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Introduction to Industrial Disc Pack Couplings

Jun 22, 2020 4:01:48 PM

Maintenance-free and torsionally stiff connecting elements for smooth stable running

For many decades engineers have turned to the flexible disc pack coupling for applications requiring maximum reliability and uptime, minimizing wear on adjacent equipment, and eliminating the need for lubrication or maintenance of the coupling itself. Prior to the invention of the disc coupling, flexible couplings nearly always included either gear teeth that required periodic lubrication, or rubber and plastic parts that degraded over time and required replacement. Further, as industrial processes became more sophisticated in the latter portion of the twentieth century, the need to improve coupling balance became critical as a means of reducing shaft vibration to protect the bearings and seals of the connected equipment – something the flexible disc coupling helped with a great deal. Over time the use of disc couplings has grown into a wide variety of applications, with numerous machine designs taking advantage of their unique characteristics. This introductory article provides a brief overview of the basic construction of industrial disc couplings and what gives them the performance and longevity that so many engineers value.

Flexible disc packs are composed of thin sheet metal shims, generally stamped or laser cut in a ring shape with a series of mounting holes; normally 6-8 but more or fewer are also used, depending on the size of the coupling, with the smallest designs commonly using 4 and with any number of holes being used for the very largest of disc couplings. The shims are stacked together and held in place with bushings, and the resulting disc pack is bolted into the coupling system. Misalignment compensation depends on the flexibility of the shims themselves, and generally the larger the stack, the greater the overall stiffness of the disc pack in terms of both torsion and bending. Centering of the disc pack in the coupling can be accomplished by different means, with one common method being precision machined pockets in the adjacent mounting flanges which hold the disc packs concentrically in place by their bushings. The disc packs are mounted to their respective hubs with the bolted joints alternating between the driving hub and the driven hub. The portions of the disc pack bridging the distances between the respective bolted joints provide the flexibility and allow a single disc pack connection to pivot and compensate for an angular bend while transmitting rotation and torque. A single disc pack is generally rigid in shear, meaning that it cannot compensate for misalignment between two independently bearing supported shafts, unless used in conjunction with a second disc pack to make the opposite angular bend and complete the parallel offset. The exception to this rule is when a single disc pack is used in conjunction with a self-aligning ball bearing, which replaces the second disc pack in providing the second angular freedom of movement. The greater the distance between disc packs, the larger the parallel misalignment which can be compensated for at a given bending angle.

The stiffness in shear of a single disc pack provides one of the keys to smooth running at high speed. Most flexible couplings have some level of softness in the radial direction, which means that centrifugal forces will more easily deform the flexible element when rotating at higher speeds, leading to instability, and making vibration more likely. In contrast disc couplings have a unique ability among flexible coupling systems to hold all of their components rigidly within their rotational axes, including any spacers, drive shaft tubing or other components which might be mounted in series between the two flexible joints. This makes them an excellent choice for high speed balancing. A further advantage of the radial stiffness of a single disc pack is its ability to support intermediate devices. Most commonly this intermediate device is a torque meter, used widely in test stands and other applications which require condition monitoring. This allows for the torque meter to become an integral part of the coupling system, held in concentric rotation by the radial stiffness of the single disc pack coupling mounted on each of its driving and driven faces.

Also related to rotational speed is the number of bending cycles a disc pack can tolerate before failure. Because they have no wear or moving parts, no abrasion occurs over the course of a single rotation, and misalignment compensation is accomplished purely through the bending of the sheet metal. Steel components are generally considered to possess long term fatigue strength when able to withstand 10⁷load cycles of a given stress amplitude. Because industrial disc couplings are generally rated for misalignment and torque conditions which can be withstood for at least this number of cycles, they are considered fatigue resistant for a service life approaching an infinite number of rotational bending cycles – key to their reputation for maintenance free reliability.

Because of the diversity of applications into which flexible disc couplings are implemented, a wide variety of configurations exist, and most manufacturers offer customization and special features. Below is a short overview of common standard designs, the features of which can often be readily made into combinations of different hub styles and with the extended spacers cut-to-length, making it by no means an exhaustive listing.

The drop-out spacer coupling shown as the last item in the overview is available with a specific set of features to make the coupling compliant with API-610 which is a standard developed by the American Petroleum Institute to address design reliability in centrifugal pumps and couplings. Instead of the disc pack bolting directly to the shaft hub, it bolts to an intermediate guard ring that in turn bolts to the shaft hub. This creates a spacer cartridge which allows for easy installation and removal without any need to move the shaft hubs. This drop out feature is useful for gaining access to pumps and gearboxes for maintenance with minimal effort. The intermediate guard rings also extend through the center holes of the disc packs, providing a safety catch to prevent the intermediate tube from being thrown in the event of unexpected disc pack failure. Use of this drop out spacer feature has become popular in applications beyond those found in the oil and gas industry, such as in rotary test stands and other devices where the need for semi-frequent coupling disassembly is anticipated.

Another useful customization is a configuration with a carbon fiber drive shaft tube. This allows for industrial disc coupling systems to span longer distances at higher speeds. The combination of light weight and high stiffness of carbon fiber tubing allows for smooth running in extended shafting applications with minimal imbalance or whipping contributed by the tube. Pictured below is an example of an industrial disc coupling configured with carbon fiber tubing as well as a special plumbing system that allows fluid to be passed through the center of the coupling to provide lubrication for machine tool applications

Perhaps the most interesting add-on feature for disc couplings is a newly developed remote sensor system which can be integrated into the spacers. These new sensor systems provide wireless transmission of torque, speed, vibration and thrust load data in real time via Bluetooth connection to either a smartphone or tablet with a special app, or to a wireless gateway for integration of performance data transmission into existing control systems. This technological advancement is of significance to engineers and operators who previously experienced some difficulty in monitoring data at critical locations in the drive line, having needed to either estimate loads or run time consuming and expensive tests for periodic monitoring of loads in the past.

