Rebalancing turbopump rotors / vibration analysis

[Andrew Seltzman]

As some of you know, I bought 3 Edwards ext70 turbo pumps on ebay about 2 years back and have been in the process of restoring them.

The initial damage:
Of the 3 pumps all 3 had cracked magnetic bearings (all rotor side magnets have small cracks, and some of the stator magnets were cracked on some pumps), one pump had the retainer fall out of the bottom bearing causing the rotor shaft to crash into the housing and partly melt, one was filled with corrosion and had a worn bottom bearing and the other was mostly fine except for the bad mag bearing.

Controller:
I bought a controller for $150 on ebay a few weeks back, the controller is in perfect condition and dries the pumps.

Pump repairs:
2 pumps repaired so far, one with magnetic top bearing other with ceramic top bearing.
http://www.rtftechnologies.org/general/ ... 0turbo.htm
(note: pump shown is one with the magnetic bearing replaced with a ceramic bearing, this modification does not work at this time (vibration issues) )

The one with the magnetic bearing has vibration issues during startup, with the rotor shaft hitting the top safety bearings, however once it pushes past this, it will spin up to 90krpm very smoothly.

Vibration analysis:
A piezoelectric accelerometer is attached to the pump case and the vibration is observed. Vibration waveform is sinusoidal at 1500Hz (expected for 90krpm pump at full speed) Charge sensitive amplifiers for accelerometers on order.

Rotor balancing potential setup:
The accelerometer will be connected to CH2 of the scope while CH1 will be connected to the phase A hall sensor output. The rotor will be manually spun until the hall sensor triggers and will be marked. The accelerometer will be aligned with the phase A triggering point, and the pump will be started. The phase difference between the hall sensor and the vibration sensor will indicate the position of the imbalance.

Hardware for use:
I have 3 single axis accelerometers and 1 triple axis accelerometers. I have 3 charge sensitive amplifiers on order.

Pictures:
Scope and turbo
Accelerometer
Accelerometer mounted
scope vibration trace at full RPM


Andrew Seltzman

[richnormand]

Very interesting experiment. I'll watch closely your results.

I visited your website a few times for inspiration while I was getting my two TPH055 on line a few months ago.

Would be nice to have accelerometers mounted on all three axis and read them simultaneously to see if the rotor is vibrating up-down or sideways or in a more complicated resonnant mode on its rotational and lateral axis. Also if there is a resonnant mode with the (heavier) housing you could end up with the rotor going one way internally and the outside housing going the opposite way (or at a phase angle).

Finally, is the hall effect sensor signal speed and position independent or does it add is own signature as the speed change?

With all of this it will be interesting how well you balance your unit. Do you know how the pro rebuild place do theirs?

Keep up the good work.
Cheers.

[Andrew Seltzman]

I have a three axis accelerometer, and several single axis accelerometers, but my scope meter only has 2 input channels.

The 3 hall effect sensors give a rotor position output for the 3 phase motor (edwards pump). There is probably some phase lag, but I'll have to measure this. Not sure how the pro-rebuild places do theirs.

New equipment acquisitions:
---------------------------------------------------------
I scored a TCP015 controller on e-bay for $113 this week. It arrived and works perfectly.

One of the three charge sensitive amplifiers(endevco 2680M62-202) arrived. It converts the 37pC/g output from the piezoelectric accelerometer (endevco 2217e) to a voltage output(0.2-2mV/g gain)

New turbo pump tests:
---------------------------------------------------------
Note, for the scope pictures. the scope is set to 10:1 probe but is using a 1:1 probe, so divide all readings by 10. Also, I am not sure about the gain the amplifier is set at.

Pics:
Charge sensitive amplifier
new turbo controller for pfeiffer(known good) turbo pump
horizontal axis measurement
horizontal axis scope screen
other horizontal axis scope screen
vertical axis measurement
vertical axis scope screen
turbo controller in standby
horizontal axis scope screen in standby(1khz)

and now the edwards pump (slight unbalance):
horizontal axis measurement
horizontal axis scope screen
other horizontal axis measurement
other horizontal axis scope screen
vertical axis measurement
vertical axis scope screen

note that the vibration on the edwards pump is about 5 times worse then the pfeiffer pump.

Andrew Seltzman

[richnormand]

"scored a TCP015 controller on e-bay for $113 this week. It arrived and works perfectly"

ARRRRRGH! must have missed this one!
OK, just updated my ebay search.
I think this is the perfect one for my TPH055

Cheers and look forward to your progress with balancing the rotor.

[John Futter]

Andrew
I'm not sure if you are trying to make silk purse out of a soars ear

we had two mass specs at work with the Edwards pumps (read Micro seiki {or similar}) that whined to dangerous levels to the ears-- now replaced with Phiefffffers (is that enough f's or have I missed one)

Anyway they did this from new FWIW

[Doug Coulter]

No, John, it's not how many F's, it's how picky those _'ing Austrians are about where you put them....but I love my Pfeiffer systems myself, and they've taken some pretty serious abuse and keep on running -- quietly. There's even one in my bedroom (#2 fusor) and we don't notice it much until the little diaphragm backing pump kicks on.

My little one has had 4 serious air inrush accidents (as in broken main window kinds of things), and made a horrible noise during that, but came out fine. The big (520lps) one's had one of those, and one dose of glass bits falling on the screen, one of which went through. Again, horrible noise, but recovery with me doing nothing at all -- thank heavens, that one I bought new....not cheap.

Even those slimy little (50lps) ones that were going cheap on ebay (straight turbo, no drag stage) look like they're going to work fine.

[derekm]

Sounds like an update of the balancing gear my father used to balance industrial fans. In that set up the accelerometer fired a strobe light . By placing marks on the rotor the mark nearest the direction of accelerometer indicated the direction of imbalance. When I was a young boy he explained this facinating box of instruments to me and then later as a teenager I saw him using it. When he died it was the item of his I most desired to remember him by. If you are interest in what the kit to do this job from 50 years ago looks like I can post the pictures.