What is the problem with flexible couplings?

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Flexible shaft couplings are straightforward and relatively affordable devices that are easy to install and maintain. However, many of them face premature failures. These issues occur despite their design to accommodate slight misalignment and tough operating conditions such as temperature variations, humidity, and heavy loads. Below, we discuss several common pitfalls encountered during the assembly and maintenance of flexible couplings.

1. Excessive Misalignment Couplings are manufactured to allow a degree of misalignment and flex, but severe misalignment of the connected shafts is among the primary causes of failure. Often, a technician may skip the alignment check before installing a new coupling, assuming that if it fits, the alignment must be correct. Sometimes they Are not even aware that alignment is necessary. A clear indication of misalignment can be dust accumulation under the coupling. As a preventive measure, ensure alignment is verified before the coupling installation. Utilizing laser alignment tools can make this process fast and precise.
2. Neglecting Manufacturer Instructions It is not uncommon for mechanics to proceed with assembling couplings without consulting the manufacturer’s guidelines, leading to early failures. The saying "more haste, less speed" rings particularly true here; technicians are encouraged to take the time to properly reference and adhere to the specific installation instructions provided by the manufacturer.
3. Wrong Specifications Not all flexible couplings are created equal. Various types exist for optimal power transmission, and selecting the right one is essential. Unfortunately, this step is often overlooked, resulting in premature failures. It is advisable to keep an adequate inventory of suitable couplings and ensure that the maintenance team is familiar with their specifications. If frequent failures occur despite proper alignment, it is wise to document system conditions at those moments and assess whether a more suitable coupling is necessary based on the machine's load, torque, temperature, and horsepower.
4. Inadequate Lubrication Always adhere to the manufacturer's guidelines regarding lubrication, including the frequency and type of lubricant required. Ignoring these directives will likely lead to complications. As a precaution, remember that the coupling and bearings may require different lubricants, so reviewing the instructions carefully is paramount.
5. Insufficient Maintenance Proper maintenance is crucial for ensuring the efficiency of any system. Couplings deserve the same attention as pumps, gearboxes, and other machinery components. A regular maintenance routine should involve:
   • Cleaning and reapplying lubricant to gaskets and sealing rings as necessary.
   • Always checking alignment.
   • Removing old grease.
   • Verifying the appropriate shaft size.
   • Replacing worn or distorted elastomer components.
   • Ensuring that keys are seated properly.
   • Checking and tightening set screws and bolts in the correct order and to the required torque.
6. Lack of Knowledge Increasing awareness of coupling failures can significantly reduce the likelihood of encountering such issues in the workplace. Well-informed mechanics and maintenance teams regarding proper specifications, assembly, and maintenance of couplings will lead to improved reliability and overall enhanced productivity, reduced downtime, and likely increased profits.

By: Stan Riddle, VibrAlign

In my career, I conduct shaft alignment training for over 500 individuals annually across various facilities—from manufacturing to energy sectors. A commonality among these facilities is the use of similar types of couplings, many of which undergo premature failures. My focus here is not to highlight a specific coupling manufacturer or design but to address the prevalent errors in coupling installation and maintenance I have witnessed during my career.

A shaft coupling plays a critical role in effective power transmission. They are generally simple to install and require minimal maintenance beyond occasional inspections. Most are designed to endure slight misalignments and challenging environmental conditions. Many coupling manufacturers are willing to assist engineers with proper coupling specifications.

So, what leads to these ongoing coupling failures?

Misalignment
It is surprising to see how prevalent shaft misalignment remains today. When I began my career, there were no laser alignment tools—mechanics had to rely on straight edges or calipers. As technology improved, lasers became accessible and offered extreme accuracy. Yet, misalignment is still rampant. Many technicians skip alignment entirely, assuming that if the coupling fits together, it must be aligned. Others might not even realize alignment is required. Mechanic awareness about the significance of alignment tools can lead to considerable reductions in coupling failures.

