Date:         Fri, 20 Dec 1996 11:18:43 -0800
Sender:       Vanagon Mailing List <vanagon@vanagon.com>
From:         Todd Francis <tbf@pacifier.com>
Subject:      Viscous Coupling Fluid info

Hi all, 
  I just wanted to add to the Viscous coupling discussion. I found a 86
factory service training manual. There is a page devoted to Viscous
fluid. Here is what it says: "Shearing action between the two sets of
plates leads to an expansion of the silicon fluid and a rise in
pressure  inside the sealed viscous coupling. The rise in pressure is
controlled by a small measured quantity of air that is included in the
drum during manufacture before it is sealed. Because of the rapid rise
in temperature and pressure, lock-up occurs very quickly, in about 1/10
second or approximately 1/4 turn of the tire. Power is now transmitted
between the plates with the fluid as a medium. 
 At the point of lock-up, a self regulating effect takes over. As
practically no relative movement takes place between the two sets of
discs, the temperature inside the coupling drops and the presure inside
the coupling drops. Now a level of slip is maintained in accordance with
the speed changes between the front and rear axles.
 THIS UNIQUE SILICON FLUID IS PATENTED and has the ability to increase
its viscosity quickly when it is heated. The fluid's viscosity does
not,however, increase when the temperature decreases."
  Along with this text there is a chart that shows the temperature
Versus the viscosity of the fluid. It shows that the viscosity remains
constantly low (there are no measurments of viscostiy on the chart. it
just says low at the bottom of the chart and high at the top of the
chart.) between -30 degrees C and 100 degrees C. At 100degrees the line
goes almost straight up to high viscosity. The viscosity remains
constantly high to the edge of the chart which I would guess to be 170
degrees C (The chart is not marked for temperaature at the high end).
 There  was talk of being able to use brake fluid. I would doubt brake
fluid has the characteristics shown on this chart. The wanted
characteristics of brake fluid would be to not change viscosity when
heated or cooled. And especially not to have a radicle change of
viscosity over a temperature range of a few degrees like the viscous
fluid chart shows.
 Even if we could get viscous fluid how would we determine how much to
put in. How criticle is the air pocket volume in the coupling? VW says "
a small measured quanity of air".  I suppose you could take one apart
and measurd how much fluid is in it. Is the air pressurized when it is
put into the unit? Again I suppose you could capture the air that comes
out and measure it. (might be kinda tough but it could be done). It
looks to me that this job is out of the realm of the back yard mechanic.
I could imagine someone could spend some time researching the problem
and specialize in rebuilding these parts here in the states. Interesting
thought. 

Todd Francis
91 Tristar Syncro       recent bumper sticker sighting        
86 Westy Syncro             "Jail to the chief"