Boundary Slip Induced Elastohydrodynamic Lubrication under Zero Entrainment Velocity Condition


Student thesis: Doctoral Thesis

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Award date26 Mar 2019


The merit of full complement bearings is its high load-carrying capacity achieved through the removal of the bearing retainer to free up extra space for installing more rolling elements. However, with no retainer leads to direct contact of rolling elements and they slide against one another with the same speed but in opposite direction. As the resultant entrainment velocity which is the average velocity of the contacting surfaces is zero, this contact condition is thus termed as zero-entrainment-velocity (ZEV). While a moving surface drags the oil into the contact area, the opponent surface moving in the opposite direction takes the oil out. As a result, no lubrication film can be built and poor lubrication of such bearings is expected. Such problem is also encountered in the design of conventional ball screws. Recent studies showed that the conventional lubrication state can be influenced by a new lubricating film formation mechanism based on the boundary slip at the liquid/solid interface. Thus, this thesis proposes to make use of boundary slip to facilitate effective elastohydrodynamic lubrication (EHL) for ZEV contacts. Only a few recent experimental studies are available in the literature to study the effect of a slip (or oleophobic) surface on bearing and most of them were conducted under fairly mild running conditions. Meanwhile, corresponding theoretical works were limited to isothermal EHL analyses. There is indeed a knowledge vacuum about the effect of slip surface on lubrication under large slide-roll ratio (SRR) conditions where the thermal effect cannot be ignored. The aims of this thesis are thus to investigate the characteristic and mechanism of boundary slip (or slip surface) effect on lubricating contacts operating under large SRR conditions, and to evaluate the concept of using boundary slip to facilitate effective EHL for ZEV contacts as those existed in full complement bearings. Both experimental and theoretical methods were used in the present study.

The experimental investigation used a conventional optical EHL test rig for the observation of a ball-disc contact. Comparison of the non-slip/non-slip contact and the slip/non-slip contact operating under ZEV conditions was conducted. Lubrication failure readily occurred in the test of non-slip/non-slip contact whereas it did not happen in the slip/non-slip test. Different oleophobic coatings were evaluated to determine the most desirable one to be used. The slip/non-slip conjunction in the test was constructed by the glass disc treated with the selected oleophobic coating and a steel ball which is known to be oleophilic. Interferograms of the contact area were observed via a microscope and captured by a CCD camera, and further processed by the self-developed calibration program to obtain the lubrication film profile. Optical observation of the contact proved that good lubrication was realized.

The lubrication characteristic of boundary slip ZEV EHL was experimentally studied. Both surface speed and viscosity of lubricants were identified to be the key factors in film formation. The lubricant was noted to entrain to the EHL contact in the same moving direction of oleophilic surface but opposite to that of the oleophobic surface. The central film thickness and the speed of contacting surfaces show a linear relationship in a log-log scale. ZEV EHL can be enabled only if the surface speed is up to a certain value, and the threshold speed is dependent of the strength of boundary slip as well as the viscosity of the lubricant. In general, the smaller the slip length and the lower the viscosity, the larger the threshold speed is. A simple method to measure the slip velocity under ZEV EHL was devised. The slip velocity was found to increase with the increase in surface speed.

Theoretical investigation of ZEV EHL was conducted. A generalized Reynolds equation which takes into consideration of the boundary slip at the lubricant/solid interface. Finite element method was called for solving the slip thermal-EHL model where the boundary slip and thermal effects were coupled. To achieve fast and stabilized solution, the Newton-Raphson iteration and temperature relaxation were applied. Solution of the model under large SRR conditions was obtained.

The coupling influences of boundary slip and thermal effect were revealed. Numerical results showed that boundary slip had a great influence on the film profile. By selecting an appropriate slip length, thick lubricating film and low temperature rise can be achieved. Further investigation showed that the application of boundary slip helped to reduce the von Mises stress near the solid surface. Numerical investigation of boundary slip under ZEV EHL confirmed its positive effect on promoting lubrication for ZEV contacts. The range of working conditions for optimization of boundary slip effect on lubricating film formation was obtained and summarized.

The new idea of using a slip surface (or boundary slip) to facilitate hydrodynamic lubrication for contacts operating at ZEV conditions was validated by the theoretical and experimental studies. The idea entails the bearing contact composed of an oleophobic (slip) and an oleophilic (nonslip) surface. The success in generating effective EHL for ZEV contacts conceives a new retainerless bearing type that incorporates alternately slip and non-slip rolling elements. As such, the new bearing design extends the application of full complement bearings to those which require a wide speed range.