After a VC fails closed, it then fails open.

We suspected this issue for a long time, but now we found evidence to support our theory.
But let’s tell the story from the beginning.
A customer brought us his old Viscous Coupling (VC) in an unknown condition to our workshop, After the first visual inspection, I suggested to the customer that we could actually save the time on the test-rig, because fragments of the upper X-ring were already hanging out of the lubrication hole of the inner hub (see attached photo). I assured him that his VC had failed open (no drive) and would only reach about 50 nm friction moment.


But when the test-run started, we were all very surprised! The Viscous Coupling was not failed open, but closed (no slip at all)! It had sucked oil inside.

After testing all VCs from my own stock with obvious X-ring damage, I found one more VC in a similar condition.

This Visco clutch shows an advanced stage of failing open. The lowered moment indicates that some silicone oil has already leaked out and the Visco will soon completely be failed open.


Our Theory:

The constant hump pressure (>10 bar) in the oil-sucked Viscous Coupling causes the rubber of the X-ring to creep and slowly be pushed into the gap. As a result, the X-ring can no longer create a seal.

To cut a long story short: After a VC fails closed, it then fails open.

Testing of a “baking oven VC” #2

After having tested a so called baking oven VC months ago, I decided to test another one due to verification resasons.

I got the baking oven VC from a german guy who cooked it at 120°C with a 60K silicon fluid.

60K Visco
60K “baking oven VC”

60K VC Testing
first testrun

After the first minutes of the testrun I was absolutely sure, that the VC is missing a lot of fluid, because the chart was much to low for a high 60K viscosity. During the testrun, the air in the VC is mixing with the silion oil as tinny bubbles – which looks like super sparkling mineral water. These tinny bubbles in the fluid reduce the shear force of the silcone oil:

more air = more bubbles = lower effektive viscosity

At a temperature of 135°C the teststand stops automatically for safety resasons. After a short cooling break, we did another testrun at a higher temperature of 155°C – that’s the maximum temperature that should not do any harm to the X-ring sealings. But there was not any signs for a Hump.

Testlauf #1 (135°C)
Testrun#1 (135°C)

Teslauf #2 (155°c)
Testrun #2 (155°c)

I refilled the baking oven VC with 12,5K silicon oil to find out the amount of missing fluid. All in all 20,3g (!) fluid was necessary to get the VC in the correct range of tolerance. Two gram silicon oil increases the hump-temperature in 8°C. This means, that the baking oven VC would have had a theoretical hump-temperature of 200°C. But that’s just theortical, because practical the X-rings would have been damaged at these high temperatures.

20,3g Silikonöl nachgefüllt
+20,3g Silikonöl

Testlauf #3
Testlauf #3


It’s not always that simple, even if it seems so. Some thoughts why the baking temperature can’t be identical with the hump temperature can be found in the last section of the first baking oven test we did months ago.