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Viscous coupling unit
A viscous coupling is made up of alternating circular plates. The plates have tabs or perforations in them. The plates are mounted in a sealed drum, and are located very close to each other. The drum is filled with silicone, or some dilatant fluid. When the two sets of plates are rotating in unison, the fluid stays cool and remains in a liquid state. When the plates start rotating at two different speeds, the shear effect of the tabs or perforations on the fluid will cause it to heat up and solidify (Silicone when heated will turn into a near solid; the viscosity of dilatant fluids rapidly increases with shear). The fluid in this state will essentially glue the plates together and transmit power from one set of plates to the other. The size of the tabs or perforations on the plates, along with the number of plates and fluid used will determine the strength and onset of when this mechanical transfer will happen.
Viscous couplings are used as the center differential in some 4WD/AWD vehicles such as the Toyota Celica GT-Four, and also as a limited slip differential (LSD) in rear axles.
