I also like the belt drives... and they do seem to last almost forever... at least the Welch pumps do. If my memory serves me correctly, the Welch 1400 pump is a single stage, with a spec'd ultimate pressure around 35 microns. So getting to 20 microns is doing very well.
When buying a used pump, I actually take a McCleod gage along to test it. It has a great negotiating effect if the pump is not doing well. Also if it is dirty, but quiet, it is likely the innards are just full of sludge and a flush will work wonders. But with a vacuum test as a witness, you can probably get some sort of agreement as to what constitutes grounds for a return.
The Welch belt drives can be rebuilt, by almost anyone, and I believe you can buy needed parts either new or rebuilt from Welch. Reed valves, and seals are often all that's needed besides the cleaning.
I have tried to rebuild a couple direct drives, Varian and Edwards I think, they had poor vacuum and oil leaks. We were unsuccessful, because the leaks were caused by shaft grooves (wear) and the low vacuum came from the leaks through the oil seals. Whereas the belt drive pumps have separate shafts and rotors, the direct drives had rotors and shafts made as an integral piece, and thus are costly to fix... You need a new or rebuilt one. I actually made a new shaft for a Welch 1400...took about an hour to machine, from ground round stock - couple grooves and some a Woodruff Key recesses.
Think I wrote a bit more about this in another post, here.
Dave C.
Created on Friday, January 05, 2001 2:03 AM EDT by David Cooper