Turbo shut down with high foreline pressure

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David Kunkle
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Turbo shut down with high foreline pressure

Post by David Kunkle »

Hi all,

Been an interesting and frustrating 1 or 2 weeks. Just when I had the chamber pressure down in the 10-7 torr range. The turbo went all *BURG on me.

*Blowed Up Real Good

Made a weird noise, then- nothing in a span of 2 seconds. Tore it down and the upper bearing was the culprit. Bearings were all clumped up and welded to the inner race. Long story short, replaced both bearings and it's running again. Still plenty paranoid, but that's beginning to subside now that it's been running for about an hour, and the chamber pressure has gotten back to 10-7 torr range.

Here's the latest problem. On starting the turbo, foreline pressure started rising to about 50 mtorr like before, then appeared to drop to around 20 mtorr after awhile. After 1/2 hour, turbo started making a different tone. Chamber pressure was dropping fast, and I could tell the turbo was slowing down. The kicker is that I noticed the foreline pressure was at least 200 - maybe 300+ mtorr on the TC . Soon after, the turbo controller kicked off the turbo, and it seemed to slow to a stop just fine. The foreline pressure then dropped back to 10 mtorr (It's usual pressure with the backing pump maxed out) Counted the leak rate with all pumps off again, and got about 2 mtorr/min. in a large chamber.

What's happening? Someone in a previous post of mine said it may indicate a leak. Still don't understand why the foreline pressure goes up so high with the turbo when the foreline pump is capable of holding the whole system to 10 mtorr by itself. Mental flash here: Is it because the 200 l/s turbo is moving out lots of newly outgassed material now under high vacuum, plus the usual leak rate of 2 mtorr/min. This overwhelms the backing pump so the foreline pressure goes up and stays there? I didn't think 2 mtorr/min. was bad- but on a large chamber may be a worse rate then on a little 6" chamber? Is my backing pump too small for the size of the chamber perhaps?

Thanks for any input.
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Re: Turbo shut down with high foreline pressure

Post by John Futter »

david
Your synopsis is correct
you have leak opening up for some reason

the turbo I do not know what it is is going to be somewhere between 50 and 300 l/s

so if it is a 300L/s and your roughing pump is 12 cuM /Hr (Edwards RV12 a really big backing pump) your turbo will start to overcome this backing pump quite quickly
12000 /60/60 gives 3.3 L/s for the backing pump so your foreline pressure will rise

this is all abit simplified but it is to illustrate what is going on

if your turbo has a low speed option use it while fault finding it will put less strain on the blades and bearings
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Re: Turbo shut down with high foreline pressure

Post by David Kunkle »

Things just turned ugly again. Had the turbo running this am, it only went down to -5 torr and sat there. Not even running for 5 minutes, the chamber pressure started to rise, making bad noises, and it shut itself down as I was reaching for the switch. It stopped OK, but like I said, lots of obviously BAD noises. Doubt it would survive another start-up. First time I started it after the rebuild, there were some suspicious noises, but I attributed it to harmonics and/or not being familiar with turbo noises. And it did get down to -7 range for a while like before it blew up the first time.

I suppose I should tear it down again. Maybe it's the lower bearing. GMN bearing and I assumed it was an angular contact bearing. Maybe I should have left well enough alone and put the old lower bearing back in. Still can if that's the problem. Not enough grease? I don't get it. And there weren't that many leftover parts when I put it back together.

I'd like to buy a rebuilt and warranteed one from Lesker's, etc. But I've already looked at their prices today and still $5k+. Let me know if you have any ideas on what to do or look for.

All of which brings me back to the original problem of this post. In my first post above, I did catch the foreline pressure dropping to about 20 mtorr after it went up to about the 50-60 mtorr range after first starting the turbo. Which is what it should do eventually. Then, soon before the turbo shut down, the foreline pressure had jumped to hundreds. Same thing this last run, except the foreline pressure didn't get quite as high before the shut down and all the really unmistakably bad noises this time. So.... I'm wondering if the foreline pressure going sky-high has been from cooking bearing grease or some such thing coming from the foreline side of the turbo all these runs?
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Re: Turbo shut down with high foreline pressure

Post by Richard Hull »

You may have volatiles in the turbo, as you have noted, but have you used your mechanical pump with the turbo vlaves to the chamber wide open? I assume you have it valved off or gated. Your real leak may be on thehigh side or top of the turbo to the chamber or the chamber, itself.

