Broken magnetic bearing

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Anton Isaksen
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Broken magnetic bearing

Post by Anton Isaksen »

Hey, this my first post (excluding introduction) and i recently got a pfeiffer tph055 turbomolecular pump off of ebay. The blades would turn but quickly stop, so as expected the lower bearing was shot. That shouldn’t be a big problem to fix.
But after taking the whole pump apart for cleaning and inspection, i found that there were several severe cracks in the lower magnet of the inner magnetic bearing. It doesn’t look like it will hold up during operation.
The other magnets were thankfully fine, but it seems that i only have three options:

1. Replace the broken magnet with a custom-made one.
2. Replace the magnet or inner magnetic bearing with one from a similar pump.
3. Turn the pump into shelf decoration.

What do others with more experience think about the feasibility of these options? And do you have any other suggestions?
Broken magnet in bearing
Broken magnet in bearing
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Alexi Hammond
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Re: Broken magnetic bearing

Post by Alexi Hammond »

Don't worry about the cracks. The pump will work fine with the magnet like that. The broken pieces of the upper and lower stator magnets are pushed inward by the repulsive magnetic field of the rotor magnets and they (pieces) will stay put, firmly stuck to the middle magnet.
Alexi
Anton Isaksen
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Re: Broken magnetic bearing

Post by Anton Isaksen »

Thanks for the reply. I should probably have given a bit more information.
The lower magnet has 6 cracks (radial) in total, so it is divided into several pieces held firmly together.
It doesn’t show in the picture, but one of those magnet-pieces is also swelling a bit outwards on one side. I couldn’t get it to fit in the rotor-stack without it touching the rotor-magnets and causing friction. While trying to push the swelling piece of the magnet back into place, another tiny piece came flying out from between a crack. And i am not able to put it back again.
The piece that fell out
The piece that fell out
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Alexi Hammond
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Re: Broken magnetic bearing

Post by Alexi Hammond »

You can try to push the remaining pieces all the way in and put the pump back together without that small piece. The pump still might work fine.
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Rich Feldman
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Re: Broken magnetic bearing

Post by Rich Feldman »

If a permanent magnet break is not perpendicular to magnetization axis,
the pieces will repel each other & need to be forced together to close the gap.
A missing chunk will distort the magnetic field, which might be tolerable in some cases.

Looks like your magnets are ferrite. Maybe you can get an exact replacement.
I have no idea how hard it is to grind down a larger magnet.
But guess it would be done in degaussed state, then re-magnetized. In loudspeakers and MWO magnetrons,
I think they are magnetized _after_ being assembled with their pole pieces.

Ferrite PM materials appear to be standardized.
When I measured the flux in a loudspeaker assembly, it matched (within 1%) a simulation using FEMM,
whose materials library includes Ferrite PM with no obvious parameter to adjust the strength of magnetization.

Are your magnets magnetized axially? Radially? You can tell by feeling for NS boundaries with a paper clip, or iron filings.
All models are wrong; some models are useful. -- George Box
Anton Isaksen
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Re: Broken magnetic bearing

Post by Anton Isaksen »

It looks like it is magnetized axially. I tried to lift the circlip (bad idea) to push the pieces in and lock them in place, but all the pieces of the magnet instead came flying out as soon as i relieved the pressure. Probably because the middle magnet (as seen in the first post) was oriented in the opposite direction of the two others.

I did find the material density of the magnets by measuring their dimensions and weighing them. OD:27mm, ID:21mm, Width:4mm and Weight:7-8g. That gives them a density of around 8g/cm^3, which is closest to that of neodymium and Sm-Co magnets.
Peter Schmelcher
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Re: Broken magnetic bearing

Post by Peter Schmelcher »

I think the question is what pump trauma caused the broken magnet? The field lines are pinched between the magnets using an aluminum washer spacer. The rotor starts spinning with its ball bearing in contact then takes off and centers on the pinched magnetic field. If that ball bearing seized before spin down or something small hit and unbalanced the blades at full rpm it could put lots of heat into the broken magnet region.

