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Introduction to Industrial Disc Pack Couplings

Posted by Andy Lechner on Mon, Jun 22, 2020 @ 09:01 AM

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 107 load 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.

Tags: test stand coupling, torsionally stiff coupling, drive couplings, lamina coupling, spacer couplings, API 610 coupling, line shaft, flexible couplings, pump coupling, jack shaft, high speed coupling, non keyed coupling, torsion resistant coupling, torque test coupling, torque sensor coupling, torque tube, coupling for test stand, Precision shaft couplings, disc pack, single flex, maintenance free coupling, flexible shaft coupling, disc coupling

R+W introduces the new CHB belt drive housing for connecting servo and stepper motors to ball and roller screw actuators

Posted by Andy Lechner on Fri, May 26, 2017 @ 12:53 PM

R+W introduces the new CHB belt drive housing for connecting servo and stepper motors to ball and roller screw actuators

 With an increased demand for compact mounting solutions in the automation industry, R+W has released the CHB housing for parallel assemblies of motors and actuators.

 These box housings are available in 5 compact sizes and 2 ratios, allowing for an easier selection process during the design phase, in addition to downsizing of motors. Each unit is machined to order and fits the motor / gearbox and actuator mounting patterns.   The frame is made from solid machined anodized aluminum, offering corrosion protection and robust construction.

 The keyless clamping system inside ensures smooth operation without slippage even during fast reversals. Download the datasheet and find out more about this new mounting solution.

R+W CHB Image-1.png


More details here:



R+W America, L.P.

1120 Tower Ln.

Bensenville, IL 60106

Phone: (630) 521-9911

Fax: (630) 521-0366

Email: Info@rw-america.com

Web: www.rw-america.com


R+W has added LPZ short spacer into its disc coupling product line

Posted by Andy Lechner on Thu, May 18, 2017 @ 09:35 AM


R+W has added LPZ short spacer into its disc coupling product line 

In its effort to fill in and complete its flexible disc coupling offering, R+W has added a standard coupling adapter which provides customers with more configuration and connection options when selecting couplings for industrial applications. 

The LPZ spacer accompanies a quickly expanding product line and offers lateral misalignment capabilities in a very small package. It is available in six sizes ranging from 500 Nm to 12,000 Nm and is able to mate with any LP coupling hub R+W has to offer within the corresponding size. Both ends of this spacer connect via a precision machined stainless steel disc pack, which transfers torque through a pure friction-drive design. This ensures that shear force on the fastening bolts is eliminated and the assembly is completely backlash-free.


While this spacer is made entirely of steel, all R+W coupling components can be made from special materials and balanced for higher speeds if the applications demands it.

More details here:



R+W America, L.P.

1120 Tower Ln.

Bensenville, IL 60106

Phone: (630) 521-9911

Fax: (630) 521-0366

New Call-to-action

Email: Info@rw-america.com

Web: www.rw-america.com

R+W Safety Couplings for Wastewater Applications.

Posted by Sample HubSpot User on Fri, Mar 17, 2017 @ 13:20 PM

R+W Safety Couplings for Wastewater Applications.

ST wastewater.jpg                                            

By popular demand, R+W has developed a standard model of corrosion proof ball detent safety coupling for clarifier drives in water treatment plants. The industry requires a well sealed, high grade stainless steel mechanism to stand up to harsh conditions, resulting in the specific design criteria. The function of the ball detent torque limiter is to disengage the driveline to protect flights, chains and sprockets in the event of overload, which can result from the high gear reductions associated with the slow speed movements required in the settling tanks.


Two standard body sizes are available, with a maximum bore diameter of 2.5” and disengagement torque settings ranging from 1,600 to 21,000 in*lbs. Custom options are always available on request.


The new R+W safety coupling design features: 

  • Reliable, precise, and consistent overload protection
  • High grade stainless steel construction
  • Adjustable and tamper proof options
  • Optional switch plate and / or extension hub
  • The ultimate in torque overload protection for clarifier drives
New Call-to-action

More info here:


Mechanically Generated Friction Fits in Power Transmission Couplings

Posted by Niilo Nykanen on Tue, Jun 02, 2015 @ 08:51 AM

Mechanically Generated Friction Fits in Power Transmission Couplings 

coupling with keyway mounting

In the world of electric motors and power transmission coupling products, torque is increasingly being transferred between directly coupled shafts by means of purely mechanically generated friction rather than by positive drive connections like keys and keyways or splines. Mechanical friction fits evolved into modern standardized dimensions during the twentieth century as machine tools and cutting tooling evolved into their present states of precision and accuracy. For many decades power transmission component design lagged behind the machining world when it came to the use of mechanically generated frictional clamping, with a large portion of connecting elements still relying on keyways and interference fits to guarantee transmission.

