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Theory analyses and applications of magnetic fluids in sealing

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Friction 2023年第10期11卷 1771-1793页

作者： Decai LI Yanwen LI Zixian LI Yuming WANG State key laboratory of tribology Tsinghua UniversityBeijing100084China

Magnetic fluids are the suspensions composed of magnetic nanoparticles,surfactants,and non-magnetic carrier *** fluids are widely used in various fields,especially in sealing,because of their excellent features,including rapid magnetic response,flexible flow ability,tunable magneto-viscous effect,and reliable self-repairing ***,we provide an in-depth,comprehensive insight into the theoretical analyses and diverse applications of magnetic fluids in sealing from three categories:static sealing,rotary sealing,and reciprocating *** summarize the magnetic fluid sealing mechanisms and the development of magnetic fluid seals from 1960s to the present,particularly focusing on the recent progress of magnetic fluid *** magnetic fluid sealing technology has been commercialized and industrialized,many difficulties still exist in its *** the end of the review,the present challenges and future prospects in the progress of magnetic fluid seals are also outlined.

关键词： magnetic fluid sealing mechanism static sealing rotary sealing reciprocating sealing

Active control of friction realized by vibrational excitation:Numerical simulation based on the Prandtl-Tomlinson model and molecular dynamics

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Friction 2023年第7期11卷 1225-1238页

作者： Xiao MA Xinfeng TAN Dan GUO Shizhu WEN State key laboratory of tribology Tsinghua UniversityBeijing 100084China

Superlubricity and active friction control have been extensively researched in order to reduce the consumption of fossil energy,the failure of moving parts,and the waste of *** vibration-induced superlubricity(VIS)presents a promising solution for friction reduction since it does not require high-standard ***,the mechanism underlying the VIS remains unclear since the atomic-scale information in a buried interface is unavailable to experimental *** this paper,the mechanism of VIS was examined via numerical calculation based on the Prandtl-Tomlinson(PT)model and molecular dynamics(MD)*** results revealed that the pushing effect of stick-slip is one of the direct sources of friction reduction ability under vibrational excitation,which was affected by the response amplitude,frequency,and the trace of the ***,the proportion of this pushing effect could be modulated by changing the phase difference when applying coupled vibrational excitation in x-and *** results in a significant change in friction reduction ability with *** this way,active friction control from the stick-slip to superlubricity can be achieved conveniently.

关键词： superlubricity vibration friction control molecular dynamics(MD)

A measurement method for distinguishing the real contact area of rough surfaces of transparent solids using improved Otsu technique

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Chinese Physics B 2015年第1期24卷 354-360页

作者：宋保江阎绍泽向吴维凯 State key laboratory of tribology Department of Mechanical Engineering Tsinghua University

An experimental method of measuring the real contact area of transparent blocks based on the principle of total internal reflection is presented, intending to support the investigation of friction characteristics, heat conduction, and energy dissipation at the contact interface. A laser sheet illuminates the contact interface, and the transmitted laser sheet is projected onto a screen. Then the contact information is acquired from the screen by a camera. An improved Otsu method is proposed to process the data of experimental images. It can compute the threshold of the overall image and filter out all the pixels one by one. Through analyzing the experimental results, we describe the relationship between the real contact area and the positive pressure during a continuous loading process, at different loading rates, with the polymethyl methacrylate（PMMA）material. A hysteresis phenomenon in the relationship between the real contact area and the positive pressure is found and explained.

