The Effect of Laser Texturing of Steel Surfaces and Speed-Load Parameters on the Transition of Lubrication Regime from Boundary to Hydrodynamic

Laser surface texturing (LST) is an emerging, effective method for improving the tribological performance of friction units lubricated with oil. In LST technology, a pulsating laser beam is used to create thousands of arranged microdimples on a surface by a material ablation process. These dimples generate hydrodynamic pressure between oil-lubricated parallel sliding surfaces. The impact of LST on lubricating-regime transitions was investigated in this study. Tribological experiments were carried out on pin-on-disk test apparatus at sliding speeds that ranged from 0.15 to 0.75 m/s and nominal contact pressures that ranged from 0.16 to 1.6 MPa. Two types of oil with different viscosities (54.8 cSt and 124.7 cSt at 40°C) were evaluated as lubricants. Electrical resistance between flat-pin and laser-textured disks was used to determine the operating lubrication regime. The test results showed that laser texturing expanded the range of speed-load parameters for hydrodynamic lubrication. LST also reduced the measured friction coefficients of contacts that operated under the hydrodynamic regime. The beneficial effects of laser surface texturing are more pronounced at higher speeds and loads and with higher viscosity oil.     

Applying Micro-Texture to Cast Iron Surfaces to Reduce the Friction Coefficient Under Lubricated Conditions

The tribological properties of cast iron have been investigated to determine the effects of micro-texturing the surfaces. The micro-textured surfaces were prepared by shot blasting or milling using a shaper. The surfaces with groove patterns and mesh patterns had higher friction coefficients than the flat surfaces. The surfaces with dimpled patterns had lower friction coefficients than the flat surfaces. The results indicated that the dimpled pattern had a beneficial effect by decreasing the friction.     

环形密封整体流动模型流场解的改进

提出了零阶摄动解的一个改进算法。用通过解析延拓加上周期性条件的三次样条方法取代复快速Ｆｏｕｒｉｅｒ变换方法求周向梯度,用递推关系式算法取代解线性代数方程组。改进算法提高了计算效率,保证了计算准确度。此外,在收敛到零阶出口压力边界条件时,构造了较易收敛的减缩修正方法。     

Load Capacity for Adiabatic Finite-Width Plane Slider Bearings in the Turbulent Thermohydrodynamic Regime

A Navier-Stokes based model developed using the Legendre collocation method is used to analyze turbulent plane slider bearings with wide ranges of convergence ratio, slenderness ratio, mean Reynolds number, and a parameter characterizing the viscosity variation. The load-capacity formulations are established for turbulent isothermal and turbulent thermohydrodynamic bearings. With the equations provided in this study, designers can quickly determine the load capacity without extensive computation.     

Accelerated Natural Convergence for Pivoted Slider Bearings

A solution technique is discussed which accelerates the convergence of the pivoted slider bearing problem. The technique is based on a fully populated, three-degrees-of-freedom stiffness matrix for the hydrodynamic lubricating film. This matrix is developed using a unique perturbation method which, when used in conjunction with a finite element formulation of the Reynolds equation, yields the bearing stiffnesses in a computationally efficient manner. The resulting natural convergence of this hydrodynamic stiffness approach is examined for a typical rigid disk magnetic recording slider.     

反旋向供油环形密封的流体动力特性

对双出口反旋向供油环形密封（有进动偏心）的流体动力特性提出了一种计算方法，建立了计算模型。导出了一阶流场的常微分方程组和边界条件的解析提法，实现了边界条件的转化。计算表明，反旋向喷流导致进动频率比大大降低，从而提高了转子／密封稳定性。     

Identification of Force Coefficients from a Gas Annular Seal - Effect of Transition Flow Regime to Turbulence

