Ebay Vacuum Pump Pressure

[IRFP460]

Hello there!

A couple of years ago my parents got me a cheap vacuum pump off ebay for christmas.
Can't find the exact thing, but this one looks fairly similar: http://www.ebay.at/itm/401029815149

Back then I didn't know much about what I was doing so it had its fair share of abuse, like pumping water vapour and no dust filtering at all.
Lately I have been building a vacuum chamber for it so that I could do more than just watch the glowing discharges in a bottle. A good vacuum chamber needs a gauge of course, so I got a Pirani gauge (InstruTech Stinger).

Even with the gauge connected directly to the pump it won't read below 0.3 mBar and that got me thinking.
The pump itself promises 0.05 mBar, so realistically I was hoping for ~0.1 mBar.
I think a leak outside of the pump can be ruled out, everything is connected with proper KF16 fittings.


Does anyone of you have a similar but newer (and less abused) pump and can measure their final pressure?

Greetings,
Lukas

[Fraser]

The oil is very important in vacuum pumps. In very hard vacuum situations I recall that the oil could cause poor vacuum levels and had to be changed for fresh uncontaminated oil.

Can your pump be opened for inspection and servicing of the port valves ? Might be worth inspecting them for damage and corrosion.

Change the oil first and see how the pump performs. Do use an oil that is designed for high performance vacuum pump use or you will be wasting your time.

Fraser

[IRFP460]

A couple of weeks ago the pump got its first oil change, the stuff that came out looked like orange juice purchased at a similar time as the pump... (maybe rust?)
I got some "vacuum oil" from ebay, said it had a vapour pressure of 0.1 Pa if I remember correctly but it was only 10 bucks. Is there an oil you reccomend?

I haven't opened the pump yet, is there anything in such a cheap rotary vane pump that can be fixed with the stuff one might have lying around?

I'm not hoping for a high vacuum, as I said I was aiming for 0.1 mBar.

[jpanhalt]

Hard to say what your problem is.  With fresh pump oil, there may still be volatiles left.  Most pumps have a valve that lets you pump a little extra air to get rid of the volatiles.  Try that overnight.  If that doesn't work, look for leaks or consider that you have a ruined pump.

John

[IRFP460]

Sorry, it appears you don't understand my question.
I am not looking for a problem to fix (yet), I want to know if someone has experience if all of those pumps perform poorly of if mine is defective / below average 
I don't know about about a valve to let out extra air but now that I know what I'm looking for I might find one.

Thanks for the answers,
Lukas

[jpanhalt]

You apparently misunderstood my reply.   i have a lot of experience with typical mechanical vane vacuum pumps.   Although, I lave less experience with turbo molecular pumps, which it does not appear you have according to your description.

You have given NO information about your pump.  So, my recommendation was generic.  Basically, you said that you have a vacuum pump that is not working.   Is it a piston pump or vane pump (I assumed the latter).  How do you know it is not pumping properly versus a leak in your system?   Do you have the pump properly configured (i.e., the bypass valve is closed).  And so forth.

There is a good likelihood that your pump is ruined.

Sorry that my attempt to help you troubleshoot it fell on deaf ears.

Good luck.

John

[Rbastler]

I understood the first post in the way IRFP460 is looking for somebody with a vacuum pump in the link, to compare the maximum vacuum that can be reached in order to determine if the pump is broken or if its with all lot of stuff from china, that they simply give you wrong numbers on what the pump can do.
He never wanted to repair the pump. Not now, from what I'm reading. Just to know if there is something wrong with the pump in the first place and then go from there.

[IRFP460]

Don't worry, I'm thankful for your help, its just that I don't have any information on the pump.
It's a cheap one from china and all I can tell you is what's written on the nameplate, which is:

1 Stage Vacuum Pump
Model                                       VP 115
Free Air Displacement              1.8 CFM
Ultimate Vacuum                     5 Pa
Voltage                                    220V ~50/60Hz
Power                                      1/4 Hp

That is literally all the information I have on the pump.
As Rbastler stated correctly I am looking for someone with the same pump or one from a similar price range to determine if my pump is broken or if the manufacturer is just inventing numbers.
I assume there is no leak in the system as the system for testing my pump is just the pump itselt, the gauge and a venting valve, all of which are connected with proper KF16 flanges and were testet on a different vacuum system for leaks.

