Research On Ultra-precision Grinding Principle And Method For Complex Shapes Of Seal Rings Used In Reactor Coolant Pumps

Reactor coolant pumps are considered as the core equipment in nuclear power plants, which are used to force high-temperature, high-pressure and radioactive coolant cycling between the reactor core and the stream generator. The seal rings are important parts used to guarantee the reactor coolant pumps running in safety, stabile and reliable conditions. These seal rings are usually made of high hardness materials such as silicon nitride, alumina, silicon carbide and tungsten carbide etc. due to their excellent mechanical strength, wear resistance, high temperature resistance and chemical corrosion resistance. It makes the seal rings difficult to manufacture that the form accuracy is restricted within submicron scale and surface roughness is restricted within nanometer scale. Currently, their core manufacturing technologies are just controlled and monopolized by a few companies abroad. Therefore, it has important significance for localization of the reactor coolant pumps to investigate novel and practical ultra-precision machining principle and method.Aiming at solving the difficult problems of manufacturing the complex shape of seal rings used in reactor coolant pumps with high hardness and high form accuracy, this dissertation proposed a grinding principle and method using a cup wheel by line-contact, and the confirmatory experiment for the proposed principle and method is performed based on the existing ultra-precision grinding machine by grinding a conical surface of the seal ring. The main contents include four aspects.(1) A novel grinding method is proposed to manufacture the curved surface of the seal rings using a cup wheel by line-contact. The mathematic model for grinding the circular curved surfaces is established and the forming principle is discussed. It is found that the rotational projection curve of the cup wheel circle can be classified into7types by solving the domain of definition and defining the characteristic points. Therefore, the spatial position of the cup wheel relative to the work-piece can be obtained by calculating the principle form error, which will provide theoretical foundations for developing the ultra-precision machine.(2) A mathematic model of grinding the wavy-tilt-dam seal rings using a cup wheel by line-contact is established on base of the four-axis linkage grinding machine. The effects of the structural and motional parameters on principle form errors of the ground wavy-tilt-dam surface are discussed. Six motion control strategies are developed and a motion control strategy that makes the principle form error be restricted less than10nm is determined.(3) The experiments to grind tungsten carbide, pressureless sintered silicon carbide and reaction-sintered sintered silicon carbide wafers with high hardness are performed using diamond cup wheels by line-contact. Surface qualities of the ultra-precision ground wafers with high hardness are investigated by observing surface topography, measuring rounghness and depth of the grinding marks. It is found that smooth surface that the surface rounghness Ra less than5nm can be obtained when the diamond wheels finer than2000#are adopted.(4) A mathematical model is established for grinding the seal ring conical surface based on the existing vertical ultra-precision grinding machine using a cup wheel by line-contact. The realations between taper error and radial profile error of the ground surface and the tilt angle of the wheel spindle (pitch angle and tilt angle) are analysed. A method for high accuracy adjusting the tilt angle of the cup wheel spindle with an optical flat and a laser displacement sensor is proposed. The confirmatory experiment is performed by ulta-precision grinding a conical surface. The measurement results indicate that the taper error is4.88urad, the radial profile error is119.4nm, the circumferential profile error is231.6nm, and the surface rounghness Ra is about2nm. The machining accuracy and surface quality are superior to the demands of their technical indicators.The research of this dissertation will provide advanced machining principle and method with independent intellectual property right for ulta-precision manufacturing the seal rings used in reactor coolant pumps. It is prospected to solve one of the difficult problems during the localization process of the reactor coolant pumps, break the technical control and blockade of developed countries, improve the high-accuracy manufacturing level of large-size seal components and international competitiveness of our country.