During the field portion of a recent training class vibration readings were taken before and after the precision shaft alignment of a pump and motor. This was done with a hand held vibration meter with a single plane accelerometer. The readings were approximately as follows:
Motor NDE V-.07, H-.07, A-.11. Motor DE V-.07 H-.07
Pump DE V-.12, H-.12, A-.15. Pump NDE V-.15 H-.15
These numbers are not bad necessarily but would indicate some misalignment and some possible bearing issues in the pump. After recording the “as found” alignment readings using the Fixturlaser EVO, it was found to be out of alignment 100 mils vertically and 30 mils horizontally (1.0 mil = .001″). The corrections were made and the precision shaft alignment completed to the tolerance specifications for an 1800 rpm motor. There was virtually no looseness in the coupling prior to alignment and significant looseness post alignment.
The pump and motor were restarted and the noise level had greatly increased. It goes without saying that the vibration had also increased! The readings were approximately as follows:
Motor NDE V-.09, H-.09, A-.09. Motor DE V-.11, H-.11
Pump DE V-.18, H-.18, A-.18. Pump NDE V-.23, H-.23
So what happened?!
Here’s my take. First of all looseness in the bearings and the coupling became apparent, once the coupling strain was relieved after the alignment was completed. It is not known how long these machine components operated in this condition, but it was obviously long enough to cause significant wear in the bearings and the coupling. Therefore significant looseness in the rotating components after the shafts were collinear.
Note: looseness is not the cause of vibration, but the amplifier of vibration which explains the increase in vibration.
I was curious to know how different coupling types might react to looseness and found this interesting study. This particular coupling had 6 rubber pin inserts and the pump shaft was visibly oscillating back and forth in the pump.
Detecting misalignment using vibration analysis
A large majority of people who actively participate in CBM and Pro-Active maintenance programs will tell you that shaft misalignment will be indicated by higher running speed and/or twice running speed vibration frequency components with high axial vibration and a 180 degree phase shift across the coupling.
This is true only some of the time. Figure 1 shows several vibration spectral patterns on rotating machinery operating under misalignment conditions.
Figure 1. Various misalignment vibration ‘signatures’ with different types of flexible couplings.
Notice that the patterns are different and don’t always show running speed and/or twice running speed vibration frequency components (1x and 2x). Several controlled tests by several individuals over the past ten years have indicated that vibration spectral patterns can be different under similar misalignment conditions depending on the type of flexible coupling installed on the machinery and under certain conditions, virtually no vibration can be detected even under moderate to severe misalignment. There are several points that you should be aware of concerning the use of vibration analysis for misalignment detection :
– there typically is not a linear relationship between the overall vibration amplitude and the amount of misalignment (i.e. it is possible that the vibration could decrease as misalignment increases)
– it is possible for the vibration levels to increase after re-aligning a piece of rotating machinery.
– if the majority of vibration is occurring at multiples of running speed, ‘phase angle’ data is somewhat meaningless.