Disc Couplings - Failure Analysis (Part 3)

If history is any judge, this post will be by far the most popular in our mini-series of three blogs on disc couplings. The first post provided a broad overview of disc couplings, and the second posts covered a number of key terms used when talking about disc couplings... but this third post is on the riveting topic of disc coupling failures.

To date, the Coupling Answers blog posts coupling failures (including Jaw Coupling Failure Analysis , Gear Coupling Failure Analysis, and Grid Coupling Failure Analysis) have been extremely well-received and appreciated by our amazing customer base. One of the reasons these posts have been so well received is that so few people like to talk about failures. Fortunately, we at Lovejoy, are confident enough in our products, design capabilities, and general understanding of power transmission that troubleshooting isn't something we shy away from. (In fact, for Hannover Fair 2015, we actually hosted a Coupling Solutions Learning Lab that was loaded with actual failed couplings. This caught countless German engineers completely off-guard.)  Moving on...

Misalignment Failures

Misalignment Failure

Like many other coupling types, disc couplings often fail in with telling or "signature" ways that point to a likely culprit. As pictured at right, when a disc coupling is subject to excessive misalignment it is likely to experience disc pack cracking near the bushings, with the outer layers fracturing first.

Misalignment Failure

Note: Given disc packs generally crack from the outer layers in, it is certainly possible that a coupling will continue to transmit torque/function with cracks in the outside layers. This means that it is still important to inspect the disc pack on a periodic basis even if the coupling appears to be functioning properly. Fortunately disc packs can generally be inspected without disassembling the coupling, and possibly even when the coupling is running (by using a strobe light assuming the coupling guard is not solid steel).

If misalignment is the likely source of a disc pack failure, it is important to realign the coupling prior to restarting the system, less the issue repeat itself. (Two options to consider when realigning a coupling are laser alignment and dial indicators.) An additional step you can take if you are not using a scalloped disc pack by Lovejoy (which offers increased misalignment handling over legacy designs), you can source a replacement coupling from us!

Misalignment Failure

Torque Overload Failures

Disc Pack Torque Overload Failure

Torque overload failures in disc packs look different from misalignment failures in that the fatigue cracks tend to form in the center of the disc pack rather than up against the bushings (as pictured at right).

A second thing to look for when suspecting torque overload on this disc pack is any evidence that the disc pack layers (or laminates) are separating or ballooning away from each other in the center sections between bushings.

Bent Disc Pack Bolts

 In addition to the disc packs themselves, the bolts used to connect the disc pack to the flanges may be bent or distorted. Such distortions are another excellent indication that the coupling is seeing torque in excess of what it was designed to handle. 

Disc Coupling Hub Burst

A third and final common way that torque overload reveals itself in a disc coupling is if a coupling hub actually bursts over the corner of the keyway. This is a failure mode common across many coupling types, and the reason the coupling fails at this point is because, due to the cut keyway, this is the weakest point in the hub.

Installation & Fastener Failures

One of the fastest way to damage a disc coupling during installation is to use impact wrenches. (Do not do this!) Not only does using an impact wrench raise the potential of twisting the disc pack (pictured at right) in a destructive manner, but it also raises the possibility of friction welding the fastener bolts to either the disc pack and or the connected flange. Fasteners should be lubricated prior to installation and torqued with care.

On the other end of the spectrum, improper torque of the fasteners may cause them to loosen. This loosening can then lead to damage and wear to the fasteners as well as hole elongation and cracking of the disc packs. To avoid either over or under torquing the fasteners, be sure to read and follow the disc coupling's installation instructions. (Lovejoy's disc coupling installation instructions and videos can be found here.)

Environmental Failures

While disc couplings generally hold up very well to many environmental conditions, they are certainly not impervious to everything. As such, it is critical that coupling users understand what corrosive agents may be present for a given application, and that the disc coupling is designed to operate under those conditions. (Pictured at right is a circular, non-scalloped, disc pack layer that has been corroded.) 

Life Expectancy

Having just covered misalignment, torque overload, installation, fastener, and environmental failures of disc couplings, a natural question might be "How long is my coupling going to last?" Unfortunately, like all other coupling type, disc coupling life is not generically predictable as it is highly dependent upon the application and the misalignment of the given system it is driving. Shaft alignment does correlate strongly with longer coupling life... so taking the time to align the system properly with periodic checks should pay lasting dividends and not be overlooked.

To learn more about Lovejoy's disc coupling solutions for your applications, please check out Lovejoy's disc coupling product pages & catalogs, the non-lubricated section of The Coupling Handbook, and/or pick up the phone ask to speak with a Lovejoy application specialist.

