超高压斜盘式轴向柱塞泵柱塞副摩擦界面油膜固液耦合作用特性研究

作为液压传动系统核心动力元件的轴向柱塞泵,超高压化是其必然发展趋势与要求,然而超高压化会造成其中关键的柱塞副摩擦界面油膜形成显著的固液耦合作用,对柱塞副油膜的摩擦润滑与密封承载性能产生规律尚不明确的影响。为此,建立一种基于变形矩阵法的固液耦合作用求解方法,该方法基于有限容积法解算油膜流体润滑方程,基于有限元法实现摩擦界面变形计算节点规则化设置及变形矩阵精准计算,在此基础上建立柱塞副油膜弹性流体动压润滑数值计算模型,针对采用软硬配对的柱塞副63 MPa超高压工况下的摩擦界面油膜固液耦合作用特性进行研究,结果表明：固液耦合作用有助于减小柱塞副处轴向黏性摩擦力和泄漏流量,一个周期内柱塞副总周向黏性摩擦力大小基本不变但分布更为集中,导致产生了更大峰值的瞬时摩擦力;显著的结构变形产生于柱塞副摩擦界面两端局部位置处,因而对泄漏流量不造成影响,在超高压工况下经过软硬配对跑合,固液耦合作用有助于原本标准柱形铜套孔形成类似“喇叭口”的一种微观形貌,增大了柱塞与铜套孔的接触面积,增强了密封超高压油的能力,降低了接触应力。建立的模型及研究结果可为轴向柱塞泵超高压化设计提供指导。     

Lubrication characteristics of dual piston ring in bent-axis type piston pumps

The bent-axis type of piston pump driven by the piston rod works by the piston rod driving the cylinder block; because of this the taper angle of the piston rod and the swivel angle between the cylinder block and the shaft are important design factors. If these factors cannot satisfy the conditions for optimum design, the friction loss between the cylinder bore and the piston increases, and the pump can fail to work under conditions of severe friction and wear. Since the piston reciprocates in the cylinder bore with high velocity, at the same time rotating on its own axis and revolving on the center of the cylinder block, a decrease of the volume efficiency is generated because of the leakage between the cylinder bore and the piston. Therefore, to prevent this, the piston ring is designed to be at the end of the piston, and the friction characteristics between the piston ring and the cylinder bore require further research due to their great influence on the performance of the piston pump. Thus, in this paper, the elastohydrodynamic lubrication (EHL) analysis of the film thickness, the pressure distribution, and the friction force, have been studied between the piston ring and the cylinder bore in the bent-axis type of piston pump. The analyzed results show that the friction force is influenced by the rotating speed and the discharge pressure.     

汽车减振器油封摩擦力变化规律研究*

采用数值模拟与实验验证相结合的方法开展对汽车减振器油封摩擦特性的研究,得到不同工况下的摩擦力变化规律;在往复油封摩擦力实验台上研究了摩擦力随润滑油温度、润滑油压力和活塞杆速度的变化规律。结果表明:随活塞杆速度的增加油封摩擦力呈先增加再减小后趋于稳定的变化规律;随润滑油温度的升高油封摩擦力逐渐减小;随润滑油压力的增加油封摩擦力逐渐增大。根据油封摩擦力变化规律,可以推断出活塞杆速度较低、润滑油温度较高、润滑油压力较低时,应使得减振器油封有较大的过盈量或抱紧力,以提高汽车减振器的性能和使用寿命。实验结果与数值仿真结果基本一致,验证了混合润滑数值模型的正确性。     

Frictional Characteristics of IF-WS2 Nanoparticles in Simulated Engine Conditions

The efficacy of inorganic fullerene-like (IF) tungsten disulfide (WS₂) nanoparticles as engine oil friction modifier additive was investigated using a high-stroke reciprocating piston ring–cylinder bore bench test system. Frictional characteristics of various IF-WS₂ nanoparticle concentrations in mineral oil were experimentally analyzed in simulated engine conditions controlling oil temperature, speed, and normal load. Though the effect of IF-WS₂ nanoparticles on piston ring and cylinder bore friction was minor in low concentrations, after a running period in a mixed lubrication regime with 10% additive, a considerable reduction in friction coefficient was observed when the IF-WS₂ formulated oil was used. This reduction remains to some extent with reference mineral oil after solvent cleaning of the mating components. The results show that a thin tribofilm gradually forms on the piston ring and the cylinder bore surfaces, reducing the friction coefficient in a mixed lubrication regime. However, in order to obtain the friction reduction there seems to be a threshold in the concentration of IF-WS₂ in mineral oil and film development period. The effects of lubrication regime, contact load, speed, and surface roughness on tribofilm formation are also discussed. The presence of WS₂ tribofilm formed on the piston ring and the cylinder bore surfaces was further verified using Raman spectroscopy.     

