Research On Contact Dynamics And Related Problems Of A Space Gear Transmission System

With the development of space technology, the spacecraft requires longer servicelife and higher reliability. The space mechanism is an essential and major component ofspace vehicle. It is significant to study the key factors and suppression measures ofcontact failure for improving the reliability and service life of spacecrafts. Consideringthe contact analysis of gear transmission system being a typical representative, the gearswere treated as the research object in this paper. In the space high vacuum environment,adhesion and surface morphology of metal contact will bring important impact on thegears contact failure. The contact deformation is very complex and involves elastic,elasto-plastic and plastic deformation. Failure of the space vehicle gear in the vacuumenvironment is obviously different with that in a terrestrial environment. Thethermodynamics, contact mechanics and tribology behavior has a distinct specificity.Therefore, it is necessary to conduct further research. In this paper, as an example, thegear parameters, the surface properties, the temperature distribution and contactcharacteristics of a gear transmission system in spacecraft were explored.Firstly, a mathematical model of contact stress variation of gear along meshing linewas built, based on the Hertz contact theory. By analyzing the impacts of gear moduleand gear ratio to meshing contact stress; distribution regularities of space gear contactstress were obtained. The influence of frictional coefficient on contact stress of gearmeshing was analyzed. The main shear variation under the common function of normalforce and tangential force was obtained. The results showed that the stress changed byabout100%when the tangential force increased to a certain value. By the means of thetheoretical analysis and finite element method, the model of space gear meshing finiteelement was built, and lay the foundation for thermo-mechanical coupled analysisSecondly, based on the single rough peak and rigid plane contact theory, acylindrical rough surface contact model was established by considering the metalsurface adhesion in the vacuum environment. This model can effectively distinguish theelastic, elasto-plastic and full plastic deformation. By comparing the elasto-plasticitymodel with Lo’s full plastic model, the results showed that the difference is very smallat a small plastic index. With the increasing of plasticity index, Lo’s full plastic modelcalculation error increase, and the calculation results of the present model is moreaccurate when the plasticity index is larger than1. Based on this elasto-plasticity model, the ratio between the actual contact area and the nominal contact area and the tangentialcarrying capacity changes with the contact positive load were analyzed. The cylindricalsurface hardness, surface roughness and the impacts of dimensionless plastic index onits contact character also were studied. Aiming at the characteristics of space highvacuum, the influence of surface energy on the contact failure of gear meshing wasanalyzed. The above studies provide a foundation for the further research on the spacesmall gear contact failure mechanism.Thirdly, a thermal network of space gear driven system model was set up.Experiments of the ordinary pressure and the vacuum environment were performed, theboundary condition of the thermal network model was determined. The impacts ofoperating parameters such as the load and the rotate speed on temperatures of key nodesin the vacuum environment were analyzed. The temperature distribution regulars ofthe gear system when vacuum no convection heat dissipation were obtained. Theimpacts of load torque, rotate speed and sliding friction index on the transienttemperature rise in the gear mesh contact area were analyzed. The flash temperaturedistribution laws of the gear system were obtained. Using a built finite element contactanalysis model, thermo-mechanical coupled analysis was carried out. The relationshipbetween the instantaneous temperature and the contact stress on the tooth meshing wasobtained.Then, aiming at the difficulties of vibration spot locating in the space gear servicelife experiment, a method of locating optimize vibration spot was set up and theoptimized vibration spot obtained by this method was verified by the kurtosis indexmethod. The limitation of single time domain analysis and frequency domain analysiswere carried out in the space gear driven system fault diagnosis. The results show thatthe frequency analysis can obtain more precise diagnosis results of the health statuswhen the single time domain analysis combined with the frequency domain analysis andthe wavelet theory analyze the gear driven system vibration signal. The health diagnosisexperiment platform of the space gear system was built. In this system, the vibrationsignal in various phases of the space gear driven system was analyzed, using acombination of wavelet transform and demodulation analysis method. The result showsthat the combination of wavelet analysis and demodulation analysis can effectivelydiagnose the health status of space active components.Finally, based on the current subdivision technology and using magnetic resistancetype stepping motor, a constant braking torque load brake used in the vacuum environment was developed. An experimental platform for the space gear system wasbuilt. The experiment comparing the ordinary pressure with the vacuum environmentwas performed. Using SEM and AFM, microstructure figures of gear meshing surfaceunder the vacuum environment were observed. The contact failure of different locationin the gear tooth surface and the mutual relations of contact failure and stress wereanalyzed. In order to study the friction wear mechanism of the gear in a space craft,comparing experiments of gear in the vacuum and the ordinary pressure environmentwere carried out. The main failure mode in different environments was also studied. Theresults show that the attached wear is more likely to occur under the vacuum conditionthan under the ordinary pressure condition, and the main wear under the ordinarycondition are the fatigue wear and the abrasive wear. In order to analyze the addendumfailure of the space gear, the ion nitriding layer thickness was studied. It provides atheoretical basis for the gear optimize design.