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Granular lubrication is a new lubrication method and can be used in extreme working conditions; however, the obstacle of force transmission characteristics needs to be urgently solved to fully understand the mechanical and bearing mechanisms of granular lubrication.

A flat sliding friction cell is developed to study the force transmission behaviors of granules under shearing. Granular material, sliding velocity, granule size and granule humidity are considered in these experiments. The measured normal and shear force, which is transmitted from the bottom friction pair to the top friction pair via the granular lubrication medium, reveals the influence of these controlling parameters on the force transmission characteristics of granules.

Experimental results show that a low sliding velocity, a large granule size and a low granular humidity increase the measured normal force and shear force. Besides, a comparison experiment with other typical lubrication styles is also carried out. The force transmission under granular lubrication is mainly dependent on the force transmission path, which is closely related to the deconstruction and reconstruction of the force chains in the granule assembly.

These findings reveal the force transmission mechanism of granular lubrication and can also offer the helpful reference for the design of the new granular lubrication bearing.

This paper aims to give the guidance for the design of the bearing.

The finite element method, the multi-body dynamics method, the finite difference method and the tribology are combined to analyze the lubrication.

The performance parameters of crankshaft-bearing system such as the misalignment, the oil filling ratio and the oil groove are also investigated. Misalignment causes the pressure to incline on one side and the pressure increases obviously. Filling ratio has great relationship with pressure distribution; the factors influencing the filling ratio are also analyzed. Different oil groove models are investigated, as it can provide the theory for oil groove design, and three factors above are always combined to influence the lubrication characteristics.

The optimization of bearing system is conducted by orthogonal test and neural network, unlike the linear optimization theory. Neural network uses the nonlinear theory to optimize crankshaft-bearing system.

The purpose of this paper is to use a combination of numerical simulation and experiment to evaluate the performance of laser surface texturing (LST) in the field of gear lubrication, and to more accurately predict the lubrication characteristics of different surfaces.

The method used in this paper is developed on the basis of the deterministic solution of the three-dimensional (3D) mixed elasto-hydrodynamic lubrication (EHL) model and the model parameters are corrected by friction test. The film pressure, film thickness and friction coefficient of different micro-textured tooth surfaces are predicted on the basis of accurate 3D mixed EHL models.

The results demonstrate that the micro-texture structure of the tooth surface can increase the local film thickness and enhance the lubricating performance of the tooth surface without drastically reducing the contact fatigue life. The stress distribution and friction characteristics of the tooth surface can be optimized by adjusting the micro-texture arrangement and the size of the micro-textures.

A new evaluation method using a 3D hybrid EHL model and friction test to predict the lubrication characteristics of LST is proposed, which can effectively improve the processing economy and save time.

The peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-11-2020-0423

A free-piston engine (FPE) is an unconventional engine that abandons the crank system. This paper aims to focus on a numerical simulation for the lubricating characteristics of piston rings in a single-piston hydraulic free-piston engine (HFPE).

A time-based numerical simulation program was built using Matlab to define the piston motion of the new engine. And a lubrication mode of piston rings was built which is based on the gas flow equation, hydrodynamic lubrication equation and the asperity contact equation. The piston motion and the lubrication model are coupled, and then the finite difference method is used to obtain the piston rings lubrication performances of the FPE. Meanwhile, the lubrication characteristics of the new engine were compared with those of a corresponding conventional crankshaft-driven engine.

The study results indicate that compared with the traditional engine, the expansion stroke of the HFPE is longer, and the compression stroke is shorter. Lubrication oil film of the new engine is thicker than the traditional engine during the initial stage of compression stroke and the final stage of the power stroke. The average friction force and power of the hydraulic free piston engine are slightly lower than those of the traditional engine, but the peak friction power of the FPE is significantly greater than that of the traditional engine. With an increase in load, the friction loss power and friction loss efficiency decrease, and with a decrease in equivalence ratio, the friction power loss reduces, but the friction loss efficiency decreases first and then increases.

