When Breaking Up is a Good Thing

A customer recently called in to Lovejoy concerned about a jaw coupling failure. The unusual aspect of this issue was that three hubs broke in half in one week (see picture 1). Lovejoy’s technical team was baffled by this inquiry and began the process of analyzing the failures.

Picture 1.

This particular application had an electric motor (30 HP) driving a blower at 3510 RPM.  Initially, our engineers focused their attention on over torque. The jaw coupling used in this application had an NBR spider rated for 792 in-lbs. of torque. The application had a torque rating of 650 in-lbs. Based on the torque specs alone, one could say that the couplings should have performed well under normal conditions (for more details on calculating torque see Coupling Sizing Torque - How to Quickly Calculate).

In conjunction with the over torque troubleshooting, the end user confirmed that the alignment was within the coupling specifications. The customer used his own methods and tools to verify alignment, and he also followed the Lovejoy Install Videos and Install Guide step-by-step procedures.

As our technical team continued with the troubleshooting steps, they took the time to share additional technical details as to what can cause torque spikes.  One of possible causes is motor pauses and restarts.  The customer then mentioned that they had an electrician replace a fuse on the motor because it had blown out previously. Furthermore, the customer mentioned that there were momentary pauses that occurred after replacing the fuse, which is an indication that the motor was causing torque spikes.

Picture 2.

Although fail-safe by design, jaw couplings can fail catastrophically when hit with a large enough over-torque. (This would be a good time to remind you that OSHA requires coupling guards for all flexible coupling applications. Please be safe.)  When a jaw coupling hub fails catastrophically, it will typically fail either by shearing at the jaw legs, or by breaking open between the inside bore and outside diameter of the hub (generally along the keyway - see picture 2).

In this particular case and because of torque rating specs, it turned out the coupling breaking in half was actually a good thing.  If the couplings were to continue to transfer torque between the motor and the blower, the probability of burning the motor or damaging the bearings in the blower becomes high.   Breaking up (coupling failure) allows you to spend a few dollars to fix the problem as opposed to thousands of dollars in equipment replacement.

Coupling Peak Torque Failure at Keyway

Gear Coupling Hub Burst Due to Over Torque

Coupling hubs failing, cracking, or bursting over the keyway corners is a common failure mode that clearly indicates the coupling hit a peak torque that it was not designed or built to take. The reason the coupling hub breaks at this point is because it is generally the weakest point on the hub. (There is less material between the corner of the keyway and outside diameter of the hub than any other point on the inside diameter of the hub.)

The root cause for this over-torque failure may be that the coupling was undersized for the given application (perhaps the wrong factor of safety was used when calculating required torque), that the application saw a massive unexpected peak load (and perhaps the coupling failing may have saved more expensive equipment from failing), the coupling bore and/or keyway were oversized relative to the maximum bore and keyway combination specified from the manufacturer for the given product size, or the coupling had excessive misalignment.

Jaw Coupling Hub Burst Due to Over Torque

Identifying the peak torque load and either eliminating it from re-occurring and/or ensuring the replacement coupling is designed to handle the peak torque is critical to ensuring this failure mode is not seen again. (Unless there was a material defect in the hub, which, at least for Lovejoy manufactured couplings, is extremely unlikely... simply replacing the hub will likely result in another hub failure.)

It is also important to note that, while undersized/underrated couplings are not good and can cause failures... it is also a mistake to significantly oversize a coupling. There are a number of drawbacks to this approach, including excessive overhung loads, potentially reduced performance, and cost.... and should be avoided.

For further information on sizing a coupling, please consider reading  "Coupling Service Factors - Best Practices" and "Coupling Sizing Torque - How to Quick Calculate". For further information on misalignment, please see "Top Reason for a Coupling Failure". 

To learn more on coupling failure analysis, go to:

 

Coupling Failure Analysis - Jaw Couplings (includes hub & spider photos)

Gear Coupling Tutorial - Part V: Failure Analysis (with photos)

Grid Coupling Failure Analysis (includes photos) 

Top Reason for a Coupling Failure

And, if you still have questions or concerns... please feel free to contact a Lovejoy application engineer or specialist.

