Mixed Lubrication State Research Articles

An enhanced water-lubrication method: Friction-reducing and diffusion properties of secondary lubricants

To prevent severe friction between water-lubricated bearings (WLBs) and stern shafts under harsh conditions, a method is introduced that uses a lubrication pump to supply a small amount of secondary lubricant into the bearing for a short period of time. The effect of the secondary lubricant viscosity on friction-reducing performance is investigated by a modified block-on-ring friction and wear tester. The transfer process of the secondary lubricant within the bearing is observed with a self-designed visualization experimental platform. A flow model for capturing the diffusion of the secondary lubricant is developed, and the flow velocity equations are used as a “bridge” to link the mixed lubrication model and Volume-of-Fluid (VOF) model. The results indicate that injecting the secondary lubricant in the mixed lubrication state has the effect of rapidly reducing the friction coefficient, while supplying the lubricant in the elastohydrodynamic lubrication state tends to increase the friction coefficient. The backflow zones are primarily located at the bearing inner surface and the upper half of the fluid. The high-viscosity secondary lubricant separates into “free lubricant” and “adhesive lubricant” during the transfer process. The “oil-free zone” appears in the lubricating film due to the obstruction of lubricant diffusion by the “backflow effect”. The area of the “oil-free zone” decreases with increasing lubricant viscosity and decreasing load. This study provides a reference for friction-reducing technologies of friction pairs under water-lubrication conditions.

Non-Newtonian thermal elastohydrodynamic mixed lubrication in elliptical contact for nutation-driven double circular-arc spiral bevel gear pair

The new type of nutation reducer with double circular-arc bevel gears as the core is characterized by advantages such as high load-bearing capacity, large transmission ratio, and simple structure. The existing research focuses on the machining methods of gears and the analysis of tooth contact. This article first time establishes an elastohydrodynamic mixed lubrication model for nutation double circular-arc bevel gear. The model's accuracy is verified by comparing it with existing test results. On this basis, combined with the finite element method and tooth contact analysis, the meshing characteristics, tooth contact characteristics, and lubrication characteristics of the double circular-arc bevel gear tooth surface are analyzed. The results show that the entrainment velocity at the contact point of the tooth surface during transmission is much higher than the sliding velocity. Consequently, the meshing motion is close to pure rolling. In constant-speed transmission, the oil film thickness at the engaging-in and exit points near the edge of the tooth surface is smaller than that at other meshing points on the tooth surface. This phenomenon can easily lead to mixed lubrication and lubrication failure, as well as tooth surface wear. During deceleration, i.e., when the speed drops to 50 r/min, the oil film thickness approaches zero, indicating that the gear has entered a mixed lubrication state. The proposed model can be used for lubrication analysis and improving transmission accuracy of nutation double circular-arc bevel gears.

Modeling and analysis of 3D mixed lubrication in marine cam–tappet pair

During the operation of the marine cam–tappet pair, affected by periodic dynamic load, transient velocity, and micromorphology, it usually operates in a mixed lubrication state. Under extreme operational conditions, contact is likely to occur at the asperity of the interface, leading to problems such as stress concentration and dry contact. Considering the transient dynamics and three-dimensional roughness of cam–tappet pair, a mixed-elastohydrodynamic lubrication model is developed in this article, while the effects of surface waviness on the lubrication state are also investigated. The results show that the film thickness can be increased by 0.098 μm, and the coefficient of friction can be reduced by 10.6% when the wavelength ratio ranges from 1/6 to 6. However, when the amplitude exceeds 0.6 μm, the coefficient of friction may reach 0.10. Under conditions of low speed and high load, the film thickness decreases and contact load ratio increases. And about 30% reduction in film thickness is achieved on cam–tappet pair, potentially leading to increased wear or scuffing failure.

Fatigue life and transmission efficiency prediction of marine timing gears under 3D mixed lubrication state

The timing gears are the core transmission components of marine diesel engine, whose working conditions are complex and harsh, transient and coupled with the microscopic lubrication state, meshing in the mixed lubrication environment with lubrication-contact coexistence, local asperity contact brings stress concentration, further affects the fatigue life and friction loss. In this study, based on a three-dimensional (3D) line-contact mixed lubrication model, dynamic subsurface stress calculation, fatigue life prediction, and friction loss power study were carried out on timing gears, taking into account transient operating conditions and actual surface roughness. For the established simulation model, an equivalent simulation test of line-contact friction of timing gears was carried out, which can realize the measurement of friction coefficient under the condition of different slip-roll ratios, and make up for the deficiency that the traditional test equipment can only carry out the test under the fixed slip-roll ratio. Although the gear meshing process can only be simulated by the disc specimen due to the limitation of the specimen form, this limitation is similar to the simplified form of the simulation model, which can meet the needs of the verification test. The influence of structure and material on the lubrication state, fatigue life and friction power consumption are studied, which provide theoretical guidance for the tribological optimization design and reliability analysis for the marine timing gears.

