Hydrodynamic Lubrication Regime Research Articles

Numeric based low viscosity adiabatic thermo-tribological performance analysis of piston-skirt liner system lubrication at high engine speed

Abstract In conjunction with lubricant oil film dynamics, automotive piston's primary and secondary motion indicatively affect the engine noise and frictional losses (fuel economy). The escalation of oil film temperature due to adiabatic shear heating of low viscosity Newtonian lubricant oil film affects the load carrying capacity and viscosity of lubricating film and enhances friction losses, piston wear, and non-linear lateral motion of piston. In this study, a numerical model for piston dynamic and lubrication is presented, in which adiabatic heating effects are simulated in hydrodynamic mixed lubrication regime by coupling with 2D-energy equation. The thermo-hydrodynamic lubrication model is solved using finite difference formulation, while the piston dynamic model is solved using the modified Newton Raphson method. Furthermore, the effect of adiabatic shearing of lubricant and piston skirt design parameters on piston dynamics and lubrication performance are analyzed. Results show that the adiabatic heating significantly affects piston lateral motion and lubrication performance. Lubrication analysis is very beneficial in the improvement of engine design of piston skirt-liner and can help reduce the friction losses.

A Novel Surface Texture Shape for Directional Friction Control

An experimental study is presented to evaluate the influence of anisotropically shaped textures on the behaviour of sliding friction and sensitivity to sliding direction. The plate samples were textured with triangular sloped dimples using an ultrafast laser surface texturing technique. Reciprocating cylinder-on-plate tests were conducted with steel sliding pairs using mineral base oil as a lubricant to compare the tribological performance of reference non-textured specimen and dimpled samples. The dimples were designed with varying converging angles in the transverse y–z plane and top-view x–y plane. In this study, no dimple was fully covered in the contact area since the dimples size is much larger than the Hertzian line contact width. Stribeck style dynamic friction curves across boundary, mixed and hydrodynamic lubrication regimes were used to determine the benefit or antagonism of texturing. Observation of the directional friction effect of the anisotropic textures indicated that the converging shapes are beneficial for friction reduction, and the dimpled specimens have a lower friction coefficient particular under prevailing boundary lubrication conditions. It was also found that the real contact length variation rate is a major factor controlling the local friction response. The sloped bottoms of the textures produce effective converging wedge action to generate hydrodynamic pressure and contribute to the overall directional friction effects.

Controllable Sliding Bearings and Controllable Lubrication Principles—An Overview

Hydrodynamic and aerodynamic lubrication regimes in their controllable forms have been intensively investigated over the last two decades. With the aim of reducing friction and improving thermal, static, and dynamic characteristics of radial sliding bearings, different types of electro-mechanical actuators have been coupled to such bearings. Depending on (i) the actuator type; (ii) the actuation principle, i.e., hydraulic, pneumatic, piezoelectric or magnetic among others; and (iii) how such an actuator is coupled to the sliding bearings, different regulation and control actions of fluid film pressure and lubricant flow can be obtained. The most common actions are: (a) the control of the injection pressure to modify the fluid film pressure statically as well as dynamically; (b) the adjustment of the angle and direction of injection flow (mostly passive action); (c) the control of the sliding bearing gap and its preload via moveable and compliant sliding surfaces; and (d) the control of the lubricant viscosity. All four parameters, i.e., pressure, flow (velocity profiles), gap and viscosity, are explicit parameters in the modified form of Reynolds’ equations for active lubrication. In this framework, this paper gives one main original contribution to the state-of-the-art of radial sliding bearings and controllable lubrication: a comprehensive overview about the different types of controllable sliding bearings and principles used by several authors. The paper ends with some conclusive remarks about advantages and drawbacks of the different design solutions for controllable sliding bearings and the main challenges to be overcome towards industrial applications.

