Jim Anderson is vice president of Coupling Corporation of America, which designs and manufactures high-performance and high-misalignment couplings. For more information, visit couplingcorp.com.
Some pumps were running with high vibration levels, while the disc couplings were failing on a regular basis.
In this particular case, a close-coupled design seemed like a good fit because both shafts were more than 5 inches long, which, combined with the 8 inches between shaft ends, would give enough space to allow for a distance between hinge points of at least 18 inches. At that distance, a coupling with 0.3 degrees of angular misalignment capability would probably work, although 0.4 degrees would give a larger safety margin.
This strategy seemed like a good option until one other detail was discovered: an obstruction on the pump side that would not allow the larger diameter of the flexible element to be installed.
Figure 3. A combination of spacer and close-coupled coupling halves
Finally, a combination of a spacer and a close-coupled coupling could work. Because of the diameter restriction, a normal hub could be used, bolted to a spacer center section with a flexible element. On the motor side, without diameter restriction, a close-coupled style coupling could increase the distance between hinge points (see Figure 3).
With the dimensions of the shafts, this "hybrid" coupling layout could provide about 13 or 14 inches between the coupling's hinge points. With that much distance, a coupling with 0.3 degrees of misalignment could not provide enough extra misalignment margin to run safely. So a higher misalignment coupling would still be needed to meet the needs of the large thermal growth.
The final solution came in the form of a high-misalignment coupling with 0.5 degrees of misalignment per hinge. With that amount of flexibility, the coupling can handle about 0.115 inches of parallel offset, which is enough to have a large safety margin on the misalignment.
The plant decided to try one coupling using this style. It was installed on one of the more problematic units. After installation, the vibration issues on that unit were reduced to normal level. The only remaining question was how long the coupling could last running at those high misalignments. The first installation was about five years ago, and the coupling is still running successfully.
Since the original installation, the plant has upgraded the remaining units with the same new coupling, and the results have been similar.
See other Maintenance Minders articles here.