Friday, December 19, 2014

Grid Coupling Failure Analysis (includes photos)

Like other coupling types, grid couplings often have "signature failures" modes that can help root cause a given coupling's failure. Some of the common causes of grid coupling failures are fatigue, torque overload, lack of lubrication, misalignment, and environmental conditions.

Grid Coupling - Fatigue WearFatigue Wear

Signature fatigue wear, which can generally be viewed as normal grid coupling wear, shows up as cracks in the grid spring element approximately in the center of the grid spring element legs (as pictured at right).

With a few grid spring element legs broken in the center, a grid coupling will likely still be operational and transmitting torque through the remaining unbroken legs. However, once such a condition occurs, the coupling is operating in compromised state and the grid spring element should be replaced as soon as possible. 

Torque Overload 

Torque overload failures appear similar to fatigue wear, but the cracked grid spring element legs are not centered but rather further up or down on the given grid spring element legs.

Lack of Lubrication

Failures due to a lack of lubrication are often localized to one side of a grid spring (where lubrication was lacking) and may resemble or look like a fatigue failure. The reason for this is a grid coupling is a metal-on-metal coupling, and a lack of lubrication will lead to premature wear (or fatigue) of the grid spring element wherever there is not adequate lubrication (see How should you pack the grease in a Grid Coupling?).

Grid Coupling - Misalignment WearMisalignment

While grid couplings are a very good vibration dampening high power density coupling, they are unfortunately not very good at accommodating misalignment. They are not designed to handle any parallel shaft misalignment, and are only designed to handle about a quarter degree of angular misalignment (see How sensitive are Grid Couplings to misalignment?).

Pictured at right is an example of a grid coupling element misalignment failure.  In such a failure, the grid spring break on the outer bends of the grid spring legs. Similar to fatigue failures, a grid coupling may have broken legs due to misalignment and still transmitting torque through the unbroken legs. This is not a desirable long term state. The grid spring should be replaced as soon as possible, and to prevent such failures (or to correct from such a failure from re-occurring) it is critical that the coupling shafts be realigned and within the misalignment tolerance of the given grid coupling.


Environmental Conditions

Environmental conditions include excessive temperature and/or chemical exposure. Operational temperatures above or below the temperature range of the grid coupling seals will lead to seal damage or failure. Similarly, grease can also break down given extreme temperature exposure. Chemicals can also lead to seal damage and failure. In addition to visible damage to seals and lubrication breakdown, environmental failures may appear similar to an overload condition. 

To learn more about Grid Couplings, please read Why a Grid Coupling - Features & Benefits, Design Basics, and Element Options

To learn more about coupling failure analysis, visit:
Coupling Failure Analysis - Jaw Couplings (includes hub & spider photos)
Gear Coupling Tutorial - Part V: Failure Analysis (with photos)
Coupling Peak Torque Failure at Keyway
Top Reason for a Coupling Failure

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