关键词： real contact area total internal reflection improved Otsu method hysteresis phenomenon

Superlubricity of a Mixed Aqueous Solution

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Chinese Physics Letters 2011年第5期28卷 191-194页

作者： MA Zhi-Zuo ZHANG Chen-Hui LUO Jian-Bin LU Xin-Chun WEN Shi-Zhu State key laboratory of tribology Tsinghua UniversityBeijing 100084

A super-low friction coefficient of 0.0028 is measured under a pressure of 300MPa when the friction pair(the silicon nitride ball sliding on the silicate glass)is lubricated by the mixed aqueous solution of glycerol and boric *** morphorlogies of the hydroxylated glass plate are observed by an atomic force microscope(AFM)in deionized water,glycerol,boric acid and their mixed aqueous *** peaks of the retained liquids adhered on the surface of the sliding track are detected by an infrared spectrum apparatus and a Raman spectrum *** mechanism of the superlubricity of the glycerol and boric acid mixed aqueous solution is *** is deduced that the formation of the lubricant film has enough strength to support higher loads,the hydration effect offering the super lower shear resistance.

关键词： superlubricity water based lubricant ultra-low friction

Relationship between the real contact area and contact force in pre-sliding regime

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Chinese Physics B 2017年第7期26卷 212-217页

作者：宋保江阎绍泽 State key laboratory of tribology Department of Mechanical Engineering Tsinghua University

The pre-sliding regime is typically neglected in the dynamic modelling of mechanical systems. However, the change in contact state caused by static friction may decrease positional accuracy and control precision. To investigate the relationship between contact status and contact force in pre-sliding friction, an optical experimental method is presented in this *** this method, the real contact state at the interface of a transparent material can be observed based on the total reflection principle of light by using an image processing technique. A novel setup, which includes a pair of rectangular trapezoidal blocks, is proposed to solve the challenging issue of accurately applying different tangential and normal forces to the contact interface. The improved Otsu＇s method is used for measurement. Through an experimental study performed on polymethyl methacrylate（PMMA）, the quantity of contact asperities is proven to be the dominant factor that affects the real contact area. The relationship between the real contact area and the contact force in the pre-sliding regime is studied, and the distribution of static friction at the contact interface is qualitatively discussed. New phenomena in which the real contact area expands along with increasing static friction are identified. The aforementioned relationship is approximately linear at the contact interface under a constant normal pressure, and the distribution of friction stress decreases from the leading edge to the trailing edge.

关键词： real contact area contact interface pre-sliding regime contact force

A method for detection and quantification of meshing characteristics of harmonic drive gears using computer vision

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Science China(Technological Sciences) 2016年第9期59卷 1305-1319页

作者： MA DongHui WU JiaNing YAN ShaoZe State key laboratory of tribology Department of Mechanical Engineering Tsinghua University

A lack of accurate description of the meshing characteristics and the corresponding frictional mechanism of the harmonic drive gear has limited progress toward modeling the hysteresis stiffness. This paper presents a method for detection and quantification of the meshing characteristics of the harmonic drive gear based on computer vision. First, an experimental set-up that integrates a high speed camera system with a lighting system is developed, and the image processing is adopted to extract and polish the tooth profiles of the meshed teeth pairs in each acquired video sequence. Next, a physical-mathematical model is established to determine the relative positions of the selected tooth pair in the process of the gear engagement, and the combined standard uncertainty is utilized to evaluate the accuracy of the calculated kinematics parameters. Last, the kinematics analysis of the gear engagement under the ultra-low speed condition is performed with our method and previous method, and the influence of the input rotational speed on the results is examined. The results validate the effectiveness of our method, and indicate that the conventional method is not available in the future friction analysis. It is also shown that the engaging-in phase is approximately a uniform motion process, the engaging-out phase is a variable motion process, and these characteristics remain unchanged with the variation of the input rotational speed. Our method affords the ability to understand the frictional mechanism on the meshed contact surfaces of the harmonic drive gear, and also allows for the dynamic monitoring of the meshing properties.