Modeling, analysis and constrained control of wet cone clutch systems: A synchromesh case study

This paper addresses the problem of controlling the regime of lubrication in sliding lubricated surfaces with the purpose of wear reduction and the lifetime increase of the friction lining material. To this end, the hydrodynamic, mixed, and boundary lubrication regimes in a cone clutch system are experimentally investigated and a piecewise linear approximation of the Stribeck curve is presented. The friction-induced temperature rise and its effect on viscosity reduction, wear, and dynamic uncertainty are discussed. A modified wear model of the cone clutch is proposed by taking into account the contributions of both the mixed and boundary lubrication regimes. Using the obtained wear model, the necessity of the operation in the mixed lubrication regime is shown, and the duration of the operation is determined. The dynamical model of the system is constructed and the friction-induced instability of the system in the mixed lubrication regime is investigated and discussed. The operation in the mixed lubrication regime together with the friction-induced instability reveals the need for a closed-loop control system. Employing the obtained dynamical, frictional, and wear models, a controller design method is developed to meet the objectives and satisfy the constraints. The proposed controller design method results in a piecewise affine (PWA) feedback law obtained by solving a set of linear programming (LP) problems offline. The resulting controller is easily implementable in real time. The robust stability of the proposed closed-loop control system is proved considering the uncertainties of the dynamical system. The robust performance of the PWA closed-loop control system in the presence of various disturbances is assessed through simulation and confirmed by experiments conducted on a test rig. The case study here is the engagement process of a synchromesh system in a clutchless automated manual transmission of an electric vehicle.

Mixed-lubrication analysis of misaligned bearing considering turbulence

Abstract Some bearings operate in mixed-lubrication regime due to the low rotational speed and the heavy load and misalign due to shaft's gravity. For large bearings, the maximum film thickness is much larger than minimum nominal film thickness and turbulence may locally develop. This paper proposes an approach to analyze misaligned mixed-lubrication bearing considering turbulence. Based on the average flow model proposed by Patir and Cheng and the Ng-Pan turbulence model, a generalized average Reynolds equation is derived. The calculation procedure is established by finite difference method. The results show that turbulence remarkably increases friction coefficient, slightly increases the minimum nominal film thickness, and decreases the transition speed from mixed-lubrication regime to hydrodynamic lubrication regime.

Tribological performances of CrAlN coating coupled with different ceramics in seawater

This paper attempts to present a comparative study of tribological performances of chromium aluminum nitride (CrAlN) coating coupled with Si3N4, SiC, WC, Al2O3 and ZrO2 balls in air and seawater. The comparison results show that the tribological performance of different tribopairs is determined by the property of ceramics. Seawater plays an important role in reducing the coefficient of friction (COF) by forming tribochemical films and developing a hydrodynamic lubrication regime. The wear rates of CrAlN/SiC in air and seawater are much lower than that of the combination of CAIN and other tribopairs because of the integrated effect of tribochemical film and micro-dimples on SiC ball. It is therefore apparent that CrAlN/SiC has the smallest COF and lowest wear rate in both air and seawater.

Analysis of lubricity and cavitation problem of oil and refrigerant mixture for hermetic compressors

The study is based on the results of experimental and theoretical research, the aim of which was to reveal the potential for the decrease of friction losses in the refrigerator's compressors. Lower viscosity oils as well as decreasing crankshaft diameters should lower the said losses. However, these measures can cause an adverse effect, if hydrodynamic lubrication regime transforms into boundary one, and the bearing eccentricity ratio reaches unity. The main objective of the study is to explain why the almost ideal surfaces of shaft and bush do wear when the carrying capacity of lubricating film theoretically exceeds real loads at eccentricity ratio 0.9 or even less. This disagreement, which also is presented in the study, can be explained by cavitation only partially. This study makes an assumption that the boundary value, i.e. oil film height, at which cavitation is starting, depends on a minimum height of the lubricating oil film, i.e. minimum clearance between shaft and bush.

