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How to avoid coupling failure from design to maintenance


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.

When selected with these design parameters in mind, and when installed and operating correctly, a coupling should have no failure issues over its’ 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.

• 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. 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. 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:
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. Understanding the allowable flex for the coupling under consideration is paramount. 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 specifications, it should first be rectified with shaft realignment followed by the appropriate coupling selection.

• Selecting the wrong coupling for the 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.

• Consideration for windup:
Windup is the rotational deflection between the driver (such as a 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. 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.

MAINTENANCE
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.


The article is courtesy of Ruland Manufacturing Co. Ltd. For more information, contact RotoPrecision at www.rotoprecision.ca.