To reduce the risk of gearing faults it is essential that alignment is perfect, so that the correct lubricant is used over a long running in period, and that the lubricating oil is subsequently kept in first class condition. Causes of faults, other than those implied above, could be one or a combination of the following; incorrect heat treatment, shock loading, vibration, bad surface finish and cutting.
New gear teeth may have variations in surface smoothness and hardness, and the high and hard spots carry most of the initial loading under cyclic conditions. This local intense cyclic loading results in sub surface fatigue and fine fatigue cracks. Lubricating oil can enter these cracks and intense hydraulic pressure caused by the closing of the teeth upon reengagement can lift out metal particles from the surface of the gear.
Intense, isolated, relatively deep pitting may be found at the ends of the gear wheel teeth.
Scuffing or Scoring
If the oil film between the teeth breaks down, metal to metal contact between surface asperities can occur resulting in local fusion or welding. This is caused by high temperature generated by friction as the surface asperities move across each other under high load. As sliding continues the welded metal is torn apart and the tooth working surface be damaged.
It is normally found on the softer of the two gears in mesh and usually occurs during recession since the damage is generally found on the dedendum of the wheel and the addendum of the pinion.
If the roots and tips of the gear teeth have not been carefully relieved it is possible for the tips to dig into the roots causing wear of the wheel roots (the softer material)
Solid particles in the lubricating oil causing scoring of the teeth. Grooving of the tooth surface in the direction of sliding and possibly some of the hard particles becoming embedded in the teeth are indications of abrasive wear.
Usually confined to case hardened gears and could be caused by poor heat treatment combined with any condition which stresses the metal beyond its yield point. Flakes of hardened metal hence break away from the surface due to the high load.
Local stresses tend to form a wave in the metal which rolls ahead of the point of contact, this results in sub surface fatigue failure and could result in ridging or even flakes of metal being sheared from the surface.
"Steam Engineering Knowledge for Engineers" by Thomas D. Morton