spring grid coupling suspect

Hi, Got some vibration data in one of the ID Fan assembly, Motor is a 2 pole motor connected to the fan through spring-grid type coupling. Max speed 3000 rpm. The fan is not able to go to full speed due to high vibrations, its a VFD system (variable frequency drive). I did not see any predominant 1X freq component. More no. of multiple harmonics are seen in the spectral data

I was able to take the data at 1500, 1800, 1920 and 2100 rpm. I noticed a strange behavior in the system. The vibration signatures shows multiple harmonics. Could not see any bearing defect frequencies. On day-1 when I took the data at 2100 rpm all vibs were in limits. When I took the vib data on day-2 at 2100 rpm all vibs went up by 3-4 times on Fan DE and Fan NDE bearings.

Vib data, corresponding comparative vib signatures are provided in the attachment. Is the spring-grid type coupling the culprit...???

Any kind of inputs are welcome

Nagaraja

 

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After looking at the data provided, I would suspect that the NDE bearing-Fan is "loose". What type of bearing(s) on the fan?

Also, from past experience, a spring/grid coupling is generally not a good choice for a VFD machine, generally an elastomeric type coulpling is preferred.

From the spectrums, is this a sleeve oil bearing, if yes, have the bearing clearances been noted? What is the bearing oil temperature?

Generally clearance for sleeve bearing -about 0.038 mm per 25mm shaft diameter. (I think I calculated (0.0015 inches per 1.00 inch diameter of shaft)

To me it looks like clear signs of type C mechanical looseness at the fan bearings.  Loose fit on the shaft, or loose fit in the housing.  Are these split case taper locked bearings, or set-screw style?  When was the last time the bearings were replaced?

If the coupling were at fault, I would expect higher amplitudes of vibration at motor DE.  Also, a coupling would have to be severely worn, or misaligned to create this much vibration.  It should be noticeable with a strobe light.

Dustin, this is indeed a taper sleeve self locked roller bearing, a taper sleeve is fitted first on the shaft on which the bearing is sitting. Is it changing its position every time we roll the fan....?? each time I am seeing different pattern with change in vibrations.

Nagaraja

I would open the bearings and inspect them.  I would suspect a bearing loose on it's taper, loose in the housing, or the shaft is worn and undersized.

I recently had a unit that showed high vibration predominantly at 2X shaft speed.  It was found that the bearing came loose on the taper due to improper installation.

GKaiser posted:

Also, from past experience, a spring/grid coupling is generally not a good choice for a VFD machine, generally an elastomeric type coulpling is preferred.

It probably is best to check with the machine OEM to get recommendations for a specific machine.  I know of some VFD drives where a grid coupling is recommended and Koppers, a coupling manufacturer, does recommend them for some VFD applications.

One has to remember that elastomeric couplings can create issues of their own, mainly increased overhung load on bearings.

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