12 Reasons for coupling failure and how to avoid them

12 Reasons for Coupling Failure and Strategies to Prevent It

A coupling serves a crucial role in connecting two rotating shafts to facilitate the transfer of rotary motion and torque. For a coupling to operate effectively, it needs to meet various conditions such as performance, environmental factors, usage, and service requirements. When correctly selected with all these parameters in mind, a coupling is expected to have a long, failure-free lifespan. Unfortunately, if even one condition is overlooked, the coupling may fail prematurely, resulting in minor inconveniences, considerable financial losses, or even serious personal injuries.

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This article outlines the primary causes of coupling failures and offers practical tips on how to minimize risks associated with them.

1. Late Selection of Couplings in the Design Process

Often, couplings are chosen late in the design phase of an application, without taking into account the intricate requirements of the system. By engaging with coupling selection earlier in the design process, each criterion can be evaluated individually, which ensures that the selected coupling effectively meets the necessary functions.

2. Choosing the Incorrect Type of Coupling

When determining the type of coupling, several factors must be addressed, including application type, torque, misalignment, stiffness, inertia, RPM, shaft mounting, environmental conditions, spatial constraints, service factors, and budget. Each factor requires careful consideration to confirm that the coupling can perform as needed without failing prematurely. Evaluation should also be revisited whenever conditions change during the application’s lifetime.

3. Misalignment Conditions

An important factor during coupling selection is understanding the application's misalignment conditions—be it angular, parallel, axial, or a combination of these (known as complex misalignment). While flexible couplings are designed to adapt to such conditions, the specific type of flexibility needed varies based on the misalignment type. For instance, an Oldham coupling works well for substantial parallel misalignment but struggles with high angular or axial misalignments, whereas a single beam coupling excels at the latter two but is not stable under parallel misalignment conditions.

4. Failure to Address Excessive Misalignment

Even flexible couplings have limits. A common point of failure arises from underestimating the misalignment degree, leading to loads that exceed coupling specifications. This accelerates wear on the coupling and can lead to the premature failure of other components, such as bearings. It is crucial to align shafts correctly before installing the coupling if misalignment exceeds the manufacturer's specifications.

5. Incorrect Torque Allocation

Torque loads are often underestimated. It’s essential to consider both the maximum instantaneous torque and the steady-state torque for applications. Different flexible couplings exhibit various static torque ratings according to their design. For instance, a double disc coupling typically offers a 15-20% higher static torque rating compared to an identically sized Oldham coupling featuring an acetal disc.

6. Neglecting Windup Effects

Windup, or torsional compliance, refers to the rotational deflection between the driver and load, akin to twisting a spring. This poses significant issues in maintaining accurate positioning due to discrepancies in angular displacement across the coupling.

7. Ignoring Backlash

Backlash represents a temporary loss of motion within a coupling. When torque is exerted in one direction, the coupling deforms; upon reversing the direction, backlash occurs. Any backlash within motion control applications can be detrimental, potentially compromising positioning accuracy and system tuning. Therefore, using zero backlash couplings is advisable in these circumstances.

8. Unsuitable Dampening Characteristics

Dampening, the reduction of shock and vibration, is crucial in motion control and power transmission applications. Inadequate shock absorbance leads to wasted energy and undue stress on system components. Couplings that don’t adequately address required dampening levels are at increased risk of premature failure.

9. Overlooking Inertia Factors

Inertia plays a critical role in a coupling’s ability to resist changes in speed, affecting overall system performance. Excess coupling inertia can introduce resonance and amplify the natural frequency of a system, degrading functionality. Low inertia couplings are preferable for high-precision applications as they allow superior tuning.

10. Misjudging Maximum Shaft Speed

Neglecting the coupling’s maximum safe operating speed during design can lead to failures, sometimes with severe implications. Particularly for high-speed applications, ensuring a balanced coupling is essential as misalignment can also affect safe operating speeds.

