(Side Bands) Is it an issue in this Case? 1

[Saeedo]

Dear All,

I need some help, We have already repalced more than 3 motors in therr years time, The bearing defect is the main issue, We loost bran new bearings within 8HRS,

Is this side bands in the spectrum plots mean some thing

 

[electricpete]

We can tell from the spectrum it is a 2-pole motor. Focusing on the failures, it may be helpful if you can explain:
1 - bearing type (sleeve or anti-friction, and if anti-friction what type)
2 - horizontal or vertical motor
3 - lubrication (oil or grease, and preferably identify the type)
4 - Driven equipment. And is it a high-temperature such that thermal growth is expected.
5 - intermediate equipment (coupling type, presence of belts or gearbox)
6 - Do the failures occur while coupled or uncoupled?
7 - roughly how many horsepower (probably not important to the question, but it's difficult to visualize the machine if we don't know if it is 1hp or 10000hp)
8 - vfd present?
9 - what do the bearings looks like after the failures.
10 - If ball beairngs, is it an inboard-fixed and outboard floating arrangement, or is it a cross-locating arrangement. With cross locating arrangement when uncoupled (and tagged out) you can move the shaft noticeably, perhaps 1 / 16". We have similar symptom on cross-locating bearing motor... haven't quite figured it out yet.

Assuming these are deep groove ball bearings, there are a number of possible causes for these sidebands:
A - Lack of preload on the bearings such as from no wavey washer or improperly sized wavey washer.
B - Process oscillation at the sideband frequency (approx 10hz).
C - machine mounted on flexible base with resonant frequency similar to sideband frequency (10hz). I don't know why this occurs, but I have seen it.
D - Series of running-speed/multiple-running-speed sidebands around BPFO and aournd BPFI that masquerade as sidebands around running speed. Remember BPFO and BPFI always add up to a whole number (the number of rolling elements), so if BPFO is 0.2 orders above a running speed harmonic, then BPFI is 0.2 below a running speed harmonic and the series of 1x sidebands around these peaks can look like 0.2 sidebands around running speed harmonics. For example 8-ball bearing with BPFO=3.2, BPFI =4.8, you can have series of peaks (in orers)
0.8X = BPFI-4X
1X = 1X
1.2X = BPFO-2X

1.8X = BPFI=3X
2X = 2X
2.2 = BPFO-1X

2.8X = BPFI=1X
3X = 3X
3.2 = BPFO

3.8X = BPFI-1X
4X = 4X
4.2 = BPFO+1X

As you can see, it looks like a series of 0.2x sidebands around 1x, 2x, 3x, 4x etc, when in fact it is 1x, 2x, 3x plus series of running speed/multiple-running-speed sidebands around BPFO plus series of running speed/multiple-running-speed sidebands around BPFI. It is easy to tell the difference if you put the spectra on log scale and examine all the peaks to fit them into a pattern. For example in the above case if it is actually bearing frequencies, then we expect to see 2*BPFO = 3.6X and 2*BPFI = 10.6x also showing up. If those frequencies don't show up even on a log scale, then you can throw this scenario (D) out the window.

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(2B)+(2B)' ?

 

[Saeedo]

Dear electricpete.
Thank you for your response,

The bearing type used is Deep Grove Ball, Inboard Fixed
Bearing SKF -6319M/C3, Greased by Shell alvania RL3.
- It is Horizontal ABB 770HP Motor driving sulzer pump 12 stage for production water injection.
- All failures accrued during normal operation -Coupled

TO be honest, the problem appeared after the maintenance team changed the bearings after 3 years of continues operation. Since the last bearing change three years back, this issues appears, Some time over greasing causes the failures or the greasing type.

We fixed one newly overhauled motor by ABB and only the bearing last for 8hours, it very disappointing.

Note: The Sidebands are 5HZ as you can see around 3X and 2X. My big question is. Is there are anything to do with the electrical motor like stator, rotor, air gape or any other problems.???

Thanks

 

[electricpete]

Note: The Sidebands are 5HZ as you can see around 3X and 2X. My big question is. Is there are anything to do with the electrical motor like stator, rotor, air gape or any other problems.???

For off-center airgap, you would normally get 2*LF vibration.

For bowed rotor, or other forms of dynamic eccentricity (disymmetry rotating at 1x), you normally get pole pass frequency sidebands around 1x and 1x harmonics. Pole pass frequency for this motor would be around 2(*50-49.69) = 2*0.31 = 0.62hz.... much less than the 10hz spacing of your sidebands.

In summary, I don't see anything suggesting stator problem, airgap problem, rotor problem, or other items commonly called "electrical-related vibration"

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(2B)+(2B)' ?