Real wear process has entropy imbalance characteristic. Entropy equilibrium is a hypothetical ideal situation. In this paper, the dissipation wear model based on the entropy imbalance characteristic is proposed for improving wear prediction accuracy under mixed lubrication. Firstly, it is revealed that the imbalance between wear heat generation and dissipation leads to entropy imbalance. The prediction errors of wear rates by entropy-balanced assumption are more than 40%. Then, evaluation parameter of entropy-imbalanced degree is defined by rotating normalized entropy generation-flow coordinate. Entropy-imbalanced dissipation wear model and corresponding wear life prediction method are obtained. Finally, it is verified that prediction errors of the proposed model and method are less than 10% and 5% through 20-hour wear life experiments under mixed lubrication.

This paper proposes an accurate and applicable online measurement method of wear depth using displacement signal for vertical testers under mixed lubrication. Firstly, displacement signal is revealed to be composed of temperature drift and Brownian motion. Secondly, the first order Taylor expansion of the Brownian motion is revealed to be the unified form of wear models. Further, the online measurement method is established by introducing non-integer order and amplitude coefficient to eliminate systematic randomness. Finally, the method is verified by graphite-metal tribo-pairs on a standard tester and a high speed friction and wear testing bench, relative errors between online and offline measurement results are less than 10%, extension errors by non-integer order within its application range are less than 2%.

At present, the prediction of wear failure is often based on temperature, friction coefficient, vibration, and other single or a few macroscopic parameters to judge the contact condition of the friction pairs. However, there are still deviations between the measured results and the actual situation. In the digital twin system for studying friction and wear phenomena, in-situ measurement and characterization of topography of friction pairs is a challenge problem which does not exist in other procedural twin systems. This paper investigates the relationship between in-situ measured macroscopic parameters and topographical parameters. First, the dynamic relationship between friction coefficient and fractal parameters is derived from the view of a microscopic contact, and the wear periods are determined based on the dynamic changes of fractal parameters. The derived friction-fractal formula is based on mechanical deformation and the Majumdar and Bhushan rough surface contact model, avoiding the assumptions based on specific wear mechanisms, then having the universal property. Second, the experimental verification is carried on which demonstrates good correspondence between the obtained topographical parameters based on the friction-fractal formula with the experimental results. The obtained friction-fractal formula provides a theoretical basis for predicting and analyzing the wear status of friction pairs in the critical equipment and supports the real-time contact state characterization of the friction pairs in the digital twin system.

This paper proposes a high accuracy time-varying wear evolution method with cloud maps highlighting dynamic characteristics of surface signal. Firstly, the cloud map method is established by arrangement of measured data, thus time-varying wear evolution process is visualized on a series of plane figures for preliminary qualitative analysis. Then, high accuracy recognition of time-varying wear states is realized by cloud map shape parameters, including kurtosis and 1-D kernel density function highlighting distribution information of surface signal. The relative recognition degree by cloud map shape parameters are higher than those of friction coefficient, Root-Mean-Square (RMS) deviation and fractal dimension. Finally, high accuracy time-varying wear life prediction is realized by cloud map size growing speed highlighting waviness information of surface signal. The relative prediction error is reduced from more than 20% to less than 10% compared with friction coefficient and roughness.

The evaluation and prediction on failure of friction pair are important for improving equipment reliability and life span. In engineering practice, researchers often determine the severe-wear stage by monitoring changes in wear and the friction coefficient. However, existing criteria for evaluating the wear process are nebulous, leading to the ambiguous relationship between the measured parameters and the wear process which cannot be precisely quantified. In this study, based on dissipation theory, an exploration into the system entropy production rate variations throughout the wear process has been conducted. This investigation reveals that entropy production rate variations at different wear stages, providing a basis for segmenting the wear process. Through experimental methods, a clear transitional stage with substantial entropy production rate variations between the mild-wear and severe-wear stages has been discovered. These findings gave a novel method for predicting wear failure. The efficacy of this proposed approach has been validated by experiment, thus laying a theoretical groundwork for early failure prediction of critical components principally prone to wear failure. Furthermore, this work provides the foundation for the construction of visual data system in friction-wear digital twin systems.

