| The overrunning clutch is an important component to realize power transmission and separation of the aircraft,which needs to meet the working conditions of high speed,long life and high reliability.However,this product rely on imported for a long time,which seriously restricting the localization process of China's aviation products.In this study,several key technologies such as precision forming and surface strengthening were analysised to solve the problem of poor forming precision and short life of an aviation overrunning clutch.The precision forming technology of the aviation overrunning clutch was studied,a new type of forming technology of"wire cutting+grinding+grinding and polishing"was explored,and the key technologies such as wire cutting precision forming,precision grinding,grinding and polishing were studied.According to respective characteristics of GCr15,9Cr18 and Cr4Mo4V bearing steels used for sprag type overrunning clutch,different surface strengthening methods such as boronizing,boron chromium co-infiltration,low temperature carburizing and salt bath nitriding were used.The microstructure and performance characteristics of the strengthening layers were studied.The precision forming technology of the aviation overrunning clutch was studied.In the research of WEDM precision forming technology,the influence of peak current,pulse width time and pulse interval time on the WEDM of GCr15 bearing steel were studied.When the peak current is 2 A,the pulse width is 1?s,and the pulse interval time is 4?s,the best comprehensive effect is obtained.The surface roughness reached Ra 0.76,and the processing efficiency is 10.1 mm2/min.In the optimization of grinding process parameters,grinding process parameters(grinding depth Ap,grinding speed Vs and workpiece speed VW)were taken as optimization variables to optimize the grinding process.The results showed that when Ap=50?m,Vs=30 m/s,VW=2.1 m/min,the optimal test results can be obtained.In the research of grinding and polishing technology,the influence of different grinding polishing methods on the machining effect was studied,and the grinding and polishing parameters were optimized.The results showed that the centrifugal polishing effect is better than that of vibration polishing,with coarse grinding abrasive with inclined cylindrical shape,fine grinding abrasive with?2×10mm alumina ceramic,volume ratio with 1:5,grinding polishing time with 6h,the optimum experimental effect can be obtained.Through a comprehensive study of the sprag precision forming technology,the sprag parts finally obtained with straightness?2?m,surface roughness Ra0.13,and no surface defects such as bumping and decarburization were observed,which is greatly improved compared with similar advanced technologies in china and abroad.The effects of boronizing process parameters on the microstructure and properties of boronizing layer on the surface of GCr15,9Cr18 and Cr4Mo4V bearing steel were studied.The results showed that uniform and dense boronizing layers were obtained on the surfaces of the three materials.According to different matrix materials,the phase compositions of the surface boronizing layer on GCr15,9Cr18 and Cr4Mo4V are?FeB,Fe2B?,?FeB,Fe2B,CrB,?and?FeB,Fe2B,CrB,and Mo2B?respectively.According to different treatment temperature and holding time,the hardness of the surface of the boronizing layer varies in the range of?1630-1950?,?1830-2140?and?1940-2210?HV,and the thickness varies in the range of?33.4-318.5?,?9.3-97.2?and?14.4-147.8??m.Diffusion activation energy of the layers are 188.6,203.7 and 195.4 KJ/mol respectively.The effect of TRD pre-chromizing treatment on the microstructure and properties of boronized layer of GCr15 bearing steel was studied.A pre-chromizing layer with uniform thickness was prepared on the surface of GCr15 bearing steel by solid TRD chromizing method."TRD pre-chromizing+boronizing"treatment can improve the surface roughness of and reduce the brittleness of the infiltrated layer,so that the bonding strength between the infiltrated layer and the substrate improved greatly.The secondary hardening technology of 9Cr18 stainless bearing steel and Cr4Mo4V high temperature bearing steel were studied.After carburizing 9Cr18 stainless bearing steel at low temperature,a infiltrated layer with high density and uniform thickness was obtained.The phase composition of the surface of the infiltrated layer is mainly supersaturated martensite??'C?,cementite and chromium-carbon compound.The hardness of the surface of the infiltrated layer varies from 1430 to 1720 HV.Compared with the quenched sample without surface treatment,the hardness of the infiltrated layer is increased by 2 to 3 times.The effective nitriding layer can be obtained by salt bath nitriding of Cr4Mo4V high temperature bearing steel.The phases composition of the surface is mainly supersaturated martensite??'N?,iron nitrogen and chromium nitrogen compounds.The surface hardness of the infiltrated layer varies in the range of 1035-1245HV.Compared with the quenched sample without surface treatment,the hardness of the infiltrated layer is increased by 1.3-1.6 times.The tribological and fatigue characteristics of layers under different surface strengthening treatments were studied.After boronizing and"TRD pre-chromizing+boronizing"treatment,the steady state friction coefficient of GCr15 decreased and specific wear rate decreased by 77.6%and 88.6%respectively.After boronizing and low temperature carburizing treatment,the specific wear rate of 9Cr18 decreased by 71.6%and 93.7%respectively.The specific wear rate of Cr4Mo4V bearing steel decreased by52.5%and 81.4%after boronizing and nitriding treatment.A special fatigue performance testing machine for overrunning clutch was developed to check the fatigue performance of the infiltrated layers.The fatigue test results showed that GCr15 steel has the shortest fatigue life and Cr4Mo4V steel has the best effect under the condition of quenching-tempering treatment.Compared with the current domestic plans,the stripping width of the infiltrated layer decreased by 11.89%,36.56%,1.32%and 35.68%respectively after 160,000 fatigue performance tests for GCr15-boronizing,GCr15-TRD pre-chromizing+boronizing,9Cr18-boronizing and 9Cr18 low temperature carburizing plans.Through the research of this subject,problems of precision forming and surface strengthening of the sprag for aviation overrunning clutch have been solved.The accuracy of the sprag has reached the international advanced level,and the surface strengthening effect of the sprag has been greatly improved,which provides theoretical basis and technical support for the localization of aviation overrunning clutch. |
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