Depending on the configuration industrial disc couplings continue to be the best option for heavy-duty transmission in applications that require the highest reliability and uptime, spanning of long distances between shafts, and combinations of high power and high speed. While a variety of off-the-shelf configurations are available for standard applications, customization also abounds. Consulting with manufacturer applications engineers is always the best starting point, and R+W is ready to assist with an evaluation of project and performance requirements, and to help determine the best solution for any application. Contact us at info@rw-america.com for details.

R+W headquarters in Germany

R+W has been designing and manufacturing high performance flexible shaft couplings and torque limiters since 1990 and continues to expand its product portfolio with each passing year. With a reputation for performance, quality, and customization, it is considered by many to be the top precision coupling manufacturer in the world.

Andy Lechner

Elastomer Jaw Couplings: Not All are Created Equal

May 14, 2013 4:03:00 PM

Elastomer Jaw Couplings: not all are created equal

ek picture

Today’s mechanical drive components market is flooded with a wide variety of direct drive couplings. Many folks working with or designing drives are not familiar with the different technologies or differences in quality and precision between manufacturers. One prime example is with elastomeric jaw couplings. Elastomer jaw couplings have been around for 100 years or more. Most of these couplings use two hubs and an elastomeric spider between the jaws to transmit torque. Although they look essentially the same, the applications can vary greatly.

Traditional Jaw Couplings

Much of the market by volume of units sold consists of low manufacturing cost cast hub couplings with a rubber spider element. These hubs are used in drives such as conveyors and centrifugal pumps turning in one direction where precise shaft position is not important. Torque is usually transmitted accross the hubs by means of a keyway locked down with a set screw. Many of the machine frames are formed and welded to loose tolerances. The coupling design has plenty of play between the spider insert and jaws to accomodate shaft misalignment. This design is optimal for producing machinery where labor time to align shafts is cost prohibitive, and they are ideal for less demanding applications.

Precision Jaw Couplings

A somewhat smaller part of the market volume is precision backlash free or low backlash elastomeric couplings. The hubs of these couplings are generally machined from solid stock with clamping features incorporated into them for wear free and play free frictional connections to the shafting. This can eliminate the need for keys and keyways and also enhance performance while improving longevity.

concentricity measurement

The machined hubs typically also come with curved jaws held to a very tight level of concentricity in reference to the bore. Measuring concentricity from the jaw to the bore is what ensures smooth transfer of rotation from one jaw set to the next.

Wear resistant polyurethane inserts are then put through a secondary “match molding” process to smooth out any inconsistencies on the driving lugs, before being press fit into the hubs for a preloaded zero backlash assembly.

Being based on the same principle of mounting a resilient element between the jaws of two metallic hubs, they look similar, but are quite different in some key characteristics. Because the hubs are precision machined from solid bar stock, they are naturally more expensive than a cast or sintered hub. But considering that both coupling styles are relatively inexpensive, the cost differences are often far outweighed by the benefits in situations where high torques or shock loads must be transmitted through a small space envelope, as can happen in many pumping, conveying, crushing and grinding applications to name a few, or in cases where dynamic motion needs to be transmitted smoothly, like in servo and stepper motor driven machines. In the latter situations the precision concentricity and backlash free characteristics, and not only the stronger material condition of the hub benefit the user and the process.

Shaft alignment

One of the assumptions being made with this design concept is that that a higher value will also be placed on the overall quality of the drive line assembly, which always includes precision alignment. It is not uncommon for someone to switch to a precision elastomer jaw coupling and hear an audible clicking as the equipment runs. Because the elastomeric spider element is preloaded in the hubs and is a slightly harder material, it will rub on the metal surfaces audibly if there is significant shaft misalignment. Although lubricant can be used to address the clicking, eventually the elastomer segment may need replacement. But when alignment is addressed they can be wear-free for a theoretically infinite service life.

NEMA frame mounted motor

Many motors, gearboxes, linear actuators and pumps have precision centering features already incorporated into their mounting frames by the manufacturer for this very reason. When the coupling is installed inside a housing or bracket which takes advantage of a centering feature on both pieces of coupled equipment, then alignment is normally within the allowable limits for the precision variety of elastomer coupling.

foot mounted motor

When no such feature exists, then alignment must be checked manually. There are several approaches which can be taken to address this potential issue. For a single elastomer element coupling the shafts need to be precisely aligned with dial indicators or laser alignment systems in case the shaft alignment is not inherent to the mechanical assembly.

For situations where space is tight, split hub couplings are available to be installed laterally after the manual alignment check has been performed and any necessary adjustments have been made.

Then, in cases where precision manual alignment is just not practical, there are other options. Many manufacturers make the urethane elastomer segment available with a convex tooth geometry, which allows for a rolling action as the coupling hubs rotate under angular misalignment.

When two are used in series with a coupling spacer, the ability to compensate for parallel shaft misalignment is magnified to many times greater than with the single element version, allowing for simple visual shaft alignment, while at the same time maintaining the benefits of relatively low inertia, zero backlash and high torque density.

While these two different shaft coupling styles may look more or less the same, small differences in materials and manufacturing processes can mean big differences in terms of the end result. When considering making the switch from traditional to precision varieties of elastomer insert jaw couplings, consider discussing your requirements with the manufacturer, and make sure to check all of the specifications to get the best coupling for your unique situation.

Niilo Nykanen