Study On Fretting Damage And Dynamic Characteristic Of Facility Shaft

The shaft parts are an important transmission part of mechanical system, which are often subjected to alternating bending and torsion loads during the running of the machine.Because of the difference of the rigidity of the materials,there exists difference in the deformations of shaft and bearing/bearing seat, which leads to the fretting behavior of the contact surface and fretting damage.As an important transmission part of the JD-1 wheel/rail simulation test machine, the shaft is subjected to the vertical load and torque during the running of the test machine.Under the combined action of vertical load and torque, there exists fretting damage on the right and left interference fit surfaces between shaft and bearings, resulting in loose of the interference fit surfaces and initiation of the cracks on the surfaces of the shaft, and affecting the test results and the fatigue life of the shaft. Therefore, the research on fretting damage of the shaft is significant. As an important part of the testing machine fixture, the dynamic characteristics of the shaft, to a certain extent, affect the dynamic response of the fixture in the running of the test machine. In addition, the dynamic characteristic parameters such as the mode shape are related to the relative displacement in a certain extent, so as to affect the fretting damage of shaft. It is important to study the dynamic characteristics of the shaft to optimize the dynamic characteristics of the shaft, avoid the resonance of the fixture structure and reduce the fatigue caused by vibration, explore the fretting damage mechanism of shaft in deep.In this paper, taking the test machine shaft as an example, the mechanism of fretting damage of shaft under different vertical load, torque load and rotational speed was studied using the test machine, the torsion fatigue fracture test and fatigue crack growth test of shaft materials was carried out using fatigue testing machine, and the force analysis was conducted to analyze the damage mechanism, the dynamic characteristics of shaft were analyzed using ANSYS, which provides the reference to explore the mechanism of fretting damage of test machine shaft and other shaft parts and to make the preventive measure and to optimize dynamic characteristics. That could have massive significance. The main results and conclusions are as follows:(1) During the running of test machine, the shaft is subjected to a certain number of bending moment and shear force due to the action of the vertical load. Bending moment and shearing force distribution along the axis are changed, the values of bending moment and shearing force on the left and right interference fit surfaces are roughly equivalent. Under the braking torque, the shaft bears a certain amount of torque in the running of test machine, on the interference fit surfaces, the torque along the axis is constant, the torque on the left interference fit surface is bigger than that on the right.(2) Under the vertical load and braking torque, fretting damage appears on the interference fit surfaces between shaft and bearings. From the surface wear scar analysis, the wear forms are mainly adhesive wear, abrasive wear and oxidation wear. With the increase of the vertical load and braking torque, the plastic deformation layer at the left and right ends of shaft is thicker, and the fretting damage becomes more serious. Bending moment and shearing force distribution along the axis are changed, so plastic deformation layer thickness is uneven, The torque born on the left interference fit surfaces is bigger than that on the right,so the plastic deformation layer thickness on the left surfaces is larger than that on the right.With the increase of the load, the micro cracks generated on the interference fit surfaces are increased, and the crack propagation angle is also larger, which are easier to expand the depth of the material under the stresses. The torque born on the left interference fit surfaces is bigger than that on the right, so the micro cracks generated on the left surfaces are more than those on the right. The rotational speed has a great influence on the fretting damage of test machine shaft. From the surface wear scar analysis, the wear forms are also mainly adhesive wear and abrasive wear. At lower speed, there exist sliding friction and pitting on the wear scar, and there exist sliding friction and furrows on the wear scar at higher speed.Relatively speaking, the fretting damage of the shaft at lower speed is more serious, it is because that the running time of shaft at lower speed is longer.(3) Under the vertical load and braking torque, the fretting fatigue cracks appearing on the interference fit surfaces of the shaft have the following characteristics: (a) Fatigue crack source is characterized with multiple sources, some cracks are initialled from the surface of the material, some are initialled from the subsurface. (b) The fatigue cracks mainly propagate along the direction of the plastic flow layer in the surface layer of the material.And the crack propagation angle is generally small, and most of the crack will propagate parallel to the surface, from the point of view of the expansion trend, most of the cracks will propagate to the surface, resulting the fretting fatigue wear.(4) Under the torque load, the fracture of the shaft material is typical fatigue fracture,which has the following characteristics: The fracture section is divided into three areas: the source of the crack, the crack growth zone and the final fracture zone. The section is located in the plane of the maximum shear stress. In contrast, the final fracture zone is more rough than the other areas. Compared with 40Cr material and 45 steel materials, the ability of anti fatigue fracture of 40Cr material is better than that of 45 steel. Through the fatigue crack'growth rate test, it is concluded that the fatigue crack growth rate of 40Cr of the shaft material is can described as da/dN=5×10-15(?K)3.5903 under the action of tensile stress.Based on non probability theory, a method for calculating the reliability of fatigue crack propagation life is presented.(5) The modal analysis of the shaft under unconstraint was carried out with the finite element method. The results show that the effect of grid partition accuracy in this paper on the analysis results is very small and can be ignored. The dominant frequency of shaft was analyzed with experimental modal method. The finite element modal analysis results with solid element are more close to the experimental modal analysis results. However, there are some errors due to the simplification of the finite element method and the influence of the test environment and the testing equipment. The modal analysis of the shaft under constraint was carried out with the finite element method. Because of the constraint, the system rigidity of the shaft model is increased, and its natural frequency is also increased. Relatively speaking, the system rigidity of the shaft under the assumption of rigid support is larger than that under the assumption of elastic support, so the natural frequency of shaft under the assumption of rigid support is larger than that under the assumption of elastic support.(6) The difference of the elastic deformation between the shaft and inner ring of bearing in the running of test machine and the reciprocating relative motion are the main reason to lead to the fretting damage of shaft. From the movement analysis of the shaft and the bearing inner ring, the relative displacement is divided into displacement along the axis and displacement along the direction of rotation. The modal analysis results indicate that the 10th order stretching mode shapes and the 1st, 2nd, 3rd, 4th, 5th, 6th, 8th, 9th, 12th order bending mode of shaft affect the relative displacement along the axis direction, the 4th, 6th, 7th, 11th order torsional vibration mode affect the relative displacement along the direction of rotation.