This has been the year for coupling failures. From both pumps and centrifugal blowers, I've had a report of a coupling failure on the average of once per month. Ten of them just this year! And the causes of each one vary across the board. Only one of them I can attribute to shaft misalignment.

The most recent failure was of a relatively new(1 year old) Rexnord size E5 coupling driving a centrifugal blower. This one was due to operational problems-it kept going into surge and reversing direction. Another Woods size 8S EPDM flex element driving a water pump had been misaligned so badly it chewed up the black flex material so bad it destroyed the guard when it blew apart.

There was one Rexnord size E10 coupling flex element that was found to have cracks on the edges, so it can be considered "normal" wear. There were some undetermined failures, too. A Woods size 9E split type EPDM element was found failed one morning after filling the WWTP blower room with smoke! The motor overloaded and shut down, but the flex material rubbing on the coupling guard did a pretty good job of damaging it. We suspect there was some type of electrical on/off failure that caused the blower to shut down and re-start more than once.

Last year I replaced a Lovejoy spider type coupling complete on a centrifugal blower at a nearby WWTP and then laser aligned the shafts. Less than 6 months later, the steel hub "broke". It split right in half, causing considerable damage to the guard and other components. My supplier replaced it under warranty because there had to be a manufacturer's defect.

Another WWTP blower application had a Rexnord size E5 flex element loosen up from the hubs and rip apart. I had just verified the shafts were laser aligned less than 3 months earlier! I am sure it did not have loose coupling bolts/screws because I always check the torque when doing an alignment. There is a slight chance I may have tightened the screws too much since I didn't have my torque wrench with me, but it is more likely this was also an electrical failure(lightning?). Rexnord warns of over-tightening the bolts/screws. They insist a torque wrench always be used.

Investigation into a report of excessive vibration and noise on a 700 HP fan resulted in a Falk gear type coupling being replaced. There was little or no grease in the hubs! Another WWTP had a Wrapflex type coupling fail due to excessive wear of the hubs! All I can guess is insufficient spacing, or a loosening of the set screws.

Although not all of these failures could have been avoided, a little preventative maintenance is still the best way to go. Invest in PM, it will save you money!