Determination of the crack propagation life of a planetary gear carrier

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Determination of the crack propagation life of a planetary gear carrier

Sahrmann, Glenn J.

During flight a U.S. Army helicopter experienced a low transmission oil pressure indication with accompanying caution light illumination. There was no illumination of the transmission chip light. Large foil type chips were found in the gearbox sump. These chips impeded oil flow to the pumps causing the low oil pressure indication. Upon teardown of the main transmission it was determined that these chips were from an aluminum shim located between a support housing and a bearing housing. A large crack in the planetary gear carrier was found. Subsequently, a second U.S. Army helicopter experienced a low transmission oil pressure indication. Teardown of the main transmission module revealed the cause to be an internal oil leak. Fluorescent dye penetrant inspection of the planetary carrier revealed the presence of an approximately five inch long crack, which except for length was nearly identical to the first crack. These cracks initiated in the blend between the gear post and the plate. An ultrasonic probe was developed by the aircraft manufacturer to inspect the planetary carrier without requiring its removal from the aircraft. This paper summarizes work performed to determine the crack propagation life of a planetary gear carrier, and thus a reliable inspection interval. The work included: finite element analysis, crack growth modeling, strain surveys of a planetary gear carrier, as well as metallurgical and fractographic analysis of the cracked parts. The integration of the components of this multi-disciplinary investigation will be discussed.