viscocoupler

A viscocoupler, or viscous coupler, is a mechanical device used to transmit torque between two shafts in response to differences in their rotational speeds. It relies on a viscous fluid that is contained in an annular chamber between the driving and driven members. When the input shaft turns faster than the output shaft, the shear created in the fluid transmits torque to the output, producing a slip-dependent coupling.

Construction typically features a housing attached to the driving shaft and an internal rotating element connected to the driven shaft. The chamber is filled with a viscous fluid (commonly silicone oil). The interface often includes interleaved plates or fins to increase surface area and shear. The viscosity of the fluid, temperature, and the geometry determine the amount of torque that can be transmitted at a given slip.

Operation and characteristics: At small speed differences, only a small torque is transmitted; as slip increases, torque rises until limited by the maximum viscous shear and temperature; the device naturally limits torque differences without active control. Because viscosity decreases with temperature, heat buildup from slipping can reduce transmitted torque and reduce coupling effectiveness. Cooling can be needed in high-demand applications.

Applications and variants: Viscocouplers are used in some all-wheel-drive systems to provide a simple, force-free torque transfer between the center differential and the transfer case, and in other machinery where a passive, speed-sensitive torque transfer is desirable. They are distinct from hydraulic torque converters, which incorporate turbine and stator stages to achieve torque multiplication. The primary advantages are simplicity and robustness; the main limitations are limited torque capacity, heat sensitivity, and poorer performance at high speed differences.