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.

Reducing Coupling Downtime in Brazil

Guest Post: Doug Durham (Lovejoy, VP of International Sales)

A large multinational manufacture of household products in Brazil approached one of our distributor partners with an coupling problem. They had an array of roof mounted pumps that were exposed to all the elements of hot summer sun and torrential rain in the winter months.

These extreme climactic swings were causing the metal hubs of the original equipment couplings to rust and corrode making the coupling changes very difficult. To replace the elastomeric elements you would need to move the hubs along the shaft. This was becoming more and more difficult as time went on. 

The Lovejoy solution was to use a Jaw In-Shear style of coupling. This would allow the element to be changed without the hubs being moved. The customer was ecstatic and changed all five over the next two months. Even though the cast iron hubs of the JIS couplings were still exposed to the elements and would rust, the element and the stainless steel ring would not rust or corrode. This allows for easy replacement and limited down time. 

The maintenance manager of the facility has stated that the JIS element change takes less than 30 minutes where the old system was down for at least 24 hours. The end customer has written this up their companies best practices and will consider the JIS style coupling for all the pumps in his facility. 

On a closing note, the life he is getting from the Lovejoy JIS coupling is a minimum of three times what he was experiencing from the original OEM couplings. 

To learn more about Jaw In-Shear, you can:
- Read a blog article: Jaw In-Shear Couplings - A Straight Forward Value Add
- Visit the Lovejoy Jaw In-Shear Product Page
- Download the Lovejoy Jaw In-Shear Product Catalog
- Look up Lovejoy Jaw In-Shear Product Part Numbers

To source a Lovejoy coupling in Brazil, please visit visit Setax or a Setax approved distributor. For other parts of the world, please use Lovejoy's Find a Distributor tool.
 

About the Author: Doug Durham has several decades of power transmission engineering sales expertise. To tap into Doug's rich industry knowledge and experience, please contact him through the Lovejoy webpage and/or find a highly qualified Lovejoy distributor through Lovejoy's Find a Distributor tool. 

Jaw In-Shear Couplings - A Straight Forward Value Add

While Lovejoy's straight jaw coupling is robust, ubiquitous and economical... is is also fail safe & requires no grease/maintenance free. (Fail safe means that if and when the elastomeric spider fails, torque will continued to be transmitted as the jaws of the two hubs interlock directly. Requiring no grease means both greatly reduced environmental leakage risk, which may not be true for grid or gear couplings, and no periodic re-greasing required.)

While being fail safe is great in some applications, is not desirable or valuable for all applications. Additionally, the traditional jaw coupling requires that at least one of the two hubs to be pulled back to replace the elastomeric spider. (Note: There is a fail-safe Snap Wrap version that enables spiders to be radially replaced without moving the hub for lower speed applications.)

The Jaw In-Shear Coupling (pictured right) is a straight forward 4 piece non-fail safe design (meaning the coupling will no longer transmit torque if/after the elastomeric element fails) that is optimized for quick elastomeric element replacement (with neither jaw coupling hub being required to be moved). 

Because the coupling is non-fail safe, the coupling behaves similar to a non-calibrated fuse to prevent equipment damage in the event of torque overloads. (Note: Elastomers are not rated or designed to fail at a specific torque level, so other methods such as rated shear pins or torque limiters should be used if a true fuse is required.) Instead of the spider being compressed between interlocked jaws of each coupling hub (a traditional jaw coupling configuration), the Jaw In-Shear coupling has the hubs drawn apart with jaws aligned axially. The wide spider is thus in a shear-plane.
 
Jaw In-Shear elastomeric spiders, being radially removable, results in saved maintenance time and money. The coupling's retaining ring encloses and secures the In-Shear Spider, using an intuitive twist-lock design that will not work loose and requires no fasteners. Furthermore, the Jaw In-Shear uses Lovejoy's industry-standard L-Type and C-Type jaw coupling hubs which are both cost effective, and quite possibly already installed on the equipment (meaning a retrofit cost will be reduced to the Jaw In-Shear elastomeric spider and retaining ring. The Jaw In-Shear spider is made from a tough Urethane formula offering high durability, chemical resistance, temperature range, and torsional softness. Jaw In-Shear coupling designs are available for jaw coupling hubs with max bores up to 9.0 inches (229 mm) and 183,333 in-lbs (20,714 Nm). 

 

 

Common applications for Jaw In-Shear couplings include rolling mills (pictured at right), electric motors to centrifugal pumps, fans, blowers, gear boxes, and plastic extruding machinery. The Jaw In-Shear solves many maintenance problems and creates on-going savings & environmental risk reduction opportunities in industries like pulp & paper, petrochemical, mining and steel.

