The View Up Here

Random scribblings about kites, photography, machining, and anything else

Sucking Again – In a Good Way

Posted by Tom Benedict on 19/11/2010

The Varian Tri-Scroll pump I rebuilt some weeks ago was put into service, and was able to pull three orders of magnitude better vacuum than the pump that had been in service on that system.  Yay!  I did something right!  Of course that means the pump that had been on that system was then sent down for servicing.  Such is life.

I got a chance to open the pump yesterday, and completed the rebuild today.  In some ways it was in better condition than the first one, but in others it was worse.  The Varian Tri-Scroll is a two-stage vacuum pump.  One stage is a single scroll, and the other is a tri-scroll.  When I opened the pump I saw the famililar brown dust indicating tip seal wear, but there was also some gooey black smearing on the orbiting plate.  I took a look at the mating part and saw one of the tip seals in the tri-scroll stage was entirely missing.  I kept going, and found half the seal in the uni-scroll section was gone, too.  The tip seals consist of a spongy black rubber layer covered with Teflon.  The Teflon is what’s actually in sliding contact and providing the seal.  But when the Teflon wears through, the rubber makes contact and just turns to goo.  Clear sign of a pump left too long.

The amazing thing, though, is that it continued to pull vacuum!  It was nowhere near the <1.0×10-3 torr I was getting on the rebuilt pump, but it was still in the single digit torr range.  (Atmosphere at sea level is close to 1000 torr.)  Not bad for a pump that’s missing half its seals and is covered in rubbery goo.

The rebuild went quicker this time, of course.  First time’s a mystery, second time’s practice, third time is the charm.  Now I’m just waiting for my third scroll pump.

What this did indicate, though, is that we need a more proactive schedule for maintaining our pumps.  The difference between rebuilding a pump that’s ready and one that’s beyond ready is a the difference of several hours of goo removal.  In the not so distant future we’re going to start going through our pump systems on a regular basis to test the foreline stage.  If it can peg a TC gauge, it’s good to go.  If not, it’s time for a rebuild.  At least two of our pumps should be good to go for the next three years or so.

Unfortunately we had to send our headquarters pump station to the summit facility a few days ago along with its complete collection of vacuum fittings.  I can’t even scrounge together enough hardware to put this pump through its 24 hour burn-in run.  Ah well…  I won’t get to find out how much it sucks until next week.

– Tom

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