Effects of Vanadium Oxide Nanoparticles on Friction and Wear Reduction

In this research, rheological and tribological performance of additive V2O5 nanoparticles in a light mineral oil has been investigated. For rheological performance, the addition of 0.2 wt. % V2O5 could reduce the viscosity of the base oil for 6%. Considering the overall friction reduction in boundary, mixed, and hydrodynamic lubrication regimes, that with 0.1 wt. % V2O5 exhibited the best effect. Friction coefficient of base oil could be reduced by 33%. In terms of wear, the addition of 0.2 wt. % V2O5 showed the lowest wear rate, which is 44% reduction compared to base oil. Through Raman spectrum and energy dispersive spectroscopy (EDS) analysis, it was found that V2O5 involved tribochemical reaction during rubbing. Vanadium intermetallic alloy (V–Fe–Cr) was found to enhance the antiwear performance. This research revealed that V2O5 nanoparticles could be an effective additive to improve tribological performance.

Friction Evaluation of Laser Textured Tool Steel Surfaces

Abstract Surface textures can be defined as a regularly arranged micro-depressions or grooves with defined shape and dimensions. These textures, if they are manufactured by laser ablation process, contribute to a significant improvement of the tribological, optical or various biological properties. The aim of this paper is to analyze the influence of the surface textures prepared by laser surface texturing (LST) at the friction coefficient value measured on the tool (90MnCrV8 steel) – workpiece (S235JRG1 steel) interface. Planar frontal surfaces of compression platens have been covered by parabolic dimple-like depressions with different dimensions. The morphological analysis of such manufactured depressions has been performed by laser scanning microscopy. Influence of such created textures on the tribological properties of the contact pair has been analyzed by the ring compression test method in the terms of hydrodynamic lubrication regime. The experimental research shown that by applying of surface textures with defined shape and dimensions and using an appropriate liquid lubricant at the same time, the coefficient of contact friction can be reduced nearly to the half of its original value.

Analytical modeling of hydrodynamic lubrication in a multiple-reduction drawing die

The hydrodynamic lubrication regime is reported to exist in numerous metal forming applications, such as wire drawing and hydrostatic extrusion, but it is difficult to achieve in the drawing of large diameter rods due to the relatively low drawing speeds common for larger parts. By creating a stable fluid film between the workpiece and the die during the drawing process friction and die wear could be significantly reduced, leading to energy savings, increased achievable reductions, and increased die life. An analytical model of the hydrodynamic drawing process is proposed which considers the geometry of the workpiece and die, as well as, the material properties (including work hardening effects), and (temperature dependent) fluid properties to determine the fluid film thickness over the reduction die. This model is then used to analyze several case studies, including a multiple reduction die with high pressure lubricant supplied to the space between the dies. It is shown that a stable fluid film can be established for low drawing speeds through the combination of a multiple reduction die and a supply of lubricant at high pressures to the inlet of the dies.

Transient Analysis of the Textured Journal Bearing Operating With the Piezoviscous and Shear-Thinning Fluids

In this paper, a mixed lubrication model is presented to analyze the tribological behavior of the textured journal bearings operating from mixed to hydrodynamic lubrication regimes. In particular, the effects of fluid piezoviscosity and the non-Newtonian fluid behavior are also considered. The presented model solves the hydrodynamic lubrication problem by a mass-conserving formation of the Reynolds equation, whereas the metal–metal contact is considered by using the Greenwood and Tripp (GT) contact model which is linked with the hydrodynamic model based on the concept of Johnson's load sharing. As a result, the performance of the textured journal bearing system under different lubrication regimes, including boundary lubrication regime, mixed hydrodynamic lubrication regime, and hydrodynamic lubrication regime, can be evaluated. Using the journal bearing systems operated under the start-up condition as examples, prediction demonstrates the influences of texture distributions on friction and wear. It is found that the friction reducing effect induced by texturing is influenced by the distribution of the texturing zones. In particular, the hydrodynamic friction can be reduced when the eccentricity ratio is changed from high to low. Moreover, it appears that the shear-thinning effect of lubricant cannot be neglected in the transient analysis of journal bearing system.

