Gearbox Noise Reduction: Prediction and Measurement of Mesh-Frequency Vibrations Within an Operating Helicopter Rotor-Drive Gearbox

Abstract

This paper presents correlations between analysis and test results for a complex mechanical system. While the paper is specificially concerned with a helicopter rotor-drive gearbox, the results and methods employed are general enough to be broadly applicable to gearboxes of all kinds. The development of gearbox noise reduction technology has been the objective of an extensive U. S. Army-supported program over the past five years. The gearbox noise problem has been recast as a mechanical vibrations problem, and detailed analytical methods have been developed to treat it. Gear excitation analyses, drive-train response analyses, and empirically-based acoustic spectrum prediction methods have been developed and published in considerable detail. Rolling-element bearing stiffness prediction methods and thin-shell vibration response prediction techniques have been shown to be essential elements in the procedure. In order to verify the analytical methods, a detailed and comprehensive test program was undertaken on a complete CH-47 forward-rotor-drive gearbox, operating under normal torque conditions in a test-cell environment. At the same time, predictions were made of quantities to be measured using the previously published analytical methods. Comparisons between predicted and measured quantities show reasonably good correlation, indicating that the analytical procedures are suitable for careful use in gearbox design or redesign efforts directed at vibration and noise reduction. Areas in which the analytical methods can be improved were also identified.