Investigation on the radial micro-motion about piston of axial piston pump

The limit working parameters and service life of axial piston pump are determined by the carrying ability and lubrication characteristic of its key friction pairs. Therefore, the design and optimization of the key friction pairs are always a key and difficult problem in the research on axial piston pump. In the traditional research on piston/cylinder pair, the assembly relationship of piston and cylinder bore is simplified into ideal cylindrical pair, which can not be used to analyze the influences of radial micro-motion of piston on the distribution characteristics of oil-film thickness and pressure in details. In this paper, based on the lubrication theory of the oil film, a numerical simulation model is built, taking the influences of roughness, elastic deformation of piston and pressure-viscosity effect into consideration. With the simulation model, the dynamic characteristics of the radial micro-motion and pressure distribution are analyzed, and the relationships between radial micro-motion and carrying ability, lubrication condition, and abrasion are discussed. Furthermore, a model pump for pressure distribution measurement of oil film between piston and cylinder bore is designed. The comparison of simulation and experimental results of pressure distribution shows that the simulation model has high accuracy. The experiment and simulation results demonstrate that the pressure distribution has peak values that are much higher than the boundary pressure in the piston chamber due to the radial micro-motion, and the abrasion of piston takes place mainly on the hand close to piston ball. In addition, improvement of manufacturing roundness and straightness of piston and cylinder bore is helpful to improve the carrying ability of piston/cylinder pair. The proposed research provides references for designing piston/cylinder pair, and helps to prolong the service life of axial piston pump.     

Friction characteristics of piston rings in a reciprocating engine

The performance of a reciprocating engine can be improved by reducing the friction between piston rings and cylinder liner, which significantly contributes to the mechanical friction losses of an engine. The friction force of a piston ring pack is calculated, based on hydrodynamic lubrication theory, for the piston rings. Calculations were carried out for three sets of conditions. Oil starvation is taken into consideration in the calculation of oil-film behaviour for a ring pack. The friction characteristics of piston rings are evaluated using the frictional mean effective pressure. The friction force of a piston assembly is measured experimentally by an improved floating liner method. The effects of lubricant viscosity and engine speed on friction characteristics are investigated by both calculation and experiment.     

活塞偏心倾斜对双筒液压减振器动态阻尼特性影响分析*

考虑活塞偏心和倾斜后活塞与缸筒之间的摩擦与润滑因素,研究充气式双筒液压减振器动态阻尼特性;建立活塞偏心和倾斜的双筒式液压减振器的动态阻尼特性数学模型和动压润滑方程,求解减振器动态阻尼特性数学方程;采用雷诺空化边界条件,对Reynolds方程采用五点差分法进行离散,利用超松弛迭代法(SOR)迭代求解,得到摩擦阻尼力;分析活塞偏心距比、活塞倾斜角度,活塞倾斜时活塞运动速度、活塞半径、活塞宽度等因素对摩擦阻尼的影响,以及各摩擦因素对减振器动态阻尼特性的影响。结果表明:活塞偏心和倾斜时,随活塞速度、偏心距比、活塞倾斜角度、活塞宽度等的增加,减振器活塞与缸筒之间的油膜摩擦力均增加,而随活塞半径的增加,油膜摩擦力减小。减振器活塞发生倾斜时,会造成阻尼力的大幅增加,严重影响车辆行驶的舒适性能,且活塞速度越快,对舒适性影响越严重。     

A Model for Wear and Friction in Cylinder Liners and Piston Rings

Abstract

A one-dimensional elstohydrodynamic mixed lubrication wear and friction model is developed. The model can predict the effects of surface roughness, asperity contact, temperature-pressure-viscosity on wear, lubrication, and friction of the piston rings and cylinder liner. Wear is predicted based on the surface asperity contact pressure. The cylinder bore wear and the ring pack friction during an engine break-in are simulated and compared with the experimental results. The influence of cylinder wall temperature and surface roughness on friction and wear is investigated. The ring pack friction due to oil viscous shearing and asperity contact is found to reach its minimum at a certain oil temperature.     