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. 

Jaw Coupling Question - "How Much Life Can I Expect Out of the Spider?"

 

A frequently asked question for jaw couplings is: “How much life can I expect out of the spider?”.  The answer to this question is, unfortunately, nearly impossible to determine.  

Spider life is affected by a variety of factors (i.e. high/low temperatures, chemical exposure, operation cycle of machinery, etc).  Lovejoy does provide advice on spider inspection and replacement.  When the spider’s leg volume or thickness is at 75% of the original volume or thickness the spider should be replaced.  Inspection of this can be done visually and will result in disassembly of the coupling only if the spider needs to be replaced. 

Correct installation of a jaw coupling can greatly increase the life and performance of the elastomeric spider.  The installation process starts with the correct coupling selection.  The selection process starts with collecting the proper information about the application.  These factors would include: horsepower, RPM, shaft sizes, the type of driver/driven equipment, type of chemical exposure (if any), and temperature exposure.  After selection, proper installation and alignment are also key factors.

• For installation and alignment instructions: please check out "How to Install a Coupling" or download installation instructions from your favorite coupling manufacturer's website. Instructions should be step-by-step straightforward and speak specifically to your jaw coupling type (straight jaw, curved jaw, spacer jaw styles, and jaw in-shear, etc.)
• To learn more about coupling selection criteria, please check out "Top Ten Factors for Selecting A Coupling".
• For help with calculating required system torque, check out "Coupling Sizing Torque - How to Quickly Calculate (inclusive of Service Factor)".

Jaw couplings are not only one of the most popular and economical flexible coupling solutions in the world, but they also offer a solution that does not need lubrication.  This can be a tremendous time saver, but low maintenance couplings does not mean no-maintenance!  By following the helpful tips above (and two robust articles recommended below), coupling users can ensure that their jaw couplings are in proper condition and avoid unexpected downtime.

Recommended Follow-On Reading: For a robust report on jaw coupling failure analysis, please read: Coupling Failure Analysis - Jaw Couplings (includes hub & spider photos). For a broad overview of jaw couplings in general, please read: Jaw Coupling Overview - Features & Benefits, Design Basics, and Element Options.

Author Credit: This article was written by Charlie Mudra (National Accounts Manager, Lovejoy, Inc.)

What is Static and Dynamic Alignment?

While we have previously identified that alignment is the primary cause of premature coupling failures (see post here), a further explanation of static versus dynamic alignment is in order.

Static alignment is the condition of the machinery at rest (think of the alignment we perform when the equipment is first installed).  Static alignment gives us the opportunity to correct issues such as soft foot, gross misalignment and to bring the system to within specifications.

Dynamic alignment is the condition of the machinery during sustained operation.  Think of an electric motor moving from its mechanical center to the electrical center, the thermal growth experienced by an internal combustion engine or a shaft moving axially in response to forces in the machine train.  It is common to perform a “hot alignment check” on equipment.  A hot alignment check is when the machinery is allowed to achieve its operational steady state condition (i.e. after a compressor train has operated for a minimum of 24 hours) and is then shut down with the express intention of quickly performing an alignment verification. 

Depending in the machinery, dynamic alignment is preferred as the equipment will experience measurable changes and can result in the equipment operating beyond alignment specifications from the initial static condition.

Remember to always align equipment to the tighter of either the coupling or equipment specifications!

Top Reason for a Coupling Failure

Coupling failure issues can be traced down to multiple types of failure modes. However, I have found that a few are the primary culprits for all types of couplings.

The #1 failure mode that causes the majority of the premature failures we see can be attributed to one aspect:

IMPROPER ALIGNMENT (angular, parallel or axial)

Some of the remaining common failure modes are (in no particular order):

• Lack of lubrication (Gear and Grid couplings)
• Improper torquing of fasteners
• Excessive torque
• Environmental
• Excessive vibration induced by mating machinery

While multiple issues can occur that will lead to a premature coupling failure, it is a good idea to start with the alignment of the machinery when determining a root cause. 

