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.
Jerry Biehler
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Re: Vacuum pump rebuild

Post by Jerry Biehler » Tue May 08, 2018 3:33 am

At that point you are pretty much making the really slow equivalent of a booster pump, also known as a roots blower. They dont rely on good sealing between the lobes and the walls, there is no contact and no oil. They are there to massively move air.

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Richard Hull
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Re: Vacuum pump rebuild

Post by Richard Hull » Tue May 08, 2018 4:00 am

That's right...Roots blowers are always the first mover and shaker on giant chambered systems. Then maybe a huge mechanical vacuum pump picks up when the bulk of the viscous flow regime is taken care of by the roots. Days and days are often required to get massive chambers down to the sub micron level. Once down they are kept down with a permanent running to the mechanical pumping system and the secondary deep pumping turbo.

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 » Tue May 08, 2018 3:06 pm

I finished putting everything back together. I attached three vacuum gauges to the intake port and then started the pump. After a few seconds the top gauge was at -29.5 Hg. The thermocouple gauge was at 65 millitorr. The pirani gauge was 49.5 millitorr. The first picture shows the pump running with the pressure gauges. After a few minutes something weird started happening. The pressure started going up and bouncing around. It went as high as 150 millitorr, but it was changing really fast. I started the pump a few times and it did the same thing.

Then something weirder happened. I put my hand over the exhaust port for a second and all of a sudden the pressure really dropped. When I removed my hand the pressure went up and started bouncing around again. I placed my hand on the exhaust port longer and the pirani gauge went as low at 11 millitorr. My dad and I are trying to figure out what might be causing this.

Here is a link to video. https://youtu.be/o6wUOpPbQJs

Tyler
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IMG_0777-1.JPG

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Richard Hull
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Re: Vacuum pump rebuild

Post by Richard Hull » Tue May 08, 2018 5:54 pm

If you can hit and maintain 11 millitorr, that would be a win and a most acceptable pump. Good luck in figuring all of this out.

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.

richnormand
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Re: Vacuum pump rebuild

Post by richnormand » Tue May 08, 2018 9:52 pm

Just a guess or two:

1)
Leak between the exhaust and the intake?
Can you devise a way to see if atm pressure overtakes an oil seal or a gasket seal or a bearing seal?
The fact that it is intermittent with plugging the exhaust and (I assume) once the pump is warmed up could be due to metal expansion or lower oil viscosity creating that leak. If there is a "pumping" action on the exhaust volume putting your hand on the port gradually lower the pressure (not atm anymore) and if done long enough it will equalise and go the other way to an equilibrium (assuming a better seal than your palm).

2)
again with oil viscosity lower at higher temperature and the pitted condition of the walls and vanes and side walls could it be a loss of pumping action with vane/wall leaking or vane bouncing? Increased pressure building up in the exhaust volume could minimise that behaviour due to the smaller pressure difference? A bit like a Roots pump that although leaky by design will be able to pump large volume of air but will struggle to reach a low vacuum by itself due to all the leaks in the lobes.

Can you easily try a thicker oil to test?

3)
electrical issue affecting the gauge. Pump not grounded, stray current, EMI etc... Try to just touch the pump near the thremocouple and see if it changes. Are these difference of pressure noticeable on the other gauge, if readable?

I am not an authority on vac pumps by any stretch. Perhaps someone on this forum can comment or perhaps you could contact the folks where you got the rebuild kit and they might have experience with that brand of pumps. I was impressed by Duniway support when I had questions doing mine with their kit.

Finally, looking at your setup. The thermocouple gauge is quite close to the intake and can be coated by oil mist that influences its accuracy. This can be cleaned with acetone in mild cases.

Tyler Meagher
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Re: Vacuum pump rebuild

Post by Tyler Meagher » Fri May 11, 2018 2:32 pm

I fixed my pump problem. I installed the wrong valve on the pump exhaust port. I used the metal valve. My dad emailed the company and the metal valve is for a different version. I needed to use the rubber piece with the spring. The pump also had a slow oil leak from the lip seal because I didn’t put it in far enough. I fixed these two things and then ran the pump for an hour. The pressure was 8 millitorr on the pirani gauge. This time the pressure did not change when I put my hand over the exhaust.

