For the Best Load Control Look to Bellows Couplings

Properly selected bellows couplings result in the best control over the load in any servo application. Here are tips to ensure you choose the right size for the application.

Bellows couplings help maintain tight controls over loads.

For years, bellows couplings have been a mainstay for efficient motion systems because they offer high torsional stiffness, low moment of inertia, and minimal restoring forces under misalignment. They may help maintain tight control over loads, which is especially critical when considering that the flexible coupling often represents the point of least stiffness in an electromechanical system. In this way, couplings have a significant effect on the stability of the entire system, as well as the postional accuracy of the load. Bellows couplings benefits include misalignment compensation paired with precise transmission of velocity, angular positioning, and torque.

Most bellows couplings utilize a stainless steel tube which has been hydroformed to create deep corrugations that make them flexible across axial, angular, and parallel shaft misalignments while simultaneously maintaining the torsional rigidity inherent to a metallic tubular structure with a relatively large outside diameter. In shaft coupling applications, the stainless steel bellows absorb slight misalignments created by perpendicularity and concentricity tolerances between the mounting surfaces of the two connected components. They also absorb any axial force created by thermal expansion of the motor shaft during operation while minimizing torsional deflection and maintaining constant velocity. Exact transmission of velocity, angle, and torque, if not maintained, can compromise the operational performance of any servo motion system.

These scenarios place stress on the bellows, particularly parallel misalignment between the two shafts while transmitting torque. Lateral misalignment compensation causes the bellows to flex into an “S” shape with an angular bend at each end of the bellows that concentrates stress primarily on the end-corrugations closest to the mounting hubs. Excessive misalignment over time can harden these areas of the bellows making them brittle as they flex around circumferences. Enough torque can eventually cause the hub to tear away from the bellows during normal operation, an emergency stop, or aggressive acceleration.

Most bellows couplings use a stainless steel tube that has been hydroformed to create deep corrugations to make them flexible across axial, angular, and parallel shaft misalignments.

Torque considerations
While improved concentricity of the mounting faces of the coupled components (closer shaft alignment) can reduce lateral misalignment and ensure against failures, note that this mode of failure is closely related to the torque as well. High misalignment reduces the torque capacity of couplings. While a misaligned coupling will not normally tear until torque is applied, a precisely aligned coupling can transmit more torque than expected.

Because a range exists between misalignment and torque, bellows coupling ratings can vary. Hence, some manufacturers’ ratings are more conservative than others. For example, there are a variety of ratings for peak torque versus maximum misalignment values. Conservative coupling providers offer a shaft misalignment range in line with what the majority of electro-mechanical systems can handle, which is approximately 0.1 to 0.2 mm. Some are rated for slightly more or less misalignment. Peak torque ratings are generally similar across bellows couplings with this range of misalignment rating and a similar outside diameter. The associated torque ratings normally assume that the maximum misalignment condition will exist in the application.

This approach has been successful and normally allows for the coupling to fit well into assemblies involving the appropriately sized components. But not all coupling manufacturers use such a rating system. Some torque ratings may be inflated along with shaft misalignment tolerances. Check the documentation. Look for statements that say significant torque de-ratings must be applied in order to use a coupling’s flexibility. An improperly sized coupling can lead to significant pitfalls.

Some torque ratings may be inflated along with shaft misalignment tolerances. Check the product documentation.

Most bellows manufacturers agree on a combination of ratings that allow for a reasonable level of shaft misalignment to exist without yielding a maximum torque rating that would cause the coupling to be too large for an assembly. Some coupling manufacturers offer coupling designs with
additional corrugations and double flextures. These provide magnification of the lateral misalignment compensation within a given torque rating while maintaining a relatively high level of torsional stiffness.

Calculations to determine proper couplings
Proper bellows coupling selection should begin with a torque calculation. Quick factors to include in a bellows coupling selection are the peak torque capacity of the servo motor, multiplied by any application gear reduction ratio, and multiplied by a safety factor of 1.5. The appropriate bellows coupling should then have a torque rating greater than or equal to that of the calculated torque.

More precise calculations include the moments of inertia and actual torque required to accelerate the load by first overestimating the required torque of the application through the use of generalized service factors. Then, reduce the torque value by considering the moments of inertia of the drive and load. Inertia mismatch can be critical to coupling longevity as reflected load inertia in aggressive start/stop or reversing applications can produce significant spikes in torque. These spikes can exceed those estimated through the use of current limits into the drive amplifier.

Selection by torque is most common. However, calculating the required coupling torque rating can be skipped if their position accuracy requirements would result in a torsional stiffness value which would correspond to a torque rating in excess of the actual power requirements of the application.

A flexible coupling is typically the most compliant of components in any mechanical motion system making its torsional stiffness a critical factor in terms of maintaining positional control over the load. Since bellows couplings have the highest torsional stiffness of any servo motor coupling, they are employed in applications with high precision positioning requirements.

There are some rare cases in which the servo loop gains set high enough can result in a mechanical frequency which will excite the coupling’s natural frequency. In these instances, elevate the coupling torsional stiffness to avoid a situation where the rate at which the coupling springs back from a torsional impulse does not match when the next impulse would take place. Auto-tuning features in most modern servo drives have eliminated this potential problem for most applications. However, in some cases, this effort may be necessary. The following calculation allows for proper coupling selection based on mechanical resonant frequency.

Properly selected bellows couplings result in the best control over the load in any servo application. The selection criteria begin with ensuring that the coupling will have sufficient torque rating to accelerate the load, followed by checking that coupling misalignment tolerances are in line with practical expectations of the accuracy with which coupled shafts will be aligned.

Generally, higher misalignment tolerances can be achieved at potential compromise to torsional stiffness. However, in most applications, bellows couplings offer ample torsional stiffness. In cases where a coupling with a good mechanical fit has marginal torsional stiffness in light of stringent requirements, shaft alignment must be addressed in order to accommodate high stiffness requirements. A good rule of thumb would be to contact a coupling expert for servo coupling requirements to ensure optimal performance.

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