Elastohydrodynamic lubrication in rotational atherectomy: Fluid film thickness characteristics and reduction of vascular damage risk

Temperature rise and film thickness reduction are the most important factors in elastohydrodynamic lubrication (EHL). In the EHL contact area, interfacial resistances (velocity/thermal slips) induced by the molecular interaction between lubricant and solid become significant due to the large surface/volume ratio. Although the velocity slip has been investigated extensively, less attention has been paid on the thermal slip in the EHL regime. In this study, numerical simulations were conducted by applying three cases of boundary slips to surfaces under sliding/rolling contacts moving in the same direction for the Newtonian thermal EHL. We found that the coupled velocity/thermal slips lead the most significant temperature rise and film thickness reduction among the three cases. The velocity slip results in a lower temperature in the lubricant and solids, whereas the thermal slip causes a temperature rise in the entire contact area in the lubricant as the film thickness decreases simultaneously. Furthermore, the effect of thermal slip on lubrication is more dominant than that of velocity slip, which increases the entrainment velocity or slide–roll ratio.

This paper presents the results of a transient analysis of elastohydrodynamic lubrication (EHL) of two parallel cylinders in line contact with a non-Newtonian lubricant under oscillatory motion. Effects of the transverse harmonic surface roughness are also investigated in the numerical simulation. The time-dependent Reynolds equation uses a power law model for viscosity. The simultaneous system of modified Reynolds equation and elasticity equation with initial conditions was solved using the multigrid, multilevel method with full approximation technique. The film thickness and the pressure profiles were determined for smooth and rough surfaces in the oscillatory EHL conjunctions, and the film thickness predictions were verified experimentally. For an increase in the applied load on the cylinders or a decrease in the lubricant viscosity, there is a reduction in the minimum film thickness, as expected. The predicted film thickness for smooth surfaces is slightly higher than the film thickness obtained experimentally, owing primarily to cavitation that occurred in the experiments. The lubricant film under oscillatory motion becomes very thin near the ends of the contact when the velocity goes to zero as the motion direction changes, but a squeeze film effect keeps the fluid film thickness from decreasing to zero. This is especially true for surfaces of low elastic modulus. Harmonic surface roughness and the viscosity and power law index of the non-Newtonian lubricant all have significant effects on the film thickness and pressure profile between the cylinders under oscillatory motion.

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This paper presents an experimental study of the effect of boundary slip on the lubricating film shape and friction of an elastohydrodynamic lubrication (EHL) contact under isothermal conditions. Ball and disc pure sliding experiments were carried out with a high viscosity polybutene oil using a conventional optical EHL test rig. The film shape and friction were measured simultaneously. The results obtained from two discs with different coatings were compared. One disc was coated only with Cr, the partially reflective layer, and the other had an extra layer of SiO2 coating on top. When running under mild conditions of low load and speed, there was no evidence of any boundary slip effect. However, when the load increased, the Cr-coated disc produced lower film thickness and friction than the SiO2-coated disc. The Cr-coated surface had a larger contact angle, i.e., smaller surface energy, than the SiO2 surface, which reflects the weak bonding between the molecules of the surface and the lubricant. The study concludes that surfaces with low surface energy promote boundary slip at the EHL contact, leading to a reduction in friction and film thickness.

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The paper presents the numerical solution of line contact thermal elastohydrodynamic lubrication (EHL) with bio-based lubricant. The model comprises Reynolds equation, film thickness, load balance and energy equations with appropriate boundary conditions by incorporating viscosity–pressure–temperature and density–pressure–temperature relations. Second-order finite difference scheme is used for the discretised form and their equation. The multigrid method with full approximation scheme is used to solve the Reynolds equation along with multilevel multi-integration method for film thickness equation. The pressure, film thickness and temperature distributions for two rolling velocities and various loads with a bio-based lubricant are presented in detail. The present findings yield a reduction in the minimum film thickness for high speed. Details of pressure spike as a function of relevant parameters are given. The results are compared with earlier findings based on different methods.

Effects of boundary slips on thermal elastohydrodynamic lubrication under pure rolling and opposite sliding contacts

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This paper describes a study of point contact elastohydrodynamic (EHD) lubrication behavior at high speeds (up to 20 m s−1). Central film thicknesses were measured by optical interferometry device. The influence of slide-roll ratio and operating temperature on the central film thickness was determined. The influence of thermal effects on the reduction of film thickness was discussed via the analysis of numerical simulation method considering thermal effects. Subsequently, the experimental data was used to amend a set of unified parameters for the thermal corrections for different types of oil at high speeds.

