My vacuum pump has gone wonky!

[martinw]

Dear All,

I hope you can help.

I have a much loved small medical vacuum pump with about a 0.1HP motor. Yes, it's tiny. I have used it for many years in a vacuum bag set-up for laminating sheet goods like MDF, styrofoam, and phenolic boards.

The latest incarnation of the bag is only about six feet long and two feet wide and the pump has always worked fine once the leaks have been sorted.

Now... the bag lives on a worktop, but the pump etc lives in a drawer and is only plumbed in on the rare occasions when it is needed.

The plumbing arrangement is as follows

Bag
Dust filter
Tee to pressure gauge
Vac pump.

Worked just great for 15 years until I stupidly replumbed the dust filter backwards. Fifteen years of fine dust was shunted into the vac. pump which made a bit of a protest, but still went on running.

Being curious, I took the pump to pieces, and saw it was a rotating four carbon vane design. OK, the dust might have knocked the tips off the vanes and reduced efficiency.

The wierd thing is that if I put my finger over the tube that would go to the vac bag, the pressure gauge reads minus 0.8 atmospheres , but when replumbed to the bag, the set-up seems incapable of voiding any air and creating any kind of vacuum in the bag.

My guess is that I should run the pump for a few hours to "re-bed" the carbon vanes. It makes a horrible noise, so I have not tried it.

Any views would be really welcome.

Thank-you

Best wishes

Martin

[Al_The_Man]

I think I know the type of pump you have, they have four carbon-like blades that are flung out at centrifugal force against the chamber?
Is there a possibility you can get replacement blades from the manuf.? it is a very simple design.
Are you sure the noise is coming from the vanes?
Al.

[martinw]

Originally Posted by Al_The_Man I think I know the type of pump you have, they have four carbon-like blades that are flung out at centrifugal force against the chamber? Is there a possibility you can get replacement blades from the manuf.? it is a very simple design. Are you sure the noise is coming from the vanes? Al.

Dear Al,

Yes, the vanes are flung out by centrifugal force. I looked up the generic design having taken the thing to pieces. Quite an elegant concept.

The noise is now pretty much as it was when it was working OK. The noise only happened when I stuffed a load of crap from the filter into the pump. It now sounds pretty much as it did. It always sounded noisy before it managed to pull a vacuum, and then became pretty-much less so. It still does with my finger blocking the hose, but it doesn't in the bag.

I think it's a volume through-put problem.

The manufacturer is no longer in business. Maybe time to buy another pump.

Actually, I can see how you can "bed-in" the vanes as they rotate at the circumference, but how do you do that at the top and bottom of the vane??

Thanks

Best wishes

Martin

[Al_The_Man]

Originally Posted by martinw Actually, I can see how you can "bed-in" the vanes as they rotate at the circumference, but how do you do that at the top and bottom of the vane??

It has been a very long time since I was involved in repairing one for a friend back in the UK who had a printing business.
I am not sure what you mean by the top and bottom?
I imagine if they have a deal of wear, you may be stuck.
I am suprised that there is no one that could supply parts for these as they seemed like a decent pump and they must have been around for a long time.
Al.

[Geof]

It sounds like you took the vanes out. Did you put them back facing the same way as before? Did you also put the same vanes back in the same slots?

Your conclusion that the vanes may need to rebed if they have been mixed up is probably correct. Your enhanced noise can be coming from the vanes being switched to different slots so their fit it now different.

If you are resigned to getting a new pump just run this one and see if they will bed in okay; in practice you have nothing to lose. They really only need to bed in on the tips.

Incidentally what type of gasket was on the side(s) of the chamber? The side clearance on the vanes can be affected by the gasket thickness; too thick and you get leakage around the sides of the vane, too thin and you get the vane binding on the sides so it cannot slide freely in the slot.

[martinw]

Dear Geof,

Thank-you.

I took one vane out, out of curiosity, and returned it in the same orientation.

