June 2019

70 n BRAKES, CLUTCHES AND COUPLINGS June 2019 www.drivesncontrols.com Overcoming torsional vibrations in drivetrains V ibration can occur in any drivetrain, whether it is the low-level, naturally occurring type produced by combustion engines or electric motors as a result of their speed and number of cylinders or stator poles, or being induced as a result of other factors. Either way, if left unchecked, vibration can be the root cause of premature wear or even catastrophic failures of drivetrains. Many drivetrain problems can be traced back to some form of vibration. There are many factors that can influence the type and severity of the vibrations. Some may relate to equipment moving slightly over time, out-of- balance items, poor alignment between the different drivetrain elements during installation, or instances where the vibrations occur as a result of the natural characteristics of a motor or engine. These naturally occurring vibrations are somewhat predictable, usually low level, and the effects can often be minimised to a large extent by considering how the items are mounted or isolated. However, there may be instances where this is not possible, or where particular operating conditions or a specific configuration intensify the vibrations. For example, in a diesel engine, the number of cylinders, their firing order, and speed of operation can induce torque pulses, adding to the normal torque transmitted through the drivetrain. These pulses pass through the train in the form of torsional vibrations, which are often much more difficult to identify, because they do not exhibit the same visible characteristics as vibrations caused, for example, by misalignments. Torque pulses also add a twisting effect on the drivetrain and the various elements react in the same manner as a spring might, by releasing the energy generated by the initial twisting motion – but in the opposite direction. The rate at which this happens is determined by the stiffness of the different elements in the drivetrain which, in turn, determines the frequency of the system. The effects of the excitation introduced to the driven systemwill also vary depending on the RPM. Where this frequency matches the natural frequency of the system, resonance will occur. In normal use, the speed can pass through critical areas without issue, providing the transition is achieved quickly. If, however, the speed remains in a critical region for long periods – whether at idle or at normal operating speeds – problems resulting from torsional vibration will occur. To avoid these issues, a detailed torsional vibration review should be undertaken, and calculations made to allow a suitable drive coupling to be chosen for the application. Couplings are muchmore thanmere physical connections between drive elements. They protect the drivetrain by being able to accommodate mechanical discrepancies, such as axial, angular or radial misalignments, and also mitigating other factors such as shock loads, starting frequencies, temperature variations, environmental conditions and, of course, vibration. Selecting a drive coupling that has the correct torsional stiffness characteristics is the first step to ensuring that vibration does not become the prime cause of failure. The process of choosing a coupling should take into account all of the factors that it is likely to encounter in service. Discussing the application with a reputable coupling supplier, and providing them with the information needed to undertake a torsional analysis, will ensure that the chosen coupling will be capable of accommodating the natural frequencies, while at the same time reducing any effects from torsional vibration. If you have experienced premature failure of existing couplings, you will need to consider whether excessive torsional vibration played a part in that failure, and ensure that any new coupling has been selected following a torsional analysis of the application. One example of a coupling designed to absorb high levels of torsional vibration and shock loading, and to protect drivetrains against overloads, is Reich’s Arcusaflex. This is a flexible flywheel coupling that provides a torsionally soft connection between a driver and a driven machine. The flexible torque transmission is achieved using a disc-shaped rubber element that is subjected to a torsional load and both absorbs high torsional vibrations and compensates for major misalignments. The inside diameter of the rubber disc element is vulcanised directly to a taper hub or bolt-on sleeve. A toothed profile on the circumference of the element provides an almost backlash-free, positive plug-in connection to the coupling flange. Shaft-to- shaft connections where required, can be achieved by shaft couplings consisting of standard flywheel couplings equipped with a second hub. n If not controlled effectively, torsional vibrations can damage machinery. David Proud, general manager of Reich Drive Systems UK, explains how to overcome the effects of vibration using drive couplings with a high torsional flexibility. Reich’s Arcusaflex coupling provides a torsionally soft connection between a prime mover and the driven machine Software tools are available to help choose the most appropriate coupling for an application