Article: Avoiding Coupling Failure - Considerations in Design Selection

Mechanical couplings have a principal use in the connection of rotating shafts for the transfer of rotary motion and torque. As with all mechanical devices, a coupling must match its’ intended purpose and application parameters, including many different performance, environmental, use and service factors. All must be satisfied for the coupling to work properly. When selected with these design parameters in mind, and when installed and operating correctly, a coupling should have no failure issues over it’s’ lifetime. However, when one or more of these is not met a coupling can prematurely fail, resulting in either a small inconvenience or possibly serious financial loss or personal injury. This article provides a view of the primary reasons couplings fail, and the steps that can be taken to minimize the risk of failure.

• Selecting the coupling too late in the design process: Far too often motion control couplings are selected exceedingly late in the application design process and without meeting the complex requirements of the system. Couplings are a critical component in determining and achieving overall system performance. Early selection will reduce errors along with the potential for premature coupling failure.

• Selecting the wrong type of coupling for the application: Coupling selection involves a number of design criteria including: application, torque, misalignment, stiffness, inertia, RPM, shaft mounting, environmental factors, space limitations, service factors, cost and others. All criteria must be considered and addressed in the selection process to ensure that the coupling will work properly without premature failure. This is important both in the initial coupling selection and if conditions change in the application over time.

• Selecting the wrong coupling for the application misalignment conditions: To avoid premature coupling failure it is critically important in design selection to match the correct coupling to the misalignment condition or combination of conditions present. Shaft misalignment may be angular, parallel or axial, with further complications when any combination of these occurs (complex misalignment). Flexible couplings are typically designed to compensate for specific application misalignment conditions. An oldham coupling is well suited for handling relatively large amounts of parallel misalignment with low capability to compensate for angular misalignment and axial motion. A single beam coupling, in contrast, easily accommodates angular misalignment and axial motion with a lower capability to compensate for parallel misalignment.

• Failing to correct excessive misalignment: Excessive misalignment between joined shafts is one of the most common reasons for coupling failure due to the creation of loads that surpass the coupling specifications. All flexible shaft couplings are designed to allow for misalignment of one or more types and to varying degrees of flex. Understanding the allowable flex for the coupling under consideration is paramount. In addition to possible premature coupling failure, keep in mind that any coupling that is designed to bend during misalignment will produce bearing loads. Misalignment beyond coupling specifications introduces the possibility of accelerated wear and the potential for premature failure in other system components such as bearings. When misalignment exists beyond the specifications of the coupling manufacturer it should first be rectified with shaft realignment, followed by the appropriate coupling selection.

• Selecting the wrong coupling for the torque in the application: Couplings are frequently under specified when careful consideration is not given to torque in the application. Design selection must take into account not only the steady state torque but also the maximum instantaneous torque, particularly when torque varies as is in starts and stops. In some cases it might be appropriate to also consider a degree of torsional compliance to dampen torque shock loads and peaks. Flexible couplings have different static torque ratings depending on the design type. For example, in looking at a specific coupling choice where all other application design factors are within the ratings of two alternatives, a double disc coupling will typically offer a 15-20% higher static torque rating over an identically sized oldham coupling with an acetal disc.

Beam Coupling: This beam coupling has failed near the center and represents what may occur in a torque overload condition. A torque in excess of the coupling design limits was applied to illustrate this example. Beam coupling failure may also occur in applications with parallel misalignment because the single beam must bend in two different directions simultaneously, creating larger stresses in the coupling that could cause premature failure.

Bellows Coupling: This bellows coupling has failed in a deep convolution closer to one hub than the other and represents what may occur in a misalignment situation. This type of failure can occur with an excessive degree of misalignment exceeding coupling design limits, or when the coupling is improperly installed and the misalignment (again within specifications) is not uniformly distributed across the bellows.

Oldham Coupling: This oldham coupling has experienced a failure in the center disc and represents what may occur in a torque overload condition. A torque in excess of the coupling design limits was applied to illustrate this example.

• Consideration for windup: All couplings have windup, also known as torsional compliance or torsional rigidity. Windup is the rotational deflection between the driver (e.g. motor) and the load. Think of it as winding up the coupling like a spring. The most significant problem with windup in a servo application is maintaining accuracy of location due to a difference in angular displacement from one end of the coupling to the other. Windup may also introduce resonance in the system and cause the servo to become unstable if improperly tuned.

• Consideration for backlash: Backlash refers to play in couplings and is essentially motion that is lost. The effect of backlash is an interruption or uncoupling in the transfer of power between the driver (e.g.: motor) and the load. Backlash is not acceptable in motion control applications, the most significant consequences being lack of control in positioning accuracy and difficulty in tuning the system. In a motion-centric application such as a servo, backlash introduces timing problems that can cause the coupling to be excessively moved forward and backward, introducing stress that can lead to premature failure. For these reasons zero-backlash couplings are ideally suited to servo applications.

