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.
 

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.

Custom Engineered Flexible Couplings

While many coupling manufacturers have impressive standard product catalogs, as Murphy would have it, your application requires something special, a bit different, or a retrofit to a legacy design. For such custom coupling situations, leading coupling manufacturers have engineering teams on standby. 

Speaking specifically to Lovejoy, following principles of Quick Response Manufacturing (which focuses on optimizing high mix low volume manufacturers for speed), our coupling engineering team is actually comprised of two separate teams: the "Modified Catalog Team" and the "Engineered to Order Team"  (or "ETO"). 

Both are cross functional in nature. The "Modified Catalog Team" focuses on orders that are relatively simple variations or tweaks to existing product design. This team is comprised of application engineers, drafts persons, and technical customer service personnel. Their goal is speed. 

The "Engineered to Order Team" is comprised of Lovejoy's former Quality Manager, several design engineers, a manufacturing engineer, and a quoting engineer/technical customer service representative. This team is charged with tackling the hairy, messy, and complex requests.

When orders come into Lovejoy's customer service department, if a new design is required, it is funneled to one of these two teams based on a series of seven questions. If the answer is "no" to all seven, the order goes to the "Modified Catalog Team". If the answer is "yes" to any, the order goes to the "Engineered to Order Team". 

While "Modified" orders are expected to be turned around quickly, "ETO" couplings can take some time to design. By dividing work out in this way, quicker "Modified" orders are prevented from getting stuck behind nasty "ETO" ones. 

Once orders get to the "ETO" team, much like a hospital emergency room, orders get triage. Our former Quality Manager, who has limited purchasing authority, goes through the order to understand and ensure every requirement that must be met. If inspection gauging is needed that is not available, he can preemptively place it on order even before the design is complete. Similarly, he can get forging and other long lead items on order before designs have been finalized. (Dual path sourcing and design finalization allows precious time to be saved for our clients.) Once triage is complete, orders are assigned to design engineers who often then directly engage customers to work through any concerns and to receive print approvals, and everyone within this team is co-located within a few feet of each other to ensure quick coordination and communication. 

If you have a challenging or unique coupling opportunity, be it elastomeric (jaw, curved jaw, S-Flex, torsional) or metallic (gear, grid, disc), we would welcome an opportunity to serve you and show you our "ETO" Quick Response Work Cell in action. 

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

Flexible Coupling Basics - A Quick Primer

Speaking from 30,000 feet, power transmission couplings are devices used to connect two shafts together, transmitting system torque from one shaft to the other. (When one shaft spins, the coupling's job is to make the other shaft spin.) Within the huge array of coupling solutions, couplings can be broadly broken down into two primary types: rigid (which we will briefly touch on) and flexible (which we will then dive into). 


Rigid couplings (an example pictured left) are exactly that... rigid. They firmly connect the two shafts together without any additional features or capability of accommodating/handling system misalignment. Rigid couplings are generally simple and cost-effective for applications where misalignment is not a concern.

In contrast to rigid couplings, flexible couplings (the focus of this article) have an integrated flexing element or design component that allows for some degree of misalignment handling & management. Within the vast flexible coupling world, couplings can further be broken into two major sub-groups: elastomeric couplings and metallic couplings.

Elastomeric (flexible) couplings - Elastomeric couplings are couplings that include a flexing rubber or plastic element to both accommodate misalignment and dampen system vibrations. Within the elastomeric coupling subset, there are many style and designs. This post will focus in on and provide a quick cliff notes overview of the 3 major elastomeric flexible coupling product types: compression loaded, shear loaded, and torsional.

(Note: Lovejoy's "The Coupling Handbook" takes a much deeper dive into everything this blog post covers, and the Mechanical Power Transmission Association also has an excellent 7 page PDF document, title Elastomeric Coupling Primer, that would make for great follow-on reading as well.)  

