On behalf of a powerplant:
Fan 1485 RPM 17 blades 1MW motor suddenly after summer stop, 2 days operation, indicate 5 times normal ov vibration, normal is 0.5-0.8mm/s analysis indicate looseness as peaks are 1xRPM 1.8mm/s 2xRPM 1.7mm/s 3xRPM 0.8mm/s. Findings are that flakes of the metalization of the blades 0.7-0.8 mm thick is falling off, bubbling and missbehaving, beneath is aluminium. Blades are replaced, fan is back to normal. Why the vibration of multiples of running speed? Nothing loose was found in foundation, bearing or otherwise. Seen something similar? I can´t say I have. Olov

I have seen a rotor bow (1X forcing function) cause harmonics on a sleeve bearing motor.

My thoughts are as follows. The forcing function is 1x. For small 1x forces, the bearing oil film acts like a linear spring and all we would see is 1x vibration. For very large 1x forces, the shaft movement within the clearance creates impacts against the bearing wall once or more per revolution. Now the bearing is acting more like a non-linear looseness and creates the harmonics. In addition to bearing clearances, there may be various other mechanical joints that can act like looseness, to an increasing extent as the force increases. These may include fan bearing-to-housing, pillow block splitline, machine feet to base, looseness of grout underneath bedplate etc.

TIOMOAICBTWA. I'm interested to hear other comments. Maybe if you clarify the specifics of the machine others may have more direct comments. For example type of bearings, type of housing, overhung/between-bearings, coupling type, others may have more relevant experience on similar machines.

Pretty ordinary exhaustfan, on groundfloor antifriction bearings, my guess is overhung but I have to check that. It has got a revised shaft as it was found to be operating near shaft critical and went ballistic, if i am correctly informed, only 1-2 years old. Manufacturer like shimpack type coupling but I can´t say certain on this. I guess you can be correct Pete, also trying to findout if the number of guidevanes can be a hint so it could be a airflow thing and that flexing of the blades could make them flaky. I think it would need to be a considerable amount of work that would make the blades flaking so it would have disturbed the neighbours by the sound of it..... Olov

quote:

suddenly after summer stop, 2 days operation, indicate 5 times normal ov vibration, normal is 0.5-0.8mm/s analysis indicate looseness as peaks are 1xRPM 1.8mm/s 2xRPM 1.7mm/s 3xRPM 0.8mm/s.

What does "summer stop" mean? What does "2 days operation" mean in reference to summer stop?

quote:

Blades are replaced, fan is back to normal. Why the vibration of multiples of running speed? Nothing loose was found in foundation, bearing or otherwise.

Since the blades have been replaced and the fan has returned to normal , are you asking why it HAD the harmonics before the blades were replaced or does it still have the harmonics now after the blades have been replaced?

Ralph,
It´s a municipal heatplant, we have reasonably warm in summer so it´s shut down summertime. Fans have been running some short time as work has been done during summer. Fan was inspected during the summer stop by the manufacturer, nothing was noted, 2 days after starting for the winterseason the event took place and the alarm level on the monitoring was passed, measurements analyzed, fan shut down and inspected... Yes, blades are swapped and a very lo level of 3xRPM can be seen, none 2x and normal 1xRPM as before problems. So yes, question is why was the multiples present when the vibration went up, it´s not obvious to me or the plant guys where they came from. Obvious could be that the blades were loose? Blade surface flaking could come from that or chemical from furnace cleaning or some other reason like fluid dynamics. If that was the case that they were loose, it should have been found when the blades were swapped. I found this to be a strange case and was wondering if somebody seen something similar. Just making blades from aluminium and plating them so thick that the plating can fall off makes me surprised. Is that normal? Fans I normally work on is real steel and those blades get pretty thin anyway sometimes. Olov

Thanks Oli.

Maybe the fan sat there down and some type of chemical attached itself to the blades and "ate" through the coating and weakened it to the point that when it was started up, the weakened spots began flaking off causing an imbalance to appear and because of the design of the system (which may not be all that bad), caused an increase of the 1x amplitude to begin and the increase in the harmonics' amplitudes followered. No real problem may be present other than if 1x increases, so do the harmonics. This is sort of common in most any weak built fan system ( not to say this one is weak), 1x increases, so do the harmonics.
As far as the blades being loose, seems like that would have shown up before being shut down for the summer. Seems like the replacement of the blades, points only to imbalance from the flaking.
What type of coating is on the blades, fiberglass? Looks like stainless steel blades or solid fiberglass blades would be better if this flaking happens often. Even regular steel, unless there is high corrision fumes present, might work better than coated blades.

Only my opinion and I could be totally wrong again.