Pfeiffer TMU 521 Bearing Replacement

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Bob Reite
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Pfeiffer TMU 521 Bearing Replacement

Post by Bob Reite »

In hopes this might be useful to others. I will describe how I replaced the bottom bearing in a Pfeiffer TMU 521 turbopump. My parts kit came with no instructions, just a bill of materials. The kit is for several different models, so some parts are not relevant to the TMU 521, and others need to be retained while carefully dismantling the pump to avoid damage to parts that will be reused.

Tools needed. For the most part ordinary tools, such as a couple of flat blade screwdrivers, a set of metic Allen wrenches, a small pin punch to hold the rotor and nitrile gloves. The two special tools needed are a tool that exactly fits the oil reservoir cover on the bottom of the pump and a pin spanner that will work in a recessed space.
tools.jpg
I machined the tools out of scrap angle iron. The tool on the left is for removing the cover. To use long screwdriver is inserted through the hole. The cover is on tight. That's why I needed an exact fit to avoid damaging the slot. The tool on the right is the spanner. I carefully measured the distance between the holes on the bushing and the clearance needed. I used a milling machine for everything except the final turning of the pins. For that I used a 4 jaw chuck in my lathe and turned the pins down to the final diameter.

To begin, the turbopump is removed from the system. Remove the motor controller as described in the operating manual, it is held on by just two cap head Allen bolts. Do not remove the Phillips head screws, those just hold on the cover to the electronics inside.

Place the turbopump high vacuum side down and remove the slotted cover. Use a pair of small flat blade screwdrivers to pry out the oil cartridge. You don't have to be too careful with this part as it will be replaced with a new one from the kit. This is what you will see at this point.
cap_oil_cart_removed.jpg
The next step is to remove the bushing with the pin spanner. This is not down very tight. I had no trouble doing it by hand with the wings on my fabricated tool.
retainig_ring_removed.jpg
Carefully remove and retain the large black O ring pictured. Also retain the black O ring that you will find in a groove on the bottom of the bushing just removed.

Now remove the cone nut. Yes, that cone shaped piece is actually a nut! It has left hand treads!. Set the turopump on it's side, with the flat part of the motor housing on the bench. If the high vacuum flange has a screen fitted, carefully remove it now. Use a metric Allen wrench to turn the cone-nut clockwise. To keep the rotor from turning, use a pin punch that just fits into one of the holes in the rotor to hold it across one of the braces. If you happen to have a long metric screw that fits one of the tapped holes in the rotor, so much the better. Slowly turn the cone-nut clockwise. You will feel it loosen and the rotor slowly moving toward the high vacuum inlet of the pump.

At some point the rotor will seem to bind. Do not be alarmed. The magnetic bearing at the top is pulling the rotor to it's fixing point at the top of the pump. This is the reason to go slowly during the removal of the cone nut. Do not even think about disturbing the screws for the top bearing. Pfeiffer put "security screws" here for a good reason, it would take specialized equipment to get this bearing adjusted correctly.
cone_nut_removed.jpg
Now that the cone-nut has been removed, set the pump back on the high vacuum flange and carefully remove the various orange and blacks O rings. Take frequent photos as you go so, that you can reassemble in reverse order. Replace any matching ones from the kit, even if the originals look OK. Reuse undamaged ones (All mine were in good shape) for which no replacements were provided.

At this point the easiest way to remove the bearing is to loosen and remove the seven Allen head bolts holding the motor stator assembly to the turbo body. Use a couple of flat blade screwdrivers to carefully pry loose the assembly. There is a large O-ring that will make it a bit tight.
pry_up_motor_stator.jpg
It will be easy then to grasp the bearing and remove it by hand, I did not have to use a bearing puller. Be sure to make note of the orientation. This is a thrust bearing as well as an axial bearing and the load is only correct in one direction. While at this state of disassembly, look for any signs of damage inside, I did not see any fortunately.