Shaft Keys & Keyways - "Be aware of the little details”

Guest Post: Jim Mabrey, Lovejoy Field Sales Representative 

An Original Equipment Manufacturer (OEM) reported having issues with the Lovejoy SX6 Disc Coupling, specifically citing that a coupling vibration existed and it appeared to be leading to premature bearing failure on a 250Hp AC Motor driving a Blower. 

The immediate assumption was that the disc coupling hubs were not sufficiently balanced and that they needed to be replaced. Unfortunately, upon replacement, the freshly balanced hubs did not eliminate or address the vibration issue at hand.

Lovejoy SX6 Disc Coupling

Diving deeper, the question was then asked about the shape of the keyway. It was at this point that it was discovered that the OEM was using a square key provided by their Motor OEM on the motor shaft. 

The prevailing North American industry bore standard (ANSI/AGMA 9002-B04), used by Lovejoy, is to supply hubs with a chamfer key slot in order to reduce the stress on the hubs. Unfortunately, the square key used by was distorting the hub enough to make the coupling unbalanced. Once an ANSI/AGMA 9002-B04 compliant shaped key was used, the problem went away. 

The Lovejoy OEM disc coupling user now requires all keys provided by motor manufacturers to be made with a chamfer per ANSI/AGMA 9002-B04. 

About the Author: Jim Mabrey has many decades experience in the mechanical power transmission industry and has been a longtime supporter and champion of Lovejoy coupling solutions. To find a Lovejoy representative in your neck of the woods, please visit Lovejoy's Sales Representative Search Tool. 

Can a good coupling protect against ignored maintenance?

Guest Post: Fred McWilliams, Lovejoy Canada

Early one dark rainy morning in early 2010, I received an irate phone call from the head of production at one of the steel mills I support. He wanted to know why our disc coupling had come off a 250hp motor shaft in a catastrophic failure at 4.00 am that morning.  

While, fortunately, no one was injured in the failure, his production has stopped and he wanted me to visit his plant immediately. He was too “hot up”, for me to get any further information. I told him I would come right away.

While driving to the plant, I received a call from a fluid coupling manufacturer's representative, who told me that he had also been summoned, as his fluid coupling had also been part of the failure.

 As Lovejoy Canada supplies this steel mill with SX255-6 Disc coupling assemblies (consisting of 3 hubs and 2 disc packs), that bolt either side of a fluid coupling at this facility, I now could understand why the fluid coupling representative was also involved in this issue. 

Upon arriving at the job site, both the fluid coupling manufacturer's representative and I were astonished to see the condition of the 250 hp 449T frame 3 ¾” diameter shaft. It was bent almost at a right angle and there was severe heating scars on it.

Upon talking to the maintenance crew, we found out that this particular motor had been making “grinding noises”, for some time and nothing had been done to find out why this was occurring, due to being unable to have Production stopped.

Both the fluid coupling representative and I suggested that the motor be thoroughly examined to see if there was a reason for the bent shaft. It was obvious to us that there had been a failure within the motor that must have caused excessive overheating to the point that the motor shaft had deformed and stopped suddenly, causing the fluid coupling assembly to come off the shaft and break through the coupling guard.

Lovejoy Canada, the fluid coupling manufacturer, and the motor supplier were asked to replace the damaged parts ASAP so that production could be re-started as quickly as possible.

Lovejoy Canada had our coupling components on our shelf in our Mississauga Ontario warehouse and they were shipped to the customer that afternoon.
Within a couple of days the drive assembly was completed and installed.

A report on the motor found that a rear bearing had seized, causing excessive overheating and deformation of the motor shaft. The new assembly has now been running successfully for over four years, and is “Red Flagged” for regular maintenance. A crucial maintenance/production lesson has also been learned.

While I never did get an apology from the Head of Production for his irate phone call, I did derived a great deal of satisfaction from being able to help him quickly troubleshoot the failure and get his production again.

So to directly answer the subject question, “Can a good coupling protect against ignored maintenance?" The answer is obviously and unfortunately "No". While no equipment will last very long if it is neglected and abused, using high quality products within their designed working envelope (like the Lovejoy SX255-6 disc coupling) will help keep equipment running for a considerable length of time. Again, using quality products should never be an excuse to ignore regular system maintenance.


About the Author: Fred McWilliams has served as Lovejoy Canada's Eastern Accounts Manager for well over a decade. To tap into Fred's rich industry knowledge and experience, please contact him through Lovejoy Canada or reach out to him directly on LinkedIn.