The Coupling Handbook

The Coupling Handbook

The goal of this handbook is to assist you with the process of sorting out the myriad of coupling styles that

exist to select the one best suited to your application. This handbook is not a textbook. There are several of

those in print which do a great job and are very useful for coupling designers. What we are attempting to do is

to provide down-to-earth useable knowledge. We want to arm you with information that you need to utilize

the variety of styles that exist in flexible couplings to your best advantage and solve real world problems.

Forward

As CEO and owner of Lovejoy, Inc., I am very proud to be leading our company into its second 100 years. After

considering the many suggestions for ways to commemorate our 100th anniversary, I felt the best one was to

create something that could have lasting value to our industry and our customers. From this goal came the

idea of our "Coupling Handbook".

During our first 100 years of existence, Lovejoy engineers, product managers and field service people have

accumulated a lot of knowledge about flexible couplings, including practical experience not found in

textbooks. Our Handbook is intended to transfer that knowledge to the people who can best make use of it --

the designers, machine builders and maintenance people who work with couplings every day.

Like Lovejoy itself, this handbook will always be a work in progress. There is always more to learn. To that end,

we welcome input from you, the reader, as to how we can improve the contents of this book in the future. We

want it to become a living, changing document that will be updated over the years to better assist its readers

with the selection, installation and maintenance of all types of flexible couplings. Between successive editions,

we will post new and updated material on the industry-wide informational website we sponsor:

couplings.com. Parts of this book also will be available for training purposes on Lovejoy's website: lovejoy-

inc.com.

I hope you will find our efforts informative, helpful and worthwhile, and that you will offer your comments,

knowledge and experience to help us continually make it better.

With thanks to you for making our success possible.

- Mike Hennessy, Chief Executive Officer

Preface

Lovejoy is very proud to be celebrating its 100th anniversary at the start of the new millennium. To

commemorate this occasion, we created a handbook for those people who are involved with mechanical

power transmission, and specifically with the general purpose couplings used in that field.

The majority of those who leave engineering school are confronted with daunting challenges. For one, they

must bridge the gap between theoretical textbooks and the practical realities of design engineering in industry

today. Engineers spend only a small portion of their time dealing with flexible couplings.

With the notable exception of gear couplings, industry-wide common designs for flexible couplings really do

not exist. Each coupling designer developed a coupling with a unique geometry and set the ratings based on

that coupling's abilities. This contrasts with other power transmission components such as chain, v-belts,

motors, and bearings where standards exist. Each of the manufacturers produces these products to standards

and in many instances even use the same nomenclature.

The goal of this handbook is to assist you with the process of sorting out the myriad of coupling styles that

exist to select the one best suited to your application. This handbook is not a textbook. There are several of

those in print which do a great job and are very useful for coupling designers. What we are attempting to do is

to provide down-to-earth useable knowledge. We want to arm you with information that you need to utilize

the variety of styles that exist in flexible couplings to your best advantage and solve real world problems.

Lovejoy has been manufacturing couplings since 1927. More importantly, we have the greatest breadth of

coupling types offered by any single manufacturer in the world. We have the applications experience with

couplings to talk about all the most popular designs out there, even the ones we don't sell. Since flexible

couplings are our strategic focus, we feel you will find this handbook to be a valuable resource.

I. Introduction A. Why a Flexible Coupling?

A flexible coupling connects two shafts, end-to-end in the same line, for two main purposes. The first is to

transmit power (torque) from one shaft to the other, causing both to rotate in unison, at the same RPM. The

second is to compensate for minor amounts of misalignment and random movement between the two shafts.

Belt, chain, gear and clutch drives also transmit power from one shaft to another, but not necessarily at the

same RPM and not with the shafts in approximately the same line.

Such compensation is vital because perfect alignment of two shafts is extremely difficult and rarely attained.

The coupling will, to varying degrees, minimize the effect of misaligned shafts. Even with very good initial shaft

alignment there is often a tendency for the coupled equipment to "drift" from its initial position, thereby

causing further misalignment of the shafts. If not properly compensated, minor shaft misalignment can result

in unnecessary wear and premature replacement of other system components.

In certain cases, flexible couplings are selected for other protective functions as well. One is to provide a break

point between driving and driven shafts that will act as a fuse if a severe torque overload occurs. This assures

that the coupling will fail before something more costly breaks elsewhere along the drive train. Another is to

dampen torsional (rotational) vibration that occurs naturally in the driving and/or driven equipment.

Each type of coupling has some advantage over another type. There is not one coupling type that can "do it

all". There is a trade-off associated with each, not the least of which can be purchase costs. Each design has

strengths and weaknesses that must be taken into consideration because they can dramatically impact how

well the coupling performs in the application.

This handbook will be a guide to assessing the features and limitations of the many standard types of couplings

on the market. Before we enter into a discussion about all of the evaluation factors to consider in selecting the

right coupling type, let's review some basic terminology that will be used in this handbook.

B. Basic Terminology

ANGULAR MISALIGNMENT: A measure of the angle between the centerlines of driving and driven shafts,

where those centerlines would intersect approximately halfway between the shaft ends. Coupling catalogs will

show the maximum angular misalignment tolerable in each coupling. A coupling should not be operated with

both angular and parallel misalignment at their maximum values.

AXIAL: A projection or movement along the line of the axis of rotation. Example: Sliding the hub in either

direction may change the position of a coupling hub, on its shaft. Thus affecting its axial position on the shaft.

AXIAL DISPLACEMENT: One type of misalignment that must be handled by the coupling. It is the change in

axial position of the shaft and part of the coupling in a direction parallel to the axial centerline. Can be caused

by thermal growth or a floating rotor. Some couplings limit this displacement and are called limited end float

couplings.

AXIAL FORCES: The driver or driven equipment can generate axial forces (thrust) in which case the coupling

will pass those forces to the next available bearing with thrust capability. Because of the inherent construction

of some couplings, forces may be generated in the axial direction when operating at high speeds or under

misalignment. Such forces can place additional loads on the support bearings.

AXIAL FREEDOM: This characteristic allows for variation in coupling position on the shaft at time of

installation.

BACKLASH: The amount of free movement between two rotating, mating parts. If one half of a coupling is held

rigid and the other half can be rotated a slight amount (with very little force), you have some amount of

backlash. The freedom of movement, or looseness, is the backlash and may be expressed in degrees. Backlash

is not the same as torsional stiffness.

BORE: The central hole that becomes the mounting surface for the coupling on the shaft. Close tolerances are

required. Bores/shafts are not always round, although that is the most common shape. Other bore types can

include hex, square, d-shaped, tapered, and spline. A spline bore is one with a series of parallel keyways

formed internally in the hub and matching corresponding grooves cut in the shaft. Spline bores and shafts

most commonly conform to Society of Automotive Engineers (SAE) standards.

DAMPING: Some couplings greatly reduce the amount of vibration transmitted between driver and driven

shafts because of the damping capacity of an elastomer in the coupling. It is a hysteresis effect that will

generate heat. The coupling must dissipate this heat or risk losing its strength by melting down. The stiffness of

the elastomer affects the rate at which vibration is damped. All-metal couplings, for the most part have poor

damping capacity.

DISTANCE BETWEEN SHAFTS: The distance between the faces (or ends) of driving and driven shafts, usually

expressed as the "BE" (between ends) dimension or "BSE" (between shaft ends) dimension.

FACTORS OF SAFETY: The coupling designer applies these factors to compensate for unknown elements of the

product design. The factors can compensate for temperature, material variations, fatigue strength,

dimensional variations, tolerances, and potential stress risers to name a few.

FAIL-SAFE: A fail-safe coupling is one that will continue to operate for a period of time after the torque-

transmitting element has failed. This is characteristic of couplings in which some portion of both halves

operate in the same plane, allowing direct contact between those portions. An example of this is the jaw

coupling, in which driving jaw faces push the driven jaw faces through an elastomer in compression between

them; if the elastomer breaks away, the driving faces simply advance to push the driven faces directly.

FINITE LIFE VS. INFINITE LIFE IN COUPLINGS:

All couplings fall into one of these two categories:

1.). Finite-life couplings are those that wear in normal operation, because of using sliding or rubbing parts to

transmit torque and compensate for misalignment. This group includes jaw, gear, grid, sleeve (shear), nylon

sleeve gear, chain, offset and pin & bush types. These types usually have lower purchase costs than infinite-life

couplings. They won't last as long, but their life span may be sufficient for the life expectancy of the

application. Periodic maintenance is required.

2). Infinite-life couplings (a name given to "non-wear" couplings) transmit torque and compensate for

misalignment by the distorting of flexing elements. The distortion results in fatigue stresses rather than wear,

and the couplings are designed and rated to operate within the fatigue capabilities of the coupling material.

"Infinite life" couplings do not necessarily last forever. This group includes tire, disc, diaphragm, some donut

types, wrapped-spring, flex-link, and most motion-control types. "Infinite life" couplings remain infinite only as

long as the load, including those caused by misalignment, is kept within the coupling's design capabilities. An

overload will fail an infinite-life coupling (but may only reduce the life of a finite-life coupling). Infinite-life

designs are most often used on maintenance-free systems where maximum torque requirements - including

transient, cyclic and start-up torque – are known.

HORSEPOWER: The unit of power used in the U.S. engineering system. It is the time rate of doing work. For

power transmission it is the torque applied and rotational distance per unit of time. Applied torque causes a

shaft and its connected components to rotate at a certain RPM (revolutions per minute).