Effect of the Texture Shape Aspect Ratio on Friction Reduction in a Hydrodynamic Lubrication Regime

Effect of the Texture Shape Aspect Ratio on Friction Reduction in a Hydrodynamic Lubrication Regime

Design and Development of Permanent Magneto-Hydrodynamic Hybrid Journal Bearing

Fluid film bearings (FFBs) provide economic wear-free performance when operating in hydrodynamic lubrication regime. In all other operating conditions, except hydrostatic regime, these bearings are subjected to wear. To get wear-free performance even in those conditions, a hybrid (hydrodynamic + rotation magnetized direction (RMD) configured magnetic) bearing has been proposed. The hybrid bearing consists of square magnets to repel the shaft away from the bearing bore. Load-carrying capacities of four configurations of hybrid bearings were determined. The results are presented in this paper. The best configuration of hybrid bearing was developed. A test setup was developed to perform the experiments on the fluid film and hybrid bearings. The wear results of both the bearings under same operating conditions are presented.

Noninvasive Parameter Identification in Rotordynamics Via Fluid Film Bearings—Linking Active Lubrication and Operational Modal Analysis

In recent years, theoretical and experimental efforts have transformed the conventional tilting-pad journal bearing (TPJB) into a smart mechatronic machine element. The application of electromechanical elements into rotating systems makes feasible the generation of controllable forces over the rotor as a function of a suitable control signal. The servovalve input signal and the radial injection pressure are the two main parameters responsible for dynamically modifying the journal oil film pressure and generating active fluid film forces in controllable fluid film bearings. Such fluid film forces, resulting from a strong coupling between hydrodynamic, hydrostatic, and controllable lubrication regimes, can be used either to control or to excite rotor lateral vibrations. If “noninvasive” forces are generated via lubricant fluid film, “in situ” parameter identification can be carried out, enabling evaluation of the mechanical condition of the rotating machine. Using the lubricant fluid film as a “noninvasive calibrated shaker” is troublesome, once several transfer functions among mechanical, hydraulic, and electronic components become necessary. In this framework, the main original contribution of this paper is to show experimentally that the knowledge about the several transfer functions can be bypassed by using output-only identification techniques. This paper links controllable (active) lubrication techniques with operational modal analysis, allowing for in situ parameter identification in rotordynamics, i.e., estimation of damping ratio and natural frequencies. The experimental analysis is carried out on a rigid rotor-level system supported by one single pair of pads. The estimation of damping and natural frequencies is performed using classical experimental modal analysis (EMA) and operational modal analysis (OMA). Very good agreements between the two experimental approaches are found. Maximum values of the main input parameters, namely, servovalve voltage and radial injection pressure, are experimentally found with the objective of defining ranges of noninvasive perturbation forces.

Performance evolution of fully and partially textured hydrodynamic journal bearings lubricated with two lubricants

This study investigates the evolution of the main bearing performance of partially and fully textured hydrodynamic journal bearing. The viscosity effect is also analysed by the mean of numerical simulations for two types of oil: the oil 1 (ISO VG 32, 31.3 cSt at 40 °C) has a lower viscosity than oil 2 (ISO VG 100, 93 cSt at 40 °C). Reynolds equation is solved by finite difference and Gauss-Seidel methods with over-relaxation for various operating conditions. It is shown that, under hydrodynamic lubrication regime, the improvement of the most important characteristics (the friction coefficient and minimum film thickness) of a textured journal bearing depend strongly on the lubricant viscosity and the journal rotational speed. The fully textured journal bearing is highly favorable at very low speeds while the partially textured journal bearing is more suitable for slightly higher speeds. The gain in bearing performance due to the texturing of the bushing disappears at a critical speed of the journal and then, for higher rotational speeds, the presence of textures becomes detrimental.