Study on the Dynamic Characteristics of Gears Considering Surface Topography in a Mixed Lubrication State

Based on mixed lubrication analysis, considering the influence of rough interface contact stiffness, contact damping, and interface friction on the gear transmission system, the relationship between interface contact and the overall performance of the gear transmission system has been established. First, the surface topography is characterized using statistical parameters of rough surfaces, and the contact stiffness and damping for tooth surfaces with different roughnesses are calculated. Subsequently, a six degree of freedom gear tribo-dynamics coupling model is developed. Finally, the established tribo-dynamics model is employed to investigate the relationship between surface roughness and the overall performance of the gear transmission system. This study provides a more intimate connection between the contact interface and the general behavior of the gear transmission system, enabling a better representation of real-world engineering problems. The research findings reveal that contact stiffness and damping decrease with increasing surface roughness. Higher roughness leads to greater gear vibration amplitude. Moreover, elevated surface roughness results in intensified meshing force and more significant energy loss. Surprisingly, when the roughness is appropriate, gears with rough surfaces lose less energy than those with smooth surfaces.

Coupling analysis of timing gear dynamics and three-dimensional mixed lubrication under multi-branch shafting of marine diesel engine

Timing gears are the fundamental transmission components of marine diesel engines, and their operational reliability and efficiency are significantly impacted by their health status. The performance of timing gears is influenced by external excitations from the branch drive shaft and load input, as well as internal excitations caused by time-varying meshing stiffness and transmission errors. Therefore, it exhibits pronounced nonlinear characteristics as a result of the coupling effect between variable dynamic loading and micro-mixed lubrication induced by surface roughness and shaft vibration. The timing gear of a specific marine 20 V diesel engine is selected as the subject of investigation in this study, taking into account the impact of various types of internal and external comprehensive excitation. The three-dimensional (3D) mixed lubrication state analysis model, influenced by the coupling dynamic effect, was expanded based on the dynamic load and speed fluctuation in conjunction with the 3D line contact mixed lubrication model. The results demonstrate that the accuracy of the model can be validated through vibration testing, simulation testing, and empirical formulas for elastohydrodynamic lubrication (EHL). Additionally, the meshing process results in significant rough peak contact between the shaved surface and the ground surface, leading to potential lubrication failure. By implementing an optimization scheme that incorporates shock absorbers, the maximum contact area ratio of the four actual machining surfaces is reduced to only 16.06 %. This effectively mitigates the issue of severe rough peak contact, enhances the lubrication condition of gear pairs, and offers theoretical guidance for tribological optimization design in marine diesel engine timing gears.

Predictions of friction and temperature at marine cam-tappet interface based on mixed lubrication analysis with real surface roughness

The working conditions of cam-tappet pairs in marine diesel engines are directly influenced by the engine output power, the operational speed, the temperature, as well as the components surface micro-morphology, etc., which cause the cam-tappet pairs work in the mixed lubrication state, thus the interfacial friction, pressure and temperature rise are vital to engine performance, efficiency, and durability. An interfacial friction–temperature prediction model for the cam-tappet pairs, considering the effects of transient working conditions and the real surface roughness, is developed in the present study, based on the theories of the transient mixed EHL and the heat transfer under the condition of a fast moving heat source. The numerical results of surface temperature rise are compared with those from the Blok formula, and a good agreement is found. The obtained results show that the presence of 3D roughness may lead to a decrease in the lubricant film thickness, and the surface temperature rise of tappet may exceed 700 K, which is close to the material scuffing temperature, causing the surface failure due to scuffing and wear. If increasing the cam speed and base circle radius within certain range, it may lead to the increment of film thickness and reduction of surface temperature rise, thus the lubrication effectiveness is increased. In addition, using cast aluminum bronze may significantly reduce the surface temperature rise and improve the interfacial characteristics.