A surface topography characterization parameter system based on fractal parameters has been established, and several estimation methods for these fractal parameters have been suggested accordingly. Since scale dependence exists in these conventional methods, it is necessary to find an estimation method for characterization parameters with uniqueness. An estimation method for ideal fractal parameters for multi-scale measurement of polished surface topography is proposed in this study. Polished surfaces of two materials, WC-Ni and 9Cr18Mo, are measured under multi-scale for frequency component analysis. This study proposes an estimation method for ideal fractal parameters based on a modified determination method for the scale-free region and the decomposition of frequency components into three classifications. The reasonable results verify the existence of ideal fractal parameters: for the WC-Ni surface, ideal fractal dimension D = 1.3 and scale coefficient G = 2.23×1020 μm; for the 9Cr18Mo surface, ideal fractal dimension D = 1.2 and scale coefficient G = 3.33×1033 μm. Additionally, it is revealed that the scale-dependent components conform to the same regulation on the same instrument by comparing the results of two materials. The conclusions of this study are expected to support tribology research and mechanical engineering related to surface topography.

Normal displacements are measured by sensors on current wear apparatus to represent wear depth. In practice, measured normal displacements are interfered by actual working condition factors such as thermal expansion and mechanical vibration, resulting in a puzzle of relationship between wear depth and measured normal displacements. This paper provides a characterization method of such relationship under mixed lubrication condition. By introducing Taylor expansion to measured normal displacements, Archard model and dissipation model are unified as the first order of such Taylor expansion. A non-integer fractal order D is determined to give the function relationship between wear depth and measured normal displacements. The fractal order D is verified to be a characterization parameter of measurement quality based on a series of wear experiments. Further analysis proves that the fractal order D is capable to be a prediction parameter of wear depth, the prediction errors are below 15

The gas–liquid miscible backflow pumping seal (G-LMBPHS) is a non-contact mechanical seal that is suitable for high-speed bearing chambers. However, the tribological properties and wear mechanisms of the frictional pair of G-LMBPHS in an oil–air environment have not yet been comprehensively studied. In this study, the tribological properties of six frictional pairs, consisting of three hard materials (18Cr2Ni4WA, Al2O3 coating, and Cr2O3 coating) and two soft materials (metal-impregnated graphite [Metal-IG] and resin-impregnated graphite [Resin-IG]), were analyzed using a disc-on-disc tribometer. An oil–air environment was created using a minimal quantity lubrication (MQL) system and a closed chamber. The results show that the COF of the four frictional pairs consisting of two coatings and two graphites decreases gradually with increasing rotational speed, and the frictional pairs composed of Al2O3 coating and Resin-IG and Cr2O3 coating and Resin-IG have the lowest COF between 0.022 and 0.03. Therefore, the frictional pairs of G-LMBPHS are in a mixed lubrication condition. The lubricant in the oil–air environment is adsorbed and stored in pits on the surface of graphite and coatings, enhancing the hydrodynamic effect of the spiral grooves and reducing the COF by up to 45%. Metal-IG has better wear resistance than Resin-IG, and the frictional pair consisting of Cr2O3 coating and Metal-IG has the lightest wear. This study provides an important basis for the selection of G-LMBPHS frictional pairs in oil–air environments.

针对航空发动机轴承腔气液两相环境非接触式机械密封启动过程的磨损问题,提出高压侧具有引流槽、可实现零泄漏的润滑密封端面结构.基于雷诺方程建立润滑膜流场分析模型,求解计算具有动压-润滑组合槽的机械密封性能,并与普通螺旋槽机械密封进行了性能对比,讨论高压侧引入润滑槽对液膜厚度、液膜刚度、泄漏率以及摩擦性能的影响规律,通过高速性能试验及摩擦磨损试验验证计算的准确性和端面的减磨效果.端面结构在低速阶段的接触摩擦试验显示,具有组合槽的密封端面在相同的启停工况下端面摩擦因数可以有效降低50％-75％,高速性能试验结果显示,具有组合槽和仅有动压槽的机械密封在工况范围内均能保持理想的负泄漏率,说明气液两相润滑机械密封能够在工作环境中处于理想的泵送状态,实现了对润滑油的绝对密封效果.外侧深槽与动压浅槽组合的机械密封端面结构可以显著改善端面摩擦磨损状况,可为高速和超高速轴承腔气液两相机械密封端面减磨优化设计提供参考.

