Abstract
Abstract It has been shown before that liquids can slip at a solid boundary, which prompted the idea that parallel-surfaces bearings can be achieved just by alternating slip and non-slip regions in the direction of fluid flow. The amount of slip at the wall depends on the surface tension at the liquid–solid interface, which in turn depends on the chemical state of the surface and its roughness. In the present study a heterogeneous surface was obtained by coating half of a circular glass disc with a coating repellant to glycerol. A rotating glass disc was placed at a known/calibrated distance and the gap was filled with glycerol. With the mobile surface moving from the direction of slip to non-slip region it can be theoretically shown that a pressure build up can be achieved. The pressure gradient in the two regions is constant, similar to that in a Rayleigh step bearing, with the maximum pressure at the separation line. The heterogeneous disc was placed on a holder supported by a load cell thus the force generated by this pressure increase can be measured accurately. Tests were carried out at different sliding speeds and gaps and the load carried was measured and subsequently compared with theoretical calculations. This allowed the slip coefficient to be evaluated.
Highlights
In lubricating systems, where the bounding solid surfaces are very close together and one of the dimensions of the fluid column is much smaller than the other two, a number of simplifying assumptions can be made, which reduce Navier–Stokes equations to the form given by Eq 1. p (1) x zIt is assumed that the flow takes place along direction x, and axis z is perpendicular to the bounding surfaces
In this study the load support of the bearing formed by the un-coated glass surface sliding against the heterogeneous surface was measured and the results compared with theoretical values
A bearing system was obtained by sliding an untreated glass disc against a pin half coated with a layer which is not wetted by glycerol, the fluid used in this study
Summary
In lubricating systems, where the bounding solid surfaces are very close together and one of the dimensions of the fluid column is much smaller than the other two, a number of simplifying assumptions can be made, which reduce Navier–Stokes equations to the form given by Eq 1. Profile across the film thickness, with the approximation of two constants. Finding those constants and the full velocity profile can be done if some assumptions regarding the conditions of the interaction between the fluid and solid at the two boundaries are made. One of the hypotheses made in deriving this equation is that there is no slip between the fluid and the solid surfaces. This hypothesis is a cornerstone of lubrication and remains the foundation of Reynolds equation for lubrication. Once the velocity profile is known the fluid flow can be derived and using the continuity of flow principle the pressure gradient can be derived [1]
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