Primary Prediction of Oil Film Cavitation Between Rotating Friction Pairs with Various Types of Surface Textures

It has been confirmed that bionic surface texture can effectively improve the lubricating and tribological performance of the sliding surface, and the texture cross section shape and depth are important parameters affecting the lubrication performance of the texture. However, there is a lack of effective optimization method for a texture cross section shape and determining the texture depth of a random texture cross section shape. Hydrodynamic lubrication performance model of a single texture under the uncompressible Newtonian fluid lubrication conditions was established based on the two dimensional steady-state Reynolds equation and texture depth equation, and finite difference method was adopted to discrete and solve the distribution of oil film pressure on the textured surface as well as analyzes the impact of different texture cross section shapes and depth on oil film pressure distribution and surface load capacity. The numerical simulation result shows that the textured surface can obtain a better hydrodynamic lubrication performance by guaranteeing that the average texture depth of a random shape of texture cross section equals the initial thickness of oil film. In addition, the shape of a texture cross section can be optimized based on the method of smaller a texture depth and deviation of a texture depth so as to obtain the optimal hydrodynamic lubrication performance of textured surface.

Non-contact resistance and capacitance on-line measurement of lubrication oil film in rolling element bearing employing an electric field coupling method

Bubble dynamics and cavitation intensity in milli-scale channels under an ultrasonic horn.

To address wear failure in screw pump stator and rotor friction pairs, this study constructed a numerical model of a rhombus-like micro-dimple texture on friction pair surfaces based on the scale structure of rhombus rattlesnakes. The model was based on the fluid dynamic pressure lubrication mechanism. The CFD method was used to calculate the bearing capacity, friction coefficient, flow field pressure distribution, and flow trace distribution of an oil film carrying surface. The effects of the area rate, depth, shape, and angle of the rhombus-like dimple texture and the actual well fluid viscosity of shale oil on the surface lubrication performance of screw pump stator and rotor friction pairs were analyzed. The results demonstrated that increasing the texture area rate and the angle of the long sides and decreasing the texture angle resulted in a decrease in the oil film surface friction coefficient and an increase in the average pressure and net bearing capacity as well as the hydrodynamic lubrication performance. The average pressure increased and then decreased as the texture depth increased, while the friction coefficient of the oil film surface initially decreased and then increased. At a texture depth of 20 μm, the friction coefficient reached its lowest value while the average pressure and net bearing capacity of the oil film reached their highest value, which resulted in optimal hydrodynamic lubrication performance. When the texture depth became greater than 20 μm, vortices were gradually formed within the texture, which decreased the hydrodynamic lubrication performance. When the area rate of the rhombus-like dimple texture, depth, angle between long sides, and angle were, respectively, equal to 27%, 20 μm, 74°, and 0°, the net bearing capacity of the oil film was maximized, the friction coefficient was minimized, and the hydrodynamic lubrication performance and anti-wear effect reached their highest values. The increase in the viscosity of the actual well fluid could enhance the dynamic pressure lubrication performance and improve the bearing capacity.

Effects of textured cylinder liner piston ring on performances of diesel engine under hot engine tests

This paper describes a three-dimensional computational model of oil film between a friction pair to investigate the characteristics of both single-phase and two-phase flow of the oil film in hydro-viscous drive. For the single-phase oil film, the distribution of pressure is very regular from inlet to outlet of the friction pair; its value decreases gradually. On the other hand, the temperature in the middle part of the oil film is considerably lower and the velocity increases at a faster rate near the outlet and has a parabolic profile, which is mainly caused by both the shear stress and extrusion force. By comparison, the physical phenomena at the outlet of the oil film are entirely different for two-phase flow with cavitation. For two-phase simulation of flow with cavitation, we first obtain the volume fraction of air bubbles at rotation speeds of 500, 1000, 2000, 3000, and 4000 revolutions/min. With increase in the rotation speed, the volume fraction of air bubbles increases, and their maximum value becomes even greater than 10%. Furthermore, due to cavitation, the torque transferred by the oil film is no longer linear with the rotation speed; its value decreases gradually. These results are important in the study of hydro-viscous drive and its applications; they shed a new light on the mechanism of power transmission through oil film in the presence of cavitation.

