Edwards EXT255H turbomolecular pump repair
You get what you pay for, but sometimes bad luck is not terminal
I recently acquired an Edwards EXT255H turbomolecular pump on "too good to be true" terms.
The seller described it as "unable to achieve good vacuum", which led me to believe that a seal was shot or something similar was at fault. That was not the case.
The moment I opened the box I knew something was wrong. There was a faint smell of burning, which made me very nervous. If the motor windings had burned out, there was nothing I could do to fix this pump. Thankfully, it wasn't that.
I realized that the motor was not at fault shortly after the minor panic. How? When you pick up a turbomolecular pump and spin it(the body) by hand, the rotor's inertia should more or less decouple the pump's rotation from its own.
This didn't happen with this pump.
At that moment I knew it was either the bearings, or the pump seized and the rotor was grinding against the stators. So, I took it apart.
Yup. That's the bearing.
A certain kind of catastrophe happened here
Bearing cup assembly and remains of the bearing
Found the issue... missing blade!
Rest of the body looked relatively ok
Found the missing blade
The first, logistically elongated issue to tackle was the bearing. For some reason, Edwards decided to go with a completely custom bearing made by Barden UK - an 8x22x10.319mm(!), half-shielded oddity.
Obviously, there was no way I was going to obtain a part internal to Edwards - even through Edwards, it would cost more than what I paid for the pump and controller combined.
The solution was found in the form of a generic bearing supplier that offered regular 708 8x22x7mm bearings in a P4 precision grade, rated at 135 000 RPM, which is far higher than 60 000 that this pump runs at. The bearings themselves were quite affordable
, though shipping was not.
Because the 8mm ID and 22mm OD matched, it was a simple task of machining two spacer rings to "fill in" the remaining 3.319mm of the larger bearing. I made the spacers out of aluminium.
The second, less obvious issue, was the missing blade. If the pump was to be spun up with the missing blade, it would shake itself apart before it even reached full operating speed, resulting in truly irreparable damage.
Since I don't have a precision dynamic balancing system, my next best option was to simply remove another blade on the opposite end, to regain symmetry at the cost of(some) pumping speed. This ended up working quite well.
And the third, even less obvious issue was the oil wicks. Because the original bearing has disintegrated catastrophically, lots of abrasive material shot into the oil wicks, and even if I were to clean them, they would still risk significantly damaging the new bearing.
This meant that I needed to make my own oil wicks, which I thought was relatively complicated to source materials for, but it ended up being very reasonable. Regular, unpigmented soft polyester felt with a thickness of 2mm matched the original wicks in texture, look, feel, and wicking performance almost perfectly.
And since it sits below the foreline, outgassing wouldn't be an issue(none spotted so far).
Below you can watch a video that details the entire repair and reassembly process.
KF16 foreline/backing port
Because all of my foreline(and some high vacuum) equipment is KF16, I didn't have any KF25 to hose adapters, or tees. And since I wanted to test it now, not in a month, I designed and manufactured my own KF16 foreline port.
The thread is an M28x2, around 3mm long. There's a seat for an o-ring to seal the port to the body of the pump.
The design is available below.
EXT255H KF16 backing port
If you happen to obtain a pump that suffered the same fate as this one, you may be able to rescue it and squeeze the last drops of life out of it on the cheap. If the manufacturer tells you it's scrap, try to prove them wrong! :)