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Gear transmission is the most important form of transmission in mechanical transmission. It has a series of features such as reliable operation, long service life, high transmission efficiency, compact structure and constant transmission ratio. However, due to improper design, manufacture or improper use, the gear transmission often fails during the working process. There are usually gear teeth broken, working tooth surface wear, fatigue pitting, tooth surface gluing and plastic deformation, etc., any failure will lead to abnormal gear operation.
1 tooth broken gear tooth break can be divided into fatigue break, overload break, and random break. Usually, the break of the involute spur gear is the full tooth break, while the break of the helical gear and bevel gear is generally partial. Broken.
1.1 Fatigue fracture teeth work like a cantilever beam, the bending stress σF generated at the root of the tooth after loading is the largest, and it is the alternating stress. In addition, there is a large concentration change in the transition part of the tooth root. Under the repeated bending stress, when the stress value exceeds the bending fatigue limit of the material, that is, the root of the tooth on the tension side of σF>σlim Fatigue cracks occur at the fillet. As the number of load cycles increases, the crack gradually expands, causing the stress on the remaining section of the gear to exceed its ultimate stress, causing instantaneous breakage.
There are many reasons for the fatigue fracture of the gear teeth, such as insufficient estimation of actual load during design, improper selection of gear materials, failure to guarantee processing quality, precision, etc., in order to prevent fatigue breakage during gear tooth work, gear design and processing When installing, it is necessary to optimize the gear parameters, improve the surface finish of the root, increase the radius of the root radius and adjust the transition zone of the root fillet, eliminate the machining marks at the place to reduce the stress concentration of the root; increase the shaft and The rigidity of the support is to reduce the degree of local overload of the tooth surface; the gear pair material should be correctly selected, and appropriate heat treatment should be performed to obtain a better microstructure; the gear is subjected to cold working such as shot peening and rolling to improve the tooth surface. Hardness and maintain the toughness of the core.
1.2 Overload broken gears have serious overload or impact load during the working process or during the manufacturing and installation process, the precision is poor, the gears are partially loaded or subjected to large impact, and overload breakage may occur. The gear teeth may be broken when subjected to heavy load or severe wear and the tooth thickness is thin. For example, in the helical gear transmission, the contact line on the gear tooth working surface is a diagonal line. After the gear teeth are loaded, if there is concentrated load, partial breakage will occur; in addition, if the manufacturing or installation is poor or the rigidity of the shaft is insufficient. When the bending deformation is too large, the teeth will be partially overloaded, and the teeth will be partially broken. For example, the driving gear of a locomotive motor suspension device is broken, and the closed holes and inclusions formed during the forging process of the blank are found in the root fracture zone, which seriously weakens the strength of the zone and causes the fracture. The overload fracture is often different from the fatigue fracture, which is characterized by the fact that the fracture location is not fixed and the section is rough.
In order to prevent the occurrence of gear overload and breakage, when designing the transmission, various factors causing serious overload should be fully considered, and appropriate safety devices such as safety couplings should be used; sufficient contact accuracy should be ensured during installation; In the middle, it should prevent large hard objects from falling into the meshing teeth to avoid impact load. In addition, necessary quality control for the gear manufacturing process, such as microcrack detection on the root of the tooth, may also reduce the possibility of overload breakage of the gear teeth. Sex. In addition, gears should be strictly guaranteed in the process of manufacturing and manufacturing, and stress concentration should be avoided as much as possible.
2 Tooth surface wear and wear is a phenomenon in which frictional loss occurs in the material on the surface of the tooth contact due to the relative sliding between the meshing tooth surfaces during the meshing transmission. No matter how high the hardness of the tooth surface, the tooth surface will wear after the gear is working. The entry of abrasive materials (such as sand, iron filings, etc.) is undoubtedly the direct cause of the increase in tooth surface wear.
The most effective way to avoid excessive tooth wear is to replace the semi-open or open drive with a closed drive. However, whether it is closed, semi-open or open drive, proper surface treatment, such as carburizing and quenching, quenching and tempering (normalizing) high-frequency surface hardening, etc., can help to improve the hardness and wear resistance of the tooth surface.
Wear varies according to its mechanism and characteristics, with slight wear, moderate wear, excessive wear, abrasive wear and corrosion wear. Slightly worn and slow, uniform, smooth and shiny after the wear. The causes of slight wear are the roughness of the contact tooth surface and the viscosity of the lubricating oil, the mismatch between the working load, the working speed of the tooth surface, etc. These reasons cause the thickness of the lubricating oil film between the tooth surfaces to be insufficient, resulting in the peak of the micro-convex body of the tooth surface being Smoothing or plastic flattening, after the gear is polished, a lubricant with a higher viscosity can be used to form a suitable oil film thickness without affecting the life and performance of the gear. When the medium wears, the material of the lower tooth surface of the gear tooth line is lost, and a nearly continuous line appears at the line position. Because the gear is limited by working speed, load, temperature, lubricant and lubrication factors, it can only be Boundary lubrication or work close to the boundary lubrication condition can prevent moderate wear by improving lubrication conditions and increasing oil film thickness. When excessive wear occurs, the tooth tooth surface material is ground, the tooth thickness is obviously thinned, the tooth profile shape is destroyed, the gear life is reduced, excessive wear is caused by poor or failure of the lubrication system and the sealing device, and the system has serious vibration and impact. Caused by the use of appropriate sealing forms and lubrication devices, can also increase the working speed, reduce vibration loads, change the geometric parameters of the gears, material, accuracy, tooth surface roughness. Abrasive wear is characterized by a relatively uniform streak along the sliding direction on the tooth contact surface. This is due to the wear caused by the abrasive falling into the meshing tooth surface, which can appropriately improve the hardness and smoothness of the tooth surface and reduce the sliding between the tooth surfaces. Measures, correct choice of lubrication, and maintenance of lubricants to reduce abrasive wear. Corrosive wear of the tooth surface of the tooth is uniformly distributed corrosion pit, the working tooth surface is accompanied by abrasion marks along the sliding direction, causing corrosion and wear due to chemical and electrochemical reaction between the active component and the tooth material in the lubricant. . Due to friction or scouring, the plaque is abraded or washed away to form corrosion and wear. To prevent corrosion and wear, the corrosive medium must be controlled. An appropriate amount of corrosion inhibitor can be added to the lubricating oil to reduce the reaction speed of the electrode. The device should have a good sealing form to prevent the lubricating oil inside the machine from being contaminated by external water, acid and other harmful substances.
