A Study Of Fretting Behavour Of Interference Fitting Of Wheel-set Subjected To Bending

To this day, the fretting problem between the shrink-fitted surfaces of wheel-set is still not fully understood, which directly affect the safety of train running, and this problem is in dire need to be solved. In this paper, a real wheel-set was cut in situ to take the wheel-seat and wheel-hub apart. The fretting damages of the surface of the wheel-seat were analyzed and the damage rules were revealed. A newly bending fretting device was also produced, by which the fretting rules of shrink-fitted surfaces of wheel-set subjected to rotating bending about distribution of fretting slipping areas, slipping displacements, slipping paths and slipping velocities were analyzed combining finite element method. At the same times, a similarity relationship of fretting amplitudes of the interference fitting surfaces between the prototype wheel-set and its geometrically similar scaled-model was deduced based on the dimensional analysis theory. At last, a structure design method to decrease the displacements of shrink-fitted wheel-set was presented.The main conclusions are as follows:1. The damage mechanism of shrink-fitted wheel-set1) After long-running under rotating bending load, the two sides of shrink-fitted surfaces of wheel-set were damaged due to fretting. The width of the two damage areas were all almost20mm, and on which red and black wear debris were covered.2) Because of the differences of the displacements and contact pressure, the fretting damage areas were comprised by three sub-areas with different characteristics. The three sub-areas were respectively gross slip regime, mixed fretting regime and partial slip regime from outside of the shrink-fitted surface of wheel-seat to inside.3) A thicker debris layer covered in the gross slip regime, and the slip regime presented the main wear mechanisms of abrasive wear, oxidative wear and delamination. In the mixed fretting regime, the traces of ploughing and plastic deformation flow could be observed on the fretting scars, and debris generation between the contact interfaces and severer degradation were visible. The wear mechanism during this regime was the combination of the plastic deformation, abrasive wear, oxidative wear and delamination. In the partial slip regime, the fretting scars presented slight damages.2. The fretting behaviours of shrink-fitted wheel-set1) The finite elements analysis showed that gross slipping occurred on the two sides of shrink-fitted surfaces of wheel-seat under the axial load. The area and the average displacements of the outside slip regime were greater than that of the inside slip regime.2) The nodes in the gross slip regime occurred axial and transverse relative displacements at the same time. The composite displacements were different everywhere.3) During wheel-set running under a cycle of rotating bending load, the nodes in the fretting regime of shrink-fitted surface of wheel-seat slipped back and forth one time. The slipping states transformed in sequence from gross slipping state to partial slipping state, and to gross slipping state. The slipping track was approximately spindle.4) The fretting behaviours of the shrink-fitted surface of PMMA shaft were accordant with that of the outside area of wheel-seat shrink-fitted surface. Because the magnitude of interference of the PMMA shaft was smaller, the fretting nodes of the PMMA shaft underwent in sequence gross slipping state, partial slipping sate, gross slipping state, partial slipping sate under a cycle of rotating bending load.5) According to the four kinds of basic motion modes of the fretting, namely, tangential, radial, torsional, rolling motion, the fretting between the shaft-hub fitting surfaces would be considered to be a kind of composite tangential fretting combining both axial and transverse motion. In another hand, in dealing with the load type of the fretting fatigue, it could be then considered as the composite fretting combining bending, tension-compression and torsion.3. Similarity relationship of fretting amplitudes between scaled-model wheel-set and prototype wheel-setA similarity relationship of fretting amplitudes of the shrink-fitted surfaces between the prototype wheel-set and its geometrically similar scaled-model was deduced based on the dimensional analysis theory. The material of the prototype wheel-set was the same as the scaled-model. The results showed that when the size of the model wheel-set, the magnitude of interference and the axle load were1/n,1/n and1/n2of those of the prototype one respectively, the fretting amplitudes of the interference fitting surfaces of the model wheel-set was1/n of that of the prototype one. At the same time, when the scaled-model wheel-set material was different of that of prototype one, the Poisson’s ratio and the friction coefficient of their materials were two important factors to affect the fretting displacements.4. A structure design method of shaft to reduce the fretting displacements of wheel-set subjected to bendingA structure design method of shaft to reduce the fretting displacements of inside area of wheel-seat shrink-fitted surface was discussed. The distribution of shaft bending deformation energy could be changed through reducing the diameter of the middle of shaft, sequentially to reduce the bending degree of the wheel-seat and the fretting displacements. A RD2wheel shaft was designed by this method. The newly designed shaft was analyzed and the results showed that the displacement of inside area of wheel-seat shrink-fitted surface was reduced.