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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


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