Categories: Shaft Alignment,Machinery Diagnostics,Condition Monitoring,Machinery Maintenance,Other Topics

Alignment and Vibration: What is the correlation?

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By on July 27, 2016

This is a subject that comes up relatively frequently with our customers and observations from the field.  The purpose of today’s blog post is to illustrate what can and often does happen.  While misalignment can and does present with easily identifiable vibration signals; often it does not.  Misalignment may be present in a machine, developing forces detrimental to machine and component life; while at the same time NOT producing a vibration signature easily distinguishable from unbalance or looseness.  Further, depending on machine design and duty, misalignment may be present and not produce substantially elevated vibration at all.

In the case presented today, excessive vibration was identified; however the vibration did not appear initially to be caused by misalignment.  Let’s take a look:

The machine tested is a 10hp overhung chilled water pump servicing a building.  The machine was initially run for vibration testing and was audibly loud.  Since we were onsite for an alignment class, the pump was intended to be checked for misalignment and corrected if found out of tolerance.  An initial vibration test using a OneProd Hawk revealed the following:

Hawk before

Note: misalignment was not specifically diagnosed.

The pump was shut down and a visual inspection performed, which revealed the following:

Coupling overview

Installed, the coupling element is clearly distorted, and black rubber shavings are present below the coupling on the base.

Coupling insert

Removed, the coupling element damage is even more obvious.

The motor/pump was aligned and the coupling element replaced.  Here are before and after alignment data:

Alignment as found Alignment as left

You should notice the very high angularity in the horizontal plane (-5.8mils/1”).  This level of angular misalignment can be seen in the wear pattern of the coupling element; the pattern is angular and the entire insert is twisted or distorted in shape.  The motor and pump was left in very good alignment condition as documented on the right with all green coupling icons for this 1800 RPM machine.  For reference, the tolerance used is documented at the bottom of the saved results screens, 0.7mils/1” angularity and 4.0mils offset.

Vibration testing following the alignment job revealed the following:

After                                                          Before

Motor DE Bearing after  Motor DE Bearing before

For comparison, the highest levels seen on the motor drive end were reduced by 93%!

Hawk final overview

All levels across the entire machine were reduced to very good levels, resulting in a machine that is now in good condition for long reliable service.  By the way, the noise level in the room was also so substantially reduced that we could actually talk at normal conversational volume (there was only one pump in the room).

This case illustrates that the most important and reliable source of alignment data is actual alignment measurements.  This particular coupling was so degraded it was producing strong unbalance and looseness indications which basically served to mask the real underlying misalignment problem.  To complicate matters further, sometimes misalignment does not even generate substantial vibration at all.  Again, this can be dependent upon machine design.  The one thing we know for sure in all cases is that misalignment will produce excessive forces that serve to prematurely wear primary components such as bearings, seals, and couplings.

If you want to always be sure to diagnose misalignment correctly, take shaft alignment measurements and there will never be a doubt.

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