An explanation of frequency features enabling detection of faults in equally spaced planetary gearbox Liu Hong a , Jaspreet Singh Dhupia a, ⁎, Shuangwen Sheng b a School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798, Singapore b National Renewable Energy Laboratory, 15013 Denver West Parkway Golden, CO 80401, United States article info abstract Article history: Received 11 April 2013 Received in revised form 22 October 2013 Accepted 30 October 2013 Available online 6 December 2013 Equally spaced planetary gearboxes are important power-train components for varied engineering systems. Their failures can result in significant capital losses and pose safety concerns. The vibration measurements perceived by a sensor mounted on the gearbox housing can provide valuable diagnostic information without normal gearbox operation interference. However, such vibration based monitoring techniques are difficult to implement in planetary gearboxes because of the complex nature of measured vibration spectra that is a result of planets revolving with respect to the stationary sensors mounted on the gearbox housing. Previous research with simulations and experiments using such measurements has reported distinct sideband patterns in the resulting vibration spectra, which differ significantly from the spectra of a normal fixed-axis/parallel gear pair system. In this paper, Fourier series analysis is used to explain these distinct sideband patterns that contain rich diagnostic information. The results obtained are useful to understand the cause of the observed vibration behavior in both healthy and faulty planetary gearboxes and identify the locations of additional frequency components introduced by the damaged gear in a complex measured vibration spectrum. Thus, the formulation presented in this paper can assist in developing robust feature extraction algorithms for early detection of planetary gearbox failures. The theoretical derivations presented in this paper are validated by both dynamic simulations and experiments on a dynamometer test bed using a 750 kW gearbox damaged during its operation while installed in a wind turbine. The predicted frequencies for observed faults in the annulus and sun gears of the gearbox are vividly presented in the experimentally measured frequency spectrum. © 2013 Elsevier Ltd. All rights reserved. Keywords: Planetary gear set Modulation Gear fault Diagnosis 1. Introduction Planetary gearboxes are commonly employed in heavy-duty powertrains for energy, maritime, and aerospace applications. Some of the typical applications include wind turbine gearboxes, planetary reducers for ships, and transmission gearboxes of helicopters. Any failures with these planetary systems have a potential to cause expensive repair and overhaul, large unscheduled downtime, and safety concerns. The perceived vibration response of sensors fixed to the annulus gear of planetary gear systems can provide useful diagnostic information pertaining to the health of such systems [1–3]. Therefore, it is essential to understand the vibration behavior of healthy as well as faulty planetary gearboxes for development of robust condition health monitoring techniques that can detect faults at earlier stages. For an error-free fixed-axis/parallel gear pair, the vibration spectrum contains only dominant components at the gear meshing frequency (MF) and its harmonics. If there is any gear damage (e.g., a tooth on one gear develops a spall or a crack), it results in modulations of the amplitude and frequency in the original vibration measurements that are periodic at the shaft rotation frequency Mechanism and Machine Theory 73 (2014) 169–183 ⁎ Corresponding author at: Jaspreet Singh Dhupia, Assistant Professor, Division of Mechatronics and Design, School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798. Tel.: +65 6790 5506; fax: +65 6792 4062. E-mail address: djaspreet@ntu.edu.sg (J.S. Dhupia). 0094-114X/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mechmachtheory.2013.10.014 Contents lists available at ScienceDirect Mechanism and Machine Theory journal homepage: www.elsevier.com/locate/mechmt