Cleaning an Edwards EXT250 turbomolecular pumpTags:
Recently, I’ve soaked my EXT250 turbomolecular pump in rotary vane forevacuum pump oil by not following the proper shutdown procedure. In this note I describe my attempt at repairing it.
The bottom ceramic bearing on EXT250 has an oilpan that’s basically a sponge saturated with oil with two small prongs that touch the shaft; the oil is aspirated by centrifugal force. The oilpan is connected to the internal space of the pump, and so when the forepump oil started to backstream (as a liquid!), it went mostly there.
When I’ve disassembled it, I discovered that the sponge is completely destroyed and unusable. However, that doesn’t mean the pump itself is beyond repair.
I’ve bought some freon, HCFC-141b—it’s technically a refrigerant, but its most useful property is that stays mostly liquid near room temperature; as all freons, it is an extremely good organic solvent.
Then, I’ve took all furniture (screws, oilpan cover, etc) off the pump, except I did not take the rotor off. (It’s not necessary, and it’s far too easy to screw up.) I’ve cleaned the furniture in an ultrasound bath using some kind of proprietary basic detergent.
Update 2015-11-01: it is actually fairly simple to take the rotor off on the Edwards EXT250 pump, as it is made from a single piece of aluminium and thus there is no way to mess up balancing.
The pump having all of its orifices open, I’ve flushed the bulk of the oil by pouring copious amounts of HCFC-141b into it. (Don’t be me and don’t let it go to the floor when it pours out—it will carry the forepump oil and that oil will be nigh impossible to get off the floor.)
After that, I’ve covered most of the orifices back, filled it with HCFC-141b and shaked it for a minute or two. Then, I have let the solvent escape. I have repeated this several times.
After that, I’ve mounted it upright and flushed it with HCFC-141b again, with the idea that it is best for any traces of oil to accumulate near the outlet than inlet.
After this procedure, the pump was completely clean from all traces of oil. HCFC-141b can generally carry off the oil by just splashing around a surface, however, as the pump has a lot of nooks and crannies, I went with a more thorough procedure. The exposed surfaces look completely virgin: they’re mirror shiny and the tool marks are clearly visible.
After cleaning, I have transplanted the oil sponge from another identical pump. So far it works great: spins up to ultimate RPM and bottoms out my Pirani gauge!