Image courtesy of Ruland Manufacturing

1 .Don't choose your coupling based on habit or price alone.

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.

Tips 1 through 3 courtesy of Carl Fenstermacher, Ringfeder Corp.

Torsionally flexible claw couplings from Ringfeder Corp. compensate for axial, radial, and angular misalignment.

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. 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.