Horsepower (HP) is converted to torque as follows:

T = the torque in inch-pounds

Where T = BHP x 63025/RPM

BHP = the motor or other horsepower

RPM = the operating speed in revolutions per minute

63025 = a constant used for inch-pounds; use 5252 for foot-pounds, and 7121 for Newton-meters

The metric system uses kilowatts (kW) for driver ratings. Converting kW to torque:

Where T = BHP x 84452/RPM

T = the torque in inch pounds

kW = the motor or other kilowatts

RPM = the operating speed in revolutions per minute

84518 = a constant used when torque is in inch-pounds. Use 7043 for foot-pounds, and 9550 for Newton-

meters

KEYWAY: A rectangular opening formed by matching rectangular slots cut axially (lengthwise) along both the

coupling bore and shaft. A square or rectangular metal key is then inserted into the opening to lock the

coupling and shaft in position. Torque is transmitted from shaft to coupling through the keyway and key.

LENGTH THROUGH BORE: The effective length of the bore in the hub, or that portion of the length that is

useable and may be attached to the shaft.

OUTSIDE DIAMETER: The largest effective diameter of the coupling.

OVERALL LENGTH: The largest effective length of the complete coupling assembly.

PARALLEL MISALIGNMENT: A measure of the offset distance between the centerlines of driving and driven

shafts. Coupling catalogs will show the maximum parallel misalignment tolerable in each coupling. A coupling

should not be operated with both parallel and angular misalignment at their maximum values.

RADIAL: Any projection outward from the center of a shaft or cylindrically shaped object, or any motion along

that line. The centerline of the projection or motion normally passes through the axial centerline of the object.

REACTIONARY LOADS: When two shafts are offset (parallel misalignment), the coupling's radial stiffness will

cause a broadside force to be exerted on the shafts. This is called a "reactionary load", as it causes the shafts

to bend slightly in reaction to the broadside force. It may also be called a "restoring moment", as a force

produced by the coupling in an effort to restore, or correct, the parallel misalignment.

RESTORING MOMENT: see REACTIONARY LOADS

SERVICE FACTORS: Multipliers that are assigned to common applications to compensate for their typical load

characteristics. These are used for the purpose of guiding coupling size selection to a torque rating that will

allow for unforeseen demands those characteristics might make on the coupling. Such characteristics can

include peak torque, start-up torque, transients or cyclic torque, or any other empirical factor.

Among couplings that have no wear parts (see Finite/Infinite life), service factors are intended to prevent

premature failure due to overload damage. Among couplings that use wear parts to transmit torque, service

factors are intended to prevent premature failure of those parts due to accelerated wear or degradation.

Caution: Resist the temptation to specify in excess of the published service factors. An oversized coupling will

not perform better or last longer, but will be unnecessarily expensive and force the system to waste energy.

Always base coupling size and service factor on the actual torque requirements at the point of installation

within the drive system.

SET SCREW: A headless screw, with hexagon shaped socket, used over a keyway to keep the key stock in place

and prevent the coupling from moving axially along the shaft. It can also be used for torque transmission on

low torque applications

STIFFNESS

STATIC TORSIONAL STIFFNESS: A resistance to twisting action (rotational displacement) between driving and

driven halves of the coupling. (The opposite - low resistance to twist - is termed "torsional softness") Stiffness

is expressed in lb.-inch/radian and measures the amount of angular displacement about the coupling's axis of

rotation at its static torque rating. Even seemingly stiff all-metal couplings can have some degree of torsional

twist.

TORSIONAL SOFTNESS: Torsional soft or hard is determined by dividing the dynamic torsional stiffness by the

nominal coupling torque rating. Values greater than 30 are hard (very stiff). Values between 10 and 30 are

torsionally flexible. Values less than 10 are considered very soft.

DYNAMIC TORSIONAL STIFFNESS: It is the relationship of the torque to the torsional angle under the load of

actual operation. The dynamic stiffness will be greater than the static. The dynamic torsional stiffness can be

linear, a constant value, or non-linear, an increasing value.

TOLERANCES: The amount of variation permitted on dimensions or surfaces of machined parts. It is equal to

the difference between maximum and minimum limits of any specified dimensions

TORQUE: In rotary motion it is the force multiplied by the radius, to the axis of rotation, at which the force is

applied. Force (F) multiplied by radius (r) = F * r = Torque. In English units (F) is in pounds and (r) is in inches,

expressed as in.-lbs. In metrics (F) is in Newtons and (r) is in meters, expressed as Newton-meters (Nm).

TORSIONAL VIBRATION: The periodic variation in torque of a rotating system. Some causes of torsional

variation are the geometry of the rotating parts of internal combustion engines, cyclic and irregular torque

demands of the driven equipment, and variations in the output of certain types of electric motors at startup.

C. Coupling Evaluation Factors

These are attributes that affect the type of coupling best suited for an application. This is a long list of

evaluation factors. For any one application there may be only three or four attributes which are extremely

important. In fact it would be difficult to satisfy more than a half dozen attributes with any one coupling. It is

important to narrow the requirements for an application down to only the most critical attributes that come

into play.

In the next chapter we summarize the major coupling types discussed in the materials and provide some

ratings of each coupling type against these factors.

Adaptability of Design - Some couplings are available in a variety of configurations (e.g. drop-out spacers,

flywheel mounts, vertical applications, special lengths, brake drums). These alternatives can be important to

users who want to standardize on a particular type of coupling design, but need to adapt it to suit different

application requirements.

Alignment Capabilities - Different couplings have different limitations as to the amount of angular

misalignment, parallel misalignment or axial displacement each can accommodate. First, determine the

amount of misalignment that can reasonably be expected between the two pieces of equipment to be coupled

and let that guide or influence coupling selection.

Axial Freedom - Indicates how much movement can be accommodated by the coupling along the axis of the

two shafts, without compromising the coupling's ability to operate at rated torque and without imposing

reactionary loads on the bearings. This is important in two situations. The first is when the BE dimension is

very small and coupling hubs need to be installed further back from the shaft ends. The other is when axial

float in the shafts is characteristic of system operation. This can include requirements for slider-type couplings

or limited end float couplings.

Backlash - Also defined in the basic terminology section. Backlash is usually not desired in applications where

precise positioning of the shafts is important.

Chemical Resistance - The ability of the coupling components to withstand chemicals in the environment

around it, either mists, baths, dusts, etc.

Damping Capacity - The ability of the coupling to reduce the torsional vibrations transmitted from one shaft to

the other.

Ease of Installation - Some couplings are more complex and take more time to properly install and align. This

might be a concern if large numbers of couplings are to be installed or if they will need to be replaced or

moved frequently.

Fail Safe or Fusible Link - Fail-safe can be important in any application where unexpected stopping of the

driven equipment might jeopardize safety, incur high expense in downtime or scrapping of material in process.

If the equipment can be operated for a while longer, until a more opportune time for maintenance can be

scheduled, fail-safe is extremely valuable. The flip side of this is the application where the user actually wants

the coupling to disengage the drive if the element should fail. This is sometimes referred to as a "fusible link"

function being performed by the coupling. There are some drives where the possibilities of severe torque or

system overloads are high. In order to protect the driver/driven equipment, a fusible link coupling may be

preferred.

Field Repairable - Means that the key components are serviceable on-site so that the entire coupling does not

have to be replaced.

High Speed Capacity - Usually refers to speeds over 3000 RPM. If the coupling fits the application but its

standard off-the-shelf model is not rated for the RPM required, determine whether the coupling can be

economically changed to bring it up to the necessary speed. Sometimes it's a balance issue and sometimes it's

a strength issue due to centrifugal force.

Maintenance Required - Consider not only the frequency of maintenance that a coupling may require, but also

how long it may take to do the work. For instance, lubricated couplings will require periodic checks of the seals

and lubricant. And when the time comes to replace any components and/or the grease, you usually have to

put in new seals.

Number of Component Parts - The more parts a coupling has, the more complex it is, and the more potential it

has for problems. This often means it will take more time to install or disassemble for repairs or maintenance,

will require more spare parts to stock, and will be more costly to balance.

Reactionary Loads Due to Axial Forces - Some coupling designs inherently generate axial forces during normal

operation. Make sure shafts and bearings will be able to withstand the reactionary loads that these forces will

impose.

Reactionary Loads Due to Misalignment - A coupling's ability to accommodate misalignment is evaluated in the

context of the reactionary loads that will result. When misaligned, sometimes even within their rated levels,

each coupling has general propensities for sending reactionary loads (whether axial or radial) through the

system. If shafts are small, or not well supported, or bearings are not substantial enough, these reactionary

loads can cause problems.

Reciprocating Drivers and Loads - Due to torsional pulses generated by reciprocating engines (most notably

diesels) as well as certain kinds of pumps and compressors, coupling selection is generally limited to a few

elastomeric types capable of damping the pulses and providing reasonable service life.

Temperature Sensitivity - This relates to the highest and/or lowest temperatures within which the coupling

materials can operate and provide normal service life.

Torque Capacity to Diameter (Power Intensity) - Couplings with equivalent torque-transmitting capacity can

vary in diameter. Size alternatives within the same torque range may become important in applications where

space is limited or if weight/inertia is a factor.

Torque Overload Capacity - Some couplings have the capacity to deal with brief torque overloads many times

the running torque, others will fail at only a few times the nominal rating. If you expect to see high startup

torque for instance and the drive starts and stops many times each day, you would probably want to have a

coupling which has good capacities in this area.

Torsional Stiffness - Defined in the basic terminology section, this is an attribute that is neither good or bad, it

just depends on the application and what is needed. You just need to be careful to select a coupling type that

has the proper level of torsional stiffness, in balance with the other performance features it provides.