关键词： harmonic drive gear gear engagement computer vision image processing kinematics analysis uncertainty analysis

Rapid thin-layer WS_(2) detection based on monochromatic illumination photographs

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Nano Research 2021年第3期14卷 840-845页

作者： Xiangmin Hu Cuicui Qiu Dameng Liu State key laboratory of tribology Tsinghua UniversityBeijing100084China

The thickness of two-dimensional(2D)nanomaterials shows a significant effect on their optical and electrical ***,a rapid and automatic detection technology of 2D nanomaterials with desired layer-number is required to extend their practical application in optoelectronic *** this paper,an image recognition technology was proposed for rapid and reliable identification of thin-layer WS_(2) samples,which combining a layer-thickness identification criterion and a novel image segmentation *** criterion stemmed from optical contrast study of monochromatic illumination photographs,and the algorithm was based on Canny operator and edge connection *** optical identification method can seek out thin-layer WS_(2) samples on complex surfaces,which provides a promising approach for automatic search of thin-layer nanomaterials.

关键词： WS_(2) thickness identification image recognition sample detection

An experimental method for quantitative analysis of real contact area based on the total reflection optical principle

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Chinese Physics B 2019年第5期28卷 158-165页

作者： Zhijun Luo Baojiang Song Jingyu Han Shaoze Yan State key laboratory of tribology Department of Mechanical EngineeringTsinghua University

The simulation of real contact area between materials is foundationally important for the contact mechanics of mechanical structures. The Greenwood and Williamson(GW) model and the Majumdar(MB) model are the basic models in this field, which are widely accepted and proven to be valid in many experiments and engineering. Although the contact models have evolved considerably in recent years, the verifications of the models are most based on the indirect methods such as electrical conductivity and contact stiffness, because of the lack of effective methods to directly measure the variation of contact surface. In this paper, the total reflection(TR) method is introduced into the verification of contact *** experiment system based on TR method is constructed to measure the real contact area of two PMMA specimens. The comparison analysis between the results of experiment and models suggests that the experiment result has the same trend with simulation, the MB model has better agreement with the experimental result because this method can take into account the variation of radius and the merging of asperities, while the GW model has a huge deviation because of the dependence on resolution and the lack of considering the variation of radius and asperity's merging process. Taking the interaction of asperities into account could give a better result that is closer to the experiment. Our results and analysis prove that the experimental methods in this paper could be used as a more direct and valid method to quantitatively measure the real contact area and to verify the contact models.

关键词： real contact area Greenwood and Williamson (GW) model Majumdar (MB) model Persson’s model total reflection (TR) method

Experimental Research on Effects of Process Parameters on Servo Scanning 3D Micro Electrical Discharge Machining

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Chinese Journal of Mechanical Engineering 2012年第1期25卷 114-121页

作者： TONG Hao LI Yong HU Manhong State key laboratory of tribology Tsinghua UniversityBeijing 100084China

Servo scanning 3D micro electrical discharge machining (3D SSMEDM) is a novel and effective method in fabricating complex 3D micro structures with high aspect ratio on conducting materials. In 3D SSMEDM process, the axial wear of tool electrode can be compensated automatically by servo-keeping discharge gap, instead of the traditional methods that depend on experiential models or intermittent compensation. However, the effects of process parameters on 3D SSMEDM have not been reported up until now. In this study, the emphasis is laid on the effects of pulse duration, peak current, machining polarity, track style, track overlap, and scanning velocity on the 3D SSMEDM performances of machining efficiency, processing status, and surface accuracy. A series of experiments were carried out by machining a micro-rectangle cavity (900 μm×600 μm) on doped silicon. The experimental results were obtained as follows. Peak current plays a main role in machining efficiency and surface accuracy. Pulse duration affects obviously the stability of discharge state. The material removal rate of cathode processing is about 3/5 of that of anode processing. Compared with direction-parallel path, contour-parallel path is better in counteracting the lateral wear of tool electrode end. Scanning velocity should be selected moderately to avoid electric arc and short. Track overlap should be slightly less than the radius of tool electrode. In addition, a typical 3D micro structure of eye shape was machined based on the optimized process parameters. These results are beneficial to improve machining stability, accuracy, and efficiency in 3D SSMEDM.

关键词： micro electrical discharge machining(micro EDM) servo scanning machining 3D micro-structure process parameter