Interference fits, wherein the diameter of the male component is actually slightly larger than the diameter of the mating female component, do have their merits when it comes to ensuring reliable backlash free transmission.  But they can be difficult to assemble, as the shaft must be cooled with a cryogenic such as liquid nitrogen, or the bore needs to be expanded with heat. If either high heat or extreme cold are used, safety is a key concern during the assembly process. Heating a metal component will also often change its mechanical properties such as temper, so cooling the male fitting part is preferred when maintaining the material properties is a concern. Due to the inconveniences associated with interference fits, most newly designed frictional clamping coupling hubs are made using compression by means of screw thread fasteners instead. 

This type of mechanical connection is very coupling with clamping hubpredictable and can be made by applying a set amount of torque to radial or axial screws. For easy assembly the fit between the male and female part typically should not have overlapping diametrical tolerances, but should also not allow for excessive clearance. When the fits can be made by hand, proper tightening of the fasteners is all that is necessary to create a friction fit which will reliably and predictably transmit torque for the life of the machine.  A shrink disk style fit on a female member of a coupling will maintain concentricity and will compensate for tolerances on a male shaft while maintaining the ability to transmit torque across a range of diameter dimensions and torque values. This type of conical clamping hub can be manufactured in different styles to promote modularity, ease of assembly and concentricity between shafts, often allowing for easier balancing for high speed applications. 


The fastener assisted friction fit has streamlined design of coupling with shrink disc hubcouplings as it allows for the use of smooth shafts which are not subject the imbalance and complications shaft key or spline tolerances can cause. The next time you are sizing and selecting rotating components, consider going with keyless frictional fits and reap the benefits of modern machine construction.


Bellows Coupling White Paper


Tags: elastomer insert couplings, engineered clamping systems, frictional clamping devices, drive couplings, jaw couplings, flexible couplings, bore diameter, bore tolerance, elastic couplings, flexible jaw couplings, keyless locking, elastomer couplings

Balanced High Speed Precision Couplings

Posted by Niilo Nykanen on Fri, Mar 06, 2015 @ 14:02 PM


Balanced High Speed Precision Couplings

As a manufacturer of precision couplings, we spend quite a bit of time on high speed applications. Here are a few basic tips about coupling balance.

A properly balanced drive component should not induce excessive vibration into the drive line. This means that the mass of the component rotating about the axis of the shaft must be evenly distributed in such a manner as to maintain an even centrifugal force about the axis of rotation. The magnitude of the centrifugal force generated by a rotating component varies with the location of the center of mass of the drive component versus the axis of rotation and orientation of the moment of inertia. 


A simple way to think of static balance about an axis is to imagine playing on a see-saw in a playground.  Two people of equal mass can sit at an even distance apart from each other and maintain balance. Or one person of a heavy mass can sit close to the pivot point and a person of a light mass can sit far away from the pivot point and maintain balance. A rotating mass works essentially the same way. As an object with constant mass moves away from the axis of rotation, the centrifugal force increases exponentially. In physics, this is simply illustrated with a moment of inertia calculation (I=mr^2).

In practical manufacturing, rotating parts are made with evenly spaced jaws, spokes, etc.  Balancing comes into play when manufacturing processes cause imbalance or additional parts such as clamping screws or shaft keys are added. Adding or subtracting mass in one area of a part causes an imbalance. This must be corrected by adding or subtracting mass on the opposite side of the part. Manufacturers most often do this by drilling holes into the part near the outside diameter so that less material will need to be removed resulting in a more structurally sound part versus removing a lot of material near the axis of rotation.

Precision parts such as machined coupling hubs are designed with the balancing holes in a location so they can be produced in quantity in a numerically controlled process without balancing each part individually. This provides cost savings and consistency to customers. Quality small to medium sized machined hubs should come with a speed rating of about 10,000 RPM. Many of these hubs can be finely balanced for speeds between 30-80,000 RPM depending on size. The larger in diameter and more massive a hub becomes, the lower the practical balance speed will be.


