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. Another source of error is choosing a coupling based on price rather than performance requirements. Driven to reduce machinery costs, engineers may shortchange the application by being overly thrifty in their coupling choice. While this approach may reduce upfront component costs, extensive and expensive backend warranty claims can wreak havoc on an OEM's bottom line and product reputation.
Another error is picking the wrong size: Choosing a coupling that is too large or too small for an application causes problems every time. An engineer must know the forces and loads to which a coupling will be exposed. Simply guessing at load requirements based on motor torque or belt capacities, for example, and then upsizing or downsizing the coupling, causes either design overkill or serious under-design. Either way, the overall machine is inefficiently designed, ultimately costing both OEM and user more to operate.
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 the 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. For example, mounting a torsionally rigid, backlash-free, high-speed disc coupling with a keyway and setscrew negates the backlash-free nature of that coupling. A keyless locking device would serve the coupling's intended purpose better. On the other hand, mounting a highly flexible jaw coupling with a keyless locking device could be overkill based on the rough and flexible nature of that coupling style.
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? How about environmental conditions, such as ambient temperature, moisture, and corrosives? Knowing the answers to these questions with regard to the application and cross-referencing against available couplings will result in selecting the most ideal coupling for the application. Remember: More than one coupling type may work.
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. If the application requires reversing torques or direction plus precise positioning between the driving and driven shafts, keyways are inappropriate. Instead, a coupling with setscrew or clamp-style hubs is the best solution for precision applications. One more note — if you need zero-backlash, get zero-backlash. Many types of couplings, including flexible bellows shaft couplings, allow no backlash, while others styles permit some backlash to occur.
5. Watch for under or overrating.
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 overemphasized.
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.
Know that choosing the correct coupling is not the end of the job. It is equally important to install the coupling properly, verifying that design considerations were correct. For example, is there a greater degree of misalignment than originally specified? After installation, regularly inspect the application assembly to ensure that design parameters are consistently maintained and that no system component, coupling wear, contamination, or other detrimental factors develop.
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.
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.
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.
Tips 1 through 3 courtesy Carl Fenstermacher of Ringfeder Corp. 4 and 5 — Brent Caldwell of Servometer. 6 and Common Pitfalls warning — Charles Henrickson of Ruland Mfg. 7 and 8 — Galen Burdeshaw of Baldor Electric Co. 9 and 10 — Andrew Lechner of R+W America.
Contact information for technical sources:
Ruland Manufacturing Co. Inc.
Baldor Electric Co.