= Gear surface pitting
<Gear surface pitting> is local loss of tooth-flank material associated with repeated contact loading and <mechanical fatigue>. The pits alter the surface geometry and can produce debris in the lubricant. This failure mode concerns the loaded flank; it must be distinguished from a tooth breaking through its section. <Hertzian contact> provides a basic elastic contact-stress model, while actual fatigue depends on the material, lubrication, load history and surface condition.<Predicted Effect of Dynamic Load on Pitting Fatigue Life for Low-Contact-Ratio Spur Gears by David G. Lewicki>[Summary, printed p.1; Summary of Results, printed p.11, dynamic loads and surface-pitting fatigue predictions]{ref}
A practical inspection can record a <gear pitting area fraction> $\phi=A_{\rm pit}/A_{\rm inspected}$, provided the inspected flank region and image scale are fixed. An original example with0.80mm² of projected pit area in a40mm² inspected region gives $\phi=0.02$, or2%. The same fraction in a smaller region need not represent the same damaged volume or depth. Consistent illumination and a stated area convention are needed before comparing successive inspections.
A2009 NASA study provides a motivating experiment. It analyzed24 sets of tapered involute spur pinions meshing with face gears in an endurance program. Vibration measurements were compared with an in-line oil-debris sensor whose coils detected passing metallic particles. Repeated inspections connected the signals to actual tooth-surface damage. The paper found debris responses for the pitting failures, while vibration indicators differed in their ability to detect different damage patterns.<Gear Fault Detection Effectiveness as Applied to Tooth Surface Pitting Fatigue Damage by David G. Lewicki and others>[Printed pp.1–5, test gears, instrumentation, inspection history and indicator comparison]{ref}
The experiment illustrates why one spectral amplitude is insufficient to identify pitting. The damaged geometry, its position among the teeth and the transmission path all affect a <gear vibration spectrum>. Combining repeated surface inspection with vibration and debris measurements tests a damage interpretation more directly than labeling every rise at <gear mesh frequency> as a pitting event.
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