In this paper, only qualitative analysis was performed on the tribological difference between conventional crankshaft engine and HFPE, instead of a quantitative one.

This paper contributes to the tribological design method of HFPE.

No social implications are available now, as the HFPE is under the development phase. However, the authors are positive that their work will be commercialized in the near future.

The main originality of the paper can be introduced as follows: the lubrication and friction characteristics of the new engine (HFPE) were investigated and revealed, which have not been studied before; the effect of the HFPE’s special piston motion on the tribological characteristics was considered in the lubrication simulation. The results show that compared with the traditional crankshaft engine, the new engine shows a different lubrication performance because of its free piston motion.

When the clearance oil film of hydrostatic bearing friction pair is in critical lubrication state, the phenomenon of zero flow of local lubricating oil will aggravate the oil film temperature rise, which needs to be solved.

In this paper, the critical lubrication parameter equation and the oil film temperature rise mathematical model are derived for the new type q1-205 double rectangular cavity hydrostatic bearing. Based on a combination of theoretical analysis, simulation and experimental verification, this paper analyzes the flow characteristics and temperature rise characteristics of the lubricating oil when the hydrostatic bearing is in a critical lubrication state under different operating conditions and finally obtains the critical lubrication state of the oil film.

This study found that the numerical simulations and the derived formulas agree with the results. When the oil film is in critical lubrication, the cross-section side flow of the oil side is almost zero. The heat cannot be taken away in time, resulting in the local temperature rise of the oil film, which causes serious heat accumulation.

It is concluded that the operating condition parameters corresponding to the critical lubrication state provide a theoretical basis for the selection of actual hydrostatic bearing operating conditions, which is of great scientific significance.

Piston ring dynamic problem plays an important role in the lubricant characteristics of a reciprocating engine, which lead to engine wear and the increased consumption of lubricating oil. A cavitation analysis of the piston ring lubrication with two-dimensional Reynolds equation has rarely been reported owing to the complex working condition. The purpose of this study is to establish a precise model that can provide guidance for the design of the piston ring.

In this paper, a cavitation model and its effect on the piston ring lubrication was studied in a simulation program based on the mass-conserving theory which is solved by means of the Newton–Raphson method. In this study, some models such as mixed lubrication, asperity contact, blow-by/blow-back flow and cavitation have been coupled with the lubrication model.

The established model has been compared with the traditional model that deals with cavitation by using the Reynolds boundary condition algorithm. The cavitation zone, pressure distribution and density distribution between the piston ring and the cylinder have also been predicted. Studies of the changing trend for the pressure distribution and the cavitation zone at few typical crank angles have been listed to illustrate the cavitation changing rule. The analysis of the results indicates that the developed simulation model can adequately illustrate the lubrication problem of the piston ring system. All the analyses will provide guidance for the oil film rupture and the reformation process.

A two-dimensional cavitation model based on the mass-conserving theory has been built. The cavitation-forming and -developing process for the piston ring–liner lubrication has been studied. Non-cavitation occurs in the vicinity of top dead center and bottom dead center. The non-cavitation period will be longer in the vicinity of 360° of crank angle. The density distribution in the cavitation zone can be obtained.

The study aims to the distribution rule of lubricating oil film of full ceramic ball bearing and improve its performance and life.

The paper established an analysis model based on the fluid–solid conjugate heat transfer theory for full ceramic ball bearings. The distribution of flow, temperature and pressure field of bearings under variable working conditions is analyzed. Meanwhile, the mathematical model of elastohydrodynamic lubrication (EHL) of full ceramic ball bearings is established. The numerical analysis is used to study the influence of variable working conditions on the lubricant film thickness and pressure distribution of bearings. The temperature rise test of full ceramic ball bearing under oil lubrication was carried out to verify the correctness of simulation results.

As the speed increased, the oil volume fraction in full ceramic ball bearing decreased and the surface pressure of rolling element increased. The temperature rise of full ceramic ball bearings increases with increasing speed and load. The lubricant film thickness of full ceramic ball bearing is positively correlated with speed and negatively correlated with load. The pressure of lubricating film is positively correlated with speed and load. The test shows that the higher inner ring speed and radial load, the higher the steady-state temperature rise of full ceramic ball bearing. The test results are in high agreement with simulation results.

