Stephen, note that although some stainless steels are not magnetic, others are. The 300 series aren't, the 400 series are.
Also it seems to me that the ball on track system, which we agreed should be equivalent, might be more convenient than an actual BB for the magnetic vs not magnetic tests you're planning. Michel 2009/9/1, Stephen A. Lawrence <[email protected]>: > > > Horace Heffner wrote: >> >> On Sep 1, 2009, at 10:51 AM, Stephen A. Lawrence wrote: >> >>> >>> >>> Michael Foster wrote: >>>> I haven't been following this thread very closely, so if my input is >>>> repetitive, shoot me. Stephen's idea seems like a good test of >>>> whether this phenomenon is thermally or magnetically driven. Has >>>> anyone tried this with non-magnetic bearings? >>>> >>> >>> Horace did, and as I recall he got a null result. His reason for trying >>> it was specifically to test whether it's a magnetic or EM effect, rather >>> than a thermal effect. Fly in the ointment is that stainless does have >>> rather different thermal (and other) properties from plain steel which >>> could have accounted for the failure with stainless. >> >> The effect is clearly magnetic. The current drop through the current >> sense resistor when running vs not running, for the magnetic bearings, >> clearly shows this. There is a back emf. > > Indeed; I'm certainly convinced. It was a "chronic skeptic" I know to > whom I mentioned the stainless/stainable result who started going on > about the thermal properties of stainless, and how it seemed to him that > it still *could* *be* a thermal effect. > > The bit with the iron wire around the bearing, if it did anything > useful, would tend to show that it's the material of the bearing *race* > which matters, rather than the material of the balls. And that would be > nice to know. > > Come to think of it, if one could obtain two identically sized bearings, > one stainless and one "stainable", I wonder if it would be > straightforward to pop them apart and swap the balls? That might be a > quicker, easier, and more conclusive way of arriving at the same result. > > >> >> There is *no* sign of torque with the stainless bearings. If fact >> they brake when current is applied, probably due to arcing. The >> braking is somewhat reduced when graphite powder lubricant is used, >> but there is still no sign of torque whatsoever for the stainless >> bearings. It is apparent there is a similar braking action for the >> magnetic bearings, but their torque clearly overcomes the drag >> increase due to arcing. >> >> I have a specific series of follow-up experiments planned, but am >> doing other things at the moment. >> >> Best regards, >> >> Horace Heffner >> http://www.mtaonline.net/~hheffner/ >> >> >> >> > >
