Keeping Machinery Alignment Records
by John Piotrowski
     Over the past twenty years, the level of awareness concerning the importance of accurate and precise shaft alignment has increased dramatically. In would therefore appear that shaft alignment seems to have taken a more important role when installing and maintaining machinery, but this alleged perception may be somewhat difficult to prove since it is quite rare that you can find any historical records on the alignment of rotating machinery in industry. Condition Based Maintenance programs incorporate periodic collection of vibration data, oil samples, and infrared imaging where a comparative analysis is made to detect unhealthy changes that occur in these parameters indicating potential problems with the machinery. But if you ask these same people who engage in these highly successful programs to show you their alignment records, rather than information, you typically get a blank stare and a quizzical look asking ... "Why should we keep that information?"
     There are several good reasons why historical alignment records should be kept and why shaft alignment should be checked periodically. Most people assume that once you align a drive system, it stays aligned forever. This can't be further from the truth. One chemical plant has annually checked the alignment of their rotating machinery for the last three years and have discovered that 80% of their machinery doesn't stay aligned for long periods of time. How did they find that out? They kept records of their work and compared the 'as found' alignment to the 'as left' alignment the last time it was checked. Do you do this? If not, why not? If you were to shut down one of your drive systems today and check the alignment, how confident are you that the randomly selected piece of rotating machinery is aligned? If you keep alignment records and find that a drive system has shifted its position, what caused the shift to occur? If it is a pump, compressor, or turbine, could there be an excessive amount of static or dynamic piping strain? Is there poor contact between the machine case and baseplate due to a 'soft foot' condition causing shim packs to work loose shifting the machine? Is the foundation shifting its position over long periods of time? Not only should shaft position measurements be kept, but also information on the preliminary checks such as : shaft or coupling hub runout conditions, 'soft foot' information and shim shapes, and static piping stress tests.
 
     Figure 1 shows a typical 'Preliminary Checklist' data recording sheet to record this type of information. Figure 2 shows a typical (and actual) 'As Found' and 'Final' alignment recording sheet.
Figure 1. Sample recording sheet for preliminary checks prior to aligning the shafts.
Figure 2. Sample report showing the as found and final alignment of the equipment.
     Not every alignment job is straightforward and in special cases, information on how the alignment measurements were taken can be invaluable. At a steel plant, a fairly complex finishing mill screw down drive train comprising several machine cases flexibly coupled together was experiencing premature failures on several components. A integrated flexible coupling and drum brake assembly was installed between two of the machine cases. The plant had a laser alignment system but was unable to use it here since the laser beam could not shoot a straight line of sight through the stationary brake shoe assembly surrounding the drum. Figures 3, 4, and 5 show how the shaft alignment measurements were taken using a face and 'inside' rim reading to capture the measurements. These photographs were then used to generate a procedure requiring a mix of both laser and dial indicator measurement systems to align the unit.
Figure 3. Capturing a face dial indicator reading on a brake drum.
Figure 4. Capturing an 'inside' rim dial indicator reading on a brake drum.
Figure 5. Measuring the face and inside rim of the brake drum.
     Another company was experiencing failures on the bearings and mechanical seal of a motor-pump drive system. Initially, the motor and pump was removed from the baseplate, the bearings and seals were replaced, and the unit re-aligned. After two more failures within a 10 month period, the alignment was checked before the motor and pump was removed only to find that the 'as found' alignment measurements did not match (or even come close) to the 'as left' alignment after the last rebuild. Upon further investigation, it was discovered that the pump foot bolts were loose and a 'soft foot' condition existed on all the pump feet and the suction and discharge piping was exerting a considerable strain on the unit slowly shifting its position over time. Since the alignment measurement system they were using forced them to name one machine stationary and the other movable, the technicians named the motor as the movable machine never bothering to check the foot bolts on the pump.
     I was once told ... "If something is important enough to measure, then it's important enough to write it down and save it." Keeping historical records of the soft foot conditions, runout on the shafts and coupling hubs, piping stress checks, as found and final off-line shaft alignment position measurements, and machinery dimensions can take some time and effort to compile and record but there is a good chance that this information might save you or one of your co-workers as much and possibly more time than it took to generate the records in the first place.

 

 

This article is provided courtesy of Turvac Incorporated.
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