Any fixes for an ingestion of oil?

How to get to vacuum, what the classes are, and what is needed for what job.

Re: Any fixes for an ingestion of oil?

Postby chrismb » Mon Dec 13, 2010 1:47 pm

No idea whether I can dry it. And I have no real plans to be tearing it apart for a clean-up, hence me asking if anyone has simply dumped solvent in their pumps before, for an approximation of a clean.

I am inclined to make up a 3 phase supply (which I was planning to do anyhow) then just run this one at low speed, as we don't really need the full bore of these turbo pumps in the micron range of the experiments done, we're really only need the molecular-drag performance in the pump (if a pump has one, which this V70 does, whereas we need to throttle a 'straight' turbo molecular pump).

I know Doug is wanting to push for lower pressure (and wants me to do likewise), but the lower the pressure the lower the reaction rate (more than linear) so there is a trade off which is probably at a pressure a little bit lower than the Pachen minimum. I'm operational in the range low e-4 torr to e-2 torr and that is as much as I'm after for now.

I did an FTIR on the oil deposits and it was just straight oil-like, so I'll chuck some hexane in it sometime and see what happens.....
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Re: Any fixes for an ingestion of oil?

Postby MrBain » Tue Dec 14, 2010 5:21 am

Drying with Hexane is a thought. Certainly easier to dry than oil.

I have some practical experience with that sound on larger vacuum systems (15 to 100 hp ranges). The reason I mention this is because that if you are 'flashing' material in the pump there is a good chance that over time you will cause excessive wear because this flashing condition will tend to eat away at the metal and leave it pitted. It is common in my field to see this sort of thing. The pressures you are seeing might also indicate the same thing is occuring as it seems close to the vapor pressures of some oils...this problem would also become worse as runtime friction increased the temperature of the oil.

Our systems only turn 1750 rpm (and only go down to about 100 microns or so), your turbos do much higher revs.. as doug stated earlier deflection might also be a concern. I persanally don't have first hand experience with turbo mol pumps.

The whole point being...over time the oil could be really bad for the gear. Raising the operating pressure at the inlet of the impellar to above the vapor pressure of the oil should do the trick until it gets warm...then you would have to raise the pressure again to compensate for the heat.
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Re: Any fixes for an ingestion of oil?

Postby Doug Coulter » Wed Dec 15, 2010 12:57 pm

I'd think the main concern about blowing solvent through there would be taking the lube (very thin oil, but low vapor pressure) out of the bearings, at least the ones that have any -- some don't. Turbos are a place where the bearing art has reached it's highest point. Some are maglev bearings, others exotic carbides and so on, some with lube, some not. So if I was going to pour solvent through one, I'd do it while it was lying on its side and rotate it to wash the vanes etc but not get the bearings.

Turbos really scream in rotational rate. The tiny ones we're playing with here are 1.5 k -- RPS, not RPM! (that's 90,000 rpm) bigger ones go slower of course, but in general they are right at the point where the best metals are used in rotors and don't have much further safety margin from simply flying apart. Larger pumps, such as my tpu-070 or the really big 6" one I have, turn a little slower - the big boy goes 833 rps.

Turbos don't do much in viscous flow pressures (like normal air pressures) but really kick when getting into molecular flow ranges. In the former range you can talk of a pressure - atoms actually hitting one another and helping push them to the pump. In molecular flow, the atoms are so far apart they rarely hit one another, and you can't really call it a "pressure" so much any more. Another way of expressing it is called "mean free path" which is how far the average atom travels before colliding with another. When mean free path gets big (centimeters up) then the turbo really shines.

This is because at these crazy speeds, it's not acting like a pump or a fan so much as a tennis racket. Individual atoms hit the blades, which are moving faster than the atoms are at the current temperature, and simply knock them down further in the pump. In fact, that's why they have to turn so fast in the first place. If you could turn one that fast at STP, it would melt from the friction with the gas -- and it's easily possible to ruin one by letting in air while it's at speed. The moan from the hypersonic blade tips is a sound you never forget -- even if your wallet can handle just buying a new pump (thousands to tens of thousand$).

This is why there's an interest in these little ebay turbo-only pumps for about $100 -- if we make a good controller for them, we get turbos on the cheap this way, and there are a lot of reasons we want them, main among them being no oil contamination in the experiement, but also, speed of pumping, convienience and so on. They're just better than oil diffusion pumps for almost everything. And at 100 for a used one, if we can skip having to buy the couple thousand buck controller (we can, and I'm working up a real slick design now to toss in the mix) then this gets into reach for even teenagers to have a vacuum system.

The one place the oil diffusion pumps really shine is in big industrial stuff where you're taking a huge chamber down to fiddlin good vacuum over and over to do things like make CD's, or other mass production vacuum coatings, since they don't cost much more when made huge -- And the extra valving etc you need to get in the tank while leaving the pump "hot" is not much consideration in cases like that. Nor is the need for water cooling. But for a home guy, those things are nasty issues. Valves are very expensive, and the plumbing hassle is not one everyone can deal with -- my shop lacks running water for example, and the run time you can get pushing water through a bucket full of "blue ice" isn't all that long before it heats up too much.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Re: Any fixes for an ingestion of oil?

Postby chrismb » Wed Dec 15, 2010 2:12 pm

It's oil in the bearings that I am thinking is the contamination. So it is putting solvent into the bearings I am interested in experience of. V70 are ceramic. I don't know if they need oil.

I am not overly concerned... I did pull it off a scrap pile so it might have been there, on the pile, due to worn bearings, but I'll presume contamination until the crackling noise is eliminated. It does run, and it does pull vac, but the controller stops running it after a few moments and I reckon it is pulling too many amps for the controller.

On the subject of bearings, I am working with a company that makes vacuum cleaner motors, and the powerful ones these days run to 100,000rpm on steel bearings. There is some special grease they use, but it is only a very small amount and remains for a long time. The behaviour of steel bearings at that speed is a fascinating subject and there are various 'dangerous' resonances that the motor has to spool through quick due to the suspension modes of the rotor and bearings becoming mis-shapen, but it is possible to run steel bearings that fast.
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Re: Any fixes for an ingestion of oil?

Postby Doug Coulter » Wed Dec 15, 2010 6:35 pm

Right, some of the "ceramic" bearings need or want no oil at all. The little TPU-050's have oil, as I've taken one apart (at least in the bottom bearing).

We call those critical frequencies the "whirling speeds" in mechanics and yes, you don't want to sit on top of one -- I notice my big turbo has a couple of those, and you can tell by the noise it makes. The first steam turbines found out all about that one....they had to kind of float the bearings with a frictional coupling to the main body to damp that out as one dodge around that. Eg instead of bolting the stationary part of the bearing to the case, they put it between some washers and let it slide around a little during those times, changing the whirling energy into heat while that was going on.

To be sure, regular ball bearings can handle an amazing amount of speed. Dremel tools go 20k rpm (which is a heck of a lot slower than a turbo), but -- don't have to do it 24/7 either. Not that I've worn out bearings on one, but my oldest one might only have a few hundred actual use hours on it, something my turbos get in 4 days or so.

You should measure the current if you think that's it -- you're probably right about that, but it's nice to be sure. Does it get hot?
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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