Drives & Controls June 2022

34 n MECHANICAL POWER TRANSMISSION June 2022 www.drivesncontrols.com C ouplings are sometimes overlooked compared to components such as motors, gearboxes and clutches, but specifying a suitable coupling can have a major impact on the performance of a powertrain. Once the main elements of a powertrain have been specified, it’s important to review coupling options to see how the relative performance of different styles may affect your design. When development schedules are tight, designers may be tempted to specify the most easily available coupling. This may be a false economy because the wrong coupling can lead to problems including premature wear of components, poor efficiency, vibrations, and inadequate torque to turn the shaft and coupling. This may necessitate costly and time-consuming redesigns. Misalignment Misalignment is the variance between the ideal and actual position and attitude of two shafts. Depending on the application, couplings may have to accommodate angular, axial or radial misalignment. Misalignment is often caused by manufacturing tolerances, but can be further affected by thermal imbalances, wear, settlement and creep during use. The optimal method for gauging misalignment between two shafts is to measure the system both at operating temperature and when cold. Assessing the tolerances of other elements of the powertrain is also instructive. Failure to accommodate misalignments can increase the wear of components such as motor bearings, and can shorten coupling service lives. Typically, as misalignment increases, the torque capacity and service life of a coupling will decrease. Accommodating higher levels of misalignment usually requires a longer coupling. For example, a bellows coupling with more bellows will accommodate higher degrees of angular misalignment. Radial misalignment can influence the length of a coupling dramatically, because it must bridge two angles to join the shafts. However, some coupling designs can break these rules, allowing designers to accommodate large amounts of misalignment in a limited space. In general, introducing increased coupling flexibility to accommodate misalignment reduces positioning accuracy. The key is to strike a balance between these two factors, based on the application. Torque To improve the torque rating of a coupling, it usually needs to be bigger. Materials and design also have an influence. A stainless- steel bellows coupling will offer more torque capacity, size-for-size, than an Oldham coupling. This is because the Oldham design has an internal plastic disc that limits its torque capacity. Another consideration is how torque is being applied in the powertrain. The catalogue torque rating of a coupling is based on its performance at a constant speed in a single direction. However, if you introduce acceleration, deceleration or changes in rotational direction into an application, you reduce the coupling’s torque capacity. For example, a coupling with 10Nm torque rating will drop to 3 or 4Nm with acceleration or deceleration in alternating directions. In such applications – such as in robots or pick-and-place machines – a larger coupling will be needed. This is the measure of a coupling’s resistance to torsional rotation. High stiffness is important for applications where Choose your couplings with care Ensuring efficient, reliable connections between shafts is paramount for system reliability and to avoid costly redesigns. David Lockett, managing director of the couplings manufacturer Huco, explores what designers need to consider when specifying couplings. Mechanical couplings come in a wide variety of technologies, sizes and capacities, each tailored to specific applications.