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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.