II. First Steps in Coupling Selection Selecting the right coupling is a complex task because operating conditions can vary widely among

applications. Primary factors that will affect the type and size of coupling used for an application include, but

are not limited to: horsepower, torque, speed (RPM), shaft sizes, environment conditions, type of prime

mover, load characteristics of the driven equipment, space limitations and maintenance and installation

requirements. Secondary but possible essential factors can include starts/stops and reversing requirements,

shaft fits, probable misalignment conditions, axial movement, balancing requirements or conditions peculiar to

certain industries.

Because all couplings have a broad band of speed, torque, and shaft size capabilities, those criteria are not the

best place to start. First, determine what attributes beyond those basic criteria will be required for your

application. If none stand out then simply choose the lowest cost that fits those basics. Almost always, though,

there will be other considerations that will narrow your alternatives down to certain types of couplings.

As we review those other considerations that guide coupling selection, we will omit rigid types and focus on

flexible couplings.

A. Types of Flexible Couplings

Many types of flexible couplings exist because they all serve different purposes. All types, however, fall into

one of two broad categories, Elastomeric and Metallic. The full range of coupling types in both categories, and

the special functions of each, will be discussed thoroughly in later chapters. The key advantages and

limitations of both categories are briefly contrasted here to demonstrate how they can influence coupling

selection.

1. Elastomeric

Couplings in this category include all designs that use a non-metallic element within the coupling, through

which the power is transmitted. The element is to some degree resilient (rubber or plastic). Elastomeric

couplings can be further classified as types with elastomers in compression or shear. Some may have an

elastomer that is in combined compression and shear, or even in tension, but for simplification they are

classified as compression or shear, depending on which is the principle load on the elastomer. Compression

types include jaw, donut, and pin & bushing, while shear types include tire, sleeve, and molded elements.

There are two basic failure modes for elastomeric couplings. They can break down due to fatigue from cyclic

loading when hysteresis (internal heat buildup in the elastomer) exceeds its limits. That can occur from either

misalignment or torque beyond its capacity. They also can break down from environmental factors such as

high ambient temperatures, ultraviolet light or chemical contamination. Also keep in mind that all elastomers

have a limited shelf life and would require replacement at some point even if these failure conditions were not

present.

Advantages of Elastomeric Type Couplings

• Torsionally soft

• No lubrication or maintenance

• Good vibration damping and shock absorbing qualities

• Field replaceable elastomers

• Usually less expensive than metallic couplings that have the same bore capacity

• Lower reactionary loads on bearings

• More misalignment allowable than most metallic types

Limitations of Elastomeric Type Couplings

• Sensitive to chemicals and high temperatures

• Usually not torsionally stiff enough for positive displacement

• Larger in outside diameter than metallic coupling with same torque capacity (i.e. lower power density)

• Difficult to balance as an assembly

• Some types do not have good overload torque capacity

2. Metallic

This type has no elastomeric element to transmit the torque. Their flexibility is gained through either loose

fitting parts which roll or slide against one another (gear, grid, chain) -sometimes referred to as "mechanical

flexing"-- or through flexing/bending of a membrane (disc, flex link, diaphragm, beam, bellows).

Those with moving parts generally are less expensive, but need to be lubricated and maintained. Their primary

cause of failure is wear, so overloads generally shorten their life through increased wear rather than sudden

failure. Membrane types generally are more expensive, need no lubrication and little maintenance, but their

primary cause of failure is fatigue, so they can fail quickly in a short cycle fatigue if overloaded. If kept within

their load ratings, they can be very long-lived, perhaps outlasting their connected equipment.

Advantages of Metallic Type Couplings

• Torsionally stiff

• Good high temperature capability

• Good chemical resistance with proper materials selection

• High torque in a small package (i.e. high power density)

• High speed and large shaft capability

• Available in stainless steel

• Zero backlash in many types

• Relatively low cost per unit of torque transmitted

Limitations of Metallic Type Couplings

• Fatigue or wear plays a major role in failure

• May need lubrication

• Often many parts to assemble

• Most need very careful alignment

• Usually cannot damp vibration or absorb shock.

• High electrical conductivity, unless modified with insulators

B. Application Considerations

Sometimes selection of coupling type is guided by application, falling into one of five categories; General-

Purpose Industrial, Specific-Purpose Industrial, High-Speed, Motion Control and Torsional. In each of these

application categories there would be elastomeric, metallic membrane flexing, and mechanical flexing types.

Once the coupling type is selected, there may be variations to consider within that type. For example, gear

couplings offer a wide variety of configurations to combine coupling functions with other power train

requirements, such as shear pin protection or braking. It is always a good idea to understand as much as

possible about the two pieces of equipment to be connected. Let the driven equipment and the driver dictate

the needs of the coupling. For example, is there a shock load or a cyclic requirement that may lead to an

elastomeric coupling? If low speed and high torque are involved, that means a gear coupling is likely best

suited. High-speed machinery will lead to a disc or diaphragm coupling. Diesel drivers need the benefits of

torsional couplings for best results. If the equipment is susceptible to peaks or transients, the application may

want high service factor or a detailed analysis of the coupling torque capabilities. That brings us to the list of

requirements that will impact the coupling selection.

The charts below will help provide the path among all the couplings for most types of rotating equipment. The

charts are organized into three sections. The first is a list of "Information Required" for the best possible

selection of a coupling. It reflects the selection process used by the OEM equipment designer, the

engineer/contractor, the coupling specifier, or the trouble-shooter. For other situations, short cuts are

sometimes taken towards the conservative side. The second is a chart of "Coupling Evaluation Characteristics"

such as torque, bore and misalignment. The third is the chart showing "Coupling Functional Capabilities”. They

are the attributes of the various couplings that go beyond the numerical information.

C. Coupling Evaluation Charts

Information Required

1. Horsepower

2. Operating speed

3. Hub to shaft connection

4. Torque

5. Angular misalignment

6. Offset misalignment

7. Axial travel

8. Ambient temperature

9. Potential excitation or critical frequencies (Torsional, Axial, Lateral)

10. Space limitations

11. Limitation on coupling generated forces (Axial, Moments, Unbalance)

12. Any other unusual condition or requirements or coupling characteristics.

The first seven items of the list above will allow a coupling selection if a service factor is used. The risk of

relying on service factors is the possibility of ending up with an oversized coupling or one that is missing an

essential feature. All the remaining information, where applicable, allows the coupling to be fine-tuned for the

application.

Some types of couplings designed to do a specific job will have a further list of needed information. For

example, a slider coupling has to have the sliding distance and the minimum and maximum BSE dimension.

Note: Information supplied should include all operating or characteristic values of connected equipment for

minimum, normal, steady-state, transient, and peak levels, plus the frequency of their occurrence.

Information Required for Cylindrical Bores

1. Size of bore including tolerance or size of shaft and amount of clearance or interference required

2. Length

3. Taper shaft (Amount of taper, Position and size of o-ring grooves if required, Size and location of oil

distribution grooves, Max. pressure available for mounting, Amount of hub draw-up required, Hub OD

requirements, Torque capacity required)

4. Minimum strength of hub material or its hardness

5. If keyways in shaft (How many, Size and tolerance, Radius required in keyway, Location tolerance of keyway

respective to bore and other keyways)

Types of Interface Information Required for Bolted Joints

1. Diameter of bolt circle and true location

2. Number and size of bolt holes

3. Size, grade and types of bolts required

4. Thickness of web and flanges

5. Pilot dimensions

6. Other

Once past the charts that follow, one can go directly to the manufacturers catalog, or can read on to learn

more about specific couplings and the other important coupling issues.

Chart 1: Coupling Evaluation Factors

Chart 2: Functional Capability Chart

III. Popular Elastomeric Coupling Types

General Elastomeric Capabilities and Types

Elastomeric flexible couplings transmit torque between the two shafts by means of an elastomeric material

(rubber, urethane, etc.) positioned between the driving and driven hubs.

The resiliency of the elastomeric material gives these couplings varying degrees of torsional softness not

available in all-metal couplings, and generally greater misalignment capability than all-metal couplings. It also

allows a single flex plane to accommodate both angular and parallel misalignment. Couplings made as metal

flexing element or metal sliding element couplings require two flex planes to achieve parallel misalignment.

Power intensity (torque-carrying capacity vs. coupling size) of elastomeric couplings is lower than that of all-

metal couplings. With no (or little) friction wear between components, however, elastomeric couplings are

considered low maintenance, although elastomer breakdown in some coupling configurations is a

maintenance issue.

Elastomeric couplings are quieter than some all-metal types. The softness of the elastomer cushions the

vibration and cyclic torque noises that result from backlash. Noise reduction can be an advantage in certain

applications, such as HVAC systems.

Because the elastomeric element handles misalignment by distorting, that action produces reactionary loads

on the adjacent shaft bearings. The reactionary loads vary in inverse proportion to the softness of the

elastomeric element. In all cases, greater misalignment will mean higher reactionary loads. Combined angular,

parallel (radial) and axial misalignments will result in the greatest reactionary load. Speed is a problem for

elastomeric couplings. The deflection of an elastomeric coupling is large for the load applied. Large centrifugal

forces may cause the element to protrude out of the coupling and hit the coupling guard.

Temperature is a restriction for elastomeric couplings. The material loses its strength as the temperature rises.

Eventually the strength reduces to zero. Temperature limits vary by type of elastomer, but generally 200 to

250 °F (110 °C) is the top end. Some elastomeric couplings may be used to dampen torsional vibration energy.

Hystersis, a characteristic exhibited by rubber with binders, allows the elastomeric material to absorb dynamic

energy. The energy is in turn lost in heat generation. If the material is able to radiate or otherwise conduct the

heat to a sink, damping will occur without damage to the coupling elastomer. If the heat builds up in the

elastomeric element it will fail or melt down. Elastomeric couplings of both the compression type and shear

type are used to control torsional vibration by damping the torsional vibration energy. The amount of hystersis

is a function of the elastomeric material as well as the stress level.