PurposeThe purpose of this paper is to use the viscosity of fluid to transmit power known as the hydro-viscous drive (HVD) to research the effect of two-phase flow on transmission characteristics.Design/methodology/approachIn this paper, a 3D computational model of oil film between friction pair was built to study the transmission characteristics of a two-phase oil film, and the distribution contours of pressure and temperature of oil film were investigated using the computational fluid dynamics technology.FindingsThe finding of the paper suggests that the distribution law of pressure and temperature of two-phase oil film is almost linear along the radial direction. However, since the physical phenomena near the outlet of the oil film are entirely different, there exists fluctuation. Meanwhile, the volume fraction of air was obtained at different rotation speeds, and the maximum value is 10.55 percent. Compared to the single-phase oil film, the torque transferred by the oil film is not linear with the rotation speed, its value decreases gradually.Originality/valueThis paper’s conclusions are very important for the study of HVD and its applications, which provide a new idea to further study the mechanism of oil film transmission and its cavitation. The development of fluid viscous speed clutch is dedicated to a large industrial fan and water pump speed regulation and energy saving. With the successful application of the technology, it will have more wide applications in different fields, such as, in steel, water, petrochemical, power plant of slag pump and exhaust fan.

By researching the influence of micro-groove texture on the surface tribological properties of the stator and rotor pair of oil production hydraulic motors, this paper aims to reduce the frictional resistance moment of the spiral pair of hydraulic motors, and further solve the problem of the difficult restart of the pump of a certain type of hydraulic-driven screw pump. According to the spiral pair of screw motors, a metal-rubber flat plate reciprocating friction model is established, and rectangular micro-grooves with different texture angles and depths are machined on the surface of the metal specimen. A combination of finite element simulation and tribological tests is used to carry out a study on the influence of different texture parameters on the friction performance of the hydraulic motor spiral pair. The results showed that at a certain texture angle, the friction coefficient of each specimen basically increases with the texture depth. When the texture depth is constant, the friction coefficient increases first and then decreases with the increase of the texture angle. The texture angle is the main factor affecting the friction coefficient. Under the same test conditions, the friction coefficient of the textured specimen can be reduced by 20.2% compared with the untextured specimen. In the metal-rubber contact pair of the stator and rotor of the hydraulic motor, the friction reduction mechanism of the texture mainly transport the lubricating medium through the micro-grooves to improve the lubricating conditions. Samples with a reasonable design of texture parameters can effectively reduce the friction coefficient of the friction pair without reducing the service life of the hydraulic motor, which is conducive to the smooth restart of the oil production system of the hydraulically driven screw pump.

During the wear and tear process of bearings, the friction coefficient between the friction pairs can be effectively decreased by employing the suitable surface texture on the frition surface. In the study, the distribution and depth variation of the surface texture were used as variables, and the genetic algorithm was used for iterative optimization to obtain the optimal texture distribution and depth. The friction and wear performance of the rectangular texture bearing sliding blocks was optimized. The depth of the texture was represented by a 4-bit binary number, and different binary numbers were set to represent different texture depths. Finally, the genetic algorithm was used for continuous iteration and evolution to obtain the optimal texture combination. The study showed that, compared with the regular texture with a depth of 0.2 μm, the friction coefficient decreased by 15.0% under the optimal texture with a non-uniform depth. Simultaneously, compared with the regular 3 μm deep texture, texture with a optimized depth makes the friction coefficient decreased by 37.5%, and the minimum oil film thickness increased by 0.979 μm. The optimal texture and oil film thickness combination obtained from the study can effectively reduce solid contact force and alleviate mechanical wear.

PurposeHydrostatic thrust bearing is a key component of the vertical CNC machining equipment, and often results in friction failure under the working condition of high speed and heavy load. The lubricating oil film becomes thin or breaks because of high speed and heavy load and it affects the high precision and stable operation of the vertical CNC machining equipment; hence, it is an effective way of avoiding friction failure for achieving the oil film shape predictionDesign/methodology/approachFor the hydrostatic thrust bearing with double rectangular cavities, researchers solve the deformation of the friction pairs in hydrostatic bearing by using the computation of hydrodynamics, elasticity theory, finite element method and fluid-thermal-mechanical coupled method. The deformation includes heat deformation and elasticity deformation, the shape of gap oil film is got according to the deformation of the friction pairs in hydrostatic bearing, and gets the shape of gap oil film, and determines the influencing factors and laws of the oil film shape, and achieves the prediction of oil film shape, and ascertains the mechanism of friction failure. An experimental verification is carried out.FindingsResults show that the deformation of the rotational workbench is upturned along its radial direction under the working condition of high speed and heavy load. However, the deformation of the base is downturned along its radial direction and the deformation law of the gap oil film along the radius direction is estimated; the outer diameter is close but the inner diameter is divergent wedge.Originality/valueThe conclusion can provide a theoretical basis for the oil film control of hydrostatic thrust bearing and improve the stability of vertical CNC machining equipment.

