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12/5/2014 Selecting the Proper Coupling | Pumps & Systems Magazine
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Written by: Andrew Svabas and Chris Scholz, KTR Corp.
Pumps and Systems, June 2009
Couplings are often forgotten until a project is nearing its end. With time running
out, users often purchase whatever a supplier has in stock instead of the best
solution for the system. Understanding the application and requirements for
coupling selection allows the user to select the best coupling solution.
Knowing the application details is one of the most critical and overlooked factors in
proper coupling selection. There are more factors to consider than just the obvious
torque and shaft size issues. For instance, exposure to shock-loading, temperature
and shear or failsafe features are often overlooked and may result in coupling
failure, damage to machines or danger to people. The chosen coupling should fit
all of the application issues.
Although several different coupling types may be used for a given application, each type can differ in cost, complexity
and performance. When selecting a coupling, first determine the space in which the coupling will be installed to
ensure the product will fit. Installing and maintaining the coupling is also critical.
Some coupling designs allow installation and maintenance without moving the surrounding equipment. These "drop-
in/drop-out designs" are often used on large, heavy equipment for ease of maintenance. Some couplings also require
periodic maintenance. Maintenance-free, non-lubricated designs are preferred and fit into most applications.
Although many coupling applications seem simple, errors can occur in the selection process. Here is a recommended
checklist to make sure the important points are covered.
1. Check the Application's RequirementsTorque is the most basic prerequisite for selecting a coupling. Whether it is an electric motor, combustion engine,
compressor or even a turbine, the driver must be understood as power, speed and torque. Torque = (Constant x HP) /
RPM. When selecting a coupling, always use the coupling's nominal torque rating. Maximum torque ratings are only
used to absorb peak torques in the system.
Operating environment is also important when selecting a coupling. Variables like chemical compatibility, ambient
temperature and frequency of operation (starts/stops) should also be considered. Added safety factor can compensate
for the durability and stresses applied to coupling components during operation if there will be excessive stopping
torques (i.e., a flywheel effect). Safety factor is additional tolerance that is added to the nominal torque requirement of
the coupling to build safety into the system and is usually driven by temperature, frequency and shock loads.
Other important application factors are typically physical dimensions, starting method and system alignment.
2. Understand the Coupling Design NeededUnderstanding coupling designs and limitations can help the design engineer make a final selection. Every coupling
has its limitations, which is why various coupling types are offered to fit industry applications.
For example, if an application has high misalignments, then a flexible coupling would be required to compensate for
and absorb the anticipated misalignments without risking performance issues. Each coupling has permissible
misalignment ratings that can compensate for difficult applications that cannot be fully aligned 100 percent. Motor and
pump shaft "end play" is crucial and should not exceed the permissible axial misalignment rating of the chosen
coupling. The fluctuation of axial misalignment can gradually fatigue the coupling and result in premature failure.
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Selecting the Proper Coupling
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12/5/2014 Selecting the Proper Coupling | Pumps & Systems Magazine
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In vertical applications, evaluation is needed to ensure that the coupling is capable of supporting vertical weight.
Special inserts can be designed to support such loading and prevent excessive compressive forces on the coupling.
Distance Between Shaft Ends (DBSE) is the gap between the drive and driven shafts. Coupling spacers not only
bridge larger DBSE gaps, but can also add misalignment capacity to the coupling.
Proper shaft engagement into the coupling hub is also critical whether it be a keyless frictional connection or keyed
connection. Premature shearing of keys or hub slippage on keyless shafts can cause extensive damage and costly re-
work.
Choose a coupling with caution when using variable frequency drives. Checking maximum and minimum speed
ratings is critical to ensure proper performance and longevity of the coupling.
3. Check Proper Coupling Design with Applicable CertificationsSome disk couplings are specifically designed to meet API 610 or 671 standards. For example, a double-cardanic,
dynamically balanced steel disk coupling ensures low restoring forces in case of misalignments, which extends the
service life of the motor/pump bearings and seals. Subject to their design, most all-steel couplings can be used on
drives with extreme temperatures.
In some cases, an ATEX "explosion proof" certification may be required. This standard certifies that if a disc pack or
similar coupling element were to fail under catastrophic overload, sparks would not occur and an ignition in an
explosive environment would be prevented. Europe has developed the ATEX standard for equipment in explosive
environments, and these parameters should be reviewed in certain applications. For a dusty atmosphere, the
maximum surface temperature must also be considered according to the ATEX standard.
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