AGMA 13FTM16 PDF

Visual examination of a compressor box revealed that the Low Speed (LS) Pinion exhibited pitting type defects on each of its forty seven (47) helical teeth. Review of the failed component revealed a somewhat repetitive type of damage at one end of the teeth only. Each tooth showed what appeared to be one defect at a similar location 3600 around the Pinion. Each defect was located within ~0.5 inch of the end of the helical tooth. It was noted that each tooth defect was observed on the coast side of the teeth only. Visual examination of the mating gear revealed no evidence of similar damage. While of and by itself, this pitting may not be cause for alarm, debris from the pitting can adversely affect other components in the gearbox, especially the bearings, and the stress concentration effect of the pitting, even though it is on the coast flank, could lead to partial tooth fracture in the region of the distress.

This paper presents a discussion of the causation, diagnosis and metallurgical failure investigation of this lubrication erosion failure. Our effort was aimed at identifying the nature of the pitting and providing recommendations to avoid repeat instances of this failure in this specific application and in other future designs for similar applications.

The subject is presented by way of the discussion of detailed destructive metallurgical evaluations of this specific lubrication erosion failure which the authors have conducted in order to analyze and characterize the failures. Lubrication erosion is generally limited to helical gears but the authors have also found this type of distress when evaluating damage to carburized, hardened and hard finished spiral bevel gears as well when operated under the "right" circumstances. Lubrication erosion observed on helical gears only, however, will be addressed in this presentation. Although a specific failure "case" is used as the vehicle for presentation, information has been extracted and condensed from several individual actual failure investigations conducted by the authors so that a better understanding of the specific conditions that lead to micropitting and the actual progression from micropitting to fracture can be presented.