A relatively large centrifugal blower w/ 300 HP motor, was operating with increased noise and vibration.   They thought it maybe was a bearing, but they weren't sure.  Would I take a look at it?  Although I had no previous vibration data, my vibration spectrums confirmed the bearings were not showing excessive wear.  The spectrums did show a good amount of imbalance of the rotating assembly.  The overall vibration levels were not excessive, so our first diagnosis was probable blower imbalance.  However, high axial plane vibrations pointed to a possible coupling problem or shaft misalignment. 

Although I haven't kept track, rotating the shaft keyways has made the difference countless times.  It happened to a new Lamson blower startup in Texas.  Upon startup, vibration levels were in excess of .35 in/sec.  As Service Manager, I travelled to the jobsite to investigate.  During our search for the cause, someone recommended putting the shaft keys in line instead of 180^O apart.  Problem solved. So it can work both ways. Now I know enough to have the local contact make the changes and save me a service trip!     

Coupling failures cause more vibration & noise problems than people realize.  Many times customers call me to come over and 'fix' the blower, or motor, bearings where I simply remove the coupling guard and find the coupling had failed.  An easy fix they could have completed themselves.  Make coupling inspection a mandatory part of your PM program. You'll not only save time and money, you could prevent a catastrophic failure!