Couplings are relatively inexpensive, but misalignment can create forces that negatively affect not only the coupling but also bearings, seals, gears, and other components.

Improper Assembly
I frequently ask mechanics whether they take a moment to read the installation instructions that accompany the coupling. More often than not, they reply negatively. Some might be in a rush, feel they are familiar with the information, or simply believe it is not crucial. However, if all technicians followed the manufacturer's installation instructions, many coupling failures could be avoided.

The insights from the manufacturer's engineering team regarding coupling performance should be taken seriously.

Lack of Lubrication
Lubricated couplings need lubrication—this is essential. While individuals would seldom allow their car engines to run dry of oil, they frequently overlook lubrication for couplings until malfunction occurs. Proper downtime is necessary to implement preventive maintenance, including lubrication of couplings. When downtime is limited, consider switching to non-lubricated coupling options.

Improper Coupling Specification
There are various flexible coupling configurations available, all designed for specific applications. Despite manufacturers being available to guide on design specifications, couplings are sometimes mismanaged by using whatever is readily available. When frequent failures occur, inspect the machine's design to verify the coupling's suitability for horsepower demands, startup torque, load considerations, temperature changes, etc.

Poor Maintenance Practices
A coupling deserves to be treated with the same precision as other vital machinery components, regardless of its appearance. Avoid using excessive force when mounting or removing the coupling. Replace worn gaskets, clean, and re-lubricate periodically. Incorporate alignment checks into preventive maintenance routines. Ensure the mounting shaft is of correct size and that elastomers showing wear or deformation are replaced. Regularly inspect set screws and bolts.

Good maintenance practices applied to pumps, gearboxes, and bearing assemblies should similarly be applied to couplings.

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Further reading:
The Ultimate Buyer's Guide for Purchasing All Flanged Tee
Why Choose Ductile Iron Gate Valves for Thailand?

Education
If you have read this far, I trust that your interest in why couplings fail is genuine. As a shaft alignment educator, I find that almost every maintenance mechanic is eager to excel in their work and takes considerable pride in their skills and expertise.

By deepening our understanding of coupling failures and adapting our maintenance approaches, we can enhance the reliability of couplings, improving the performance of the machinery and, consequently, the products and services they support.

Recent Developments in Couplings

Couplings are practical devices that connect and transmit torque between two shafts, playing a pivotal role in numerous machines. For example, pumps depend on couplings to maintain the power required to transport liquids and gases effectively. As industries strive to increase productivity, pump requirements are becoming more demanding, pushing couplings to perform at optimal levels.

The Survivor series of couplings by R+W exemplifies this trend. These disc pack couplings utilize friction to transmit torque efficiently through the disc packs, mitigating issues associated with stress concentration and chatter, which can lead to early failures.

Another approach to enhancing flow rates is by increasing the operating speed of existing equipment. Couplings must then be capable of withstanding these heightened speeds. The Thomas XTSR series from Rexnord illustrates this phenomenon. Its three-piece design ensures a minimal footprint while facilitating straightforward installations.

The coupling includes an anti-flail ring to secure the center member in case of failure and overload bushings to alleviate stress during startup, preserving the lifespan of the disc packs. Additionally, optional torque overload systems can be implemented.

Besides aiding in fluid transport, couplings are essential in generating electrical power. Bearings in these systems face the challenge of stray electrical charges that could potentially harm mechanical components. The design of Zero-Max CD Couplings provides electrical insulation while safeguarding mechanical components from overloads, and its composite discs are resilient against extreme environments.

To effectively transfer generated power, rigid couplings, such as those offered by Oren Elliott Products, are available in various materials and configurations to meet specific operational needs.

R+W
www.rw-america.com

Rexnord
www.rexnord.com

Zero-Max
www.zero-max.com

For more information on the GERWAH product line, click here.

To delve deeper into flexible coupling options, please visit Restrained Flexible Coupling.