Hi  Syncronauts and   all you  normal  Vanagoneers  (if  you're at   all
interested in VCs :-)

On Thu, 1999-04-22 08:02:49 -0700, Davidson wrote:

> Locks up at 100 degrees???? !!!!!!

Wolfgang probably  meant degees centigrade.   But it seems the theory of
how viscous couplings  work is either not very  well understood or there
are  several  theories which  contradict each  other.  This is  a common
practice nowadays when some company is filing a patent, because it makes
it more cost intensive for any competing company to disregard the patent
protection (or to "reinvent" something very  similar but still different
enough and thus not  patent protected).  List member  Tim Smith (are you
listening, Tim?  :-) claims to have two  papers describing two  of these
contradicting  theories.  Ages ago he promised  to scan them and to make
them available to the list,  but his scanner is  still seeming to refuse
to work.

One well known fact about viscous couplings  is that they are containing
a series of  perforated metal disks which are  not contacting each other
and which are  alternatingly attached  to the  input and output  shafts.
Another fact is   that there is some  special  viscous liquid (silicone)
floating around and between these disks.   These two facts can easily be
proven by opening an old viscous coupling (kids,  don't try that at home
... you  better just believe  me :-).  The  third fact is that a viscous
coupling realy  works,  and this can  be  proven by  driving a Syncro in
difficult terrain :-).

Now, on to the   theories.  On this  list  (as well as elsewhere)  there
basically are circulating three theories:

1-st theory:   Differing rotational velocities  on the  input and output
shafts causes  friction in the silicone fluid  and thus  heat.  The heat
causes the silicone to expand which in turn presses the metal disks upon
each other and this direct contact causes the viscous coupling to lock.

In my opinion  this is  clearly  bogus.  Pressurising any liquid   which
contains  submerged  metal  plates   will not force   these  plates into
contact.   This, too,  is a  simple  fact from simple physics  which can
easily been proven.  What's more, these disks are bare metal, no pads or
such, as in a normal clutch.  If the friction caused by bare metal being
pressed on bare metal would be essential for the  operation of a viscous
coupling, they would do a poor job.  Within very short  a time the metal
surfaces would be polished and friction would be reduced to almost zero.

2-nd  theory:  Differing rotational  velocities  on the input and output
shafts causes  friction in the silicone fluid  and thus heat.   The heat
causes the  silicone to become  stiff thus causing the perforated  disks
(now the perforation starts to make  sense, see?)  to rotate at the same
speed.

3-rd theory:   Differing rotational  velocities on  the input and output
shafts causes shearing of the viscous fluid  and it's that shearing (and
not the heat)  which causes the silicone to   become stiff and thus  the
disks to rotate   at  the same   speed.  Of course,   before the viscous
coupling locks up  totally, there will be some  friction, and  thus some
heat.

So the main  question to tell  these two theories  apart is whether this
heat is the cause for  the locking of  the  viscous coupling or just  an
inevitable technical by-product.  If you  ask me,  I simply don't  know.
But currently I would put my money on the third theory, because I really
can't see how quite  a few cubic   centimeters of silicone  fluid  could
change their temperature in such a rapid an uniform way as the operation
of a  viscous coupling does  indeed require  (we're talking about  split
second timing here).  Perhaps  I will know  more when Tim will ever  get
his scanner ready (hint, hint :-)

Hope this clarified a few things.

Sincerely
 Rainer

'89 Caravelle GL Syncro 16"

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