Hello:

Reference to John Piotrowski article "Why Shaft Misalignment Continues to Befuddle and Undermine Even the Best Predictive and Pro-Active Maintenance Programs". I agree with him about the difficulty to predict misalignment, especially in diaphram coupling. Many cases I have personally recorded with severe misalignment, while the vibration and other PDM tools did not give any warning. The question which I want to ask is: Do you recommend to add alignment check in centrifugal pumps at a 6 months PM interval.

Best Regards, (name & company witheld by request)

Thank you for your e-mail. We are asked this question very frequently.

As you have come to realize, shaft misalignment disguises itself very well on rotating machinery. It is possible to have slight, moderate, or even considerable misalignment present during operation and not be able to detect this condition, or at least its severity, using vibration analysis or other NDT methods. Knowing this, and understanding that equipment can and does shift its alignment condition, one comes to the conclusion that alignment should be check periodically to verify accurate long term alignment. I would recommend that alignment checks on new machinery be performed somewhere between 2000 and 4000 hours of operation (i.e. 3-6 months running 24 hours/day). A record of the final alignment should be compared to the as found alignment after this time period and if more that 1 mil/inch of misalignment occurred, that the unit be realigned. Allow another 2000 and 4000 hours of operation to elapse and check the alignment again. If more than 1 mil/inch of misalignment occurred again, I would recommend looking for causes to explain why the shifting is occurring such as: excessive piping strain, shifting of foundations or baseplates, excessive soft foot conditions, or off-line to running machinery movement. For equipment that has been in service for several years I would recommend that alignment be checked annually. If after several years, it is observed that equipment does not shift its position, then alignment (as well as other) checks could be made every 3-5 years. Successful long term alignment involves more than taking measurements with brackets and indicators or laser/detector systems and moving machinery. It requires a thorough understanding of foundations, machine case to baseplate interfaces, why flexible couplings should NOT be engaged during the alignment measurement process, movement of machinery from off-line to running conditions, piping, ductwork, or conduit stresses, and record keeping to track the progress of the machinery in a facility. For additional information, you may want to review the republished article entitled "Importance of Machinery Alignment Records" in the Technical Info section. Would you be interested in allowing us to post your question and this answer on our web site in the Technical Info section? If so, let us know. Best of luck and let us know what you find when you start checking the alignment of the equipment in your facility.

John Piotrowski

January 2001