Frictional modes of barrel shaped piston rings under flooded lubrication

A friction force measurement system using the floating liner method was developed to study the frictional behavior of piston rings. The measurement system was designed to control the effect of the secondary piston motion and to control temperatures of the cylinder wall and oil. The friction force between the barrel shaped piston ring and the cylinder liner was measured under flooded oil supply conditions. The measured friction forces were classified into five frictional modes with regard to the combination of predominant lubrication regimes (boundary, mixed and hydrodynamic lubrication) and stroke regions (mid-stroke and dead centers). The modes were identified on a Stribeck diagram, where the friction coefficients were evaluated both at mid-stroke and at the dead centers.     

Temperature—friction characteristics of cylinder lubrication in large,slow, cross‐head marine diesel engines under boundary lubrication conditions

In large, slow, cross‐head marine diesel engines research has increasingly shown that the lubrication regime between piston rings and cylinder liner at top dead centre is of the boundary lubrication type due to the high gas pressure, low sliding speed, and high temperature. This means that the tribological properties of piston ring, cylinder liner, and cylinder lubricant in these types of engine under boundary lubrication conditions should be considered simultaneously when friction and wear between the piston ring and cylinder liner are studied. Until now there has been no standard method to evaluate boundary lubrication performance. There are a few traditional methods used in lubricant research, but their results are not correlated with service conditions. It is important to find a suitable method to evaluate the boundary lubrication performance of lubricants at the laboratory testing stage or before the engine testing stage. The important parameters, such as sliding speed, normal load, materials of the contacting pairs, and lubricant, need all to be controlled. In this paper a systematic experimental procedure, the ‘five times heating and cooling test’, is introduced to assess lubricant properties under boundary lubrication conditions. Most of the parameters mentioned above are controlled. The model contact, of pin‐on‐plate form, is made from the actual piston and liner materials used in a large‐bore, slow, cross‐head marine diesel engine. The temperature characteristics of different blends of lubricants are investigated under boundary lubrication conditions using a pin‐on‐plate reciprocating test rig. These blends of lubricants have the same additives but different base fluids; they nevertheless fulfil the physical and chemical requirements of a real marine diesel engine. The test temperature range is from room temperature to the working temperature of the top piston ring. The experiments show that there are different temperature—friction characteristics for lubricants with different bases and the same additive package and there are also different temperature—friction characteristics during heating up and cooling down for each blend. Single‐base lubricants have more promising temperature—friction characteristics than those of a blend of a high‐viscosity base and a low‐viscosity base at high temperature.     

二甲醚发动机活塞二阶运动-润滑耦合模型建立及数值分析

通过改造潍柴WP6发动机的供油、燃烧系统,开发出一台二甲醚发动机。试验采集二甲醚发动机燃烧参数,针对活塞运动和润滑分析,建立活塞裙部系统以及由曲轴、连杆、活塞组成的多体系统的耦合润滑模型。基于拉格朗日乘子法建立多体动力学方程,并采用有限单元法对润滑模型进行求解。将耦合计算结果与实验对比,验证模型准确性。根据现有二甲醚发动机活塞裙部的结构参数,研究不同活塞结构参数对敲缸及润滑的影响。结果表明,活塞间隙越小时,活塞二阶运动的空间越小,导致敲缸现象减弱,磨损降低,但较小的间隙会产生比较高的油膜剪切力,微凸体表面接触也会加剧,引起较大的摩擦力及摩擦功耗;活塞裙桶形面最高点的位置也会影响活塞裙-缸套系统的运动和润滑特性;桶形面与活塞销中心的距离越小越容易满足力与力矩平衡条件,有利于降低敲缸以及摩擦损耗。     

Effect of Bio-Lubricant and Biodiesel-Contaminated Lubricant on Tribological Behavior of Cylinder Liner–Piston Ring Combination

This article addresses the issue of friction and wear characteristics of diesel engine cylinder liner–piston ring combinations under different lubricating conditions using a pin-on-disc wear tribometer. The discs were made out of actual engine cylinder liner material using a casting process. Pins were made out of top compression ring material. The tests were conducted on a pin-on-disc tribometer for wear and friction characteristics of the cylinder liner and piston ring combination with diesel-contaminated rapeseed oil–based bio-lubricant, diesel-contaminated commercial synthetic lubrication oil (SAE 20W40), biodiesel-contaminated commercial synthetic lubrication oil (SAE 20W40), and used (150 h) commercial synthetic lubrication oil (SAE 20W40). Experimental results demonstrated that the rapeseed oil–based bio-lubricant and biodiesel-contaminated synthetic lubricant exhibited better performance in terms of wear, friction, and frictional force under similar operating conditions. Thus, usage of newly formulated bio-lubricant and biodiesel in the long run may have a positive impact on engine life.     