Recommended Follow-On Reading: For a full review of jaw couplings, and additional commentary on Jaw In-Shear couplings, please read: Jaw Coupling Overview - Features & Benefits, Design Basics, and Element Options

Jaw Coupling Overview - Features & Benefits, Design Basics, and Element Options

Why a Jaw Coupling? 

Lovejoy first started manufacturing their signature straight jaw couplings in the late 1920s, and, today (more than 85 years later), Lovejoy continues to sell hundreds of thousands of jaw couplings every year. What makes the jaw coupling so practical and timeless that it continues to be as relevant today as they were during the great depression?

First and foremost, jaw couplings are popular because they are versatile & robust. They truly are and make for an excellent general all-purpose industrial power transmission coupling. They operate in a wide band of temperatures, can handle angular misalignment, can handle reactionary loads due to misalignment, have good torque to outside diameter capability, have good speed capability, good chemical resistance (spider dependent), and decent dampening capability.

They are easy and straight forward to install. A basic jaw coupling is comprised of only 3 main parts. Two hubs, and an elastomeric spider or insert. (And, yes, we call them spiders, because, well... they look kind of like spiders... even though our spiders don't always have 8 legs.) For a step by step video tutorial of jaw coupling installation, please feel free to watch this video.

Jaw couplings are fail-safe. While, you should periodically check your jaw coupling spiders for wear and replace them (good jaw coupling rubber lasts a very long time, but it does not last forever)... inevitably, someone will forget to check or service a jaw coupling and the spider will wear out or fail. In this case the coupling hubs will interlock and continue carrying the load...though without any dampening capability or other benefits that the spider provided... affording an operator the opportunity to quickly shut down a system. (Note: If this happens, you need to replace both the spider and the two hubs as soon as possible, as the hubs will have worn on each other and no longer be useable.) Hub on hub operation without a spider is extremely sub-optimal, and not recommended for continued system use (unless in the midst of a true emergency situation).

Jaw couplings are ubiquitous and low cost relative to other alternatives. Distributors in every corner of the globe stock the product, most likely just a few miles from you. (You can find one here.) Given how common they are, this means not only finding a jaw coupling will be easy... but should also be able to quickly service or replace parts should something unexpected happen.

Quick side note: Dimensionally interchangeable product does not mean equivalent performance or quality... particularly when it comes to the elastomeric spider quality. Do not be fooled. Several competitors of Lovejoy do sell dimensionally interchangeable jaw coupling products. These products are no more a "Lovejoy jaw coupling" than a store brand cola is a "Coca-Cola". Please buy carefully.(Lovejoy® is an active registered and defended trademark of Lovejoy, Inc.)

Additional Benefits

In addition to jaw couplings being considered fail-safe and all the other benefits already discussed above (dampening capability, chemical resistance, etc.), jaw couplings are also considered maintenance-free as they require no lubrication. Unlike gear, grid, or chain couplings that require periodic reapplication of grease... as long as the spider remains intact, a jaw coupling will continue to work without additional maintenance required.

Jaw couplings, in their traditional form, transmit their torque through the elastomeric flexing element in compression (meaning the spider legs get squished). From an elastomeric element perspective, this is preferable for torque transmission because elastomers can generally take much more punishment and load (inclusive of handling peak loads) in compression (squeezing) than the alternative... which is in-shear (getting pulled or stretched).

Given the jaw coupling has been accepted as the bread and butter of industrial couplings, spreading far and wide, a final benefit of the design is that there are now a tremendous number of specialty jaw coupling variations for just about every industry you can imagine... providing tremendous design flexibility for equipment designers and maintenance engineers alike.

Hub Design Basics

Standard jaw couplings comprise of two hubs (one hub pictured at left), and an elastomeric spider. The hubs generally are bored to a clearance fit tolerance, with a single keyway and set screw, to enable quick installation.

The standard straight jaw hub material for small to mid-range sizes (L035 up through around an L190) is powder metal. Powder metallurgy is the process of blending, compacting, and heating (sintering) very fine metal particles. The advantage of forming the hubs this way is that they can be pressed very closely to finished net shape, minimizing the time and costs of machining, as well as virtually eliminating scrap. Powder metallurgy is a proven mature technology that Lovejoy and others in the field have been using for decades. (More on powder metallurgy can be learned at the Metals Powder Industry Association website.) 

Larger standard jaw hubs (above L190) are traditionally manufactured out of cast iron, which is also a proven mature technology, as this technology is more cost effective for the given sizes and quantities seen in this product range.

Stepping just outside the standard, aluminum hubs (either cast or from bar stock) as well as stainless steel (from powder metal, cast, or bar stock) are also very popular. Aluminum is particularly popular because it is light and does not rust, while stainless steel is popular in food and pharmaceutical environments.

