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Big axial fan vibration unbalanceable

Big axial fan vibration unbalanceable

Big axial fan vibration unbalanceable

Good morning,
i posted this thread before thread384-389985: Axial vibration unbalanceable Fan but i lost my connections in while and the thread was closed.
Please help me giving some indication about this problem because i really don't know ho to solve it.

  • Axial vibration found especially in a belt driven fan
  • Big vibration in axial (until 16 mm/s)
  • I tried balancing using axial magnitude and phase with no good results
  • I tried balancing using horizzontal and vertical mag/phase with no result
  • The phase is quite stable
  • i check for dumper status and vibration is not relatoed with that (some fan have new dumpers, some other i change them)
  • I checked for structure status (cracks, loosenes, etc..) and nothing related found
  • With radial indicator i checked for the runout of motor and fan pulley founding very low values (6/100 mm max)
Have you any idea what other test to do in order to uderstandt the phenomenon?
Do you think using ODS could be usefull for this issue?

Thank you

RE: Big axial fan vibration unbalanceable

We had high axial vibration at run speed on a large induced draft fan. It ended up being a resonance in the back plate of the fan wheel. There were stiffening vanes on the back plate that had corroded away, reducing the natural frequency into the run speed range. We found this using ODS and testing with a shaker. We measured bearing housing stiffness and created a rotordynamics model of the fan. We replaced the fan wheel with a new one and the vibration went away.

Johnny Pellin

RE: Big axial fan vibration unbalanceable

Hi JJPellin,
thank you for your good answer and your experience.

What do you exactly intend for " There were stiffening vanes on the back plate that had corroded away" ? What type of stiffening vanes are you speaking about?
This is a good point i didn't check but i'd like to understandt if your vanes are exactly vanes i'm thinking about.

RE: Big axial fan vibration unbalanceable

From your other post:


i tried to balance the fan with no result. Effectively i change vibration only in location referred for balance work.
I see in your recent post you attempted several more balance with different points. Is it repeatable that you can effecitvely bring down the vibration at the monitored point to a low / stable value, but not the other points?

If so, this seems to me consistent with tilted fan wheel which creates something like a couple unbalance. If you are using single plane balance solution, you can create a static unbalance which will provide equal/opposite response at the particular point you are measuring for your solution, but it won't bring the whole machine down because these static and couple components remain and do not cancel at other points.

I also note that you have already performed "check for fan skew". So hmmm. How much runout did you see? Perhaps the symptom of being able to balance only the monitored location also might also apply to pulley runout or misalignment? (that's an open question to the group,...I'm not sure).

Are the two fan bearings both on the pulley side of the fan, or is there one bearing on each side of the fan?

(2B)+(2B)' ?

RE: Big axial fan vibration unbalanceable

The stiffening ribs on the back of the fan were 1-1/2" by 1-1/2" angle iron welded to the back plate of the wheel. I have attached a picture showing the condition of these stiffeners as-found during the overhaul. We had also tried to balance our fan to correct the vibration. The balance was not effective. We did not have phase data on this fan and could not confirm that the phase was stable.

Johnny Pellin

RE: Big axial fan vibration unbalanceable

Are you absolutely sure this is unbalance? The reason I ask is that in almost every case of high axial vibration on a belt driven fan the cause has been misalignment. It's quite possible that the motor could be aligned to the belt but the fan may not be. My rule of thumb has always been if the axial vibration is more than the radial vibration (horizontal OR vertical) then something is misaligned.

Ron Frend

RE: Big axial fan vibration unbalanceable

Thank you guys for your support.

  • electripete: What do you suggest for a couple unbalance? This fan have a very thin impeller so it is disk form, when you suggest to balance in a couple method, you are talking about fan impeller as 1 plane and pulley as second plane?
  • JJPellin: Thank you for explanation
  • ronfrend: I'm just searching what it could be without any restriction. One clear things is the standard balancing procedure to reduce high axial 1X have no effect. Starting from this point every considerations can take place. I checked for the belt alignment and it was good (i used this similar laser device)

  • To clomplete diagnosis i performed some test as shown in a pictures found some values out of tolerance (in red into Table)

    There is a picture for fan, schematic where measurements done and values reported. All descriptions unfortunately are in Italian but if you have a problems to identify the mechanical parts related i can translate them.

    RE: Big axial fan vibration unbalanceable

    Hey vibsofistio,
    I hope you had a merry Christmas - not working too hard?
    Looks like you have an installation alignment issue on the taper roller bearings. The inner cup on both bearings and the inner race on one bearing are out of tolerance in the axial direction. If you have bearing mislignments that will give a high 1X axially. I suggest refitting the bearings.
    I did also see you have a high axial runout on the front of the impeller - is this a new unit?

    Ron Frend

    RE: Big axial fan vibration unbalanceable

    Hello ronfrend,
    everithing's fine, eat a lot, drink no so much, anyway all was nice.
    Hoping you had the same cheers!!!
    Anyway, coming back to work and ronfrend, yes i work too much bigsmile.
    What you said was exactly my first impressions and i provided a mechanical operations to fix the problem.
    The results? worst then before; spectrum is exactly the same and the axial components are bigger (until 24 mm/s rms)than before the mec operation.
    Unfortunately my head is very hard and i continue to have idea probably this problem should be related to mounting problems to fix.
    Anyone have idea what control or procedure to make (IE make same measurements to verify it coming back to misalignment bearings) dimensional control as made by me?

    Thanks to all

    RE: Big axial fan vibration unbalanceable

    Hi Vibsofistio, I had a quiet new year's eve. In Houston after a long flight so missed the celebrations but no hangover on New Year's Day. Result. bigglasses

    I'm at a loss as to why this is happening. From what you say you have a high (24mm/s) 1X axially at fan speed - is this ONLY on the fan shaft? In my experience there are only a few things that can cause this:
    1) Shaft to belt misalignment on the fan shaft pulley to the belt
    2) Bearing misalignment
    3) Bent shaft (but that would also give a fairly high 1X radially)
    4) Fan not mounted square on the fan shaft
    5) Pulley not mounted square on the fan shaft

    If you don't see any of these defects the only other thing I can think of is some form of aerodynamic force causing causing a high axial loading.

    Good luck and Happy New Year


    Ron Frend

    RE: Big axial fan vibration unbalanceable

    Good morning guys,
    i want to update you because finally the problems is solved and i'd like to share with you the impression about and also because this information could be usefull for you (this is the forum scope... sharing experiences).
    I attach spectras waterfall for evidence.
    The customer made this steps:
    1. Balancing of pulley
    2. Change bearings and Bearing housing
    No balancing of the fan.
    Customer said the problem was the pulley unbalance but in my opinion the problem solved for the replace of bearing and bearing housing.

    Please could you write your opinion about this SOLVED problem?

    Anyway in the next weeks i'm going to investigate in details

    good work to all

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