Meng Yonggang

A rheological model for lithium lubricating grease

The rheological properties of a lithium lubricating grease were examined under various settings of shear rate, time and temperature with an emphasis on the thixotropy of grease. A HAAKE RV 20 rheometer was employed in the experiments, utilizing a cone and plate configuration. Precautions were taken when scheming and carrying out the test in order to eliminate the effects of slip at the wall, fracture and other forms of disturbance. Flow curves and thixotropic hysteresis loops were obtained. Based on the measurement results, a new rheological model involving a structural parameter has been proposed. The characteristics of the proposed model and its implication for lubrication analysis were discussed.

Effects of external electric fields on frictional behaviors of three kinds of ceramic/metal rubbing couples

Abstract In this paper, effects of external electric fields on frictional behaviors of Al2O3/brass, Al2O3/stainless steel and Al2O3/carbon steel couples under boundary lubricating conditions were studied on a self-made plate/plate type tribotester. Emulsion of 1 wt% zinc stearate dispersed into deionized water was used as lubricant in the experiments. The experiment results have shown that external electric fields affect the friction coefficient and its fluctuation of each rubbing couple substantially. The three couples showed different rates of change in friction coefficient and different tendency of change as the field was turned on and off during the rubbing process. Results acquired from experiments carried out in a wide range of external voltage, from 0 to 110 V, indicating that friction coefficient of Al2O3/brass couple increases monotonously with the intensity of the external electric field, although no linear relationship was found between them.

Compression Enhanced Shear Yield Stress of Electrorheological Fluid

Shear tests of an electrorheological fluid with pre-applied electric field and compression along the field direction are carried out. The results show that pre-compressions can increase the shear yield stress up to ten times. Under the same external electric field strength, a higher compressive strain corresponds to a larger shear yield stress enhancement but with slight current density decrease, which shows that the particle interaction potentials are not increased by compressions but the compression-induced chain aggregation dominates the shear yield stress improvement. This pre-compression technique might be useful for developing high performance flexible ER or magnetorheological couplings.

A dynamic rheological model for thin-film lubrication

In this study, the effects of the non-Newtonian rheological properties of the lubricant in a thin-film lubrication regime between smooth surfaces were investigated. The thin-film lubrication regime typically appears in Stribeck curves with a clearly observable minimum coefficient of friction (COF) and a low-COF region, which is desired for its lower energy dissipation. A dynamic rheology of the lubricant from the hydrodynamic lubrication regime to the thin-film lubrication regime was proposed based on the convected Maxwell constitutive equation. This rheology model includes the increased relaxation time and the yield stress of the confined lubricant thin film, as well as their dependences on the lubricant film thickness. The Deborah number (De number) was adopted to describe the liquid-solid transition of the confined lubricant thin film under shearing. Then a series of Stribeck curves were calculated based on Tichys extended lubrication equations with a perturbation of the De number. The results show that the minimum COF points in the Stribeck curve correspond to a critical De number of 1.0, indicating a liquid-to-solid transition of the confined lubricant film. Furthermore, the two proposed parameters in the dynamic rheological model, namely negative slipping length b (indicating the lubricant interfacial effect) and the characteristic relaxation time λ0, were found to determine the minimum COF and the width of the low-COF region, both of which were required to optimize the shape of the Stribeck curve. The developed dynamic rheological model interprets the correlation between the rheological and interfacial properties of lubricant and its lubrication behavior in the thin-film regime.

A finite element approach to plasto-hydrodynamic lubrication in cold forging☆

Abstract A plasto-hydrodynamic lubrication model dedicated to metal forming processes is presented in this paper. A finite element formulation for the fluid-solid interaction problem is given, as well as an effective calculation procedure for solving the resulting non-linear equation system. A computer simulation of the processes of lubricant film formation and transportation in the upsetting of a metal cylindrical billet illustrates the validity of the proposed model. The thickness of the entrapped film at the instant of depression closure and its variation as compression proceeds are discussed and compared with the results given by previous researchers.

Experimental Research on Boundary Slip of Confined Liquids at Micro/Nano Scale and Effect of Shear Rate and Viscosity

In order to research the flow and transportation property of liquids between solid and liquid at micro/nano scale, a commercial Atomic Force Microscope was modified by attaching a 45micro meter diameter glass ball to the end of cantilever to form a ball-disk contact, which was used to investigate boundary slip effect of confined liquids under different velocity. Solid surface specimen was prepared as hydrophilic Si (100) and hydrophobic OTS film surfaces on Si (100) verse different viscosity sucrose solution. The relationship between shear rate and boundary slip are discussed carefully. Results indicated that, slip boundary conditions are found for all different viscosity sucrose solution both on hydrophilic and hydrophobic surface, and slip length of liquid on solid surface with poor wetting property was obviously larger than that with good wetting property. Further more, the degree of slip is found to increase with shear rate and liquids viscosity for the two surfaces, which shows that boundary slip is shear rate and viscosity dependent. The results will be important for the research and control of the flow properties of confined liquids.

Simulation and Experimental Validation of the Effect of Surface Texture on Fluid Film Formation

A surface texturing process with the photolithography and wet etching methods was established. By optimizing the process parameters, surface textures with regular dimples on steel plates were obtained. The textured steel plates were tested on Falex friction test machine against lapped smooth brass plates under different loads. Test results of friction coefficient have shown that both of the dimple dimension and distribution patterns influence the friction characteristics remarkably, and some of the tested textures result in lower friction than the non-textured samples. In order to reveal the mechanism of surface texturing and to reduce the cost of experiments, a numerical simulation program which can deal with mixed lubrication analysis has been developed, based on the Patir & Cheng’s average flow model the Kogut & Etsion’s friction model. The tendency of the simulation results of friction coefficient is consistent with the experimental one for some typical load cases. Therefore, the design and optimazation of surface texture in tribological applications can be assisted by numerical simulation of mixed lubricantion.