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Bearing friction torque

Posted by cristian 
Bearing friction torque
October 06, 2015 04:54PM
I have some cheap Chinese ball bearings that I would like to use for a corexy system. While they seem to roll smoothly, I would like to get an idea of their friction torque under different loads.

Has anybody tried to measure bearings friction "at home" already? I have a couple of ideas about how to do it, but any previous experience would be welcome.
Re: Bearing friction torque
October 06, 2015 06:40PM
... you can wind a string around, measure the pulling force ... and calculate the effective g-cm value with the radius ...

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Re: Bearing friction torque
October 07, 2015 01:43AM
... you can wind a string around, measure the pulling force ... and calculate the effective g-cm value with the radius ...

As it is written, this is what I would do to measure the friction in the absence of load. Doing it under load requires probably a more complicated system (something like this for example, where the friction is given by the difference of M1 and M2) but maybe somebody has simpler ideas in mind?
Re: Bearing friction torque
October 16, 2015 02:58PM
Okay, I did my homework. I set up a system like this

and I tried to measure static friction torque. It turned out to be quite a difficult thing to measure "at home" on 608 ball bearings, because of all the possible alignment errors between the bearings and the line as well as the "noise" on the measure, given by the non homogeneous static friction in relation to the rotation angle of the bearings.

The measurements I made are enough to get an order of magnitude of bearings friction torque, but unfortunately they are not enough to extrapolate a full graph in relation to the load: after spending an afternoon playing with bearings, line, cans of water and a pipette, my girlfriend started to think that I went insane so I had to stop.

The experiment was simple: two weights M1 and M2 (water cans) connected with the line (0.5mm diameter) over two bearings (608zz, 22mm diameter), initially with M1 == M2 and then M1 was increased drop of water by drop of water until the system started to move. Reality is of course not as simple as that, since the line is partially elastic and not exactly at the center of the bearings, two bearings are involved, weights oscillate, etc. In brief, these are the measurements (in gram-force):
Initial mass for M1 and M2              Final mass for M1          Difference
  59                                            65                       < 6
1062                                          1109                        47
2063                                          2103                        40
2063                                          2115                        52
4064                                          4137                        73
4064                                          4122                        58
4064                                          4132                        68
The load on the bearings should be approximately given by L = (M1 + M2) / sqrt(2). Since M1 ~= M2, we have L ~= M2 * sqrt(2).

Given the error in the measurement and the intrinsic noise of the system, standard deviation will be considerable (the measurements are so few that standard deviation cannot be calculated for most loads). The relation between friction torque and load is probably not even linear in such a wide range. Excluding the first measurement where the friction was too low to be measured "accurately" and the load was not interesting for practical purposes, the worst ratio friction/load case seems given by the second measurement, with L =~ 1500 and the friction torque (per bearing) being T ~= (47 gram-force / 2) * (22 mm / 2) ~= 2.5 N * mm. If the bearings are used "naked", without changing their diameter (and the thickness of the line or belts is considered as negligible), the resulting friction is approximately 1.5% of the applied load. Looking at the measurements, it seems that friction increases with load (as it should be) but the ratio friction/load improves overall.

Given that I paid those bearings about 0.30€ each, this is a great result. However in a system with a lot of bearings (for example a corexy where you have at least 8 bearings) and long belts, this is probably not so negligible, because the load on the bearings depends on the tension of the belts which may be considerable.
What do you think?

Next step is measuring belt and line elongation as a function of load.
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