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Hi guys

looking for some advice regarding an overhung fan that has been having an ongoing vibration issue since installation. 200kw motor 1480rpm. Fan runs with acceptable vibration levels at 70% but get worse at we get close to 90% before dropping down slightly at 100%. Initially i would have thought a resonance issue but i would like to know if this is a Air flow/turbulence issue and what thing to expect. I've added a word document with relevant data if anyone wishes to have a look. 


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@Registered Member posted:

Look at your spectrum and all those harmonics, Can you get a waterfall plot of the axial fan nde?

Hiya, unfortunately i don't have the software to be able to create a waterfall plot. i think it would look rather like a standard resonance graph where there would be a dramatic high amplitude in the axial axis versus the others. i guess changing the Fans damping would alter its natural frequencies anyway right? 


AFAIK, resonance dont have harmonics.

This problem is present since installation, then presumably its a new machine, yes its posible a resonance, also posible its a installation problem, a damage bearing during transportation, what does the OEM said?, do you have spectrums to compare from new to now?

you dont have axial from DE bearing, can you get that one, do a phase analisis and post results here.

Some posible ways of investigating resonace is to do a bumptest to the the machine, get phase data of 1x during castdowns, measure 1x across the bases looking for discrepancies across baseplate.


sorry to say this, but from overalls and 1 spectrum no way is posible to give an advice.

Dear James, 

Resonance happen when one frequency of machine (in this case 1xrpm)  is almost same that one natural frequency in the base plate, or one component near to the fan. bump test is recommended in machine points , in the same direction of highest amplitude. 

if the problem is resonance, the amplitud will change when you modify the rpm.. I  believe is not our case.

the flow can create variation in amplitud if you have looseness, please, if you have two channel data colector make cross phase analysis, near to he high vibration point. 

next step is check internal cleareances, run out of shaft, coumpling condicion, an and try to make balance of rotor.. 

if turbulence is affecting the system, check looseness in damper (air inlet flow control system)

some cases are not easy to find de cause, but if there is one problem, also there is one solution. 




Thanks for the reply. Firstly I thought it was resonance because of the high 1x which does change with the speed of the fan, however I don't know enough about the role of air flow on vibrations and if that would make a difference if for example the air flow inlet was restricted or if turbulence was an issue..

I'll do a bump test and try and check internal cleareances, run out of shaft.. 



Is that the "fixed bearing ?

Do you have phase measuring capabilities  with you analyzer?

In the video I //think// I see the pillow block wobbling axially.

I'd be very interested in the contruction of the fan pedestal top plate.  Details like  thickness, and confirmed presence of any welded stiffeners in close proximity to the bearing bolts to start.


I'd also take a bunch of spectrums and 1X phase vertically all over the top plate.

Hiya Dan

So this is the floating bearing , the NDE bearing. And unfortunately no I can't get phase readings from my vibration sensor which is rather annoying.

Yes you're right, from the video it looks like the pillow block is moving axially back and forth with the shaft. 

Okay I'll see what I can find out as I've not got that information to hand. 


I would agree that an resonant structure in the fan axial direction with frequency near 90% of full speed is possible. The "deck of cards" shim pack under bearing housing and possible thin support base are suspects for correction.

The video indicates a rocking vibration mode. The low visual frequency is related to camera/phone video rate; correct?


As stated by others, resonance is typically speed related, not present across the entire speed range

The plates on which the fan bearings are mounted is typically twice as thick as the actual top plate. The top plate is not thick enough to support the axial movement. Both videos you provided clearly show this axial movement

By the looks of the fan base, there is very little stiffness on the top plate. I have had success by putting "C" channel the entire length of top plate, from each bearing bolt on the underside of top plate, parallel with the fan shaft. The "C" channel must fit tight to underside of top plate.

IF the fan is new, I would be talking with the fan OEM to get them involved in their design flaw.


"As stated by others, resonance is typically speed related, not present across the entire speed range"

Hi Dave glad to hear from you, okay so from what you are saying I don't understand why the initial vibration peak changed after additional duct work was added. It initially had a vibration problem that was worse at 80% and then after the additional ductwork was added it changed the point of the worse vibration to 90% so to me that would have suggested a speed related resonance issue but due to the change in mass it affected where the natural frequency was?  

Yes you are right  the videos show alot of axial movement, I didn't realise that the thickness of the top plate would have that much of an effect but now that you mention it it does look rather thin. Would this likely be the main cause of the axial movement, a lack of stability in the top plate ? Yes the fan is relatively new ( one year old)   the manufacturer is involved so I will suggest someone having a look at that as an option.

Thanks alot 


I faced similar issues with fans in paint shop of a car manufacturing indsutry.

because of the poor base any time there was paint deposition on the impeller the vibration shooted like anything.

We kept on doing impeller cleaning & balancing, did result in the vibration reduction, but after 10-15 days again the same problem.

In your case its the looseness getting excitation because of structural weakness.

If you correct the looseness the vibration might reduce but next time any other issue will get amplified because of the poor base.

Do correct the base. Strengthen it rock solid.

As discussed above the possibility of resonance in the axial direction, run/up coast down is crucial, also if the motor is powered by a VFD, you can vary the speed (higher/lower) from the current speed and check if the axial vibration is dropped.

In many cases, high vibration due to unbalance causes transient events in the time waveform that translates into looseness type 3, it doesn't mean you have a looseness problem. When levels are reduced after a balancing correction (correction away from the resonant frequency), harmonics will disappear.

Last edited by Registered Member