A Novel Approach to Reduction of Frictional Losses in a Heavy-Duty Diesel Engine by Reducing the Hydrodynamic Frictional Losses

An important parameter in the reduction of fuel consumption of heavy-duty diesel engines is the Power Cylinder Unit (PCU); the PCU is the single largest contributor to engine frictional losses. Much attention, from both academia and industry, has been paid to reducing the frictional losses of the PCU in the boundary and mixed lubrication regime. However, previous studies have shown that a large portion of frictional losses in the PCU occur in the hydrodynamic lubrication regime. A novel texturing design with large types of surface features was experimentally analyzed using a tribometer setup. The experimental result shows a significant reduction of friction loss for the textured surfaces. Additionally, the textured surface did not exhibit wear. On the contrary, it was shown that the textured surfaces exhibited a smaller amount of abrasive scratches on the plateaus (compared to the reference plateau honed surface) due to entrapment of wear particles within the textures. The decrease in hydrodynamic friction for the textured surfaces relates to the relative increase of oil film thickness within the textures. A tentative example is given which describes a method of decreasing hydrodynamic frictional losses in the full-scale application.

光干渉による潤滑膜厚計測

Optical interference is one of the most powerful tools for lubrication studies, since it enables in-situ measurements of lubricant film thickness under practical conditions. After various improvements, optical interferometry has been developed to apply not only to thick (e.g., micrometer-scale) films in hydrodynamic lubrication regime but also to thin (e.g., nanometer-scale) films in boundary lubrication regime. This paper reviews the principles of measuring lubricant film thickness by optical interferometry and some technical tips on methods using white-light spectroscopy.

Statistical Analysis of Surface Texture Performance With Provisions With Uncertainty in Texture Dimensions

The performance of surface textures with dimensional uncertainty due to the manufacturing process is investigated with statistical models. The uncertainty parameters are geometrical dimensions (i.e., dimple diameter, area ratio, and dimple depth) and the performance parameters include the friction force, the load-carrying capacity, and the coefficient of friction. The results show that logarithmic models provide an excellent fit to the data and can explain more than 99.98% of the variance in data. The most critical geometric parameter for the coefficient of friction and the load-carrying capacity is found to be the dimple diameter, whereas the most critical geometric parameter for the friction force is the area ratio. Manufacturing errors that follow normal distribution with three-sigma quality are found to be insignificant. Under the conditions simulated, it is determined that a dimple diameter of $1883~\mu \text{m}$ and a dimple depth of $5.5\sim 6.5~\mu \text{m}$ yield optimal performance when operating in the hydrodynamic lubrication regime. The area ratio is the key parameter and must be determined based on the requirements of the load-carrying capacity and the coefficient of friction.

Sliding friction in the hydrodynamic lubrication regime for a power-law fluid

A scaling analysis is undertaken for the load balance in sliding friction in the hydrodynamic lubrication regime, with a particular emphasis on power-law shear-thinning typical of a structured liquid. It is argued that the shear-thinning regime is mechanically unstable if the power-law index n < 1/2, where n is the exponent that relates the shear stress to the shear rate. Consequently the Stribeck (friction) curve should be discontinuous, with possible hysteresis. Further analysis suggests that normal stress and flow transience (stress overshoot) do not destroy this basic picture, although they may provide stabilising mechanisms at higher shear rates. Extensional viscosity is also expected to be insignificant unless the Trouton ratio is large. A possible application to shear thickening in non-Brownian particulate suspensions is indicated.

A single asperity sliding contact model for molecularly thin lubricant

Molecularly thin lubricants are important in protecting the recording head and the rotating disk in a magnetic storage hard disk drive from mechanical damage induced by contact. Direct contact is more likely to occur at lower head-media spacing, which is the distance between the rotating magnetic disk and the head that reads/writes the data, and is highly desirable to be extremely low (in the nanometer range) for Terabit/in2 areal densities. Solid contact mechanics of the head-disk interface has been well addressed through extensive experiments and modeling, by neglecting the effect of the molecularly thin lubricant. Considering the important role of the lubricant, it is necessary to investigate the contact mechanics that include the lubricant. The present study develops a mechanics-based model to account for the nanoscale phenomena including slippage, nonlinear viscosity and bonded lubricant fraction, thus providing a measure to bridge the hydrodynamic lubrication and solid contact regimes at the nanoscale. Results show that the friction coefficient increases with the bonded ratio, which correlates with experimental observations.