They can be had without the possibility of outside influence or also with a great deal of electronics or hydraulics. One thing that all self-locking differentials have in common, is one or two multiple-disc clutches on either side of the axle shaft gear wheels.
The basic construction is already very old, thus well tried and tested. The, in oil running disc-package develops, when necessary, sufficient friction to hold the two drive-surfaces together with a force of approx. 25 to 70 percent. Should traction problems arise, e.g., between the two sides of a driven axle, more torque is distributed to the wheel with the better grip.
Of course, this not only applies to differentials between two wheels (transverse), but also to differientials between the two axles (longitudinal). Originally the self-locking took place mechanically, thereby, when, through the tension of differing wheel speeds, the two halves of the divided casing for the offset bevel gears is spread apart, then the clutch is operated. (see figure 3).
Even at the time of the VW-Beetle, this type of locking differential could be ordered. Directly installed by the factory, it functioned well, indeed, with increasing mileage it became lost some of it's efficiency. Nowadays, a solution like this would no longer be acceptable. Nevertheles, as you can see in the above picture, the mechanics have been carried over into modern day.
So, what remains are the two multiple-disc clutches. They are now however, when required, operated by hydraulic pressure. Of course, a great deal of electronics is responsible for making the decision. First of all, there are the wheel-sensors, which call up the system. To do this, sufficient hydraulic pressure has to be created through a suitable pump. It would be ideal, if this system could be used for other purposes as well.
In the event of it being triggered, a decent amount of pressure is applied to the ring-piston (see figure 2), which presses the package of multiple discs together. Different from today's systems, in the past it could not be modulated and was simply activated when the wheels reached a certain speed difference. These locking differentials were only intended to help you to get out of an awkward situation, thus, they were effective up to an adequate speed, of let's say, 35 km/h.
In the meantime, this type of locking differential also supports, e.g., the ESP-track stabilization up to quite different speed ranges. They are also not only hydraulically operated, but also through magnetic clutches. As a longitudinal lock, the actual gearwheel offset can be done away with and have only one clutch to connect the front- and rear axles with each other, a further development of the Haldex-clutch. 07/13