Tribological study on hydrostatic slipper bearing with annular orifice damper for water hydraulic axial piston motor

Hydrostatic slipper bearing is an effective way to maintain a fluid film between slipper pad and swash plate that slide against each other, and thereby mitigate direct surface-to-surface contact in water hydraulic axial piston motor (WHAPM). The hydrostatic slipper bearing with an annular orifice damper is proposed, and the reaction force of the bearing in WHAPM is investigated. The effects from the friction within the cylinder bore, the dynamics of the piston, and the centrifugal force of the piston–slipper assembly are examined. The characteristic equation of the hydrostatic slipper bearing with an annular orifice damper is formulated, where the effects of various geometric parameters (e.g. damping length, supporting length, and clearance between the piston and the cylinder bore) are reflected. The relevant criterion for designing the hydrostatic slipper bearing can then be established. Results of the theoretical analyses indicate that (a) the friction coefficient, the swash plate angle, and the inertia and centrifugal loads (generated under a high motor rotating speed) would have significant influences on the reaction force; (b) an appropriate swash plate angle can help eliminate the fluctuation of the reaction force; (c) the load-carrying capacity of the hydrostatic slipper bearing is more sensitive to the damping length than to the supporting length of the piston; (d) a short damping length can help enhance the load-carrying capacity; (e) a small clearance between the piston and the cylinder bore would help improve the adaptive ability to the varying load for the hydrostatic slipper bearing, when clearance between the slipper pad and the swash plate ranges from 5 to 20 μm. Experimental studies of the slipper pads sliding against the swash plates are conducted at a custom-manufactured test apparatus, given different material combinations and design methods. The experimental results indicate that the hydrostatic slipper bearing with an annular orifice damper would decrease the possibility of the severe wear between the slipper pad and the swash plate in comparison with the hydrostatic clamping ratio bearing in the WHAPM, and the CRA laser cladding (compared to the ZrO₂·MgO-plasma-sprayed coating and the stainless steel 2Cr13) is a promising candidate as the tribo-material when sliding against composite materials in water lubrication system. The hydrostatic slipper bearing with an annular orifice damper has been successfully applied to a WHAPM developed at the Huazhong University Science and Technology. The result demonstrates that the developed bearing has a satisfactory tribolgical performance, and can be extended to the manufacture of water hydraulic axial piston pumps.     

矩形密封圈的增效运行参数研究

建立矩形密封圈的混合润滑模型,分析工作压力、活塞杆运行速度和密封件粗糙度对轴向往复用矩形密封圈的摩擦力矩、泄漏量的影响.结果表明:过大的密封压力会对密封件造成损坏,使得摩擦力和净泄漏量极速增大;往复速度的增加会使摩擦力线性增大,直线往复密封的净泄漏量随着表面粗糙度的增大表现为越来越大的增量.利用田口实验设计方法对矩形密...     

考虑磨损进程的轴向柱塞马达柱塞副泄漏研究

轴向柱塞马达广泛应用于航空航天、工程机械液压作动系统,容积效率是其重要指标。然而其柱塞副承载润滑状态恶劣,发生的磨损会导致泄漏损失增大,进而使马达的容积效率低于理想设计值。建立了轴向柱塞马达柱塞副磨损退化进程模拟模型,获得了其从跑合磨损到稳态磨损过程中承载界面轮廓的变化,基于此,对自然磨损状态下柱塞副泄漏行为进行分析。结果表明,随着缸孔的磨损,柱塞副泄漏量呈先增大后减小的趋势,柱塞副运行400 min时达到稳态磨损阶段,此时泄漏量相比初始泄漏量增加了2.3%。指出了柱塞副磨损状态和泄漏损失变化的映射关系,对提高轴向柱塞马达的容积效率和实现轴向柱塞马达磨损预测性维护具有一定指导意义。     

Tribological property investigation on a novel pneumatic actuator with integrated piezo actuators

