In this type of failure pits are formed on the tooth flank. These pitted areas can occur on the addendum as well as on the dedendum side of the tooth. Initially a crack is formed in tooth flank, which may be due to various reasons. One reason may be due to high localization of stresses. Into this crack, oil will be forced due to the oil film pressure. If the movement of the tooth load is such that it closes the mouth of its crack thereby preventing the oil from escaping, the pressure of this trapped oil increase more and more as the load progresses further. This will extend the crack along the line of maximum stress and a pit will be formed.
Failure due to plastic flow:
Pitting is particularly observed with case hardened gears and with quenched and tempered gears. With softer steels, when the loading is high enough to exceed the yield stress, plastic flow of material takes place and the tooth form is damaged.
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