Gear tooth stiffness reduction measurement using modal analysis and its use in wear fault severity assessment of spur gears Isa Yesilyurt a, * , Fengshou Gu b,1 , Andrew D. Ball c,2 a Kocatepe Universitesi, Usak Muhendislik Faku ¨ltesi, Usak 64200, Turkey b Maintenance Engineering Research Group, Manchester School of Engineering, University of Manchester, Manchester M13 9PL, UK c Manchester School of Engineering, University of Manchester, Manchester M13 9PL, UK Received 14 November 2002; revised 14 January 2003; accepted 14 January 2003 Abstract Due to excessive service load, inappropriate operating conditions or simply end of life fatigue, damage can occur in gears. When a fault, either distributed or localised, is incurred by gears, the stiffness and consequently vibration characteristics of the damaged tooth will change. A possible non-destructive technique for damage detection and severity assessment can be derived from vibration analysis. This paper presents the use of vibration analysis in the detection, quantification, and advancement monitoring of damage incurred by spur gear teeth. The stiffness of a single spur gear tooth is analysed theoretically, and due to the difficulties in measuring the gear tooth stiffness, an experimental procedure based on the modal analysis is developed to assess the severity of the gear tooth damage. A pair of spur gears was tested under accelerated wear conditions, and conventional time and frequency domain techniques are applied to the gear vibrations to indicate the presence and progression of the wear. The developed modal stiffness assessment technique is then used to quantify the resulting wear damage to the spur gear teeth. q 2003 Elsevier Science Ltd. All rights reserved. Keywords: Gear damage; Spur gear tooth stiffness; Modal analysis; Gear wear; Gear vibration 1. Introduction Gear tooth damage can be caused by a variety of factors including inadequate lubrication, inappropriate operating conditions or specifications, material insufficiencies, and manufacturing or installation problems. Effective lubrica- tion of gear systems is of critical importance because it prevents direct tooth contact, reduces friction and vibration levels, removes heat generated in the mesh, and protects the gears from corrosion. When the tooth surfaces are subjected to excessive stress conditions, tooth surface failure may occur. This can cause removal and/or plastic deformation of the contacting tooth surfaces [1–3]. In some cases, surface- fatigue cracks occur in plastically deformed regions under excessive contact stress, and these can also be caused by scuffing or wear failure [2,4,5]. Once initiated, crack propagation is accelerated by the hydroulicing effect of gear lubricant and tangential tractive force. If surface deterioration is not corrected in the early stages of development, catastrophic tooth failure may result. Tooth surface failures are progressive failure mechan- isms and can be detected at macroscopic level by the naked eye. In some cases, an individual gear tooth may be weaker than others on the same gear due to bending fatigue, shock loading or an internal void. Bending fatigue failure may start with a small crack either on the free surface of the gear tooth [1,6,7] or below the tooth surface, where the maximum shear stress occurs [8]. Once initiated, it can progress rapidly giving little warning of gear deterioration and resulting in an abrupt breakage. In general, tooth damage causes a reduction in gear tooth stiffness, and severity of tooth damage can be assessed by considering the reduction in its stiffness. Tooth stiffness is a key parameter in gear dynamics in determination of factors such as load-carrying capacity of gears, dynamic tooth loads, vibration characteristics of geared system, and many 0963-8695/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0963-8695(03)00011-2 NDT&E International 36 (2003) 357–372 www.elsevier.com/locate/ndteint 1 Tel.: þ44-161-275-4458; fax: þ44-161-275-4433 2 Tel.: þ44-161-275-4347; fax: þ44-161-275-4372 * Corresponding author. Tel.: þ90-276-263-4195; fax: þ 90-276-263- 4196. E-mail addresses: iyesilyurt@isbank.net.tr (I. Yesilyurt), fgu@fs1.eng. man.ac.uk (F. Gu), adball@fs1.eng.man.ac.uk (A.D. Ball).