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 Requirements
Torque 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 Needed
Understanding 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.
Figure 1: Magnetic couplings with ceramic containment shrouds
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 Certifications
Some 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.
4. Check for Any Special Requirements
Extended shafts, large DBSE, non-standard bores/keyways, splines, tapers and hydraulic-fit mounting can all affect coupling selection. Certain applications, including food grade, offshore corrosive environments and the petrochemical industry, require the coupling to be corrosion proof.
Also consider performance properties like failsafe couplings that need to continue driving the application after a catastrophic failure or non-failsafe couplings that need to stop driving.
5. Select a Quality Coupling Manufacturer Who Makes the Right Products
Request company catalogs or research websites to learn about the products a company offers. Engineering support along with installation instructions and drawings should be available if needed. Always follow the selection procedures that each coupling manufacturer outlines. Coupling suppliers can answer additional questions over the phone.
Some companies will perform complimentary torsional vibration analysis to ensure that the coupling will withstand normal system operation. This is a great option for people with critical applications or OEM machine builders and is important because the moment of inertia of the driver and driven component can create torsional vibration. In these cases, a simple calculation can select the correct coupling that moves the critical resonance out of the operating range of the application.
Figure 2: Split hubs permit easier removal of the coupling from the shaft without moving the driver or driven equipment.
6. Find a Supplier Who Can Deliver the Product in a Reasonable Time and Cost
Select a supplier with a broad base of knowledge and experience with coupling applications and the applicable technical issues. Start the selection process as early as possible to allow ample time to enlist technical support from the manufacturer, if required. This will ensure that the final choice is the best coupling solution for the application and is received onsite within the required timeframe.
Selecting the correct coupling can be challenging. With the correct information and a basic understanding of the issues involved, this complex task can be reasonably managed.