- Pumps & Systems, April 2008
Many pump couplings are designed to be a fuse that fails before major damage occurs to expensive pump systems. Premature wear or failure is typically the first symptom of a problem. The most frequently encountered causes of coupling problems are:
- Misalignment
- Improper fit or assembly
- Overload
- Abnormal environmental conditions
- Torsional Vibrations
Misalignment
Volumes have been written about misalignment, so we will highlight some of the most frequent causes. Misalignments between the motor and pump are unavoidable and are often created during the unit's assembly. Poor initial alignment (for example, using a straightedge), distortion from pipe strain or an improper/weak base support are typical causes of misalignment. Even if both components are properly aligned, subsequent misalignment may occur from extreme temperature variations during operation.
To compensate for these errors, engineers and maintenance technicians use a flexible coupling that permits some axial, radial and angular misalignments. This shaft coupling range meets special requirements that the manufacturer and end user must consider when selecting the proper coupling.
A prime example of these special requirements is hot medium pumps where extreme temperature variation causes thermal growth and resulting shaft displacements unsuitable for single-cardanic or single flexible insert designs. In these applications, excessive shaft misalignment reduces the service life of the coupling, bearings and/or seal systems. A double-cardanic, or two flexible inserts with a spacer design, can compensate for excessive misalignment (see Figures 1 and 2).
Misalignment indicators:
- Changes in the running noises and/or vibration
- Premature wear of the coupling insert or cracking of disk packs
- Shaft seal leakage
- Premature bearing failure
Recommended solution:
- Shut down the unit
- Check for misalignment issues, i.e., loose baseplate bolts, a broken mounting foot, heat expansion of the components or change in the assembly gap of the coupling
- Check the coupling insert for wear, and replace if necessary
- Consider using a double-cardanic spacer coupling
Improper Fit or Assembly
There is no doubt that a properly selected shaft coupling extends the service life cycle of a pump system. However, service life cycle costs should also be considered.
Taper lock bushings may seem the preferred choice from a stocking and maintenance standpoint. However, be aware that the design is not failsafe. Taper lock bushings require strict assembly procedures, or failure is possible. A clamping hub or split hub design may be more reliable and easier to assemble.
The purchase price of a single-cardanic coupling is less than a double-cardanic design. However, if purchase price is the sole determining factor in selection, service costs may increase significantly. Restoring forces are lower on double-cardanic couplings, so the coupling inserts and mating components (for example, bearings and seal systems) are replaced less frequently, reducing service costs. A split hub design eases assembly and disassembly of a double-cardanic coupling.
Maintenance cost savings increase exponentially when a split hub design is used. In many cases, a split hub design does not require additional assembly and disassembly of the coupling since the coupling can dismount from the shaft without moving any equipment (see Figure 2).
Improper fit or assembly indicators:
- Movement of the hub on the shaft
- Loosening of the coupling's fastener to the shaft
- Changes in the assembly gap between hubs of the coupling
- For disk couplings, check for loose disk pack bolts and/or nuts
- Damaged or broken components
Recommended solution:
- Shut down the unit
- Confirm shaft sizes and tolerances for fit
- Tighten all component fasteners to the appropriate tightening torques. For added safety, use a thread locking device or liquid hardener to prevent bolts from backing out.
Overload
During preliminary coupling selection, overload conditions that can cause premature coupling failure are often overlooked. Safety factors are generally selected based on normal operating conditions. Sizing the coupling for peak motor starting torques (in addition to considering the inertias of the pump rotors or the drive train of piston pumps) could avert a failure. When selecting the coupling, thoroughly review the peak power the pump will experience during abnormal conditions, i.e., start-up, shutdown, upsets, frequent start/stops, etc. Evaluating the extremes of the driver and the driven can help pinpoint potential problems before the system is assembled.
For applications demanding extreme availability (such as refining applications), a proactive way to prevent trouble is to monitor a coupling's performance with a torque-measuring or infrared device for identifying abnormal conditions in critical pump systems. These tools can be used to detect abnormal torque or wear conditions, even alerting the operator when to change an insert (see Figure 3).
Abnormal Environmental Conditions
It is important to identify high ambient temperatures that may occur at the coupling. Sometimes heat can even travel through a shaft and cause coupling failure. Disc couplings are often the best solution with their all-steel design. Jaw-coupling inserts are also available in alternate materials that can withstand temperatures up to 480-deg F. There are also insert materials that are more resistant to abrasives such as sands or chemical attacks (see Figure 4).
Abnormal environmental indicators:
- For couplings with elastomeric inserts, look for premature wear or deterioration of the coupling insert
Recommended solution:
- Confirm the ambient temperature to the insert chosen and replace if necessary
Torsional Vibration
One of the hidden causes of failure a coupling may see is torsional vibration, often mistakenly identified as a "coupling problem." Remember that a coupling is a fuse that typically indicates a problem exists somewhere in the system. If coupling size is simply increased until the coupling failure goes away, torsional vibration could remain in the system, fail the next weak link and result in higher repair costs.
Torsional problem indicators:
- Couplings with elastomeric inserts
- Disk couplings:
- Look for cracking or breaks in the individual disk packs or bolts
To resolve this issue, most coupling manufacturers can evaluate the system using a torsional vibration analysis to identify at what speed the resonance occurs. They can then recommend a running speed and/or a coupling with the proper amount of stiffness to move the resonance out of the RPM operating range of the unit.
Recommended solution:
- Shut down the unit
- Check all the coupling components for damage or wear and replace if necessary
- If torsional vibration is apparent, have the coupling manufacturer conduct a torsional vibration analysis
- Tighten all coupling fasteners to the appropriate tightening torques
- Adjust the RPM setting to operate outside the resonance zone
Conclusion
In the pump industry, the coupling is often one of the last items selected. Couplings are often considered a "commodity item," and the objective is to find the lowest cost, quickly delivered coupling that can transmit the required torque. Often the pump manufacturer's general purchasing agent may be responsible for coupling selection. This is a difficult mindset to change until problems arise in the pump system, and selecting the next size coupling is not always a viable solution.
Selecting a coupling for worst case operating conditions can save money, headaches and downtime. Nominal torques, peak torques, engine torque curves, misalignment, heat transfer, torsional vibration, transmittable torques, key stress, shaft tolerances, assembly and maintenance should all be factored into coupling selection.
Selecting the right coupling will help improve a pump system's efficiency and reduce overall downtime.