11. Selecting for Electrical Isolation Incorrectly

Electrical isolation prevents unwanted electrical current transfer between mechanical components while permitting mechanical energy transfer. Couplings like Oldham or jaw types can serve this function when fitted with non-metallic or polymer inserts, and some coupling types can be manufactured using electrically isolating materials.

12. Confusion Between Fuse Couplings and Fail-Safe Couplings

Fuse couplings cease energy transfer upon failure, while fail-safe couplings continue operating post-failure. For instance, jaw couplings can be viewed as fail-safe since even when the spider fails, the jaws interlock for ongoing power transmission. It’s vital to clarify which type is needed during the design phase.

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This information is provided courtesy of Ruland Couplings and Shaft Collars.

For assistance in choosing the right coupling for your requirements, reach out to our Couplings & Drives expert.

Common Assembly & Maintenance Mistakes

Understanding Why Coupling Failures Occur Frequently

Flexible shaft couplings stand out as uncomplicated, economical devices that are not difficult to install or maintain; however, many still experience premature failures. These breakdowns happen despite the design intent to accommodate slight misalignments and endure various challenging operating conditions, including temperature fluctuations and humidity.

To optimize performance and productivity, consider these six frequent assembly and maintenance mistakes that compromise coupling efficiency.

Misalignment Issues

Although couplings accommodate some degree of misalignment, excessive misalignment between connected shafts is a primary failure source. Mechanics may neglect to check alignment before coupling installation, assuming that fitting implies proper alignment. Dust accumulation underneath the coupling can indicate alignment issues.

Prevention Tip: Make alignment assessment a non-negotiable step before coupling installation. Laser alignment tools simplify this task to ensure speed and accuracy.

Ignoring Manufacturer Instructions

Technicians frequently assemble couplings without consulting the manufacturer's guidelines, leading to premature failures. The adage, "more haste, less speed," resonates especially with coupling installations; technicians should be encouraged to thoroughly follow installation instructions without rushing.

Choosing Incorrect Coupling Specifications

With various types of flexible couplings available, it’s crucial to select the right one for optimal power transmission. Unfortunately, incorrect selections can lead to premature failures.

Prevention Tip: Maintain a diverse inventory of the right couplings in your spares, ensuring your maintenance team understands different specifications. If failures recur despite proper alignment, document system conditions at the time of failure to enable selection of a more suitable coupling for your machinery’s load, torque, temperature, and horsepower.

Inadequate Lubrication

Always follow the manufacturer's lubrication guidelines regarding both frequency and lubricant type. Adhering to these specifications is straightforward; failure to do so will result in problems.

Prevention Tip: Remember that lubricants for the coupling may not be suitable for bearings; always ensure compliance with the guidelines.

Poor System Maintenance Practices

Without appropriate maintenance, the overall performance and efficiency of your system can deteriorate. Couplings are as critical as pumps, gearboxes, and bearing assemblies, deserving equal levels of care. A regular maintenance schedule should include:

• Cleaning and relubricating gaskets and sealing rings, replacing them as necessary
• Checking alignment
• Removing hardened grease
• Ensuring appropriate shaft sizes
• Replacing damaged or twisted elastomers
• Verifying that keys are seated correctly
• Checking and tightening set screws and bolts in the proper sequence and torque specifications

Empowerment Through Knowledge

The better informed your maintenance team is about the causes of coupling failures, the less likely they will face coupling-related issues in operation. Ensuring your mechanics understand the significance of proper specifications, assembly, and maintenance will enhance equipment performance, boost productivity, minimize downtime, and potentially increase profitability.

For a technical partner dedicated to enhancing your productivity and journey towards improving profitability, Nexxis stands out in the Australian industrial sector by providing innovative, cost-effective strategies for inspection and testing equipment, yielding better financial and operational results. For further details, reach out to Nexxis today.

If you seek additional information, please visit Restrained Flexible Coupling.