The Coupling Handbook: Part II

First Steps in Coupling Selection - Types, Considerations, and Charts

Selecting the right coupling is a complex task because operating conditions can vary widely among applications. Primary factors that will affect the type and size of coupling used for an application include, but are not limited to: horsepower, torque, speed (RPM), shaft sizes, environment conditions, type of prime mover, load characteristics of the driven equipment, space limitations and maintenance and installation requirements. Secondary but possible essential factors can include starts/stops and reversing requirements, shaft fits, probable misalignment conditions, axial movement, balancing requirements or conditions peculiar to certain industries.

Because all couplings have a broad band of speed, torque, and shaft size capabilities, those criteria are not the best place to start. First, determine what attributes beyond those basic criteria will be required for your application. If none stand out then simply choose the lowest cost that fits those basics. Almost always, though, there will be other considerations that will narrow your alternatives down to certain types of couplings.

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

A. Types of Flexible Couplings

Many types of flexible couplings exist because they all serve different purposes. All types, however, fall into one of two broad categories, Elastomeric and Metallic. The full range of coupling types in both categories, and the special functions of each, will be discussed thoroughly in later chapters. The key advantages and limitations of both categories are briefly contrasted here to demonstrate how they can influence coupling selection.

1. Elastomeric

Couplings in this category include all designs that use a non-metallic element within the coupling, through which the power is transmitted. The element is to some degree resilient (rubber or plastic). Elastomeric couplings can be further classified as types with elastomers in compression or shear. Some may have an elastomer that is in combined compression and shear, or even in tension, but for simplification they are classified as compression or shear, depending on which is the principle load on the elastomer. Compression types include jaw, donut, and pin & bushing, while shear types include tire, sleeve, and molded elements.

There are two basic failure modes for elastomeric couplings. They can break down due to fatigue from cyclic loading when hysteresis (internal heat buildup in the elastomer) exceeds its limits. That can occur from either misalignment or torque beyond its capacity. They also can break down from environmental factors such as high ambient temperatures, ultraviolet light or chemical contamination. Also keep in mind that all elastomers have a limited shelf life and would require replacement at some point even if these failure conditions were not present.

Advantages of Elastomeric Type Couplings
• Torsionally soft
• No lubrication or maintenance
• Good vibration damping and shock absorbing qualities
• Field replaceable elastomers
• Usually less expensive than metallic couplings that have the same bore capacity
• Lower reactionary loads on bearings
• More misalignment allowable than most metallic types

Limitations of Elastomeric Type Couplings
• Sensitive to chemicals and high temperatures
• Usually not torsionally stiff enough for positive displacement
• Larger in outside diameter than metallic coupling with same torque capacity (i.e. lower power density)
• Difficult to balance as an assembly
• Some types do not have good overload torque capacity

2. Metallic

This type has no elastomeric element to transmit the torque. Their flexibility is gained through either loose fitting parts which roll or slide against one another (gear, grid, chain) -sometimes referred to as "mechanical flexing"-- or through flexing/bending of a membrane (disc, flex link, diaphragm, beam, bellows).  

Those with moving parts generally are less expensive, but need to be lubricated and maintained. Their primary cause of failure is wear, so overloads generally shorten their life through increased wear rather than sudden failure. Membrane types generally are more expensive, need no lubrication and little maintenance, but their primary cause of failure is fatigue, so they can fail quickly in a short cycle fatigue if overloaded. If kept within their load ratings, they can be very long-lived, perhaps outlasting their connected equipment.

Advantages of Metallic Type Couplings
• Torsionally stiff
• Good high temperature capability
• Good chemical resistance with proper materials selection
• High torque in a small package (i.e. high power density)
• High speed and large shaft capability
• Available in stainless steel
• Zero backlash in many types
• Relatively low cost per unit of torque transmitted

Limitations of Metallic Type Couplings
• Fatigue or wear plays a major role in failure
• May need lubrication
• Often many parts to assemble
• Most need very careful alignment
• Usually cannot damp vibration or absorb shock.
• High electrical conductivity, unless modified with insulators

B. Application Considerations

Sometimes selection of coupling type is guided by application, falling into one of five categories; General-Purpose Industrial, Specific-Purpose Industrial, High-Speed, Motion Control and Torsional. In each of these application categories there would be elastomeric, metallic membrane flexing, and mechanical flexing types.

