Study On Ground Surface Topography And Grinding Affected Layers In Bearing Ring Raceway Grinding Process

High performance bearings are core components of major equipments and precision devices. However, the backwards of independent research and design capabilities of high speed, heavy load and high precision bearing are becoming to be a soft spot of restricting the development of the equipment manufacturing industry in China.comprehensive upgrade the level of bearing’s research and design and the level of manufacturing process have already become a key problem which the bearing industry have to face. The raceway surface is working surface of bearings, the surface quality and dimensional accuracy will direct influence on work performance and life of bearings. The raceway grinding process is one of the key manufacturing processes. The influences produced in grinding process on raceway surface quality will extended into bearing products, and further influence the service behavior of bearings. Grinding is a high specific energy process, which need more energy put into removing unit volume materials. Most of the energy invested is transferred into heat and produce high temperature and its influences on material qualities are also hardly avoided.Therefore, in order to obtain the best condition of raceway surface quality and improving grinding technology level, it is a key issue is how to choose a reasonable grinding parameters.In the evaluating indicators of surface qualities, ground surface roughness and heat affected layers are chosen as the key research objects. Take the microscopic interaction mechanism in grinding contact zone as a breakthrough point. The ground surface topography and grinding affected layers were studied both theoretically and experimentally. In the work of this paper, the numerical relationship between grinding parameters and ground surface roughness and thickness of grinding affected layers, was established, which has certain theoretical significance. An experimental platform were built and bearing ring raceway grinding experiments were conducted and a bearing ring raceway grinding process planning method considering roughness, grinding affected layers and grinding efficiency were taken into considered, which has certain practical significance. There are following several contents in the research of this paper.(1) A deep research and analysis has been carried out on microscopic interaction mechanism in grinding contact zone and interrelated machematical models were established. Based on the assumptions of grains’ size following normal distribution and the grains’ location following random distribution, the number of grains in unit grinding wheel volume and maximum undeformed chip thickness have been developed. The grinding contact zone has been redefined to be a space region of "grinding contact length×grinding width×maximum undeformed chip thickness" and there are four stages while a grain experiences contact zone:uncontact, sliding, plowing and cutting. The mathematical relationships between the starting locations&lengths and the sizes, protruding heights, locations of grains, have been analyzed. The maximum undeformed chip thickness was obtained by solving a equation of making the removed material volume from all cutting grains microscopically, equaling with specific removal rate macro scopically. Based on the analysis of these four stages, the integrating ranges of grain size, protrude height and location of each type of grains, and the numbers of each type of grains were obtained. The contace zone were discreted and the numbers of each type of grains in each interval were calculated the the distributions of each type of grains were obtained.Theoretical analysis and calculation results showed that the number of uncontact grains is a half of total grain number and reduces along grinding contact length, sliding grains are existing along whole grinding contact zone and its number is increasing along grinding contact length, plowing and cutting grain are starting occurrence at some place in grinding contact length. The research on the microscopic interaction mechanism has been established an theoretical basis of the following work.(2) Models of ground surface topographies and roughness were established considering grinding wheel dressing and wear effects and the mathematical relationship between grinding and dressing paremeters and ground surface roughness was established. Shape line LA was defined on the workpiece surface and describes surface topography. The grains on the wheel surface go through successively and change the shape of LA.Based on the relationship of grain protrude height, size, location and penetration depth, considering the stochasitic nature of grains’distribution, the grain’s size, protrude height and locations at normal and width of workpiece surface have been determined. After all the grains passed, the ultimate shape of LA and ground surface roughness Ra, were obtained. Comparison results between calculation and experiment data showed in good agreement. The influence of grinding parameters on ground surface roughness was analyzed. Take the grinding wheel dressing and wear effects into consideration, dressing and wear shape line Ldw was established and the ground surface topographies and roughness considering dressing and wear effects were obtained.Before a grain changes LA, its shape was changed by Ldw. A dressing lead single factor experiment was carried out to validate the models and the influence of dressing parameters on ground surface roughness was analyzed. The relationship between bearing raceway grinding parameters and ground raceway surface roughness then was obtained by applying relative methods. Results showed that, bearing raceway surface roughness only relative with precision grinding stage. Since the grinding depth of precision grinding is small, raceway roughness is affected by bearing ring spin velocity the most. Based on the relationship between dressing parameters and ground surface roughness, the rationality of dressing parameter can be evaluate, and based on the relationship between grinding parameters and ground surface roughness, a set of reasonable grinding parameter can be chosen to satify process requirements on roughness.(3) A relationship between grinding paremeters and grinding dark layer thickness has been established. A method of process planning comprehensive considering dark layer thickness, roughness and working efficiency has been proposed. The mechanism of grinding white layer generation has been analyzed. Single plowing and cutting grain forces models were developed. Combining with the distributions of each type of grains on grinding contact zone, the total heat flux distribution was obtained. Combining with the heat partition ratio distribution, the heat flux into workpiece was obtained. An FEM model was developed, the heat flux into workpiece was loaded on the grinding surface and the grinding temperature field and dark layer thickness was obtained. The relationship between bearing raceway grinding parameters and dark layer thickness was then obtained. Results showed that the dark layer thickness was influenced by grinding depth the most. Base on the relationship between grinding paramters and ground surface roughness as well as dark layer thickness, a method of porcess planning comprehensive considering roughness, dark layer thickness and working efficiency, was proposed. Based on a single grain cutting FEM model, the generation mechanism of white layer was analyzed. Results showed that, even though the population temperature in grinding contact zone is not reaching the phase-transition temperature, the white layer can also be emerged bacause single grain cutting. In this situation, the characters of white layer on workpiece surface are random and discontinuous.(4) An experimental platform of bearing ring raceway grinding of angular contact ball bearing B7008C inner ring, and a series of experimental research were carried out. The tangential grinding force was obtained by measuring the power of grinding wheel drive motor. The grinding temperatures were obtained by a modified screw type thermocouple method. The metallurgical structure of ground surface material was observed under metalloscope and the thickness of dark layer can be obtained. All of the above measurement data were compared with calculation results which showed in good accordance.