Entrance and Stationary Roughness Effects on the Load Carrying Capacity of a Wide Wedge Gas Bearing

Abstract

The objective of this work is the determination of the effects of surface roughness amplitude and inlet conditions on ultra-thin, compressible, isothermal, infinitely-wide gas bearings. The method of study is numerical in nature and consists of a second-order accurate finite difference solution of the Reynolds equation of lubrication without molecular slip for a range of bearing numbers spanning six orders of magnitude. The motivation for this work comes from the magnetic disk drive industry where ever decreasing head flying heights are being sought to increase the recording density. Past studies by these authors of the infinitely-wide air bearing with stationary roughness have shown that, unlike in the case of a smooth bearing, the load peaks at a finite bearing number comparable to that at which current rigid disk drives operate. This suggests that it may be possible to arrange the roughness pattern in such a way as to cause the slider to separate from the hard disk more rapidly, minimizing wear to both surfaces. A wedge bearing with stationary sinusoidal roughness is studied for different roughness amplitudes and two phases. It is shown that for all configurations considered, the load exhibits a peak unlike in the case of the smooth bearing where the load monotonically reaches a peak at an infinite gas bearing number. Two rough sliders with flat tapers smoothly attached to their leading edge are also studied to answer questions regarding the role that the inlet condition plays in the resulting magnitude and behavior of the generated load. The leading taper allows the bearing to dynamically determine the entrained flow rate and maximum pressure as well as to self prescribe the inlet condition at the leading edge of the first roughness wave. The inlet conditions prescribed by the developing flow in the flat taper region still give rise to a peak in the load. The addition of the smooth taper, however, causes an overall decrease (increase) in the load when the roughness waves are entirely above (below) the plane of the taper compared to the results of the rough bearing with no taper. It is demonstrated that all the considered roughness patterns result in a peak load at a finite bearing number. Of special interest are two bearing configurations: one composed of a smooth taper followed by a transversely roughened slider and the other is a rough slider with a transverse roughness pattern whose slope at the inlet of the bearing is negative. Both are shown to achieve a maximum lifting force at low bearing numbers, providing rapid separation while alleviating the narrow rail manufacturing problem.