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Sun Gear Fault Detection on an OH-58C Helicopter Transmission

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Sun Gear Fault Detection on an OH-58C Helicopter Transmission

Adrian Hood  Darryll Pines 
Research Assistant  Dean and Farvardin Professor 
Alfred Gessow Rotorcraft Center  A. James Clark School of Engineering 
Department of Aerospace Engineering  University of Maryland 
University of Maryland  College Park, MD 20742-3015 
College Park, MD 20742-3015 

Detecting seeded faults on a full-scale helicopter transmission is the focus of this paper. Two methods to isolate the dynamics of an individual sun gear, in an effort to assess its condition, are developed and validated on an OH-58C helicopter transmission’s planetary reduction stage. Planetary indexing is used to geometrically synchronize dynamic measurements with the positions of the meshing teeth. This provides the opportunity for source/signal mapping that can lead to increased sensitivity, allow-ing faults to be detected earlier and thus increasing the available time for corrective action. Special measurement and data processing techniques are needed to .lter out the effects of the planet gears, bearings, input spiral bevel stage, and other components in and around the gearbox. Two sun gears, which were damaged in previous fatigue tests, serve as the focus of this current work. A vibration sep-aration (VS) algorithm, for the non-sequential OH-58C transmission was developed and the resulting signals analyzed. A non-VS based method, which uses only the time synchronously averaged data and takes advantage of the signal/source mapping required for VS is also developed. The FM4 condition indicator is used to compare the two damaged cases with the baseline. Both algorithms were successful in identifying the damage on the sun gear with multiple faults. Sun gear damage was con.rmed by the presence of sun mesh groups.
Introduction

In the last 30 years, researchers have attempted to de-velop qualitative and quantitative helicopter gear fault de-tection methods. (Refs. 1–17) Typically, a system is com-pared to a previous undamaged state over many .ight hours and various tools are used to detect when the system de-viates from the baseline. The effectiveness of the method-ology is governed by it is ability to discriminate between changes due to operating conditions versus those truly due to damage. False alarms are potentially costly since its pos-sible that a helicopter may be unnecessarily removed from service for inspection and/or overhaul. In addition, the sys-tem should not be so insensitive that many faults are missed, leading to accidents and possibly the loss of lives. Thus, the number of false alarms, which can be as high as 1 per 1,000 .ight hours, must be kept to a minimum. (Ref. 18) In or-der to increase the probability of a correct diagnosis, it is best to learn as much about the system as possible. This is accomplished by understanding the gear system’s funda-mental properties and how 1.) they are affected by damage and 2.) how the damage manifests in measured signals.
 
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