High Vibration in motor in horizontal direction mounted on CT structure of Pultruded FRP

[Apsp]

High vibration has been observed horizontally in motor DE&NDE as well as in gear box in a newly build 4 cells Cooling tower of Pultruded FRP material after commissioning of 20 month.
High vibration has been observed in motor mounted on 3rd cell and vibration is 12 to 14 mm/ses (RMS Value) at 570 rpm and 8 to 10 mm/sec at 1x rpm of motor.Vertical and axial vibration of motor is very less ie. 2 mm/sec and 2.5 mm/sec at 570 and 1x rpm speed.
Rest of motos mounted at other cells(1,2&4 nos) have normal vibration in each direction.
Details of equipment-
Motor rpm-1488
Gearbox ratio-13:1
Fan dia-336"
No of Fan blade-8
We have checked all nut bolt tightening of structure,did replacement of motor, gearbox,fan assembly but vibration not reduced.
I am surprised that vibration is less in 1x rpm and high on 570 rpm and it is not reducing.
Please suggest.

 

[Tmoose]

Were detailed vibration measurements included in acceptance testing?
Did the vibration increase slowly, or all at once ?
What does the manufacturer have to say?

What details RE: operating or atmospheric events have occurred in the last 20 months that are being withheld?

I suspect there are issues with any or all of these -
the support structure.
The fan balance ( hollow blades love to fill with water or debris )
The couplings or driveshaft unless belt driven.

 

[Apsp]

Fan blades already have replaced with new blades.
Drive shaft also has been replaced with coupling and rebalancing has been performed also. But no effect on vibration.

Fft showing mechanical looseness on 570 rpm but we already have checked all base casting,structure joints,foundation joints of gearbox,motors.
A mysterious force coming from somewhere.dont know.
I can share overall vibration reading on 570 or 1x rpm.

 

[BrianE22]

Is there a coupling or is the fan directly mounted on the motor shaft?

 

[electricpete]

There is no obvious match for the 570cpm frequency. Do you have a more complete description of the gear including tooth counts? Do you have additional description of the equipment bearings (sliding or anti-friction and if anti-friction, do you have part numbers).

> did replacement of motor, gearbox,fan assembly but vibration not reduced

This, combined with lack of obvious match for the 570cpm frequency makes me think maybe this is a resonant frequency excited by some kind of broadband excitation.

Is the 570cpm directional (much higher in one direction than others)?

To narrow things down, maybe run the motor uncoupled to see if the frequency is still there.
Also perhaps do some probing to measure vibration of the support structure and adjacent areas to get a rough idea of where that frequency is present and how high.
Photo of the equipment is always appreciated, if you can post it.





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(2B)+(2B)' ?

 

[drawoh]

Apsp,

How rigid is your mount structure?

--
JHG

 

[Tmoose]

Were detailed vibration measurements included in acceptance testing?

 

[Tmoose]

Did the vibration increase slowly, or all at once ?

 

[Tmoose]

What does the manufacturer have to say?

 

[Tmoose]

1488 input rpm / 13 = 114 fan rpm

https://midwesttowers.com/wp-content/uploads/crossflow-cooling-tower-diagram-2-800x445.png

I'd do a static stiffness test on the "bad" frame and a "good" frame pulling horizontally on the eyebolt on top of the motor in the "motor horizontal" direction.
The stationary reference point needs to be chosen based on information stubbornly withheld to date.

In addition to measuring sideways displacement I'd put a good level on top of the motor looking for tipping, which is an indication of the torsional stiffness of the FRP pultrusion frame.
Since no pictures or sketches have been provided I am forced to envision/guess about the frame.
I'd guess it runs from the gearbox out to the drive motor.
I'm kind of expecting the pultruded FRP frame resembles a 1950 truck frame with C channel side rails and a few simple cross-members stock on here and there.

https://www.autopaper.com/images/15535/photo/img8829_47879.jpg

Torsional stiffness is L-O-W. << Less than 10% of one that had closed section/box rails and well done cross-members including an X member.
If some invisible internal de-lamination has occurred the stiffness is even lower.

 

[JJPellin]

We have used motion amplification video to diagnose problems with a cooling tower fan motor. It can help to detect where the relative motion is happening which can lead you to the weak point in the structure. I have attached an example of one part of that video analysis on our fan.

Johnny Pellin

 

[electricpete]

> I have attached an example of one part of that video analysis on our fan.

It looks like a whole lot of relative motion (at the frequency shown) between the motor feet and the c-shaped channels they are mounted to.
I'm curious what that represents... loose hold-down bolts, spongy shim pack, plastic shims?

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(2B)+(2B)' ?

 

[Tmoose]

Op Joined eng-tips Nov 18.
Last post on this thread Nov 18.
Last login Dec 13-ish.

Maybe they fixed it.