Vacuum pump rebuild

Every fusor and fusion system seems to need a vacuum. This area is for detailed discussion of vacuum systems, materials, gauging, etc. related to fusor or fusion research.
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Dennis P Brown
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Re: Vacuum pump rebuild

Post by Dennis P Brown » Wed May 30, 2018 10:07 am

Well, 35 microns isn't very good for a two stage pump but that would be usable if the pressure didn't then go up. Are you certain there are no leaks? Vacuum gauges can and do fail (not that that is the issue but don't rule it out.) If you did not have new oil, than the ballast air run may fix the worst of the issues. The inner workings of that pump have experienced serious corrosion. From previous pics, the inner vane surfaces for the pump case does look rather poor. That could be an issue for holding vacuum.

Jerry Biehler
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Re: Vacuum pump rebuild

Post by Jerry Biehler » Thu May 31, 2018 5:53 am

Really, he is lucky he is getting what he is with the condition of the pump.

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Richard Hull
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Re: Vacuum pump rebuild

Post by Richard Hull » Thu May 31, 2018 7:16 am

Yeah, A bold and brave effort at raising the dead and a very teachable venture. Such resurrections are typically doomed. I imagine a key vacuum forming fraction of that pump's mass went out over the years in the oil and in the removal of rust from the pitted metal. However, they are still at it.

I find it tough to imagine an oil change made the pump fail to do what it did in the graph earlier!!

Looking closely at the photo of the pump at 350+ millitorr...........check out the non-right angle tilt and the cockeyed KF clamp. Is this simply a bad connection?? All the KF clamps I have and have used are nice and parallel when clamped properly. I assume the KF on the pump is in the same parallel plane as the top of the pump body? The elbow to gauge clamp looks fine!

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

Tyler Meagher
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Re: Vacuum pump rebuild

Post by Tyler Meagher » Sun Jun 03, 2018 1:55 am

My pump worked great! The pressure was below 1 millitorr on my MKS gauge after an overnight run. The experiment I just finished running was to learn about something called vapor pressure.

My experiment was to replace the VacOil 20 with cheap oil from Amazon. The vacuum oil you use for air conditioner repairs. The same as Samuel. I ran the pump overnight with a blank off. I then recorded a pump down curve with the oil and pump still hot. I measured the oil temperature at 66°C inside the case. The temperature of the pump body was 77°C. The inside of the pump by the vanes must be much hotter. The pressure could not go below 500 millitorr.

Next I changed the oil with VacOil 20. I ran the pump blanked off overnight. With the oil and pump still hot I ran another pump down curve. This time the pressure could not go below 50 millitorr.

I changed the oil again with new VacOil 20. I ran the pump blanked off overnight again. This time the pump could get down to just below 5 millitorr.

What I have learned is the vapor pressure of the oil can determine the lowest pressure the pump can go. The vapor pressure depends on temperature. When the oil is hot the vapor pressure goes up. Different vacuum oils have different vapor pressures. Vapor pressure is what causes the oil to leave the pump inlet. The ultimate pressure I reached in each run was where the vapor pressure matched what the pump could remove.

The bad news is I have to completely disassemble and clean everything if I want to get the same performance as before.

Tyler
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pumpdown all.jpg

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