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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

Mounting a Sprocket or Pulley to a Torque Limiter

May 21, 2014 7:06:00 PM

Mounting a Sprocket or Pulley to a Torque Limiter

As a manufacturer of torque limiters for indirect drives, we are often asked to provide some assistance when it comes to mounting the torque limiter inside a drive sprocket, pulley, or gear. R+W does offer to provide torque limiters as a complete package, with the drive attachment pre-mounted according to customer specifications, but we are also happy to provide customers with some guidance when it comes to doing this on their own, including providing machining drawings for a do-it-yourself or third party project. In this article we’ll focus on mounting a roller chain or timing belt sprocket to a torque limiter, as they represent the vast majority of requirements for indirect drive torque limiters.

The process is really straight forward, and can usually be performed in any machine shop with a milling machine and a lathe. Once the correct torque limiter body size has been selected, based on the required bore diameter and disengagement torque, it must be compared dimensionally with the size of sprocket being used, in order to determine whether it can be mounted directly to the torque limiter, or if it will need to be offset mounted. While a drive sprocket with a smaller diameter than the torque limiter output flange can be offset mounted on a separate bearing, and attached using an adapter plate as needed, it is usually easier and less expensive to choose a sprocket that will fit directly.

Most quality torque limiters include a bearing (1) between the base of the clutch and the output flange. This helps to ensure that the driving and driven portions of the torque limiter are properly guided within the rotational axis after disengagement. In order to protect the bearing from moment loading, the belt or chain tension (2) needs to be well centered over the bearing, unless it will be supported by an external bearing, as is the case in offset mounted systems. Centering the tension over the bearing in the torque limiter often requires that an additional pocket be machined into the drive attachment, so that the torque limiter can be sunk into drive attachment to a depth that places the bearing underneath the drive medium (i.e. belt or chain). R+W offers an allowable load centering range, in terms of a distance from the end face of the torque limiter. This load centering range, dimension “S” from the R+W safety couplings catalog, is the range in which the center of the chain or belt must reside for smooth, sound operation.

Once the dimensions have been selected, machining the mounting features into the sprocket or pulley is fairly straight-forward. First an inside pilot diameter is bored into the sprocket on a lathe. This bore should be precise to match the centering diameter on the torque limiter output flange. The centering diameter is referred to as dimension “E” in the R+W safety couplings catalog, and it is the contact surface which ensures that the sprocket will be well centered around the drive axis. An ISO H7 tolerance is recommended for the pilot diameter, which normally runs between half a thousandth to just under two thousandths of an inch oversized from the nominal diameter, depending on the size, with the larger tolerances applying to larger diameters. The pilot bore should also be concentric to the pitch diameter of the pulley or sprocket. Lathe jaws with a flat grip for the tips of the sprocket teeth and a machined recess to hold the face plane of the sprocket perpendicular to the axis of rotation allow for a simplified setup.

Next, any relief needed for centering the chain or belt tension over the bearings in the torque limiter is turned into the sprocket’s face. This diameter must be greater than dimension “G” from the R+W safety couplings catalog, and is less critical than the pilot bore diameter, since it is only for clearance. It is important however that the sprocket or pulley is square in the lathe chuck, since the turned face in the resulting pocket needs to rest flat against the face of the torque limiter output flange in order for the drive to run smoothly.

Finally, the clearance hole bolt circle is drilled into the sprocket on a milling machine or drill press. The size and number of holes can be taken from dimension “H” in the catalog, and the bolt circle diameter taken from dimension “F” in the catalog. Using a rotary table makes this process quick and easy. These holes can be counter-sunk or counter-bored to save axial space if installation space is at a premium.

Once the sprocket or pulley is finished, it is ready to be mounted to the torque limiter output flange. Inspect the machined surfaces to ensure that they are clean and free of nicks, burrs, and debris. Slide the drive attachment over the centering pilot on the torque limiter and rotate, while applying gentle pressure, to ensure a proper fit. Little to no relative movement between the drive sprocket or pulley and the torque limiter output flange should be possible, aside from rotation. Insert the mounting screws and ensure that they are finger-tight. Evenly tighten the screws in a crosswise pattern, applying 1/3, 2/3, and finally 3/3 of the recommended tightening torque for the size and type of screw being used.

The drive attachment is now machined and properly mounted, and is ready for installation.
As always, don’t hesitate to contact your coupling experts with questions about proper sizing, selection, and handling of torque limiters and safety couplings.

Andy Lechner

Solving Problems with Torsionally Stiff Couplings

May 23, 2013 12:08:00 AM

SOMETIMES YOU NEED A FLEXIBLE COUPLING THAT'S TORSIONALLY STIFF

One of our customers in Michigan has a short application story we'd like to share with you. It provides a great example of how precision bellows couplings can help solve machine performance problems.

MICHIGAN CUSTOM MACHINES, NOVI, MI (http://www.michigancustommachines.com/)

Engineer: Brian Nugent

"Michigan Custom Machines builds end of line functional test machines, primarily for the diesel and automotive industries. This particular application required a 400 lb flywheel to mimic the inertia of a diesel engine driveline. The flywheel was mounted to a shaft that needed to be coupled to a custom camshaft which is actuating a fuel injector.

One of the biggest challenges on this application was stiffness of the coupling. We have history with this application and have found if the coupling has wind up, even though it is zero backlash, it will affect how the test is performed by allowing the instantaneous rpm to droop momentarily within part of a revolution.

For this application I called (R+W) direct, also passed information back and forth through e-mail to him. Prior to this application I have always known who R+W was, mainly through word of mouth within the custom test machine community and the internet.

We used a Model BK1/6000/XX. It was a custom model - the adapters were custom to fit the different shaft sizes and the shaft locks we used.

It was known when sizing this coupling, the BK1/6000/XX could not handle as much torque as a previous disc type coupling used, but the stiffness was higher. After installing the BK1/6000/XX, an instrument was used to see how much phase change is seen during operation from one side of the coupling to the other. The BK1/6000/XX showed 1/3 of the phase difference or wind up compared to the previous couplings used. This resulted in more consistent testing of the fuel injector.”

CUSTOM MADE TO FIT

Normally for this 6,000 Nm application we would have worked with our BK3/6000 shaft coupling, but it became clear as we were checking the fits that a special solution would be needed in order to accomodate existing space restrictions. So it was decided that this solution would be based instead on our BK1/6000 basic bellows coupling. One of many custom mounting arrangements we work with here at R+W is to insert special flanges inside the bellows so that keyless locking devices can be used without extending the coupling length. Flat head cap screws allow us to maintain a low profile while still maintaining the structural integrity needed for high torque transmission.

If you have a difficult shaft coupling application you'd like some help with, don't hesitate to contact our applications engineering group at applications@rw-america.com.

Andy Lechner