Overhung fan

Hi guys, happy weekend ")

i need some help regarding one critical equipment in our plant.

It's an Overhung Fan having high overall and ge.

At lower frequency side of the spectrum, i can see running speed harmonics as well as of sub-harmonics of 1/3x , this has occurred after assembly and replacement in March 2016.

During the balancing work in march, i found so many weights attached to the fan from previous balancing work, we ignored and continued with balancing(adding another weight) , and could not lower the 1x  than 5mm/s . 

We continued to run it,( time is money) and we will schedule to replace the fan by any chance. 

Another question, Is this high vibration caused by looseness between shaft and bearing inner race? 

Aziz 

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"another question, Is this high vibration caused by looseness between shaft and bearing inner race?" Unquote

Was there a problem or potential problem seen, concerning the installation of the bearings, but was it decided that "it" was no threat, during the "assembly in March 2016"?

If there was something noticed during assembly, it very well could be increasing looseness.

Quote:

"during the balancing work in march, i found so many weights attached from previous balancing work, we ignored and continued with balancing(adding another weight) , and could not lower the 1x  than 5mm/s ." Unquote

Too many weights are a historical indication of balance problems, IMO.

Did you test to see if the fan is running at or near a critical frequency. Balancing is rather difficult, at best, when running at its critical frequency. Many weights sometimes is an indication of this. An easy test is to monitor, in live mode with low Fmax and low resolution, the 1x amplitude of the fan as it coasts down.

But a normal stored tach light coastdown tracking of amplitude, rpm, and phase is better.

I see that maybe the motor has the highest amplitude at 1x (on the point "1" H.
What was the motor Drive End readings?

Which bearing position on the fan did you use during the balance job?

What was the axial NDE fan bearings amplitude?

Excessive looseness, as will some other "things", will hinder the balancing procedure. Were your phase readings erratic or steady? Amplitude reading? Steady or varying?

What are the sidebands off of the running speed and running speed harmonics? They appear to be less than 1/3x and more than 1/4x.

You spoke of seeing 1/3 sub harmonics. Which data set do these show in?

1/3x, with harmonics of 1/3x, are a good indication of bearing to shaft looseness IMO.

Thanks and Have a Great Day,

Ralph

 

 

Harmonics of running speed and a lot impacting on time waveform. It look like bearing loose making bearing problem. So, how about clearance of outer race and housing and fit clearance between shaft and inner race.

How about vibration at DE side of motor? and what is the reason for balancing? did the fan working well before replacement and assembly in March 2016?

Sometimes one finds a large number of weights on an overhung (or not) fan when the balancing was not optimal (I've seen it when the factory (slow speed balance machine) weights were off, too, perhaps due to fits and other work.).  Single plane balance?  Try a couple weight?  

Of course, as mentioned being near a critical speed, if applicable, can make it more difficult.  Not taking into account variability due to flow conditions can hurt.  Thermal effects for hot fans.

  This looks very similar to a fan balance problem I have had at a customer. The fan configuration is identical. Our root problem is that there is too much flex in the motor base, and it then cracks welds in the foundation. This allows excessive relative motion between the motor and fan shaft, damaging the bearings. I would run ODS checks to see if there is flexing in the fan motor base, and impact tests to see if you are near a critical speed. Once you know for sure you have a solid base, replace the bearings, laser-align the installation, and remove all of the balance weights from the fan and re-balance from scratch. 

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