Based on the plot below, from a coastdown of a fan, would one be safe to assume the fan system is running above its first critical and close to approaching the second?

Resonance, probably yes, why in rotating parts?
Do some knock test to verify.
Olov

Without phase I would not be willing to draw that conclusion....

Ralph,

That would be my guess, but like you know and Steve said-without phase....

Good Luck,

Danny

I put some "stiffeners" in a few places and as soon as I get a little more time I will post the coastdown plot from the results of the stiffeners as well as the balance results.

Time was short so I may have trouble getting the phase reading unless the coastdown phase plot was saved, not sure if it was right now, I was not running the 2120. but I'll check.

The rotor-shaft-bearings assembly weighs about 500 pounds and is overhung belt driven. What would be a guess as to the amount of weight needed to balance this, say if it is at or close to a critical and also say if it is not?
We balanced it yesterday, so the required weight is known.

Reason for all these questions is the thing has to be balanced quite regular (not by me, some other company does it normally), so the owner of the fan wants me to try to solve why.

Ralph is it a NYB

Barry and All,

I don't know if it is a NYB or not. I was helping with this job "long distance" over the phone and internet.
The first coastdown plot I have in my original question was drawn freehand from data over the phone. It looked to me like it was running well above critical speed and that is why I asked you guys.
I may be totally wrong in my idea of coastdown amplitude plots and any suggestions from the following MS Excel plots based on the actual data from the fastbal program will be appreciated.

This message has been edited. Last edited by: Ralph Stewart,

As_Found_Readings_Start_rpm_versus_peak.xls (21 Kb, 36 downloads)

Ralph, you could ask for a bump test made at the rim in axial direction. At 3 and 6 o´clock and some max 200 Hz span. Made with a fist or very soft rubber mallet. Would reveal what you need to know. The scetch smells critical or disk wobble critical but never say for sure. Best regards Arne

Hello All,

Here are some numbers from the fan of the original question of "Is this above the critical speed", during and after the balancing, also after bracing.

Original was: 4.5 mils at 357 degerees
Trial Weight: was 1 ounce at 45 degrees
Correction Weight was: 0.99 ounces at 46 degrees
Final Trial Run was: 0.5 mils at 164 degrees

Are these numbers typical with something running below, at or above its critical speed? If it were below critical would not the 1st trial weight been placed somewhere around 130 to 175 degrees opposite direction of rotation using the CSI 2120?

Ralph,
Based on my experience with the 2120, running below the first critical, 130 to 175 degree correction would be about right. I find that if I subtract 30 degrees and place my weight 180 from there will get me real close (357 - 30 = 327 - 180 = 147) most of the time. If it goes completely through the first resonance, instead of -30 degrees, shouldn't I subtract 180 and goe 180 which would put me back at 357 for my correction. Now my question is, if you are into your second critical, would you add that degree of phase shift to get back to your light spot. Which in your case would be another 49 degree phase shift into the second critical.
Just some thoughts,
Ronnie

You have to align (physically or virtually) your measurement transducer system, your balance plane, and the trigger coordinates to use the phase shift correctly.

If you balance multi-mode systems the rules are similar. Except you must consider mode shape when balancing and interpreting the data.

When balancing well above the critical place the weight on the high spot. General statement -- specifics should be taken into account, like more than one mode or not a balance issue, etc.

Hi Bill,

quote:

You have to align (physically or virtually) your measurement transducer system, your balance plane, and the trigger coordinates to use the phase shift correctly.

Are you saying the transducer and the tach trigger light have to be in the same plane? As in my case, both were in the horizontal direction, facing the same direction with the transducer on the inboard bearing, for single plane balance.

quote:

General statement -- specifics should be taken into account, like more than one mode or not a balance issue, etc.

More than one mode of what?

Are you also saying it is incorrect to say this particular fan possibly was running above a critical even though the correction weight placement indicates it is? Can't the "critical arae" of a system be influenced by other factors other than total rotor imbalance, ex: system stiffness, system mass, and not just the rotor's critical speed itself?

Or am I reading your reply wrong?

Having the transducers aligned should put the coordinates together if there are no instrument lags. The balance plane 0 indicator needs to line up with the trigger location on the shaft, too.

Mr. Lynn asked about more than one critical; this means more than one mode.

The stiffness and mass go into making the critical where it is along with other factors. Damping plays a part. On overhung (hung over?) fans polar inertia and speed play a part (gyroscopic stiffening). On other machines the process parameters (e.g. pressure differential) may influence the criticals.

Putting a weight approximately 45 behind the vibration vector (lagging the vibration by 45 degrees) would say that the machine was above a natural frequency. 45 degrees would also imply some amplification from the critical, operating at approximately the ‘half power point.’

Thanks for the info Bill.