Fault Detection on a Full-Scale OH-58 A/C Helicopter Transmission

Abstract

Detecting seeded faults on a full-scale helicopter transmission is the focus of this work. Methods to isolate the dynamics of an individual sun gear, in an effort to assess its condition, are developed and validated on an OH-58 helicopter transmission's planetary reduction stage. This area has been shown to be challenging because the planetary system does not allow for direct measurements of the sun gear. Instead, special measurement and data processing techniques are needed to filter out the effects of the planet gears, bearings, input spiral bevel stage, and other components in and around the gearbox. Planetary indexing is used to geometrically synchronize dynamic measurements with the meshing tooth's position along its pressure line. This provides the opportunity for source/signal mapping that can lead to increased sensitivity, allowing faults to be detected early and thus increasing the available time for corrective action.

Accelerometers mounted along the transmission housing, acoustic transducers distributed about the test cell, and an oil debris monitoring system are all used to analyze three seeded fault cases. Transmission components, (two sun gears and a single planet bearing), which were damaged in previous fatigue tests, serve as the focus of this current work. Two vibration separation (VS) algorithms, tailored to the three planet OH-58A, and the four planet, non-sequential OH-58C transmissions, were developed and their resulting signals analyzed. In addition, a geometrically synchronized measurement method to transmission diagnostics is also developed. This non-VS based method uses only the time synchronously averaged data and takes advantage of signal/source mapping required for VS. Eleven commonly used condition indicators are used on both global and separated signals and their results tabulated.

All three damage detection algorithms were successful in identifying the damage on the sun gear with multiple faults. Sun gear damage was confirmed by the presence of sun mesh groups. Detecting the single tooth spall continues to be a challenge. Also demonstrated is the ability for the vibration separation methods developed to isolate components.

Safety and cost are the main motivators for helicopter Health Usage and Monitoring Systems (HUMS). During flight, critical components are subjected to sustained vibratory and impulsive loads requiring the need for frequent inspections. Methods that can reduce this time and effectively detect faults in their infancy are highly sought. Actively monitoring the transmission's health can provide the benefit of detecting damage early and possibly avoid catastrophe. In addition, active monitoring provides an updated assessment of a component's condition which can possibly increase its life when compared to scheduled replacement times. The methods proposed for gear tooth diagnostics can be integrated in an overall helicopter HUMS program with the main objective of cost-effectively improving the safety of both civil and military helicopters while reducing the cost of ownership.