I still can't work out how the top and bottom parts of the vane manage to seal.

Anyway, if I let the pump run to try and bed the chipped vanes in again, would you recommend a closed or open end to the pump inlet. My guess is a closed one, but I do not know why.

Best wishes,

Martin

[martinw]

Originally Posted by Al_The_Man I am not sure what you mean by the top and bottom? I imagine if they have a deal of wear, you may be stuck. I am suprised that there is no one that could supply parts for these as they seemed like a decent pump and they must have been around for a long time. Al.

Dear Al,

Sorry, I didn't explain myself at all well throughout this thread. Maybe sheer frustration had a hand in that.

Will reply shortly,

Best wishes,

Martin

[lgalla]

Martin,sounds like you didn't clean the filter in 15 years.Also sounds like you have an attachment to the little sucker.Pour an epoxy block around it and have it as a conversation piece of a friend who passed away.Time for E-bay.
Vacuum pumps can be touchy.My 1Hp pump is not to be shipped with oil as upon startup the vanes will break.Rotating the pump manually will place the oil where it should be.Keep this in mind if getting a pump on E-bay.
larry

[HuFlungDung]

Vane type pumps do wear over time, ie., the housing itself can go out of round, which adds noise to the operating pump. The vanes can actually jump over the wear ramps inside the housing. This allows the compressed gas to leak back into the chamber behind, and represents inefficiency.

If it is the type of pump which I am picturing, the 'seal' as it were, is controlled by the gap between the rotor and the housing. I'm assuming the rotor is offset to one side, and barely clears the housing, with the inlet and outlet ports being on either side of this closest approach. Reboring the housing will smooth up the surface, however, it will also increase the gap between the rotor and the housing. This increases the amount of 'carry-through' between the inlet and exhaust, and so the pump is less efficient after such work is carried out. In some cases, the end-plates bolt pattern can be reworked to move the rotor closer to the housing again, or an eccentric sleeving operation could be carried out on the bearing bores.

[Geof]

Originally Posted by martinw ....I still can't work out how the top and bottom parts of the vane manage to seal....

The vanes seal on the tip because they are held out against the housing by centrifugal force. They seal on the sides simply because there is such small clearance; in fact on an oil-less pump they leak a little bit. They don't seal on the inner end, they seal against the side of the slot mostly because the pressure differential pushes them back; some vane pumps I have seen had small vent holes leading down to the front of the slot to sort of guarantee pressure was down there.

From your comments it sounds like you filled a pump with dust, took it apart and did not clean anything; the vanes are still in the same slots.

Take it apart, mark everything as to slot and orientation and clean everything assiduously.

It sounds like an oil-less pump so it should be assembled totally clean, oil free and dry.

Now that I know you did not mix up the location of the vanes but did, maybe, fill the pump with dust, my diagnosis is that one of the vanes is stuck. This will allow the pump to pump down a small volume but not a large volume and it will also makes the pump noisy. The stuck vane comes clear of the eccentric housing every revolution and then makes contact again causing a very loud rattle.

[Al_The_Man]

I wonder how they lap the blades in at the factory?
The fact they are made of such a friction free material, I would think it would take too long to bed them in by running.
Unless they are precisely machined?
Al.

[Geof]

Originally Posted by Al_The_Man I wonder how they lap the blades in at the factory? The fact they are made of such a friction free material, I would think it would take too long to bed them in by running. Unless they are precisely machined? Al.

This is probably a situation similar to running in the cylinders and piston rings in a car engine. You have two not-completely-perfect surfaces rubbing against each other and the imperfections in each polish each other to a smoother state. When you replace the piston rings it is necessary to hone the cylinders to get back to two non-perfect surfaces once more bedding in. If you do not do this the new rings do not run-in correctly. I would suspect that if the vanes were replaced in one of these pumps a similar thing would have to be done to the housing, rough it up so you are replicating the original bedding-in conditions.