The performance of cylinder liner-piston ring (CLPR) affects the efficiency of marine engine greatly. An experimental study on textured CLPR was conducted to seek insight into the operation reliability of CLPR. Three types of surface texture, groove texture on cylinder liner, dimple texture on piston ring, and co-texture on both sides, were processed. A series of tests were then carried out on an engine tester. Three characterization parameters, contact resistance, worn surface topography on cylinder liner, and cylinder pressure were used to describe the CLPR tribological properties and air tightness. The results showed that the three textures all improved tribological properties. Compared to the non-textured surface, the contact resistance of the three textures increased 30.7%, 71.7% and 98.4% on average. The wear resistance of dimple texture was weaker than that of groove texture. Air tightness was closely related to oil film pressure. At dead centre, the cylinder pressure of the three textures respectively was optimized by 4.6 kPa, 7.8 kPa and 12.3 kPa averagely. The co-texture was shown to combine the advantages of groove texture with the advantages of dimple texture, which provided the surface with improved oil film distribution and more efficient debris trapping, resulting in the optimum tribological properties and air tightness among the three textures.

High-speed and heavy-load tribological properties of hydrostatic thrust bearing with double rectangular recess

The working principle and motion process of an aviation wet clutch are analyzed. The initial velocity before the friction pair engaged is solved. The transient Reynolds equation is modified, and an oil film bearing capacity model and a micro-convex bearing capacity model are derived. The film thickness equation between N friction pairs and a pressure-plate is derived. A dynamic engaged model of springs, pistons, friction pairs, and pressure plates are established. The torque balance equation is established of two pairs of friction pairs. The friction torque, rate of change in the oil film, and law of relative change in speed are obtained. The results demonstrate that the spring preload and the viscosity of the lubricating oil have a significant influence on the engagement characteristics. Increasing the quality of the friction plate will reduce the time of engagement, whereas the quality of the friction plate has slight effect on the friction torque characteristics and oil film thickness. The initial speed generated by the collision process will reduce the output speed, sharply increase the torque peak at the lock, and increase the shift shock.

The purpose of this paper is to study the oil film and friction characteristics of valve plates with a micro-textured surface and to explore the influence of textures of different shapes and sizes on the valve plates. Firstly, on the basis of thermohydrodynamic theory, this paper established the lubrication model of the oil film on the valve plate pair of swashplate axial piston pumps, according to the Reynolds equation. Secondly, the micro-texture was added to the mathematical model of the valve plate pair’s oil film. A combination of the energy equation, oil-film-thickness equation, elastic deformation equation, viscosity–pressure and viscosity–temperature equation, the finite difference method, as well as the relaxation iteration method, was used to solve the problem, and the textured and non-textured valve plate surfaces were simulated. The nephogram of the oil-film-thickness distribution, elastic deformation distribution, oil-film-pressure distribution and oil-film-temperature distribution were generated. Then, the control variable method was used to change the cylinder rotational speed, tilt angle, oil viscosity, initial oil film thickness and other parameters to analyze their effects on oil film characteristics. In addition, the friction characteristics of non-textured surfaces, square textured surfaces, triangular textured surfaces and circular textured surfaces were compared and analyzed. It was found that the textured surface of valve plates can obviously improve friction efficiency under the same operating conditions. The square texture, especially, is the preferable shape, rather than the triangular texture and the circular texture, and the friction performance is at its best when the texture depths are between 20 and 50 . The results provide a theoretical basis for the design and improvement of the valve plate.

To assess the prediction accuracy of existing cavitation models for in-nozzle string cavitation, elucidate the accumulation characteristics of the vortex flow field on the bubbles, and explore the influence of non-condensable gas in the string cavitation inception and development, a modified cavitation model has been proposed. This modified model is expected to be more precise in predicting string cavitation. In this paper, numerical simulations are carried out in conjunction with visualization experiments, where discrete-phase particles represent the non-condensable gas. Results indicate that the current Zwart–Gerber–Belamri cavitation model fails to account for the effect of the non-condensable gas factor in the modeling process, leading to inaccurate predictions of string cavitation within the nozzle. To address this issue, modification of the mass source term of the cavitation model is proposed based on the Rayleigh–Plesset equation and the bubble surface pressure equation. The modified model demonstrates better accuracy in predicting string cavitation inside the nozzle. The gas nuclei inside the fuel tend to accumulate around the vortex core, influenced by the internal vortex flow field, which further contributes to the string cavitation inception and development. This study proposed and evaluated a more accurate cavitation model and further improved the cavitation initiation and development mechanism.