In the gear transmission system which is the most prone to failure in coal mining machinery, the failure ratio of various parts are: gear 62%, bearing 17%, shaft 10%, box 7%, fastener 3%, oil seal 1%. It can be seen from this that among all the parts of the gear transmission system of coal mining machinery, the failure ratio of the gear itself is the largest. Therefore, the occurrence of gear failure in the gear transmission system of coal mining machinery is the key to the normal operation of coal mining machinery.
The gears in the coal mine mechanical transmission system are the key parts to transmit motion and power. According to the statistics of the common fault types, they can be divided into broken teeth, pitting corrosion, gluing and wear.
1) Broken teeth
The gear tooth of coal mine machinery will bear continuous alternating load when it is engaged and driven. The gear tooth is equivalent to cantilever beam. Therefore, the stress at the root of the gear is the largest. When this multiple bending stress and stress concentration exceed its bending fatigue limit, the fatigue crack will appear at the root of the gear, and the crack will continue to expand, when it can’t bear the effect during the period When the force is applied, fatigue fracture will occur at the root of the tooth.
In addition, because of the gear processing error, assembly error and short-time overload or impact load, thestress intensity factor exceeds its stress limit in the operation process, which will make the gear crack or fracture.
2) Pitting on tooth surface
In the meshing process of coal mine mechanical gear, the meshing surface of the gear teeth has relative rolling and relative sliding at the same time. The tooth surface will produce shear stress on the contact surface in the relative rolling process, and tensile and compressive stress on the contact surface in the relative sliding process.
When the gears are meshed and rotated, the speed of the meshing point on the surface of the driving gear increases gradually from the tooth root to the tooth top, but the speed of the meshing point on the surface of the driving gear decreases gradually from the tooth top to the tooth root. The relative speed of the contact surface between the driving wheel and the driven wheel is zero at the node. Therefore, the sliding direction of the driving gear surface deviates from the node, while the driven gear surface glides towards the node. In this way, the rolling and sliding directions of the driving wheel and the driven gear top are the same, and the contact surface is under pressure stress, while the rolling and sliding directions of the two gear top are opposite The contact surface is under tensile stress.
The repeated action of the above two stresses on the meshing surface of the gear will lead to the formation of shear stress in the depth of the tooth surface. When the shear stress of multiple cycles exceeds the shear fatigue limit of the gear, the surface of the gear will have cracks. At the same time, the lubricating oil in the cracks will also increase the cracks under the action of the meshing pressure. With the continuous meshing and rotation of the gear, the cracks will continue to expand, and finally appear on the tooth surface The metal peels off to form a small pit, which is called pitting. When pitting is expanding and each of them is connected into a piece, there will be more flake metal peeling on the tooth surface, that is contact fatigue peeling.
The metal spalling caused by the pitting of the tooth surface will seriously affect the normal operation of the gear, and even cause the gear to break when the spalling is serious.
3) Gluing of tooth surface
Most of the gears in the gear transmission system of coal mine machinery are running at high speed and heavy load. The high temperature produced in the running process will break the lubricating film, and the meshing tooth surface will contact directly and produce the fusion welding phenomenon, which will cause the groove mark on the tooth surface in the running process. For low-speed and heavy-duty gears, although there is no high temperature produced by high-speed gear rotation, it is not easy to form a good lubricating oil film on the surface, so there will also be fusion welding due to heavy load.
4) Tooth surface wear
Tooth surface wear is essentially the phenomenon that the meshing surface materials of gears are consumed. The common wear of coal mine mechanical gear is abrasive wear and corrosion wear. When the gear is under heavy load and insufficient lubrication, its meshing surface can not form a good lubrication film, which will directly contact and form wear. When metal particles or other hard particles are sandwiched between the contact surfaces of gears, abrasive wear will occur, which will obviously change the tooth profile. Because of the deterioration of the lubricating oil and the humidity of the mine, the lubricating oil is corrosive, which will speed up the rusting of the gear tooth surface and increase the wear rate of the gear.