Used properly, dial indicators (lower cost & pictured above) and laser alignment tools (premium/higher cost) are both effective tools to consider when aligning a coupling.

Recommended Follow On Reading: For a deep dive into specific failure modes, inclusive of photographs, check out the following four articles below.

 

Coupling Failure Analysis - Jaw Couplings (includes hub & spider photos)

Gear Coupling Tutorial - Part V: Failure Analysis (with photos)

Grid Coupling Failure Analysis (includes photos) 

Coupling Peak Torque Failure at Keyway

Coupling Failure Analysis - Jaw Couplings (includes hub & spider photos)

Jaw couplings are an incredibly robust and popular solution for transmitting torque between two shafts. With tens of millions of applications currently running around the world using jaw couplings, this coupling type is one of the most trusted power transmission devices on the planet... however, like almost anything else, they can and do fail. When they do, whatever the application is and whatever corner of the world you may be in, it is important to be able to quickly understand why the coupling failed... as well as what needs to be done to ensure your application is safely up and running as soon as possible.

Fortunately, not only are jaw couplings robust, cost effective, popular, and straight forward to use… but they are also fairly straight forward to trouble shoot if/when they fail. The reason jaw couplings are straight forward to trouble shoot is because jaw coupling failures often leave behind a telltale “signature”. (These "signatures" are why coupling application engineers are particularly quick to request photographs when trying to root cause a failure with a client.)

This post will explore the most common jaw coupling "signature" failure modes, and hopefully help you get started in identifying and correcting any jaw coupling failures you may come across. All failures discussed in this post will fall into one of five categories: normal wear, torque overload failure, misalignment failure, environmental failure, or vibration failure. 

(Note: This post does not address manufacturer quality failures, such as casting defects or poor elastomer quality. It is important to remember that interchangeable products do not mean equivalent quality, so it is important to source your couplings from a trusted industry leader.) 

Should you care to learn more about jaw coupling basics before carrying on with failure modes, please check out: Jaw Coupling Overview - Features & Benefits, Design Basics, and Element Options.

 

Normal Wear 

 

The image at the top of the page (and at right) is actually an example of normal even elastomeric jaw coupling spider wear for a coupling that is operated in one direction only. The wear is even and linear, indicating that misalignment does not appear to be an issue with this specific application.

 

When a jaw coupling runs in one direction, only every other spider leg is put under compression. (If the application were to run in both directions, called a reversing load, you would expect to see similar wear on every leg of the spider.) 

 

While this wear pictured is to be expected, this coupling spider has worn to the point where it does need to be replaced. As a general rule, you should replace a spider when at least one of the elastomeric legs have compress or worn down to 75% of their original thickness

 

And how fast does it take for the coupling spider to be compressed to 75%? Unfortunately, this cannot be calculated as it is entirely application specific. The environment, peak torques, selected service factor, misalignment, system vibrations, starts/stops... will all play a role in determining how long it takes for the spider to compress to 75%.   

 

Bonus Suggestion: If you are running an application in only one direction, and discover wear on your spider similar to the one pictured above (3 legs compressed 25% or more)... if you don't have a replacement spider readily available, you can rotate the spider one leg rotation (moving the 3 uncompressed legs into the position the 3 compressed legs were), and continue running the application until your replacement spider arrives.   

 

Overload Failure

 

When sizing and selecting a coupling it is always important to identify the appropriate service factor for the given application, and include it when calculating the coupling sizing torque. (For a great tutorial on how to calculate coupling sizing torque, check out: Coupling Sizing Torque - How to Quickly Calculate.)

Overload or over-torque occurs when the coupling is subjected to peak torque load(s) in excess of the calculated coupling sizing torque (which, again, if you don't understand... please click the link above). This can happen because the coupling was undersized (usually the case), or because something terribly wrong and unexpected happened in the system. In the case of the coupling being undersized, it is important to not just replace the failed coupling, but to go through the selection process again... selecting a coupling that meets the necessary requirements of the application. 

