Under-loaded and cold ball-bearing extremely high friction?
Under-loaded and cold ball-bearing extremely high friction?
(OP)
We're trying to find the cause of very high losses in a custom high-speed brushless motor. The losses occur at light/no load with a cold motor. As soon as the motor heats up to normal operating temperature (approx 80 deg case) all is good. The motor's rated speed is 12000 RPM (at 1kW).
Temperature measurements seem to point at the oversized front bearing we're using to make these motors survive the massive amount of vibration from the (unavoidably unbalanced) tool it's swinging around. The front bearing is the first thing that heats up according to our sensors, so either the bearing or the shaft is getting hot quickly.
The bearing is an NSK 6304 and it is under-loaded running the machine at working speed but unloaded. The static friction of the bearings we verified to be very close to the ideal as calculated by SKF's (web) prediction tool even when cold. The hot running friction seems pretty ok too, SKF's tool predicts about 11-45W of losses depending on load at given the power consumption that looks not too far off. But a 'cold start' we're losing about 300-400W somewhere for the first minute or so... Can a skidding / under-loaded bearing have 10 to 40 times the friction torque of it's cold 'starting' torque?
Temperature measurements seem to point at the oversized front bearing we're using to make these motors survive the massive amount of vibration from the (unavoidably unbalanced) tool it's swinging around. The front bearing is the first thing that heats up according to our sensors, so either the bearing or the shaft is getting hot quickly.
The bearing is an NSK 6304 and it is under-loaded running the machine at working speed but unloaded. The static friction of the bearings we verified to be very close to the ideal as calculated by SKF's (web) prediction tool even when cold. The hot running friction seems pretty ok too, SKF's tool predicts about 11-45W of losses depending on load at given the power consumption that looks not too far off. But a 'cold start' we're losing about 300-400W somewhere for the first minute or so... Can a skidding / under-loaded bearing have 10 to 40 times the friction torque of it's cold 'starting' torque?
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RE: Under-loaded and cold ball-bearing extremely high friction?
Johnny Pellin
RE: Under-loaded and cold ball-bearing extremely high friction?
Both bearings are press-fit into an aluminium flange. The much smaller rear bearing fixes the shaft axially (nut on shaft clamping the inner ring). The front bearing is free floating on the shaft (can move it by hand when cold, but not enough to spin the shaft in the bearing).
> Some motors use wave springs to place a light axial load on the bearings even when cold.
This motor doesn't use any wave springs (in an attempt to reduce running friction and save battery on extended runs).
> How is the bearing lubricated?
NSK standard grease.
> Is this a single speed motor or is it variable speed? What speed are you running at start-up when the high load condition is noted?
It's meant to run at a fixed speed (with varying load). The exact speed is a little dependent on tool specifics, but say 12000-15000 RPM. The controller is set to run that regardless of load, including at startup.
RE: Under-loaded and cold ball-bearing extremely high friction?
I think your bearing is experiencing radial overload from a combination of the outer race interference, the dilation of the inner race at the high speeds and the rapid thermal expansion of the bearing balls at high start-up speed. It becomes a runaway process. The overload generates heat which causes more overload which generates more heat. I would reduce the interference fit on the OD as much as possible and use the controller to ramp up to full speed gradually.
Johnny Pellin
RE: Under-loaded and cold ball-bearing extremely high friction?
RE: Under-loaded and cold ball-bearing extremely high friction?
RE: Under-loaded and cold ball-bearing extremely high friction?
https://www.skf.com/us/products/rolling-bearings/p...
There are a few things to consider based on application. If high vibration and load direction reversal are requiring an interference fit of the outer race you really should consider making the housing material out of steel or other material with similar CoTE so that your interference doesn't change so much over the operating temperature range. Also consider using a retaining compound instead of an interference fit.
Angular contact bearings can operate with preload and will be more tolerant in your application.
Use heaters to preheat the motor prior to operation.
RE: Under-loaded and cold ball-bearing extremely high friction?
If internal clearance is a problem, maybe one option to explore is larger internal clearance class bearing. (C3 instead of C0 or C4 instead of C3).
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(2B)+(2B)' ?
RE: Under-loaded and cold ball-bearing extremely high friction?