Experiments to identify the leakage and dynamic force coefficients of a plain annular gas (air) seal are presented. The test rig consists of a rigid shaft and a seal housing supported from elastic rods. Two orthogonal impact guns excite the test seal and housing, and displacement and acceleration sensors measure the seal dynamic forced response. A frequency domain parameter identification technique allows accurate estimations of the system stiffness and damping force coefficients. Tests with no journal rotation and at 3600 rpm are conducted for pressure ratios (P_s/P_a) from 1.5 to 3.0 for centered and eccentric (50 percent of clearance) seal conditions. In all cases, measured mass flow rates and identified direct damping coefficients steadily increase with increasing pressure ratios. The direct stiffnesses, however, exhibit a peculiar behavior at and around a pressure ratio of 2.0. As the pressure ratio increases from 1.5 to 2.0, the direct stiffnesses decrease and become negative. Then, as the pressure ratio rises to 3.0, the stiffnesses steadily increase and become positive. A bulk-flow model, which includes the effect of transition flow from laminar to turbulent, demonstrates reasonable agreement with the experimental observations.     

往复运动机构的摩擦过渡

往复运动机构在运动速度较高时能够保持流体润滑状态,但当速度降低到一定程度,运动副间的润滑形态则由流体润滑状态过渡到边界润滑状态,其摩擦系数陡增,振动和噪音加大。当在润滑剂中加入抗磨添加剂时,这种现象立即消逝。此实验提示：抗磨添加剂对于发动机润滑油是必不可缺的,对于解决往复运动机构的爬行是行之有效的。     

A Two-Wavelength Model of Surface Flattening in Cold Metal Rolling with Mixed Lubrication

A new model of surface flattening is developed for cold metal rolling in the mixed regime. Longitudinal surface roughness is modeled by two separate wavelengths. The new model follows the asperity crushing analysis of Sutcliffe (1999) for unlubricated rolling but additionally includes a hydrodynamic model to account for the effect of the lubricant. The effect of various parameters including speed, reduction in strip thickness, roughness wavelength and lubricant properties is examined. The results show similar behavior to previous models of mixed lubrication, with a speed parameter A_s having the most influence, and confirm the results for unlubricated rolling that the short wavelength components of the surface roughness persist more than the long wavelength components. The predicted changes in roughness are in good agreement with experiments.     

Experimental Study on the Sealing Mechanism of Bidirectional PTFE Lip Seals

Seals made of polytetrafluorethylene (PTFE) are resistant to heat and chemicals and can run dry, but they do not pump back fluid. PTFE lip seals with a spiral groove do pump back fluid but only in one rotational direction. PTFE lip seals for both rotational directions (bidirectional) are not available on the market. At the Institute of Machine Components (IMA) of the University of Stuttgart, PTFE lip seals with bidirectional sealing aids were developed and analyzed. During the research, the leak-tightness in static and dynamic conditions was continuously improved. The latest design is leak-tight in both rotational directions. The sealing mechanism for this design is explained. The resulting seals can be used in conditions where elastomeric lips seals will fail.     

Load Capacity for Adiabatic Infinitely Wide Plane Slider Bearings in the Turbulent Thermohydrodynamic Regime

A two-dimensional Navier-Stokes-based model developed using the Legendre collocation method is used to analyze turbulent plane slider bearings with wide ranges of bearing configuration, mean Reynolds number, and a parameter characterizing the viscosity variation. The load-capacity formulations are established for turbulent isothermal and turbulent thermohydrodynamic bearings. With the equations provided in this study, designers can quickly determine the load capacity without extensive computation.     

Friction-Reducing Surface-Texturing in Reciprocating Automotive Components

A model is presented to study the potential use of micro-surface structure in the form of micro pores to improve tribological properties of reciprocating automotive components. The Reynolds equation and the equation of motion are solved simultaneously for a simplified “piston/cylinder” system with surface texturing. The solution provides the time behavior of both the clearance and the friction force between the “piston ring” and “cylinder liner” surfaces. It is shown that surface texturing can efficiently be used to maintain hydrodynamic effects even with nominally parallel surfaces. It is also shown that optimum surface texturing may substantially reduce the friction losses in reciprocating automotive components.     

