Geometric discontinuities of the solid boundaries at the inlet of narrow passages, as for slider bearings, piston rings etc. generate local phenomena where inertia plays an important role. Assuming the lubricant is carried by a moving surface towards the contact zone, the flow characteristics at a short distance ahead of the inlet are investigated. The velocity distribution across the lubricant layer and the shape of the free surface are analyzed, when the surface tension is taken into account. The differential equation of the free surface shows that the thickness of the liquid layer has a special value (He = 3) at the point where the incoming flow is reversed. The pressure at the inlet of the film is then calculated, in terms of the dynamic pressure of the unperturbed flow. It is shown that the inlet pressure head depends on the shape of the free surface (through He), on the pressure gradient and on Reynolds number. Subsequently, the drag generated by the pressure head is determined. After the flow entering the gap and the expelled flow were obtained, the problem of bearing starvation or flooding was examined, thus leading to a new criterion for lubricant supply conditions.

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