Viscocouplers

A viscocoupler, also known as a hydroviscous coupling, is a torque-transfer device that transmits power between two shafts through the viscous shear of a filled fluid. It typically consists of an outer and an inner rotor connected to the driving and driven shafts, with a viscous fluid, such as silicone oil, sealed in the space between them. Fins or plates on the rotors increase the surface area available for shear.

The operating principle relies on the shearing of the viscous fluid when there is a difference in rotational speed between the input and output shafts. As one shaft speeds up relative to the other, the fluid shears and a torque is transmitted to the slower shaft. The amount of torque transferred depends on the fluid viscosity, the temperature (which changes viscosity), the clearance between rotors, and the surface area. Viscocouplers provide a gradual, smooth engagement and inherently limit slip, with transmission increasing as differential speed grows, and diminishing as the speeds converge.

Applications of viscocouplers are mainly in automotive drivetrains, particularly in all-wheel-drive systems and some limited-slip arrangements. They are favored for their simplicity, lack of a conventional clutch, and self-contained operation. They can automatically adapt to changing traction conditions without electronic control, but their torque capacity is limited, and performance is sensitive to temperature and aging of the fluid.

Advantages include compactness, simplicity, and self-regulating behavior. Limitations include reduced response speed, potential heat buildup, dependence on fluid viscosity, and limited effectiveness at high speeds or high torque. Variants and refinements focus on optimizing viscosity, clearances, and thermal management to broaden usable ranges.