Gear shaft failure analysis and improvement

2019 - gear shaft failure analysis and improvement 04 - Johnson jia (29 Qinchuan Machine Tool & Tool Group Share Co. , Ltd. ) Gears is an important transmission form in mechanical rotation, one of which has high transmission efficiency, reliable operation, transmission ratio accurately the advantages of compact structure, long service life, widely used in machine tools, automotive products such as, gear with tooth meshing transmission torque and the power, under high alternating bending stress and contact stress, tooth pastry erosion and agglutination caused by the failure, and even broken teeth. Gear shaft of a gear box ( See figure 1) , in the process of the work load is bigger, the material is 18 cr2ni4w, belongs to the high strength alloy carburizing steel, high strength, good toughness and excellent performance, good hardenability, the main technological process: forging & rarr; Normalizing & rarr; Turning & rarr; Hob & rarr; Carburizing & rarr; To carbon & rarr; Quenching & rarr; Coarse grinding & rarr; Gear grinding & rarr; Fine grinding. Heat treatment requirements for carburized layer depth of 1. 2 mm, hardened 58 HRC. Parts of the heat treatment process curve is shown in figure 2. One gear shaft use break a year later, the tooth of intact, in & phi; 90 mm between cylindrical and tooth fracture. We analyses the broken axis, from gear design, machining, heat treatment, etc, this thesis presents to avoid similar situation from happening again. 2. The crack analysis ( 1) Chemical composition of broken parts to chemical composition analysis, the sample, as shown in the chart components conform to the standard. 18 cr2ni4w toughened learn composition ( The mass fraction) （ %） （ 2) 59 ~ 60 HRC hardness of surface hardness, the core hardness of 42 ~ 44 HRC, conform to the technical requirements. （ 3) The fracture morphology ( 4) Fracture analysis of fracture is a typical conchoidal pattern, from the macro morphology of the fracture, fracture surface is composed of three area with different characteristics, 1 area as the source of crack, the half moon, black surface, the area of about 6 was; 2 area for crack growth area, the surface is delicate porcelain; 3 areas for instantaneous fault zone, roughness, bump is bigger, with radial stripes, accounts for about 50% of the total area. Belongs to the typical fatigue crack fracture surface. The gear shaft rupture occurred in heavy cutting slot, the section on a small piece of black area 1, show that the crack occurred earlier, scrutinize the breakup of parts, heavy cutting groove finish is very poor, have deep processing tool, in the process of heat treatment here produced a small crack, crack on the surface of oxidized in the process of heating, cooling, thus been black. Cracks in heavy cutting groove, hard to find. In use process, gear shaft under cyclic loading, the crack expands gradually, slower expansion at this time, the forming region 2 ( Extended area) , as a result of 18 cr2ni4w steel hardenability is very high, the heart of 42 ~ 44 HRC hardness after quenching, high hardness, so extension area without obvious rings. Fatigue crack growth to a certain extent, the axial cross section of narrow, lead to insufficient strength, cause instantaneous overload, cause fracture, the section has the characteristics of fast fracture, due to the high hardness axis, and more stress at work, so the formation of the final fracture zone of the area is larger, accounts for about 50% of the total area. 2. Improvement measures fatigue damage to the defect of great sensitivity, high stress concentration generally originated in part of the surface of the part or defect, such as surface crack, soft point, inclusions, transition and sharp corner mark, etc. Lead to fatigue failure of working stress is very low, often below the yield strength of material. Break in the use of a year, visible defects on the surface of the gear shaft caused the high stress concentration, reduced the fatigue strength, greatly shorten the service life of the gear shaft, 18 cr2ni4w steel on the stress concentration is more sensitive, especially for those to be attention. Parts of section change place, can produce stress concentration phenomenon. Therefore for the stepped shaft in the cross section size change place fillet transition should be adopted, and should not be processed into heavy cutting groove, the easy problems between paragraphs stepped cylindrical increase R3 ~ R5mm arc transition, in order to reduce stress concentration. Circular arc transition processing quality is easier to control, not easy machining defects. Improve the surface finish of parts, also can effectively reduce the stress concentration. 3. Conclusion gear shaft parts structure, the machining quality, heat treatment process, the material chemical composition and so on various aspects of adverse factors will lead to the failure of shaft, so only the structure of the improvement of the design of the parts, strict control of raw material composition, improve the machining accuracy, improve the quality of heat treatment process and so on comprehensive consideration, to ensure the quality of the shaft, prolonging the life of the shaft. After the improved gear shaft no fracture phenomenon occurred again.