The next test is to run the pump for 24 hours and record the pressure. My dad is helping me write my first python program to record the pressure on my PC.

Tyler
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IMG_0799-1.JPG
Exhaust valve
IMG_0801-1.JPG
Lip seal not in far enough

richnormand
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Re: Vacuum pump rebuild

Post by richnormand » Fri May 11, 2018 6:27 pm

Congrats!
If you click with Python there is a very good online course from MIT via the Edx program (assuming you have time).
I would not leave the thermocouple sensor as shown in your previous photo for a 24 h run. Pirani probably OK. I am sure Richard would know.

From your valve geometry was it because the metal valve was leaking too much atmospheric pressure in the internal exhaust port at the exit of the pump?

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Richard Hull
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Re: Vacuum pump rebuild

Post by Richard Hull » Fri May 11, 2018 8:22 pm

Placing a TC gauge tube right at the inlet to a mechanical pump and letting it run for protracted periods is a bad idea. The only time I recommend this is to determine what you have when you first test your pump. Do not let it run for more than it takes to bottom out the gauge in a couple of minutes. From that point, if the pump needs work or is a piece of junk, never run with the TC gauge tube at the head until some corrective action is taken and you need to see if it has improved.

The TC gauge in my system is about 12" from the pump just before the first conflat valve to the rest of the system. I let the pump run for 2-3 minutes with the valve closed. By this time it is down to 12 microns. I then open the valve to the diff pump and the TC usually rises only slightly but quickly drops back to 12 microns.

Due to my leaky fusor chamber when I open the diff pump to fusor valve, the TC gauge immediately goes to near atmosphere and the pump down continues for about 3 minutes when the entire system, by the TC gauge is below 20 microns.

I only now turn on the power to the fusor chamber's Baratron and the diff pump boiler heater. It can take 5-10 minutes before the diff pump drags the system down into the 10e-4 range. Here is where I can begin fusison.

I have, on two occasions, removed the TC gauge tube and filled it with MEK and shaken it for a minute or two to remove any oil vapor deposits. The fluid comes out clean and clear at 12" from the pump that runs for hours at a time. Once re-installed there is no improvement or decrement noticed in its function from before the cleaning.

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.

Rex Allers
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Re: Vacuum pump rebuild

Post by Rex Allers » Fri May 11, 2018 10:38 pm

Tyler,

Fantastic work in bringing a very cruddy looking pump back from the dead. Looks great now and your measurements indicate it is working very, very well. You can be proud.

-Rex
Rex Allers

Tyler Meagher
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Re: Vacuum pump rebuild

Post by Tyler Meagher » Wed May 16, 2018 2:57 pm

Thanks everyone for all your comments. I ran the pump for 24 hours. I was happy to find the pressure at 9.5E-4 torr on the pirani gauge. The thermocouple gauge was at 0. The pump was very hot when I touched it. I tried a second MKS 901p pirari gauge. It was 2 millitorr after about an hour. I understand the high quality oil in my pump is helping with the low pressure.

The picture below is my finished pump with a few parts left over. The broken plastic parts and magnets in the picture are suppose to be on the motor coupling. The safety valve needs this to work. I removed the safety valve and the anti-suck back valve. These are the other left over parts in the picture. I epoxied over the safety valve hole to the intake port. You can see this in the previous posts. Now I understand how this feature works. I know I need to vent the intake when I turn off the pump. I have a vent valve I will connect outside to replace the parts inside.

Rich, I am not sure about the metal valve issue. I also took the pump body off to fix the lip seal. I tightened the bolts when I put it back together. My dad thinks I should take the pump apart and put the metal valve back in for a test. Then take it apart again and put the spring and rubber valve back. I’m not so sure.

I’m working on a python based vacuum controller so I can log the pressure on my laptop.

Tyler
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IMG_0833.JPG
Finished pump

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