High 2X axial vibration in uncoupled 2 pole sleeve bearing motor 5

[edison123]

On testing an uncoupled, 2 pole (3000 RPM, 50 Hz) motor on sleeve bearings, I found the following axial vibration frequencies (mm/sec RMS) on DE bearing top cover.

Overall - 3.91 mm/sec RMS
0.04 at 1500 RPM
0.89 at 3000 RPM
3.00 at 6000 RPM
0.05 at 9000 RPM
0.07 at 12000 RPM

I confirmed it is not electrical vibration since the overall DE axial vibration did not drop immediately upon cutting off the motor supply. The motor test bed is a machined bed in which I have tested many high speed motors successfully, so no bed issues.

The overall axial vibration in NDE bearing top cover was only 1.5 mm/sec RMS. (I see normally axial vibrations on both bearings are almost the same).

Vibrations in DE & NDE Horizontal and Vertical were also less than 1.5 mm/sec RMS.

Any reasons why this high 2X axial vibration only in DE bearing ?

I am also posting this in mechanical forum to get their views, so don't RF me.

Muthu

http://www.edison.co.in

 

[edison123]

Hi Tmoose - It was overall axial velocity during coasting up. At 3000 RPM, the bearing top horizontal and vertical were also measured (but only at center points of your photo) and were well under 1.5 mm/sec rms.

Muthu

http://www.edison.co.in

 

[Steven L. Reid]

I know what my father wou;d have said.

Rotor balanced at some RPM.

Now out of balance at some other RPM.

Something loose in Rotor (Cage Bar?).

 

[edison123]

Feedback from the client

Decoupled motor vibrations (RMS Velocity in mm/sec) at Motor Opp Drive End (MND) and Motor Drive End (MD) in V/H/A.

Coupled motor vibrations (peak to peak displacement in microns / RMS velocity in mm/sec) at Motor Opp Drive End (MND), Motor Drive End (MD), Pump Drive End (PD) and Pump Opp Drive End (PND) in V/H/A.

The axial pressing of the top cover plate at locations A1 to A3 did not change the the axial vibration at location A4 of the motor DE bearing in both decoupled and coupled conditions. The top cover plate was not found vibrating severely.

The axial vibrations are stable from decoupled to coupled. Though the client is happy with the motor performance (when compared to high pre-refurbishment vibration levels of 9 to 11 mm/sec rms), I am still curious about the source of 2X axial DE bearing vibration.

Thanks epete for your valued tip. In future, we will do this test in our shop.


Muthu

http://www.edison.co.in

 

[edison123]

pete - Update.

Based on the performance of the last repaired motor, today we received another identical motor from the same client with the same vibration issues. This time, I will do the axial load resonance test of the 'flimsy cover' during the uncoupled run. Other than this test, do you think any other checks are needed? This time, I want to bring down the DE axial to less than 2 mm/sec rms.

Muthu

http://www.edison.co.in

 

[Tmoose]

Hi Edison 123,

As mentioned in my 6 Feb 20 17:53 post I'd definitely measure the vertical and horizontal vibration on the DE bearing at speed. Front, center and rear. Please see the attached image . Spectra at least. 2X amplitude and phase if possible.

Until with those measurements it is proven that the entire bearing housing is translating with similar amplitude and phase in all 3 horizontal and all 3 vertical locations it is a capital mistake to assume they are.

My hunch , like yours, is the axial vibration >> 50% of VERTICAL and Horizontal readings strongly suggests there is something structural going on here.
In my opinion Standard bearing housing centerline readings leave the analyst with one eye closed and both hands tied behind his/her back.

https://theawesomer.com/tardis-optical-illusion/495364/

https://d7hftxdivxxvm.cloudfront.ne...6967226a0900/larger.jpg&width=1200&quality=80

 

[electricpete]

I don’t have any special insight on this one, since it doesn’t match the pattern of that motor I linked where the flimsy plate was resonant.

I agree with you guys, it does still smells like resonance of some kind.

If you do a bump test, the condition of the rotor (rotating or stationary) can make a difference. i.e. the stator may slide back and forth axially under the rotor without moving the rotor while running, but not while stationary (no oil film). So a bump test while running is ideal from that standpoint BUT you probably won’t be able to bump it hard enough to create a 2*LF bump response large enough to stand out compared to the running 2*LF. So a compromise would be to bump test it while it’s coasting down before it comes to rest.


=====================================
(2B)+(2B)' ?

 

[edison123]

Thank you, Tmoose and pete. I will keep your valued suggestions in mind. This motor is in lot worse shape than the earlier one, so it will take more time to refurbish. I will update when we do the 3000 RPM run.

Muthu

http://www.edison.co.in

 

[Tmoose]

Now there is a second motor with High 2X axial vibration ?