Visco-Hydraulic Clutch
The visco-hydraulic clutch, generally called "visco clutch", holds special significance among the clutch designs discussed here. So far it has not been applied in vehicles as a starting or shifting clutch but as a self-regulating axle or transfer case (Chapter 3.5), i.e., as a locking device for these components. If used as a main clutch in its present design, slip would have to occur permanently in order to transfer a torque, which would be disadvantageous.
The figure shows the fundamental design of a visco-hydraulic clutch, which basically consists of a number of axially-displaceable lamellae. Inner and outer lamellae are each respectively connected to a clutch outlet. The clearances are filled with a highly viscous liquid (generally silicon oil). By the visco-hydraulic principle, torque transmission takes place as a result of the fluid dynamic forces acting between the individual lamellae.
The tensile stresses resulting from the shearing of the fluid film, linearly depend on the dynamic viscosity ηdyn and the difference in velocity relative to the film thickness according to Newton's Law of Tensile Stresses.
When applied to the visco-hydraulic clutch, the transferred torque is given by:

Eq. 3-19

where zVC = number of active areas
Δω = differential velocity between outer and inner lamellae
ro = outer radius
ri = inner radius
c = clearance
In a visco-hydraulic clutch, this approach for laminar flow is generally applicable to smooth lamellae since, the critical Reynold's Number can be hardly be achieved as a consequence of high dynamic viscosity.