The pump has no bybass, ballast, etc and you were right about your assumption, it is a vane pump.

Greetings,
Lukas

[Zero999]

A couple of weeks ago the pump got its first oil change, the stuff that came out looked like orange juice purchased at a similar time as the pump... (maybe rust?)

That doesn't sound good. In my limit experience of vacuum pumps, that might explain why it's not performing as well as expected.

It is possible the specification was exaggerated in the first place but without measuring the vacuum when it was brand new vs now, you'll never know.

[johansen]

its a single stage pump, i would take it apart and wash the parts out with whatever solvent you have available and change the oil.

concerning the cheap two stage units, i don't recommend taking them apart unless you know the vane(s) is broken. there are 5 plates of metal, each screwed to the next one and there are no alignment pins to keep them in position, very difficult to put them back together and get the pump running.


a two stage rotary vane pump such as the welch duo seal units, belt driven at about 500 rpm.. theoretically these units are good to the vapor pressure of the oil, 1 micron . you can find them on ebay for a few hundred $ (probably not in Germany though). there is mostly nothing to wear out in them and i would buy the cheapest one you can get and take it all apart and fix it. there are repair kits you can buy, but you can make the gaskets you need from a sheet of paper soaked in vacuum oil. tolerances are not critical, but some say you should use cigarette paper to set the clearance between the rotor and the offset cylinder it rotates in.

[SeanB]

Just remove the outer cover on the vacuum side, after draining the oil and wipe off the crud from inside,using a clean cloth and put the unit back together, and put in new oil. The oil you can buy vacuum pump oil, or just use regular synthetic 0w60 motor oil, in this use just as good.

Before taking it apart for cleaning run a little spray oil through the pump with it running, using a restriction to make it pull some suction, and then leave it running for around an hour with inlet closed off, to get all the crud out of the chambers and the oil hot. Then clean and put fresh oil, and also remove the metal splash plate and make sure it is cleaned and rust free as well, it will drop contaminants into the oil.

[calexanian]

These pumps only have about a 40 micron blank off ultimate pressure. By the time you put a chamber on it you will be very disappointed with its performance. Additionally the input ports are very small so transmittance and conduction are very low. They are fine for vacuum drying, refrigerant removal, and epoxy use, etc. I have tried to use them as backing pumps for small diffusion pumps and they work, but really slowly. If you want to have a good baseline get a Welch 1402. Rebuild kits for them are cheap. Even an old tired one will handle a higher gas load than one of those little ones. I don't recommend a 1400 for a vacuum chamber. They are very clean pumps, but cannot handle a large gas load. Good for fixed applications and backing small booster pumps for larger diffusion or turbo pumps and kinda wimpy by themselves.

[IRFP460]

I have now taken the pump apart, cleaned it, polished some surfaces and put it back together.
The pump dosen't appear to be broken, there are no visible scratches on the sealing surfaces.
One problem I've noticed is the play between the rotor and the stator, you can get a slit of ~2mm between them. I guess the construction is similar to that of the 2 stage pumps that johansen described.
I've readjusted the stator to form a tighter gap and now i get down to 0.25 mbar in about 30 min.
I couldn't exchange the oil for something better yet, but some shell S2 R 100 is on its way.

Thank you for all your tipps,
Lukas

[johansen]

The pump dosen't appear to be broken, there are no visible scratches on the sealing surfaces.
One problem I've noticed is the play between the rotor and the stator, you can get a slit of ~2mm between them. I guess the construction is similar to that of the 2 stage pumps that johansen described.
I've readjusted the stator to form a tighter gap and now i get down to 0.25 mbar in about 30 min.
I couldn't exchange the oil for something better yet, but some shell S2 R 100 is on its way.

that is really interesting such a huge gap like that. i presume the rotor has a minor diameter of about 40mm, and the stator is about 55mm diameter? so the vanes have about 15mm of travel as they rotate?

were the bolts tight when you took it apart? its also possible these pumps are assembled by people who don't understand how they work.

