“Tiger stripes” on a high speed pinion made of a carburized SAE 931 0 steel were investigated. The
stripes were on lines of action on the load side of the teeth coinciding with different angular positions of
the gear mesh. Scanning Electron Microscopy (SEM) of the affected areas showed fused metal particles,
with a diameter of 1 -3 microns, and gas pockets. The morphology of the damage was typical of electric
discharge damage shown in ANSI/AGMA 1 01 0-E95. This indicates that the stripes were in fact electric
discharge damage. Microhardness surveys on a metallurgical transverse section of a tooth showed a
hardness loss due to the discharge, with load side surface hardness even lower than 58 HRC. The cause
of the “tiger stripes” and potential damage to the gear tooth were analyzed.

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