January 12, 2017
Are Flexible Couplings Designed to be Misaligned?
By James Pekarek
A common misbelief that occasionally comes up in our precision shaft alignment training classes is that, elastomer insert flexible couplings are designed to allow misalignment between rotating machinery shafts. This is not the case. Although this type of coupling may be more forgiving to the effects of misalignment, they are definitely not intended to be operated that way.
These couplings are beneficial to help protect the driving and driven machines against the harmful effects produced due to, shock loads, hard starts, vibrations, and for machines aligned to thermal offset targets that “grow” into alignment due to thermal growth. They are also designed to act like a fuse, as they will “self-destruct” to break the connection in the event of a failure on either the driver or the driven side. This would prevent massive shock loading from passing through to the other piece of equipment and potentially causing a failure on the other side.
Flexible couplings benefit from precision alignment just like other types of couplings or rotating machinery. Decreased MTBF, longer life expectancy, longer bearing and seal life. Just because a flexible coupling can tolerate some amount of misalignment, does not mean it doesn’t need to be precision aligned.
During a training class at a university in 2013, we checked the alignment of 3 chill water pumps at the central plant. Each was driven by a 250 HP, 1800 RPM electric motor and an elastomer insert coupling.
The lead mechanic stated that they did not need to be aligned because they had new coupling inserts installed 2 weeks ago and he aligned them with a straight edge. It was obvious there was an alignment issue as the coupling inserts had considerable wear after only 2 weeks of operation as indicated by the pile of shredded elastomer all over the floor under the coupling.
The mechanic told me they have to replace them every other month because “the motors are just so powerful”. As suspected that was not the issue. We checked the alignments of all 3 pumps with their Fixturlaser NXA Pro Alignment System and found that all were severely out of tolerance.
Within 90 minutes we had all three pump sets finished and back into service.
A few months later, I contacted the university and spoke with the maintenance manager. He informed me that the three chill water pumps were running perfectly and they have not had a coupling failure since being properly aligned. He also mentioned there was not any buildup of coupling debris to be found.
First hand proof that all couplings will benefit from proper precision shaft alignment.