Chapter 20 A Rational Basis for Determining Vibration Signature of Shaft/Coupling Misalignment in Rotating Machinery Changrui Bai, Surendra (Suri) Ganeriwala, and Nader Sawalhi Abstract Shaft misalignment is the most common fault in rotating machinery besides unbalance. A poorly aligned machine can cost a factory upward of 30–40% in machine down time, replacement parts, inventory, and energy consumption. Vibration analysis is prompted as a most common methodology for determining misalignment while a machine is in operation. Considering the importance of alignment, the vibration spectrum of misalignment lacks a consensus and is elusive. This work is an evolution from the research performed on a large body of the vibration data to determine a unique vibration signature for shaft/coupling misalignment while operating under varying conditions such as speed, type and level of misalignment, coupling types and machinery dynamic stiffness. The data is analyzed from tests conducted on different machinery fault simulators operated at several shaft speeds, types of couplings, shaft diameters, structural stiffnesses, and multiple misalignment configurations. The results indicate a confusing picture of misalignment vibration signature. In this paper we present the results of vibration data analysis and outline an approach for vibration analysis of the shaft/coupling misalignment of rotating machines. This includes uses of rotor frequency response function and physics based predictive model. Keywords Misalignment · Vibration spectrum · Machinery dynamics stiffness · Misalignment forces · Rotor dynamics model · Diagnostics model · Machinery fault simulator 20.1 Introduction When two misaligned shafts are joined together by a coupling, the machine structure is subjected to deformation (strain). The deformation will be different at each angle of rotation, depending on the type and level of misalignment. The corresponding stress will depend on the stiffness of the machine structure. The Fourier transform of the rotation angle dependent deformation (or stress) curve will contain several terms. When the machine starts turning, the varying stress will produce varying forces which lead to vibration at each of the Fourier components, assuming a linear relationship. The problem is complicated even further due to the inherent nonlinearities of the machine. Vibration signatures are widely promoted for studying machine malfunctions. However, the literature does not present a clear picture of signature characteristics uniquely attributable to misalignment [1–4]. One of the reasons for inconsistency is that a rotor exhibits several critical speeds over a range of frequencies used to characterize misalignment spectra. When fault frequencies fall near associated resonances their amplitude gets amplified and when they fall near anti-node the amplitudes are attenuated. This is a report of a systematic series of experiments designed to determine the consistent features, if any, of vibration signatures for misaligned machinery. Operating and design parameters such as shaft speed, shaft diameter, type and level of misalignment and coupling types all have been reported to exhibit the effect on shaft misalignment. In this report we systematically varied coupling type or stiffness while all other parameters were held constant. The machine was fault-free with the exception of deliberate misalignment, which was also varied systematically. Baseline vibration data were recorded for each test condition. The figures below pictorially depict the common two types, parallel and angular, shaft/coupling misalignment. Here a centrifugal pump driven by a motor is shown to be misaligned. One can easily see that under any of two conditions when motor starts driving the pump, there will be stresses on the entire structure. The level of stress will obviously depend on coupling type and structural stiffness. The results will show the effect (Fig. 20.1). C. Bai · S. Ganeriwala ( ) · N. Sawalhi Spectra Quest, Inc., Richmond, VA, USA e-mail: suri@spectraquest.com © The Society for Experimental Mechanics, Inc. 2019 D. Di Maio (ed.), Rotating Machinery, Vibro-Acoustics & Laser Vibrometry, Volume 7, Conference Proceedings of the Society for Experimental Mechanics Series, https://doi.org/10.1007/978-3-319-74693-7_20 207
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