An Evaluation of Starting-Load Capacity and Static Friction in Babbitt-Lined,Hydrodynamic Journal Bearings

A recent application called for starting 127-mm (5-in) diameter, babbitt-lined hydrodynamic journal bearings under unit loads of about 10.34 MPa (1500 psi). In this application, the load was imposed only a few seconds before rotation could be started and hydrostatic jacking was not permitted. This paper presents the results of a test effort in which one full-size bearing was subjected to several hundred starts under unit loads ranging from 3.4 to 15.5 MPa (500 to 2250 psi). The dwell time between load application and start of rotation was varied in the test from less than one second to more than one hour. The paper also provides a review of the relevant literature, as well as calculations of the contact zone temperature during starts under boundary lubrication conditions.     

A Mixed Flow Model for Lubrication with Emulsions

An oil-in-water emulsion in the inlet zone of a concentrated contact is modeled by treating the oil particles as flattened cylinders surrounded by water. In an independent flow model, the oil and water flows are coupled only through the pressure gradient. However, the model leads to anomalous behavior with regard to the flow of water. To overcome this problem, corrections to the pressure gradients due to interactions between the oil and water were derived. Both models showed that the emulsion became concentrated because the higher viscosity oil was preferentially drawn into the conjunction. The net effect was similar to an inlet starved of oil. The inlet film thickness predicted by the interactive theory was in good agreement with Dow's experimental measurements for an EHL contact.     

Hydrodynamic Pressure Generation in a Pocketed Thrust Washer

The objective of this study was to experimentally investigate hydrodynamic pressure generation in surface-pocketed thrust washers. A novel method of pressure mapping was developed to allow for in situ measurement of the pressure generated by surface modifications. Thin-film pressure transducers, located just below the thrust washer surface, were used to measure pressure variations as a function of the operating conditions. Contour maps showing the cavitation region and the location of peak pressure were clearly displayed. The experimental work presented maps the pressure profiles with real-time, high-resolution sensors. The thin-film pressure transducers were used to investigate the pressure interactions between surface features. In addition to the experimental setup, a model of the contact was developed using ANSYS FLUENT. Cavitation, friction, film thickness, and load support were all compared with experimental results and the two were shown to be in good agreement. The model demonstrated an accurate prediction of the pressure profile but varied slightly with the predicted load support of the thrust washer. The simulation was then used to optimize the pocket density for the experimental operating conditions. The optimal bearing design had the highest load-carrying capacity with a low friction coefficient.     

A Simplified Model to Study EHL Film Collapse During Rapid Halting Motion

A theoretical model for describing the EHL film thickness during rapid deceleration is presented. The theory is based on the pioneer work of Ertel (1939) and Grubin (1949), who gave the first analytical solution for the elastohydrodynamic lubrication of a line contact under stationary operating conditions. An extension is made here for rapid halting motion. The proposed model is well adapted when the halting period is small in comparison to the transit time (i.e. 2b/u, ratio between the contact width and the rolling speed). This work completes the model of Glovnea and Spikes (2001b), appropriate for slow halting motion but which suffers from experimental fitting, and the model of Chang (2000) that is more suitable for speed or load oscillations at a wavelength close to the transit time.

This behavior implies that stop-start, reciprocating or rapidly halting machine components may be able to maintain a separating film for longer than would be expected based on steady-state EHL theory. An application to a ball bearing arrangement in a space mechanism is finally made in order to assess the model capabilities.     

A System-Approach to the Elastohydrodynamic Lubrication Point-Contact Problem

The classical EHL point contact problem is solved using a new “system-approach,” similar to that introduced by Houpert and Ham-rock for the line-contact problem. Introducing a body-fitted coordinate system, the troublesome free-boundary is transformed to the fixed domain. The Newton-Raphson method can then be used to determine the pressure distribution and the cavitation boundary subject to the Reynolds boundary condition. This method provides an efficient and rigorous way of solving the EHL point contact problem with the aid of a supercomputer and a promising method to deal with the transient EHL point contact problem. A typical pressure distribution and film thickness profile are presented and the minimum film thicknesses are compared with the solution of Hamrock and Dowson. The details of the cavitation boundaries for various operating parameters are discussed.     

