by Bill Holtz, Rexnord Industries


Where high coupling speed ratings are required, a disc or gear coupling is the best choice due to compact size and inherently balanced designs. In addition to the relatively high speed ratings of the standard designs, only these two coupling types can be dynamically balanced for increased speed ratings (see Figure 2).

Figure 2

Figure 2

An added advantage for disc couplings is low potential unbalance, which makes their balance more repeatable. In other words, the balance achieved on the balance machine with a disc coupling will have the best chance of being repeated in the field.

Torsional Stiffness

Each type of coupling has unique stiffness characteristics that can be used to meet application requirements  (see Figure 3). In applications such as an indexing conveyor or a paper machine drive, where the drive output and input need to be closely in phase, a torsionally stiff coupling like a disc or gear coupling may be the better choice. Disc couplings, although not as stiff as gear couplings, offer the benefit of no backlash. To reduce the severity of shock loads, elastomeric couplings can be used to provide softer starts than a gear or a disc coupling. Many of these designs can exhibit torsional windup of up to 6 deg at rated torque.

Figure 3

Figure 3

Torsionally soft couplings may also be employed to avoid vibration from operating at system natural frequency, either by shifting the system natural frequency or by dissipating the vibrational energy associated with system resonance. Grid couplings are a good compromise of soft and stiff, offering the strength of steel and the softness of an elastomer. This coupling design can reduce transmitted vibration by about 30 percent compared to a gear or disc coupling.


In our comparison, where a spacer coupling is required, a bonded tire or disc design may be the best choice for the lightest weight options (see Figure 4). For higher horsepower or close-coupled applications, particularly for extremely high horsepower applications, gear couplings will be the lightest weight option by far, resulting from their high torque density.

Figure 4

Figure 4. Weight

Torque Density

Torque density (see Figure 5) is expressed most accurately as torque capacity per unit volume. We will express torque density modified as meaning coupling torque rating/coupling weight. 

For a flexible coupling, nothing compares to a gear coupling for transmitting the most torque in the lightest and smallest package.

Figure 5

Figure 5. Torque Density


Disc couplings offer the highest temperature rating due to their entirely metallic construction. The other coupling types are limited by either their elastomeric components such as flex elements or seals or by the temperature limitations of their lubricants (see Figure 6).

Figure 6

Figure 6. Temperature