What is the problem with flexible couplings?

The function of a coupling is to connect two rotating shafts, for the transfer of rotary motion and torque. For a coupling to work at its optimum efficiency, it must match all required conditions, including performance, environmental, use and service factors. If all of these factors have been taken into consideration when selecting a coupling the coupling should have no failure issues over its lifetime. However, if just one of these factors is not met, a coupling can prematurely fail, causing anything from a small inconvenience, to a significant financial loss, and even the potential of personal injury.

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This article will help you to identify the main reasons why couplings fail and provide you with useful information and advice to help you minimise the risk.

1. Selecting the coupling too late in the design process

Couplings are often selected extremely late in the application design process, without meeting the complex requirements of the system. By considering couplings early on in the design process, each criteria can be considered individually, ensuring that the coupling chosen is suitable for the functions required.

2. Selecting the wrong type of coupling for the application

Several criteria must be considered when deciding on a type of coupling, including the type of application, torque, misalignment, stiffness, inertia, RPM, shaft mounting, environmental factors, space limitations, service factors, and cost. Each criteria must be individually considered to ensure that the coupling will be suitable for the application and not result in premature failure. This process of evaluation must also be repeated for any change in conditions throughout the application&#;s lifecycle.

3. Selecting the wrong type of coupling for the application misalignment conditions

An essential consideration when selecting a coupling is the misalignment conditions of the application. This may be angular, parallel or axial, or a combination of more than one misalignment (complex misalignment). Flexible couplings should be considered in these conditions, although the type will depend on the type of misalignment present. For example, an oldham coupling is suitable for large amounts of parallel misalignment, but cannot tolerate a high level of angular or axial misalignment, whilst a single beam coupling can withstand large amounts of angular and axial misalignment, but not parallel.

4. Failure to correct excessive misalignment

Even flexible couplings which are designed for use on misaligned shafts have their limits. A common point of failure is the under-estimation of the degree of misalignment, creating loads that surpass the coupling specifications. This causes the coupling to wear at an accelerated rate, and has the potential to cause other components, such as bearings, to also fail prematurely. Where misalignment exists beyond the manufacturer specifications for the coupling, this should first be rectified with shaft realignment before installing the coupling.

5. Selecting the wrong coupling for the torque in the application

The torque of an application is frequently under-estimated. The maximum instantaneous torque for the application needs to be considered, in addition to the steady state torque. Flexible couplings have different static torque ratings depending on the design type. For example, a double disc coupling will typically offer a 15-20% higher static torque rating than an identically sized Oldham coupling with an acetal disc.

6. Lack of consideration for windup

Windup is also known as torsional compliance or torsional rigidity, and is present in all couplings. It is the rotational deflection between the driver and the load, similar to winding up a spring. The most significant problem with this is maintaining accuracy of location due to a difference in angular displacement from one end of the coupling to the other.

7. Lack of consideration for backlash

Backlash is the loss of motion momentarily in a coupling. For example, when torsion is applied in one direction, the coupling bends and compresses under that stress. When the direction of torsion is changed, backlash is experience within the coupling. Any amount of backlash in a motion control application could be detrimental to the application, potentially causing lack of accuracy in positioning, and difficulty in tuning the system. Zero backlash couplings should be considered in these scenarios.

8. Selecting a coupling with the wrong amount of dampening

Dampening refers to the minimisation of shock and vibration and is particularly important in motion control and power transmission applications to reduce the waste of energy and the unnecessary stress on system components. Shock dampening helps to reduce the effects of impulse loads, minimising shock to the motor and other sensitive equipment. The potential for premature coupling failure can be accelerated when the selection of coupling type does not fully take into consideration the dampening levels required.

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9. Lack of consideration for inertia

Inertia refers to the coupling&#;s resistance to change in angular velocity, and governs the tendency of the coupling to remain at a constant speed in response to applied external forces (eg torque). Too much coupling inertia in an application can seriously degrade the performance of the entire system by introducing resonance and adding to the natural frequency of the system. A low inertia coupling can allow the system to be tuned to a higher performance level, and is a very good choice for high precision applications.

