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Introduction Thin-wall traction drive gears for locomotives are subject to large traction, friction and impact loads during operation, so high strength, hardness, impact toughness and dimensional accuracy are required to ensure safe operation of the locomotive and gears. Service life.
The thin-wall drive gear of a heavy-duty locomotive has the main dimensional parameters and strength check parameters. The thin-wall drive gear is tapered with the motor main shaft. The maximum inner diameter is 824mm, and the minimum wall thickness is only 155mm. 2 It can be seen that the gear is subjected to high wheel traction during operation, so the contact stress and root stress are high, and it is prone to spalling failure or fracture failure. To this end, it is necessary to analyze the factors affecting the mechanical performance of the thin-wall drive gear, and improve the design and manufacturing process to improve the overall mechanical performance of the gear.
1 Factors affecting the mechanical properties of the gear 11 The impact of metallurgical quality on gear performance The metallurgical quality of the material is the basis for the strength of the gear, and the metallurgical factors affecting the performance of the gear include chemical composition and high and low defects.
The high and low magnification defects of the gear have an adverse effect on the mechanical properties and heat treatment of the gear. Therefore, improving the metallurgical quality of the steel can ensure the performance of the gear.
The purity of steel is the key to ensuring the quality of forgings. Vacuum deoxidation smelting process can improve the purity of steel. Through secondary refining, the content of hydrogen, oxygen and nitrogen in the steel can be controlled to a very low level, and the content of S and P can be greatly reduced, the amount of non-metallic inclusions can be significantly reduced, and the morphology of inclusions can be improved, thereby improving Mechanical properties of steel.
12 Influence of residual stress on gear fatigue strength The residual stress on the root and tooth surface has a great influence on the mechanical properties of the gear. There are three sources of stress on the root surface: one is that the interference assembly of the gear will cause a large tensile stress on the root surface; the gear adopts the taper interference fit, and the assembly method includes the electromagnetic induction hot sleeve assembly process and the overall thermal assembly. The process, regardless of the assembly process, is heated. If the heating temperature or the heating part is not well controlled, it is easy to cause the local heating temperature of the gear inner hole to be high, causing the roundness of the gear inner hole to change. After assembly, the coaxiality of the gear and the shaft may be poor, or the root of the tooth may be partially affected. Large tensile stress causes fatigue. The second is the residual tensile stress of the subsurface heat treatment. Third, the tooth root is subjected to periodic bending stress under the action of periodic load during the operation, so that the surface of the tooth root will be subjected to periodic maximum tensile stress.
The traction gear has only one tooth in operation when it is working.
Therefore, the stress at the tooth root of the running gear is derived from the superposition of the above three stresses. Since the gear is periodically engaged by the carrier during operation, the gear is prone to bending fatigue fracture, and the main influencing factor is the first source of stress.
The existence of residual tensile stress will greatly reduce the fatigue strength of the gear. On the contrary, the residual compressive stress has a great effect on improving the fatigue strength of the gear. The residual compressive stress of the tooth surface and the root can reduce the peak value of the bending tensile stress. Improve bending fatigue performance. Moreover, when there are already fine cracks in the material, the residual compressive stress can suppress the crack propagation. When the depth of the residual compressive stress layer is about 5 times of the crack depth, the influence of the crack can be eliminated. It can be seen that increasing the residual compressive stress on the gear surface can greatly improve the fatigue strength and service life of the gear. Therefore, you must choose the appropriate interference, control the heating temperature and assembly process.
13 Effect of heat treatment on gear performance The comprehensive mechanical properties of the gear are closely related to the quality of the material heat treatment. Gears require good surface hardness and core toughness.
The contact fatigue strength of the gear is an indicator of its hardness, and the bending fatigue strength of the gear is an indicator of its toughness. Research at home and abroad shows that the hardness of the gear core should be in the range of 40HRC.
Surface carbon content affects gear performance by affecting retained austenite, martensite and carbide structure. Studies have shown that in the case of high cycle times, the fatigue life increases with the increase of retained austenite, but after a certain period, it decreases with the increase of retained austenite.
The fatigue life decreases as the carbide level increases. Therefore, it is necessary to control the formation of carbides.
In summary, the locomotive traction drive gear must start from the raw materials, adopt strict process management methods, appropriate interference, heat treatment process and assembly process to ensure good mechanical properties and bending fatigue strength, and prevent Sudan gear impact breakage. Case.
2 Process plan 21 raw material selection.
Taking into account the contact, bending, gluing strength of the gear and the impact strength during the operation of the locomotive, and considering the working conditions of the gear pair, the gear material should have high and uniform hardness and wear resistance, high elastic limit, contact Fatigue strength, sufficient toughness and good hardenability.
