1975-02-01

Early Detection of Defects in Rolling-Element Bearings 750209

This paper presents the results of studies which demonstrate the feasibility of the High-Frequency Resonance Technique for defect analysis of rolling-element bearings in virtually any bearing system. Specific emphasis was placed on helicopter engine and transmission applications. The HFRT is a method for separating ball-pass or roller-pass frequency peaks from background noise by isolating and demodulating high frequency spectrums of accelerometer signals. The process is also called envelope detection.Tests were conducted with bearings from a UH-1 helicopter transmission to show the effectiveness of the HFRT explicitly for that application and implicitly for other bearing applications. The resonance peaks for these tests were selected by inspection of raw data signals and were found to correlate reasonably well with predictable bearing race natural frequencies. The test bearings were mounted in a rotating dynamic response test rig and run under various conditions of speed, load, and bearing housing design. Both good bearings and bearings with artifically-induced discrete defects were tested. Signals were recorded from accelerometers located in various positions. The resulting data was analyzed using the HFRT, and showed as much as an order of magnitude increase in the amplitude of the envelope-detected ball-pass or roller-pass frequency peaks for the bearings with the defects as compared to the same bearings without defects. Variations in speed and load were found to have relatively little effect on the defect signals' magnitudes.Similar signals were recorded from accelerometers mounted externally on an operating UH-1 helocipter main rotor drive transmission during ground tests. This data, analyzed using the testing criteria developed from the results of the test rig investigations, shows that the HFRT can readily detect bearing defect signal components in the presence of high background noise levels, such as those which would be encountered in a turbine engine. In combination with the test rig results, these results infer that the HFRT is not only feasible for use with the bearings of a UH-1 helicopter transmission, but may also be applied to other bearing systems as well.

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