So what does an overload failure look like? Well... when something is pushed to the point of failure, something is going to fail... and, in the case of flexible jaw couplings, the weak point is the elastomeric spider or insert. Pictured below is a classic jaw coupling overload failure. When the overload occurs, the legs of the jaw hub will pinch/shear off the spider legs under compression. (Please jump back up to the "normal wear" section above if you don't understand why only 3 of the 6 spider legs would have sheared in the example below.)

Jaw couplings are fail-safe by design (meaning they can continue to run after a failure), and this is because, even after the elastomeric spider legs are sheared away... the metallic hubs will interlock and continue to transmit torque. This is not a good long term condition for a system, but can be a useful feature during short term cases of emergency. The metallic jaw hubs will rub and wear away on each other (without lubrication), so quickly identifying the root cause of the failure (which may be an undersized coupling) and replacing the element and/or coupling is important.  

While straight jaw spider overload failures generally take the form of complete shear (pictured above and below left), curved jaw couplings can show spider overload failures either in complete leg shear off (like straight jaw) or in "squared off" spider legs (pictured below right).

Though fail-safe by design, when jaw couplings are hit with a large enough over-torque, jaw coupling hubs can and do fail catastrophically. (This would a good time to remind you that OSHA requires coupling guards for all flexible coupling applications. Please be safe.) 

When a jaw coupling hub fails catastrophically, it will typically fail either by shearing at the jaw legs (pictured at right), or by crack open between the inside bore and outside diameter of the hub (generally along the keyway, as pictured below). Coupling hubs generally cracking along the keyway because there is less metal between the keyway and the outside diameter of the hub than at any other spot of the inner bore. The keyway is the weak link, and, specifically, the corners of the keyway are the weakest link inside the keyway. This failure mode is common to a large number of coupling types, and generally referred to as a "keyway burst". 

The two pictures above represent a classic "keyway burst". The burst occurred at the corner of the keyway, with the crack then being mirrored 180 degrees from that keyway corner on the other side of the hub. (The picture at left also highlights that the crack split through the setscrew hole. With clearance fit bores, a set screw hole generally needs to be inserted over the keyway, making the keyway just that much more of a weak link.)

 

Misalignment Failure

 

Second only to transmitting torque, flexible couplings are designed to accommodate for system misalignment. Couplings are designed and rated to handle a certain amount of angular, parallel, and axial misalignment... but users do not always stay within these published ratings. The tell tail sign that misalignment is causing premature jaw coupling is that the jaw coupling spider is wearing unevenly... either at an angle or not across the full face of the spider legs. Spider "dust" may also be present in coupling or coupling guard.

The spider legs under compression (shown at right) look like pizza wedges. This is a classic misalignment issue, so replacing the coupling alone will not address the underlying misalignment that is present in the application. The two shafts connected to the coupling must be either realigned (preferable), or a different coupling that can handle the misalignment needs to be selected. 

(Note: Misalignment is very hard on a power transmission system, including on the system's bearings, and can lead to premature system failure. It should be minimized whenever possible... irrespective of if the coupling can handle it or not. A future post will cover shaft alignment in depth, but using a dial indicator kit is a cost effective approach for aligning couplings... especially when they come paired with a free calculation app. Laser alignment systems can be even more precise, but are generally much more expensive.)

 

Environmental Failure

 

The two primary environmental issues that cause jaw coupling failure are temperature related (above or below the rated temperature of the elastomer), or chemical related (where the elastomer is not compatible with a chemical present in and around the coupling). 

On temperature... pushing an elastomer outside its rated temperature range, can and will lead to premature spider failure. While these failures can look similar to an over-torque, if caught early enough, they can also be identified by cracks in the elastomeric spider's legs. (Hint: If no suitable spider can be found to handle the temperature of the system, consider looking into all metallic couplings... which are broadly summarized in this article: Flexible Coupling Basics - A Quick Primer).

