We know that gear oil belongs to transmission system oil, which is generally divided into two categories: vehicle gear oil and industrial gear oil. Therefore, in terms of common gear forms in vehicles and industrial equipment, there are roughly seven categories: spur gears, bevel gears, helical gears, helical bevel gears, worm gears, and hyperbolic gears.

For us ordinary lubricant practitioners, sometimes the theory is difficult to understand and comprehend. But in order to clarify some truths, we need to briefly mention some basic concepts of lubrication theory.

Lubrication states are divided into fluid lubrication, mixed lubrication, and boundary lubrication. Once the lubrication state of the gear exceeds the boundary lubrication, the gear will be damaged and fail. The forms of gear failure are measured by the surface roughness of the tooth surface when it is damaged. The degree of damage, from light to heavy, is: abrasion, pitting, bonding, peeling, abrasive wear, ripples, and ridges. It can also be seen here that pitting corrosion is actually a relatively mild gear damage. In recent years, the concept of 'micro pitting corrosion' of gears has been hyped up in wind power gear oil. When it comes to wind power gear oil, it is always referred to as micro pitting corrosion. I feel that it seems to be related to some foreign gear manufacturers and lubricant producers artificially setting the threshold for wind power gear oil, and launching a related testing equipment in advance has deprived a considerable number of lubricant producers of their right to speak. Of course, this viewpoint can be debated.

The friction surface is completely separated by a lubricating oil film, and the surface protrusions do not directly contact each other. This is fluid lubrication, which is further divided into fluid dynamic lubrication and fluid static lubrication. This is the ideal state of lubrication. Under high loads during equipment operation, the viscosity of lubricating oil increases (which we call the viscous pressure effect), and metal materials undergo elastic deformation. Fluid dynamic lubrication at this time is called elastohydrodynamic lubrication. When there are no special circumstances, the lubrication state of gears is basically mixed lubrication, that is, elastic fluid dynamic lubrication plus boundary lubrication.

Speaking of pitting corrosion: pitting corrosion is a surface fatigue phenomenon that may occur on the tooth surface after a certain period of operation under cyclic stress. The main factors affecting pitting corrosion are gear design and material selection, which are only related to gear oil. If the gear ratio is not 1:1, high-speed gears may experience pitting corrosion first. If the gear pair is made of different materials, pitting corrosion may only occur on one gear.

Micro pitting is actually a type of surface fatigue or contact fatigue, where shallow pits with a depth of about 20 µ m appear on the surface of the friction pair, known as micro pitting. The internal cause is the presence of physical or chemical defects in the metal surface, such as lattice defects, interstitial atoms, compositional segregation, etc.