Viscous Coupling Unit

Viscous Coupling Unit

Viscous coupling are used in Land Rover specification Borg Warner transfer gearboxes to control the center differential during conditions of low traction.

A viscous coupling unit (VCU) is a sealed steel drum within which there are a series of slotted metal discs. Half of the discs are splined directly to the ID of the drum and the other half are splined to the VCU's output hub - very much like an automatic transmission clutch assembly but with no friction material. The drum is filled with a silicone fluid and sealed. The special silicone fluid has the ability to become very viscous when agitated by the shearing action of the inner and outer slotted discs. This creates a rapid temperature rise in the fluid. At this point, the viscosity is such that the discs "lock up" and the entire VCU - drum and hub - rotate as one.

In essence, the VCU automatically locks the center differential. There is, therefore, no need for a mechanical diff-lock such as is fitted to the LT230T. The locking action is nearly instantaneous. Once traction is gained, the shearing action ceases and the viscosity quickly falls back to normal.