bigBADbenny wrote:Bc, would you consider a refurbed uprated fluid diff to have a shorter service life than stock refurbed?
Eg in the 10-15kg/mm region, not quite under steering monster eg 20kg/mm
First principles evaluation.....
Let's say the there were a new set of tyres on the front of a Spec-B and a worn set on the rear.
Rolling diameter of a new set of 215/45/18 tyres is 650.7mm Tread depth is 8.3mm
Rear tyres are at legal minimum depth of 1.5mm.
Front rolling diameter 650.7mm rear rolling diameter 637.1mm
Vehicle is doing 110kph on freeway.
At that speed the average tyre rotational speed is 906.3 RPM
The centre diff case is rotating at 3535 RPM
The front output shaft of the Centre diff is turning slower and the rear output shaft is turning faster (as the rear tyres are worn)
The rear output shaft is turning at 3572 RPM.
The inner and outer viscous clutch plates are rotating at 38 rpm relative to each other. Or, 3.95 radians per Second
The standard diff will transmit about 40N.m of torque before slipping.
So the power (or heat) generated within the Centre Differential by the viscous coupling slipping due to differential action is
P = T x Omega
= 40 x 3.95 or about 160 watts.
(Note, having just rebuilt the Centre Differential out of Flat6Estate's car, the metal parts were showing signs of blue oxide in the thrust washer area, so things were getting warm?)
160 watts of heat needs to be dissipated by the centre differential assembly into the gearbox oil that surrounds it, remembering that the gearbox oil is also being heated up by the other rotating losses within the gearbox.
Increasing the slip stiffness of the centre differential to 10 to 15 Kg.F (100 to 15o N.m) will increase the heat dissipation of the centre differential proportionally.
Presently, I do not have knowledge of what is the critical temperature that the silicone fluid must reach to commence degradation for the standard fluid or the replacement fluid.
We can see that the driving force is really the mismatch of rotational speeds across the differential.
If a higher force Viscous Coupling Centre Differential were to be used, one should take greater maintenance of the tyres to ensure the same brand and type of tyres are used and that the tyre pressures are set correctly.
Minimising tyre mismatch would be the key to successfully using a higer specification centre differential.
Cheers!