On chemical failures... if an elastomeric spider is in contact with chemicals that it is not compatible with, the spider will generally rapidly deteriorate and fail, sometimes breaking off in pieces. Coupling manufacturers will generally include compatibility guidelines in their catalogs for each elastomer material relative to traditional chemicals that a coupling may come in contact with. 


 If you know you coupling is going to be in regular contact with a fluid, it is important that you check the manufacturers catalog to ensure compatibility and/or call the manufacturer to confirm if their catalog does not include such information. 

Ensuring temperature and chemical compatibility are two of the top ten factors for selecting a coupling, and should never be minimized or ignore.

 

Vibration Failure

 

Vibration failures are one of the easiest to identify, but can be a bit challenging to overcome. As seen in the photos at right, in this failure mode, the spider legs heat up and melt from the inside out... extruding or "oozing out" elastomeric material once the outside of the elastomer is finally compromised.

What causes this failure is a heat buildup in the spider caused by the spiders attempt to absorb damaging high natural frequencies in a system, generally caused by a reciprocating engine (diesel, gas, natural gas, liquid propane), pump, or compressor. These system natural frequencies can wreak havoc on a system (and coupling spiders), and there is actually an entire class of couplings (known as torsional couplings) designed specifically to tune a system above or below the system's natural frequency.

Torsional couplings are generally much more expensive than jaw couplings. Because it is possible that a jaw coupling can (by the use of its elastomeric element) play the role of a torsional coupling (tuning the system above or below its natural frequency)... many users will try to insert a jaw coupling in the place of a true torsional coupling. 

Unfortunately, if/when the jaw coupling is not able to effectively tune the system above or below its natural frequency, the elastomeric element will overheat (failing to dissipate all the heat being created as it attempts to absorbing/dampening the natural frequency vibrations)... leading to the elastomeric spider oozing.

When this type of torsional failure is seen the "right response" is to contact your coupling manufacturer and request a torsional analysis for your application. The manufacturer will ask you for a number of pieces of information on your system, run some calculations, and offer you a specific torsional coupling designed specifically to tune your system out of harm's way. This solution will almost certainly cost significantly more than the jaw coupling it replaced.
 The "alternative response" (which may or may not work, and could take a considerable amount of time) is to start buying and trying different jaw coupling variations, hoping for the best. With a bit of luck, the changes will lead to the system being tuned away from the systems natural frequencies. (Changes that may work include changing the hardness of the spider, going harder or softer, or changing the size of the coupling, up or down, of course taking care not to undersize or significantly oversize the coupling.)

Ultimately, using a jaw coupling as a "poor man's torsional coupling" is an inexact science that could prove to be a cost savings if you are patient and can afford the time. Jaw coupling really are not ideally suited to serve this function, but they can work. Oozing indicates "it not working", so you need to try something different. Just replacing the spider with a like spider will not fix a torsional vibration problem.

 

Still Have Questions? 

 

If this article did not help you better understand a jaw coupling failure and you still have questions, please give us a call or send us an email (with photos) so we can help you trouble shoot further. 

Alternatively, if this article was helpful for... we'd love to know that as well, so please leave us a note at the bottom of the page.

And if you liked this article, but you'd like to get a "second opinion" from another source... we recommend you check out the Mechanical Power Transmission Association's Common Causes of Jaw Coupling Failure publication. (Note: We are an active part of this association, and did play a significant role in drafting the document. You will find its advice to be largely similar to this post, though certainly presented in a much more formal manner.)

For a deeper dive into jaw couplings, we recommend reading: Jaw Coupling Overview - Features & Benefits, Design Basics, and Element Options.

For further information on coupling failures, we also recommend reading:

Gear Coupling Tutorial - Part V: Failure Analysis (with photos)

Grid Coupling Failure Analysis (includes photos) 

Coupling Peak Torque Failure at Keyway

Top Reason for a Coupling Failure