I recently bought a cute little Leybold/VAT kf40 gate valve.
Condition looks pristine, and I'm sure the gate seals fine. There is however, a pressure spike every time I open or close it, jumping from about 10^-6 to 10^-5 torr and back.
The gate is actuated by a linear rod being modern in or out by the handle, so I assume it is just o-ring sealed, and no bellows.
Could one lubricate this shaft with a few drops of diffusion pump oil? It seems to have been oiled before, unless that oil residue is just contamination.
What are your experiences with manual gate valves? Should I try disassembling the rod seal, or accept it as it is?
Pressure spike when cycling gate valve
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- Dennis P Brown
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Re: Pressure spike when cycling gate valve
I have a manual gate valve and the shaft uses no oil or vacuum grease; these are generally a bad idea since they are just contamination. So if the o-ring seals are good, oil/grease is of no use. If the o-rings are bad then vacuum grease can improve the performance but at a cost to how clean the system remains. A jump in that range is trivial for a fusor.
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Re: Pressure spike when cycling gate valve
Getting to the axle seals would require punching out multiple pins. I was given a bit of Santovac-5, applied to the shaft, and this easily gave me an order of magnitude in the better direction.
When a shaft is slid from outside air directly into vacuum, I guess there will always be a pressure increase, as by definition the surface moisture needs to flash off.
It's also difficult to diagnose, as the valve casing is a large and strange shape, so it will take quite some time to pump out anyway.
Thanks for the advice anyway!
When a shaft is slid from outside air directly into vacuum, I guess there will always be a pressure increase, as by definition the surface moisture needs to flash off.
It's also difficult to diagnose, as the valve casing is a large and strange shape, so it will take quite some time to pump out anyway.
Thanks for the advice anyway!
Re: Pressure spike when cycling gate valve
If the valve seat is biased to one side, you could try putting that side to the main chamber and have the shaft side towards the pump. That might help.
Re: Pressure spike when cycling gate valve
I think bellows sealed is the only way to go...
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Re: Pressure spike when cycling gate valve
I have a Varian butterfly valve that uses an o-ring shaft seal. I never rotate it because if I do it looks like a virtual leak for a day or so. Vacuum grease reduces the problem but a no o-ring system is better if you want a high vacuum.