Other considerations such as torque transfer at high speed need to be examined as well. Centrifugal forces at high speeds will begin to open clamping hubs for example. Since the screws of clamping hubs are normally tightened to a torque which takes them near their tensile yield, the effects of centrifugal forces applying additional screw tension as the clamp opens must be considered. This is why in medium to large sized hubs, conical clamping rings are often favored, since they offer better symmetry and better stress distribution as a result of the forces applied to them at high speed.

R+W makes a lot of standard and custom couplings for high rotational speeds, some of which rated to handle speeds in excess of 150,000 rpm.

The coupling shown at left is one of our miniature designs. Higher torque versions are available on request.  Contact us with your high speed coupling requirements and get the ultimate connection.

 Bellows Coupling White Paper

Tags: balanced shaft coupling, high speed coupling, coupling for test stand

Mounting a Sprocket or Pulley to a Torque Limiter

Posted by Andy Lechner on Wed, May 21, 2014 @ 12:06 PM


Mounting a Sprocket or Pulley to a Torque Limiter

sprocket mounted to 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.


load centering over torque limiter


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. torque limiter dimensions


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. 


sprocket with callouts

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.  




sprocket mounted on torque limiter

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


 Video Demonstration of Safety Couplings

Tags: torque overload, ball detent torque limiter, torque limiter mounting, ball detent coupling, flange for torque limiter, torque limiting sprocket, torque test coupling, adjustable coupling, servo torque limiter, safety couplings, ball detent clutch, torque limiter, mounting torque limiter, safety coupling output flange

How to Calculate Mass Moment of Inertia when Sizing Torque Limiters

Posted by Niilo Nykanen on Wed, Jan 22, 2014 @ 16:16 PM

Technical Book on Safety and Overload Couplings


As a manufacturer of safety couplings and torque limiters, we are often asked to provide some assistance in calculating the moment of inertia of various loads, in order to aid in selecting the appropriate disengagement torque settings.  Once the driving and driven inertia values are known, acceleratin and deceleration rates are used to estimate safety coupling torque adjustment ranges. 

Moment of inertia can be described as a mechanical property of the mass of a solid object that quantifies the required torque needed to change the angular velocity of the object about an axis. This inertia is also known as the mass moment of inertia, first moment, or, rotational inertia.  This moment can be used to calculate how much energy it takes to get an object rotating which is especially useful in mechanical drive lines with motors and engines.  When sizing a torque limiter or safety coupling, it is important that the disengagement torque value be set to a value higher than what is required to accelerate the load up to speed.  It is also helpful to be aware of the torque levels resulting from abruptly decelerating a rotating mass, which is where torque limiters and safety couplings come into play, protecting driveline components from the excessive torques resulting from an unintended blockage stopping a machine too rapidly.  Moment of inertia often denoted as (I) is a unit of mass multiplied by area squared (M*A2). In the case of a two inch diameter rotating shaft with a mass of one hundred pounds, moment of inertia can be calculated per the example below.  

1 22 equation 1

Calculating the moment of inertia for a 0.1 meter diameter shaft with a mass of 1000 kg is done in the exact same manner.

0.1m diameter 1000 kg shaft inertia

Finding the moment of inertia for a tube or hollow shaft is very similar. In the next example, we will find the value for a 0.5 meter outside diameter tube with a 0.3 meter inside diameter. The mass of the tube is 500 kg. 

moment of inertia of tube

The calculation for a moment of a square or rectangular shaft or shape can also be useful. In this example, the moment of a one inch by one inch square shaft with a mass of 1000 pounds will be found.

moment of inertia of a square shaft

The examples above are handy for calculations involving sizing of mechanical drive components, and not just safety couplings. Inertia ratios of rotating shafts and motor rotors are critical to proper operation of many drives. These calculations are all done about the axis of typical shaft rotation and cannot be used for a different axis.  It is important to note that this is the mass moment and the calculation gives us a property of mass at a distance from the axis of rotation. As with all rotating objects, the moment increases exponentially as a mass moves away from the axis of rotation.  These basic facts of physics are critical for machine design and have been a primary guiding principle for centuries of mechanical engineering.

