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基于恢复时间的剪稀模型及其在弹流润滑中的应用
中文摘要

减少运动副间的摩擦损失是提高机械零件效率、减小能量损耗的主要途径。摩擦副间润滑油品的摩擦特性研究至今仍然是工业界和学术界关注的热点。润滑油品在高应力、高剪切率、高温等条件下,其粘度和状态发生的变化均会影响油品宏观的摩擦特性。因此,油品在某工况下的流变特性研究是上述热点问题中的难点。本学位论文以此为出发点,从理论和实验两个方面探究准确预测各类油品流变特性的模型和方法。 本文首先从几十年来在流变研究中广泛使用的一个假设,即剪应变率近似等于两表面的速度差与油膜厚度的比值入手,分析了不同尺度的弹流润滑副内的非牛顿流体热流变润滑特性。通过分析接触区内油品温度和广义粘度的变化规律,发现了一个以前未报道的、完全违背预期的事实,进而引出剪应变率对广义粘度的影响。结果显示,在尺度较大或其它温度梯度较大的场合,上述关于剪应变率的假设并不成立。特别地,对于粘度较高的聚合油,即便在接触副尺度较小时,该假设也会误导油品的流变研究,干扰我们对油品流变特性的认识。 接下来,关注到最近国际上关于哪个流变模型更好的争论,开展了适用于多类流体的流变模型研究。用球-杆间相互接触、通过和恢复的过程模拟剪切流动时流体分子间的作用,依据球-杆恢复到未受剪状态的时间,建立了全新的半解析流变模型。通过与著名的Eyring和Carreau-Yasuda模型的比较,指出这三个流变模型的表达式具有一致性,但新模型的流变参数的个数更为理想。进一步,使用新流变公式建立了点接触热流变弹流润滑的数学模型,模拟了生物油squalane、聚合油PA0 650和矿物油Shell Turbo 33等三种不同类型、不同粘度油品的流变曲线。结果显示,三种油品使用新模型得到的摩擦系数随滑滚比变化的曲线与实验曲线均吻合得较好,表明本文提出的新流变模型是正确的,且其应用不受油品类型和粘度范围的限制。 为了进一步验证新流变模型的正确性和其广泛的适用性,本文将新模型分别应用于反向滑动的点接触副和同向滑动的圆锥滚子接触副的热流变弹流润滑理论分析。数值仿真结果表明,新模型预言的高粘度聚合油钢对玻璃反向滑动的弹流润滑副内有典型的、由热粘度楔导致的表面凹陷;且随着钢球反向速度的增加,表面凹陷的尺度逐渐增大,热弹流油膜的第二压力峰逐渐左移,并最终占据压力主峰的位置。通过对接触区内油品受到的剪稀、热稀和热粘度楔效应的综合分析,解释了这些润滑行为的合理性和正确性。进一步,对三种油品开展了钢对玻璃反向滑动的大量实验研究,从多个角度定性地比较了实验和理论结果的一致性,验证了反向滑动工况下,新流变模型的正确性和实验数据的可靠性。另外,使用新流变模型分析了考虑力矩平衡的修形圆锥滚子间弹流油膜的热流变润滑行为,指出当载荷偏置时,滚子会发生偏斜,导致滚子端部效应增强;而增加修形滚子的凸度能减弱滚子的端部效应。因此,为了防止零件局部润滑失效而发生偏磨,滚动轴承中滚子母线的适当修形和齿轮装置中齿面的修形对其润滑状态都是有益的。 最后,自行设计制造了球-盘接触任意滑滚比下的膜厚和摩擦力测量装置,实现了润滑油品在点接触多种工况下油膜膜厚和摩擦力的同步测量。使用该装置分析了标准参考油的摩擦特性,研究了三种不同长度的链状结构PAO油品的流变特性,以及同一等级不同分子结构的四种油品的摩擦特性。结果显示,合成油的摩擦系数整体上低于矿物油,且油品的摩擦特性与分子结构密切相关。 关键词:流变;恢复时间;弹流润滑;摩擦力;热粘度楔;分子结构