Based on the fluid–solid conjugate heat transfer theory and combined with Reynolds equation, lubricating oil film thickness formula, viscosity temperature and viscosity pressure formula. The thermal analysis model and EHL mathematical model of ceramic ball bearings are established. The flow field, temperature field and pressure field distribution of the full ceramic ball bearing are determined. And the thickness and pressure distribution of lubricating oil film in the contact area of full ceramic ball bearing were determined.

The peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-05-2023-0126/

This paper aims to address the influence of lubrication methods on operational characteristics, power losses and temperature behavior of gears and bearings. It contributes to the improvement of resource and energy efficiency of geared transmissions.

Experimental investigations were performed at a gear and bearing power loss test rig. Thereby, dip lubrication, injection lubrication with injection volumes from 0.05 to 2.00 l/min and minimum quantity (MQ) lubrication with an injection volume as little as 28 ml/h were considered. Measurements were evaluated in terms of no-load and load-dependent power loss, bulk temperatures and mean gear coefficients of friction.

Results show strongly reduced no-load gear and bearing losses for lubrication methods with low lubricant quantities. Load-dependent losses are similar to conventional lubrication methods and tend to be lower at high speed. This is related to higher bulk temperatures, as the heat dissipation of lubrication methods with low oil quantities is limited. Limited thermal load limits were shown to be extended by LowLoss gears.

Systematic investigations were conducted to evaluate the influence of dip, injection and MQ lubrication on power loss and temperature behavior of gears and bearings. The results of this study support further research on needs-based lubrication methods for gearboxes.

In a harmonic drive during assembly of its components like strain wave generating (SWG) cam, flexspline (FS) and circular spline, a gap is formed between the cam’s outer surface and the FS cup inner surface due to mismatching. This gap, which is known as “Coning”, plays a vital role in the flow of lubricant at that interface. This paper aims to analyse the coning phenomenon and the lubrication mechanism.

In the present investigation, the geometry of the coning gap and its variation with the SWG cam rotation are established. Essentially, the deflection of FS cup and deformation of SWG cam (bearing outer race) are derived to find the gap due to coning. Next, the hydrodynamic lubrication equation is solved to get pressure profiles for this gap under suitable boundary conditions assuming non-Newtonian lubrication.

Methods of estimating the coning gap and lubrication pressure profiles are established. Effects of non-Newtonian terms (coupling number and non-dimentionalized characteristic length) and SWG length (finite, long and short) on pressure profiles are also shown. All analyses are done in non-dimensionalized form.

Establishing the geometry of coning and non-Newtonian hydrodynamic lubrication aspects in the coning in the FS cup and SWG cam interface are the originality of the present investigation.

The purpose of this paper is to analyze the problems of tilting-pad thrust bearing static instability and lubrication performance under static bistability.

The static equilibrium state of tilting-pad thrust bearing is analyzed, and key parameters are extracted from the general lubrication model. Then, a distribution area of bearing static equilibrium points is achieved by solving the model. The area is divided into three sub-areas which represent monostabillity, bistability and instability, and an unstable boundary of the area is discovered. By these findings, a reversible lubrication failure phenomenon is explained. A calculation method is proposed to obtain bearing lubrication performance under the bistability.

The variation of working conditions can lead to migration of unstable boundary and static instability. After resuming the working conditions, unstable boundary will resume in situ, and the bearing will operate steadily again. Moreover, there is a big difference between the two groups of lubrication performance solutions under the bistability.

The static stability acceptance test of tilting-pad thrust bearing should be implemented under start–stop, accidental excitation and other conditions before service to prevent such lubrication failure phenomenon. Moreover, fulcrum position of the bearing pad should be kept away from the bistable area.

This paper has preliminarily revealed a static instability mechanism of tilting-pad thrust bearing, analyzed bistability lubrication performance and proposed some suggestions to improve the bearing stability.