Assessing the failure of friction pairs is a crucial approach to prevent accidents. Researchers have endeavored to forewarn potential accidents through monitoring wear and friction coefficient. However, these parameters indicate abrupt changes only after the failure of the friction pair has already occurred. Thus, it is imperative to develop a wear prediction method that can anticipate the wear and friction process. This study investigates the variation of the system energy during friction and wear and proposes a criterion for categorizing the wear process based on the dissipation theory. Moreover, this study summarizes the entropy production rate change characteristics of diverse wear processes and proposes a method to predict wear failure in advance based on these features. The proposed wear failure prediction model is demonstrated through application examples. This model provides a theoretical and computational foundation for preventing wear failure of critical components and supports wear failure prediction of subsequent friction pair digital twin systems.

The service life of products is significant for ensuring stable and reliable performance of these products, such as spacecraft, military equipment and electronic equipment. Such service life is generally evaluated by measuring the working duration of products before losing efficacy or breaking down through tests under the same working condition with the real working condition. However, with the rapid increasing improvements in reliability and of products, the service life of products had been lengthen remarkably, the corresponding time and financial costs of the conventional simulated products life tests were continuously increasing, and such costs were becoming too high to be accepted. On this basis, the concept of accelerated test was suggested as a potential solution in which appropriate equivalent of products service life should be selected. The tests were accelerated by adjusting the changing rate of the equivalent, so that the test durations were shortened under a certain equivalent. In 1980s, the Accelerate Life Test（ALT） and the Accelerate Degradation Test（ADT） based on ALT, which estimated the reliability and service life of products under common stress levels through the accelerate test data under higher stress levels were suggested. ALT and ADT were the most common accelerated test methods, in which the failure possibility function were regarded as the equivalent. In other accelerated tests, the selected equivalent also included surface topography and condition parameters. As for accelerated wear test, there were numerous models for quantitative prediction of wear, which was a typical form of the degradation process of materials, the equivalent of wear could be selected based on these quantitative wear models. This paper aimed to propose an accelerated wear test method based on an accurate quantitative wear model. The investigations were conducted based on friction and wear with experiment condition of surface contact and mixed lubrication by the standard apparatus for typical mechanical seal material pair 9Cr18Mo/M234A0. The structure of tested seal ring was modified to reduce temperature rise and strengthen the fluid dynamic pressure effect. In this paper,the accelerated wear model was established with the entropy increase as equivalent of wear, based on the dissipation wear model which considered the wear as an irreversible thermodynamic process. The dissipation wear model was verified with the mentioned seal pair, revealing a linear relationship between wear rate and entropy increase. By modifying the entropy increase rate under the same entropy increase, the wear test duration was shortened. On this basis,this accelerate wear model was verified with equivalent wear tests with long time/low entropy increase and short time/high entropy increase. This model was also compared with accelerate wear model based on Archard model. The result of which revealed that wear test of 200 min duration could be equally replaced by a wear test of 10 min duration with an error of 2.22%, and the maximum relative error of other time was 4.75%. The feasibility and the accuracy of this accelerate wear model was proved preliminarily by the test results of this study. Further investigation revealed that the wear mechanism was invariable during the tests in this paper.

