10 tips for using couplings in your application

Choosing the correct coupling for the task at hand is no easy task. It means asking the right questions before specifying a particular style. Choices abound, and more than one coupling type may be suitable for any given application. Here we share with you expert tips and tactics to make it easy.

Courtesy: Machine Design (Frances RichardsElisabeth Eitel | Dec 01, 2009)

Because engineers are creatures of habit, choosing a coupling type is often a matter of having selected that same type for a previous project. However, because not all couplings are created equal, specifying out of familiarity often mismatches equipment needs with coupling capabilities.

2. Determine the best way to mount coupling to shaft.

The method by which a coupling is mounted on the shaft may determine the success or failure of that coupling, regardless of whether it is  right for the job. Traditional keys, keyways, and taper bushings work well in unidirectional applications with minimal shock or reversing loads. For reversing loads and shock applications, keyless locking devices are the preferred mounting method because keyless devices are backlash-free.

3. Ask yourself what the coupling should do.

Perhaps most important is to ask oneself, “What should this coupling do?” Further questions: Does it need to transmit high or low torque? Is the application high or low speed? Does it need to be maintenance free? How about backlash free? Are there misalignments between components to be compensated, and by how much? Does the application require the coupling to absorb shock? How crucial is cost? What about weight?

4. Be aware of correct terminology.

Inch-pounds or inch-ounces? It may seem obvious, but the units of the coupling’s torque rating are commonly confused. Getting the spec wrong can cause you to miss the proper coupling choice by more than an order of magnitude. Another area of confusion involves the use of keyways: Keyway couplings are for high torque, not high precision.

Underrated couplings: Using a coupling with a torque rating insufficient for the application can damage or break the coupling, compromising the desired transmission. Overrated couplings: If a flexible shaft coupling is chosen with a torque rating higher than required, it may be unnecessarily bulky and stiff. Typically, the higher the torque rating of the coupling, the larger and less flexible it is.

6. Proper installation cannot be over-emphasized.

Selecting the right coupling for an application can be a complex process, but need not be overly time consuming. The best approach is to carefully consider all design criteria. Typically, these include torque, shaft misalignment, stiffness, rpm, inertia, space requirements, and shaft mounting. A coupling that addresses all of these issues will ultimately perform as required in the application.

7. Improper selection means wrong type and wrong size.

The most common mistake that engineers make when specifying couplings and shafts is improper selection, which includes choosing the wrong coupling type and size. Many coupling styles are available in the marketplace, and chances are that more than one type of coupling will work well for a given application. Conversely, there are likely many couplings that will not work well for that same application. Understanding application requirements and balancing those against functional advantages of available couplings will help identify the ideal product.

8. Know the difference between right and wrong.

Ideal coupling selection is one in which the coupling is installed quickly and then forgotten for years and years. It is a coupling that does not fail due to application demands. Nor does it transfer stress or failure to mating components. It does not require scheduled downtime to maintain reliability.

The wrong coupling is the exact opposite and is usually easy to spot. It takes hours or days to install. It requires near perfect, time-consuming alignment practices and requires frequent lubrication through scheduled downtime. When the wrong coupling fails, it often fails catastrophically, without warning, leading to extended, unplanned, costly downtime. Upon misalignment, the wrong coupling causes premature failure of mating equipment such as bearings, gears, or shafts. Rather than serving as a fuse in the machine, the wrong coupling in an overloaded machine transfers failures to more expensive components such as motors or gearboxes.

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.

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 servo coupling 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.

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.

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