Once the coupling type is selected, there may be variations to consider within that type. For example, gear couplings offer a wide variety of configurations to combine coupling functions with other power train requirements, such as shear pin protection or braking. It is always a good idea to understand as much as possible about the two pieces of equipment to be connected. Let the driven equipment and the driver dictate the needs of the coupling. For example, is there a shock load or a cyclic requirement that may lead to an elastomeric coupling? If low speed and high torque are involved, that means a gear coupling is likely best suited. High-speed machinery will lead to a disc or diaphragm coupling. Diesel drivers need the benefits of torsional couplings for best results. If the equipment is susceptible to peaks or transients, the application may want high service factor or a detailed analysis of the coupling torque capabilities. That brings us to the list of requirements that will impact the coupling selection.

The charts below will help provide the path among all the couplings for most types of rotating equipment. The charts are organized into three sections. The first is a list of "Information Required" for the best possible selection of a coupling. It reflects the selection process used by the OEM equipment designer, the engineer/contractor, the coupling specifier, or the trouble-shooter. For other situations, short cuts are sometimes taken towards the conservative side. The second is a chart of "Coupling Evaluation Characteristics" such as torque, bore and misalignment. The third is the chart showing "Coupling Functional Capabilities”. They are the attributes of the various couplings that go beyond the numerical information.

C. Coupling Evaluation Charts

Information Required
1. Horsepower
2. Operating speed
3. Hub to shaft connection
4. Torque
5. Angular misalignment
6. Offset misalignment
7. Axial travel
8. Ambient temperature
9. Potential excitation or critical frequencies (Torsional, Axial, Lateral)
10. Space limitations
11. Limitation on coupling generated forces (Axial, Moments, Unbalance)
12. Any other unusual condition or requirements or coupling characteristics.

The first seven items of the list above will allow a coupling selection if a service factor is used. The risk of relying on service factors is the possibility of ending up with an oversized coupling or one that is missing an essential feature. All the remaining information, where applicable, allows the coupling to be fine-tuned for the application.

Some types of couplings designed to do a specific job will have a further list of needed information. For example, a slider coupling has to have the sliding distance and the minimum and maximum BSE dimension.
Note: Information supplied should include all operating or characteristic values of connected equipment for minimum, normal, steady-state, transient, and peak levels, plus the frequency of their occurrence.

Information Required for Cylindrical Bores
1. Size of bore including tolerance or size of shaft and amount of clearance or interference required
2. Length
3. Taper shaft (Amount of taper, Position and size of o-ring grooves if required, Size and location of oil distribution grooves, Max. pressure available for mounting, Amount of hub draw-up required, Hub OD requirements, Torque capacity required)
4. Minimum strength of hub material or its hardness
5. If keyways in shaft (How many, Size and tolerance, Radius required in keyway, Location tolerance of keyway respective to bore and other keyways)

Types of Interface Information Required for Bolted Joints
1. Diameter of bolt circle and true location
2. Number and size of bolt holes
3. Size, grade and types of bolts required
4. Thickness of web and flanges
5. Pilot dimensions
6. Other

Once past the charts that follow, one can go directly to the manufacturers catalog, or can read on to learn more about specific couplings and the other important coupling issues.

Chart 1: General Coupling Evaluation Factors
Chart 2: General Functional Capability Chart
Chart 3: Lovejoy Specific Preselection Guide

Go To Next Section - Part 3: Popular Elastomeric Coupling Types - Compression Loaded, Shear Loaded, Combination, and Torsional
Go Back To Handbook Index

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