This “split-hub” bellows coupling from R+W America allows assembly of the coupling on equipment where shaft ends cannot be moved to accommodate assembly of couplings with a one-piece hub. The stainless steel bellow exhibits very high torsional rigidity with zero backlash.

Another issue is surprise deflections. Flexible shaft couplings are designed to transmit torque, while providing compliance in some combination of compression/extension, bending, and offset. Unanticipated deflections can shorten coupling life. In short: Understand (or at least conservatively estimate) the operating deflection modes when choosing a coupling.

Tips 4 and 5 courtesy of Brent Caldwell, Servometer

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.

Tip 6 courtesy of Charles Henrickson, Ruland Manufacturing

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.

Bellows-type flexible shaft couplings from Servometer feature low elastic windup, zero backlash, zero cyclic speed variation during 360° rotation, and minimal side loading.

For example, attributes such as stiffness, misalignment capability, ease of installation, ease of maintenance, temperature capability, inherent balance, and speed capability all influence coupling reliability during operation. Too often, the overall nature of the application is underestimated and incorrect service factors are applied — because shock loads, brake loads, shaft deflection/bending, sensitivity to windup, and cycle rates are commonly unforeseen. These criteria are generally addressed through selection service factors and proportioned against the nameplate or demand horsepower. If the selection service factors don't reflect the demands appropriately, then the coupling will likely suffer from premature fatigue failure.

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.

Tips 7 and 8 courtesy of Galen Burdeshaw, Baldor Electric Co.