The driving gear material is 17CrNMio6DIN17210. Its chemical composition and composition segregation should meet the requirements of Table 3 and Table 4.
17CrNMio6 is a vacuum deoxidized steel. The non-metallic inclusions in the material meet the requirements of the German DIN50602 standard, and the quality grade can fully meet the standards of ME grade materials. One of its characteristics is that the content of S and P is greatly reduced, thereby greatly reducing the content of plastic and brittle inclusions, and the small component segregation improves the heat treatment performance and mechanical properties of the material, thereby improving the fatigue resistance of the workpiece; The second characteristic is complete deoxidation, the maximum oxygen content is less than 20ppm. In addition, 025035 Mo is added to the material, which effectively reduces the temper brittleness of the workpiece.
22 Reducing the assembly interference Since the gear ratio of the driving gear and the transmission center distance have been determined by the user, it is impossible to reduce the root stress by increasing the diameter of the root circle. Therefore, the stress at the root can be reduced by reducing the interference of the assembly.
Through the interference calculation, the minimum effective interference of the transmitted load (considering the influence of the joint surface roughness) is 0132mm; according to the other type of locomotive, the gear of the similar structure is used for the safety of the assembly for more than half a year. The surplus is reduced from 02010221mm at the beginning of the design to 01350150mm.
23 Strengthen the heat treatment process control.
The gears are carburized and quenched by multi-purpose furnaces, that is, carburizing and tempering are completed once in the furnace, so that all heat treatment processes of the workpiece are in a protective atmosphere, which not only eliminates oxidative decarburization, but also ensures the formation of surface non-martensitic.
In order to achieve the heat treatment requirements of the gear, the heat treatment process is: paint anti-seepage #preheating# carburizing, high temperature tempering # quenching #清洗# twice low temperature tempering # check. High temperature tempering temperature is 650? The tempering time is 210min and the quenching temperature is 850? , quenching time is 60min, low temperature tempering temperature is 180? The tempering time is 300min. Two low temperature tempering can effectively eliminate stress and retained austenite. Carbide precipitates in the workpiece during high temperature tempering, which effectively reduces the carbon content in the austenite, prevents the retained austenite from exceeding the standard, and improves the carbide morphology to obtain a better quenched structure. This ensures good surface hardness and core toughness. Gear performance test results.
The surface hardness is slightly lower, but is greater than or equal to the lower limit of the specified value. The core hardness is close to the upper limit. The surface carbon concentration of 8 corresponds to a mixed structure of a large number of lath martensite partially acicular martensite, which has good comprehensive strength properties.
The surface hardness, core hardness, mechanical properties and metallographic structure of the gear after heat treatment have met the specified requirements.
24 enhanced shot peening shots have a good effect on increasing residual compressive stress. Enhanced shot peening not only eliminates the tensile stress generated during machining, but also minimizes the adverse tissue effects of the gear surface during heat treatment; while improving the gear due to continuous tool marks or grooves, holes and transition fillets Stress concentration caused by structural factors, thereby increasing the fatigue life of the gear.
The driving gear was subjected to surface-enhanced shot peening by the Beijing Aeronautical Materials Research Institute. The steel shot was first sprayed and then the glass shot was used for secondary shot peening. After the shot peening, the residual stress test was carried out.
Test conditions: CrKaB target, (211) diffraction crystal plane, diffraction 2? Corner 156? . The relevant method is set to peak, the equipment is Finnish X3000 stress analyzer, the X-ray tube voltage is 28kV, and the X-ray tube current is 6MA. The test site is the tooth root.
The residual stress of the gear surface after shot peening is compressive stress, the value is between 9041237MPa and the residual compressive stress is more than 800MPa, which fully meets the technical requirements of the gear.
25 improve the assembly process using the overall thermal assembly process, the heating temperature is less than or equal to the low temperature tempering temperature of the gear. The heating temperature is strictly controlled to avoid excessive local tensile stress caused by excessive local temperature of the gear. The actual contact area of ​​the mating surface is greater than 80, and the surface roughness is Ra0804#m, and the mating surface is clean.
3 Conclusion For thin-walled drive gears, factors affecting its performance include structural dimensions, raw materials, residual stress, surface hardness, core hardness and surface roughness. By analyzing these factors affecting the performance of the gear, the manufacturing process plan of the driving gear is proposed in a targeted manner, and the CNC machining technology is used in the field milling, and the right tooling G42 is used for the up-cut milling. Climbing is beneficial to improve the quality of machining. In CNC machining, it is a frequently used mode, that is, G41 mode is often used in programming.
3 Conclusion This paper systematically discusses the application of tool radius compensation, which has certain guiding significance for CNC teaching, scientific research and operation personnel. Due to different systems, different methods for dealing with various problems are not the same.