Hi to all, I have an overhung fan showing high 1x axial vibration measured from the two pillow block housing.

Initally, my concern was bad installation, but discussion with the bearing supplier, indicates that installation isn't the issue. His call was based on relative low operating temperature of the two bearing blocks, approx 60 degC.

My question now is the high axial vibration due to inadequate structure rigidity aka foundation issue or due to fan unbalance? Is there a way for me to determine which is which? I seem to be having looseness, (harmonics in the axial direction.) Is temperature a sufficient indication of status of bearing in the plummer blocks? (SNU615 - bearing SKF22212EK)
thanks in advance...........

overhung_fan_FFT.doc (84 Kb, 68 downloads) Word document format

Temp may be a good indicator if the misalignment is severe.

With 30 mm/sec, I think this is 15 mils displacement, in the axial direction and about 7 mils in horizontal, I would lean toward unbalance. But what does the waveform show, a sine wave pattern or what?

A common symptom of unbalance in overhung rotors is axial vibration. Hopefully the illustration will explains why.

What does the way form look like?
What are the readings vertical, horizontal?
What is the phase reading like at turning speed in the radial plane?
What are the phase readings on the bearing axial from side to side, top to bottom?
Are the bolts tight?
What does it feel like?
What has changed since the bearing installation?
Was the problem there before?

Just a few questions I would be asking myself.

quote:

Originally posted by Muah:

My question now is the high axial vibration due to inadequate structure rigidity aka foundation issue or due to fan unbalance? Is there a way for me to determine which is which?

It could be either way. Low dynamic stiffness could be due to closeness to resonance (likely in axial direction) or it is just low static stiffness. Of course, in any case if unbalance is reduced, vibration response will also be lowered.

Based on axial spectra you probably have clean imbalance. In order to assess actual amount of unbalance in this rotor a trial weight should be attached and sensitivity measured. Then unbalance (lb-inch) could be calculated, ISO grade derived and compared with OEM specs. If it is within the specs then low dynamic or static stiffness is the cause. But I'd start with a bump test.

David

Don't know your environment but 60 C looks hot.
Waylon I liked your description of whirling of shafts, how would the waveform look like?

Thanks all for your feedback. and sorry for not getting back sooner.
Last friday, what i did was place a choke on two side of the frame, and the vibration in the axial direction was halved!!
Prior to this high readings, the springs in the isolator was found to be damaged, and was replaced with rigid mountings, that is when all the high vibrations started showing.
Hence my inital question to all, which came first? Unbalance? causing the excessive vibration but was dampened before by the 'bouncy' springs? or rigid support not being rigid enough, causing the slight imbalance in the impeller to be amplified?
And since adding a choke to restrict axial movement of the fan-motor frame, the axial vibration has halved. I am gearing towards a bad support resulting in excessive vibration all-round.
As for waveform, my analyzer doesn't do waveform, so I can't collect any waveform data.
But interestingly, will waveform analysis help me decide if it was unbalance or insufficient support?
As for resonance, wouldn't i see a more directional high vibration amplitude?
thank you all

What size and weight is your fan? If they have put it on springs and isolators there are probably some huge forces inherent to the system. Since your fan shaft is greater then 2" inches it must be some "monster" running above the natural frequency of the unit. With defect springs the natural frequency will change and can bring it close to operating speed.

What is a choke?

Pardon all.

What is the purpose of having isolators/dampers?

Is it for isolation of vibrations between system and surroundings?

Is if for reducing the vibration of inherent high vibration equipment?

Changing the natural frequency of the system?

thanks all

Choke = Wedge.

Hi Waylon,
some how or the other, i am only able to seeing the diagram you posted now. From your diagram, wouldn't radial vibration be more significant compared to axial? Is there a general ratio i can expect to see?

