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Originally Posted by pberku
However, I modified the procedure to reflect what I believe is a more realistic real-life scenario.
When the bms is mounted in the clutch it is FIXED at both ends, so, when heated, being fixed at both ends, it CAN NOT expand along its longitudinal axis. It can only expand upwards (bend).
The stand alone bms on the other hand, not being fixed DOES elongate along its longitudinal axis, as well as bend-up. It is the bending that causes the fan to activate, not elongation, therefore the bms that is affixed at both ends will bend faster, as no energy is wasted elongating it. Consequently, it will activate the clutch at a lower temperature.
So in real life, if you heat up a stand alone bms, the conclusions will be invalid. Now going back to your own testing, there probably was nothing wrong with the bms's themselves.
Phil
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but even though your results “appear” to be contrary to my findings, how do you explain Ritter’s results? He just mounted them on the car and tested the complete assy in the ‘good-old-fashioned-way’.
He had three ‘bad’ ones that didn’t pull in at rad temps of over 110C. That’s the real world test, and they failed. Mine didn't work either when mounted on the car. My tech has never seen one work correctly and he’s been repairing MBs for 30+ years.
So now to your experiment. Let me say that I have a few vfc here at home and I took them out to look carefully at how the bms mounts into the aluminum housing. In both cases, they both have
just under an 1/8” of wiggle room. The bms is NOT pinned horizontally and can move left and right.
But even if it couldn't, it does NOT expand horizontally. It expands via the coefficient of differential expansion (CODE).
Now to the calcs. The coefficient of expansion (COE) of brass and steel is 19 and 13 ppm/deg C respectively. So that means that as temperature is raised from 20C to 100C (80C difference) that the following happens: Brass expands 0.00152” and steel expands 0.00104”. The difference is 0.000480”. The (CODE) is less than 1 mill or 1/1000 of an inch and that is in the direction of the 'steel' laminant.
Even if the expansion was ALL horizontal (which it can’t be) it would be only 1.5 mills!
So the conclusions are:
1) The bms is not fixed horizontally, there’s apx 1/8” clearance end to end to end.
2) The CODE results in less than 1/1000”. The COE of brass yields only 1.5 mill and steel 1.0 mill.
I will repeat your complete vfc “in-the-kettle’ experiment but I don’t expect to see any difference. The bms is NOT captive and and the principle of "BMS" does not depend on the mechanical properties of the "mounting' device.
Implicit with any of these experiments is the fact that the bms need NOT COMPLETELY bend to open/release the clutch. And as jbaj007 pointed out, he shaves the pin about 1mill to cause it to activate with less ‘bending’ than it would require if the pin is full length. The point is that maybe at 88C (assuming it was FULLY bent at 91C) might be enough to engage the clutch.
But we are still with the quandary that 91C still corresponds to over 110C in the radiator to get 91C at the bms.