Damping of torsional energy in a power transmission system can also be accomplished by means other than

the flexible coupling. Frictional dampers, viscous dampers and torque converters are all used. The

characteristic of damping exhibited by these couplings is different from torsional tuning of a system. Torsional

tuning uses the dynamic torsional stiffness of the coupling to establish a low torsional critical speed.

Torsional stiffness of a coupling is a mechanical property of the coupling materials, modulus of elasticity, and

the geometry of the coupling element. Metal couplings usually depend on the spacer piece or floating shaft to

lower the resilience or torsional stiffness. Torsional stiffness is described as the torque necessary to deflect a

coupling in the circular direction. When dealing with power transmission couplings, it is usually measured in

inch-pounds per radian (Newton Meters per radian in metric). Rubber in shear and rubber in compression

provide the lowest torsional stiffness. Note that the geometric configuration of the coupling will determine the

loading. The unit may not be acting like a torsional spring just because we are applying a torque load. Other

elastomers in the plastic range are progressively stiffer. Coupling materials like urethane, and Zytel® make for

stiff couplings. They have little resilience, but carry more compressive load. Choice of materials in designing

elastomeric couplings is a balance between resilience and load carrying capability. Resilience is helpful for both

cyclic loading and misalignment capabilities.

Types of Elastomeric Couplings

Elastomeric couplings classify into three main types by the way their elastomeric element transmits torque -

i.e. the element is either "in compression", "in shear", or a combination of the two.

Compression Types. This type of elastomeric coupling is characterized by a design in which the driving and

driven hubs rotate in the same plane, with parts of the driving hub pushing parts of the driven hub through

elastomeric elements positioned as cushions between them, but not attached to either hub. As torque is

transmitted, the elastomeric elements are being compressed. Parallel offset misalignment is accepted via

compressive distortion of the elastomer material. Angular misalignment is accepted via sliding or distortion of

the elastomer material depending on the method of securing to the hubs.

Compression type couplings generally offer two advantages over shear types. First, because elastomers have

higher load capacity in compression than in shear, compression types can transmit higher torque and tolerate

greater overload. Second, they offer a greater degree of torsional stiffness, with some designs approaching the

positive-displacement stiffness of metallic couplings. However, greater torsional stiffness generally produces

higher reactionary shaft loads when the coupling is subject to parallel misalignment.

Shear Types This type of elastomeric coupling is characterized by a design in which all parts of driving and

driven hubs rotate in different planes, with the driving hub pulling the driven hub through an elastomeric

element attached to both hubs by various methods. These can include clamping, intermeshing teeth, or by

bonding to metallic brackets that are bolted to the hubs. As torque is transmitted, the elastomeric element

absorbs some of the torque force by being stretched through twisting. The design accepts misalignment

through the deflection and distortion of the elastomeric member and also through sliding, if the elastomeric

member is attached to the hubs through the use of intermeshing teeth.

Shear type couplings generally offer two advantages over compression types. First, they accommodate more

parallel and angular offset while inducing less reactionary load to the bearing. This makes them especially

appropriate where shafts may be relatively thin and susceptible to bending. Second, they offer a greater

degree of torsional softness, which in some cases provides greater protection against the destructive effects of

torsional vibration. Greater torsional softness generally produces lower reactionary shaft loads when the

coupling is subjected to misalignment.

The in-shear design also allows the coupling to act as a "fuse" to protect the driver and driven equipment from

torque spikes or system overloads which might cause damage elsewhere.

Combination Shear and Compression Type This type of elastomeric coupling transmits torque between hubs

through an elastomeric element in-shear, but transmits torque from hub to element (and back again) by

compression between hub teeth and intermeshing teeth formed into both ends of the element. Misalignment

is accommodated primarily by the sliding of the elastomer against the hub teeth (similar to a gear coupling).

1. Compression Loaded Designs

Jaw Couplings

A classic example of compression-type couplings, first patented in 1927, is the jaw coupling. It is still one of the

most widely used flexible couplings in the world and one of the lowest cost couplings available.

Since elastomeric technology was not what it is today, the spiders were originally made from materials such as

leather. Now a wide array of materials are available. Typical applications include pumps, gearboxes,

compressors, fans/blowers, mixers, conveyors, and generators, usually driven by an electric motor. Jaw

couplings usually are not recommended for engine-driven, frequent stop-start or reciprocating loads because

they are not designed to dampen torsional vibration. However, they might be able to serve such applications if

the proper service factors are used in sizing the coupling. Damping capability depends largely on the geometry,

type and amount of elastomer used.

Its design is simple, usually involving only three parts. Both driving and driven hubs have two to seven jaws

(thick, stubby protrusions) formed around their circumferences, pointing towards the opposing hub. When the

hubs are brought together, jaws from both hubs mesh loosely with each other. Gaps between them, and

sometimes the central inner space between the hubs, are filled with an elastomeric material, usually molded

into a single asterisk-shaped element called a "spider". The legs of the spider protrude radially to become the

cushions between the jaws. Some designs of Jaw couplings use blocks or tubes of rubber that are placed in

between the opposing jaw faces and must be held in place through the use of a retaining collar, or the hubs

have enclosed cavities into which the elastomer is placed.

In general, the greater the surface area (and volume) of the elastomer in compression, the higher the torque

rating of the coupling. Exploded view of Jaw coupling

Torque is transmitted from one shaft to the other through the compression of the elastomer between the

driver hub jaws and the driven hub jaws. Since the jaws between the two hubs rotate intermeshed in the same

plane, this design is called "fail-safe". If the elastomer should fail, the coupling will still transmit the torque,

albeit quite noisily given the metal-to-metal contact. This is still the preferred alternative for some

applications, where the equipment is critical to a production process and cannot be allowed to stop.

Some degree of permanent compressive set is normal as elastomeric elements age in service. This is a helpful

feature for Jaw couplings; when permanent set reduces the element's original thickness by 25% or more, it

provides a visual sign that the element should be replaced.

Another helpful feature unique to Jaw couplings is that compression is applied only to the spider legs or load

cushions forward of the driving jaws - trailing legs or cushions behind the driving jaws remain relaxed.

Accordingly, when compressive set reaches maximum in the driving cushions, the spider's trailing legs or

cushions can be advanced into the driving position. Thus, in most applications, jaw couplings carry a builtin set

of replacement elastomers, which can be used to reduce replacement costs. Note that couplings applied in

reversing drives or those with frequently varying torque usually relinquish this benefit.

Jaw coupling torque ratings are primarily limited by the elastomer material's compression strength, not the

jaw/hub strength. Thus, a jaw coupling can handle brief or infrequent torque spikes above the nominal rating

far better than the elastomer in-shear designs. It would take a torque of 6 or 7 times the nominal rating of

rubber elastomers to break off the hub jaws. If you change the spider from natural rubber to Hytrel® which has

much greater compression strength, the torque rating for the coupling is magnified 2 to 3 times. By contrast,

an elastomer that transmits torque through a shearing action cannot absorb torque any greater than 3 or 4

times its nominal rating without tearing.

Other features of jaw couplings include; no metal-to-metal contact for quiet operation, resistance to

oil/grease/dirt/moisture in many tough environments, simple to install and align, and low maintenance

requirements. Many variations of jaw couplings are possible, ranging from flywheel designs, spacer couplings,

special hub materials as well as a variety of elastomeric materials to choose from. In addition, jaw couplings

are one of the lowest cost couplings.

There are some limitations to jaw couplings. Their angular and parallel misalignment is more limited than with

in-shear designs. When misaligned they introduce fairly significant reactionary loads on the shafts. Maximum

bore is limited by two factors: the inside diameter of the jaws and the length through bore of the hub.

Generally, the bore (shaft diameter) should be no greater than the length of shaft engagement in the hub.

Maximum axial float accommodated by jaw couplings is limited to about 10% of the axial thickness of the

spider. Most designs have backlash or free play between the fit of the elastomer/jaws and are not suited to

motion control applications. Temperature capacity is usually no greater than 250°F (121°C), that keeps them

out of some applications. Vertical applications are difficult since standard hubs are clearance fit bores with

only one set screw, thus hubs must be modified in order to grip the shaft tightly enough.

Jaw Coupling Types

Several different types of jaw couplings are available to serve different application requirements. Most of

them fall into two general categories: a "straight side" type, in which the jaw side faces are flat and straight;

and a "curved jaw" type, in which the jaw side faces have a cupped shape.

A. Straight-Side Type

Straight-side jaw couplings are available in sizes with bore capacities from 1/8" (4mm) up to 2-7/8" (73mm).

This coupling type is used for light to medium duty applications with a maximum torque capacity of 6,228 in-

lbs. (704 Nm).

Small size hubs are made from sintered iron, while larger sizes are cast iron hubs. Neither sintered iron nor

cast iron can be welded to by normal methods.

• Angular misalignment will vary from ½ to 1° maximum depending on the material used. (Materials

discussed later.)

• The same goes for parallel misalignment capability, which will vary from .010" to .015" with different spider

materials.

Standard straight-side jaw couplings offer several alternatives in spider constructions in addition to the basic

asterisk-shaped solid spider or open-center spider spiders, both of which are held captive naturally within the

assembled coupling. (Open-center spiders simply allow greater axial freedom for installation on shafts with a

close BE dimension.) Alternatives include collar, ring-in-groove, block, and in-shear. Collar Types are those

fitted with elastomeric elements that are installed and removed externally. Such elements usually take the

form of a linear spider in which the legs are molded into a single strip of elastomeric material that is wrapped

around the assembled coupling so that the legs drop into the spaces between intermeshed jaws. These wrap-

around spiders require a circular collar around the coupling's circumference to prevent the elastomeric strip

from being flung off by centrifugal force. Typically, the collar is a stamped steel ring held in place by three

retaining screws to one of the hubs.

