Borg Warner viscous, Care & Feeding

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Originally Posted by isuzurover

From the rheology paper it would seem that this statement is completely incorrect.
Which holds with physics - liquids decrease in viscosity when heated, gases increase.

So your list would more correctly be:
Shear increases viscosity of the silicone, however this is countered by a decrease in viscosity with heat, however with enough heat then you get enough of a pressure increase inside the VC to induce the hump.

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Yup, you're totally correct - I dug out a dog eared copy of Hilliers "Fundamentals of Vehicle Technology" and I quote" "the combination of an increase in air pressure due to fluid expansion and a drop in fluid pressure (between plates) causes the plates to be pushed together. This causes metal to metal contact and at this point(the hump point) the coupling temporarily departs from it's viscous operation"
The VC in the BW is very stiff, almost locked anyway in viscous mode when static. I haven't taken a BW to bits, but it's worth noting that some VC have plates that are spaced ie don't slide. Therefore don't know whether thats the case with this unit.

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Originally Posted by superquag

POD,
My main concern is to ensure the VC replacement is going to enjoy a long and stress-free life !
Maybe some sort of cooling system for the transfer fluid, or even just increasing its volume would make a differance...

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superquag,
Probably stating the obvious but - and it would appear to involve not much engineering - If you routed the oil pick pipe out of the casing and through a small oil cooler (something like a motorbike oil cooler) mounted on the Transfer case and back in?
The pump's not high cap or high pressure so something simple with minimal pumping losses might work.....

So it looks to me that in conclusion we could answer the original question from all the data.

1 Avoid different sized tyres FtoR.

2 Avoid front wheelspin, to ensure the VC is heated as little as possible. My guess is that this would include avoiding dry sand as much as possible. My TC has a temp sensor which has never come on but LR must have put it there for a reason, as it is only in 3.9 versions of RRC.

3 I had to get this in. Change the oil very frequently to avoid wear on the output shaft 24Kk being mentioned under hard use and use synthetic to run the case as cool as possible. Maybe marginal for the VC but positive for the TC.
Regards Philip A

Page 6 of this pdf is useful (specifically the viscosity and shear sections)

http://www.silicone.jp/e/catalog/pdf/fluid_us.pdf

Silicone fluids show a much slower decrease in viscosity with increasing temperature.

I am just shocked that one of the Ashcrofts would be (partially) wrong about something!!!

Yup - I think Phillip has summed it up. Vindicates his point that well lubed is well cool.

Izusurover and I shall retire our text books.
Those who where mislead that there was no centre diff in a BW transfer box are now corrected.
Superquag can go buy the right size tyres and a s/hand oil cooler off a Harley.

But I might have a chat with the missus about silicone.....:)

In all fairness Isuzu, Ashcrofts are the experts in cog & chain gearboxes, and as the VC is not a fixable item ..... (nor is it a "real" gearbox part!) :eek:

I like the idea of a simple oil cooler, - had missed the existance of the pump which makes it even easier. (Was thinking of how to use natural convection or external pump, - more to go wrong)

I've heard that synthetic oils run cooler/more slippery so that makes sense. As does the idea that the VC relies on the fluid to moderate and dissipate the heat it generates... - Changing it more often than specified can't hurt either.

PhilipA, your second point hit the painful spot.... Past history of my Black Hole is the last 70K+ involved a LOT of beach & sand-dune work. I can vouch for that in the amount of fine white stuff still inside every nook and cranny.! And switches:mad:
Over here we don't have mud, clay and heavy soils such as is common on the Eastern End. We have SAND, coarse, fine, all of it dry and rather slippery. If its wet sand, thats because the ocean is on top!

So, thank you to all for clarifying the Cause of Death, Bloody beach sand!

James in Gosnells.

The EH, '95 Vogue SE, with *working* EAS & cruise control, stuffed VC... and still don't like the heavy steering despite being RWD for now...

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Originally Posted by Hobbes

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But I might have a chat with the missus about silicone.....:)

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... Brave man... shall we send flowers to the funeral ?:wasntme:

Yes - it might get a bit vis(c)ous..........:eek: