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Ball bearing drag increase in magnetic field


Danijel Gorupec

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Some two yers ago I was designing a small, cheap electromagnetic clutch. I noticed that magnetic field lines would cross (radially) through a ball bearing, but I didn't expect much problems with that. However, it proved that the magnetic field increased the ball bearing drag much more than I expected. I had to redesign the clutch to include an aluminum ring around the ball bearing to provide some magnetic insulation (this increased complexity of the clutch considerably)... Until now I don't understand why did magnetic field increased the drag that much - forces involved by magnetic field were well under the ball bearing limit (normal mechanical forces of the same magnitude, radial or axial, would not increase the drag noticeably).... What do you think?

 

I have a gut feeling that if I place a steel plate in a magnetic field (parpendicular to the plate) and if then I try to roll a steel ball over that plate, the ball would roll with increased drag. But I have no real idea way would that be. Maybe it is something basic and obvious...

 

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How does it happening? Is it enough that stell ball moves through a magnetic field, or must it also rotate to generate the drag?

I did not have time at the moment to experiment much, but it seemed as if the drag is present at very low speeds and does not depend on speed - however, I might be wrong on that.

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Motion between a conductor such as the ball bearing and a magnetic field induces eddy currents in the object, which creates a magnetic field (this is an application of Lenz's law). The two magnetic fields oppose each other and provide damping. It's the same concept that you use in a hybrid or electric car for regenerative braking, where the energy from the induced current is stored.

 

http://en.wikipedia.org/wiki/Eddy_current_brake

 

As the videos Sensei posted show, the effect is quite dramatic.

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As the videos Sensei posted show, the effect is quite dramatic.

 

I have personally performed test with 2 meter long Aluminum straight pipe, 1 cm diameter.

And couple (quantity does matter!) neodymium cylindrical magnets, 8 mm diameter, 4 mm height each.

With 5-6 such magnets, I was receiving steady 12-13 seconds time spend on moving to the other side of pipe (pipe vertically of course). That's 19-20 times longer than free fall.

Free fall 2 meters takes ~0.64 seconds normally (and that's delay when we pass anything but these magnets through my pipe).

 

I will make video when will buy new Full HD camera..

 

Results (today performed again to be sure):

1 magnet: ~16 seconds

2 magnets: ~21 seconds

5 magnets: ~12 seconds

Edited by Sensei
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Thanks...

 

I guess, because both of you pointed to the same thing, that this is the answer. I knew about this effect, but I was probably too quick to dismiss it, expecting that it would be negligible... What confused me is the fact that half of the clutch is just a piece of metal that is rotating inside the magnetic field. The drag generated is noticable and I expected it. However this relatively small ball bearing immersed into a relatively lower magnetic field generated several times more drag than the rest of the clutch - that is why I got under impression that maybe something else is happening.

 

I guess that if I run the numbers I would get the result that I observed. In any case, it was surprising to me (I am thinking... balls inside bearing not only circulate around, but are also rotating around themselves - maybe this somehow increases the effect).

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I expect the hysteresis to create a worse drag than eddy currents do, as a gut feeling. Roll bearing steel is ferromagnetic and has an important remanence, making hysteresis losses strong.

 

A simple test if still possible: hysteresis loss would create a torque at small speed also, while eddy currents create a torque proportional to the speed (it would stop increasing under conditions improbable here).

 

Though, eddy currents are nasty in ferromagnetic materials, because the induction can be high and the permeability increases the AC resistance.

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Hi Enthalpy... possibly yes. I remember my surprise when I discovered the serious drag at ridiculously low speeds. The clutch was intended to run at only 60RPM so I didn't expect much of eddy-current generated drag.

 

If you say that hysteresis drag is predominant at lower speeds (I did not know about this) then maybe this is the reason. I am only not sure what hysteresis exactly do you have in mind (the clutch was DC powered). I suppose that you are referring to the fact that balls rotate and this is what creates the changeable magnetic field on them?

 

Is this hysteresis effect essentially the same thing as the following effect I have in mind: When a steel ball is immersed into a magnetic field, it gets magnetized in the direction of that magnetic field. If the ball is then quickly rotated for a small angle, its magnetization vector also gets rotated because it takes some short time (energy?) for magnetization to realign. Therefore for this short time the steel ball will generate torque trying to return itself to the original position.... Is this effect I am describing real, and if yes, does it have a name? Hysteresis?

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Yes, that's it. Ferromagnetic materials can be "hard" or "soft" (historically observed at hard and soft steel, and you bearing steel is very hard), meaning that is takes some extra field, hence energy, to reverse their induction, and this is nearly independent of time.

 

Because hysteresis losses (in joule per magnetization cycle) don't depend on the frequency, but eddy currents create power losses proportional to F2 (at mechanical and mains frequency domain), hysteresis is the more probable at 60rpm.

 

Yes, you can understand it as torque in a rolling ball (in addition to understanding it as energy), where the ball's magnetization isn't aligned with the external field. You might perhaps feel with your hands a very small angle where the bearing has a spring behaviour before acting as a brake; this won't happen with eddy currents. And the torque will be felt even under 60rpm.

 

The ball bearing has a remarkable shape where the contact between the balls and the races has a small area that concentrates the magnetic flux to a high induction, and extremely hard steel that is a bit difficult to demagnetize, though it's not a good permanent magnet neither.

 

Hey, if you ilke such bizarre magnetic setups, I believe Branly's coherer still needs an explanation 130 years after it was introduced!

http://en.wikipedia.org/wiki/Coherer

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