Theoretical life prediction of tribo-pairs such as seals, bearings and gears with the failure form of wear under mixed lubrication depends on quantitative analysis of wear. Correspondingly, the wear life test depends on an accelerated wear test method to save the time and financial costs. Therefore, the theoretical basis of accelerated test design is a wear model providing a quantitative relationship between equivalents and accelerated test duration. In this paper, an accelerated wear test design method based on dissipation wear model entropy analysis under mixed lubrication is proposed. Firstly, the dissipation wear model under mixed lubrication is verified by standard experiments as a theoretical basis. Then, an accelerated wear test design method is proposed, taking the entropy increase in the dissipation wear model as an equivalent. The verification test shows that 20 times acceleration could be reached by adjustment of the entropy increase rate. The effect of entropy increase rate gradient of duty parameters is also discussed, revealing the fastest acceleration direction. Finally, the advantages and disadvantages of the proposed method are discussed. The results in this paper are expected to contribute to long life predictions of tribo-pairs.

Scale-free region is a specific frequency region only in which the fractal characteristics for surface topography exists. The uniqueness in fractal characterization exists in theory; however, by incorporating the conventional methods, various scale-free regions can be obtained for the same surface profile, resulting in the non-uniqueness of fractal characterization and reconstruction. Therefore, this paper aims to solve the non-uniqueness problem of the scale-free region. Firstly, the origins of such non-uniqueness are revealed, including random components introduced by manual selection under single-scale and information distortion affected by measuring frequency under multi-scales. Then, a filtering method based on an equal PSD amplitude Weierstrass-Mandelbrot function is proposed to extract the length and particular fractal components of the scale-free region. The proposed method is verified by acquiring the unique scale-free region length l 0 and calculating the unique fractal dimension D with the extracted fractal components. Additionally, measurement influences are discussed, including measuring frequency and length. Further application on surfaces with different grinding processes and roughness levels is also conducted to test the practicability of the method. In summary, the necessary conditions for measuring the scale-free region are revealed. On this basis, obtaining method for the scale-free region is proposed.

Internal spiral groove dry gas seal (ISGDGS) are used to seal special positions of rotating machinery. There is an urgent need to address the problems caused by contacting of rotating and static rings during high-speed rotation of ISGDGS. The main aim of this study is to investigate the influence of spiral groove structural parameters on ISGDGS sealing performance. Fluid-solid-thermal coupling method was used to analysis the relationship of sealing performance (opening force, leakage, friction power, and gas film stiffness) and spiral-groove structural parameters. The test analysis method is used to study the sealing performance under four different structural parameters, and the obtained trends are the same as the analysis results. According to the analysis results, the value range of structural parameters for optimal sealing performance is proposed This thesis has provided a deeper insight into the influence of the structural parameters of end face on the sealing performance, and provides a basis for its optimal design.

考虑热与变形对油气两相动压密封自振稳定性的影响,建立基于油气两相动压密封自振稳定性数学模型,采用流固热耦合有限元方法,研究油气比、转速、压差和O形圈阻尼等参数对油气两相动压密封受干扰后的轴向、角向自振稳定性能的影响.结果表明:转速较低时轴向自振稳定性较好而角向自振稳定性较差,转速高时两者相反,O形圈阻尼较低时轴向自振稳定性较差而角向自振稳定性较好,O形圈阻尼高时两者相反,因此在极端转速和取极端O形圈阻尼的情况下轴向或角向临界频率较小,不利于油气两相动压密封自振稳定;压差越大轴向临界频率越大,轴向自振稳定性越好,但角向临界频率越小,角向自振稳定性越差;随着两相介质油气比的增大,轴向临界频率减小而轴向临界质量增大,油气比在0.1～0.15时临界频率、质量以及转动惯量较大,密封综合自振稳定性能较好.

在高参数工况下,超临界二氧化碳(以下简称SCO2)动压密封的端面容易发生热弹变形,从而影响动压密封性能,针对该问题,建立了SCO2动压密封热流固耦合数值分析模型.在考虑了粘性耗散的基础上,求解了密封环温度场,采用CO2真实物性数据求解了流体膜压力场,将温度场和流体膜压耦合到密封环上,求解了密封端面的热弹变形;对比研究了热变形和弹变形对热弹总变形的影响,分析了转速、压力和温度对密封端面热弹变形的影响规律,提出了减少热弹变形的方法.研究结果表明:SCO2在动压密封高温下宜减小密封环的温差,以减小端面变形,高压下宜采用高弹性模量材料以减小端面变形,高转速下依靠动环热变形与弹性变形互相抑制关系以减小端面热弹总变形;密封环的端面最大轴向间隙与介质温度呈线性关系增大,与压力呈线性关系减小;转速则使其先减小,后增大.