Ring-in-groove types, sometimes called "Snap-Wrap", are similar to collar types except that wrap-around

spider is held in place with a Spiralox retaining ring that snaps into a groove that is molded into the spider's

perimeter. This version is only available in NBR spider material and the maximum speed is 1750 RPM. Standard

hubs are used. The ring is removed easily with needle-nose pliers.

These features are ideal for those situations where the shaft ends must be positioned closely together, yet the

shaft diameters are greater than what can be accommodated in the open center type spider.

The compression block types serve heavy-duty applications that require shaft size and/or torque ratings

beyond the capability of standard Jaw couplings. Usually of larger diameters, these designs transmit torque

through independent blocks of elastomeric material, in cube, oval, or wedge shapes. Sometimes called load

cushions or elastomer cylinders, these blocks are individually inserted into the spaces between the assembled

coupling's intermeshed jaws, and held in place by a steel collar. This design offers the advantage of easily

changing its torsional stiffness by varying the hardness and design of the blocks. The compression block jaw

coupling is available with a maximum bore capacity of 12.0" (300 mm), and torque up to 1,000,000 in-lbs.

(113,000 N m). Common applications include compressors, large fans, blowers, mixers, and municipal or

irrigation pumps.

In-shear spiders, the newest improvement in spider design, completely change the way the jaw coupling

functions. These spiders are axially twice as wide as standard spiders for straight sided jaw hubs, so instead of

allowing the jaws of both hubs to intermesh in the same plane, they push the hubs apart so the jaws rotate in

separate planes, and in axial alignment hub-to-hub. This arrangement causes the radially removable elastomer

to transmit torque through a combination of shear and compression method. The spider is held in place with a

floating stainless steel ring, which locks into special grooves in the OD of the spider.

As with the collar and snap-wrap designs, the jaw in-shear allows easy removal and replacement of the spider

without disturbing the hubs. There are no fasteners to worry about either since the retaining ring slides into

grooves in the spider. One available design fits standard straight-sided jaw coupling hubs on the market which

makes it an easy retrofit design. Another version uses special hubs with many shorter, stubbier jaws and a

special elastomer, but achieves the same concept in features/benefits.

The primary benefits are (1) simplified maintenance (2) non-failsafe operation (3) greater angular

misalignment capacity of 2°, and (4) greater torsional softness.

This coupling should only be used for electric motor driven applications, most commonly centrifugal pumps,

fans, mixers, gear boxes, and plastic extruding machines.

Special Hub Materials and Designs

Jaw coupling hubs are typically made of sintered iron or, for larger sizes, cast iron. Neither can be welded to by

normal methods. In some applications customers will desire to weld the connection of the hub to the shaft, or

weld another component such as a shaft collar, sprocket or pulley to the diameter of the hub. Special materials

such as 1018 steel, 303/316 stainless or 660/464 bronze are possible to meet those and other unique

application requirements. The torque and misalignment ratings do not change based upon the hub material.

The elastomer spider determines those ratings.

Light Hubs: This category of the standard jaw coupling uses hubs made from aluminum or other light metals. It

provides for a significantly lighter coupling if lower inertia is important. When an application calls for better

corrosion resistance than sintered or cast iron, but not the expense of stainless steel, aluminum is a good

alternative. Light material hubs use the same spiders as the standard straight sided jaw.

Special modifications such as clamped hubs, bushed hubs, extra long or shorter than standard hubs, and

pinholes are possible. The use of clamped hubs or bushings with couplings is common. Generally these are

advantageous when the application requires a firmer grip on the shaft than is provided with clearance (slip fit)

bores and one or two set screws. These include vertical drives, motion control, or equipment with high levels

of vibration and shock loads.

Elastomer options

When jaw couplings were invented, elastomeric technology was not what it is today, and spiders were

originally made from natural materials such as leather. A wide array of materials are now available, including

several non-rubber-based elastomers that offer light weight, chemical resistance with the ability to be molded

into complex shapes. They are also economical to manufacture and use. Generally, rubber-based spiders are

more resilient and better for cyclic loading and misalignment capabilities, while synthetic spiders make for

torsionally stiffer couplings that can carry a more compressive load.

1. NBR (Nitrile Butadiene Rubber) a.k.a. Buna-N -- is the standard and most economical material for jaw

coupling spiders. It offers the best combination of temperature and chemical resistance, misalignment, and

damping ability. Rubber has the best resiliency in bouncing back from deformations that occur in cyclic or

heavy shock loads. This is the only material suitable for reciprocating engine applications.

Most sizes of NBR spiders are 80A-shore hardness and are black in color. Temperature range is -40°F (-40°C) to

212°F (100°C). Also referred to as "SOX" by some manufacturers. This material will allow the Jaw coupling to

experience a torsional wind-up at full torque load of 4°-10°, depending on the coupling size.

Another attribute of natural rubber products used in compression is that they take a permanent "set" or loss

of volume after just a short time in operation. This does not become a performance problem until the spider

thickness is anything less than 75% of its original size, at which point it should be replaced. This limits their

selection in motion control/precision applications since increased free-play in the coupling results from the

"set". Shelf life of natural rubber elastomers is 5 years.

2. URETHANE has a 1.5 times greater torque capacity than NBR due to its greater compressive strength (either

40D or 55D shore hard ness is used) as well as better abrasion/wear characteristics. It holds up better to

environmental conditions such as ozone, ultravio let, and some oils-chemicals versus the NBR. It is limited to -

30°F (-34°C) to 160°F (71°C) temperatures however, and should not be used in heavy cyclic or start/stop

applications since the damping ability is limited. The in-shear spider is a slightly different type of urethane and

is rated for -30°F (-34°C) to 200°F (93°C). Urethane spiders typically are blue color and offer a shelf life of 5

years.

3. HYTREL® increases the torque capacity of the jaw coupling approximately 2½ times versus the NBR with its

higher compressiveload carrying ability. These spiders are a tan or cream color with a 55D shore hardness. This

material provides the best chemical resistance as well as a temperature range of -60°F (-51°C) to 250°F

(121°C). However, as with Urethane, it should not be used in appli cations where cyclic loads, frequent

starts/stops, or regular shocks and vibrations occur. The shelf life is 10 years. Angular misalign ment is only

1/2° versus the NBR and Urethane that are both 1°.

4. BRONZE is not an elastomer, but is one of the options available for those high temperature requirements

(up to 450°F) which most other materials are not capable of. Most commonly, bronze is selected in salt

water/marine applications. It is only to be used for slow speeds, less than 250 RPM, since the coupling will

prematurely wear from metal-to-metal contact otherwise.

5. NYLON is a good electrical insulator, holds up well under heavy continuous loading, and may be substituted

where bronze is too noisy. The torque rating is the same as for Hytrel®.

6. VITON® is a synthetic rubber that has a temperature range of -65°F to 450°F with a durometer of 75-85A

scale. It provides the high temperature capability of bronze with excellent chemical resistance. The torque

rating is the same as for NBR and may be slightly derated depending on the application conditions.

7. ZYTEL® is a fiberglass reinforced compound with excellent resistance to most chemicals and corrosion. It is

three times more torsionally stiff than Hytrel® and can operate in temperatures ranging from -40°F (-40°C) to

300°F (149°C).

Curved Jaw Couplings

While the straight jaw coupling is known around the world, there is also another design that has wide

acceptance, primarily in Europe and Asia. It is generically referred to as the curved jaw coupling. This jaw

coupling product is available in sizes covering bores from 5/32" up through 5-11/16" (145mm) and torque

from 35 in-lbs. up to 66,375 in-lbs. (7,500 Nm).

While the coupling still consists of two hubs and a spider in the center that is under compression, the main

difference is in the geometry of the jaws and the corresponding spider legs. The intermeshing faces of a radial

curvature, giving them a concave or cupped shape. This provides a built-in encapsulation of the spider legs by

the hubs. The corresponding spider legs are crowned, or curved both axially and radially to follow the jaw face

shape, making them similar to a gear tooth in geometry.

The jaw and spider curvature has two important benefits. First, by encapsulating the spider legs, it permits

higher speed ratings compared with similar size straight-sided jaw couplings. It also extends angular

misalignment capacity to 1.3° for some sizes.

Most curved jaw hubs have four jaws vs. three for similar sizes of straight-sided, with the jaws pushed farther

out toward the perimeter of the hub. This enables the spiders to have large open centers. The design

characteristic's combine to allow larger maximum bores in most cases and to accommodate close "BSE"

dimensions.

The curved jaw design also results in some special limitations. Due to encapsulation, radially removable spiders

(wrap around, block) cannot be used. The damping capacity of the design is lessened under greater loads. The

overall length of the coupling is usually greater than the similar straight-sided jaw coupling. The type of

sintered iron commonly used in the smaller sizes is much denser, translating into heavier couplings. And

finally, spacer couplings can only be achieved by using extended hub lengths, this adds a lot of weight and still

does not allow for a true drop-out section.

Spider types

The standard material is urethane for all curved jaw spiders. There are simply three different shore hardness

which yield differing levels of torque capacity. Each of the shore hardness numbers are color-coded for easy

identification, blue for 80-shore, white/yellow for 92-shore, and red for the 98/95-shore. All of the spiders are

an Open Center Type (OCT). The urethane composition allows for a maximum temperature rating of 212°F

(100°C) versus the 160°F upper limit for the L-type urethane. Some manufacturers also offer Hytrel® as an

alternate material as well.