Place the motor stator assembly back on the main body, but don't install any bolts yet. Place the new bearing on the rotor shaft. While the tolerances are tight, it is not a press fit, you should be able to just use your fingers to expose enough thread for the new cone-nut to grab. If it does need to be pressed down, only press against the inner race, using a piece of brass tubing that just clears the rotor shaft. Tighten the cone-nut counter-clockwise slowly just a turn or so. Now reinstall and tighten the seven Allen head bolts to secure the motor housing back to the main body. Set the pump back its side and continue to tighten the cone-nut counterclockwise. At some point you will feel the rotor try to turn. Use the pin-punch or long screw to keep the rotor from scraping as it begins to clear the stator blades. Light to moderate resistance will be felt as the nut draws the rotor back into place resisting the force of the top magnetic bearing. The correct tightness is achieved when all of a sudden you encounter much higher resistance as the rotor bottoms out. Unfortunately I was never able to obtain torque specs for the various fasteners, I just had to rely on my memory on how tight things were when I removed them. Pfeiffer used left hand threads on the cone-nut because their engineers felt that a left hand thread would tighten, not loosen, during normal operation.

Place the pump back on the high vacuum flange and reinstall in reverse order O rings and washers that we removed, with new if they came with the kit. Reinstall the bushing. It is correct if a hole in the side is about half covered. Finally install the replacement oil cartridge and the cover.
Turn the pump upright and verify that the rotor spins freely with no rubbing. It should take at least a minute to come to rest after giving it a push by hand. Reinstall the screen if one was provided. Reinstall the controller electronic package and the pump will be ready to place back in service.
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richnormand
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Re: Pfeiffer TMU 521 Bearing Replacement

Post by richnormand »

Nice info.
Thanks.

About to do this on my TPH 055.
It is very noisy at the start. However once at speed it is nice and quiet until shutdown.
Once the speed is reduced the noise comes back.

The shaft does have some play at the bottom so I assume the noise comes from the worn bearing having too much wear.

Cheers.
Matt_Gibson
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Re: Pfeiffer TMU 521 Bearing Replacement

Post by Matt_Gibson »

I always wondered what noises are normal and what noises aren’t. Also, what’s a normal time period for start up and stop…

Mine makes different noises at different speeds. Never any sort of squealing or squeaking.

-Matt
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Richard Hull
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Re: Pfeiffer TMU 521 Bearing Replacement

Post by Richard Hull »

Turbos tend to make noise during start up and depending on mounting, all have resonant points during startup which can be disturbing for a few seconds as they come into and speed out of resonance. All noises are vibratory and never squealing or rasping or scratching. The important thing is that they all go virtually silent at running speed.

Likewise they go through a short vibratory resonance in slow down. They can get downright ear piercing high pitched as they slow down if under deep vacuum. Mine is very annoying high pitched sound for about 3 minutes in the middle of its 10+ minute slow down. Under vacuum they can take many minutes to slow to a full stop. Sometimes, I think it has stopped many minutes after shut down, but drawing close to it I can still hear it turning (Gotta' be real close to it.)

My pump is a used, original, one-owner pump, controller and cable with 2400 hours on it when I obtained it. I did properly lube the oil pad when I got it. It has served me well, indeed.

Richard Hull
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Matt_Gibson
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Re: Pfeiffer TMU 521 Bearing Replacement

Post by Matt_Gibson »

Mine gets pretty quiet at full speed, but I usually keep it in standby since that is enough to go below 0.01Microns (my gauge limit). No vibrations can be felt when I touch the turbo.

Getting up to speed only takes a few minutes. It seems to take forever (an hour) to come to a full stop.

Looks like I’m good. I have a spare that needs a new bearing, so I’ll probably try out the Alpine Bearing to see how well that goes.

-Matt
Matt_Gibson
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Real name: Matt Gibson

Re: Pfeiffer TMU 521 Bearing Replacement

Post by Matt_Gibson »

Just wanted to say that I successfully removed my old (shot) bearing from the spare using these instructions. I’ll order a replacement bearing from Alpine to see how they do.

Should mention that I have the TMH071P.

Thanks!
Matt
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