Balancing response at structural looseness/resonance

Dear members ,

Yesterday i have balance a fan ,it has dominant 1X r.pm 1500 .Initial balancing attempt is not success earlier two days before and production people given limited time due to their production demand . Yesterday i have received a call from the plant and after stopping the fan ,we observed some cracks on discharge duct and broken nut bolts at duct suction and discharge flange area .After repair we have examine static balance and found good condition and after motor trail with out belts .I have found at Motor side 1X after the repairs and corrections .Earlier readings are changed in Vertical direction with out any corrections ,when the fan running before stopped for the balancing . 

Then after we have started Two plane fan balancing but the results are not satisfactory .Balancing response is slow and time consuming .We have handed over the fan to the Production people ,because of the production demand .

Please refer the below attached documents and share your opinions and suggestions .

 

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Balancing report
Motor running without belts
Reading before stopped for balancing
Earlier readings
Original Post

Apparently both motor and fan operate at same speed (1500 rpm) with connecting drive belts? The vibration waveforms have unsteady amplitude, so modulation or beating can make balancing a challenge. Make sure that belt pulleys/sheaves are in good condition and alignment and tension are correct. You might consider reducing belt tension to allow for belt slip to reduce fan speed, so you can get good balance data on fan.

Walt

Dear Member ,

It is  a Belt driven and the Motor and Fan running at same speed  and Belts tension is good .Belts and Pulleys / sheaves alignment and condition are good .Any way i will check ,when the opportunity come for Balancing .It is mounted on soft foundation .

Just a week before , i had Balanced Two Overhung Blowers Running at 3000 r.p.m and resonating at the speed (Vibration 100 mm/sec in Horizontal direction and 20 mm/sec in vertical direction   and not responded for initial balancing attempt and it had directional resonance .i had change the stiffness of the structure by Welding stiffners in 'X' shape on pedestal in Three directions .It is responded well and vibration decreased to 5 mm/sec in Horizontal direction and 3 mm/sec in Vertical direction .Those Blowers are running smoothly till now .

Is there any such issue for this I.D Fan.This fan also not responded for initial Balancing ,after that we found cracks on discharge duct and damaged Nuts and Bolts at Suction duct flange .After the repairs ,it is responded and the response is not to the satisfactory .And the trail weight location is the same angle as first balancing attempt .But the response is little ,Fan had responded after Correction weights attached .

Please verify once again the balancing report.So that you could understand better .I will consider all the feed  back ,when i get opportunity to Balance the Fan .

Kishore kumar posted:

Dear Member ,

It is mounted on soft foundation . Has the movement of the soft foundation been compared to the movement of the fan bearings' balancing positions (amplitudes and phase)? Is the "soft foundation" springs or rubber pads?

Just a week before , i had Balanced Two Overhung Blowers Running at 3000 r.p.m and resonating at the speed (Vibration 100 mm/sec in Horizontal direction and 20 mm/sec in vertical direction &nbsp and not responded for initial balancing attempt and it had directional resonance .i had change the stiffness of the structure by Welding stiffners in 'X' shape on pedestal in Three directions .It is responded well and vibration decreased to 5 mm/sec in Horizontal direction and 3 mm/sec in Vertical direction .Those Blowers are running smoothly till now . This must have been a "hard foundation" fan since strengthening the "structure" changed the "response" and allowed for a better balance, HUH?

Is there any such issue for this I.D Fan.This fan also not responded for initial Balancing ,after that we found cracks on discharge duct and damaged Nuts and Bolts at Suction duct flange .After the repairs ,it is responded and the response is not to the satisfactory .And the trail weight location is the same angle as first balancing attempt .But the response is little ,Fan had responded after Correction weights attached .Was the "damage" to the duct work confirmed to have been caused by the unbalance movement of the actual fan pedestal or from the movement of the "soft foundation" and the fan pedestal combined?

Please verify once again the balancing report.So that you could understand better I will consider all the feed  back ,when i get opportunity to Balance the Fan .After reading your balance report carefully, as suggested, I notice some things, especially the "reference run data", that raises a question or two in my thinking.These questions are:

1. I am not sure if the "OBH" position is the bearing closest to the fan or closest to the sheave, because Some analyst use "OB" as next to the sheave. I, personally consider "OB" as next to the fan.

2. I see, in the reference data, where the phase between the OBH and IBH points are almost the same, (70 and 73). The amplitudes are also fairly close, one to the other, (54 and 42). I asked myself, Why would I not have tried a single plane balance first rather than a complicated 2 plane balance procedure? Just my thinking.

3. After running the numbers from the balance report through CSI's balance program, I came up with corrections weight of 102 @ 215 degrees and 67 @ 70 degrees. This would have been with the removal of the original trial weights.These numbers are based on the assumption that the tach light and transducers are sitting in the same plane and facing the same direction.

4. I also read in the report where, for the trim run data, the weight was changed(?) from 80 and 18 to 70 and 20 at almost the same positions of the 1st correction weights, but not sure if the trim weights of 70 and 20 were added to the 1st correction weights or replaced the 1st correction weights.

With an amplitude of 54 mm/sec Peak, and that seemingly converts to near 30 mils Peak to Peak, and unless some of this movement is being absorbed or shared or caused by the soft foundation, this is extremely high, IMO.

Is there some way you might could temporarily stiffen the "soft foundation" long enough to see just how out of balance the fan really is?

 

Only my opinions and I could be totally wrong.

Thanks and Have a Great Day and a Happy New Year,

Ralph

 

Dear Ralph ,

you have shared a very good opinion , i would like to thank for that .

1. Has the movement of the soft foundation been compared to the movement of the fan bearings' balancing positions (amplitudes and phase)? Is the "soft foundation" springs or rubber pads?

It is mounted on Rubber pads .

2.This must have been a "hard foundation" fan since strengthening the "structure" changed the "response" and allowed for a better balance, HUH?

Yes , that fan is on rigid foundation .

3.Was the "damage" to the duct work confirmed to have been caused by the unbalance movement of the actual fan pedestal or from the movement of the "soft foundation" and the fan pedestal combined?

May be you are right ,But the Fan responded to Balancing after Duct cracks repairs and replacing damaged Nuts and Bolts   .Initial Balancing has not get any response ,after that we inspected the structure ,we found these  problems .

4. I also read in the report where, for the trim run data, the weight was changed(?) from 80 and 18 to 70 and 20 at almost the same positions of the 1st correction weights, but not sure if the trim weights of 70 and 20 were added to the 1st correction weights or replaced the 1st correction weights.

1 st Trim weights are not removed , 2nd trim corrections suggested in Fast Bal 2 at 80@215 on first plane and 25 @ 24 but the vibration increased after that  i removed the weights and placed at 70@215 and 20@345. After that vibration reduced .

5.I asked myself, Why would I not have tried a single plane balance first rather than a complicated 2 plane balance procedure? Just my thinking.

It is in balancing condition while checked for static balancing without belts .And it is not responding well for Single plane Balancing .

 

Image result for happy new year

 

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