ISSN   1004-0595

CN  62-1224/O4

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考虑接触点运动的偏心轮-滚子副运动学及润滑特性研究

Kinematics and Lubrication Characteristics of Aneccentricwheel-Roller Pair Considering the Contact Point Motion

  • 摘要: 在凸轮-滚子副润滑分析中,为求解接触区的卷吸速度对凸轮-滚子单元进行运动学分析,发现接触点除了绕凸轮轴轴心作定轴转动外,还会因为滚子升程的变化来回摆动,产生牵连运动. 卷吸速度的计算会采用忽略接触点运动的简化处理方法,但产生的误差鲜有研究. 使用点的合成运动法,将复杂运动转化为简单运动的叠加,简化了计算过程. 为更好的厘清凸轮与滚子间的运动学关系,将偏心轮-滚子作为凸轮-滚子的1个特例进行运动学分析,得到考虑接触点运动的卷吸速度解析解. 结果表明:考虑接触点运动的卷吸速度最大相对误差为8.89%,接触点的运动对卷吸速度计算的影响不可忽略. 讨论了偏心轮-滚子单元偏心距、偏心轮半径、滚子半径和偏心轮转速对卷吸速度计算结果的影响,发现偏心距较大时接触点运动的卷吸速度相对误差影响较大. 建立偏心轮-滚子副非牛顿点接触时变润滑模型,数值模拟了接触点的运动对润滑状态的影响. 发现偏心轮角度为0°时,y/a=0截面的中心膜厚差距最明显,相对误差可达6.30%,同时接触点的运动会给第二压力峰带来细微变化,因此不可以忽略接触点运动带来的影响.

     

    Abstract:
    Cam-roller followers are widely used in the valve train of internal combustion engines in recent years. They can significantly reduce the power loss and improve the lubrication state. However, the cam-roller pair still faces harsh working conditions as the load, the entraining velocity and the radius of curvature change drastically. This often causes severe wear problems on the cam surface and cause a complex lubrication state.
    In the lubrication analysis of the cam-roller pair, the kinematic analysis of the cam-roller unit was carried out for solving the entraining velocity. It was found that the contact point not only rotated around the center of the camshaft as a fixed axis of rotation, but also oscillated from side to side as the roller moves up and down. This swinging produced a convected motion which effect the result of entraining velocity. The use of the synthetic motion method of points simplified the calculation process by transforming complex motions into a superposition of simple motions. The central idea is that relative velocity equals absolute velocity minus convected velocity.
    In lubrication analysis of the cam-roller pair, a simplified method that ignored the contact point motion was used by some people in the calculation of entraining velocity, but the resulting errors had rarely been studied. In order to clarify the kinematic relationship between cam and roller, the eccentric wheel-roller was taken as a special case of cam-roller for kinematic analysis, and the analytical solution of the entraining velocity considering the contact point motion was obtained. In order to explore the effect of contact point motion on the results of the entraining velocity, the relative error ψ was defined. It can be seen that the relative error was 8.89% when rotation angle was 0°. The difference was obvious that could not be ignored, so the contact point motion must be taken into account in the calculation of the entraining velocity.
    In the process of calculating the entraining velocity, there were four parameters that would affect the results of calculations: eccentricity, the radius of eccentric wheel, the radius of roller and the rotating speed. Therefore the effects of the four parameters on the calculation results had been discussed. It could be seen that the eccentricity had the most obvious impact on the relative error, and when the eccentricity increased from 5.0 mm to 7.5 mm, the relative error increased from 8.89% to 14.33%.
    It was known that the entraining velocity effected the film thickness directly. The higher the entraining velocity, the more lubricant would be involved into the contact zone. Therefore, the impact of the contact point motion on the lubrication state had been studied. A non-Newtonian transient lubrication model of the eccentric wheel-roller pair was established to simulate the effect of the contact point motion on the lubrication state. Since the central film thickness was a strong function of the entraining velocity, the trend of central film thickness was similar with the trend of entraining velocity. The relative error was 6.30% when rotation angle was 0°. Accordingly, the effect of the contact point motion could not be ignored on the analysis of lubrication state.

     

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