Magnetic bearings

[Johnny Pneumatic]

Can anyone tell if this: http://www.top-flite.com/accys/topq5700.html

Is suspended from only one side, or both? I've looked for other photos, ones showing both sides at the same time, but it's been fruitless.

If it's not double sided in its levitation, is there a way I could make such; without needing a computer to control active electromagnetic bearings? Two strong NIB magnets is what I want to use to support a free spinning shaft. The reason? Secret.

 

[Ririon]

Can anyone tell if this: http://www.top-flite.com/accys/topq5700.html

Is suspended from only one side, or both? I've looked for other photos, ones showing both sides at the same time, but it's been fruitless.

If it's not double sided in its levitation, is there a way I could make such; without needing a computer to control active electromagnetic bearings? Two strong NIB magnets is what I want to use to support a free spinning shaft. The reason? Secret.

My guess is that this device is "single sided". You could probably do it double sided with superconductors and magnets. Read up on the Meissner effect.

Good luck! PM me the secret so i can buy some shares before you go public. (Kidding, broke...)

Ririon

 

[bruto]

It is double-ended. There's no practical way a single magnet could hold that shaft from one end. It would immediately pull the shaft to itself, and it would never hold the shaft horizontal. It's just a pointed shaft mounted through the prop with two fairly powerful, and well matched, magnets adjusted so that the points of the shaft almost touch the magnets. Remember that this is a device for balancing model plane props, which are pretty light. An ingenious static balancing device, but I think for any serious rotation it would come apart from even the slightest imbalance, and there could be no load on the rotating shaft, so it couldn't do much work. Static balancing does not require that the prop or wheel be spun.

Make sure you click on the product review, which shows more detail, and also notes that the device is sensitive to drafts.

 

[Johnny Pneumatic]

It is double-ended. There's no practical way a single magnet could hold that shaft from one end. It would immediately pull the shaft to itself, and it would never hold the shaft horizontal. It's just a pointed shaft mounted through the prop with two fairly powerful, and well matched, magnets adjusted so that the points of the shaft almost touch the magnets. Remember that this is a device for balancing model plane props, which are pretty light. An ingenious static balancing device, but I think for any serious rotation it would come apart from even the slightest imbalance, and there could be no load on the rotating shaft, so it couldn't do much work. Static balancing does not require that the prop or wheel be spun.

Make sure you click on the product review, which shows more detail, and also notes that the device is sensitive to drafts.

You think it's possible to use two of the hocky puck-sized NIB magnets talked about here: http://www.dansdata.com/magnets2.htm
to make a large double-ended free in the air shaft?

 

[bruto]

You think it's possible to use two of the hocky puck-sized NIB magnets talked about here: http://www.dansdata.com/magnets2.htm
to make a large double-ended free in the air shaft?

I don't know how big or how powerful the magnets need be, but I would guess it depends on what you intend to hang from the shaft. I'm guessing that for light duty the magnets do not need to be very big. If I can, I might even head out to the shop tonight and see what happens if I try to suspend a bicycle spoke between a couple of disc magnets, and I'll report back if I do. But whatever size magnets, I don't think that you will ever get a magnet-only shaft, at least of this simple design, which lacks any radial constraint, to withstand useful rotational speeds without some further device, magnetic or otherwise, to keep it from precessing or otherwise wandering whenever some tiny imiperfection of design or surrounding atmosphere puts it off balance. I'm guessing that to get a magnetic bearing that functions for any kind of driven or driving shaft, you'd need a powerful magnet or set of magnets that contain the shaft in both radial and axial directions, and perhaps compensates on the fly for variation. Again I'm guessing, but my guess is that this would be very difficult to achieve with permanent magnets, and very expensive as well, owing to the need to machine them to precise shapes in the right orientation. I imagine you could use electromagnets, which would be much easier to design, since the cores are machinable, and there's even the possibility of sophisticated on-the-fly feedback correction, but of course you need a good bit of energy to power them, probably losing the advantage of the bearings unless your need for frictionless bearings is separate from the overall need to conserve energy.

It is useful to remember in all this how very little rotational resistance an ordinary ball bearing has. Dipping iinto my ever-handy 1932 Edition of the New Departure Handbook, I note that they estimate the frictional torque of a "1202" bearing (that's a standard-issue single row ball bearing of 15 mm. bore, 35 mm outside diameter, with 9 balls, steel separator, and no grease seal), operating at 1140 rpm and a load of 150 pounds, is .00062 horsepower. That's a pretty good sized bearing, and I'd expect the tiny little bearings that are found in scientific instruments and the like to be about as close to frictionless as you can get without a lot of expense and trouble.