Run your mechanical pump only, all valves open, and find out what your chamber goes down to. If it hits and holds 10 microns, your system is good and I would suspect the turbo, only then.

This is why I like the no moving parts, air cooled diff pump.

Richard Hull
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Re: Turbo shut down with high foreline pressure

Post by David Kunkle »

John Futter wrote:david
the turbo I do not know what it is is going to be somewhere between 50 and 300 l/s

so if it is a 300L/s and your roughing pump is 12 cuM /Hr (Edwards RV12 a really big backing pump) your turbo will start to overcome this backing pump quite quickly
12000 /60/60 gives 3.3 L/s for the backing pump so your foreline pressure will rise
I get the concept now. My turbo is 200 l/s, and the roughing pump is a Trivac D4A that amounts to 2.1 l/s. I can see how a chamber leak can back up the roughing line. But with the turbo going out, I'm not sure what's what till I get a good pump in there.
If your experiment needs statistics, you ought to have done a better experiment.

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Re: Turbo shut down with high foreline pressure

Post by David Kunkle »

Richard Hull wrote:You may have volatiles in the turbo, as you have noted, but have you used your mechanical pump with the turbo vlaves to the chamber wide open? I assume you have it valved off or gated. Your real leak may be on thehigh side or top of the turbo to the chamber or the chamber, itself.

Run your mechanical pump only, all valves open, and find out what your chamber goes down to. If it hits and holds 10 microns, your system is good and I would suspect the turbo, only then.

This is why I like the no moving parts, air cooled diff pump.

Richard Hull
I've only run wide open because I've been trying to see how low a chamber pressure I can get.

In case I wasn't clear, I was wondering: if the turbo bearings are going bad, can they be cooking off grease and maybe outgassing the phenolic cage around the bearings, etc. into the foreline? The pump didn't seem to be hot anywhere, even when it seized up the first time.

As far as the mech pump only at wide open, it will go down to 10 microns and as low as 5 microns. I have an Iridium ion gauge that's rated from 4E-10 up to 1E-1 torr, and it usually agrees very well with the TC. I have pulled the uncleaned chamber down to 5E-7 torr. I'm still not convinced I don't have a leak, but with the turbo acting up, I'm not sure if anyone can tell right now.
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Re: Turbo shut down with high foreline pressure

Post by John Futter »

David
did you buy the bearings from the turbo pump co or a bearing supplier?????

the two are very different the turbo Co pays to have the bearings greased with special apiezon grease with little vapour pressure or a synthetic

more later Ham sked to take
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Re: Turbo shut down with high foreline pressure

Post by David Kunkle »

The bearings came from GMN Bearing. They are high speed, high precision, and I greased them with a small supply of turbo bearing grease that came with the pump.
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Re: Turbo shut down with high foreline pressure

Post by David Kunkle »

Interesting day. Appears that I have a handle on everything going on in this post now. It's a little from column A and a little from column B.

Column A: Decided to read leak detection sections again in a few of my vacuum books. Decided to try the soap bubble method using higher pressure- 35 psi- instead of 15 and as one book recommended- patience. Wait and watch. Found some relatively fine bubbles from a weld that I had trouble with before. Couldn't find a darn thing over the rest of the chamber. I'll be tearing the whole thing apart and dragging it back to the weld shop. Enough of putting up with that. And therefore, the constant high foreline pressure- even though it would pull down to E-7 torr range- for awhile before it backed up the foreline .

Column B: Tore down the turbo again, and it's the upper bearing- again. Relatively intact, but the plastic cage split in the center into 2 intact rings. And the centers of the cage where it split in half were black/burned, and they smelled like it too! I think that validates my theory that something from the foreline side of the turbo was jacking up the foreline pressure.

Called an engineer at GMN Bearings. He said when the cage splits, it usually means the forces aren't being distributed through the bearing properly. He walked me through the pump's engineering, and we discovered all the forces on the upper bearing are put through the top and bottom of the inner race only- none on the outer race, meaning it requires an axial bearing and not an angular contact bearing. I assumed they were the original bearings when the pump first blew up. The old bearing was angular contact, so that's what I replaced it with. It's becoming fairly obvious that neither bearing was original to the pump. There is literally no info on this pump on the internet, but I did come across some old paperwork and a catalog with similar model #'s. I estimate this pump is from early 70's to mid 80's at the newest.

