Metallurgical Examination of a Failed Spiral Bevel Gear
Abstract
A failed spiral bevel transmission gear from an Army cargo helicopter was first sent to the primary contractor for analysis and then the Army Research Laboratory. The failure occurred during a training flight at Ft. Meade, Maryland, where a reported 'loud bang and shutter' forced an immediate landing. Subsequent inspection of the Number 2 engine revealed a 15-20 degree section of the gear had fractured and penetrated the transmission housing. The gear was fabricated from X-2M steel, a high hot hardness alloy. Light optical microscopy of the failed gear section revealed characteristics consistent with a fatigue failure, including a relatively flat fracture surface and beach marks. The fracture origin was located at a defect characterized by a darkened half-moon shaped region. The darkened topography was considered evidence that a pre-existing crack was present during the manufacturing process of the gear. Energy dispersive spectroscopy of the blackened surface revealed the presence of sodium, indicating the crack was open to the black oxide finish process. The pre-existing crack was oriented perpendicular to the direction of grinding, indicating the possibility of a grinding crack. Metallography confirmed that a pre-existing crack was a grinding bum, since evidence of rehardening and retempering were observed. Also a possible contributory factor to crack propagation was the presence of a carbide network in the carburized case of the X-2M steel part. Metallographic examination of the damping ring groove area (fracture origin location) showed a deeper than acceptable carburized case. It was later learned that this region was not masked, and was subject to a double carburization by mistake. Further visual examination of the failed component revealed small grinding cracks at the comer of the tapered surface.
Document Details
- Document Type
- Technical Report
- Publication Date
- Apr 01, 1996
- Accession Number
- ADP010188
Entities
People
- Marc S. Pepi
Organizations
- United States Army Research Laboratory