The Visco Lok® LSD is a traction optimised speed-sensing limited slip differential. A self-contained silicone-fluid filled reservoir and shear pump generates a hydraulic pressure proportional to a speed difference across the axle. The speed difference between the feed disc and the fluid filled channel of the pump disc and the resulting fluid shear forces moves the fluid from the reservoir to the apply piston. The hydraulic pressure creates an axial force via the apply piston, compressing the clutch pack. The resulting locking torque through the clutch pack transfers torque to the wheel with better traction. A variable volume reservoir compensates for the fluid thermal expansion.
See Viscous LSD.
The viscous coupling or viscous LSD is often found in all-wheel-drive vehicles. It is commonly used to link the back wheels to the front wheels so that when one set of wheels starts to slip, torque will be transferred to the other set. The viscous coupling has two sets of plates inside a sealed housing that is filled with a thick fluid. One set of plates is connected to each output shaft. Under normal conditions, both sets of plates and the viscous fluid spin at the same speed. When one set of wheels tries to spin faster, perhaps because it is slipping, the set of plates corresponding to those wheels spins faster than the other. The viscous fluid, stuck between the plates, tries to catch up with the faster disks, dragging the slower disks along. This transfers more torque to the slower moving wheels (the wheels that are not slipping).
(Source: How stuff Works http://www.howstuffworks.com)
The VL disc joint is most commonly used with a flange connection where it is bolted to the gearbox.
The VL (Verschiebegelnk Loebro) is a ball plunging joint suitable for the inboard joint of rear-wheel drive CVJ sideshafts. With maximum 22 degrees articulation plunge of 50mm and very small rotational backlash it is ideal for front driven vehicles with manual gearbox and for rear axles.
This VL Joint is used for stem connections to the gearbox or the wheel hub. Its compact design provides the minimum outer diameter for the VL joint. For rear driven axles, the VL can also be used as wheel side joint.