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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. 

coungerweights

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.

mks

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

CALCULATING MOMENT OF INERTIA

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

 

MTBF APPROACHING INFINITE

 

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

Basics of Angular Acceleration and Rotational Moment of Inertia

Posted by Niilo Nykanen on Fri, Jun 14, 2013 @ 14:22 PM

  Bellows Coupling White Paper

 

Angular Acceleration and Moment of Inertia in Machine Design 

As a provider of flexible drive couplings and ball detent safety clutches, we are often asked to provide a bit of assistance in calculating application torques, especially for customers looking to retrofit existing equipment.  In order to aid in the process of estimating torques, we'll review one of the basic calculations used to estimate the torque required to accelerate a rotating mass to a certain speed over a given time.  

Angular acceleration (α) can be defined as angular velocity (ω) divided by acceleration time (t). Alternatively, pi (π) multiplied by drive speed (n) divided by acceleration time (t) multiplied by 30. This equation yields the standard angular acceleration SI unit of radians per second squared (Rad/sec^2). The equation below defines the rate of change of angular velocity.

 equation 1

ω = angular velocity in the standard SI unit of radians per second (Rad/sec), 1 radian = 57.3 degrees

t = acceleration time in seconds

π = 3.1416

n = drive speed in revolutions per minute RPM

 

In the next example, angular velocity will be calculated for acceleration from 0 to 60 RPM in one second. Note that 2π radians per second = 60 RPM.

 calculation 2

 

This calculation is very useful in machine design because angular acceleration multiplied by rotational moment of inertia equals torque.  Keep in mind that the exact moment of inertia can be difficult to calculate based on complex geometries in real drive lines, and other variables such as friction are not considered in the next calculation.  Nonetheless it is still very helpful in approximating torque requirements or establishing baseline minimum values for component sizing purposes. 

 calculation 3

J = moment of inertia in kg∙m2

T= torque in N∙m

N= force in Newtons

kg= mass in kilograms

m= lever arm radius in meters

 

In the final example below we will use the angular acceleration we found above to calculate torque on a flywheel with a 1 meter radius and 1000 kg mass. 

 calculation 4

As we can see, if a flywheel with a 1 meter radius and 1000 kg mass were to be accelerated to 60 RPM in one second, it would require 3141.59 Newton meters of input torque.

 

 

I hope you found this refresher on calculating angular acceleration to be helpful.  If you have questions pertinent to the sizing and application of shaft couplings or safety clutches, feel free to contact our applications engineering department.

 

applications@rw-america.com

 

Bellows Coupling White Paper

Tags: rotational moment of inertia, acceleration torque, angular acceleration, motor acceleration, drive acceleration

Taking Advantage of Coupling Configurability

Posted by Niilo Nykanen on Thu, May 30, 2013 @ 16:05 PM

  Bellows Coupling White Paper

 

Configurable coupling systems save time and money

Many machine builders are well aware of the advantages of using configurable components in the design stages. As projects progress, specifications often change and builders must adapt components to fit new constraints. Drive and electrical components must be flexible to adapt to new operating parameters or machine chassis dimensions. Electrical designers often make use of DIN rail mounted devices which can be interchanged quickly on the production floor or in the field. Circuit breakers, fuses, and power supplies can be easily changed out in the event that a different motor or sensor is installed. From the mechanical perspective it is usually coupling elements which need to be changed to adapt to new dimensions.  This is why QD or similar bushings are typically used with V-Belt sheaves.  If drive speeds change, a fairly quick sheave swap is all that is required. If a new gearbox or actuator needs to be installed, drive couplings in various lengths and bore diameters are often required in order to help get everything tied together.  Finding direct drive couplings with compact dimensions and creative mounting configurations is normally fairly easy.  But in some cases layout changes involve longer distances between mechanically connected equipment, requiring something a little more specialized.

 

BELT DRIVE

 

 Belt drive or direct drive

With longer distances between rotating components, designers are often in a position to choose between belt or chain driving, and using a direct drive line shaft system.  A direct drive line shaft coupling typically provides for stiffer power transmission than belts, which can be especially advantageous in applications that require precision timing and positioning or frequent changes in rotational direction. Line shaft couplings are also low maintenance compared with belts which need to be changed at regular intervals, just like car tires for optimal performance. But in the past there were occasionally major drawbacks to using direct drive line shafts over belts in some applications.  Assembly with steel shafting and standard couplings generally requires intermediate support bearings and is not very well suited to higher drive speeds over long distances.  Most preassembled torque tube styles of line shaft couplings are also built to order rather than being stocked by many large industrial supply companies. In the past a typical prefabricated torque tube style line shaft would need to be rebuilt if length or shaft sizes changed. 

 

old style line shaft

 

New configurable line shaft couplings

In more recent years, prefabricated, variable length, telescoping line shaft couplings have been brought to the industrial market in order to address the growing trend toward designing with direct drives. A variable length line shaft with removable hubs solves the issue of shaft sizes and lengths changing (within adjustment range) as a machine is built or upgraded. With a variable length line shaft, the overall length can be changed in minutes by simply loosening and tightening a couple of machine screws. Jaw style hubs or flange mounted bellows coupling hubs can be swapped out with stock parts in less time than it takes to change a v-belt sheave and re-tension belts. Additionally, common size adjustable line shaft couplings are stocked by many distributors and ready to ship with hubs just like sheaves and belts.

 

Adjustable Line Shaft Coupling

 

This new take on configurable coupling component technology ultimately saves cost in labor and/or materials over time. While newer components produced in smaller quantity can have higher pricing up front, they can actually lower the cost of maintenance and overhaul down the road. Making use of a more precision product can also help to increase the rate at which a product is manufactured. As we all know, increased productivity and lower service costs ultimately decrease the time over which a return on initial investment is seen. This makes money for our employers, which is something I’ve found they enjoy universally.  

 

Bellows Coupling White Paper

Tags: spacer couplings, line shaft, telescoping shaft, jack shaft, drive shaft, line shaft coupling, adjustable coupling, torque tube

Solving Problems with Torsionally Stiff Couplings

Posted by Andy Lechner on Wed, May 22, 2013 @ 17:08 PM

 

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.

 

  Bellows Coupling White Paper

 

 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.  

mcm photo

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.”

 

 

bk1

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. 

 

 

  Bellows Coupling White Paper

Tags: test stand coupling, torsional stiffness, torsionally stiff coupling, coupling torsional stiffness, torsional rigidity, drive couplings, bellows coupling, torsion resistance, torsion resistant coupling, torque test coupling, torque sensor coupling

Elastomer Jaw Couplings: Not All are Created Equal

Posted by Niilo Nykanen on Tue, May 14, 2013 @ 09:03 AM

 

Elastomer Jaw Couplings:  not all are created equal

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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 coupling 2

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 coupling

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. 

       

 

 

 

 

 

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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. 

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convex elastomer

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. 

 

 

 

 

 

ekzWhen 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.

   Elastomer  Coupling Sizing Program

Tags: drive couplings, jaw couplings, flexible couplings, pump coupling, elastic couplings, elastomer couplings, spider coupling