Drives & Controls June 2022

35 www.drivesncontrols.com June 20 22 MECHANICAL POWER TRANSMISSION n a high signal integrity or positional accuracy are required. Conversely, reduced torsional stiffness plays an important role when accommodating higher misalignments, or where torsional damping against shock loads is desirable. Once again, designers must consider their application’s needs and compromise between accuracy and accommodating misalignment. A bellows coupling offers high torsional stiffness, making it ideal for precision applications, but is limited at accommodating misalignment. However, a plastic double-loop coupling with a plastic element swaged between two hubs, offers high misalignment capabilities but low torsional stiffness, and is not unsuitable for precise operations. Stiffness Torsional stiffness is not as important for couplings with frictional loads – such as pumps, shutter doors and other machinery – because synchronisation of the shafts is not an issue. However, with inertial loads, torsional stiffness is important because it stops what is being driven from “lagging” behind. If it is not torsionally stiff under load, a coupling can act as a spring. This will delay the transfer of motion slightly, which is undesirable for signal integrity or positional accuracy. For example, inertia between the motor and the driven load is an area of potential instability for a coupling, which can be influenced by torsional stiffness. A coupling placed between the rotor of a motor and the rotor of a pump, both of which are inertial loads, will act like a spring. As the motor starts, it will twist the coupling before motion is transferred, causing lag which will produce instability if the effect is too great. As the inertial load increases, the effect becomes more pronounced. These instabilities can build on each other to create resonance, which can reduce the service life of the entire powertrain because vibrations place undue stress on critical components. While impossible to eliminate this phenomenon entirely, it can be minimised by tailoring the torsional stiffness or load of the coupling by choosing appropriate designs, materials and sizes. It’s worth considering the risk during specification, so that the correct tolerance is designed in. Footprint While it might seem obvious, it is vital to allow adequate space for a coupling. Every design will be different, but typically the outside diameter of a coupling should be at least twice the shaft diameter. Length depends on the type and level of misalignment that must be accommodated. An Oldham coupling will typically be three times as long as the shaft diameter. The size of the coupling will also be influenced by the peak torque capacity required. For example, joining 3mm shafts and allowing 0.5 degrees of angular misalignment, needs a coupling that’s about 12.7mm long. While it may be tempting to assume that joining two shafts will be simple, it’s not always the case. Some couplings suppliers offer customisation services to help engineers to meet the demands of specific applications. They can modify existing designs or produce entirely new ones. Furthermore, the ability to produce working 3D-printed examples rapidly for testing purposes can cut development times. n For an example of couplings in action, see page 37. Oldham couplings have two hubs and a central torque disc, and combine precision with reliability