3 tooth surface pitting pitting is a common failure mode of closed gear transmission. When gear test and machine overhaul, pitting and pit can be seen on the tooth surface. For the hard tooth surface of carburized broken fire, you can see Go to some frosty pattern block or appear a large piece of metal peeling off. When the gear teeth of the wheel enter the meshing, the contact of the tooth surface of the wheel teeth will generate a large contact stress under the action of the normal force, and the contact stress disappears after the disengagement. For a fixed point of the tooth profile, it is subjected to a contact stress that approximates a pulsating change. If irregular contact fatigue cracks appear on the tooth surface of the contact stress σH>σlim, the metal particles on the surface of the tooth surface fall off as the crack propagates and expands, forming a pit-like pit. Often appear first on the root surface near the pitch line. This is because the contact fatigue strength of the tooth root surface near the pitch line is low, and the relative sliding speed of the mesh is low, the lubricating oil film is not easily formed, and the frictional force is large. For the above reasons, in order to prevent pitting corrosion prematurely, it is possible to increase the hardness of the tooth surface, increase the viscosity of the lubricating oil, lower the surface roughness value, and reduce the pitting corrosion to extend the life of the gear. However, for gear transmission with low speed, it is advisable to use a lubricant with a higher viscosity; however, for gear transmission with higher speed (such as circumferential speed v>12m/s), it is recommended to use oil lubrication, so it is recommended to use low viscosity lubrication. oil.
4 tooth surface gluing in high-speed heavy-duty gear transmission, due to the large pressure between the meshing tooth surfaces, the relative sliding speed is higher, the heat generation is large, the instantaneous high temperature is generated, the lubricating oil film is broken, the tooth surface is directly contacted, and dry friction or Semi-dry friction, and this friction will further increase the temperature, so that the inherent fusion welding adhesion occurs locally in the tooth surface, and then tears in the sliding direction to form a transfer of the surface material between the two tooth surfaces. The tooth surface is glued. It has two kinds of cold gluing and hot gluing. In the low speed and heavy duty soft tooth surface gear transmission, due to the large partial pressure of the tooth surface, it is possible to invalidate the lubricating oil film, causing the tooth surface metal to directly contact and plastically deform, and the contact surface The metal molecules interdiffuse and partially recrystallize to produce local weld adhesion. The bond points are torn apart when tangentially sliding to form a cold glue. In the high-speed heavy-duty gear transmission, the tooth surface temperature is high, and the lubricating oil film between the meshing teeth is broken due to the action of high temperature and high pressure, causing welding and tearing of the tooth surface metal contact points. The softer tooth surface is torn to form a groove, and the harder tooth surface adheres to the metal deposit of the torn film to form a thermal glue.
The main measures to prevent or reduce the bonding of the tooth surface: (1) use angular displacement gear transmission to reduce the sliding coefficient at the beginning and end of the meshing; (2) reduce the modulus and tooth height to reduce the sliding speed; (3) use the pole (4) Select the gear pair material with good anti-gluing property; (5) Keep the appropriate hardness difference between the large and small gears when the materials are the same; (6) Improve the hardness of the tooth surface and reduce the surface roughness value. Especially in the design of heavy-duty high-speed transmission gears, in addition to the sufficient bending strength and contact strength of the gear teeth, it should also ensure that the tooth surface has a certain anti-glue bearing capacity, that is, when designing the transmission gear, The peak load checks the gear gluing load capacity, not the average load. At the same time, practical measures (such as injection lubrication) should be taken to control the temperature of the meshing zone while ensuring good lubrication.
5 tooth surface plastic flow tooth surface softer gear, heavy load may produce plastic flow of the tooth surface under the action of friction, thus destroying the correct tooth shape. Since the frictional directions of the tooth top and the tooth root are opposite to each other on both sides of the pitch line of the driving gear tooth surface, a groove is formed near the pitch line; the driven wheel is opposite, and the frictional force direction is opposite, so that the vicinity of the pitch line is formed. Ridge. This damage often occurs in low speed, frequent start and overload drives.
Appropriately increase the hardness of the tooth surface, and use a lubricating oil with a higher viscosity to reduce or prevent the plastic flow of the tooth surface. Reducing the tooth surface roughness, appropriately selecting the material and hardness of the main and driven gears, and performing proper running and the like can also prevent the plastic flow of the tooth surface.
In addition, a room temperature oil grading austempering process can be used to obtain an austenite ductile iron structure. The Austrian-bead ductile iron structure has good mechanical properties of high strength, high elongation and high impact value, and has high bending fatigue strength and good wear resistance. This process has been applied to the final transmission large gear of Taishan-120 small four-wheel tractor. After 6 months of operation, there is no broken teeth and chipping, and there is no crack or pitting on the tooth surface.
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