High-Speed Elastohydrodynamic Lubrication by a Dilute Oil-in-Water Emulsion

When a concentrated contact is lubricated at low speed by an oil-in-water emulsion, a film of pure oil typically separates the surfaces (stage 1). At higher speeds, starvation occurs (stage 2) and the film is thinner than would be expected if lubricated by neat oil. However, at the very highest speeds, film thickness increases again (stage 3), though little is known for certain about either the film composition or the mechanism of lubrication, despite some theoretical speculation.

In this article, we report the film thickness in a ball-on-flat contact, lubricated by an oil-in-water emulsion, at speeds of up to 20 m/s, measured using a new high-speed test rig. We also investigated the sliding traction and the phase composition of the film, using fluorescent and infrared microscopy techniques.

Results show that, as the speed is increased, starvation is followed by a progressive change in film composition, from pure oil to mostly water. At the highest speeds, a film builds up that has a phase composition similar to the bulk emulsion. This tends to support the “microemulsion” view rather than the “dynamic concentration” theory.     

A Parametric Study on Friction Instabilities in Mechanical Face Seals

The present article aims to analyze and evaluate how dynamic parameter design influences the behavior of mechanical face seals and improves their performance by detecting undesirable phenomena like stick–slip and ringing. Those phenomena occur in situations of boundary lubrication.

A new lumped parameter model is proposed to study mechanical face seals used in automotive cooling water pumps in order to examine how dynamic parameters such as mass, moment of inertia, and stiffness are involved in stick–slip phenomena. The aim of this work is to provide a practical design tool to predict stick–slip conditions and to choose the optimal mechanical specifications. Numerical simulations demonstrate how the critical service conditions vary with design parameters and permit tabulation of the results.     

Interactive Effect between Organic Friction Modifiers and Additives on Friction at Metal Pushing V-Belt CVT Components

The interactions between organic friction modifiers (FMs) and other additives in a continuously variable transmission (CVT) fluid (CVTF) are investigated with the goal of optimizing friction management of metal pushing V-belt CVTs. Three types of FMs (oleic acid, oleyl alcohol, and glycerol mono-oleate [GMO]) were formulated in poly-α-olefin (PAO) and a fully formulated CVTF, and the friction performance was evaluated in a reciprocating test apparatus (TE77). To estimate their effect on the major frictional components in the CVTs, a steel belt–pulley and a torque converter clutch, the tribotests were carried out with both steel–steel and paper–steel sliding configurations. Then, the posttest materials were assessed by surface analysis techniques to observe the chemical nature of any reacted layers at the surface.

The results indicate that the friction on the steel surface was significantly influenced by a combination of the FMs and the CVTF additives as well as the functional group of the FMs. Although oleic acid and GMO typically present greater friction reduction than oleyl alcohol under most conditions, oleic acid did not decrease friction at the steel–steel contact with the presence of the other additives in the CVTF formulation; the friction reduction effect was impaired by the presence of other additives. Surface analysis of the posttest specimens implied that it was interactions with the calcium detergent that inactivated the FM effect of oleic acid.     

Comparison of Power Losses and Temperatures between an All-Steel and a Direct Outer Ring–Cooled,Hybrid 133-mm-Bore Ball Bearing at Very High Speeds

Next-generation aircraft engines will have to face more stringent requirements for reliability, thrust to weight, efficiency, environment protection, and profitability. These requirements affect all engine modules and components, including rolling element bearings. To cope with the above-mentioned requirements, next-generation aircraft engine main shaft bearings will operate under higher loads, speeds, and temperatures and increased reliability. In addition, lighter weight components are desirable. Hence, new material and cooling technologies including weight- and stress-optimized designs need to be developed.

In this article, the experimental investigation results of a novel main shaft ball bearing featuring ceramic balls, direct outer ring cooling, squeeze film damping, as well as surface-nitrided raceways are presented. Bearing rig testing under typical aircraft engine flight conditions has been performed. Savings for oil flow quantity of more than 45% and for power loss of more than 15% were identified. Outer ring temperature reductions of more than 20 K were achieved due to the use of ceramic ball material and the direct outer ring cooling concept. The ultra-high-speed capability of the bearing was demonstrated. Rotational speeds of 24,000 rpm were achieved at bearing temperatures below 200°C. The fundamental experimental results including oil and bearing temperature distribution, power dissipation, and bearing efficiency are presented. In addition, experimental power loss and temperature results are compared with data for a conventional all-steel bearing.     

Analysis of Turbulence and Convective Inertia in a Water-Lubricated Tilting-Pad Journal Bearing Using Conventional and CFD Approaches

The influence of turbulence and convective fluid inertia in a water-lubricated journal bearing was investigated using two types of models: a “conventional” solution based on traditional lubrication theory (Reynolds equation) and a more rigorous computational fluid dynamics (CFD) program containing a full Navier-Stokes solution. The calculations reveal that turbulence accounts for around 50% of the load capacity in the water-lubricated bearing studied, highlighting the importance of accurate characterization of turbulence in such applications. Convective inertia, also referred to as transport inertia because it depends only on the spatial parameters within the film rather than time-dependent journal motions, was found to lower the static film pressures (load capacity) by about 6% compared to an inertialess solution.

