AGMA 08FTM05 PDF

Aerospace gears require post case-hardening grinding of the gear teeth to achieve their necessary accuracy. Tempering of the case hardened surface, commonly known as grinding burn, occurs in the manufacturing process when control of the heat generation at the surface is lost. A gearbox with minimal service time was removed in service from an aircraft, disassembled, and visual inspection performed. Linear cracks along the dedendum of the working gear tooth face were found in three adjacent teeth. A detailed inspection of the gearbox found no other components with distress. AGMA 2007-C00 provides details of the temper etch process and exclusively uses a Nitric acid etch process, which is typically used in production quality inspections. The incident gear was processed for grinding burn using an Ammonium Persulfate etch solution. Quality records documented variation in chemical concentration levels during the time the failed gear was manufactured. A design of experiments was conducted to understand the effects of the factors and interactions that impact the capability of the Ammonium Persulfate process used in production to detect griding burn. Presented are the metallurgical findings, load distribution analysis of actual geometry, crack propagation analysis, and design of experiment results of the Ammonium Persulfate etch process.