Improving Tribological Performance of Porous Polyimide by Surface Polishing

Because of its self-lubricating qualities, porous polyimide (PPI) has been used as a crucial cage material for space bearings; however, overcoming the failure followed by blackening of PPI remains a challenge. Herein, we developed a surface smoothening approach to optimize the tribological performance of PPI in an oil-impregnated state. Pre-polishing using sandpaper and then with Al2O3 paste enables the fabrication of nanoscale rough PPI surfaces with significantly reduced friction and surface wear and complete suppression of the blackening phenomenon. Compared with polishing using sandpaper or Al2O3 paste, the compound polishing method created a substantially smoother surface. It exposed more polytetrafluoroethylene at the outermost surface, improving the liquid and solid lubrication properties of the contact interface under a mixed lubrication state. The further analysis indicated that decreasing the size of PPI surface pores during pre-polishing using sandpaper was crucial in resisting the adsorption of residual polishing particles.

Lubrication failure analysis of camshaft bearings in V6 diesel engine

This contribution investigates the lubrication characteristics and failure causes of camshaft bearings belong to a V6 diesel engine. Numerical simulation based on the theory of elastohydrodynamic lubrication and flexible multibody dynamics is performed to analyse the lubrication characteristics of camshaft bearings. The results reveal that lubrication performance of different bearings of camshaft shows great differences. For a certain bearing, the mixed lubrication state and elastohydrodynamic lubrication state alternately occur in one engine cycle. The duration of mixed lubrication varies greatly in different bearings. For the bearing closest to the timing drive gear, the total pressure is dominated by the asperity contact pressure, and the predicted safety factors of the asperity contact pressure, hydrodynamic pressure and oil film thickness are less than 1, which means that this bearing will have lubrication failure.

Robust and universal macroscale superlubricity with natural phytic acid solutions

Using superlubricants to reduce friction and wear can dramatically reduce energy consumption and economic losses in industrial production. In this paper, we report a natural and environmentally friendly lubricant based on phytic acid solution (PA-0) that can achieve robust macroscale liquid superlubrication between PDMS and glass interfaces, and the superlubrication state can be maintained for at least 13 h. Significantly, its superlubrication can also be extended to other friction pair materials, reflecting its robustness and universality as a superlubricant. The calculation of the lubricating film thickness reveals that the superlubrication realized by the PA-0 solution belongs to the mixed lubrication state that comprises boundary lubrication and hydrodynamic lubrication. These findings may advance the practical application of liquid superlubrication.

Feature Recognition on Friction Induced Vibration of Water-Lubricated Bearing under Low Speed and Heavy Load

Under low speed and heavy load operating conditions, the marine water-lubricated bearing (WLB) is often in a mixed lubrication state. This leads to abnormal friction-induced vibrations and noise in underwater vehicles. An empirical mode decomposition (EMD) based method for identifying friction-induced vibration characteristics of WLB was proposed, and friction-induced vibration experiments of WLB were carried out. The results showed that the specific pressure and lubricant temperature significantly affected friction-induced vibrations while the cooling water flow hardly had an effect. Studying the monotonicity of the eigenvalues of IMF energy, time-domain standard deviation, bias angle, and center of gravity frequency can visualize the trend of the bearing friction-induced vibration signals. Among them, the critical point of friction-induced vibrations could be analyzed by the inflection point of IMF energy, standard deviation, bias angle, and the sudden increase of the center of gravity frequency. The study results were valuable for revealing the friction-induced vibration mechanism of WLB and providing an important reference for design optimization.

Effect of the grease thickener on tribological properties of Si3N4/GCr15 contact interface and the performance in hybrid ceramic ball bearing

Si3N4 hybrid bearings are considered the most promising motor bearings due to their characteristics of electrical insulation, high speed, low friction coefficient, and wear resistance, which can better meet the requirements of driving motors of electric vehicles. However, the tribological mechanism between the ceramic-grease-steel complex interface is unclear. Three kinds of greases, namely lithium, polyurea, and calcium sulfonate complex grease, were synthesized from poly alpha olefin 40 as base oil. The effects of grease thickener morphology on the tribological properties of Si3N4/GCr15 contact surfaces at different slip speeds were investigated, and the vibration and friction torque performances in 6204 hybrid ceramic balls were evaluated. The results showed that the thickener could form physical deposition film and friction film to protect the worn surface at low speed. The friction and wear conditions were significantly improved at high speed under lithium grease and polyurea grease lubrication, being a mixed lubrication state. In presence of lithium grease, the tribological performance of the Si3N4/GCr15 interface and the vibration performance, impacted condition, starting torque, and friction torque of the hybrid ceramic ball bearings were all the best. Because lithium grease is subjected to continuous shear of the ceramic ball, which releases the base oil stably, and forms a thick oil film with the broken thickener in the center of the raceway, the bearing is in a mild lubrication state. Therefore, for Si3N4 hybrid ceramic ball bearings, lithium grease is a lubricant with good application prospects.

