Experimental detection of additional harmonics due to wear in journal bearings using excitation from a magnetic bearing Athanasios Chasalevris a,n , Fadi Dohnal b , Ioannis Chatzisavvas a a Technische Universität Darmstadt, School of Mechanical Engineering, Institute for Dynamics of Structures, 64287 Darmstadt, Hessen, Germany b ALSTOM Ltd., TTT Rotordynamics, 5401 Baden, Switzerland article info Article history: Received 26 April 2013 Received in revised form 25 November 2013 Accepted 1 December 2013 Available online 7 December 2013 Keywords: Wear Bearings Detection Magnetic bearing abstract An experimental rotor bearing system with an elastic rotor mounted in worn journal bearings is designed and investigated. The system is operated at run-up and run-down conditions and the response is analyzed with emphasis on passage through the first critical speed. Wear introduces additional sub- and super-harmonics in the response signal compared to the intact system. Time histories are analyzed in the time–frequency domain and bispectrum analysis is also performed in order to extract information on the influence of wear depth to these additional harmonics. Wear in one journal bearing is artificially produced and two cases of wear depth are investigated, one of 20% of bearing radial clearance and one of 40%. & 2013 Elsevier Ltd. All rights reserved. 1. Introduction Wear in journal bearings is considered as a defect developed after long-term operation of rotor bearing systems. Hydrodynamic journal bearings, supporting rotating shafts for a long period of time, are the cause of significant wear in the bearing surface. The detection of the new operating radial clearances is of great importance since the defect leads to a decrease of the bearing load carrying capacity and alters the dynamic behaviour of the rotor bearing system. Wear as a defect and its influence on the dynamic characteristics of the system is an object that has been investigated mostly during the last three decades and for its detection there are also contributing researches in the literature. State of the art concerning behaviour of worn bearings and methods to detect or even identify presence and extent of wear in journal bearings is presented herein. The onset and development of wear in plain hydrodynamic journal bearings under repeated stop/start cycles have been studied experimentally by Mokhtar et al. [1]. Wear was easily discernable, but localized changes in diametric clearance, surface finish and roundness of the bearing bore were measured after varying numbers of operating cycles had been completed. A study of wear location within the bearings showed that it was caused entirely by the sliding motion occurring during start-up and that no significant contribution to the wear process was a result of shutdown. This group also observed that, once an initial rapid wear phase was completed, the surface finish of the hardened steel shaft was reproduced in regions of the bearing surface subjected to continuing wear. Dufrane et al. [2] investigated a worn journal bearing and established a model that describes wear geometry. This worn model was not of circular type. Two models of wear geometry were established to be used in further analysis of the effect of wear on hydrodynamic lubrication. These wear models are not of circular type. The first of the proposed models is based on the concept of imprinting in the bearing and the second one is based on a hypothetical abrasive wear model with the worn arc at a radius larger than the journal. Dynamic characteristics of the fluid film in journal bearings of various types, including worn bearings, where evaluated by Vaidyanathan and Keith [3]. Wear effects in journal bearing performance for turbulence and laminar regimes were examined theoretically by Hasimoto et al. [4]. A study on the effect of non- circular wear pattern on the stability of a rigid shaft was performed from Kumar and Mishra [5], using two journal bearings with turbulent flow, while thermodynamic analysis of worn fluid film bearings was performed by Fillon and Bouyer [6] concluding that a benefit of wear is the lower temperature of operation. Measurement of wear in journal bearings was discussed by Ligterink and de Gee [7] through a distinction between stationary and non stationary contact conditions using the Holm/Archard's wear law [8] that defines the influence of the bearing load to the wear rate. Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/triboint Tribology International 0301-679X/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.triboint.2013.12.002 n Corresponding author. E-mail addresses: chasalevris@sdy.tu-darmstadt.de (A. Chasalevris), fadi.dohnal@power.alstom.com (F. Dohnal), chatzisavvas@sdy.tu-darmstadt.de (I. Chatzisavvas). Tribology International 71 (2014) 158–167