Hydrodynamic pressures calculated by the conventional Reynolds solution were initially about 30% lower than those of the more rigorous CFD model for the water-lubricated bearing operating in the turbulent regime. The mesh spacing of the conventional model was refined and a method was developed to adjust the turbulence model within the Reynolds solution as a function of the pivot Reynolds number. These refinements brought the calculated bearing load capacities and power losses of the conventional Reynolds model into better agreement with those of the CFD model for a broad range operating conditions.     

High-Temperature Friction and Wear of Boron Steel and Tool Steel in Open and Closed Tribosystems

More and more components in automotive, material processing, and mining industries are operating under harsh conditions involving high temperatures and high contact pressures. Tribotesting for such applications is done using both open (one surface meeting a fresh countersurface) and closed (one surface follows the same track on the countersurface) test configurations. In order to enable development of new materials and processes intended for such conditions, there is a need for better understanding pertaining to tribological phenomena occurring under these different test configurations.

In this work, friction and wear characteristics of quenched and tempered tool steel sliding against boron steel (22MnB5) have been studied. The experiments were conducted using a specially designed hot strip tribometer (HST) under dry conditions at room temperature and 400°C in open as well as closed configurations. Scanning electron microscopy/energy-dispersive spectroscopy, and X-ray techniques were carried out to analyze the worn surfaces. Additionally, the results from the closed test configuration were compared to previous tests carried out with the same materials and parameters using a pin-on-disk (POD) test rig. The results have shown that wear was reduced at higher temperatures as well as with repeated sliding on the same contacting surfaces (i.e., closed configuration) compared to those with an open configuration. A good correlation of wear mechanisms and coefficient of friction between closed configuration tests and those carried out with the POD test rig were observed especially at 400°C.     

Influence of Currents from Electrostatic Charges on WEC Formation in Rolling Bearings

White etching crack (WEC) early bearing failures occur when the rolling contact is subjected to a so-called additional load such as an electrical current flowing through the bearing, in addition to the pure rolling load (p_Hz). Tests on rolling bearings showed that a low electrical direct current flow, such as that resulting from electrostatic charges, can lead to WEC failures in oil-lubricated roller bearings and greased ball bearings.

The WEC formation in the performed tests was dependent on the current, electrical polarity, load type (rotating or stationary ring load), and bearing load. A black oxidation of the WEC critical bearing ring led to a significant increase in lifetime. Based on the findings, the failure hypothesis “cathodic WEC fatigue” for electrical direct current-initiated WEC failures was established.     

Singularity and workspace analysis of three isoconstrained parallel manipulators with schoenflies motion

This paper presents the analysis of three parallel manipulators with Schoenflies-motion. Each parallel manipulator possesses two limbs in structure and the end-effector has three DOFs (degree of freedom) in the translational motion and one DOF in rotational motion about a given direction axis with respect to the world coordinate system. The three isoconstrained parallel manipulators have the structures denoted as CuuUwHw-//-CvvUwHw, CuRuuUhw-//-CvRvvUhw and CuPuUhw-//- CvPvUhw. The kinematic equations are first introduced for each manipulator. Then, Jacobian matrix, singularity, workspace, and performance index for each mechanism are subsequently derived and analysed for the first time. The results can be helpful for the engineers to evaluate such kind of parallel robots for possible application in industry where pick-and-place motion is required.     

圆柱滚子式三叉杆万向联轴器热弹流脂润滑特性分析*

为了分析圆柱滚子式三叉杆万向联轴器圆柱滚子与滑块槽之间的脂润滑状况,基于Ostwald模型建立线接触热弹流脂润滑数值计算模型。结合联轴器的稳态工况参数,采用多重网格法、逐列扫描法对润滑油膜压力、膜厚和平均温升求解;探究不同输入轴与输出轴轴线夹角、不同输入轴回转半径、不同圆柱滚子半径、不同初始黏度和不同流变指数对联轴器热弹流脂润滑特性的影响。研究结果表明:减小输入轴与输出轴轴线夹角、减小输入轴回转半径、减小润滑脂初始黏度有利于降低二次压力峰和平均温升,但膜厚减小;增大圆柱滚子半径有利于降低二次压力峰和平均温升,增大膜厚;增大流变指数,有利于增大膜厚,降低温升,但二次压力峰增大。因此,根据实际工况选择较小回转半径,适当减小轴线夹角和增大圆柱滚子半径,适当减小润滑脂初始黏度和增大流变指数,均有利于改善联轴器润滑特性。     

Effect of geometrical parameters in micro-grooved crosshatch pattern under lubricated sliding friction

Tribological test was carried out using a pin-on-disc geometry with textured SKD11 pin on bearing steel disc, under sliding in paraffin oil. Micro-grooved crosshatch pattern has been fabricated with various angles and widths. The effects of geometrical parameters on friction were mainly examined in mixed and elastohydrodynamic lubrication. The results show that friction control can be achieved by fabricating the micro-grooved crosshatch pattern on a contact surface. It is observed that each geometrical parameter of texture influence on friction, especially decrease of groove aspect ratio and increases of groove sliding length show friction reduction performance. Crucial parameter G_l was proposed for micro-grooved crosshatch texture. The friction mechanism is explained by micro fluid flow with limited theoretical approach.