针对刷丝与转子摩擦生热、刷束的局部过热、热量传入轴承腔体等影响机器性能的问题,对柔性丝刷式密封的生热、散热、隔热性能进行了研究.建立了大过盈变阻力柔性丝刷式密封和传统金属丝刷式密封CFD多孔介质数值模型,对比了柔性丝和金属丝刷式密封摩擦生热性能、沿挡板的传热性能、阻碍热量进入轴承腔体的隔热性能,并探究了工况参数和结构参数对刷封隔热性能的影响规律;开展了柔性丝刷式密封摩擦力测量试验和摩擦生热试验.研究结果表明:碳纤维刷式密封摩擦生热较低,沿挡板散热效率较高,对轴承腔的隔热性能较好;增大压差和温度,出口热流量升高,不同转速区间对热流量影响不同,通过增大刷束厚度、降低前、后板间隙会提高隔热性能;试验和数值结果吻合良好,误差为9.4％.

波形弹簧在航空航天机械密封等精密且紧凑的结构中发挥着重要作用.除刚度和弹簧力外,恒刚度极限、给定压缩量下的最大等效应力和最大径向变形等承载特性也影响着波形弹簧性能.采用接触非线性求解方法,分析外径、波厚、波宽和波高等结构参数对承载特性的影响.为了设计方便,提出并推导最大径向变形量的计算公式,利用试验和数值分析验证公式准确性.采用正交分析方法,得到承载特性的敏感因子.研究结果表明,恒刚度极限随着外径、波厚的增加而增加,随着波宽、波高的增加而减小;最大等效应力大小随着波宽、波厚的增加而增加,随着外径、波高的增加而减小;最大径向变形量随着外径的增加而减小,随着波高的增加而增加.

为了揭示油气混相回流泵送密封(OG-RPS)结构开启过程的动态泄漏特性和开启特性,分别进行4种典型密封结构的开启过程运转试验.测量记录密封开启过程的动态泄漏量,进行泄漏特性和开启特性对比分析.结果 表明:OG-RPS的泄漏率变化分为3个明显的阶段,分别对应密封结构开启过程的3种状态,通过泄漏率变化可有效监测密封结构开启过程.随着线速度的增大,密封结构未开启时泄漏率逐渐增大,泄漏率为正值;密封结构不完全开启时泄漏率逐渐减小,泄漏率为正值;密封完全开启时泄漏率逐渐减小,泄漏率为负值.槽数为12个、槽深为5μm和槽坝比为0.7的OG-RPS结构密封性能和开启性能最佳.研究成果为OG-RPS的优化设计、理论分析验证及实际应用提供了基础,并建立了1种基于泄漏率监控密封结构开启过程的方法.

针对密封端面参数结构选取不当导致密封高速旋转时动静环烧毁的问题,对螺旋槽泵出型动压密封端面参数进行了优化.基于气相流体控制方程,采用了流固热耦合有限元分析方法,考虑动静环变形以及热变形对流体的影响,研究了螺旋角、槽数、槽堰比、槽坝比、槽深等端面参数对开启力、泄漏量、摩擦功耗以及气膜刚度等密封性能的影响,提出了螺旋槽泵出型动压密封设计优化方案;采用试验研究方法,验证了计算模型的准确性;对比了运转前后密封端面微观形貌图,发现试验密封端面几乎无摩擦磨损,验证了密封可行性.研究结果表明:螺旋角在15°～20°、槽数在12个～16个、槽堰比在0.5 ～0.6、槽坝比在0.65～0.75、槽深在7μm～10μm时,密封开启力、气膜刚度较大,泄漏率、摩擦功耗较小,密封综合性能最佳.