Surface particle lubrication and wearing mechanism for asphalt pavement with aeolian sand in deserts

• Wear and lubrication mechanism of aeolian sand on asphalt pavement was studied. • Rolling-sliding friction exists between tire/road surface with particulate contaminants. • Angularity of the sand accelerates the wear of the asphalt membrane on pavement. • The wearing rate is proposed to evaluate the third body wear effect of aeolian sand. Particulate pollutants have a significant lubricating and abrasive effect on asphalt pavements. In this research, an experimental evaluation using an indoor accelerated wearing apparatus was used to simulate the wearing of Aeolian sand on asphalt pavements in a desert area. The British Pendulum Tester and a 3D scanner have also been used to examine the changes in skid resistance and texture morphology over time. Then, to evaluate the wearing process of sand particles on pavements, the wearing rate was proposed, and ultimately, a prediction model for the pavement with aeolian sand was constructed. The results shows that the wear rates of clean pavement, 50%Ψ and 100%Ψ of sand are 25.6%, 28.6% and 33.8% respectively. In terms of particle lubrication, the pavement is in a mixed lubrication state at 50%Ψ of sand; and in a fluid lubrication state when aeolian sand completely covers asperities of the pavement. From the wearing point of view, tire wearing on clean road is two-body wear; when there is aeolian sand between the tire/road contact interface, it is abrasive wear. The skid resistance prediction model developed by integrating the Pearson Correlation and Ridge Regression methods to identify-five texture indexes (ZMPD, Z Ra , λ q , Z Rs , S dr ) and the British Pendulum Number (BPN) can effectively predict the lubricating and wearing effects of aeolian sand on the pavement.

The study of the vibration under different lubrication states based on Stribeck curve

The tribology system is a complex nonlinear dynamical system. To study the vibration under different lubrication states, the experiments on full-film lubrication state and mixed lubrication state are designed based on Stribeck curve. In full-film lubrication state, vibration signal energy does not increase, but in mixed lubrication state, vibration signal energy increases with the increase of loading force. Under the condition of full-film lubrication, the autocorrelation function of vibration signal is similar to that of noise signal, which has smooth characteristics. In mixed lubrication state, the autocorrelation function of vibration signals has the characteristics of narrow convergence. Results reflect that for smaller loads there are no friction-induced vibrations due to full-film lubrication and for higher loads the friction-induced vibration comes into existence as the rough peaks between pin and disk surfaces come into play.

Coupling analysis of turbulent and mixed lubrication of water-lubricated rubber bearings

Water lubricated rubber bearings (WLRBs) are widely used in steam turbines, submarines and other large equipment. Due to the low viscosity of water, water-lubricated bearings are usually in a turbulent and mixed lubrication state at operation. Therefore, a turbulent mixed lubrication model of WLRBs was established by developing an extended average Reynolds equation and includes turbulent, and its rationality was verified by experiments. The results reveal the effect mechanism of turbulence on mixed lubrication under different load, rotational speed, axial misalignment and radius clearance. The contact pressure of WLRBs presents chaotic distribution after considering turbulence. The results also give an optimal axial misalignment and radius clearance that can maximize the turbulent force. The numerical simulation can provide some guidance for the theoretical analysis of WLRBs.

Numerical and experimental study on the effect of surface texture with roughness orientation considered under a mixed lubrication state

Purpose The purpose of this paper is to investigate the effects of surface texture with roughness orientation considered on tribological properties under a mixed lubrication state numerically and experimentally. Design/methodology/approach Based on the average Reynolds equation and asperity contact model, the impacts of surface texture parameters and roughness orientation on lubrication properties have been calculated using finite difference method. Tin–bronze samples with various prescribed surface texture geometric parameters and roughness orientation were fabricated by laser surface texturing technique, and the tribology performance of the textured surface was studied experimentally. Findings The effects of surface geometric parameters and roughness orientation parameters have been discerned. The experimental observations are in good agreement with the numerical prediction, which suggests that the numerical scheme adopted in this work is suitable in capturing the surface texture and roughness effect under mixed lubrication state. Originality/value By meticulously controlling the surface roughness and surface texture geometric characteristics based on the laser surface texturing process, samples with prescribed surface texture parameters and roughness orientation consistent with that in theoretical studies were fabricated and the theoretical model and results were verified experimentally.