And for the lower bearing, the forces go thru the top of the inner race, then thru the bottom of the outer race, requiring an angular contact bearing- which is what I used. But.........I installed it upside down. Yeah...I'm 2 for 2. He said he's surprised the lower bearing didn't come apart before the upper one installed like that. So, I've put my turbo window shopping on hold for now. Another $200 or so is still a lot cheaper than a whole other turbo system.

Between no leaks and a properly functioning turbo, I can't wait to see what it pulls down to when it gets back together.
If your experiment needs statistics, you ought to have done a better experiment.

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Re: Turbo shut down with high foreline pressure

Post by Jerry Biehler »

Usually the bearings are ceramic ball type. I think you may have the wrong bearing and I am rather certain it is not angular contact.
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Re: Turbo shut down with high foreline pressure

Post by David Kunkle »

Jerry Biehler wrote:Usually the bearings are ceramic ball type. I think you may have the wrong bearing and I am rather certain it is not angular contact.
I thought the newer turbos used the ceramic. I believe this model is from the 80's at best. Both bearings had steel balls- even though they were probably not original. May run better or far longer with ceramic except for the $$$ involved.

When you say certain it is not angular contact, do you mean upper, lower or both? The upper bearing certainly doesn't seem to be angular contact. But the lower certainly seems to need angular contact the way the engineer explained it to me.
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Re: Turbo shut down with high foreline pressure

Post by Peter Schmelcher »

Bearing manufacturing is a go big or go home industry so most companies you find are engineering support and resellers that mark up the bearing prices. I got a small Pfeiffer (actually three) missing the bottom end ball bearings and was considering taking a chance with ceramic dentistry ball bearings for around $30. I ended up buying working turbo for $75 before I decided on the replacement bearing and moved on. I had concluded that ceramic balls are the best choice because they won’t weld to the race and at high rpm maintain their diameter tolerance better.

Best of luck.
-Peter
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Re: Turbo shut down with high foreline pressure

Post by Jerry Biehler »

Ceramic bearings have been around for a long time and they have been in turbos for a long time too. Angular contact bearings must be used in pairs, that is just how they work. Good bearings are expensive. The angular contact bearings in the headstock of my lathe are something like $1500. I think I spent $300 on the bearings for a surface grinder I had and that was after getting New Old Stock sets off ebay.

You might check to see what the guys who build model turbine engine use, they do use ceramic bearings, I know.

Trying to go cheap on stuff like this NEVER ends well. You could try cheap bearings again but when it fails it could just lock up destroying the pump and not failing as gracefully as it did.
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Re: Turbo shut down with high foreline pressure

Post by David Kunkle »

The GMN engineer took a look at Seltzman's website where he rebuilt this pump. He changed his mind and says both bearings are angular contact, and he's comfortable with the bearing #'s that we used last time. Now, he's guessing it may have been a greasing issue - or lack thereof. That seems to make the most sense at this point. I'm actually hoping that's the case. Got 2 new bearings of the same #'s (pre-greased from GMN at no extra cost. Wish I'd known that first time around!). Definitely more grease from GMN than what I had in there on the first try. I was paranoid about too much grease. It's all back together and spins freely. Now I just have to get my chamber back from the weld shop - anytime now.
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Re: Turbo shut down with high foreline pressure

Post by David Kunkle »

Got tired of waiting to get my chamber back, so I decided to cap off the turbo with a reducing flange and my ion gauge to see if this thing's any good. Started it up just sitting on its legs connected to the backing pump. Not exactly what I was hoping for. It seems to start, although there *might* be a lag before making its usual whiny turbine sound. Ion gauge goes from about 7 micron to start and goes down to a measly 3 or 4 microns, but then stops as the turbo whine disappears. The gauge starts moving upward from there, and I shut it off. No tripping or codes from the power supply. I removed the bottom cover and it still spins freely. I've started it at least 6 times hoping there was a little too much grease in the bearings. Each time I start it, it seems to get a little lower by 1/2 to 1 micron.

