Viscous coupling unit

[1] The device consists of a number of circular plates with tabs or perforations, fitted very close to each other in a sealed drum.

The first mass-produced viscous couplings for a permanent 4WD off-road-capable vehicle were in the AMC Eagle, which was produced from 1980 to 1988 model years.

[2] The AMC Eagle's single-speed model 119 New Process central differential used a viscous coupling filled with a liquid silicone-based material.

[3] It linked the front and rear differentials for quiet and smooth transfer of power to the axle with the greatest traction, on wet or dry pavement.

Tony Rolt is often credited with the original idea and development as applied to automatic couplings in vehicle transmission systems, particularly four-wheel drive, working with Freddie Dixon at the time.