Compression Loaded: Let's start the elastomeric compression loaded discussion with the bread and butter all-purpose industrial coupling: Lovejoy's very own straight jaw coupling. This coupling comprises two hubs with straight jaws that are interlocked with an elastomeric spider (we call it a spider because, yes, it looks like one) in between the two hubs (with the spider serving as the flexing element) transmitting torque in compression. One feature of this coupling is that it is considered "fail-safe". If/when the spider eventually wears out (which, after a full service life it will, as any/every rubber based product will eventually break down), the metallic jaw hubs can then continue to carry the load (though not as smooth, and with a bit more noise). (Note: It is certainly preferable to replace a spider before it fails, generally when it has been compressed to 75% its original size... as this is much cheaper and quicker than replacing both hubs and the spider, which will be required if the hubs start to wear on each other if the coupling is run in the absence of a functional spider.)

The basic jaw coupling (known as an L-line, and pictured above next to "Compression Loaded" title), has many variations for specific applications. These include aluminum (lightweight) and stainless steel (for food and pharmaceutical applications) hubs, special spiders made for high temperatures, high torques, chemical and oil resistance (and even a bronze spider for high torque, low speed applications), quick change out radial spiders, drop out spacers, and a variation (known as a jaw in-shear & pictured left) that turns a "fail safe" jaw coupling (where the elastomer is in compression) into a "non-fail safe" coupling (where the elastomer is a combination of both in-shear and in compression, preferable for applications where torque transmission should cease should the coupling not be operating at full capacity).

In Europe, a curved jaw coupling variation has become the standard industrial coupling, and Lovejoy sells a large number of curved jaw couplings into Europe through Lovejoy's German affiliate. Lovejoy also replaces a significant number of curved jaw coupling components on equipment in the United States that has been imported from overseas. (Note: Just like Lovejoy's L line, curved jaw couplings are manufactured, finished, and readily available from Lovejoy's Downers Grove, IL manufacturing facility.) While curved jaw couplings cannot be turned into jaw in-shear like straight jaw (due to their tooth profile), one nice feature of curved jaw couplings is that, by tightening their tolerance and using a very stiff elastomer, the curved jaw couplings can be turned into a very affordable backlash free coupling. (Backlash refers to the looseness of fit of a coupling, which is generally undesirable in very precise motion control applications.) 

Shear Loaded: In addition to the Jaw In-Shear couplings, there are several other popular elastomer in-shear designs. These popular designs include the tire (or tyre) coupling, and the sleeve coupling (or S-Flex coupling, pictured left). A common trait among all of these shear loaded designs is that system torque transmission will cease if/when the elastomer fails. In this capacity, the coupling is acting similar to a non-calibrated fuse. (Elastomers generally are not rated or designed to fail under specific load conditions, and should not be used or trusted to be a fuse. However, they are often designed as the weak point in a power transmission system... and will fail if there is a major system lockup. Solutions to include adding rated shear pins and/or a torque limiter to the coupling design are available if a user is looking/requiring their coupling to act explicitly as a fuse.)

Torsional: The third and final elastomeric coupling grouping to cover are torsional couplings. Mechanical power transmission systems can have devastating natural frequencies (think Tacoma Narrows Bridge Collapse). Diesel engine applications are one of the most common applications where natural frequencies need to be managed. The goal of a torsional coupling is to tune the system above or below its natural frequencies... and both torsionally soft (incorporating soft rubber) and torsionally hard (often incorporating hard plastics) are available to tune the system. 

Selecting the proper torsional coupling is not a trivial task (generally involves a formal torsional vibration analysis), and it is highly recommended that you consult with a coupling manufacturer's staff prior to making your own product selection. (Lovejoy offers multiple types of torsional couplings, and can be interchangeable with specific other manufacturers.)

Metallic (flexible) couplings - Metallic couplings are different from elastomeric couplings in that they do not employee elastomeric (soft) materials to provide coupling flexibility & dampening. The breadth of metallic coupling offerings is massive (covered in depth in The Coupling Handbook), and this post will focus in on and provide a quick cliff notes overview of the 2 major metallic flexible coupling product types: lubricated and non-lubricated.

Lubricated metallic couplings achieve flexibility through loose fitting parts rolling or sliding against one another, while non-lubricated metallic couplings achieve flexibility through a flexing or bending of a metal component itself. Lubricated couplings are generally less expensive, but do require periodic maintenance/more lubrication, and will eventually "wear out". Non-lubricated are generally more expensive, require minimal maintenance, and categorizes as having theoretical "infinite life" (no metal on metal wearing parts).  