Technical Book on Safety and Overload Couplings

Tags: rotational moment of inertia, coupling inertia, moment of inertia, drive shaft inertia, inertia calculations, how to calculate inertia

Proper Shaft Fits for Precision Coupling Devices

Posted by Niilo Nykanen on Thu, Aug 22, 2013 @ 15:09 PM


Proper Shaft Fits for Precision Drive Components


When sizing high performance drive components, proper shaft fit is an important consideration in helping to maintain concentricity and balance in the rotating equipment. In v-belt drives, chain drives and other low-cost / low-speed systems, designers often give little thought to shaft fit. Many manufacturers of these types of low cost drive components are simply relying on a shaft key and set screw to transmit torque and retain the hub on the shaft, and other types of misalignment absorbing elements help to take up any eccentricity resulting from a relatively large clearance between the shaft and the bore.  But when it comes to high-performance / high-speed drives and “smart” motors, such as servo and steppers, shaft fit becomes increasingly important.

When researching a possible vendor for zero backlash components with engineered clamping systems that can be assembled by hand, it is imperative that they are able to provide a shaft tolerance as well as a published chart which verifies the shaft and bore fits required. As a general rule of thumb, the overall diametrical clearance between the shaft and the bore should be around 10-50 µm (.0004-.0019”) for smaller diameter shafts, and as shaft diameters increase beyond 80mm (3.150”), larger clearances become allowable, and more annulus is required around the ID of the bore to ensure easy installation. If the fit is correct, a bit of oil is all that is required to easily slide a hub onto a shaft. 


DIN Fit Tolerance Charts


 Many engineers are inclined to specify interference fits for high performance drives, which in a solid hub design would require either sub-zero cooling of the shaft and / or, more commonly, heating of the bored and keyed hub. This type of fit dates back to times before frictional clamping systems became well established in the power transmission industry, and is intended to eliminate problems associated with loose shaft fits and backlash.  Requiring this type of work can create a maintenance and installation problem, and is not necessary in most modern applications incorporating engineered clamping systems. But the importance of precision shaft fit remains when it comes to frictional clamping style hubs. 


Frictional Clamping Hubs

In the case of clamping collarstyle hubs, which offer the advantage of very easy installation and removal, if the fit clearance is too large between the shaft and bore, once clamped, the hub can become eccentric to the shaft.  This can cause an imbalance and or misalignment issue in addition to the potential for failure of the driveline. In self centering clamping systems, often used for high speed / high power systems, excessive fit clearance can reduce the transmittable torque of the shaft hub connection.  But with proper shaft fits and screw tightening torques applied, they offer superior performance to simple keyway hubs, and far easier handling than interference fit hubs. 


clamping hub (clamping collar)       self centering clamping hub (self-centering clamping system)


Manufacturing Standards

There are several standardized shaft tolerance charts commonly used in industry such as ANSI B4.1-1967 (R2004) for imperial units and ISO tolerance system per DIN 7160 (8.65) for metric units.  Precision drive components should have a clearance fit which is just out of range of a transition fit such as an H7/h6 fit per DIN 7160 (8.65) chart. For example on a 32 mm diameter shaft, this would mean that the shaft could be 0-16 µm under 32mm.  The bore could be 0-25 µm over 32 mm. This would result in a diameter difference of 0-41 µm (.0000-.0016”).


32H7     32h6


If the shaft and bore were both the same size, assembly would be difficult. In practical manufacturing, this would almost never happen. A good machinist would normally use a H7 go/no-go plug gauge and an h6 go/no-go ring gauge. The go portion of the plug gauge would not practically fit into the bore by hand unless the bore was slightly larger than 12 mm exactly. Vice-versa, with the ring gauge and shaft, when checking a turned part with a ring gauge, it would not fit by hand unless the shaft was a bit smaller. If either no-go plug or ring gauge fits, the parts are scrapped. This method of machining/checking shafts and hubs results in parts with a precise fit which can be assembled by hand. 

For more information on proper shaft fits and precision couplings for high performance drive applications, contact applications@rw-america.com.