This guy repaired a turbo pump using ceramic skate board bearings and just omitted some bent blades during assembly. I imagine he would find some ring magnet that is close or glue the broken one back together and just use it.
https://www.youtube.com/watch?v=jL0fXUZ4wFQ&t=24s
Anton Isaksen
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Re: Broken magnetic bearing

Post by Anton Isaksen »

I am also wondering how the magnet got damaged without anything damaging the rotor blades. I think it is made of Samarium-Cobalt, since the damaged parts have a shiny look and a matching material density. And it would also make sense to use Sm-Co magnets, as they are very resistant to demagnetization and corrosion.

Using FEMM and my arduino hall sensor, I should be able to approximately determine the flux density and material type of the magnets. Maybe I can get a matching or a similar replacement of another material (ferrite or neodymium), but I doubt I'll be able to machine it on my own if the dimensions don't match.

It will probably be hard to glue all the pieces correctly together, and I assume a special type of glue or epoxy is needed to not hurt the vacuum, but it might be doable.
Al Nejati
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Re: Broken magnetic bearing

Post by Al Nejati »

Looks like your magnets are ferrite. Maybe you can get an exact replacement.
The magnets are unlikely to be ferrite. I have a TMH260 which has a very similar-looking magnetic bearing, and it's Sm-Co.
Doug_Gourlay
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Re: Broken magnetic bearing

Post by Doug_Gourlay »

not sure how reliable this reference page is, but they cite Neodymium magnets for the EXT70 rebuild.
Is there some reason to believe that they would be subjected to unusually high temperatures to require Sa Co?

http://www.rtftechnologies.org/general/ ... 0turbo.htm

I wonder what the strength of the magnets would be.

I emailed Storchmagnetics, about a 'custom' set of ring magnets, as indicated on the page and they didn't seem to have a clue what I was taking about.

Doug
Andrew Seltzman
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Re: Broken magnetic bearing

Post by Andrew Seltzman »

At the time when I made that post I believed them to be NdFeB, however I didn't have any equipment (xrf or similar) to verify that; they could have been SmCo. The problem with removing the magnetic bearing and replacing it with a mechanical bearing is that the rotor balance will change and cause vibration. I never got this edwards pump up to full speed; although a fun project, that turbo was well beyond salvaging.

In short,
Bad magnetic bearing in rotor=> bad turbo pump.
Bad magnetic bearing in stator=> possibly reparable.

I did have some luck replacing the original ceramic turbo bearing in a pfeifer TPD011 with a ceramic bearing from VxB bearings (I think). It does make it up to full speed but you can tell that there is a little imbalance in the rotor, thought this was pre-existing.
Andrew Seltzman
www.rtftechnologies.org
Jerry Biehler
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Re: Broken magnetic bearing

Post by Jerry Biehler »

I have two pfeiffers with the same problem, a 500l/s and a 900l/s.

No, it will not run right with cracked magnets. I have found that out the hard way. Severe vibrations during spin up, initially it was tolerable and eventually it just got scary.

The magnets look like unplated Nd magnets to me. I contacted my Pfeiffer rep and he is asking is magnets are still available but I have not gotten any response on that subject. Im not holding my breath on this one.

The magnets on these can be damaged by the turbo being dropped. The shock is enough to fracture them when they are hanging in the spider.

I thought about installing a bearing at the top but like Andrew said, it has its own issues, balancing and needing a high vac compatible grease. Most turbos that are full ball bearing only have the bearings exposed to the foregone vacuum.

But for me it's not worth it to mess around. The pumps I have a pretty big and a crash could be dangerous. Im getting a newer maglev 2150l/s from a friend thats going to replace the 920. I just need to figure out adapting ASA 6 to ISO250F. This is for a coating system, not a fusor!
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