As early as 1917, the American inventor Melvin L. Severy got a patent granted for a viscous fluid coupling. However at this time the only oils available were viscous oils of mineral origin, which are badly suited for transferring high torques. That’s because on the one hand, viscosity decreases when temperature rises, and on the other hand, higher temperatures cause disintegration.
Only the wonders of modern chemistry made the breakthrough of the idea finally possible. Now silicone oil could be produced synthetically. Therefore a fluid was available which is resistant to highest temperatures and also loses very little of its viscosity when warmed. Even so, using silicone oil is at best a compromise solution, since its pseudoplasticity is actually unwanted.
Thus higher difference revolution speeds (shear rates) cause an apparent decrease in viscosity, followed by a digressive increase in the transferrable drive torque. The opposite behavior, called “dilatant”, would be ideal for the use in a viscous-coupling. Sadly there are no known dilatant fluids which react similarly chemically stable to silicone oil when used under the special strains of a viscous-coupling.
The basic idea was taken up again by “Harry Ferguson Developments” in the 70s and the modern viscous-coupling, using silicone oil as a transfer medium, was born. In the automotive industry, the viscous-coupling was first used primarily as a torque converter, a vibration damper and as a viscous cooling fan. In the beginning of the 80s, new applications in 4WD vehicles were found after lots of development work:
The Viscous limited slip diff (VLSD):
Speed-sensitive differential lock
A middle differential and a differential lock between front and rear axle
By the way the first vehicle with a viscous-transmission was not the VW-T3 Syncro but the American AMC Eagle, which is said to be the precursor of nowadays’ SUVs. It was produced between the years 1979 and 1987.
The free inner volume of a viscous-coupling is filled to about 90% with silicone oil.
The input variable for the transferrable drive torque in the viscous-coupling is exclusively the varying differential speed between the two axles. Concerning the transmission behavior we have to distinguish between two different modes. In principle, a Viscous-coupling first goes through the "Viscose-mode" and can switch to the so called "Hump-mode" in case of continuous stress.
As soon as the external and inner plates rotate at different speeds, cohesion causes inner friction inside the molecules of the silicone oil, which tries to equalize the differential speed again. In this process the silicone oil is exposed to additional shearing forces at the holes and slots between the plates which are rotating against each other.
Silicone oils are pseudoplastic fluids, which means their viscosity decreases with increasing shear stress. This causes a digressive transmission behavior in the drive torque. The transferred torque depends mainly on the momentary viscosity of the silicon oil and the geometry of the set of plates. Then again the momentary viscosity depends on the base viscosity, the temperature and the shear stress.
SOLID STATE FRICTION
The inner friction that occurs in Viscose-mode causes the silicone oil to warm up. Since silicone oils have high thermal expansion (about fourty times that of aluminum), the inner pressure rises inside the hermetically sealed viscous-coupling. During this process it’s mostly the degree of filling (e.g. 90%) of the viscous-coupling which influences the speed at which the pressure increases. In case of a steady differential speed, the contained air therefore gets more and more compressed and creates a solution with the silicone oil, until an effective filling level of 100% is reached. In this state the inner pressure rises abruptly, so that further energy input would destroy the viscous-coupling, which is designed for a maximum inner pressure of approx. 100 bar. This causes the viscous-coupling to go into Hump-mode. In the past the Hump-effect was thought to be a dilatant behavior of the silicone oil. However the Hump-effect has nothing to do with sudden changes in viscosity since silicone oil is not dilatant but instead the opposite, namely pseudoplastic.
If the gaps become too narrow, the silicone film tears, causing mechanical friction between the plates. Hence the transmitted torque suddenly increases, whereby the differential speed quickly declines. On the road this means that a vehicle that got stuck can now either be freed or that the motor dies. The temperature falls and the viscous-coupling changes back to Viscose-mode. The Hump-mode is meant for a momentary increase of traction in extreme situations, but it is also a constructive self protection mechanism of the coupling from overheating.
The same phenomenon occurs in 90% of old T3-Viscos in the test run before overhauling:
That of extreme hardening:
However this very common “extreme hardening” of the T3-Visco is no sign of aging but a construction problem. This specific extreme hardening of the T3-Visco only occurs when the viscous-coupling has sucked gear oil from the front differential. The problem of “oilsucking” was noticed at Steyr-Daimler-Puch (SDP) in the late 80s, which is why it was also investigated in a Dissertation.
The remarkable conclusion was the following: At wintery outdoor temperatures, a static vacuum forms in the viscous-coupling due to the high thermal expansion of silicone oil. This vacuum is enhanced during start-up. Because of the vacuum the viscous-coupling sucks in portions of gear oil in the cold-running phase and breaks over time. This problem does not depend on the mileage. In short-distance operation an extreme hardening of the viscous-coupling can already happen after 2000 km (approx. 1243 miles). However the results of this investigation came too late in the summer of 1990. Therefore SDP did not make any further effort in solving the problem.
A simple test setup with a manometer shows the virulence of the issue.
In order to prevent the “oilsucking” caused by the vacuum, the viscous-coupling is filled through a special valve.
This makes it possible to insert a slight static excess pressure into the viscous-coupling. Moreover we can adjust the static pressure to the ambient temperature via the special valve.
Comparing it with the SDP factory adjustment back then, nowadays we need another filling quantity and an adjusted viscosity. Yet with help of a test bench, working out this modification is a solvable task.
Steyr-Daimler-Puch had the durability of the viscous-coupling tested in a Dissertation under specified operating conditions. The investigations showed that the service life of a viscous-coupling depends on the differential speed, the power loss and the height of the Hump-torque. On principle we have to distinguish between two separate service life- categories though: