Behavior of Traction Oils under Impact Loads

The squeeze film forming ability of traction fluids is studied under impact load by a falling bearing steel ball against a stationary cylindrical surface or flat anvil made of mild steel, bearing steel, or aluminum. The effect of the pressure-viscosity coefficient and of the viscosity is investigated for plastic and elastic impact.

For soft materials, oils with high pressure-viscosity coefficients maintain much greater surface separation under impact load. The time fraction of separation by the oil film increases linearly with the product of the pressure-viscosity coefficient and the yield pressure of the anvil material. It decreases with an increase in surface roughness. Similar results are observed under impact on stationary or rotating surfaces. These results can be used to estimate the solidification behavior of traction oils. In the case of the elastic impact for hard materials, the coefficient of restitution of the ball is influenced mainly by the squeeze film effect, which is governed by viscosity itself.     

圆弧齿轮热弹性流体动力润滑数值解

根据圆弧齿轮啮合点具有二维表面速度的点接触弹流模型,在变网格下联立求解能量方程、雷诺方程和弹性位移方程等,获得了齿面接触区内的温度、压力和润滑膜厚度分布。计算考虑了润滑剂粘度和密度随温度压力变化和挤压膜效应等因素。用恩瑞模型进行了齿面摩擦分析,讨论了工况变化对齿轮润滑性能的影响。结果表明：在中等载荷或转速时,圆弧齿轮的润滑膜厚度是渐开线齿轮的3倍以上,而齿面摩擦因数比渐开线齿轮小近一个量级;圆弧齿轮沿齿宽方向的滚动速度对润滑膜性态起主导作用,沿齿高方向的滑动速度对润滑膜温升及膜厚影响不大;齿面挤压膜效应对压力分布有一定影响。     

Combined effects of piezo-viscous dependency and non-Newtonian couple stresses in wide parallel-plate squeeze-film characteristics

According to the Stokes micro-continuum theory of couple stress fluids together with the consideration of viscosity variation with pressure, the combined effects of piezo-viscous dependency, and non-Newtonian couple stresses in wide parallel rectangular-plate squeeze-film characteristics have been presented. An approximate analytical solution is obtained using a small perturbation method. Comparing with the iso-viscous and Newtonian case, the squeeze film performance is apparently improved. In addition, the improved characteristics are further emphasized for the squeeze film plates operating with a larger viscosity–pressure parameter, a non-Newtonian couple stress parameter and a lower film thickness.     

Effect of Piezoviscous Dependency and Couple Stress on Squeeze Film Lubrication Between Parallel Stepped Plates

Theoretical analysis of the effect of piezoviscous dependency and couple stress on squeeze film lubrication between parallel stepped plates is presented in this article. According to the Stokes microcontinuum theory of couple stress fluids, the modified Reynolds equation is derived by considering viscosity variation along the film thickness. The standard perturbation technique is used to solve the nonlinear Reynolds equation and an approximate analytical solution for the film pressure is obtained. It is found that the effect of couple stresses and pressure-dependent viscosity variation increases the load-carrying capacity and lengthens the squeeze film time.     

Squeeze film characteristics of conical bearings with combined effects of piezo-viscous dependency and non-Newtonian couple stresses

In this paper, the analysis of squeeze film characteristics of conical bearings with combined effects of piezo-viscous dependency and couple stress fluid is presented. On the basis of the Stokes microcontinuum theory of couple stress fluid model and Barus experimental research, a modified Reynolds equation is derived, the standard perturbation technique is used to solve the highly non-linear Reynolds equation and approximate analytical solution is obtained for the squeeze film pressure, load carrying capacity and squeeze film time. According to the results obtained, the effect of viscosity pressure dependency on the squeeze film lubrication of conical bearings with couple stress fluids is to improve the load carrying capacity significantly and lengthen the squeeze film time as compared to iso-viscous Newtonian case.     

Synthesis and Characterization of Alkyl-Bridged Bicycloheptanes as Traction Fluids

The traction fluid is a critical component of a toroidal-continuously variable transmission (T-CVT). As the medium that transmits power through the toroids, the traction fluid needs to provide a high traction coefficient and retain low dynamic viscosity at cold temperatures; this is a challenging combination of properties. A comparison of a variety of fluids shows a broad correlation between the traction coefficient and the fluid's low temperature viscosity, or pour point. This work investigated a series of novel compounds as traction fluids through chemical synthesis and the measurement of their relevant physical properties. Specifically, four new alkyl-bridged bicycloheptane fluids have been synthesized and refined to high purity. Their traction coefficients, measured with a ball-on-disc traction apparatus, are comparable to those of commercial fluids over the relevant range of temperature and pressure. Their dynamic viscosities at low temperature, however, are higher than the viscosity of commercial fluids and exceed the value of 3 × 10 ⁴ cP at ?40°C. These bridged bicycloheptanes also exhibit a correlation between their low-temperature viscosity and traction coefficient. The reasons for this correlation are discussed, and the effect of the molecular structure on viscosity and traction coefficient is investigated. This analysis finds semi-quantitative relationships between fluid properties and the molecule's volume, stiffness, and ring structure.     

Prediction of pressure–viscosity coefficient of lubricating oils based on sound velocity

The pressure–viscosity coefficient is an important parameter in tribology. Experimentally, it is calculated using the high‐pressure viscosity measurement. Also, the adiabatic bulk modulus is calculated using the sound velocity in the lubricating oil. Several lubricating oils are considered on the group basis such as traction oil, mineral oil, polyalphaolefin oil, perfluoropolyether oil and glycerol, depending on their molecular structure. Experimental pressure–viscosity coefficient is compared with the adiabatic bulk modulus. It is found that the pressure–viscosity coefficient increases exponentially with the adiabatic bulk modulus, and the relationship depends on the molecular structure of the lubricating oils. This study proposes two equations to predict the pressure–viscosity coefficient from the adiabatic bulk modulus based on sound velocity, one for the traction oil, and another for the paraffinic mineral oil and the polyalphaolefin oil. Copyright © 2009 John Wiley & Sons, Ltd.     

Numerical analysis of TEHL line contact problem under reciprocating motion

This paper presents a full numerical analysis to simulate the thermal elastohydrodynamic lubrication (TEHL) of steel–steel line contact problem under reciprocating motion. The equation system is solved using multigrid techniques. General tribological behaviors of TEHL under reciprocating motion are explained. Comparison between thermal and isothermal results reveals the importance of thermal effect in prediction of the traction coefficient and film thickness. The influences of frequency, stroke length, and applied load on the variations of film thickness, pressure and traction coefficient during one working cycle are discussed. Furthermore, the influence of slide–roll ratio on tribo-characteristics of oil film under same entraining velocity is revealed.     

Molecular dynamics simulations of elastohydrodynamic lubrication oil film

This paper reviews recent research in molecular dynamics studies of the traction properties of hydrocarbon fluids under elastohydrodynamic lubrication, focusing on the technical problems that arise on making predictions of the traction properties of an oil film with a submicron thickness at the actual sliding contacts of the machine elements by at a nanoscale molecular simulation. The effect of the oil film thickness and shear rate are examined including the result of a submicron thickness simulation of the oil film using a tera‐flops computer. The mechanism of the phase transition of the fluids under high pressure, the boundary slip, and the momentum transfer related to the molecular structure of the fluids are also presented. Copyright © 2010 John Wiley & Sons, Ltd.     

Effects of viscosity‐pressure dependency on the non‐Newtonian squeeze film of parallel circular plates

On the basis of the Barus experimental research, the viscosity of an incompressible lubricant grows exponentially with its pressure. Therefore, the squeeze film issues between parallel circular plates lubricated with a non‐Newtonian couple‐stress fluid are reexamined in the present study by considering the effects of viscosity‐pressure dependency. According to the results, the influences of viscosity‐pressure dependency raise the load capacity and lengthen the approaching time of the plates. Numerical results under different values of the viscosity parameter and the non‐Newtonian parameter are also provided in tables for engineering applications. Copyright © 2012 John Wiley & Sons, Ltd.     

The Elastohydrodynamic Traction of Synthetic Base Oil Blends

The aim of this study was to investigate the elastohydrodynamic (EHD) properties of lubricant blends. Three base fluids of very similar viscosities, a polyalphaolefin, a diester and an alky lated aromatic, have been obtained and their EHD film thickness and traction behavior measured at a range of pressures. Blends of these fluids have been prepared and the influence of blending on film thickness and traction has been investigated. Traction measurements were conducted at film thicknesses between 100–200 nm and thermal analysis was incorporated to correct for in-contact shear heating. The blends showed a broadly linear relationship between the inlet pressure-viscosity coefficient and blend composition. Isothermal traction comparisons revealed that traction is not an additive property of lubricant blends.     

Squeeze film characteristics of finite journal bearings: couple stress fluid model

A theoretical study of squeeze film behaviour for a finite journal bearing lubricated with couple stress fluids is presented. On the basis of the microcontinuum theory, the modified Reynolds equation is obtained by using the Stokes equations of motion to account for the couple stress effects due to the lubricant blended with various additives. With the Conjugate Gradient Method of iteration the built-up pressure is calculated, and then applied to predict the squeeze film characteristics of the system. According to the results evaluated, the rheological influence of couple stress fluids is physically apparent. Compared with the case of a Newtonian lubricant, the couple stress effects increase the load-carrying capacity significantly and lengthen the response time of the squeeze film behaviour. On the whole, the presence of couple stresses improves the characteristics of finite journal bearings operating under pure squeeze film motion. The rheological effects of couple stress fluids agree with previous works.     

Experimental investigation of transient and thermal effects on lubricated non‐conformal contacts

In this work, thermal and transient effects on non‐conformal lubricated contacts are investigated through experimental analyses. Experiments between a ball and a plane surface of a disc are described. Friction coefficients and film thicknesses are measured (the film thickness only for the glass‐on‐steel contact). A paraffin base mineral oil is used as a lubricant. First experiments are carried out under steady‐state conditions. To include effects due to different thermal properties of contacting materials, a steel‐on‐steel and a glass‐on‐steel contact with different slide‐to‐roll ratios are tested. If the contacting materials have different thermal properties, as in the case of a glass‐on‐steel contact, thermal effects like the temperature–viscosity wedge action could clearly be shown. It is found that the friction coefficients are influenced by the slide‐to‐roll ratio and the thermal properties of the contacting materials. Under transient conditions, the entraining velocity is varied with a sinusoidal law. Squeeze effects explain ‘loops’ of friction and film thickness found also in previous works. The formation of friction loops is related to the measured film thickness differences. However, also under non‐steady‐state conditions, thermal effects, like the temperature–viscosity wedge action, influence the friction coefficients. Copyright © 2007 John Wiley & Sons, Ltd.     

Combined effects of piezo-viscous dependency and non-Newtonian couple stresses in wide parallel-plate squeeze-film characteristics

According to the Stokes micro-continuum theory of couple stress fluids together with the consideration of viscosity variation with pressure, the combined effects of piezo-viscous dependency, and non-Newtonian couple stresses in wide parallel rectangular-plate squeeze-film characteristics have been presented. An approximate analytical solution is obtained using a small perturbation method. Comparing with the iso-viscous and Newtonian case, the squeeze film performance is apparently improved. In addition, the improved characteristics are further emphasized for the squeeze film plates operating with a larger viscosity-pressure parameter, a non-Newtonian couple stress parameter and a lower film thickness.     

A critical evaluation of film thickness‐derived pressure–viscosity coefficients

A single parameter, the pressure–viscosity coefficient, α, quantifies the pressure dependence of the viscosity of the liquid in elastohydrodynamic lubrication (EHL). Most published values of α have not been obtained from measurements of viscosity as a function of pressure. Rather, these effective pressure–viscosity coefficients have been derived from the measurement of the EHL film thickness, a more difficult procedure. In this article, five well‐characterized liquids that should be Newtonian in the EHL inlet are identified for which film‐derived coefficients have been reported. These coefficients are compared with coefficients derived from published viscosity correlations and new viscosity measurements. The film‐derived coefficients are found to not be an accurate representation of the piezoviscous response. The procedure of deriving a pressure–viscosity coefficient from a film thickness measurement does not offer an alternative to the simpler and easier viscometer measurement. Copyright © 2014 John Wiley & Sons, Ltd.     

The role of fragility in EHL entrapment

Experimental measurements of time dependent film thickness in entrapped liquids, measurements of viscosity under pressure, and simulations using realistic pressure–viscosity models contribute to improved understanding of the mechanisms of entrapment formation and persistence. The ambient viscosity and pressure–viscosity coefficient affect entrapment only as much as they are predictors of behavior at much higher pressure. Fragile liquids, such as lubricating oils, experience rapid increase in sensitivity of dynamic properties to temperature and pressure as the glass transition is approached. The fragility property of lubricants appears to be of overwhelming importance to entrapment which experimental evidence indicates will reduce starting friction.     

Influence of degradation on the film‐forming properties of polyolester oils

The film thicknesses of two polyolester degraded oils were measured over a wide range of temperatures to investigate the influence of degradation on film‐forming properties. The results have been interpreted in the light of the idea that degradation of an ester lubricant can produce two different types of product which have opposing influences on film‐forming properties. One is the formation of smaller, polar molecules, such as acids, which may result in a decrease in effective pressure—viscosity coefficient. The other is the formation of larger, bulky molecules such as sludge, which result in increased pressure—viscosity coefficient. The effective pressure—viscosity coefficient decrease with degradation of the oil thus is made up of an earlier stage involving a series of reactions, followed in a later stage by an increase.     

挤压条件下弹流油膜的摩擦学现象研究

纯挤压条件下弹流油膜的研究已证实了油膜的中央凹陷。研究利用常载荷下的钢球在较小的初始间隙下冲击附有润滑油的玻璃盘,在接触区外围出现了外围凹陷,而不是熟知的中央凹陷。研究结果表明当初始的冲击间隙较大时,油膜的压力分布和厚度以及中心压力-时间曲线中的峰值和接近结束时的中央凹陷都与以前的自由落球问题相似。随着初始的冲击间隙减小,最大压力从接触区中心转换到圆周外围区,相应地,中央凹陷变得越来越不明显而在外围区出现了圆周凹陷,进一步的数值分析,发现当油膜足够厚时在小的初始间隙条件下也出现了外围凹陷。这主要是当初始的冲击间隙变小时,中央油膜厚度比大冲击间隙条件下润滑油被"冰冻"时薄很多,中央凹陷不明显;在外围区域,表面间隙很小,局部挤压效应变强,因此形成了外围凹陷。     

Pressure viscosity coefficients and traction properties of synthetic lubricants for wind turbine gear systems

Synthetic lubricants are increasingly used to provide equipment reliability for wind turbine gear boxes. The majority of synthetic lubricants used today are based on polyalphaolefins. In gear systems where contact pressures are high, the pressure viscosity coefficient and traction values of the lubricant are important fundamental properties. A comparison of these properties for a wind turbine lubricant based on a polyalphaolefin and two lubricants based on polyalkylene glycols has been undertaken. Pressure viscosity coefficients were calculated from viscosity measurements made using an ultra‐high pressure falling needle viscometer at pressures up to 50 000 psi. Significant differences in properties were observed with both polyalkylene glycol lubricants showing lower pressure viscosity coefficients and much lower traction values. A calculation of the film thickness values in the Hertzian contact zone suggests that polyalkylene glycol lubricants may provide elastohydrodynamic films that are approximately 25% thicker than polyalphaolefin lubricants. Copyright © 2012 John Wiley & Sons, Ltd.