Tribology International 41 (2008) 221–233 Operating characteristics of the bump foil journal bearings with top foil bending phenomenon and correlation among bump foils Yong-Bok Lee à , Dong-Jin Park, Chang-Ho Kim, Seung-Jong Kim Korea Institute of Science and Technology, Tribology Research Center, Seoul 136-791, Republic of Korea Received 18 January 2007; received in revised form 14 June 2007; accepted 2 July 2007 Available online 29 September 2007 Abstract In many previous studies, the experimental results show the ripple traces of the post-test top foil due to the top foil deflection differences between the areas touched with and without bump foil. This phenomenon represents clearly the behaviour of the top foil motion. Therefore the top foil deflecting appearance should be taken into consideration for more reliable estimation of the bump foil bearing behaviour. However inherent top foil bending effect was ignored under assumption to the rigid body able to act only vertically. In this paper, a numerical analysis and its experimental investigation were studied with the intention to abstract the static and dynamic characteristics of the bump foil bearing considering the top foil bending effect and correlation among bumps. The bump and top foil inserted between the journal and the bearing housing have each mechanism as a shock absorber and journal supporter, and each mechanism was analysed using numerical method, respectively. In the case of the top foil, the deflection of that and Coulomb damping generated by relative friction was taken into consideration theoretically. In addition bump foil analysis was executed to be assumed to have a structure that flexible bumps with corrugated shape are connected with each other. This analysis verifies that the stiffness at the fixed end where the friction forces between the bearing housing and bump foil superpose is more than that at the free end. Using this structural analysis, the overall analysis of the bump foil bearing was executed through coupling process. In parallel, the experimental process was carried into execution with the bump foil bearing that L/D is 1. The rotating speed was from 10,000 up to 30,000 rpm and the load was 50 N. By comparing the analysis with the experimental results, the effects of the structural characteristics of bump foil were presented. r 2007 Elsevier Ltd. All rights reserved. Keywords: Bump foil bearing; FDM; FEM; Structural damping; Frequency response function; Uncertainty analysis 1. Introduction The bump foil bearing is simple with respect to structures and has received a large amount of interest because the bearing can endure severe environment such as high temperatures and speeds. This bearing supports the journal, using the hydrodynamic effect of thin air film generated by rotating of journal and the load carrying effect of flexible bump makes it possible to deflect elastically. The bump foil bearing can guarantee stability and accommodate the expansion of the rotating journal due to high temperatures and centrifugal force generated by high speeds. Many studies on the bump foil bearing were executed. Walowit calculated analytically the structural stiffness of the bump using the circular beam equation about the inner mounted bump by assuming plane strain [1]. The bump stiffness equation suggested in that study did not consider the friction effect between the bumps and housing or bumps and top foil. Heshmat analysed numerically the bump foil bearing by using Walowit’s equation for bump stiffness [2]. He assumed that the bump foil is an elastic foundation and used the compressible Reynolds equation about thin air film between the journal and the top foil. For ARTICLE IN PRESS www.elsevier.com/locate/triboint 0301-679X/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.triboint.2007.07.003 à Corresponding author. E-mail addresses: lyb@kist.re.kr (Y.-B. Lee), djpark@kist.re.kr (D.-J. Park), kimch@kist.re.kr (C.-H. Kim), sjongkim@kist.re.kr (S.-J. Kim).