Pneumatic piston–cylinder actuators are commonly used in industry for a variety of automation and robotics applications. In order to suppress leakage, these actuators comprise seal rings which unfortunately introduce friction and affect the positioning accuracy and output force. This article investigates vibrations of the seal generated by integrated piezo actuators to reduce friction force. For this, two piezoelectric stacks are integrated in the cylinder and used to excite vibration modes. This concept was studied in a compact cylinder pneumatic actuator with a bore diameter of 5 mm and a stroke of 10 mm. Dry friction measurement shows a 52% reduction from the original friction force at a driving frequency of 18.29 kHz and vibration amplitude of 0.05 μm. In the wet friction experiments, the friction force can be reduced by 54% from the original wet friction with vibrations at amplitude of 0.04 μm.     

气动平衡器O形密封圈与气缸润滑特性分析

为了降低O形密封圈与气缸内壁之间的摩擦力,气动平衡器装配过程中在气缸内壁涂上一层润滑脂。基于脂润滑相关理论,构建O 形密封圈和气缸的有脂润滑运动方程,通过MATLAB中PDE模块求解脂润滑模型,得出膜厚、接触应力在密封圈与气缸接触方向上的变化规律。基于ANSYS建立密封圈的瞬态结构有限元分析模型,以密封圈的运动速度和工作压力为恒定条件进行数值模拟,得出不同压缩率、摩擦系数以及行程方式条件下最大接触应力的时域特性。     

航空作动器主密封结构参数化分析*

航空作动器主密封属于往复密封,在飞机调整姿态时起重要作用。以航空作动器常用的斯特封为例,以摩擦力和反向泵回率作为密封评价指标,通过正向(EHL)和逆向(IHL)求解雷诺方程的方法建立往复密封混合润滑模型,推导往复密封系统的摩擦力和反向泵回率的计算方法。根据理论分析,分别对密封唇口的表面形貌参数、唇口与活塞杆倾角进行结构参数化分析。结果表明:选定的表面均方根粗糙度、唇口与活塞杆倾角均满足反向泵回率大于0,且摩擦力与表面均方根粗糙度、油测倾角成正比,与空气侧倾角成反比。通过分析得到了该型斯特封唇口的最优结构参数,为往复密封圈的结构优化提供了参考依据。     

转速对EHA泵柱塞副柱塞位姿及泄漏量影响仿真分析

斜轴式轴向柱塞泵作为电动静液压作动器（Electro-hydrostatic actuator,EHA）液压系统的重要元件,高转速、高工作压力是实现其小型化、轻量化的重要手段。柱塞副是斜轴式轴向柱塞泵中重要的两对摩擦副之一,对斜轴式轴向柱塞泵的工作状况起着直接影响。随着缸体转动,柱塞在缸体柱塞孔中的位姿朝着各个方向不断变化,柱塞副泄漏量也随之变化。为准确得到高速高压下柱塞副的泄漏量并给出解释,充分考虑柱塞受力情况,通过对离散化的柱塞副油膜雷诺方程和力平衡方程进行迭代求解,使用柱塞端面偏移量来确切描述了柱塞在缸体柱塞孔中的位姿随转速的变化情况,据此给出了柱塞副泄漏量模型,分析了转速变化对柱塞位姿和柱塞副泄漏量的变化的影响。结论为EHA泵的设计提供了适当的理论指导。     

柴油机缸套表面微沟槽织构润滑性能仿真分析

针对缸套表面织构微沟槽形貌,建立了缸套-活塞环摩擦副混合润滑理论模型,并采用MATLAB编程计算来分析微沟槽形貌参数对其润滑摩擦性能的影响规律。结果表明：缸套表面微沟槽可以形成很好的油膜压力,有效地改善缸套-活塞环间的润滑状态;随着微沟槽角度的增大,最小膜厚比逐渐增大,其润滑效果也越来越好,综合考虑摩擦润滑性能和机油耗性能情况下,最佳的微沟槽角度为60°。在上止点附近,面积占有率变化Sp对量纲一摩擦力影响较大;在其他区域,面积占有率对摩擦力影响不大;综合考虑油膜厚度与摩擦力,当Sp=0.15时效果最好。随着微沟槽深宽比e的增大,量纲一摩擦力不断增大,当e从0.025增大到0.150时,平均量纲一摩擦力增大了2.3倍,但深宽比过大,润滑效果将会减弱。研究结果认为,最佳深宽比的范围为0.05~0.08。
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