Experience has led us to rate gears by calculating their pitting resistance and bending strength. These calculations do not provide for other types of destructive gear conditions.
The standards that we use provide design information for the “pitting resistance of gear teeth, based on the contact stresses that arise when the teeth are in mesh.” When the contact stresses exceed the permissible stresses for the material, small particles of the tooth detach. Pitting is described as initial, micro, or macro. Macro-pitting itself is categorized as initial, progressive, destructive, or flake. The pits will originate on the tooth surface where there is a combination of pressure and relative negative sliding. When the particles detach the surface has a pockmarked appearance. Pitting, therefore, is a wear mechanism that results from cyclic compressive load stresses that exceed the material’s endurance limit.
Pitting is described as a surface fatigue failure. Pits propagate inwardly and show irregular linear markings, while exfoliations/flaking run parallel to the surface. Their growth from the origin area is on a crack path that fans out to form an arrow-shaped cavity. The cracks will almost always enter the surface at a shallow angle of about 10º, penetrate a few thousandths of an inch, and then exit at a similar shallow angle. The rare exception takes place when there is a failure due to a very hard and large inclusion; in this instance the crack commences at the inclusion and joins with the pitting crack that progresses at a steep angle from the original pit. The extent of the pitting is influenced by the film thickness. Tests indicate that there is a significant difference in the life of the pits with different lubricants, but the character of the pits remains relatively unchanged. When there is a localized overstressed area on the tooth flanks, the load will attempt to redistribute itself by removing the high spots. This action creates small pits from 0.015” to 0.032” in diameter. Once the high spots have been removed and the load redistributed the phenomenon usually ceases.
One of the difficulties with pitting is identifying whether the pitting is initial or progressive. To be classed as initial pitting the pits must be localized and smaller than 0.032” in diameter. An early examination should be made and the progress of the pitting recorded. Ignoring small cracks and pits can lead to significant problems. When the pitting area extends along the tooth face, good alignment is indicated. Inclusions play a major role in propagating the pits.
Micro-fissures and flaking at a depth of approximately 0.0004” are a prelude to pitting and the condition is known as “grey staining.” The condition is also known as frosting, or micro-pitting. This appearance is caused by a multiplicity of shallow surface cracks appearing in a localized area. This form of pitting has become more prevalent with the increased use of surface hardened gears, case carburized, or nitrided made of high quality steels. Micro pits tend to appear when the operating limits of a case hardened gear are approached. High surface loads and heat, combined with a thin lubrication film, are conducive to this form of pitting. In large gears, such as those used in wind turbines, micro-pitting has been eliminated by changing the profile and lead modifications. The changes reduced the hard contacts at the end of the contact area. When a micro-pitting safety factor is applied, it is in relationship with the lubricant film thickness and not as a linear function of the applied torque. Micro-pitting can be predicted by the ratio of lubricant film thickness to mean-surface roughness. Both the pinion and wheel have to be verified. Although initial frosting can show up on the addendum, the first signs usually appear on the dedendum. At best the condition can progress into a scuffing failure, at worst cracking progresses between the grains and teeth fracture.
A micro pit results from the joining of shallow surface cracks within the intergranular oxidation zone (IGO), which is the surface oxidation that occurs along a carburized steel’s grain boundaries—a condition known as exfoliation. The pitting usually occurs in the slower running second and third stages of multiple stage gear units. The conclusion to this series will appear in the November 2008 issue of Gear Solutions magazine.