Gear failure generally occurs on the tooth surface, rarely in other parts. According to the basic form of gear failure in work, gear failure can be divided into two categories: tooth surface wear and tooth fracture. Tooth surface wear refers to the process of gear consumption due to the continuous friction of surface materials in meshing engineering. The possible failure forms of 20CrMnTiin use are as follows, which are analyzed as follows.
（1） Wear of tooth surface
There are two kinds of tooth surface wear: abrasive wear and running in wear. After the tooth surface is worn, the tooth profile will be deformed, resulting in vibration, impact and noise. When the wear is serious, the tooth may be broken due to the too thin tooth thickness. Abrasive wear is the main failure mode of open gear drive. The new gear pair, because of the roughness of the surface after machining, only part of the peak contacts when it is loaded. The pressure at the contact is very high, so during the operation, the wear rate and the wear amount are both large. When the wear reaches a certain degree, the friction surface is gradually smooth, the pressure decreases, the wear speed is eased, and this kind of wear becomes running in. People intentionally make the new gear pair run in under light load to create conditions for subsequent normal wear. However, it should be noted that after running in, the lubricating oil must be cleaned and replaced.
Measures to improve wear resistance:
improve the tooth surface hardness of spiral;
improve the lubricating conditions, add antifriction additives into the lubricating oil, and keep the lubricating oil clean.
(2) Pitting and spalling of tooth surface
There are pitting corrosion on the tooth surface of spiral bevel gear. Generally, there are small pits near the tooth root. The reason is that in the meshing process of gears, there are both relative rolling and relative sliding, and the friction force of relative sliding is in the opposite direction on both sides of the node, which results in pulsating load. The effect of these two forces results in fluctuating cyclic shear stress at the depth of gear surface layer. When the shear stress exceeds the shear fatigue limit of, the surface will produce fatigue cracks. Crack growth will eventually make small pieces of metal on the tooth surface peel off, forming small pits on the tooth surface, which is called pitting.
Measures to improve the contact fatigue strength of gears:
improve the hardness and reduce the roughness of tooth surface;
The viscosity of lubricating oil should be selected reasonably, and the lubricating oil with higher viscosity should be used (the practice shows that the lower the viscosity of lubricating oil is, the easier it is to penetrate into cracks, and the faster the pitting corrosion grows) to reduce the dynamic load;
adopt positive displacement gear transmission to increase the comprehensive radius of curvature.
(3) Gluing of tooth surface
When the driving spiral bevel gear and the driven spiral bevel gear have strong friction due to the poor lubrication conditions, it is easy to produce the scuffing damage. In order to prevent the scuffing, the hardness and finish of the tooth surface should be increased properly, and the active lubricating oil should be used.
The methods to improve the anti tooth surface gluing are as follows:
reduce modulus, tooth height and sliding coefficient;
lubricating oil with extreme pressure additives;
adopt tooth profile modification to improve transmission stability;
Improve the hardness and reduce the roughness of the tooth surface of the driving spiral bevel gear;
When the material is the same, keep the proper hardness difference between the driving spiral bevel gear and the driven spiral bevel gea
(4) Plastic deformation
When the transmission load of the spiral bevel gear is too large and the working stress of the tooth surface exceeds the yield limit of the material, the plastic flow occurs on the tooth surface, which causes the groove at the pitch line of the active spiral bevel gear and the convex ridge of the driven spiral bevel gear.
The methods to reduce or prevent the plastic deformation of tooth surface are as follows:
properly improve the hardness of spiral bevel gear tooth surface;
Use high viscosity lubricating oil.