10. Selecting the wrong coupling for the application shaft speed

Failure to consider the coupling&#;s maximum safe operating speed during the design stage can quickly result in failure, sometimes with tragic consequences. A balanced coupling is essential in high speed applications. Any degree of misalignment can also affect the coupling&#;s safe operating speed.

11. Selecting the wrong coupling for electrical isolation

Electrical isolation is the separation of two mechanical components to prevent electrical current transfer, whilst still allowing mechanical energy transfer. Oldham and jaw couplings can be electrically isolating when non-metallic or polymer inserts are used, and other types of coupling can also be manufactured in electrically isolating materials.

12. Selecting a fuse coupling instead of a fail-safe coupling, or vice-versa

A fuse coupling disallows further energy transfer upon failure, whereas a fail-safe coupling is designed to continue working, even after failure. For example, a jaw coupling would be considered fail-safe, as even if the spider fails, the jaws of the two hubs interlock, allowing continued power transmission. Both have their uses, but it is important to establish which type is required for the application during the design stage. 

Those clever chaps at Ruland have put together a 5 question quiz to test your coupling failure knowledge. All it takes is two minutes to complete, so why not give it a go. Have fun and remember, failure is a building block to success!

Information courtesy of Ruland Couplings and Shaft Collars.

For help and advice on choosing the right coupling for your application, contact our Couplings & Drives expert.

Flexible shaft couplings are simple and relatively inexpensive devices that are not difficult to install or maintain, yet many fail prematurely. These failures also happen despite the devices being specially designed to tolerate slight misalignment and to cope with tough operating conditions including temperature extremes, humidity, loads and more. Here are six common mistakes made in assembly and maintenance of flexible couplings.

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1. Misalignment&#;While couplings are designed to allow some misalignment and varying degrees of flex, excessive misalignment between joined shafts is one of the most common reasons for failure. Often, a mechanic will not check the alignment before installing a new coupling and assumes that if the coupling fits, the shafts must be aligned properly. Sometimes the mechanic does not even know that alignment is required. One of the telltale signs of coupling misalignment is the presence of dust under the part. Prevention tip: Make alignment a prerequisite prior to coupling installation. Laser alignment tools make the task easy, quick and accurate.
2. Not Following Manufacturer Installation Instructions&#;All too often, mechanics assemble couplings without referencing the manufacturer&#;s instructions and this can lead to premature failure. The old saying &#;more haste, less speed&#; is especially true when it comes to installing couplings, and technicians should be urged to refer to the specific manufacturer&#;s installation directions, adhere closely to them and not rush the process.
3. Incorrect Coupling Specifications&#;Not all flexible couplings are the same. There are many different types and for optimum power transmission. The right coupling must be selected. Unfortunately, this does not always happen, and the result is premature coupling failure. Prevention tip: Make sure there are appropriate couplings in the spares inventory and that the maintenance department is aware of the various design specifications. If the operation is experiencing frequent coupling failure despite the alignment being in order, it is recommended that engineers document the system conditions at failure so a more appropriate coupling can be used that is more suited to the machine&#;s load, torque, temperature, horsepower, etc.
4. Insufficient Lubrication&#;Always follow the manufacturer&#;s specifications about lubrication&#;both the frequency of lubrication and the type of lubricant required. Failure to adhere to these instructions will lead to problems. Prevention tip: Remember, the coupling and the bearings may not be able to use the same lubricant, so always check the instructions carefully.
5. Poor System Maintenance&#;Without proper maintenance, the efficiency and effectiveness of a system will be compromised. A coupling is just as important as a pump, gearbox, bearing assembly or any other component of precision machinery and must be given the appropriate level of care and maintenance. A regular maintenance schedule should include the following:
   • Clean and relubricate gaskets and sealing rings and replace if necessary.
   • Check alignment.
   • Remove hard grease.
   • Ensure shaft size is appropriate.
   • Replace worn or twisted elastomer.
   • Check for properly seated keys.
   • Check and tighten set screws and bolting in the right order and to the torque requirement.
6. Lack of Information&#;The more knowledge about why couplings fail, the fewer chances there are to experience issues relating to coupling failure in the workplace. The better informed the mechanics and maintenance team are about the importance of the correct specification, assembly and maintenance of couplings, the more reliable the machinery will be. This will lead to better equipment performance, enhanced productivity, less downtime and likely improved profitability.