The Coupling Handbook: Part I

Introduction - Why a flexible coupling?, Basic Terminology, and Evaluation Factors

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.

Go To Next Section - Part 2: First Steps in Coupling Selection - Types, Considerations, and Charts
Go Back To Handbook Index

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