Effectiveness of Impact-Synchronous Time Averaging in determination of dynamic characteristics of a rotor dynamic system Abdul Ghaffar Abdul Rahman a , Ong Zhi Chao a,⇑ , Zubaidah Ismail b a Mechanical Engineering Department, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia b Civil Engineering Department, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia article info Article history: Received 1 April 2010 Received in revised form 25 June 2010 Accepted 1 September 2010 Available online 17 September 2010 Keywords: Modal analysis Rotor dynamics Dynamic characteristics Gyroscopic effect Impact-Synchronous Time Averaging abstract Dynamic characteristics of rotor dynamic systems, namely natural frequencies, damping and mode shape, vary with rotation speed due to gyroscopic effect. To consider such effect, modal analysis is required to be performed while in operation. However, to generate the frequency response functions, it is not viable to perform data reduction in frequency domain as noise and harmonics excited by rotating forces dominate the spectrum and hide the natural frequencies. The full set of dynamic characteristics is difficult to obtain. A novel technique, referred to as Impact-Synchronous Time Averaging (ISTA), is developed in this study to screen out the noise and harmonics. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Efficient operation of rotating machines often requires high rotational rates, which gives rise to concerns that the system will resonate at unacceptable large amplitudes and become unstable. A comprehensive study of rotor dynamics is needed to overcome the problem of vibration, resonance and failure of structure in order to create faster and highly productivity of rotating machines [1]. Machines with a non-rotating shaft behave much like structures. However, once the rotor is spinning, the modes are no longer planar. With radially symmetric bearings, the rotor center traces out a circle. The rotor whirls either in the same direction as rotation, or against rotation, resulting in both forward and backward whirl modes. When a gyroscope is spinning rapidly, it has a large amount of a conserved physical quantity called angular momentum. Angular momentum is a special measure of rotational motion that cannot be created or destroyed. It can only be transferred between objects. If a twist is applied to the gyroscope around its axis of rota- tion, it will either spin faster or slower, depending on which way the gyroscope is twisted. A gyroscopic effect occurs in rotating structures whenever the mode shape has an angular (conical/rocking) component. For forward whirl, as shaft speed increases, the gyroscopic effects essentially act like an increasingly stiff spring on the cen- tral disk for the rocking motion. Increasing stiffness acts to increase the natural frequency. For backward whirl, the effect is reversed. Increasing rotor spin speed acts to reduce the effective stiffness, thus reducing the natural frequency. In the case of the cylindrical modes, very little effect of the gyroscopic terms was noted, since the center disk was whirling without any conical motion. Without the conical motion, the gyroscopic effects do not appear. Thus, for the soft bearing case, which has a very cylindri- cal motion, no effect was observed, while for the stiff bearing case, which has a bulging cylinder and thus con- ical type motion near the bearings, a slight effect was noted [2]. 0263-2241/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.measurement.2010.09.005 ⇑ Corresponding author. Address: Department of Mechanical Engineer- ing, Engineering Faculty, University of Malaya, 50603 Kuala Lumpur, Malaysia. Tel.: +60 3 79675270; fax: +60 3 79675317. E-mail addresses: agar@um.edu.my (A.G.A. Rahman), zhichao83@ um.edu.my (O.Z. Chao), zu_ismail@um.edu.my (Z. Ismail). Measurement 44 (2011) 34–45 Contents lists available at ScienceDirect Measurement journal homepage: www.elsevier.com/locate/measurement