When it was disassembled for the last set of pre-greased bearings, I did wash all the parts with soap and water including the base with the windings. There was a lot of crud in the base from the previous burnt and broken bearing cage. Put everything in the oven at about 150F to make sure everything was dried out before assembly. Any ideas on what's wrong this time? At least there aren't any bad grinding noises so far like last time.
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Re: Turbo shut down with high foreline pressure

Post by Andrew Seltzman »

There is usually a ~1-2sec delay from when the power button is pushed until the turbo starts up, this is normal. As far as the failure to start, this is definitely due to too much grease on the bearings. Since the turbo is driven by a 3 phase induction motor with basically no feedback(except motor current), unlike a modern brush-less motor driven turbo, the controller doesn't know there is extra load on the bearings due to the excess grease. I wouldn't try to remove the existing grease though, it just needs to distribute it better.

When I had this problem after a re-grease, I would wait until it sounds like its spinning the fastest then turn the controller off/on again quickly while it's still spinning, this will reset the start up frequency ramp and will get the turbo spinning faster each time.
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Re: Turbo shut down with high foreline pressure

Post by David Kunkle »

Hi Andrew. If I couldn't figure this out, I was going to send you an email tonight anyway.

I did like you suggested, and it seemed to help. After a while, I just let it run thinking getting it up to temp would make the grease flow easier. After a while, the gauge started dropping and rpm's started going up fast until it hit about E-6 micron range. No strange noises, etc.

Just as I went in to check to see if the pressure had dropped further.....bang. Came to a screeching halt in 2 seconds. WOW! Just when I thought I was OK... because I'm pretty sure the 1st rebuild was lack of lubrication- odd noise right off the bat that only got worse. I have no doubt it's the upper bearing again.

This time it wasn't even hooked up to the chamber- just a reducer to the ion gauge. The backing pump by itself on this setup was getting me to about 7 microns. I did notice the foreline pressure jump up again as the turbo was dying. Not sure what it was just before. Still thinking grease, burning bearing cage, etc is making the foreline pressure jump up to 150+ microns. But, in case it isn't, can the backing pump be too small causing the foreline to back up, killing the turbo? I'd think it would only make it noticeably slow down for a while and not make it go bang suddenly. With that tiny little chamber this time, I don't see how the turbo could flow enough to back up the foreline that much that quick this time. I didn't think even that pressure should kill a turbo or the turbo should shut itself down. Roughing pump is a D4A (127 l/m or 4.5 cfm) Turbo is 200 l/s.

Kinda fed up now at $250/whack X2 now at installing new bearings. Seriously thinking about getting a different turbo setup. At this point, I just want to figure out if the backing pump could be too small so it doesn't happen again with a different pump.
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Re: Turbo shut down with high foreline pressure

Post by Jerry Biehler »

I am telling you, you have the wrong bearings. Angular contact bearings are no good unless you have a way to set the preload, and in this situation it needs it more than ever. In a turbo you really dont need angular contact since there is no real axial load other than the weight of the rotor. Standard deep groove radial bearings are all you need. Pick up a set of cheap ceramic ball bearings off ebay and install those and use a good low vapor pressure grease, maybe one of the krytox series of greases. 30% grease fill, no more. That may even be a bit much.
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Re: Turbo shut down with high foreline pressure

Post by Richard Hull »

There is always a no moving parts diff pump option. Works every time. No controller, no cables, no bearings, no motors.

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Re: Turbo shut down with high foreline pressure

Post by David Kunkle »

You've convinced me to try a standard ceramic, Jerry. Can't go too wrong anyway for $65 for ebay ceramic bearing and grease. Still lot cheaper than a new turbo setup. I decided to re-use the original lower bearing. Partly because I couldn't find a ceramic in that weird size, and partly because there's never been a hint of trouble from the lower bearing all 3 times it's blown up.

The original upper bearing has no info on it indicating that it's angular contact. But now that it's blown apart, it's easy to see the outer race has an inner lip where the ball bearings actually rest on the # stamped half. I was thinking that indicated an angular contact bearing? So, only thing that concerns me is Andrew said that original bearing in there worked fine for him for several years.

This thing is old enough that there is NO info out there, and no one that rebuilds turbos will touch it due to its age, lack of info and parts.