Also available for curvedjaw applications is the No Backlash (NBL) spider. This is simply a special, thicker spider

that can be used with the standard hubs to provide a snugger fit for those low backlash requirements. It only

provides a true zero backlash up to 10% of the rated torque of the spider. It is available in two-shore hardness

(92-yellow and 98-red). Some manufacturers also offer a special hub, often referred to as the "GS" style, for

use with the NBL spiders. The GS hubs are of similar geometry to the standard curved jaw hub except the jaws

are slightly oversized to make the intermeshing of the three components a true interference fit. This style can

either be pre-assembled at the factory or assembled by the user with the aid of a lubricant since the

components are so tightly fitted. It provides full zero backlash performance for motion control applications up

to 10-25% of their rated torque, depending on the size.

Special Considerations for Selection

Because this coupling was designed in Europe, it uses the DIN 740 rating methodology, which gives you a

Nominal (Tkn) as well as Maximum (Tkmax) rating. Nominal torque Tkn is the steady state design torque for

the coupling. Maximum torque Tkmax is a cyclic torque capability for 100,000 cycles or 50,000 reversing

cycles.

In terms of the selection process, it means that the Service Factors are unique for the curved jaw coupling.

There are independent factors which must be multiplied by the nominal torque of the application to arrive at

the design torque. Only when the coupling/spider Tkn and Tkmax ratings are both greater than the respective

nominal and design torque (calculated for the application) do you have the proper size coupling. The urethane

spiders, while rated for a maximum temperature of 212°F, have a de-rating factor that must be applied to their

misalignment capability. This takes effect at any condition above 86°F.

Donut Shaped Elastomeric Couplings

This style of coupling was developed in 1970 for use with diesel engines. The donut shaped elastomeric

coupling consists of a rubber donut fastened with cap screws to hubs. The hubs provide the shaft connection.

The elastomer mounts in between the hubs to transmit the torque and allow misalignment. Metal inserts

(either aluminum or steel) are bonded into the elastomer and provide a durable material through which the

fasteners attach to the hubs. The elastomer donut is precompressed between the fasteners to make certain

that the torque is always transferred in a compression mode. The elastomer is stronger in compression than in

tension. By preloading the donut any tensile forces merely relieve the compression and do not put the unit

into a tensile load-carrying situation. Donuts can have a square, rectangular, octagonal or other cross-section

design. They do not have to be round.

Donut couplings can have one hub that is smaller than the other to fit inside the donut. It is called the

cylindrical hub. The donut is fastened to the inner or cylindrical hub by radial fasteners. The other hub is a

flanged hub to which the donut is attached by axial fasteners. The elastomer uses metal inserts that transfer

torque by friction between the metal inserts and the metal hubs then through the elastomer to the next set of

fasteners attached to the other hub. The torque path alternates from one leg of the donut to the next. The

fasteners are tightened to make a high friction joint and avoid loading the bolts in shear. Donut couplings that

use the cylinder and flange hub system have bore limits on the cylindrical hubs compared to other couplings of

similar torque capabilities.

One way to eliminate the cylindrical hub limitations is to use a wraparound type of elastomer. The inserts are

all devised to use radial cap screws to fasten the elastomeric element to alternating hubs. This one does not

have the flywheel plate option or material options for the element.

Another design has spider shaped hubs with arms that are at the same diameter as the bolt circle within the

donut. Once again the attachment alternates from one hub to the other, but the fasteners are all axial. Torque

is carried by an elastomer in compression and is transferred to the hub via metal tabs inserted in the rubber.

Torque is also transmitted by the friction between the bolt sleeve inserts and the hubs. The torque path is

from hub to insert to elastomer to insert to hub. The elastomer carries the load in compression on alternate

legs. This design is not available with flywheel plates or stiff elastomer materials.

Donut type couplings can handle a load in either direction as the load shifts to alternate legs still in

compression. Even more importantly the donut can accommodate alternating loads and cyclic loads without

backlash. There is windup in elastomeric couplings. These couplings when constructed of rubber exhibit a

quality of hystersis. That quality enables the coupling to dampen the vibration energy that passes through the

coupling.

Elastomers for Donut Type Couplings

The base elastomer is a natural rubber with binders. It is suitable to about 190°F temperature before it loses

strength. When the temperature increases the coupling must be derated. The formulations of this elastomer

are identified by the shore hardness. Each successively harder rubber carries more torque, but is torsionally

less resilient.

Alternate elastomers include Hytrel® and Zytel®. Each is considerably more stiff than rubber. The change in

materials will mean an increase in normal torque capability. The change in material may require the coupling

design to change in order to accommodate the fastening of the Donut coupling with all bolts axial elastomer to

the metal hub.

Pin & Bushing Type Couplings

Pin & Bushing couplings transmit torque through cylindrical or barrel shaped metal pins that are enclosed in

elastomeric bushings. The elastomeric bushing covers one half the pin while the other half has stepped

diameters with a threaded end. The shaft connections are flanged hubs drilled to hold the bushing or the

threaded end. It can be done with the bushings all in one flanged hub or they can be alternated from side to

side. The bushing is inserted into a cylindrical hole while the threaded and stepped end is inserted into a

stepped hole with counter bores on either side. A nut is attached to the threads to hold the bushing in place

during operation.

The elastomers are compressed into the holes and may have a shape that permits easy installation. Elastomers

can be rubber, the original bushing type, Viton®, or urethane type materials. Hubs are cast iron, steel or

stainless steel. Pins are steel or stainless steel. The elastomer cushions shock loads and compensates for

misalignment. Pin and bushing couplings are inherently fail-safe with the pins continuing to transmit torque

when the bushing is worn. The bushing can both wear and fatigue from usage. Pin and bushing couplings are

non-lubricated.

The pin and bushing coupling are high capacity vs. their size. The capacity or torque capability is directly

related to the bolt circle diameter, number of pins, and the type of elastomer. They are designed to make it

easy to replace the pins and bushings which are the wearing components.

2. Shear Loaded Designs

Shear-Type Donut (Sleeve)

The original patent on this design was issued in the late 1950's. The shear type donut coupling (sometimes

called a sleeve type) was marketed heavily to the pump industry, in particular the ANSI chemical process pump

segment. A strong following was built up which continues to this day. Much like a jaw coupling, the shear type

donut is simple in design. A standard coupling is composed of three components, 2 flanges and 1 sleeve. The

sleeve (a short, spool-shaped, tubular element) has serrations molded around the perimeter of each end,

which mate with corresponding serrations molded into both hub flanges. This puts the element in-shear

between the two flanges, so the torque is transmitted through the twisting of the elastomeric sleeve. There

are several features to this coupling which translate into tangible benefits to the user:

• Because this design is double-engagement, it is radially very soft and produces very little reactionary load

on bearings and shafts when misaligned. However, misalignment will shorten sleeve life.

• The torsionally soft design of an in-shear elastomer helps to dampen out most peak overloads and prevent

vibratory torque from going back to the driver.

• The sleeve has a large open center, which allows close positioning of the shafts.

• The torque overload capacity of this coupling is only 3 or 4 times the rated torque (the point at which the

sleeve will tear, round-off the teeth, or "pop out"), versus the 6 or 7 times for a jaw coupling. Thus it provides

the "fusible link" protection characteristic of in-shear couplings.

The shear type donut style coupling is best suited in the following applications:

• Where system alignment may be hard to maintain over a period of time, and the coupling needs to tolerate

the drift.

• Where the motor and pump are on a common base plate but there is no pump mounting bracket involved,

i.e. a "non-piloted" pump application.

• Where shafts are closely coupled (i.e. minimal BE dimension).

• Where shafts are relatively small for the torque loads, or the bearings are light duty.

In general, the shear type donut coupling will work well on electric motor driven applications with uniform

loads such as; centrifugal pumps, blowers and fans, screw compressors, some conveyors, line shafts, and

vacuum pumps. Care should be taken however, that shear type donut couplings are not used under the

following conditions:

• Where loads have high-inertia, especially if they produce variable torque loads, or where overloads/spikes

are expected to be greater than 2X nominal ratings.

• Where reciprocating engines, compressors or pumps are involved. Shear type donut couplings do not

respond well to torsional vibrations. • Where the coupling will operate regularly at less than 25% of its rated

torque. The sleeve teeth will wear prematurely due to the rubbing action against the flange if too lightly

loaded. This can be a concern particularly with the Hytrel® sleeves since they have such high ratings.

There are five manufacturers of this design. All produce their product to be fully interchangeable. However,

serrations in sleeve ends and hub flanges must mate, so components from different manufacturers may not

always fit together properly. This is due to the tolerance that is built into each company's initial design

criterion (i.e. how tight or loose they want the fit between components to be), and the state of wear of the

tooling that produces the sleeves and flanges. Mixing of components from different manufacturers must be

avoided if at all possible.

Flange designs

A. J-type

A basic, economical flange, the J-type is available only in four smaller sizes 3 through 6, with smaller bore

models cast in zinc alloy and larger bore models in cast iron, all limited to the lower torque sleeve materials

(discussed later).

B. S-type

Provides a greater variety of sizes, from 5S to 16S, with all flanges made of cast iron. Characterized by extra

cast-in thickness projecting from the inner face of the hub, which allows greater through-the-bore shaft

engagement, S-type flanges allow larger bores than available with J-type flanges, and can be used with all

sleeve materials.

C. B-type

This flange is modified to accept an industry-standard bushing. Offered in sizes 6B through 16B. The use of a

bushing limits the bore capacity of the coupling, but provides a better grip on the shaft. It can also simplify the

stock room of many users, if they use bushings on other P.T. components. Due to the torque limits of the

bushing, Btype flanges cannot be used with higher torque sleeves.