During the operation of the industrial chains, serious starvation may happen so that the oil film is often too thin to effectively separate the bush-pin surfaces, resulting in surface damage of the pin since the bush surface is harder. In this paper, the effects of oil supply and equivalent radius of curvature on the variations of pressure, film thickness and temperature in the contact area are studied based on the thermal elastohydrodynamic lubrication (EHL) theory and a finite line contact lubrication model under oil starvation condition is employed. It is found that oil starvation and the increase of equivalent curvature radius will aggravate the stress concentration at the end of the contact area, and more oil supply is required in the contact area if the thermal rise is considered. An oil starvation mechanism of finite line contact EHL is proposed, in which in addition to the direct influence of oil supply, the interaction of film thickness, pressure and temperature in the contact area also play important roles. The reduction of minimum film thickness is proposed to measure the degree of oil starvation at the ends of the contact area. The oil starvation process of finite line contact is divided into four typical stages.

In this study, a modified mixed lubrication model is developed with consideration of machined surface roughness, arbitrary entraining velocity angle, starvation, and cavitation. Model validation is executed by means of comparison between the obtained numerical results and the available starved elastohydrodynamic lubrication (EHL) data found from some previous studies. A comprehensive analysis for the effect of inlet oil supply condition on starvation and cavitation, mixed EHL characteristics, friction and flash temperature in elliptical contacts is conducted in a wide range of operating conditions. In addition, the influence of roughness orientation on film thickness and friction is discussed under different starved lubrication conditions. Obtained results reveal that inlet starvation leads to an obvious reduction of average film thickness and an increase in interasperity cavitation area due to surface roughness, which results in significant increment of asperity contacts, friction, and flash temperature. Besides, the effect of entrainment angle on film thickness will be weakened if the two surfaces operate under starved lubrication condition. Furthermore, the results show that the transverse roughness may yield thicker EHL films and lower friction than the isotropic and longitudinal if starvation is taken into account. Therefore, the starved mixed EHL model can be considered as a useful engineering tool for industrial applications.

Contact–fluid interfacial slippage in hydrodynamic lubricated contacts

Elastohydrodynamic lubrication (EHL) is an important branch of the lubrication theory, describing lubrication mechanisms in nonconformal contacts widely found in many mechanical components such as various gears, rolling bearings, cams and followers, metal-rolling tools, traction drives, and continuous variable transmissions. These components often transmit substantial power under heavy loading conditions. Also, the roughness of machined surfaces is usually of the same order of magnitude as, or greater than, the estimated average EHL film thickness. Consequently, most components operate in mixed lubrication regime with significant asperity contacts. Due to very high pressure concentrated in small areas, resulted from either heavy external loading or severe asperity contacts, or often a combination of both, subsurface stresses may exceed the material yield limit, causing considerable plastic deformation, which may not only permanently change the surface profiles and contact geometry but also alter material properties through work hardening as well. In the present study, a three-dimensional plasto-elastohydrodynamic lubrication (PEHL) model has been developed by taking into account plastic deformation and material work-hardening. The effects of surface/subsurface plastic deformation on lubricant film thickness, surface pressure distribution, and subsurface stress field have been investigated. This paper briefly describes the newly developed PEHL model and presents preliminary results and observed basic behavior of the PEHL in smooth-surface point contacts, in comparison with those from corresponding EHL solutions under the same conditions. The results indicate that plastic deformation may greatly affect contact and lubrication characteristics, resulting in significant reductions in lubricant film thickness, peak surface pressure and maximum subsurface stresses.

Impact of film thickness on off-state current of bottom contact organic thin film transistor has been investigated using two dimensional numerical simulations and analytical model. Off-state current of the device reduces by six orders of magnitude as film thickness is scaled from 45 nm to 10 nm, with rate of reduction in off-state current being slow first and then significantly higher. An analytical model for off-state current has been developed to offer an insight into above results of off-state current, and the model predictions are found in good agreement with reported experimental results. The developed model is especially important for the device with smaller film thickness as at such film thicknesses, space charge limited current model is inadequate to explain off-state current of such devices. A horizon for scaling device channel length through a reduction in film thickness only has been explored using an analytical model and simulation results. Off-state current of a shorter channel length (L) device can be comparable to a relatively longer channel length (i.e., L + δL) device if the fractional reduction in film thickness becomes equal to square of the fractional reduction in channel length. Following such reduction in film thickness successively for a number of steps, an expression for film thickness corresponding to the device with a desired value of channel length has been developed and verified with simulation results. Although the device with larger film thickness has a problem of poor subthreshold performance, it, in general, has advantage of better mobility. To alleviate this problem of the device with larger film thickness, a stack contact device has been proposed. An investigation of its subthreshold performance shows that its off-state current can be significantly lower as compared to conventional contact device.