Failure Mechanism Theory And Accelerated Life Testing Method Research For Space Lubrication Harmonic Drive

Present work is based on the Civil Aerospace Advance Research Program and theproject “The theory and method of reliability design of a new model drive componentsand the systematic” which had been supported by National Natural Science Foundationof China. Due to the development of aerospace technology, long-life, high-precision,high-reliability, high-power density and high-stability become fundamentalperformances for modern space driving mechanisms. However, the reliability andservice life of space drive mechanism is directly influenced by fundamental theoreticalresearch, especially for the dynamic service behavior and failure mechanism of themechanism under complex space environment, there still lack of complete theoreticalframework and experimental study for them. All these issues have become the weakpoints during the development process of long-life space drive mechanism, andconstituted the major causes of frequent on-orbit malfunctions of space drivemechanism, furthermore, become the stumbling block for the development of modernspacecraft.Therefore, this paper takes lubricated harmonic drive, the most typical componentsin space drive mechanism, as the research object, applying mixed lubrication theory,conducts simulation analysis on the failure mechanism of the harmonic drive,establishes the failure model and based on above, proposes the optimal acceleratedlife testing method. At last, the comprehensive performance testing system of spacelubricated harmonic drive is set up and the relevant tests are carried on to verify theeffectiveness of the proposed method. The main research content is composed of thefollowing six parts:(1) Lubricated harmonic drives for space application are prepared for testing, thetooth surfaces of circular spline and flexspline are lubricated by mixed diamond-likecarbon films (WC-DLC:H) and PFPE-based grease (Braycote601). The wave generatorbearing is lubricated by MoS2-Ti composite films and polytetrafluoroethylene retainer,the contact surface between flexspline and wave generator is lubricated only byBraycote601. Meanwhile, the mechanical and tribological properties of the lubricatingmaterial are tested.(2) In order to obtain proper mixed lubrication model to study the failuremechanism of the lubricated parts, Reynolds equation is first deduced under isothermal condition. Then, film thickness equation of line contact taking surface roughness intoconsideration is established and in which the relationships of viscosity–pressure,viscosity–temperature, density–pressure and no Newtonian fluid property for thelubricant are considered. The relative motions and load distributions of contact surfacesbetween the inner flexspline and wave generator bearing, circular spline and flexsplinetooth surface are analyzed, and the equivalent lubrication model is established. Basedon above, the mixed lubrication simulation is conducted with the film thickness ratio (λ),load distributed generated by asperity contact (Wc) and relative contact area (Ac) as therepresentative variables.(3) After obtain the mixed lubrication simulation results, the failure mechanism ofthe grease lubricated contact parts and the effects of rotation speed, load, temperature ontheir lubrication status is discussed. The results release that the rotation speed has themost significant effect comparing to load and temperature. Moreover, the effects of thetemperature and load on asperity contact are more serious at a lower speed while theeffects decrease with increase in the rotation speed. The analysis results become thetheoretical basis for the determination of accelerated stress and accelerated factor duringaccelerated life test.(4) The factors effect backlash, torsional stiffness and transmission accuracy of theharmonic drive and how they generate those effects are analyzed. The computationalformulas for calculating backlash, torsional stiffness, transmission accuracy and wearclearances between friction pairs in the harmonic drive are presented.(5) The wear model of MoS2-Ti composite film and polytetrafluoroethylene werefitted through the data obtained from wear tests, and the possibility for conductingaccelerated test for the two lubricating materials are analyzed based on the wear model.The effect of asperity distribution on friction coefficient during contact in mixedlubrication is investigated. And then the adhesive wear model characterized by frictioncoefficient in mixed lubrication is proposed based on Johnson-Williamson mixedlubrication model and Archard adhesive wear model. Furthermore, based on the wearmodel and film thickness equation of line contact elastohydrodynamic lubrication, theaccelerated life test schemes for full film lubrication and mixed lubrication are proposed,respectively. Finally, accelerated life test scheme for the harmonic drive is obtained,including the choice of accelerated stress and accelerated factor and establishment of theextrapolation relationship between accelerated life and regular life.(6) A precisely testing method for transmission accuracy, transmission efficiency, backlash and torsional stiffness of the harmonic drive is studied and correspondingexperimental system for in-circuit and in-situ test of harmonic drive under thermalvacuum environment is set up. A serial of5000-hours thermal vacuum life tests arecarried out, and the test results verifies the validity of the proposed theory.