• Selecting a coupling with the wrong amount of shock absorption and dampening: In a mechanical power transmission application dampening refers to minimizing the transfer of shock and vibration. Dampening is particularly important in motion control and power transmission applications to reduce undesirable vibration which wastes energy and creates harmful stresses on system components. Shock dampening helps reduce the effects of impulse loads, minimizing shock to the motor and other sensitive equipment. Couplings must not contribute to system vibration and may be selected based on the dampening effects desired. One type of coupling that dampens well is the zero backlash jaw coupling comprised of an elastomeric "spider" and two hubs. The spider, available in multiple durometers, provides the desired application dampening and can be selected based on the magnitude of the impulse load. The potential for premature coupling failure can be accelerated when the selection of either the overall coupling type or the spider material are incorrect.

• Consideration for inertia: Inertia is a body’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 (e.g.: torque). In a power transmission system, inertia is determined by mass and distribution about the axis, a factor determining drive torque specification. Selection of a coupling for a servo drive system where couplings start and stop intermittently requires consideration of inertia, in addition to zero backlash and torsional stiffness. Selection also requires an understanding of the driven-system inertia values and their affect on the coupling. Too much coupling inertia for a given application can seriously degrade the performance of the entire system by introducing resonance and adding to the natural frequency of the system, with possible unintended consequences. A low inertia coupling can allow the system to be tuned to a higher performance level and is a very good choice for precision applications.

• Selecting the wrong coupling for the application shaft speed: Application speed is another very important factor in selection. When a coupling’s safe operating speed is not addressed in the design criteria it can quickly result in failure, sometimes with tragic consequences. In high speed applications the use of a balanced coupling is essential. It is also important that consideration be given to coupling stiffness since speed also causes deflection. Pay particular attention to the manufacturer ratings for speed, never adversely alter the dynamic balance of a coupling before or after installation, and remember that any shaft misalignment can significantly affect a coupling’s safe operating speed.

• Selecting the wrong coupling for electrical isolation: Electrical isolation is the principle of separating functional components of mechanical systems to prevent the movement of currents while mechanical energy transfer is still maintained. Extraneous electrical currents can be a serious problem in the control of servo systems when passed between drive and driven components. Oldham and jaw couplings are electrically isolating when nonmetallic and polymer inserts are utilized. Other coupling types can also be manufactured in electrically isolating materials.

•Selecting a fuse coupling instead of a fail-safe coupling, or vice-versa: A fuse coupling disallows energy transfer upon failure while a fail-safe coupling is designed to stay engaged. Some applications require a fail-safe coupling to protect personnel or equipment. For example, one might use a fail-safe coupling in a material handling application where an interruption in the flow of material might introduce a safety or process issue if the coupling were to fail. Jaw couplings are considered fail-safe because, even if the spider fails, the jaws of the two hubs interlock, allowing continued power transmission. In contrast, an oldham coupling with a similar failure mode of it’s’ center disc will disengage and not allow continued power transmission. Each has its merits depending on the application, operator safety or other factors.

Any recommendation for resolving a motion control or power transmission problem requires a thorough understanding of the application and system design factors, as well as the nature of the problem itself. That said, below are examples of a few reported problems and alternatives to consider.

* Please Note: Considerations are based on limited information. All specific design factors must be considered before making a final or alternative selection.

• Does the application require high torsional stiffness?
• What are the accuracy requirements?
• Does the application require dampening or shock absorption?
• How much misalignment is present in the design? Is it angular? Parallel? Axial? Complex?
• Does the coupling need to be the break-first point in the system? Does it need to be fail-safe?
• Is electrical isolation a requirement?
• What is the maximum torque applied to the coupling?
• At what speed or speeds will the coupling be operating?
• In what temperature will the coupling operate?
• Are there other environmental factors for the application (e.g.: chemicals, wash down, vacuum)?

You can prepare the most elaborate meal imaginable yet failing to serve it properly will send your guests home disgruntled. The best design effort and attention when selecting a shaft coupling is essentially meaningless if the coupling is installed improperly or if the actual application parameters are outside of original design criteria. Far too many times a coupling is installed hastily or without regard for the manufacturer specifications, leading to premature failure. Follow coupling installation instructions to the letter as this is where common mistakes typically occur. Basic coupling installation instructions might include:

• Prepare the coupling and shafts prior to installation; clean mating parts; oil shafts lightly.
• Check to ensure that any misalignment between the shafts is within the coupling's ratings.
• Follow all instructions for fasteners, specifically the tightening sequence and torque requirements.
• Don’t install the coupling too far left or right of the center line. Center any misalignment along the length of the coupling.

• Don’t install shafts too deep or too shallow in the hubs for the specific coupling in use. Some couplings require a minimum gap between shafts. In most cases the shaft depth within the hub is specified by the manufacturer based on the coupling design.

• Don’t introduce additional stress on the coupling by compressing or stretching it upon installation. Couplings must always be installed in their free-state.

These basic guidelines are intended to reinforce the importance of proper coupling installation, thus reducing the possibility of premature failure. Always refer to the specific manufacturers instructions when performing a coupling installation.

The keys to avoiding coupling failure are correct coupling selection utilizing all application design criteria, proper Installation and periodic system maintenance. Consider all of the application requirements early in design as this will reduce the risk of selecting the wrong type of coupling. Install the coupling properly, verifying that design considerations were correct. For example, is there a greater degree of misalignment than originally specified? Last, regularly maintain the system to ensure that design parameters have been consistently maintained and that no wear, contamination or other detrimental factors have been introduced to any system components. If a problem does arise after the application is in operation gather and document all possible details. This will uncover the problem and a corrective solution can be implemented.