Analysis for Hydrodynamic Wedge-Platform Thrust Slider Bearing with Ultralow Surface Separation

Abstract

For the case of ultralow surface separation, in a hydrodynamic wedge-platform thrust slider bearing, the outlet zone and a portion of the inlet zone are in boundary lubrication, while most of the inlet zone is in the multiscale lubrication contributed by both the adsorbed boundary layer and the intermediate continuum fluid film. The present paper first presents the mathematical derivations for the generated pressure and carried load of this bearing based on the governing equation for boundary lubrication and the multiscale flow equation. Then, the full numerical calculation is carried out to verify the analytical derivations. It was found that the mathematical derivations normally have considerable errors when calculating the hydrodynamic pressure distribution in the bearing, owing to introducing the equivalent parameter λ bf , e which is constant in the inlet zone; however they can be used to calculate the carried load of the bearing when the surface separation in the outlet zone is sufficiently high. The study suggests the necessity of the numerical calculation of the hydrodynamic pressure and even the carried load of this bearing. It is also shown that owing to the fluid-bearing surface interaction, the pressure and carried load of this bearing are significantly greater than those calculated from the classical hydrodynamic theory.

Multiscale Elastohydrodynamic Wedge-Platform Thrust Bearing with Ultra Low Clearance

“…As done by Wang and Zhang [12], the present study uses the non-continuum nanoscale flow equation to describe the pure adsorbed layer flow and the multiscale flow equations to describe the flow of the sandwich film. By this way, the solution is sufficiently fast to satisfy the engineering sizes of the bearing.…”

“…By considering both the surfaces as rigid, Wang and Zhang [12] studied the hydrodynamic wedge-platform thrust bearing with ultra low clearance where there are only the physically adsorbed layers (with the continuum fluid film vanishing) in the outlet zone, close to the outlet zone also only exist the physically adsorbed layers, and in the remaining area of the inlet zone occurs the sandwich film consisting of both the adsorbed layers and the intermediate continuum fluid. When both the surfaces are considered as elastic, this kind of lubrication mode may also exist except the reduced area of the pure adsorbed layer owing to the surface elastic deformation, as shown by Fig.…”

“…Some mathematical equations are fundamental and may be repeated in this section. When the intermediate continuum fluid film is present between two adsorbed layers as shown by the sandwich film, the governing equation is [12]:…”

“…here: bf = hbf/h, p is the hydrodynamic pressure, x is the coordinate in the flow direction, u is the sliding speed and positive,  and  are respectively the bulk density and the bulk viscosity of the fluid, the adsorbed layer, q0 = j+1 / j (j is the separation between the (j + 1) th and j th fluid molecules across the adsorbed layer thickness) and q 0 is averagely constant, n is the equivalent number of the fluid molecules across the adsorbed layer thickness, n-2 is the separation between the neighboring fluid molecules across the adsorbed layer thickness just on the boundary between the adsorbed layer and the continuum fluid film, and the formulations for F1, F2 and ε were shown in [12]. When the intermediate continuum fluid film is absent because of too small surface separations and there is thus only the physically adsorbed layer across the whole surface separation, the governing equation is [12]:  are respectively the average density and the effective viscosity of the adsorbed layer across the whole surface separation, and S is the parameter accounting for the non-continuum effect of the adsorbed layer.…”

“…In a hydrodynamic thrust bearing, when the surface separation is ultra low, there will be the pure adsorbed layer area in the outlet zone, while in the remaining areas the sandwich flows occur consisting of both the adsorbed layer flow and the intermediate continuum fluid flow. By using the multiscale analysis, Wang and Zhang [12] studied the performance of the hydrodynamic wedge-platform thrust bearing working in this condition by assuming rigid surfaces. They showed that when the surface clearance in the outlet zone of this bearing is below 5nm, the hydrodynamic pressures can be increased by 2 orders by the effect of the adsorbed layer for the strong fluid-bearing surface interaction, compared to the classical calculations; The dimensional pressures in this bearing can be more than 100 MPa, so large as to cause the surface elastic deformation significantly greater than the surface clearance.…”