D. T-type

Similar to the B-type for on industry standard bushing, the T-type is a standard flange modified to

accept another industry style of bushing. There are two ways to mount the bushing to the flange. The first way

is from the serration side (rear) or from the same side of the flange as the shaft is inserted initially (front). As

with the B-flanges, the T-type cannot be used with high torque sleeves due to the limits of the bushing ratings.

E. SC-type

Intended primarily for pump applications, these flanges are separable from their shaft-mounted hubs by

removal of four hex-head cap screws axially installed through each hub. This enables the flange-and-sleeve

assembly to drop out so routine pump maintenance can be performed without disturbing pump or motor

mounting and alignment. Various sizes for Spacer Flanges and Spacer Hubs can be mixed/matched to provide

ANSI standard separations of 3-1/2", 5" and 7", and dozens of other non-standard shaft separations as well, in

coupling sizes 5 through 14. SC type flanges and hubs can also be used in combination with other flanged hubs

to create a half-spacer coupling. Any of the available sleeve materials can be used.

Elastomer (Sleeve) Types

A. Materials

EPDM is the standard material used. It is a rubber-like compound that allows the sleeve to twist as much as 15°

at full torque. It has the highest temperature rating (275°F/135°C) of the sleeves available. It provides good

resistance to most commonly found chemicals and is not affected by dirt or moisture. This sleeve is a dull black

color. Sleeves have angular misalignment capability of 1° and parallel misalignment ranging from .010" (size 3

coupling) to .062" (size 16). Neoprene® sleeves, which also can twist as much as 15° at full torque, offer better

chemical resistance than EPDM, especially to oil, but is rated only for a max. temperature of 200°F(93°C). The

color of the sleeve is black with a shiny finish and a green dot for easy identification. As with EPDM,

Neoprene® sleeves have angular misalignment of 1° with parallel misalignment ranging from .010" up to .062"

.

HYTREL® is a polyester elastomer designed for high torque and excellent chemical resistance. It carries four

times the torque of the EPDM/Neoprene® materials but is limited to ¼° angular misalignment and parallel

misalignment from .010" (size 6) up to .035" for size 14 couplings. It only twists to about 7° at full torque. The

Hytrel® material is orange in color.

B. Sleeve Designs

One-Piece Solid sleeves are identified by material as JE (EPDM), JN (Neoprene®), and H (Hytrel®) types. They

are the least expensive of the rubber sleeves, available in sizes 3 to 10 for JE, and 3 to 8 for JN. For Hytrel®,

they are available in sizes 6 to 12.

One-Piece Split sleeves are identified by material as the JES (EPDM) and JNS (Neoprene®) types. They are used

for applications where the shafts are positioned closely together and the sleeve must be "peeled away" for

replacement. They are available in the same sizes as the JE and JN sleeves.

Two-Piece Split sleeve are made up of two completely separated halves. For the E (EPDM) and N (Neoprene®)

styles, a retaining ring is used to prevent the sleeve from bowing outward or being flung off under speed. The

HS (Hytrel®) is such a rigid material that the ring is not necessary. The E sleeve is available for size 5 - 16

couplings, the N sleeve for sizes 5 - 14, HS for sizes 6 - 14. This design provides the greatest ease of installation

and replacement.

Clamped Elastomer in Shear Couplings

Corded tire types

This design came about in the late 1950's as a solution for dealing with transient torque peaks and shock loads

in diesel-driven pumps. Named for their resemblance to an auto tire, this design consists of two flanged hubs

equipped with clamping plates, which grip the coupling's hollow, ring-shaped element, by its inner rims.

Furthering the similarity, tire coupling elements usually are rubber derivative elastomers with layers of cord,

such as nylon, vulcanized into the tire shape. The coupling transmits torque through the friction of the clamp

applied to the inner rims of the tire and a shearing of the element. Slippage of the coupling may be expected

to occur at about four times the rated torque.

The two significant limitations to the corded tire type coupling are speed and space constraints. As speed

increases, the coupling exerts axial forces on the shafts due to the centrifugal forces working on the elastomer.

And the geometry of the tire itself makes for a large outside diameter for its torque capability. A design

variation includes an inverted tire coupling in which the tire element arcs inward toward the axis, thus

overcoming the centrifugal forces at speed. This affords 10-30% higher RPM service, depending on its size.

The corded tire coupling is torsionally soft and can dampen vibration. High radial softness accommodates

angular misalignment up to 4° and parallel offset up to 1/8". Rare among elastomeric couplings is its capability

to allow a certain amount of axial shaft movement. These properties give corded tire designs a wide variety of

applications including those driven by internal combustion engines. This coupling is offered in spacer designs

as well as with hubs which can accept bushings.

Bonded Urethane Tire

This design was first marketed in the 1970's and has found success primarily in the process pump industry

because of several features that the corded tire lacks. The design utilizes a urethane material that is bonded to

two half-circle metal rings (a.k.a. "shoes") which are then bolted to the two hubs. Torque is transmitted from

the hubs through the shoes/bonded joint and then the shear-plane of the split urethane tire.

The design offers advantages such as radial removal of the element halves, high angular misalignment

capability (4°), and shock load cushioning. In its standard close coupled configuration, it can span greater BSE

lengths than most in-compression couplings, and it also has the large opening in the center of the tire to allow

complete flexibility in positioning shaft ends. The outside diameter (OD) of this design is also smaller than the

Corded Tire type for similar shaft and torque capacities.

Spacer couplings are achieved by using the same shaft hubs and simply extending the lengths of the steel

shoes onto which the elastomer is bonded. Hubs can also be reversed in their mounting orientation to further

add to the BSE permutations possible. Bushings are also commonly used on this style of coupling. A heavy duty

elastomer option (25% more torque) is available, but it reduces the misalignment capacity by 50%.

It has proven to be ideal in applications such as pumps, screw compressors, blowers, mixers, crushers, and

general power transmission drives.

Limitations of the urethane tire type include the large number of fasteners required for installation and

removal of the elastomer, and the fatigue of the element and the bond between steel and elastomer under

torsional vibrations.

3. Combination Shear & Compression Loaded Designs

Jaw with Elastomer In-Shear

Another design of elastomeric jaw coupling completely changes the way the jaw coupling functions. Instead of

the jaws of the hubs interlocking, the use of an in-shear spider pushes the hubs apart and aligns the jaws of

each hub along the same axial plane. Thein-shear spider then is twice the axial width of a standard spider, and

it is loaded in shear rather than in compression. This spider provides certain features different from common

jaw couplings.

• Radial removable spider

• In-Shear design for non-failsafe operation

• No metal-to-metal contact should the elastomer fail

• No need for tools to install or replace the elastomer

• Non-lubrication benefit of an elastomeric coupling

• 2° angular misalignment

A floating ring encases the outside of the spider and locks into special grooves on its OD. There are several

designs on the market, with only one manufacturer offering the benefit that this special in-shear spider is used

with regular jaw coupling hubs, same as for the in compression design. Urethane is the most common

elastomer material available. It has a combination of durability, chemical resistance, and torque/load carrying

strengths.

This coupling should only be selected for electric motor driven applications. The most common ones include

centrifugal pumps, fans, mixers, gearboxes, and plastic extruding machines.

Torque ratings and service factors are unique for this version of a jaw coupling.

Gear with Elastomer

This is a gear coupling like the continuous sleeve gear coupling, except the sleeve is made from a slippery

elastomeric material. The advantage of this type of sleeve material is the no lubrication feature. They are

limited in torque, speed and size. The limits are imposed because the hub tooth to sleeve tooth friction

eventually exceeds the elastomers inherent lubrication capability.

The coupling consists of a molded nylon continuous sleeve with internal gear teeth that match gear teeth on

the periphery of a metal hub. The metal hubs are made from steel bar stock or powdered metal. The

combination of molded sleeves and powdered metal pressed hubs are a very economical coupling

combination. The hubs are held in the sleeve by spiral rings. The hubs are mounted on the shaft with the

traditional clearance fit key, set screw or a clamped type split hub. The nylon sleeve has high torsional stiffness

and is resistant to chemical attack. The hub tooth is crowned to obtain the misalignment capability. Backlash is

designed to be at a minimum in this style of coupling. Misalignment capability varies from 1° to as much as 5°

depending on the manufacturer.

Nylon sleeve couplings are used for motor/generator sets, pump sets and other light to medium duty industrial

applications. Often they are used on the front power take-off of internal combustion engines because they are

small and lightweight. The coupling configuration permits vertical and blind assembly when needed. Speed

capabilities under light loads can reach 5000 RPM, misalignment to 5° and ambient temperature to 150 °F. The

bore capability is usually less than 2 inches for the popular sizes, however like all clearance fit couplings some

versions are available to 4 inches bore. For torque capabilities refer to the manufacturers catalogs as the

torque is tied to the speed rating as much as to the physical properties of the coupling.

4. Torsional Couplings

Several designs of couplings were developed to solve the problem of damping torsional vibration. The primary

source of those vibrations are diesel engines, but there could be other sources. The torsional vibration travels

through the coupling to the connected equipment. The vibrations can damage both the connected equipment

and the coupling itself. A discussion of torsional systems and vibrations are included in the Applications section

of this handbook.

Couplings

The primary torsional coupling uses a resilient elastomer as the flexing medium. All of the couplings described

in the elastomeric section of the handbook have been used on torsional service with varying degrees of

success. The elastomeric types described in the following section are the couplings with the best attributes for

torsional service.

The elastomer shape used in the coupling is very important for damping and damping is an important attribute

for the torsional coupling. The most successful shapes are radially loaded cylinders, toruses, and spheres. In

addition, the thickness may be much larger than found in conventional elastomeric couplings. Sharp corners

are usually avoided in the torsional designs to reduce stress concentrations.

