Surface roughness-induced hydrodynamic gas lubrication theory including rarefaction effects is used to explain the effects of surface roughness on the magnetic head-medium spacing. Since video-recording operates at h/σ (film thickness/standard deviations of composite roughness) ≤ 3, the spacing increases with the roughness because the head surface remains at the distance on the order of 3σ from the mean of the tape surface. Data processing tapes operate at h/σ ≥ 4, and the increase in spacing with the roughness is found to be due to roughness-induced squeeze films. Squeeze motion can be generated by the isolated high asperities on the tape surface and bearing load variations due to a moving roughness (in shear flow) which are expected to increase with an increase in the surface roughness, and modulations of surfaces from other instabilities. Increase in signal-to-noise ratio and in amplitude variation for a rougher tape is also explained by the surface roughness variations. Influence of surface roughness on the head-disk spacing is also analyzed. Recommendations are made for an optimum roughness orientation and magnitude and whether the roughness should lie on the stationary or moving surfaces.

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