Gear Tooth Breakage — Failure Analysis & 8D Correcti...

Symptoms Tooth fractured at root fillet; metallic debris in oil Knocking/rattling noise; GMF sidebands in vibration spectrum Affected Components Spur, helical, bevel gears in transmissions, gearboxes Case-hardened steel (20MnCr5, 8620); shafts, bearings Detection Methods Vibration Spectrum: GMF sidebands, harmonics Oil Debris: Magnetic plug, spectrometric Fe analysis MPI: Surface cracks at root fillet Metallurgy: Microstructure, hardness profile, case depth SEM: Beach marks = fatigue; cleavage = overload Possible Causes Bending fatigue: cyclic stress > endurance limit at fillet Overload: single-event excessive torque Pitting → spalling → breakage: contact fatigue reducing cross-section Case-core interface failure: wrong case depth Grinding burn: residual tensile stress Misalignment: load concentration at tooth end 5-Why Example Problem: 3rd gear helical tooth broke at root fillet after 15,000 km Why break? → High-cycle bending fatigue — beach marks visible Why fatigue? → Cyclic bending stress > material fatigue limit Why high stress? → Fillet radius 0.3mm vs 0.5mm spec (30% higher stress) Why undersized? → Hob tool worn beyond allowable limit Why not replaced? → Tool life tracked by parts count, not profile measurement TRC: Stress concentration at undersized root fillet MRC: Hob cutter life management relied on parts count, not tool geometry verification Fishbone (6M) Man: Increased...