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Rotary vane vacuum pump repair?

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LutzH

Wed Dec 23 2009, 07:34PM

Registered Member #1721 Joined: Sat Sept 27 2008, 08:44PM
Location:
Posts: 136

Hello:

While cheap, these are dam good pumps. They are marketed under several names, but its the same pump. Mine is a Dayton bought from Grainger in the late 1980's. Yes it still runs and I still have it. I have never changed any internal components durring all this time. The secret is when in doubt, change the oil, even though mine gets little use I have changed the oil about 7 times.

The gas ballast (The Brass Thumb Screw) is important, for 98% of the stuff I do I just leave it open, or "cracked". It prevents atmospheric H2O from condensing in the oil. With the ballast closed mine will still pull down to 100 microns! The inlet plumbing really limits the gas flow, and you will not get 3 cfm with it in place at vacuum. I put in a hose barb with a 3/8 inch hole, the only drawback is "Suck-Back" in other words pull a vacuum on something, and turn off the pump and watch the something fill with pump oil yuukkk. There is a steel ball float under the fitting but it will not seal against the hose barb. I thought about lapping the end of the hose barb so that it will, but instead I just added a 1/2 inch ball valve. If its still a worry then I can add a trap.

My main pump is a rebuilt Hyvac 45 17cfm pump which should last about 100 years , but I still use this reliable small pump for many apps.

klugesmith

Mon Feb 15 2010, 06:51AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

I just had my biggest Doh! moment of the year.

Things were going well with the DV-85 pump. Yesterday was the first time I ran it for more than a few seconds. Had a donated Convectron gauge and was testing some vacuum connection parts & methods for the science workshop program (while waiting for KF-25 fittings to hook up the fancy pump). With a KF-16 ring set between the gauge and the 1/2" ball valve, I reached an indicated vacuum of 20 millitorr after a few minutes for the new & old plumbing (and gauge) to settle down.

That was fantastic, but when I tried to repeat it this morning the motor could hardly turn the pump. The machine had reverted almost to its as-scrounged state. Well I needed it for a Sunday afternoon session at the science workshop. So drained the oil, opened the case, and messed with the all-too-familiar cartridge bolts as reported above. Included brief motor runs & strategic hammer-taps that had seemed to make things settle into alignment, last time. That seemed to do the trick, but after a minute or two of run time (at science workshop) the pump started dogging again.

Back home after a generally productive session, I was eager to tackle the pump, but reluctant to repeat the oily disassembly job. How about running in different positions? With pump standing vertically on its nose (motor end up), it seemed to run a bit smoother. Now stand it on its tail, which had been handy when the case was open to access the cartridge.

But this time the case was closed & full of oil. The bent-pipe handle on these pumps is screwed directly into the "crankcase" and serves as the air exhaust port. Or, in motor-down orientation, the oil drain port. At least the oil drains slowly because air has to glug in through the same pipe -- if the pump is not turning! When I pulsed the motor for a second, it pressurized the case & drove the whole pint of oil out through the handle, in a stream like that from a garden hose. I think this is where Homer Simpson would say, Doh!.

I don't have enough clean oil to refill the pump, so am shut down for a few days. Will review all the helpful suggestions here, and re-consider what defect of material or assembly could make a good pump (as attested by Lutz) turn bad.

Hazmatt_(The Underdog)

Tue Feb 16 2010, 12:11AM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

I'm betting you need to replace the drive shaft. But clean it out and inspect all the feedthoughs for the shaft. Re-torque the bolts evenly criss cross and get a torque wrench if possible to make sure its all even.

Pretty obvious suggestions, but that's what I would try.

plazmatron

Tue Feb 16 2010, 12:39AM

Registered Member #1134 Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351

It seems kinda odd that when you tighten up those bolts that the rotor should seize.

As you have pointed out this is a precision part, so I would be willing to bet that something is in there that shouldn't be. Be it grot, rust or decomposed oil.
I'm thinking perhaps there is a thin coating of something either on the ends of the rotor, or the plates that seal against the ends (I notice that the plate shown has a stain on the inside that makes it look as though the pump has been left standing for some time)

For a parts with tolerances like that, the thinnest crud on there (like that stain) might be enough to bind the rotor to the stator.

So it looks like cleaning is the order of the day. Try acetone or IPA first, and if you can get rid of all those stains, re-assemble and try again.

If they aren't going to budge, you can try polishing them out using fine steel wool, and oil. If you choose to do this don't rub vigorously at the stain area itself, you must polish the entire face of the part as evenly, in fairly large circles.

Once your done, clean the parts with acetone or IPA and re assemble.

Sounds like dirty work, but its the best I can come up with, and certainly wont hurt.

Les

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