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Shaft alignments suffer from
lack of apprentice programs
by John C. Robertson
As a consultant, I often work with people who are involved with shaft alignments and, in many cases, have not been taught the basic skills of this highly demanding technology. This is a task that requires some careful study before attempting to align two or more components in order to make them run smoothly with little or no vibration.
In this day of laser alignment units with an accuracy that is within 1/460th of a millimeter, why do we have so much trouble aligning equipment to within plus or minus 0.003 inches, which most manufacturers specify as design? I think that can be answered very simply by pointing out there is a dire need for experienced millwrights to provide on-the-job training skills to the new generation of millwrights and mechanics now on the shop floor.
But where are these teachers to be found? Most of them have retired or left the workplace and have, unfortunately, taken their skills with them. It's no good turning to high schools or technical colleges because there is very little hands-on skills experience available there. Having taught in technical colleges, I know from experience that hands-on mechanical plant maintenance training is not on most curricula. Manufacturers of rotating equipment do not help either as they still include in their technical manuals the steel rule and feeler gauges method to align shafts.
Considering the equipment failure rates due to shaft misalignment, consider the following basic points to ensure millwrights and mechanics are fully aware of the unseen conditions that exist, and in most cases, contribute to the root cause of many shaft alignment failures. With these basics in mind, Strategic Work Systems developed a precision maintenance training program designed to train students to correctly align shafts using the reverse dial indicator graphical method, rim and face indicator method, and laser alignment techniques.
The student is taught how to identify and correct shaft alignment problems by:
1) Fully understanding the construction of foundations and installation of bedplates.
2) Being able to recognize the symptoms of misalignment in machines.
3) Understanding why misalignment causes failures in motors, pumps, turbine generators, gear drives, fan units, compressors and diesel engines.
4) Defining shaft alignment and determining acceptable tolerances for machines having various loads and speed conditions.
5) Understanding what soft foot is and how to correct it.
6) Understanding what causes shaft runout and how to correct it.
7) Understanding the effects of thermal growth in machines and how to compensate for this if the design figures are not available.
8) Understanding what indicator bracket sag is and how to compensate for this on the dial indicated readings.
9) Understanding how to set up dial indicators to make the correction moves accurately.
10) Practicing the reverse dial indicator method of shaft alignment and plotting the results graphically.
11) Practicing the laser method of shaft alignment.
12) Being able to recognize the effects of piping stresses and how to correct them using a simple, step-by-step checkout system.
This course is for those who want to learn how to successfully implement this highly skilled technique in the workplace.
Misalignment isn't just a problem with multiple shafts aligned through a coupling; it can also be a major problem with valves in piping systems. If we stop to think about it, when a pipe has to be pulled into place to join with a flange or socket of a valve, the force generated to keep the pipe and valve together is sufficient to twist the valve body. If we consider the internal construction of a simple swing check valve, the valve disc is hung on a shaft that operates between two bearings on each side of the valve body. When the body twists because of the influence of the piping misalignment forces, the two disc shaft bearings get out of alignment with each other. This causes the disc to get out of alignment with the valve seat and generates enough friction to cause the valve disc to hang up.
This problem is more common than most people think, but it's amazing how many people can remember in detail the number of valves and pipes they have personally had to pull into place in order to complete the connection.
The worst case I experienced was tracking down a misaligned, 24-inch-diameter pipe from a booster pump discharge to the main feed pump suction nozzle on a main steam generator feed pump. That pipe was 9 inches out of alignment and responsible for a series of pump seal failures, main bearing runouts, disintegrated couplings and sheared bolts. The overall pump units had to be deliberately misaligned to meet the vibration specifications. The amazing thing was this was a startup problem (10 years before) and all the maintenance mechanics knew about it but did not recognize it as a problem.
These types of problems will continue to raise their ugly
heads because maintenance personnel and management
haven't been trained to think problems through from a root cause failure analysis standpoint. It's imperative that apprenticeship programs get restarted in the United States to provide training
in industrial maintenance and workshop practices. This must also be accompanied by formal training in the applied sciences
to enable apprentices to understand how machines work. If
they don't understand how machines are designed, they'll be "fixers" for the rest of their working lives. If such apprenticeship programs aren't put into effect soon, it's predicted American industry will be severely handicapped by massive equipment failures around the year 2005 because of the lack of properly trained maintenance people.
Pay now, or youll pay later.
John Robertson is the maintenance reliability specialist for Strategic Work Systems, a consulting firm with offices in Greenville, S.C., and Mill Spring, N.C. For more information, call , e-mail or visit www.swspitcrew.com.
MRO Today. Copyright, 2000.
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