Ceramic bearing is apparently already on its way judging by the email I just got. Though I just decided I'd talk to the engineer at GMN again over this latest disaster. We studied the original bearings again, and I am quite convinced both bearings are angular contact. Apart from the physical characteristics these bearings have, he told me the "S" stamped on them means angular contact. No one mentioned that before. He offered to have me send him the all the bearings so they can examine them and determine why they are failing. So at this point, I'm not sure how a radial bearing will work better. Unless someone replaced the factory bearings with these angular contact, and they somehow managed to break-in and/or last for several years for Andrew.
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Re: Turbo shut down with high foreline pressure

Post by David Kunkle »

Richard Hull wrote:There is always a no moving parts diff pump option. Works every time. No controller, no cables, no bearings, no motors.

Richard Hull
How low can most diff pumps go?
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Re: Turbo shut down with high foreline pressure

Post by Nick Peskosky »

David,

Have you considered that the upper bearing (regardless of type) continues to fail on you due to the rotor balance of your pump being out of alignment? An unbalanced rotor axle coupled with high RPMs experienced by the pump when it has reached full speed may be causing uneven loading of the bearings within their races. Unfortunately, balancing a pump is non-trivial and is usually left to professional vacuum technicians.... It's just something you may want to consider before you continue to pour money into failed bearing assemblies.

Most diff pumps can easily hit 10^-6 to 10^-7 Torr in a clean system with good seals and a fresh charge of quality diffusion pump oil. As Richard often states, they are "set it and forget it" systems which serve as a low cost means of medium-high vacuum for most amateurs. Turbo molecular pumps were commercially adopted over diffusion pumps in many processes due to their oil free nature (beneficial for semiconductor/optical processing) and ability to rapidly cycle between atmosphere and high-vacuum with no ill-effects (needed for load lock applications or modern electron microscopes). After market turbo pumps are usually cost and complexity prohibitive for amateur use, by that I mean it is both costly to procure a working pump and unlikely that you will easily happen across a pump/cable/controller all at one time. For common Fusor use, either pump is suitable to provide sub-Micron vacuum levels required for deuterium admittance and often the personal selection of one pump versus the other is a function of cost, mechanical background and luck.
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Re: Turbo shut down with high foreline pressure

Post by Richard Hull »

I did not mean to poo-poo Turbos, of course. A working turbo is a wonderful thing. The key word is "working", however, and one that is not working can cause a lot of money to be sunk in a lost cause. Their would be a point for me where I would cut my losses.

Richard Hull
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Re: Turbo shut down with high foreline pressure

Post by Jerry Biehler »

Post pics of each side of the old bearing and I can tell you if it is angular contact.

I have had great luck with turbos. Got a couple of those little TMP50's that were on ebay for $100 a piece and they both work well. Installed a used Varian 301 on my SEM and a giant 2000l/s turbo on my coater. You just need to keep an eye out.
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Re: Turbo shut down with high foreline pressure

Post by David Kunkle »

Nick Peskosky wrote:Have you considered that the upper bearing (regardless of type) continues to fail on you due to the rotor balance of your pump being out of alignment? An unbalanced rotor axle coupled with high RPMs experienced by the pump when it has reached full speed may be causing uneven loading of the bearings within their races. Unfortunately, balancing a pump is non-trivial and is usually left to professional vacuum technicians.... It's just something you may want to consider before you continue to pour money into failed bearing assemblies.
Yeah, I'm starting to get the feeling there's something else going on. Pretty confident now I've had the correct angular contact replacement each time. Since the original and one replacement are now in pieces, I measured the outer races today with a mic, and the "land" on the upper part that makes it angular contact is exactly the same size as the original. I think that shoots the GMN engineer's one idea that the original might have a different degree of angular contact. In other words, all 3 bearings seem to be carbon copies.

Never considered the axle/shaft may be out of alignment. Slight vibration only to touch, and I always attributed that to coming thru the rubber hose from the backing pump. I'm guessing what you're talking about is pretty subtle though, and the turbo won't exactly go dancing across the lab. Are you talking about the weight being off like an unbalanced crankshaft or the axle getting tilted off from vertical? Any way to tell? If that's the case, this thing is toast because you can't even pay anyone to work on it due to its age.
If your experiment needs statistics, you ought to have done a better experiment.

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