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. Weight
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. 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. Temperature
As a result of speed, torque or their combination, couplings may impart axial forces or deflections into connected equipment. See Figure 7 for a comparison of relative thrust forces for typical coupling types.
Figure 7. Thrust forces
When a coupling connects two axially restrained shafts, the coupling may impart forces into the bearings. These forces should be reviewed to ensure that the thrust capacity of the bearings is not exceeded. If the coupled shafts are not fully restrained, the coupling may move one or both shafts axially in an attempt to equalize forces. Applications involving axially sensitive equipment such as sleeve bearing motors or herringbone gear sets should be reviewed to ensure that the coupling travel will not cause equipment damage. Many couplings can be supplied with accessories such as gap discs, rung spacers or limited end float rings to limit this axial travel.
In their standard configuration, only a disc or gear coupling will not impart axial forces into the equipment. Of the two, only a disc coupling will truly limit end float, since it acts like a spring in both directions to hold the shafts axially in the position they were installed. This property makes them the ideal choice for sleeve bearing applications.
In addition to the application conditions described above, consider other secondary selection factors including duty cycles, environmental conditions, degree of maintenance, overall cost and expected service life. Familiarity and availability of a given coupling design may also impact the final decision.
Manufacturer's guidelines should be followed for periodic inspections, lubrication and other areas of attention that a coupling may require. Disc couplings, for example, do not require lubrication but should be inspected during scheduled shutdowns for any fretting, cracking or spreading/separating discs, which can indicate excessive misalignment or torque.
A coupling manufacturer with a broad product line and staff of experienced application engineers will be able to provide advice that leads to an optimum coupling choice.
Pumps & Systems, May 2010