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