Spider (Elastomer) Design Basics

 

The most standard fundamental jaw coupling spider is a spider constructed of solid Nitrile Butadiene (Buna N) rubber (pictured at left). This flexible elastomer is oil resistant, resembles natural rubber in resilience and elasticity, and operates effectively in a temperature range of -40° to 212° F (-40° to 100° C). This rubber is well suited for cyclic loads (stop/start or reversing). Broadly speaking, this spider generally has a shore hardness of 80A, high dampening capacity, and good chemical resistance.

Another popular variation of the Nitrile Butadiene (Buna N) rubber spider is the open center option (pictured right). While the closed center version (pictured above) is a more robust design, and more desirable particularly for higher speed applications, the closed center does not allow for extremely close (near touching) shaft separation. Simply put, the open center spider is designed primarily for applications where the between shaft ends (BSE) distance is very tight, and there is no room for center section (though it will certainly also work for application where the BSE could accommodate a solid center spider).

A third variation is for the spider to wrap around the outside of the coupling, and then be held in place with a retaining ring. There are several ways for this concept to occur (one of which is pictured at left), with the primary advantage of this approach being that it allows for easy spider replacement (as the coupling hubs do not need to be slid back or moved on the shaft). The primary disadvantage of this variation is that it is typically restricted to lower RPM speeds only, sometimes as low as 1750 RPM based on design. (The LC design, shown at left, has a 3600 RPM max.) The other disadvantage of this design is cost (given it is a more complex design). 


Pictured at the top of this article (and also at right) are individual elastomeric cushions. In the really large jaw couplings (C and H type), these cushions are used in place of one piece elastomers, but behave very similarly. They are held in place with a collar and hardware kit (securing the collar to one of the two hubs.

In addition to the standard Nitrile Butadiene (Buna N) spiders, urethane (pictured left), Hytrel®, and Bronze are also popular materials. Urethane has a greater torque capability relative to Buna N (approximately 1.5 times), but provides less dampening effect. Its operational temperature range is also reduced on both ends of the spectrum (-30° to 160° F, -34° to 71 C). It does have good resistance to oil and chemicals, but is not recommended for cyclic or start/stop applications. 

Hytrel® spiders (pictured right) is designed for high torque and high temperature operations (-60° to 250° F, -51° to 121 C) and has excellent resistance to oil and chemicals. It is not recommended for cyclic or start/stop applications, and is also generally limited to a half degree of angular misalignment (vs generally 1 degree for Buna N and Urethane). Parallel misalignment handling ability does remain similar to Buna N & Urethane spiders at approximately 0.015 inches.

Bronze spiders are rigid, porous oil-impregnated metal inserts that are designed exclusively for slow speed applications (max 250 RPM) that require high torque capabilities. These spiders are largely not affected by extreme temperatures (-40° to 450° F, -40° to 232 C), water, oil, or dirt. Angular misalignment capability is similar to Hytrel® (generally at a half degree), but parallel misalignment handling is reduced to 0.010 inches.




Limitations

Traditional jaw couplings are limited in operating temperature to what can be handled by the elastomeric spider (as cited above). These couplings do offer moderate misalignment handling capabilities (both in angular and parallel), but some applications may require more. Radial stiffness may introduce high reactionary loads in these couplings, and vertical applications may require special hubs (if setscrew proves insufficient in keeping hubs from sliding). Lastly, traditional jaw couplings are no well suited for motion control applications because they do have backlash (there is some looseness between the jaws and elastomer... which will become more compressed over time).

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Bonus Material! (Two Additional Jaw Options)

Jaw In-Shear Couplings

One additional jaw coupling variation not discussed above is the jaw in-shear. This design allows a traditional jaw (with elastomer in compression) to quickly be turned into an elastomer in-shear, which is a non-fail safe design.


In addition to being non-fail safe, this design is also very easy/quick to inspect or change out. Should/when the elastomer needs to be replaced, simply rotate and slide the shaft collar off, unwrap the old spider, put a new one on, slide the cover back on, and twist.

The design shown is completely interchangeable with standard jaw coupling hubs, so "upgrading" to this style on requires purchasing a new spider kit (the wrap around spider and ring).

Curved Jaw Couplings

Curved jaw coupling are different than straight jaw in that their teeth have a curved profile. This design style is popular in Europe, but it is also broadly popular in motion control applications, because... when using a very stiff urethane spider... it is possible to create a backlash free coupling (sometimes known as a GS or Spidex®). (Unfortunately, one draw back of the curved jaw design is that, because of the jaw curvature, it can not be converted to a in-shear design like directly above.)

Should you have any further questions on jaw couplings, please do not hesitate to contact us by phone and/or by asking us a question at the link on the top of this page.


Recommended Follow On Reading
For a broad discussion covering the many types and categories of flexible couplings, please check out the following blog article: Flexible Coupling Basics - A Quick Primer