Couplings need to match the application demands and parameters or premature failure can occur. Ruland Manufacturing’s William Hewitson, vice president of manufacturing and engineering, and Charles Henrickson, director of marketing, look into why couplings may fail, and how to minimise the risk of this occurring.
There are many application parameters that need to be met to ensure a coupling will work correctly, ranging from performance and environmental factors, to use and service. If one or more of these is not met, however, the coupling may fail prematurely.
Coupling selection errors
There are a number a reasons why a coupling may fail, starting with errors when the component was selected:
Selecting the coupling too late in the design process: Far too often, motion control couplings – which are a critical component in determining and achieving overall system performance – are selected very late in the application design process and don’t meet the complex requirements of the system.
Selecting the wrong type of coupling: 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 these must be considered and addressed 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:
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; while a single beam coupling 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.
Bear in mind that any coupling that is designed to bend during misalignment will produce bearing loads – misalignment beyond coupling specifications can mean 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: Design selection must take into account not only the steady state torque but also the maximum instantaneous torque, particularly when torque varies. 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.
Consideration for windup: All couplings have windup (also known as torsional compliance or torsional rigidity), the rotational deflection between the driver (e.g. motor) and the load. 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 and the load, and is unacceptable in motion control applications. Consequences include 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. As a result, zero-backlash couplings are suited to servo applications.
Selecting a coupling with the wrong amount of shock absorption and dampening: Dampening is important in motion control and power transmission applications to reduce undesirable vibration. Shock dampening helps reduce the effects of impulse loads, minimising 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.
Zero backlash jaw couplings dampen well. These are 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 such as 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. It also requires an understanding of the driven-system inertia values and their affect on the coupling.
Too much coupling inertia can degrade the performance of the entire system. 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: When a coupling’s safe operating speed is not addressed it can quickly result in failure. Not only is the use of a balanced coupling essential, but it is important that consideration is given to coupling stiffness as speed also causes deflection.
It is important to pay 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 utilised, but 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 – in a material handling application, for example, 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 its centre disc will disengage and not allow continued power transmission. Each has its merits depending on the application.
To minimise the possibility of failure, there are a number of questions to ask:
• 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)?
Proper installation is essential, therefore there are a number of points to consider. But it is important to refer to the manufacturer’s instructions.
Basic coupling installation instructions could 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 centre line. Centre 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.
In most cases, motion control couplings are maintenance-free, however regular and diligent ‘system’ maintenance is important for the entire system in which the coupling is an integral component. System maintenance requirements and schedules are generally a function of the specific application, duty cycles, operating parameters, environment and other factors. Any maintenance or service plan for the system as a whole is intended to avoid component failure anywhere within the system, including shafts, couplings, motors, bearings, etc. The coupling may be adversely affected if other component operating characteristics force operation outside of design specifications.
Basic system maintenance requirements might include:
• Check for abnormal operating characteristics such as unusual noise or excessive system component temperatures.
• Check for excessive vibration or other indicators of a change in alignment within the system.
• Check for any signs of wear or looseness in fasteners; re-torque where necessary.
• When using an oldham coupling or jaw type coupling, consideration should be given to the duty-cycle of the centre disc or spider. Wear on this component may result in backlash, thus introducing system performance issues. Replace centre discs and spiders with the vendor specified part and material when the duty cycle has been exceeded or when excessive wear is noted.
The discs are low-cost items, easily replaced, and will restore the coupling’s original capabilities.
• In the event a coupling fails it is important to determine and document the conditions within the system in which the failure occurred. This will allow for appropriate corrective action, including specification of a different coupling to address any changes in the application.
The right solution
Taking the above factors into consideration should ensure the right coupling is selected for the application, and premature failure will no longer be an issue.