There are also non-resilient elastomers used for torsional couplings. Non resilient couplings or stiff torsional

couplings are used for low inertia, diesel driven equipment.

Some metallic couplings were designed with the diesel in mind. One notable one is the grid coupling described

in its own section of this handbook.

The resiliency and the torsional softness of the coupling are used to judge the coupling's ability in torsional

systems. Torsionally soft couplings are those units that have a ratio of dynamic torsional stiffness to nominal

torque of less than 30.

Donut Shaped Elastomeric Couplings

The donut shaped elastomeric coupling consists of a rubber donut fastened with cap screws to hubs. The hubs

provide the shaft connection, the elastomer mounts in between the hubs to transmit the torque and allow

misalignment. Metal inserts (either aluminum or steel) are bonded into the elastomer and provide a durable

material through which the fasteners attach to the hubs. The elastomer donut is precompressed between the

fasteners to make certain that the torque is always transferred in a compression mode. The elastomer is

stronger in compression than in tension. By preloading the donut, any tensile forces merely relieves the

compression and does not put the unit into a tensile load-carrying situation. Donuts can have a square,

rectangular, octagonal or other cross-section design. They do not have to be round.

Donut couplings have one hub that is smaller than the other to fit inside the donut. It is called the cylindrical

hub. The donut is fastened to the inner or cylindrical hub by radial fasteners. The other hub is a flanged hub to

which the donut is attached by axial fasteners. The elastomer uses metal inserts that transfer torque by

friction between the metal inserts and the metal hubs then through the elastomer to the next set of fasteners

attached to the other hub. The torque path alternates from one leg of the donut to the next. The fasteners are

tightened so make a high friction joint to avoid loading the bolts in shear. Donut couplings that use the cylinder

and flange hub system have bore limits on the cylindrical hubs compared to other couplings of similar torque

capabilities.

The donut style elastomeric coupling is primarily used on torsional damping and tuning systems associated

with Diesel drivers. In such a case a flywheel plate replaces the flanged hub. The flywheel plate is drilled to

match various SAE designated or DIN designated flywheel dimensions. The coupling is configured to dissipate

heat that is generated by hysteresis. It is also rated for a maximum torque, a nominal torque, and a vibratory

torque. Each of the values are different, the maximum torque is limited to a specific number of cycles.

Donut type couplings can handle a load in either direction as the load shifts to alternate legs still in

compression. The donut can accommodate alternating loads and cyclic loads without backlash. There is

windup in elastomeric couplings. These couplings when constructed of rubber exhibit a quality of hystersis.

That quality enables the coupling to dampen the vibration energy that passes through the coupling.

Elastomers for Donut Type couplings

The base elastomer is a natural rubber with binders. It is suitable to about 190°F temperature before it loses

strength. When the temperature increases the coupling must be derated. The formulations of this elastomer

are identified by the shore hardness. Each successively harder rubber carries more torque, but is torsionally

less resilient. The variations allow the application engineer to tune the system for critical speed as well as

torsional vibration damping. Hystersis, a characteristic exhibited by rubber with binders, allows the

elastomeric material to adsorb dynamic energy. The energy is in turn is lost in heat generation.

If the material is able to radiate or otherwise conduct the heat to a sink, damping will occur without damage to

the coupling elastomer. If the heat builds up in the elastomeric element it will fail or melt down.

Alternate elastomers include Hytrel® and Zytel®. Each is considerably more stiff than rubber. The change in

material may require the coupling design to change to accommodate the fastening of the elastomer to the

metal hub. The increase in stiffness changes the unit from torsionally soft to torsionally stiff, and as a result the

tuned critical moves from a value below operating speed to one above operating speed. The change in

materials will mean an increase in normal torque capability. Refer to the chapter on torsional applications for

more information on critical speeds and damping requirements.

Elastomer Block Compression Couplings

This type of coupling is similar to the jaw coupling in that torque is transferred from one hub to the other by

compressing captured rubber blocks. In this case the hubs consist of an external claw hub matched to an

internal pocket hub that contains the elastomer. There are several varieties of these couplings.

Variations include the shape of the elastomer blocks and the type of elastomer. The ideal shape for the

elastomer is a cylinder, loaded radially. Alternatives use rounded-off rectangular shapes. The coupling is used

for both shaft-to-shaft connections as well as shaft-to-flywheel connections. A popular application for this

coupling is the diesel driven generator. Another common application is the synchronous motor driven

compressor. Both of these applications are very high horsepower units. An example of a lower horsepower

application is the electric motor driven reciprocating compressor.

This style of torsional coupling is manufactured with OD's of 3 inches to several feet. Obviously the large size

carries a very high torque that is associated with the large generator sets and ship propulsion. The hubs can be

a casting of iron or bar stock or a forging of steel. Torsional stiffness ratio for these units is in the medium

range, which is consistent with the application requirements. A very soft unit would either not have the torque

capability or would have to be dimensionally too big to get the torque capability.

Bonded Elastomer in Shear Couplings

This coupling was developed in 1980 for diesel flywheel applications. There are two basic types and both use

an elastomer element in-shear. These couplings have a very low torsional stiffness ratio, in the range of 1.5 to

12. The normal torque capability of these couplings range from 900 inch-pounds to more than 1,000,000 inch-

pounds depending on the size and type. They are used with the largest of diesel engines and small ones when

extremely low torsional stiffness is needed. The couplings can be configured with elements in series to reduce

the torsional stiffness even more, or in parallel to increase the torque capabilities. As with other rubber

couplings there are several elastomer variations that are identified by the shore hardness. The higher the

"shore" number ithe stiffer the torsionaly coupling is.

The first type of bonded elastomer in-shear coupling is a rubber disk bonded to an inner (or driven) metal ring.

The inner ring can be combined with various hub types for fastening to a shaft. The types include tapered OD

split hubs, bolted straight bore cylindrical hubs, and special hubs for connecting to U-Joint shafting. The outer

diameter of the rubber disk is an external toothed form that slides into a circular metal ring with internal

teeth. The circular metal ring is cast aluminum to keep the weight, and therefore the inertia, low. The outer

ring OD is configured to bolt to a diesel engine flywheel.

The rubber disk has a designed shape to ensure that equal stress occurs over most of its section, thus

providing a large torsional angle and avoiding high stress in these areas. Loading at the inner ring and outer

teeth is reduced below normally accepted levels by the design of those two areas. The load is carried in shear

from its periphery to its center. This style has a non-linear torsional stiffness. The element could have some

backlash in the tooth form at the periphery, although normallyit it is a tight fit. The tooth area becomes a wear

point when the coupling is misaligned. Coupling life therefore, is dependant on the wear as well as the

torsional loading cycles.

The second type of bonder elastomer in-shear is a four-sided closed ring of elastomer with a special cross

sectional shape. The OD, the ID, and one side are flat and perpendicular to each other, the fourth side is

tapered from OD to ID in a conical shape. The torque load is carried from one side to the other via shear

forces. Both the OD and the thickness from side to side determine torque capability. The elastomer is bonded

to metal plates on each side. The plate on the flat side is configured to attach to a flywheel adapter or a shaft

hub disk at the OD. The plate on the other side matches the conical shape and is configured to bolt to coupling

hubs or half couplings at the ID. The ID may include plain bearings to carry minor radial and axial loads.

There is a wide variety of secondary couplings that are bolted to the side opposite the flywheel. They include

gear coupling halves, link couplings, and disc plates. The secondary couplings provide misalignment capabilities

not available from the primary torsional coupling. Cardan shaft adapters and clutches have also been attached

to the coupling.

The rubber element again has a designed shape to provide for equal stresses across the element. The element

has a linear torsional stiffness. There is no backlash in this style of element, however there is torsional windup.

This coupling is a non-wear configuration and coupling life is dependent on the torsional damping and

maximum load cycles.

Torsionally Stiff Couplings (Flywheel)

While torsional softness can be a benefit for elastomeric couplings, there are some applications that require

stiff elastomers. Most of the elastomeric coupling types have an alternative stiff elastomeric material. Jaw

coupling, donut shaped compression loaded, and unclamped donut in shear are sometimes supplied with

Hytrel® or other stiff elastomers such as Zytel® or urethane. The stiff elastomer is used for greater torque

capability without going to a larger size. Stiff elastomers have less resilience and may restrict the angular

misalignment capability to much lower values.

Many times the switch to a torsionally stiffer elastomer is to tune the torsional system to a higher natural

frequency. This is done on some diesel driven systems with light inertia loading. One example is a diesel driven

hydraulic pump for off-highway equipment. Torsionally stiff couplings for these applications are a significant

coupling need. The couplings are designed for attachment from a flywheel to a driven shaft. The couplings can

be of the compression type as described under the "Donut Shaped Elastomeric Coupling" section or can be a

stiff elastomeric disc loaded in shear.

The stiff elastomeric disk, loaded in shear, has a torque capability up to 21,240 inch pounds. The torsional

stiffness ratio is above 100. Normally this is a high volume molded disk to make an economical coupling for

small diesel production engines.

Shear loaded disks are molded of Zytel® or nylon with strengthening fibers. The disk is designed with boltholes

on the periphery to match a flywheel-drilling pattern. The ID is designed to mate with a coupling hub in a

sliding fit. The coupling hub can have typical gear coupling teeth with crowning or can have four to six crowned

dogs. The crowning accommodates a limited amount of angular misalignment while transferring torque from

the element to the hub. Hubs are made from steel bar stock or from powdered metal. The hub bore is usually

a spline to match a standard hydraulic pump shaft, but could be a straight bore with key.

The torsionally stiff coupling for flywheel applications is designed for blind assembly. Having the shaft hub slide

into the flywheel attached elastomer does this. This coupling type is often supplied with pump mounting

plates and flywheel enclosures.