I OPENED THE CHICHAGO BLOWER 6520 HD-5 SIZE 15 FOR SEASONAL MAINTENANCE. REPLACED THE BEARINGS AND BELTS. WHEN STARTED THE BLOWER VIBRATION VENT UP MAX. 50MM/SEC PEAK AT INBOARD AND 35MM/SEC AT OUTBOARD. IT HAD BEEN RUNNING SMOOTH BEFORE. VIBRATION IS ALL AT 1X (3500 RPM).
IS IS UNBALANCE? IF YES YES, WHY? WHAT WRONG I HAVE DONE TO IT?

Only bearing and belt replacement shouldn't change balance but can change 1X is installed incorrectly.

Bearing must have proper internal clearances. Bearing must be square with the shaft and bearing to bearing alignment a must, all without induced stressed from the base. Next, good alignment, sheaves and tension.

Bearing misalignment is the probably cause IMHO. I went on a similar fan with vibration from beneath or hell; well above 1 IPS. They spent 48 man-hours attempting to balance prior to my getting there. I spent 6 hrs on bearing alignment and belt alignment - 30 min on balance.

If you take the belts off and at that magnitude a heavy spot should swing to the bottom or strain/stress in the bearings is preventing it.

Make sure you have a floating bearing and its setting is correct.

It really sounds like a setup problem.

Did you change or damage the sheave on the blower? This can cause a 1X vibration if the sheave has runout.

What is the phase difference from the inboard to the outboard?

What type bearings were installed? Same as the ones removed? Are they tapered sleeve type? Are they pillow block eccrentric collar type? Are they pillow block set screw type?

To go from running in a good condition to running in a bad condition and all that was done was the changing of the bearings and belts, someone messed up somewhere.

Looks like a complete root cause should be done, starting with the alignment of the sheeves and backing up from there.

Is it safe to assume the highest vibration is in the horizontal direction? 3500 rpm is pretty high rpm, would not take much to change amplitude if something is not done correctly, even a small error could possibly cause trouble. What is the history of this fan as far as balancing goes? Has it ever been found to be running close to the systems critical?

Most sheaves have enough runout to generate 1x and higher order vibrations if belts are overtightened. Also check the shaft for runout at the sheave.

Could the shaft have been moved axially when the bearings were changed and now the impeller is catching somewhere inside the fan casing once per revolution? Or perhaps a key catching on the guard? This would give an FFT with a high 1x peak. Take a time waveform, if it's imbalance you will see a sine wave, if it's impeller/key catching you will see sharp impacts at every revolution.

It might be obvious but did you leave anything lying around and they got caught up on the blades?
Was any damage done to blades (bent moved out of position?
Is everything tight?
Just some ideas

Feroz,

You said the bearings/belts were replaced. Did you do anything to the fan/blade ie. any cleaning at all?
Just a thought, if nothing was done/seen of the fan/blades, any prior buildup could have dislodged itself during the replacement and cause unbalance

Regards...Rajan

Actually due to operations pressure, I swaped the blower with the boiler still under seasonal maintenance.

Its shaft runout when checked on the Lathe machine was max.0.02mm. Blowers weight alongwith shaft is 15.85Kg and 3 kg for pulley + key. I put the blower on balancing machine with pulley and key. At 500 RPM, no unbalance seen. At 750 RPM slight unbalance (1 div. of my machine scale, which we consider to be very low). But at 1500 RPM unbalance reading does to 6 div.s

I put masking tape on the blower vanes to nullify air affect.

At 1500 RPM, I saw the blower wobbling quite a lot.
It is a thin plate blower of 0.8mm thickness.

Bearings are 2 bolt oval flange type UC206.

This blower has a previous history of high vibrations which were arrested by installing four stud bolts under the motor structure. These are also in place.

I think I should try to first straighten its edges by light hammering and then balancing.

Any ideas? Thanking you.

