AGMA 09FTM12-2009 pdf free.AGMA Technical Paper The Anatomy of a Micropitting Induced Tooth Fracture Failure – Causation, Initiation, Progression and Prevention.
Until fairly recently, surface durability of gears has been defined by macroscopic pitting (macropiting) in which a crack initiates at a subsurface location where the shear stress exceeds the shear allowable, Figure 2A. When such a crack propagates to the tooth surface, a small piece (or, more often, several small pieces) of material, Figure 2C, are liberated leaving an inverted cove shaped defect, as shown in Figure 2B.
As this process is repeated, more and more pits appear and eventually the tooth surface is heavily damaged, as Figure 3A shows. Eventually, if the loads are high enough, the pitting damaged region of the tooth acts as a significant stress concentration and bending fatigue failure of the tooth may occur through the pitted region, as shown in Figure 38.
Micropitting
More recently (over the last 10 to 15 years) a microscopic pitting phenomenon, generally referred to as micropitting, has become a very problematic failure mode in certain applications. Typically where high loading is present at lower speeds under low or marginal film thickness conditions micropitting becomes a significant risk. It is important to note, however, that though it usually appears in a somewhat different presentation, micropitting is also a factor in the operation of higher speed gears as well. In the latter instance, micropitting is frequently piesent as a “hard line’ that leads fairly quickly to the formation of large spalls that may lead rather rapidly to tooth fracture failures thus the original micropitting “evidence” is often lost in the failure. Extremely careful metallurgical evaluation of the fractures can, however, often pinpoint the micropitting “connection” (Figures 14C and 15 show this type of micropitting failure).
The cause of micropitting is still not fully and completely understood. Initially, it was thought that the cleanliness of the steel might be playing a significant role; however, even where very clean steels are used, micropitting still occurs. Micropitting appears to occur at local surface irregularities including tooling witness lines (Figure 4) and general surface roughness peaks. It has been demonstrated that micropitting capacity can be improved through the use of improved finishing techniques, especially “super finishing” processes which reduce the surface finish down well below 10 RMS. The use of some extreme pressure (EP) additive oils to avoid scoring type failures has also been shown, at least anecdotally, to increase the tendency for micropitting to occur, at least with some formulations.AGMA 09FTM12 pdf download.