英文摘要

A primary way to enhance the transmission efficiency and to reduce energy loss is to suppress the friction of rubbing surfaces. Even to this day, the friction features of lubricants confined in contacts are still hot topic both in industry area and scientific researches. The friction of lubricants is sensitive to its viscosity and state under high pressure, high shear strain rate, and high temperature conditions. Thus, the rheological properties of lubricants are complex and tough to be evaluated. From this consideration, this dissertation will focus on the explorations of the rheological behaviors of lubricants theoretically and experimentally, so that to develop a new rheology model to predict accurately the rheology characteristics of lubricating oils. In this dissertation, the availability of a popular assumption employed in rheology studies in the past decades was first discussed. In this assumption the shear strain rate was defined as the ratio between sliding speed and film thickness. The scale effect on lubrication was analyzed with consideration of non-Newtonian and thermal effects. By analyzing the temperature and the generalized viscosity of the lubricant in highly pressured contacts, an unexpected prediction was found, which has not been reported in the literatures yet. Consequently, the influence of shear strain rate on the generalized viscosity was introduced. The results indicate that the widely used assumption will become invalid for large scale contacts or at other status with high temperature gradients. Especially, for the polymeric oils with high viscosity, this assumption will deliver inaccurate rheological information even at small scale contacts, misleading our understanding of lubricant rheology. Regarding to the limitations of the rheology models which are acceptable to some extend but still in open argument and discussion, exploratory work was conducted to develop a new rheology model that is able to describe the rheological features for extensive lubricants. In present work, the contact, passage and recovery of molecule interactions in the sheared fluid film were simulated by a ball-rod configuration. Based on the recovery time of the ball-rod deformation, a new semi-analytic rheology model was established. By comparing with the noted Eyring and Carreau-Yasuda models, it is found that the three models are consistent in their forms. But the new model is more favorable due to its proper number of rheology parameters. Furthermore, the new rheological equation was combined with the mathematical model of thermal elastohydrodynamic lubrication (EHL) for point contacts. The rheological features of three types of lubricants, i.e. bio-oil of squalane, polymeric oil of PAO 650 and mineral oil of Shell Turbo 33 were then obtained through numerical simulations. The results show a good agreement between the friction coefficients versus slide-roll ratio predicted theoretically and measured experimentally, suggesting that the new proposed rheological model is accurate enough and beyond the limitation of oil types. To further validate the applicability of the new rheological model, thermal EHL solutions in opposite sliding point contacts as well as in the contacts of two tapered rollers were achieved elaborately. It is found that for the thick polymeric oil in the ball and glass contacts under opposite sliding conditions, abnormal surface dimple produced by thermal viscosity wedge will be predicted by the numerical solutions. Moreover, with increase in the magnitude of ball speed, the dimple will be amplified and the EHL pressure spike shifts to left and becomes the main pressure peak eventually. By comprehensively analyzing the effects of shear thinning, thermal thinning as well as thermal viscosity wedge, the mechanisms of lubrication behaviors were then rationally acknowledged. Moreover, extensive experiments were conducted with three oils in ball and glass contacts. The agreements between experiments and numerical predictions were achieved through qualitatively comparisons. This further validates both the reliability of the new model and the experiments. In addition, using the new model, the thermal rheological behavior in modified tapered roller contacts was also analyzed by considering the moment equilibrium. When the load was applied not through the contact center, the roller will skew, resulting in local high stress at one end, whilst this situation will be improved to some extent by increasing the convexity of rollers. Therefore, it is beneficial to avoiding local lubrication failure and partial wear by proper modifications of the generatrix of rollers. Finally, a test rig was developed to simultaneously measure the film thickness and friction coefficient under various slide-roll ratios. Using this test rig, the fiction properties of standard reference oil was analyzed. Moreover, the friction behaviors of three PAO oils with different chain structures and four oils with the same grade but different structures were compared. The results show that synthetic oils contribute to the lower friction coefficient. The results also point out that the friction behavior of oils strongly depends on the molecular structures. Keywords: rheology; recovery time; elastohydrodynamic lubrication (EHL); friction; thermal viscosity wedge; molecular structure

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