[IRFP460]

It's all a bit smaller than you guessed, see pictures below.
The bolts were tight and secured with a lock washer, but I don't see how the manufacturer would make sure the slit has the correct width, picture three is the worst case scenario 
What difference does it make how far the vanes have to travel?

[calexanian]

The vanes engage the walls by centrifugal force as they spin. In better pumps they have springs and guides inside to do it. Havc pumps do not generally. Once again. They are not lab grade.

[Gregg]

It may be possible to lap the ends of the pump cylinder to remove the corrosion.  It would be even better if you can also lap both end caps.  A little lapping will also help take up any axial wear that the blades may have.  In vacuum systems every little leak adds up.
A flat surface plate with quality sandpaper and a little thin oil would be needed to lap the parts.  The fit can be checked with a thin layer of something like Dykem high spot paste (you can make your own with some thin grease like Vaseline and a little paint pigment).  Look up some Youtube videos about lapping and scraping machinery for some methods that might suit your situation.

[johansen]

those cheap pumps have springs too.

anyhow, yes you want minimum clearance, it basically sets the compression ratio. the oil that flows into the pump helps seal it. so in theory, if enough oil gets sucked into the pump with every rotation, you can practically achieve infinite compression ratio.

that's how you can get 1 micron with a 2 stage pump.

but if you have a 2mm gap between the rotor and the stator, then you can't get a good compression ratio.

the air gets pushed around by the vane. the more travel you have on the vane, larger diameter the stator, smaller diameter the rotor = more volume of air pushed out of the pump with each rotation.

It may be possible to lap the ends of the pump cylinder to remove the corrosion.  It would be even better if you can also lap both end caps.  A little lapping will also help take up any axial wear that the blades may have.  In vacuum systems every little leak adds up.
A flat surface plate with quality sandpaper and a little thin oil would be needed to lap the parts.  The fit can be checked with a thin layer of something like Dykem high spot paste (you can make your own with some thin grease like Vaseline and a little paint pigment).  Look up some Youtube videos about lapping and scraping machinery for some methods that might suit your situation.

i'm not sure he needs to do this without first checking the thickness of the stator and rotor. they should be very close, but the rotor needs to be thinner because when it heats up it expands and the pump can bind.

if there is galling on the rotational surfaces, it would be good to smooth those out, but don't remove much metal, those surfaces aren't that critical far as i know, there is oil constantly being sucked into the rotor through the system.. 

but if the rotor was .001" thinner than the stator and the stator had galling and wear where the two surfaces almost touch, then yes i would agree that you can lap the two plates.

it may not be possible for him to lap the rotor flat because the rotor may have a shaft sticking out of both ends of it.

[IRFP460]

Thank you for your suggestions, but sadly lapping is not an option, I have no equipment to make sure that rotor and stator have the same thickness.
I've polished both plates facing the stator with some 2000-grid paper, but that didn't give me a conciderable improvement, so I guess the problem really is somewhere inbetween rotor and stator.
I will test the pump with my shell S2 R 100 vacuum oil tomorrow and report if I get any changes.

There is a new member to the family, a Oerlikon Leybold D 2,5 E Trivac which gets me down to around 5 * 10^-3 mbar, so repairing the "chinapump" is just luxus by now; it is still a nice pump for doing "dirty" stuff, like pumping water etc.

Oh, and slightly OT: My trivac hasn't got the optional gas ballast, do you know where to get the valve for it?

[Ooste]

There are many good brands of vacuum pumps available that will provide years of trouble free service. I would invest in one that is at least 2 stage and has a gas ballast at the bare minimum. The gas ballast will help reduce the amount of oil changes required and can even be used to 'clear up' non condensables in the oil while ballasting the pump with it blanked off for several hours. The two pumps I use are a JB platinum 7cfm like this https://mechanicguides.com/best-micron-gauges/ for residential work and small commercial units and a YJ 11cfm superevac for large commercial systems and chillers.

Keep in mind that the vacuum pump will only perform well if you are using the proper accessories to connect to the system. Make sure you invest in large diameter vacuum rated hoses and core tools like the ones Appion makes, or you can even go wild and invest in large diameter CSST hoses which have a permeation rate of zero. Add a bluvac micron gauge and you will always be certain that the system is dry and tight when you complete your evacuation.