(Note: While the Lovejoy brand is near synonymous with the elastomeric coupling market, the company has been a major player in the metallic coupling industry for several decades. Many people are surprised to learn that Lovejoy's metallic coupling sales are on par with Lovejoy's elastomeric coupling sales, and that their knowledge of the products and applications is so great.)

Lubricated Couplings: The three major types of lubricated metallic couplings are: gear, grid, and chain. The primary form of failure for these type couplings is wear (metal on metal contact), meaning torque peaks/overloads as well as poor or improper lubrication/grease maintenance will shorten the coupling's life.

Of the three major lubricated metallic coupling types, gear couplings (where misalignment is achieved through crowning on the gear tooth surfaces) are historically the big boy on the block. Gear couplings have a very high power density (can carry huge torque loads in a small footprint), have many available custom application options (i.e. - flanged or continuous sleeves, spacers, floating shafts, limited end float, sliders, insulation), can be balanced to operate at high RPMs, and are generally lower cost than other equivalent high torque coupling alternatives. Inclusive of Lovejoy, a number of manufacturers' flanged gear couplings are half coupling for half coupling interchangeable through size 9, given they adhere to the common AGMA standard (AGMA 9008-B00: Flexible Couplings -- Gear Type -- Flange Dimensions, Inch Series). Please remember, interchangeability does not mean coupling quality, reliability, ratings, or performance characteristics are equivalent... so please proceed cautiously when selecting a vendor.

Grid couplings (where misalignment is achieved between a single spring steel serpentine grid wrapped around two flanges) are also a very well respected lubricated metallic coupling. One advantage of this coupling is that the flexibility of the grid provides it an ability to spread out impact energy over time... allowing the coupling an opportunity to reduce the magnitude of peak loads. Grid couplings can be used in both horizontal and vertical axis applications, and also have may additional feature upgrades (floating shafts, break discs, spacers, etc.). Grid couplings generally compete with large elastomeric couplings (which may be too large to fit inside the system's space constraints), as well as with gear couplings (given the grid coupling's enviable all-metal ability to modestly dampen vibration). Most major grid coupling manufacturers (inclusive of Lovejoy) have kept the majority of their grid coupling components directly interchangeable... with the exception being unique seals and gaskets. (If interchanging manufacturers, corresponding seals and gaskets should be procured from the same manufacturer to avoid potential sealing issues). 

In contrast to gear and grid, chain couplings are somewhat of a dirty step child. In full disclosure, Lovejoy does not manufacture chain couplings... so we may be a bit biased... but, generically speaking, chain couplings are found and used on unsophisticated applications (i.e. - makeshift farming equipment). Chain couplings are known for being relatively rugged and very low cost. A chain coupling consists of two sprocket hubs with a single double roller chain connecting the two hubs. These couplings are relatively easy to install, maintain, and rough align.

Non-Lubricated Couplings: Popular non-lubricated metallic flexible couplings include the disc (or disk), diaphragm, link, spiral wound, bellows, and beam coupling types. All six have a theoretical infinite life (meaning they have no metal on metal wearing parts), assuming the flexing or load carrying methods stay within the mechanical endurance limits of the flexing metal material. (Overload on these type couplings, be it continuous torque or cyclic misalignment forces, will result in fatigue failure.) 

These couplings have been historically complex to understand and evaluate... as significant stress analysis (finite element analysis) must be conducted to flush out performance characteristics (inclusive of taking torque load, misalignment,  temperature, and varying system speeds into consideration). Non-lubricated couplings generally have a higher upfront cost, relative to traditional lubricated couplings, but can offer long term "total cost of ownership" savings opportunities.

Of all non-lubricated coupling types, disc couplings (with multiple layers of flexing discs) are the most popular... and they continue to pick up steam both in new designs (inclusive of boiler feed pumps, gas and steam turbines, compressors, high-speed test stands, marine propulsion systems, and wind energy) and in replacing traditional installed gear coupling applications (where either maintenance, reliability, or environmental concerns arise). Disc couplings can excel at high speeds (where balancing and lubrication concerns put gear couplings at a disadvantage), and, furthermore, have the built in advantage of no backlash which lubricated metallic couplings cannot claim). 

