From: JP Rodkey [sidnoreo@wi.rr.com]
Sent: Wednesday, December 11, 2002 9:57 AM
To: 928
Subject: [928] Fan Clutch Madness - OR - How Does That Thing Work?
(Tedious)

I took the rest of the fan clutch apart to have a look-see.   Makes you
wonder who came up with this design.

DISCLAIMER:  This is my understanding of operation based on observation
only.  Corrections are certainly welcome.

There are four main components:
1.  housing
2.  drive shaft
3.  driven disc
4.  valve

The housing is cast aluminum and has ribs on the inside in order to engage
the silicon fluid properly.  The housing half on the engine side has a ring
of friction material applied to it. There is a steel disc sandwiched inside
the housing covers and it's driven by the input shaft.  This disc also has
ribs stamped onto it's surface.   There is a hole about 1/4" diameter on the
outer edge of the disc.   There is another hole about halfway out the
radius.   And, finally, there is a spring steel valve (more like a flap)
that covers the hole.

Here's how I think it works:  The normal condition (cold) allows more slip
of the housing as the shaft is turning.   The thermal strip on the front of
the housing rests against a pin, which in turn keeps pressure on the spring
steel, which covers the hole (the hole that's halfway up the radius) in the
disc.  While cold, centrifugal force pushes the silicon outward and helps
spin the housing thereby turning the fan that's bolted to it.

When the temperature rises to the appropriate level, the bimetal on the
front of the clutch bends and lets the pin in the center of the hub move
forward.   This motion frees the spring steel valve to open.   Now, the
silicon that's forced outward inside the housing has an escape route through
the second (the one halfway up the radius) hole in the disc - and this
produces a fluid flow which works with the vanes (ribs) inside the housing
to essentially lock up the clutch.  I believe the friction material inside
the housing is to make sure the spin can overcome the slipperyness of the
silicon.

If the silicon level falls too low inside the housing, you do not get the
fluid flow required to push the steel disc inside the housing against the
friction material in order to lock the clutch.

When the bimetal cools and straightens, it pushed the pin in the center of
the hub which in turns covers the hole in the disc and stops fluid flow
which eases pressure and no longer holds the steel disc against the friction
material.

I hope this description makes sense.   It's hard to describe what's going on
inside without actually seeing a disassembled unit or a drawing.


JP Rodkey
79 Euro
88 S4




From: Jim Bailey [mailto:jim@928intl.com]
Sent: Wednesday, December 11, 2002 6:23 PM
To: 928
Subject: [928] RE: Fan Clutch Madness - OR - How Does That Thing Work?
(Tedious)


According to Porsche..... " There is no mechanical connection between the
drive section and the driven section. The torque is transmitted only by the
viscous silicon oil in the annular gap on both sides of the drive disc. This
means that the fan speed will always be lower than the drive speed." The
working chamber is near the shaft and the pin , arm , bimetal tab all
control how much fluid is in the working chamber . The silicon has the same
viscosity hot or cold but the amount directed / retained in the "working
chamber" varies as the pin is moved in and out . Once enough fluid has
leaked out of the "clutch" it can no longer transmit sufficient torque but
it never can spin over about 3,400 rpm .

Jim Bailey
928 International
Jim@928intl.com
www.928intl.com
(714)632-9288