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December, 2009
MAINTENANCE MINUTE
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Welcome to the Centrifugal Blower Maintenance Minute,  
the 60 second newsletter designed specifically for centrifugal air & gas blower operators.  Each month we will feature a different aspect of service and maintenance to centrifugal blowers. We will review operation, troubleshooting, actual case histories, and answer reader questions.  
Soft Foot = Excessive Vibration.....
A Case History
  

A local manufacturing plant had recently replaced the output drive coupling($20K) on a Falk gearbox after it had failed.  Replacement had involved complete disassembly of the large gearbox(with 12" diameter shaft).  While the machine was down, the main drive motor, 500 HP, DC, 1400 rpm, had been sent to the motor shop for PM & reconditioning. Re-installation, shaft alignment, and startup had been completed by facility maintenance personnel.
AssemblyUpon startup, the motor exhibited excessive vibrations, especially under load. Since we had been collecting vibration data on most of his equipment regularly(including this set of machines), I was called in to determine why?  Was the gearbox causing the motor to vibrate, or was the motor at fault.        
Complete vibration analysis confirmed the overall vibration levels were minimal on the gearbox, but the motor had a high outboard vertical level and high horizontal plane level at the drive end.  The axial plane levels were only slightly higher than "normal", but did indicate a probable misalignment and/or soft foot condition. 
An alignment check of the motor/gearbox drive coupling with my Laser Shaft Alignment System confirmed the horizontal alignment was out of tolerance by almost .030".  The rear feet of the motor had to be moved over 1/8"!  Following horizontal corrections, the front foot of the motor was found to be .067" "soft".  We corrected for soft foot by adding .125" shim to the front right foot, and .020" shim to the rear left foot. The vertical alignment was further corrected by adding .100" shim to both of the rear feet of the motor to bring the shaft alignment into tolerance. The millrights had supposedly installed the shims in the same place as they had been removed. 
AssemblyA check of the vibration levels after startup, and under load, measured considerably less and within tolerance.  Outboard vertical levels decreased from .26"/sec to an acceptable .10"/sec.  Drive end horizontal levels decreased from .12"/sec to .07"/sec.  There is no evidence of rotor imbalance.     The soft foot and misalignment were obviously the cause of the higher vibrations. 
There is no substitute for good alignment practices.  A good technician can align a coupling with a straight edge to within .010"! All shaft alignments must be checked for soft foot.  This company did not use a Laser(someone had stolen it a few months ago), and they obviously did not have well trained alignment technicians.  As companies cut back on maintenance $$, and lose experienced techs to attrition, saving money with PM becomes a real uphill battle.  But there is no doubt, laser shaft alignment reduces machine vibrations and thereby prolongs the life of your equipment!  
 
� Copyright  2009 Snyder Technical Services, LLC.  All Rights Reserved
                      

Maintenance Minute is published each month by Snyder Technical Services, LLC and is based on the work of former Lamson National Service Manager, Bill Snyder.  Should you have any questions, or problems to discuss, please contact me. 
 
Sincerely,
 
Bill Snyder
Founder - Snyder Technical  Services, LLC 
Next Month: 
 
Blower Drains...Do they work?
Thought for Today:
 
  
"Good Health.....is merely the slowest  possible rate at which one can die!"