V [mm/s] 1xTS amplitudes
==========================

Fan DEH = 9
Fan DEV = 1.6
FAN DEA = 15

Fan NDEH = 15
Fan NDEV = 8
Fan NDEA = 15

(NDE = impeller side]
(DE = pulley side)

thanks

I don't know about a fixed ratio, but it is not uncommon for overhung unbalance to create higher axial than radial.

Some things to try may include:
1 - bump check to check for resonances near running speed.
2 - attempt balance. We don't know the size of your fan yet but from what I have heard here a static balance often does the trick (attempt static balancing until bearing phases show the remaining unbalance is predominantly couple).

Another possible way to visualize the effects of overhung unbalance is attached. It suits me a little better because it shows movement at the bearing housings, which will require motion about the flexibility of the bearing supports (after all, if we have infinitely rigid bearing housing, we will never see any vibration except perhaps impacting, right?). The bearing supports are shown as tall and skinny to emphasize this point. While your machine probably doesn't have the same static support flexibility suggested by the figure, it is really dynamic support flexibility to running speed vibration that counts.... and your machine may have substantial dynamic flexibility to movement of the bearing supports at running speed. I am not at all saying one is right and one is wrong... just different ways to look at it. The real-world situation will involve bending of multiple parts and more complex than either.

Let me know if there is difficulty opening the attachment (powerpoint format).

Also someone mentioned misalignment - worth considering. Is it direct or belt driven?

On your spectra if I was reading right I saw almost 30 mm/sec rms? That's pretty high.

This message has been edited. Last edited by: electricpete,

overhung_axial.ppt (20 Kb, 34 downloads)

I have seen cases where balancing an overhug overspeed trip mechanism on a tubine generator reduced axial levels by a factor of 4 with almost no change in the radial direction.

A common rule of thumb with overhung rotors is that imbalance will increase 1x vibration levels in the axial direction at the bearing closest to the rotor, with less effect (in the axial direction) on the bearing awaay from the overhung rotor. If the levels at both bearings are similar, a stiffness problem seems more likely.

Jon
Spintelligent Labs

Hi Jon,
from what i read, and what you suggest, it would seem that more likely I am having a structure issue. Since both Axial readings are almost identical. Which means both bearing blocks are moving in-sync.
=========================
Fan DEH = 9
Fan DEV = 1.6
FAN DEA = 15

Fan NDEH = 15
Fan NDEV = 8
Fan NDEA = 15
=========================

I have manage to arrange for a phase reading to be done on the fan. I would report back when that is done next monday.

anyone can point me to some reading material with regards to insufficient structure rigidity?

hi pete..........thanks for the PPT. i manage to open it and it greatly helps in my visualizing of why over-hung gives axial readings. as for misalignment of the pulley, i did a checked plus rectification of the pulley with laser, so that shouldn't be contributing to the excessive axial vibration.

thanks all for your great help.

quote:

Last friday, what i did was place a choke on two side of the frame, and the vibration in the axial direction was halved!!
Prior to this high readings, the springs in the isolator was found to be damaged, and was replaced with rigid mountings, that is when all the high vibrations started showing.
Hence my inital question to all, which came first? Unbalance? causing the excessive vibration but was dampened before by the 'bouncy' springs? or rigid support not being rigid enough, causing the slight imbalance in the impeller to be amplified?
And since adding a choke to restrict axial movement of the fan-motor frame, the axial vibration has halved. I am gearing towards a bad support resulting in excessive vibration all-round.

We might use chock instead of choke. This scenario seems odd. Hs any of this been designed? This design should be reviewed for what is needed. If springs worked in the first place, why did they change the design?\

Balancing may still be needed, but the basic design/installation should be reviewed.

Hi william,
you are right, the fan came with spring-isolators to start with, but the springs were badly corroded and weren't holding out. I had them removed and replaced with rigid support, hoping that will be sufficient to hold the fan down. We have larger fans, slower speed though and slightly smaller fans, but similar high speed without spring-isolators. And they are a-okay.
Therefore my questions, what is the real purpose of spring-isolators / dampers ?

thanks all........