One drawback of disc couplings is that they are generally less tolerant of misalignment (queue smiles from the shaft laser alignment product sales folks). Without diving too deep into disc couplings, disc packs can be circular (call it version 1.0), flat sided (version 2.0), or scalloped (version 3.0) on the outer dimension... with each revision offering improved performance. (Circular disc packs acts as a beam... stressing the extreme edges, flat sided packs avoid the curved disc pack drawbacks, and scallop disc packs both avoid the curved disc pack drawbacks and provide more flexibility/misalignment handling capability. The additional capability can be attributed to the disc pack's reduced cross-section... which requires less force to flex, translating to lower reactionary loads on the system's adjacent bearings.) All three disc pack styles are readily available on the market (though Lovejoy only sells the scalloped version). 

Diaphragm couplings were originally introduced to service very high speed, high horsepower applications in the petrochemical industry... and has since progressed to other extreme applications such as helicopter drives. Diaphragm couplings handle misalignment through use of a flexing metal plate (or series of flexing metal plates in parallel). The metal plate(s) is loaded in shear, with torque being introduced at the outside diameter of the coupling and then transferred on to the inside diameter. (The process reversed at the opposite flex point.)

Diaphragm couplings are known for their large outside diameters, and, generally, very high cost. Diaphragm couplings are generally sold as custom solutions, and there are a wide variety of options to consider... so those seriously in the market for this coupling should spend a considerable amount of time speaking with manufacturers' application engineering staffs. (Note: Lovejoy does not sell diaphragm couplings, though Lovejoy does sell API 610 or 671 disc couplings that can sometime compete for the same application.)

Taking a huge step back from size and cost of diaphragm couplings, triple wound spring couplings are generally small and much more an off the shelf mainstream product (translation: Lovejoy has lots of these in stock). These couplings handle torques generally only up to ~1800 in-lbs, 213 Nm), with bores up to 1.5 inches (38 mm), and speeds up to 30,000 RPM. They operate by having three tension springs mounted (one inside the other) to the two shaft hubs. The middle spring runs in the opposite direction of the inner and outer spring to allow the coupling to transmit torque in either direction. As each spring can flex independently, both angular and parallel misalignment can be addressed with this coupling. This coupling does have both backlash and windup, but their values are known... so these couplings can be found in use on index positioning and robotic applications.

The three link couplings is variation/cousin to the traditional disc pack coupling, where two sets of three flat strip springs are attached to a triangular plate at the inner diameter and a circular flange at the outer. Links function similar to disc legs, only they are thick enough to be loaded in compression on one side and in tension on the other. With two flex planes, this coupling was designed for lightly loaded, high misalignment (up to ~5° angular) applications with smooth non-cyclic torque loads. 

In addition to disc couplings, bellows couplings (pictured left) and beam couplings (pictured below left) are considered well suited all-metal motion control couplings (Motion control applications include shaft encoders, resolvers, all forms of servo devices, linear and ball screw actuators, robots, step motors, light duty pumps and metering devices, plotters, medical equipment, positioning tables, computers and radar.) Key features/requirements for motion control couplings are their torsional rigidity, low inertia, constant velocity, low radial stiffness, zero backlash, corrosion resistance and the capability of cyclic (repeated start/stop/reverse) activity.

Bellows couplings use thin tubular metal bellows (either one or two layers) formed with annual corrugations as the flexing element, while beam couplings (pictured left) use a single piece of aluminum or stainless steel (generally bar stock) cut with single or double flex planes. While there are many similarities between bellows and beam couplings that make them both well suited for many common applications, one notable performance variance is that bellows couplings are generally more torsionally stiff than beam couplings (though beam couplings are also fairly stiff). 

In concluding this rapid 30,000 foot overview of flexible power transmission couplings, we would invite you to take a deeper dive into this subject matter (by reading "The Coupling Handbook")... and, of course, please don't hesitate to holler if we can be of any further assistance.

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.