Experimental Study on the Coefficient of Friction of the Ball Screw

Abstract The coefficient of friction (COF) is a key factor to estimate the performance of ball screws. Pieces of research focus on the experimental study of the COF, leading to the COF chosen empirically in many studies. To acquire the COF of the ball screw, a measuring system is conducted to detect the friction torque under different preloads and rotational speeds and the effects of the applied axial load and rotational speed on the COF are analyzed. By the curve fitting method, the Stribeck curve of the ball screw is obtained. The experimental results show that the lubricating state can be divided into two categories: the mixed lubrication state and the elastohydrodynamic lubrication state. This study is beneficial to choose a suitable working condition for a different performance of the ball screw.

Experimental research on the wall-wetting effects on the frictional property of the cylinder liner–piston ring pair

The application of novel injection strategies (high-pressure injection, early injection, retarded injection, etc.) in combustion engines has made the wall-wetting problem severer. As the splashed fuel dilutes the lubricating oil, the tribological performance of the cylinder liner–piston ring pair will be affected. In this research, the viscosity and wettability tests were conducted firstly by mixing diesel into lubrication oil. It was found that the dynamic viscosity of the mixture drops with more fuel diluting the oil, and a small quantity of diesel mixed will cause a remarkable decline in lubricant viscosity; also, the contact angle shows a downward trend with the increasing diluting ratio. Then based on several typical diluting ratios, the reciprocating friction tests were carried out to measure the instantaneous friction force of the production ring/liner pair. The experimental results showed that under a mixed lubrication state, the peak friction force of the ring/liner pair occurs around the dead centers, while the minimum force occurs at the middle position of the reciprocating stroke; with more fuel diluting the oil, the bearing capacity of oil film degrades, resulting in the increase of friction force. In addition, the average friction coefficient of the ring/liner pair shows an upward trend with the increasing diluting ratio, and the Stribeck curve moves toward the upper-left, which means the lubrication condition of this pair tends to transit from mixed lubrication to boundary lubrication, causing negative effects on the frictional property of the cylinder liner–piston ring pair. Therefore, the diluting ratio should be controlled under 20%.

Contact stiffness and damping of spiral bevel gears under transient mixed lubrication conditions

Existing studies primarily focus on stiffness and damping under full-film lubrication or dry contact conditions. However, most lubricated transmission components operate in the mixed lubrication region, indicating that both the asperity contact and film lubrication exist on the rubbing surfaces. Herein, a novel method is proposed to evaluate the time-varying contact stiffness and damping of spiral bevel gears under transient mixed lubrication conditions. This method is sufficiently robust for addressing any mixed lubrication state regardless of the severity of the asperity contact. Based on this method, the transient mixed contact stiffness and damping of spiral bevel gears are investigated systematically. The results show a significant difference between the transient mixed contact stiffness and damping and the results from Hertz (dry) contact. In addition, the roughness significantly changes the contact stiffness and damping, indicating the importance of film lubrication and asperity contact. The transient mixed contact stiffness and damping change significantly along the meshing path from an engaging-in to an engaging-out point, and both of them are affected by the applied torque and rotational speed. In addition, the middle contact path is recommended because of its comprehensive high stiffness and damping, which maintained the stability of spiral bevel gear transmission.

Analysis of Mixed Lubrication Characteristics of Cycloid Pin-Wheel Transmission

Cycloid pin-wheel transmission mechanism is the key transmission component of RV reducer. During the transmission, the entrainment speed along the tooth surface, the contact load and the radius of curvature always change sharply, which has a significant transient squeeze effect on the lubrication performance. The research on available elastohydrodynamic lubrication (EHL) and mixed EHL for cycloid pinwheel transmission is mainly conducted under the assumption of steady-state, and these transient effects are ignored. Therefore, a transient hybrid EHL model for cycloidal pinwheel transmission was presented, which takes into account key variable parameters along the meshing surface, including contact load, curvature contact radius, and entrainment speed. Besides, the lubrication characteristics of the meshing points with poor lubrication conditions at different entrainment speeds were also studied. The transient parameters of the cycloidal pin gear drive used in this paper are obtained through load tooth contact analysis. To improve the computational efficiency and convergence accuracy on relatively dense meshes, a progressive mesh densification method is applied. The comparison between the transient EHL and the corresponding steady-state EHL analysis results shows that the squeezing flow effect will affect the film thickness distribution of the contact area and the meshing trajectory line, and cause contact area increase. Operating speed is a key factor affecting the lubrication performance. The lower the speed, the worse the lubrication performance, and the higher the proportion of asperities contact in the mixed lubrication state.