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Old 01-08-2013, 09:14 AM   #9 (permalink)
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Join Date: Jan 2010

I did some basic calcs to see what was happening here.

Standard spec for a 1012 oneway bearing lists a torque rating of 5.3Nm

If we assume a powerful motor like the Scorpion 4525 Ultimate/Limited the max continuous power is about 4500W. After losses if we assume an electrical and mechanical efficiency of 85% we get about 3800W at the bearing.

With a 2100rpm headspeed after reduction, the secondary shaft with the bearing is actually spinning at 2100x68/20=7140rpm


So we are just under the maximum torque handling capability of the bearing, 5 vs 5.3Nm

This is of course assuming a very loaded condition where the motor is putting out its maximum continuous rated output. With a 4 minute flight using up 4Ah of battery at an average voltage of 3.7v/cell you get 178Wh/(4min/60min/h)=2670W average power consumption. Again with 85% efficiency this is about 2270W average power transmitted through the bearing.
So average load on the bearing is more in tune with 3Nm in an aggressive flight

In a crash or in transients the peak loading of the motor can spike to 10,000W
Again with 85% efficiency this is 8500W and this can translate to 11.4Nm, far exceeding the spec of the bearing.

Important: Just because the spec of the bearing can be exceeded does not mean a catastrophic failure of the bearing. It just means the bearing can not hold the torque.
When this happens, the shaft slips momentarilty until it catches again. The more times the shaft slips on the bearing, the more wear and tear and eventual failure.
This is why you will see scoring on an unhardened shaft or marring on a hardened one.
The outer diameter tolerance of the shaft is also super critical to proper torque holding.
SAB Heli Division Rotorquest
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