Hydroviscous drives use the viscous property of fluid to develop drive torque. Basically, a shearing action can be developed in fluid between adjacent rotating disks, making it possible to transmit torque from one disk to another.

Hydroviscous drives are the only fluid drives providing full lockup at full-speed operation. They are rated from 5 to 20,000 hp at 1,800 rpm and are often used in high-inertia, high-power applications. Below 25 hp, this type of drive tends to be more costly than comparable belt or chain drives. But hydroviscous drives offer greater speed ratios (to 20:1).

All hydroviscous units must have a heat exchanger. Also, all hydroviscous drives must be controlled by either open-loop or servocontrol closed-loop circuits.

Typical applications include cycling drives on stamping presses and packaging machines; constant-tension drives and dynamometers; adjustable-speed drives; and as couplings in high-horsepower applications.