Characterization of Torsional Vibrations: Torsional-Order Based Modal Analysis

Torsional vibrations are angular vibrations of a rotating machine, typically a shaft along its axis of rotation. When these vibrations are amplified, they could lead to comfort, efficiency, noise or coupling wear problems. Torsional vibrations are important in cases in which the power needs to be transmitted using a rotating shaft or couplings, such as in the case of automotive, truck and bus drivelines, marine drivelines or power-generation turbines. Their characterization both in terms of frequency and damping ratios is quite difficult since they are influenced by several parameters such as material properties and operating conditions (temperature, load, rpm, etc). New powertrain designs, such as start-stop systems, downsized engines and lighter powertrains, increase the importance of developing an in-depth understanding of torsional vibrations. These are the main reasons behind the development of the so-called Torsional-Order Based Modal Analysis (T-OBMA) technique. As its name suggests, this technique is base upon the Order-Based Modal Analysis (OBMA) technique that showed to be very powerful for identifying the modal parameters in operational conditions in case of rotating machines during transient operations. The classical OBMA is performed by measuring both the rotational speed (by means of zebra tapes or digital encoders) and some other quantities in several points on the structure itself (accelerations, deformations, etc.). On the other hand, T-OBMA is focused on the identification of the torsional modal parameters by measuring only the rotational speed in two or more points along a driveline. The technique has been validated both in a simulation environment and in a real test scenario. In this paper the main outcomes of the simulations will be explained and the guidelines to apply it in a rotating machinery context for torsional vibrations identification will be listed.