The Effect of Slider Geometry on the Performance of a Powder Lubricated Bearing

The experimental investigation was carried out for dry contact tribosystems to elucidate the ascertained, yet uncharacterized, relationship between the coefficient of friction, η and apparent contact geometry in sliders. This paper presents the results of various experimentally evaluated slider pad width to length ratios, B/L (L is signified with direction of motion), while maintaining the unit loading constant. The empirical data is intended to provide guidance in design and in theoretical prediction of optimum bearing geometry for solid/powder lubricated systems.

In this experiment, a liquid lubricant, two different dry powders (MoS₂ and TiO₂), and four test specimens with B/L ratios ranging from 0.5 to 2.0 were used. An alternative experimental method has been used to simulate wear debris in the dry contact tribosystems, vis-à-vis dry powders, which were introduced into the interface gap to address the affinity of the wear process. Slider pads' performance characteristics as a function of the applied load (ranging from 34.5 to 345 kPa) and surface speed (0.004 to 4.5 m/sec) were determined at ambient condition. Experimental evidence shows that in the pre-hydrodynamic regime (low speed) the friction coefficient, η, decreases as B/L ratio increased from 0.5 to 1.5. Beyond a B/L ratio of 1.5, η increased with increasing B/L ratios. Optimum B/L ratio of 1 (based on measured minimum η) was identified for operation in the hydrodynamic regime. Overall reduction in η as a function of load was observed which is independent of B/L ratio and lubricant type. Also presented are the velocity effects on slider frictional performance.     

Evaluation of Ball-Bearing Performance in Liquid Hydrogen at DN Values to 1.6 Million

Experimental data were obtained in liquid hydrogen (?423 F) on two series of 40-mm-bore ball bearings utilizing various retainer materials. Effects of diametral clearance and retainer material on limiting DN value (product of bearing bore in mm and shaft speed in rpm) were investigated at thrust loads to 500 lb and at speeds to 41,200 rpm. An analysis was made to determine the effect of ball size and race curvatures on the heat generated in bearings of both series as a result of ball spin. The results, supported experimentally, indicate that higher limiting DN values at a specific thrust load could be obtained with an extremely light series (1908) bearing with open-race curvatures than with a light series (108) bearing. Successful operation to a DN value of 1.6 million was obtained with 1908 bearings (at 110 lb thrust load) using two different retainer materials. The glass-fiber-filled PTFE (polytetrafluoroethylene) retainer exhibited much less wear than the MoS₂-filled phenolic retainer at these test conditions.     

Numerical Analyses of Hydrodynamic Lubrication and Dynamics of the Rolling Piston and Crankshaft in a Rotary Compressor

In this article, numerical analyses of hydrodynamic lubrication and dynamics of the crank, rolling piston, and vane were carried out to study the tribological performance of a rotary compressor. Dimensionless Reynolds equations of journal and thrust bearings in dynamic load condition were derived and solved numerically. To deal with the lubrication of the rolling piston, the effect of the nonuniformity of tangential velocity over the bearing surface on the Reynolds equation was taken into account. In addition, combined with the analyses of dynamics and kinematics of the crank, piston, and vane, the angular velocities of the crank and piston as well as the motion mode between the piston and vane were studied. Analysis results illustrate characteristic oil film pressure distributions of the crank and piston bearings, which are different from that of common journal bearings. Under the influences of dynamic load and eccentricity of the cam, the wedge effect as well as the stretch and squeeze effect contribute greatly to hydrodynamic pressure. The relative motion mode between the piston and vane tip is not always pure sliding but accompanies rolling in some cases, which depends on the magnitude of the friction coefficient between the piston and vane tip. The analysis results are helpful for the improvement of rotary compressor design.