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
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.
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
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.
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
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
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
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.
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.
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 oilschemicals 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.
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
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).
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.
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.
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.
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