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.

How sensitive are Grid Couplings to misalignment?

Grid couplings are very sensitive to misalignment. They are designed to handle almost no parallel shaft misalignment, and only minimal angular misalignment (as called out in the Lovejoy grid coupling rating chart and the coupling pre-selection guide below).

While misalignment must be considered and proactively addresses when using a grid coupling (see Why Lovejoy Offers Shaft Alignment Dial Indicator Kits), there are several major benefits to using a grid coupling. The biggest is that it is an all metallic flexible coupling design that can transmit significant torque in a small footprint (high power density) while also (unlike metallic gear & disc couplings) providing system vibration dampening capability. Grid couplings are a well proven technology, and are readily available from stock from a few leading coupling brands (inclusive of Lovejoy). 

Lovejoy Grid Coupling Misalignment Capacity 

(Standard and Spacer)

	Max Installation		Max Operational		Nominal
	Misalignment (in)		Misalignment (in)		Gap (in)
Size	Parallel	Angular (X-Y)	Parallel	Angular (X-Y)	+/-10%
1020	0.006	0.002	0.012	0.010	0.118
1030	0.006	0.003	0.012	0.011	0.118
1040	0.006	0.003	0.012	0.013	0.118
1050	0.008	0.004	0.016	0.015	0.118
1060	0.008	0.004	0.016	0.018	0.118
1070	0.008	0.005	0.016	0.020	0.118
1080	0.008	0.006	0.016	0.024	0.118
1090	0.001	0.007	0.016	0.028	0.118
1100	0.010	0.008	0.020	0.032	0.177
1110	0.010	0.009	0.020	0.035	0.177
1120	0.011	0.010	0.022	0.040	0.236
1130	0.011	0.012	0.022	0.047	0.236
1140	0.011	0.013	0.022	0.053	0.236

While the power density of a grid coupling is enviable, one final drawback to consider when selecting a grid (or gear) coupling is the fact that it requires lubrication (grease). Unlike elastomeric flexible couplings (which generally also provide system vibration dampening capability at lower power density levels), this means your maintenance team will need to periodically re-grease the coupling through the lubrication ports on the coupling's cover and be careful to avoid and/or properly address any grease leakage environmental concerns.

Alternatives to grid couplings include disc couplings (see Why to Switch from Grid to Disc Couplings), which avoid lubrication concerns but do not offer the grid coupling's level of vibration dampening, and jaw in-shear couplings (see Jaw In-Shear Couplings - A Straight Forward Value Add) which avoids lubrication but has a lower power density.

Universal Joints - Unique Solutions for Applications with Greater Misalignment

One of the primary functions of a flexible coupling is compensating for both angular and parallel misalignment.  But what if both the angular and/or parallel misalignment in the application is greater than what most flexible couplings can handle?  This is where a universal joint (u-joint) can provide a solution!  The unique features of a universal joint (and Lovejoy’s product offering) can provide customers with the best coupling fit for any application.

U-joints provide a unique function in the role of a flexible coupling.  Most flexible coupling designs offer from 1-1/2 to 6 degrees of angular misalignment.  The u-joint has the ability to compensate 25 degrees of angular misalignment as standard and up to 45 degrees of angular misalignment with modification.  The double u-joint type can provide larger amounts of parallel offset (amount dependent on size) than most other coupling types.  This provides customers with a flexible coupling solution that can accommodate large amounts of both angular and/or parallel misalignment.

The standard D type u-joint provides up to 25 degrees of angular misalignment.  The DD type allows for angular and larger amount of parallel offset (amount dependent on size) due to its double u-joint configuration.  The HD type allows for longer life due to the induction hardened yokes used in the assembly.  The NB style uses needle bearings to compensate for applications requiring higher speeds (up to 6000 RPM).  Finally, the LOJ and JR-4 types allow up to 45 degrees of angular misalignment in hand crank type applications. 

The amount of misalignment that a u-joint can compensate for is dependent on torque of the application, RPM, and the amount of angular misalignment in the application.  Should you have any further questions, consult your favorite technical support representative for proper sizing and support.  

Recommended Follow-On Reading: For a more complete perspective on general flexible power transmission couplings in general, excluding universal joints, please read: Flexible Coupling Basics - A Quick Primer

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

Top Ten Factors for Selecting A Coupling

So you understand that a flexible coupling is a connection between two pieces of equipment used to transmit torque and compensate for misalignment... and would like to know what are the key criteria for narrowing down the near countless coupling options to just a few really great ones for further consideration? Perfect, we've got you covered.

Unless you are simply replacing an old coupling that worked well (and simply need to identify it to reorder it)... the 5 fundamental pieces of information that you need to size just about any coupling are:

• Horsepower of the motor
• RPM (at the point of the coupling)
• Shaft and keyway sizes
• Shaft separation or BSE (distance between shaft ends)
• Type of driven equipment (i.e. - pump mixer, conveyor, etc.)

In addition to these 5 fundamentals sizing factors, the following 5 fundamental application factors should also be reviewed and considered against the needs of the application:

• Operating temperature
• Chemical exposure
• Run cycle (continuous or start/stop)
• Amount of space available for the couplings
• Misalignment handling requirements (angular, parallel, & axial)

While these 10 baseline selection factors are far from exhaustive (many other system specific considerations such as fail-safe, maintenance-free, or backlash requirements may and should be considered), these 10 criteria will quickly narrow down your basket of options from dozens of coupling solutions to a select few for further review.

The following charts provide just a quick reference broadly summarizing four fundamental coupling types and their ability to accommodate angular, parallel, and axial misalignment, as well as torque, temperature, and chemical exposure. (For those looking for a bit more in-depth review, a much more complete coupling pre-selection guide... covering 12 coupling types and 12 selection criteria... can be found here.)

Additional Note on Misalignment: When evaluating coupling misalignment ratings, please note that ratings for each coupling type represent maximum allowable numbers. Couplings cannot and should not be aligned at the maximum allowable misalignment for more than one condition (i.e. - both angular and parallel misalignment occurring at the same time). Couplings misaligned beyond their allowable ratings will result in a dramatic drop in coupling life. Although coupling life cannot be specifically calculated, minimizing coupling misalignment greatly benefits coupling life.

For specific torque capacity (= Horse Power x 63025/RPM x Service Factor), temperature range, and chemical exposure resistance or capability, please consult the manufacturer product catalog for the specific coupling you are considering (and/or feel free to give us a call if you have further questions).

Lastly, in addition to these top 10 considerations, we highly recommend that you always procure couplings from a reputable coupling company that has a strong history of quality, a strong technical support arm, and an ability to rapidly troubleshoot and ship replacement product if and when something unexpected occurs in your power transmission system.