Para-Flex couplings from Baldor use a non-lubricated elastomeric flexing member loaded in shear. Torque is transmitted through a composite element system reinforced with torque-carrying tension cords.

9. Beware of inadequate torsional stiffness.

A coupling lacking adequate torsional stiffness can cause resonance and failure. This problem is becoming more common, as machines are increasingly required to rapidly simulate cam profiles, which can introduce torsional vibration to the driveline. Because couplings are almost always the most compliant component in a system, they tend to dictate the natural frequency of the entire drive axis. When this natural frequency is excited, noise, vibration, and ultimately coupling failure result. During the design process, if it becomes apparent that the drive will be required to index multiple times per second, for example, resonant frequency and coupling torsional stiffness need to be examined.

In cases where the driving and driven shafts are mounted to separate bases, shaft alignment is not determined by a coupling housing; therefore, shafts must be aligned during installation of connected components. In these situations, either laterally mounted couplings or high misalignment couplings are often required. Otherwise, high restoring forces can be transmitted onto shaft bearings, often causing shafts to break and couplings to fail.

10. Consider supplier sizing methods and shaft assemblies.

It may be tempting to match coupling torque ratings to RMS torque data provided by servo sizing software, but this does not always account for torque spikes that the coupling may receive due to reflected load inertia. To address the issue, many servocoupling manufacturers have programs and formulas of their own which, depending on the coupling design itself, also account for inertia and duty cycle data that is normally already available. These manufacturer-provided sizing methods help ensure that the coupling will have the right torque rating for the application.

When designing for simple line shafts or jackshafts, many engineers assume that the most practical approach is to fabricate the steel shafting, align center support bearings, and use single-piece flexible couplings at each end. In fact, there are a number of coupling manufacturers who supply couplings from precision-extruded tubing, which often possesses the light weight and lateral stiffness characteristics sufficient to supplant intermediate supports. These particular coupling shaft assemblies come from the factory, cut to length and ready to install as a single unit, and often with options such as length adjustability and integral torque limitation. Supplier application engineers can advise on properties such as critical speed, flexibility, weight, and inertia, depending on the required overall length.

Tips 9 and 10 courtesy of Andrew Lechner, R+W America

Avoid common coupling pitfalls

As with all mechanical devices, a coupling must match its intended purpose and application parameters, including many different performance factors. However, a design engineer must look beyond these criteria and also address issues such as the application environment, serviceability, maintenance, and speed of replacement if required — as downtime can seriously degrade many processes. A common pitfall: not understanding what a manufacturer's product specifications actually mean. For example, axial load data is sometimes determined under ideal (unrealistic) conditions, and sometimes expresses a “failure mode.” Designers must fully understand these specifications as well as design criteria for the machine under review.

Another common mistake in design selection is misidentifying the type and degree of application or system misalignment. Is the misalignment angular or parallel? Is there axial motion? Do all three conditions exist, and to what degree for each? Proper coupling selection cannot be correctly made without a complete understanding of the misalignment being addressed in the system.

Source: Charles Henrickson, Ruland Manufacturing