DIN Fit Tolerance Charts

Tags: engineered clamping systems, frictional clamping devices, drive shaft, bore diameter, bore tolerance, non keyed coupling, precision couplings, Precision shaft couplings, zero backlash clamping, keyless locking

R+W Assumes a New Position on Metallic Couplings

Posted by Andy Lechner on Fri, Jul 19, 2013 @ 10:29 AM




The concept of fatigue resistance in flexible shaft coupling design has been highly valued by R+W since its inception in 1990.  For much of its first two decades in business the company focus was almost exclusively on couplings for high performance servo drive technology.  When it comes to machinery that utilizes this type of equipment, professionals at all levels know that shut down for maintenance can be extremely costly, and that unplanned downtime can have catastrophic effects on the profitability of a process.  When properly applied, the flexible bellows coupling addresses this and a great number of other concerns in support of high speed, high accuracy machinery.  In addition to fatigue resistance it offers the benefits of high torsional stiffness, low moment of inertia, and continuous symmetry, all of which lend themselves very well to motion systems involving rapid indexing and high precision positioning – essentially making it the first choice for servo drives.  For many years R+W has been well known as a leader in bellows coupling technology.  But as the company has continued to grow and add couplings for higher powered industrial drives to its portfolio, the need for a different type of fatigue resistant metallic coupling has become apparent.  In 2013 R+W is introducing its SURVIVOR series of flexible disc pack couplings.  Not to be confused with a servo coupling, the flexible disc pack coupling is ideally suited for many of the most demanding industrial power transmission systems made.  

 disc packbellows servo coupling










Material fatigue results from a certain number of stress cycles at a certain stress amplitude.  In a flexible coupling this essentially means the number of shaft rotations at certain levels of misalignment and torque.  In the case of ferrous materials, when the stress amplitude is known and kept below the fatigue limits of the flexible element in the coupling (i.e. the misalignment and torque ratings), any number of cycles can be tolerated without fatigue.  The goal is infinite life for the product.   More than just servo driven machinery demands reliable performance.  Engineers in the petrochemical, power generation, steel and paper industries, to name a few, might consider that to be a laughable statement, and might also agree that reliable operation of their equipment is more critical today than ever.  Designed to protect drive shafting, bearings and gears from stress related to misalignment and structural changes, a flexible shaft coupling is necessarily subject to a very large number of bending cycles in its life.  More traditional designs require either periodic lubrication or replacement of wear parts in order to help relieve this kind of stress.  But this kind of frequent maintenance is simply unacceptable in some critical installations.  Metallic flexible couplings are a category which is typically designed with the intent to fully eliminate wear, based on the principle of fatigue resistance. 

R+W has been applying this concept to maintenance free bellows couplings for many years on its mission to deliver efficiency through coupling design.  While the metal bellows coupling is often scaled up into the megawatt drive power ranges for applications which demand its specific characteristics, many industrial drive applications do not involve the dynamic motion profiles of servo systems, and tend more toward continuous forward rotation.  In this category, as loads become larger and drivelines more power dense, a different set of shaft coupling characteristics can come into focus as being more suitable.  With this in mind, R+W is proud to present its LP-SURVIVOR series, for demanding industrial power transmission applications.


exploded view


There are some distinct features of the R+W version of the steel disc pack coupling, the most notable of which makes further advancements toward the goal of infinite service life.  R+W SURVIVOR series couplings transmit torque across the disc pack assemblies purely by friction.  A series of bushings are pressed together by R+W to assemble the disc packs, while precision locating features in the hubs and spacers present a concentric fit.  The bolt assemblies are then tightened through the hubs, spacers and bushings to generate the necessary clamping pressure across the faces of the disc packs to transmit all of the power by friction.  This purely backlash free friction fit serves to eliminate problems associated with stress concentration, backlash, and micro-movements, all of which can result from transmitting torque across the shanks of shoulder bolts. The frictional connection of the disc packs further increases service life, in addition to making the complete coupling assembly more torsionally stiff.

The first generation of LP-SURVIVOR series couplings consists of both single and double flex versions to mount by keyway and set screw (LP1+LP2), a double flex version with precision conical clamping ring assemblies (LP3), and a special API 610 version (LPA) which meets all of the stringent requirements for critical centrifugal pumping applications.  Two standard spacer lengths are available for each double flex version, with full customization of dimensions and materials available, depending on the specific application requirements. As with all R+W couplings, the LP couplings are available with either imperial or metric bore diameters ranging from 18 to170mm (~3/4” to 6-5/8”) and with torque capacities ranging up to 20,000Nm (177,000 in*lbs). 

Whatever the requirements may be, an R+W coupling expert is available to help in the sizing, selecting and customization of the ideal high performance shaft coupling for your requirements.  For more information on R+W’s new line of disc pack couplings, please visit our website at www.rw-america.com, or call us at (888)479-8728 to speak with a coupling specialist today.

Survivor Series Disc Pack Coupling Catalog

Tags: disc pack coupling, lamina coupling, API 610 coupling, steel disc coupling