Research On The Evaluation System Of Composite Brake Shoe With High-Friction Coefficient For Railway Freight Car

China railway freight is walking towards heavy load and high speed. The weight of axle in a freight train has been increased from21ton to23ton, even25ton in special transportation routes for coal. Performance of high friction coefficient composition brake shoe, the key part of a braking system, is tightly related with the safety of railway trains. Therefore a scientific evaluation of high friction coefficient composition brake shoe is extremely important.This paper on composition brake shoe mainly included four parts as follows: on-site use; physical and mechanical performance; friction performance and aging performance.The research starting from the on-site use of composition brake shoe deeply collected a variety of problems appeared in the use and then statistical analysis was employed for the data obtained. After statistical analysis, the condition of on-site use of composition brake shoe was known well. Eventually, the problems of composition braking shoe are included as breakage in brake shoes、shoe deformation、 bias-wearing、metal pick-up. Breakage rate:25.91%, deformation rate:3.59%, bias-wearing rate:20.29%, metal pick-up rate:42.24%.In the part on physical and mechanical performance, standard test blocks were prepared with the composition brake shoes from16companies and a couple of physical and mechanical performances were tested including impact strength, compressive strength, compression modulus, bonding strength and Rockwell hardness. The changes of physical and mechanical performance from room temperature to300℃were also investigated. Additionally, a finite element method was used to analyze the distribution of stress of brake shoe’s friction part and it is inferred the process of braking shoe breakage is deformation-cracking-plum hole’s loosing-friction part loosing. A rank correlation method was taken to judge the correlation of impact strength and brake shoe’s breakage. It was found that impact strength is significantly correlated with the breakage of brake shoe at a95%significant level. The effects of changes of impact strength on the condition of breakage were discussed. The results showed that increasing the minimum impact strength to3kJ/m2can result in the breakage lower than30%. At last, by virtue of vertical force analysis of brake shoe and the on-site use condition of a typical brake shoe, the measuring requirements of compressive strength and elastic modulus were investigated. The minimum compressive strength can be lowered to20MPa and a minimum requirement should be set for compressive elastic modulus without a measuring requirement.The research on friction performance included:comparison of testing methods for friction performance of brake shoe; analysis of factors affecting friction performance; investigation on metal pick-up. In the part of comparing testing methods, the correlation of test results and on-site use conditions of pin disk test machine、 speed testing machine、inertia brake testing machine were separately compared with brake shoes from16companies as testing samples. It is indicated test results from pin disk test machine、speed testing machine have better correlation and are relatively consistent with the on-site use conditions. The result from inertia brake testing machine, however, showed poor correlation with those from pin disk test machine、 speed testing machine and is not consistent the on-site use conditions well. As a result, pin disk test machine is recommended for the evaluation of the performance of composition brake shoe. In the analysis of affecting factors for friction performance, a comparative analysis of clustering was employed on the basis of results form pin disk test machine. Sample a with better performance and sample c with worse performance were analyzed by thermal analyzer, surface profilometer, scanning electron microscopy, energy spectrum to see the differences of thermal gravity temperature, Profile feature, surface morphology and element content of surface, which will provide some typical affecting factors from on a statistical basis. Results showed that materials with stable friction performance have higher initial thermolysis temperature, averagely318℃, and lower dual’s surface roughness, averagely N4; Fatigue wear is the main wear way and it is rare to see abrasive wear and adhesive wear; The components of friction surface mainly contain iron, oxygen and carbon element. The friction can be maintained well when the molar ratio of Fe/O is3:4and the content of carbon is around10%. Materials with unstable friction performance have lower initial thermolysis temperature, averagely281.1℃and higher dual’s surface roughness, averagely N5to N6; Significant abrasive wear and adhesive wear can be observed; The carbon content in a surface layer is far higher than10%. The ratio of Fe/O is not in a regular range and the valence of Fe is not at a stable level. In the research for metal pick-up, a comparative analysis of clustering was also carried out. According to results, three kinds of brake shoes with the most serious metal pick-up and three kinds of brake shoes with the least serious were separated into two categories. Based on the results from pin disk brake testing machine and the analysis of the measuring differences of two categories, it is found between both kinds of the brake shoesDual micro-average height of surface profile-Rpm and rockwell hardness-HRR are significant different. Rank correlation test was further taken to prove Rpm and HRR correlate with metal pick-up at a95%significant level. Scatter plots showed Rpm and HRR are individually linearly correlated with metal pick-up rate of composition brake shoe. Therefore, a prediction model about metal pick-up rate of composition brake shoe was set up with a binal linear regression equation:P=0.029423+0.050717Rpm-0.00301HRR. All parameters passed a90%test of significance and the amendment freedom factor of the model is0.8471. The model works well for the prediction of the degree of metal pick-up of composition brake shoe.The last part of the research investigated the changes of some performances of composition brake shoe in thermo-oxidative aging process. Samples were prepared individually with phenolic resin system and nitrile butadiene rubber (NBR) system. The changes of physical and mechanical performance and friction performance were tested and an aging rate kinetic equation was derived. Results showed that brake shoes with phenolic resin have a smaller degree of performance attenuation than those with NBR; the attenuation of impact performance and adhesive performance of brake shoes with NBR can be exactly fitted with Boltzmann function. Coefficient agreed is0.9915and0.9975; Dynamic analysis indicated aging rate is proportional to1/2power of oxygen content and3/2power of the active chain content in NBR.The innovations of the research are as follows: 1) It is the first time that breakage ways of composition brake shoe were proposed and then the major affecting factors were analyzed.2) It is firstly proposed that the metal pick-up of composition braking shoe is significantly positive to micro average height of surface on the dual and negative to surface hardness of braking shoe, which resulted in a prediction model for metal pick-up rate of composition brake shoe. The amendment freedom factor of this model is0.8471.3) Firstly investigated the effect of aging process on composition brake shoes with a thermosetting resin or a NBR adhesive system. A kinetic equation was derived and the affecting factors of aging rate were discussed.The research provided a useful reference for further improving the evaluation system for composition braking shoe...