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RustyCas

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Reply with quote  #1 
I mentioned before that I had a customer that replaced an alloy wheel with a steel wheel, which managed to create resonance at running speed (not good on a 3600 rpm machine).  They finally had a new allow wheel built and installed it over the weekend. I went down to align it and do an initial run.  I was thinking I'd balance if needed, but no preparations had been made to weld on this alloy.

I couldn't see any balance weights so I was worried that it hadn't been shop balanced.  But I couldn't see the backplane, and who knows, maybe they had managed to build the first welded-construction wheel in history that was perfectly balanced.

I set my meter in peak-hold mode and we rolled it up.  There is a soft-start on this fan, but it came to speed quickly.  Only let it run for about 5 secs.  They wondered why I was using such long cables.

Wow.png 


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Danny Harvey

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Reply with quote  #2 
Dam! That's really bad. How much does the rotor weigh? What HP?

Long cables was a good idea.

During preparation for a questionable start-up, I was superglueing my accels to the bearings and tywrapping the cables and it scared the engineering manager so bad that they held up for a final meeting before starting up. I told him I was just being safe, but I guess they had more inside info than I did, because they postponed the start-up and had a factory guy come in. It was bad, but I don't know how bad and he fixed it.
Alex

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Reply with quote  #3 
Have you tried to find heavy spot before startup?
Shurafa2

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Reply with quote  #4 
If my calculation is right, it is about 120 mils (0.12 inches) which you can see by the naked eye.
Please share with us the rest of the story.

Regards- Ali M. Al-Shurafa
MachDiag

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Reply with quote  #5 
I haven't seen amplitudes like that in while... thankful really.   Even with anyform magnets, it's pretty darn tough keeping the accelerometers attached when things start shaking that bad.
fburgos

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Reply with quote  #6 
Wow that's have to be some record
dnk

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Reply with quote  #7 
Over the years i had 2 major failures of overhung fans. One had a piece broken off 1 blade, the other had 1 blade broken off and several bent. In both instances the new fan was several weeks out for replacement. 
I worked in house and also performed fabrication and repairs. 
 on 1 i welded a piece on the blade, the other i fabed a blade and straightened the bent blade. 
 before running i spun the blade by hand with belts off and added wts. until it didn't stop at same spot. 
On the first fan that was enough so i could run and balance (2300 rpm).
 on the second fan still couldn't keep accelerometers on. Installed portable VFD so i could balance at lower speed and worked my way up to full speed(3600).
Both fans lasted until new fans were received and could be replaced. 
The highest amplitude was 50 mils.
fburgos

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Reply with quote  #8 
I don't remember anything over 150mm/s, there was a fan I balanced that was made in the shop it was statically well balanced.... But once started it was bad, it cracked the bearing pedestal in one of the runs, I used long cables and record bode, go to a safe place, start 5 seconds count then off, tightened again all the base bolts, add test run, start/off calculate add correction and tight all bolts... This took several runs, some new bearing pedestal, all night shift but in the morning it was less than 2mm/s
OLi

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Reply with quote  #9 
I also balanced 50 mm/s fans, one where a crowbar went thru by "mistake" so when you need to hold the transducer with your tiny body weight you know it is bad. Company record is 200 mm/s on a substantial exhaust fan where everything were cracked but the same relative substantial power cable was the only thing holding it basically. It took some time, months to get lower than 3mm/s. (Since they swapped the bearing mount and run in to new problems.) It also had on-line monitoring that obviously was of no use.
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Curran919

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Reply with quote  #10 
When I first did the math, I thought "thats not that high", but then I realised I missed a decimal in my head...

My record is 200mm/s from a fan that had one blade fly off. A new blade was leaning against the motor. They'd been waiting 2 months for an opportunity to replace it.
Danny Harvey

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Reply with quote  #11 
I probably didn't get to measure my highest because I'm afraid of really heavy things flying through the air.

This one was a misapplied dust collector system fan of about 8000 lbs.  The dust was really sandy and they used hollow blades.  They would wear a hole in one blade and it would fill with sand. 

I knew it would be bad so we tried to ramp it up.  About 600 rpm or so and 5 ips, I told them to cut it off.  THAT"S when they told me about the hollow blades and the sand. (Not my client. Filling in for a friend.)

I found the heavy blade and we drilled into it and drained out about 80 lbs of sand.  That really helped.
RustyCas

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Reply with quote  #12 
To get the complete “story” you have to ask a lot more questions than you should need to. Then again, sometimes I prefer not to know too much.
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Danny Harvey

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Reply with quote  #13 
I should have known that something like that was going to happen.

The worst balance job I ever did.
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