Turbo Pump Problem

[daniel_pflug]

Hey all,

Currently I'm working in a vacuum/x-ray lab at my university. Attached to a large bell jar assembly was a small turbomolecular vacuum pump that recently started giving up on me. It's a Pfeiffer model TMU 071 P.

I've taken it off the enormous chamber and hooked it up to a much smaller vessel, but the pressure remained constant. The same happened when I blanked it off with just a full nipple on it to hold the vacuum gauge. Everything is wired properly, and the gauge was tested working on another chamber with a different pump. The turbo will only kick it down to the .5 micron range and sometimes a little bit lower. Needless to say, this pump should be capable of a much deeper vacuum.

One thing that is worth noting: the large chamber was mainly used for thin film coating (Al I believe) and this is the pump that always ran with it. The oil has been changed regularly, but aside from that I don't know if/when any maintenance was done (eg. cleaning) as I'm a relatively new face at the lab. Just thought I'd look for some ideas on how to fix this problem; especially before trying to tear the pump apart.

[Tom McCarthy]

Any chance of leakage in the line or from the chamber itself?

Tom

[Chris Bradley]

What d'you mean 'giving up on you' and holding constant pressure?

If the turbo is running at speed, it is a constant high pitched spinning and it's pumping. If it is not spinning then the controller will likely shut it down and it won't spin at all. Are you saying it is spinning very slowly?

[Nick Peskosky]

I'm going to have to agrew with Chris, could you be more specific in what way the pump seems to be malfunctioning aside from not attaining its typical ultimate pressure? What is your current measured pressure at the exhaust of the pump? It could be an ailing forepump not necessarily the TMP which is causing a loss in compression ratio.

[daniel_pflug]

Its controller says that it is hitting its top speed and staying true in that regard. The pump spins and operates well, but just isn't pulling the same vacuum it used to. I don't believe that it's an issue with the chamber/inlet side because I've tried it on multiple chambers (in descending volume) and it has hit the same pressure for each. It is possible I suppose that there is an issue with the foreline, I haven't investigated this nearly as much.

The mech pump seems to be holding up well and is fairly new (although it does get quite a bit of usage). If there is a leak between pumps would this cause such a significant change in ultimate pressure??

[Chris Bradley]

Its controller says that it is hitting its top speed and staying true in that regard. The pump spins and operates well, but just isn't pulling the same vacuum it used to.

This is nothing to do with the turbo, then. The turbo rotor operates [should be operating] in molecular flow and the geometry of the rotor blades dictates if it is working. The only problem with the turbo is if the blades have fallen out, in which case it'll be vibrating like mad and shut down. It is the pure laws of physics, a bit like pulling out your out-board motor and looking at the prop blades. They are either there, or they are not. If they are rotating in the water and your boat isn't moving, then it ain't the out-board motor that's the problem! Untie your boat from the mooring, or get off the sand-bank first!!

This must be a connection or chamber issue.

Does the turbo have a fast-bleed option, and is this blanked off and sealed correctly, or otherwise operating correctly? (It'll be a little port, M6 or similar thread, on the side of the turbo.)