One critical component in your pump is the
coupling that might be connecting your motor to the pump. In the case of fire pump for example, it would
be a grid coupling. Like other coupling
types, grid couplings often have "signature failures" modes that can completely
make your pump fail.
In the case of fire pumps, where a flex or grid coupling is recommended (as per NFPA standards), here are 5 reasons that may cause grid coupling failures:
1) Reversing or highly fluctuating loads
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| Figure 1 - Fatigue Wear |
Fatigue failures are typically due to high start-up or impact
loads and/or in combination with reversing or highly fluctuating loads. 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 (see Figure 1 – Fatigue Wear).
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 a compromised state and the grid spring element should be replaced as soon as possible.
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 a compromised state and the grid spring element should be replaced as soon as possible.
2) Undersized Coupling
A bad combination of undersized coupling and peak
torque load(s) in excess of the calculated coupling sizing torque can cause a torque overload. In this situation, the cracks in the grid
spring element legs are not centered but rather further up or down the center
3) Lack of Lubrication
Not lubricating the coupling properly can lead to failures. In this case, the cracks are often localized
to one side of a grid spring (where lubrication was lacking) and may resemble
or look like a fatigue failure. The 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. (How should you pack the grease in a Grid Coupling?).
4) Misalignment
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| Figure 2 - Misalignment Failure |
A grid coupling is an excellent vibration dampening high power density
coupling. They are unfortunately not
very good at accommodating misalignment. Grid couplings are not designed to
handle parallel shaft misalignment, they are only designed to handle about a
quarter degree of angular misalignment (see How sensitive are Grid Couplings to misalignment?).
Figure 2 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 and the grid spring should be replaced as soon as possible. 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.
Figure 2 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 and the grid spring should be replaced as soon as possible. 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.
5) Excessive temperature and/or chemical exposure
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.
