Gear transmission works by the meshing of teeth, and the gear tooth is the part of the gear directly involved in the work, so the failure of the gear mainly occurs on the gear tooth. The main failure forms are tooth fracture, tooth surface pitting, tooth surface wear, tooth surface gluing and plastic deformation.
1.Broken gear teeth
There are usually two cases of gear tooth fracture: one is fatigue fracture caused by repeated bending stress and stress concentration; The other is overload fracture caused by sudden severe overload or impact load. Especially gears made of brittle materials (cast iron, quenched steel, etc.) are more prone to tooth fracture. Both types of fracture start at the side of the tooth under tensile stress. Increasing the transition fillet radius of the tooth root, improving the mechanical properties of the material, reducing the surface roughness to reduce the stress concentration, and strengthening the tooth root (such as shot peening and roll extrusion) can improve the fracture resistance of the tooth.
2.Tooth surface pitting
When the gear teeth work, the front meshing is under the repeated action of alternating contact stress, and several small cracks will appear on the tooth surface near the pitch line. As the crack grows, small pieces of metal will peel off, which is called tooth surface pitting. The continuous expansion of tooth surface pitting will affect the stability of transmission, produce vibration and noise, and cause the gear to not work normally. Pitting corrosion is a common failure form of closed gear transmission with good lubrication. Increasing the hardness of the tooth surface and reducing the surface roughness can improve the pitting corrosion resistance of the tooth surface. Due to the rapid wear of the tooth surface, there is no pitting corrosion in open gear transmission.
3.Tooth surface wear
When the gear teeth mesh, due to relative sliding, especially when the external hard particles enter between the meshing working surfaces, it will lead to the wear of the gear tooth surface. After the tooth surface is gradually worn, the tooth surface will lose the correct tooth shape, which will cause the tooth to be too thin and broken. Tooth surface wear is the main failure form of open gear transmission. In order to reduce wear, important gear transmission should adopt closed transmission and pay attention to lubrication.
4.Tooth surface gluing
In the high-speed and heavy-duty gear transmission, the pressure between the tooth surfaces is large, the temperature rises and the lubrication effect is poor. When the instantaneous temperature is too high, the two tooth surfaces will be partially melted and the metal will adhere to each other. When the two tooth surfaces move relative to each other, the adhesive place will be torn, resulting in banded or large-area scars on the tooth surface along the sliding direction. The low-speed and heavy-duty transmission is not easy to form oil film, although the friction heating is small, However, cold gluing may also occur due to heavy load. Use lubricating oil with high viscosity or good anti gluing performance to reduce surface roughness and form good lubrication conditions; Improving the hardness of the tooth surface can enhance the anti gluing ability of the tooth surface.
5.Plastic deformation of tooth surface
When the gear with soft tooth surface with low hardness is under low speed and heavy load, due to the excessive pressure on the tooth surface, the metal on the tooth surface will produce plastic flow and lose the original tooth shape under the action of friction. Improving the hardness of the tooth surface and using lubricating oil with high viscosity can help to prevent or reduce the plastic deformation of the tooth surface.