Top 5 Ways to Trash Your Pump

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

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.

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

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. 

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. 

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.

Fatigue 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?).

Misalignment

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

Why a Grid Coupling - Features & Benefits, Design Basics, and Element Options

Why a Grid Coupling

Grid couplings are a popular coupling option where both high torque levels and dampening requirements exist. Unlike gear and disc couplings (alternative metallic coupling types capable of transmitting a significant amount of torque), grid couplings have a unique ability to reduces vibration by as much as 30%, and cushions shock loads to safeguard driving and driven power transmission equipment. 

The grid spring element absorbs impact energy by spreading it out over time, and thus reduces the magnitude of the peak loads. This is possible because of the progressive contact that occurs between the curved profile of the hub teeth and the flexible grid. As the load increases, more of the tooth comes into contact with the flexible grid spring element. 

 

Additional Benefits

Horizontal Split Cover

Grid couplings are a versatile, proven technology with interchangeable components readily available from several major coupling manufacturers (including Lovejoy). 

Grid couplings have a high power density (transmit a high amount of torque relative to their size), and are relatively straightforward and simple to install. They also have good resistance to environmental conditions, and available for both inch and metric bores.

Design Basics

Vertical Split Cover

A grid coupling is comprised of two hubs, a grid spring element, and split cover kit (which includes two cover halves, gaskets, seals, and hardware).

Like gear couplings, grid couplings are a metal on metal flexing design, and it is critical that the coupling be packed properly with coupling grease (see How should you pack the grease in a Grid Coupling?) 

Grid couplings are available with either a horizontal or vertical split cover design. Horizontal covers are generally viewed as easier to install, while vertical covers enable a grid coupling to be run at a higher maximum speed (see What is the difference between Horizontal and Vertical Grid Couplings?).

Spacer Design 

Full Spacer Design

Grid couplings are also available in a spacer and half spacer designs, which are ideal for allowing equipment to be serviced. Such designs are particularly popular in pump applications, where a drop-out section (full spacer design) or quick disconnect (half spacer design) allows for equipment servicing without disrupting the greased grid coupling element.

 

Limitations

One of the biggest, if not the biggest, limitation of grid couplings is their limited ability to accommodate misalignment. While great at dampening vibration, they are not designed to handle parallel shaft misalignment and only designed to handle about a half a degree of angular misalignment (see How sensitive are Grid Couplings to misalignment?).

Half Spacer Design

Additionally, grid couplings are also not "maintenance-free" because they require lubrication (grease), which must be periodically checked and topped off if required. Care must also be taken to ensure that lubrication does not leak on to the ground and create an environmental concern. 

For further information on grid couplings, please see Lovejoy's grid coupling product page.

What is the difference between Horizontal and Vertical Grid Couplings?

Horizontal Split Cover Grid Coupling

The difference between "horizontal" and "vertical" grid couplings lies simply in two types of split cover designs (and their corresponding cover kits). The grid spring elements and coupling hub technology are the same. 

Horizontal covers are designed for ease of assembly and removal, particularly in tight spaces, as they can be put on after the hubs and grid spring element have been already assembled.

Vertical split cover designs, require putting the split covers on the shaft prior to putting on the shaft hubs and grid spring element. Once the hubs and grid spring element have been attached, the vertical split covers can then slid over the hubs and grid spring element and fastened together. (This also means that to completely remove a vertical split cover off a shaft, the grid spring element and coupling hubs would have to first be removed.)

Vertical Split Cover Grid Coupling

The benefit of the vertical split cover design is that it can operate at a higher maximum speed (RPMs). The Grid Coupling Performance Data chart below (which was extracted from page GD-10 of Lovejoy's Grid Coupling Catalog) has the difference in maximum speed ranges between the horizontal and vertical split covers circled in red. Based on your application, it may be required to go to a vertical split cover design if the horizontal cover design maximum speed is too low. 

Installation videos of Horizontal Split Cover Grid Couplings, Vertical Split Cover Grid Couplings, Full Spacer Grid Couplings, and Half Spacer Grid Couplings are all readily available on Lovejoy's YouTube channel, and formal installation instructions can be downloaded on Lovejoy's Installation Instructions resources webpage.

For additional information on grid couplings, to include grid coupling interchanges, please see the grid coupling product page on Lovejoy's website.