Viscous Coupler: Operation
The viscous coupler (Fig 1 ) is a housing nearly filled with a high viscosity silicone liquid and thin metal plates alternately splined to an inner and outer drum. The viscous coupler provides torque in the following modes:
- Shear mode (normal operation)
- Hump mode (locked mode)
The inner plates are slotted around the radius and the outer plates have holes in them. In the shear mode (normal operation), the plates are evenly spaced and the torque is created by the shearing of the plates through the fluid and 90-100% of the torque is applied to the rear axle. During the shear mode, a fluid flow pattern is created from this design (holes and slots). This fluid flow causes high pressure on each side of each pair of plates and low pressure between each pair of plates.
When a high speed difference (shear) occurs because of loss of traction (one axle spinning faster than the other), the silicone fluid expands as it heats from this shearing. When the silicone expands to fill the viscous coupler completely, this pressure difference is high enough to squeeze each pair of plates together. The resulting hump torque is up to 8 times higher than the shear torque. When the viscous coupler is in the hump mode, it does not lock the axles (undifferentiated 4-Wheel Drive). It controls the amount of slippage while delivering maximum power to the axle having greatest traction. Once the speed difference equalizes the fluid and plates cool down and the viscous coupler goes back to the shear mode.