by a significant torque increase. The torque is generated by pressure induced friction contact between the plates.
Where external pressure is applied (externally induced torque amplification), torque is controlled from outside according to a given control algorithm. This coupling, currently under development, provides excellent dynamic response.
When pressure is build up internally by supplying energy (self-induced torque amplification), the torque may amount to a multiple of the initial viscous torque. One application of this principle is in improving traction in ABS-compatible viscous limited slip differentials for motor vehicles. The “hump” meets the increased torque requirement under demanding starting conditions and provides thermal overload protection for the coupling itself.