Corrosive damage occurs when corrosion occurs on the gear surface. Corrosion damage is not very common in gear transmissions lubricated with ordinary mineral oil, but corrosive damage can occur when lubricating oils containing extreme pressure additives are used.
This process gradually produces corrosive damage on the tooth surface, but as long as the control is appropriate, the degree of such damage is not as important as the adhesive damage produced in other cases. The flank in a corrosive state is smooth.
Three lubrication states can be produced between the gear meshing surfaces: boundary lubrication, hybrid lubrication, and fluid lubrication.
Boundary lubrication mostly occurs in gear transmissions that operate from low to medium speed, heavy-duty gear transmissions, or gear transmissions that are subject to high shock or dynamic loads. Under the condition of boundary lubrication, the thickness of the oil film is not enough, so the contact of the metal between the two tooth surfaces cannot be completely ruled out. In the place where the tooth surface has the greatest unevenness, the metal penetrates the thin oil film, and the local metal-to-metal contact occurs. It is.
The coefficient of friction of the boundary lubrication is a combination of the coefficient of friction of the metal-to-metal contact portion and the coefficient of friction generated at the sheared portion of the oil film. The viscosity of the lubricating oil has little effect on the coefficient of friction because the true shear of the oil film is secondary. However, in some cases, high viscosity lubricants can eliminate boundary lubrication by forming thicker oil films.
Fluid lubrication occurs when a layer of oil is formed between the two sliding surfaces to prevent metal-to-metal contact. Friction is caused by oil film shear, so the coefficient of friction of fluid lubrication is a function of the viscosity of the lubricant.
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