Date: Fri, 21 Feb 1997 12:58:53 -0400 (AST)
Sender: Vanagon Mailing List <vanagon@vanagon.com>
From: smitht@unb.ca (Tim Smith)
Subject: <Syncro> VC #4: The Real Story
OK, here's how they say it works.......
some data:
fluid: silicone polymer (~paste) called di-methyl-silicone or 'siloxane'
source: Wacker Chemical and Dow-Corning
VC details: sealed container with 24 discs (12 input/12 output) which
operates at up to 100bar (1500PSI) when locked. A 5-10% air bubble MUST be
included in the filling. Locking occurs when the siloxane is heated by
slip/shear.
What happens next is weird though folks...
The heated siloxane expands, but like any oil it gets THINNER not thicker
with heat. Once the fluid has expanded enough to completely fill the VC,
and the air bubble has been pressed/dissolved into the fluid the pressure
rises very rapidly inside the VC. The rotating discs have 32 slots in one
set, and 20 holes in the other set, in an alternating stack of 24 (i think)
Because of the slots/holes the fluid between the plates is being churned up
a bit while rotating and since nothing is perfectly manufactured/symmetrical
there is a tendency for the fluid to move to one side or the other of the
slotted discs. This thicker fluid side builds up pressure and drives the
slotted disc tighter and tighter against the adjacent hole disc. The
slotted discs are free to slide on the shaft, the holed discs are held with
spacers. The VC then locks up due to 'metal on metal' contact, similar to
clutch pack in an autotrans/motorcycle.
The fluid is no longer transmitting torque!! So it cools and slips again
and heats and locks. But this all happens so smoothly it is essentially
constant. This is one of 2 ways they are felt to lock, the other idea is
that the dissolved airbubbles are not uniformly distributed, hence the fluid
shear heats non-uniformly and that causes the discs to shift and lock. Told
you it was all wierd!
The transition to locked happens fast 5RPM diff will do it, 1/4 to 1/2 turn
of the tire!! It is called the 'hump' transition and the rate at which it
happens is directly related to the air bubble volume used in filling. For a
rapid (off-road) locking use less air. The ratio of initial fluid shear
torque (prior to lockup) vs. locked torque was 2 - 4:1 in early designs.
Jeep has gotten 8.8:1 in Jeep ZJ transfer case/centre diff model NV249. (1992)
The 'discovery' of how VCs work was seemingly by chance, since they were
initally thought/designed to operate on fluid shear only, acting partly as
vibration/shock dampers, in 1950s. When VAG prototypes (1980s) were
disassembled it was noted that the faces of the discs would have wear marks
on one side and none on the other, and that the wear faces were
matched/paired up for corresponding hole/slot discs. Plexiglass models were
made and observed, then steel models with plexi windows. The 'clutch pack'
action was visible.
What does this imply for us poor folks...
1) the discs are high precision ground and surface hardened components
2) if the discs warp due to heat the VC is dead
3) if siloxane is overheated it will change viscosity permanently and the VC
is dead
4) if the seals die, and prevent pressure buildup and the VC is dead
5) if the front diff loses oil then the hot VC can't dump its heat and
unlock, so 3) happens (and the VC is...)
6) contamination of the fluid by oil or water will kill the coupling, it
operates above the boiling point of water when locking, vapour probs and oil
buggers the metal on metal lockup (as would dirt)
7) the basic torque transmission of the coupling WITHOUT lockup is 30n-m,
due to seal friction alone, so verifying a dead VC by jacking one side and
twisting a wheel while holding the other one is not proof of a good VC
8) these units may be refillable and reusable if the discs are 'unworn' and
unwarped, if we can get the fluid (looking good) and if we can get the seals
(??)
Tough act to copy, will get back to Syncronauts/others as I get more info.
Guy from 3M just called, I sent him away with the name 'siloxane' so he's
looking for their product. Dow is looking good (for a change?!) for the
fluid. 55 gal drum was smallest normal size, may be able to wangle enough
for a couple or 3 VCs??. No idea of costs either. Sorry for the bandwidth
for all other vanagon folks, but hell, I'm having fun. :) Tim