 

[bruto]

Quick and dirty experiment: a short length of bicycle spoke, sharpened at each end to a point; two small disk magnets stuck to the opposing faces of a largish vise; the vise closed in until the spoke is suspended horizontally. It snaps to attention, and stays horizontal. This is very easy even with weak magnets. What proves impossible at least in this exercise is getting the spoke to levitate between the magnets. It always pulls to one end or the other. Flipping over one of the magnets does not change this. I think this is inevitable, because if either of the magnets repels instead of attracting the shaft, it would displace it radially as well. The only possibility I can see is if the magnets are perfectly matched, and the gap very carefully chosen. While it might be possible with very strong magnets to do this, I am guessing that in the prop balancer as with my experiment, one or the other point is in contact with a magnet at any time, and they have found that the friction of this is so tiny that it is good enough (as well as much cheaper and easier to put together than instrument-grade ball bearings).

 

[Ririon]

It is double-ended. There's no practical way a single magnet could hold that shaft from one end. It would immediately pull the shaft to itself, and it would never hold the shaft horizontal. It's just a pointed shaft mounted through the prop with two fairly powerful, and well matched, magnets adjusted so that the points of the shaft almost touch the magnets. Remember that this is a device for balancing model plane props, which are pretty light. An ingenious static balancing device, but I think for any serious rotation it would come apart from even the slightest imbalance, and there could be no load on the rotating shaft, so it couldn't do much work. Static balancing does not require that the prop or wheel be spun.

Make sure you click on the product review, which shows more detail, and also notes that the device is sensitive to drafts.

Have you actually seen this IRL? My intuition tells me that you could not have something magnetic suspended in thin air between two powerful magnets. Any slight movement towards one of the magnets would result in the force from the closest magnet being larger than the force from the other one, and the object in the centre would hit the closest magnet.

However, if the system is well balanced, the only thing touching the magnet would be the tip of a needle, and the other magnet would hold the object upright. (You can buy pens that balance on their tips under a magnet. Similar principle.) This would form a very low friction bearing, but I wouldn't know if it's a significant improvement over a set of two ball bearings, except when it comes to looks and coolness factor...

So my guess is that it is double sided in the sense that it has magnets on both sides, and single sided in the sense that only one end of the propeller shaft is suspended in mid air. The other end is touching the device.

I'm tempted to get some superconductors and magnets and try to make a REALLY double sided one. Now THAT would be cool!

 

[ceptimus]

I've got one.

It has two magnets. The shaft doesn't stay suspended in mid air - it touches one of the magnets.

The magnets have a teflon pad, so that the friction at the end that touches is very low. The other end doesn't touch and so has no friction.

No arrangement of permanent magnets alone can produce stable levitation. Superconducting coils acting as magnets at cryogenic temperatures can do this - but that is because as they move slightly in the magnetic fields, it induces more or less current in the coil - producing a self-stabilizing effect.

I have a toy, called a levitron, that stays suspended in mid air. It consists only of permanent magnets, but uses spin stabilization. It's effectively a spinning top held up by magnets. When the spin winds down due to air resistance, it topples and falls. Actually, the stability is a very complicated phenomenon, and depends on the wobble of the top as well as the spin. You can read about it here:

http://www.levitron.com/

 

[Ririon]

I've got one.

It has two magnets. The shaft doesn't stay suspended in mid air - it touches one of the magnets.

The magnets have a teflon pad, so that the friction at the end that touches is very low. The other end doesn't touch and so has no friction.

No arrangement of permanent magnets alone can produce stable levitation. Superconducting coils acting as magnets at cryogenic temperatures can do this - but that is because as they move slightly in the magnetic fields, it induces more or less current in the coil - producing a self-stabilizing effect.

I have a toy, called a levitron, that stays suspended in mid air. It consists only of permanent magnets, but uses spin stabilization. It's effectively a spinning top held up by magnets. When the spin winds down due to air resistance, it topples and falls. Actually, the stability is a very complicated phenomenon, and depends on the wobble of the top as well as the spin. You can read about it here:

http://www.levitron.com/

Just what I thought. I guess you could do it with coils too, but as long as you have to deal with superconductors, I think the Meissner effect would be easier.

Picture of Meissner effect demonstration kit

Explanation by the people who sell the kit, mentioning "frictionless bearing".

 

[CaveDave]

bruto, Ririon, and ceptimus are all correct.

You can't "levitate" a shaft between two supports in a simple manner, but a pointy-ended shaft can be held with tiny friction between two magnets with only one end touching. It won't be terribly stable (for high speeds) or strong (can't handle much radial load), but it will work for the advertised purpose (static balancing of small objects).

Dave