Guide to shaft couplings

The basic function of a coupling is to connect two shafts, end-to-end, inline, for the purpose of transmitting torque or power from one shaft to another, causing both to rotate in unision. This article details the types, advantages and limitations of shaft couplings.
By Joseph L. Foszcz , PE/CPE, Senior Editor, Plant Engineering Magazine August 1, 2000
  • Types

    • Advantages

      • Limitations

        • The basic function of a coupling is to connect two shafts, end-to-end, inline, for the purpose of transmitting torque or power from one shaft to another, causing both to rotate in unison. If the shafts are perfectly aligned, there is no problem in using rigid couplings. Flexible couplings are able to compensate for minor amounts of misalignment and random movement between the two shafts. They are commonly used in industrial applications.

          Such compensation is vital because perfect alignment of two shafts is extremely difficult and rarely attained. If not properly compensated, minor shaft misalignment can result in unnecessary wear and premature replacement of other system components. Flexible couplings can, in varying degrees, minimize the effect of misaligned shafts.

          In certain cases, flexible couplings are selected for other protective functions as well. They provide a breakpoint between driving and driven shafts that acts as a fuse if a severe torque overload occurs. This breakpoint ensures that the coupling fails before something more costly breaks elsewhere along the drive train. Flexible couplings can dampen torsional vibration that occurs naturally in driven equipment.

          Each type of coupling has some advantage over another type. There is no single coupling that can “do it all.” There are tradeoffs associated with each design, including initial cost. Each type has strengths and weaknesses that must be taken into consideration, because they can dramatically impact how well the coupling performs.

          Plant Engineering magazine extends its appreciation to the Power Transmission Distributors Association (PTDA), Lovejoy, Inc., and TB Wood’s, Inc., for the use of their material in the preparation of this article. This article includes excerpts from the Power Transmission Handbook published by PTDA. The handbook is a primer on power transmission/motion control products and applications. The handbook is available from PTDA at 312-876-9461 or at www.ptda.org/catalog.

          Joseph L. Foszcz, Senior Editor, 630-320-7135, jfoszcz@cahners.com

        • Types

          • Advantages

            • Limitations

              • &HEADLINE>Elastomeric couplings&/HEADLINE>

                Advantages

              • Torsionally soft

                • No lubrication or maintenance

                  • Dampens vibration and absorbs shock

                    • Field replaceable elastomers

                      • Low reaction load on bearings

                        • Handles large misalignment

                          • Low cost for a given bore size

                            • Limitations

                            • Sensitive to chemical sand heat

                              • Low power density

                                • Not suited for positive displacement

                                  • Difficult to balance

                                    • Some types have poor overload capacity

                                      • &HEADLINE>Coupling evaluation factors&/HEADLINE>

                                      • Adaptability of design

                                        • Alignment capabilities

                                          • Axial freedom

                                            • Backlash

                                              • Chemical resistance

                                                • Damping capacity

                                                  • Ease of installation

                                                    • Fail safe

                                                      • Field repairable

                                                        • High speed capacity

                                                          • Maintenance required

                                                            • Number of parts

                                                              • Reactionary loads from axial forces

                                                                • Reciprocating driver and load

                                                                  • Temperature sensitivity

                                                                    • Torque capacity to diameter

                                                                      • Torque overload capacity

                                                                        • &HEADLINE>Metallic couplings&/HEADLINE>

                                                                          Advantages

                                                                        • Torsionally stiff

                                                                          • Good heat capability

                                                                            • Good chemical resistance

                                                                              • High power density

                                                                                • High speed and large shaft capability

                                                                                  • Zero backlash in many types

                                                                                    • Low cost per unit of torque transmitted

                                                                                      • Limitations

                                                                                      • Fail in fatigue or wear

                                                                                        • May need lubrication

                                                                                          • Often, many parts to assemble

                                                                                            • Most need careful alignment

                                                                                              • Poor vibration damping and shock absorption

                                                                                                • Electrically conductive, unless modified