"underpressure"

Feroz,
You mentioned the runout was checked in a lathe. Was it verified that the bearing fits were good. A little wear on the shaft and a new set of bearings might move your center of rotation changing the mass center displacement. We did some calculations a few years ago and .0002 TIR was enough to take 3600 RPM out of our spec using 4W/N. Although for as much unbalance as you are seeing I think there would have been an obvious groove. But I have ran across a few balance jobs that this was the case.
One place I worked at we had machinists doing the balancing in our stand refuse to run on the beaing fits so they ran on the shoulders for the bearing cap clearances and did not confirm they we running true to the bearing fit. Lets just say there were a lot of poor quality balance jobs done until that was corrected.
This might not be the problem in your case but it is one more thing to keep in mind when balancing after bearings have been changed.

Ronnie

"UNDERPRESSURE"????

Relax. You can't rush too fast. Many mistakes will be made. There is an old saying here in the USA that goes something like this: "Haste makes waste"

MAN! Only 8 mm thick. That is only 315 mils (.315 inches thick). My error on the decimal point sorry. But it was running before the work was done, it should run now, as soon as the problem is located.

quote:

Actually due to operations pressure, I swaped the blower with the boiler still under seasonal maintenance.

If I understand correctly, you are now trying to use a different fan than the one you started with? The entire fan or just the rotor and shaft?
All this balance stand data, is it from the fan from the other boiler or from the fan that you originally started with? I guess my question is: is the boiler still down or is it running with the fan from the other boiler that is still under maintenance?

You asked, "Any ideas".
I would put it back together, check all alignments and try to balance it, in place. Has anyone at your plant ever balanced this fan in the field? If so, they should have some idea of its sensitivity to balance weights.

Have you check the system for its critical frequency with a bump test or coastdown?

Good Luck.

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

wobbling problem ???

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

Yahoo! think u having a similar problem.

But what let me know the critical frequency of the structure if u check it.

Mine is a Yorkshire boiler on which this PROBLEM is installed.

Regards

MAN!! I am lost somewhere between "yahoo" and "Feroz Khan".

What have I missed? A post or something by someone?

Quote by "yahoo":

quote:

Blower is the same on which vibration discussion is underway.
Machine shop has balanced the blower (at 1500 RPM)but very little grinding on the previous welded balance weights.
I have not yet checked the system for its critical frequency with a bump test. But as I mentioned earlier it had been resonating due to which we had installed 04 stud bolts to increase its stiffness.
But what you guys say about wobbling problem. I have tried to straighten the edges but little could be done.

Quote by "Feroz Khan":

quote:

Yahoo! think u having a similar problem.

But what let me know the critical frequency of the structure if u check it.

Mine is a Yorkshire boiler on which this PROBLEM is installed.

HELP ME OUT HERE. What have I missed?

I would not be overly concerned about the fan woble you are seeing. Some woble is normal especially on small/thin steel fans and is caused by the welding process during manufacturing. I would echo the others in saying that the bearings were likely not installed properly.

8 mm is the size of prox probe; they used to be 0.3 inches (Others used to be .190 inches now 5 mm [0.197 inches). Anyway this is 0.315 inches and not 0.0315.

If the fan has a visable wobble, you may have a moment imbalance because of angular runout. Thin fans like this are sometimes balanced as a single plane. A single plane balance on the fan wheel will not correct (in general) a concentrated moment due to a wobble.

Its thickness is 0.8mm not 8mm.

If you suggest balancing, it should be I think 2 plane dynamic as ARI Guideline G-2002 says
B]two plane balance method is preffered method for balancing wheels, when the width to dia. ratio is greater than 0.30 [/B]
In my case it is 0.357 (l=150mm and dia.=420mm)

But I don't know its critical speed. If below, at or above crical, what would be the weight location?

Thanks

quote:

Anyway this is 0.315 inches and not 0.0315.

Thanks Bill. My bad on the decimal point.

Well I don't know now, Feroz said:

quote:

Its thickness is 0.8mm not 8mm.

Oh well, it's thin anyway.

Feroz,
I don't understand what yahoo is saying in his post about where he says:

quote:

Blower is the same on which vibration discussion is underway.
Machine shop has balanced the blower (at 1500 RPM)but very little grinding on the previous welded balance weights.
I have not yet checked the system for its critical frequency with a bump test. But as I mentioned earlier it had been resonating due to which we had installed 04 stud bolts to increase its stiffness.
But what you guys say about wobbling problem. I have tried to straighten the edges but little could be done.

Do both of you work at the same site. Am I confused or what. SORRY if I am.