Analysis of micropitting of prototype surface fatigue test gears

The early stages of micropitting were examined on experimental gears designed for surface fatigue studies by the Helical Gear Rating Committee of the American Gear Manufacturers Association. Optical profilometry, scanning electron microscopy, and metallographic examinations were conducted on the first prototype gearset tested. The gearset was tested in an FZG test rig. To evaluate tooth bending fatigue strength, the gears were loaded to load stage 12. A tooth broke from bending fatigue in 0.88 h (the expected time). This short runtime allowed for the examination of micropitting at an early stage of development. The distribution and morphology of micropitting over the tooth face and associated relationships with surface micro- and macrogeometry features and traction have been examined. Micropitting was primarily associated with finish geometry. A few pits were associated with sulphide inclusions and one pit was possibly initiated by a small non-metallic inclusion. Metallographic examinations revealed asperity-related lines of dark etching alterations caused by high asperity contact and traction stresses.