An endeavor has been made to investigate the effect of transverse surface roughness on the behaviour of thin film lubrication at nano scale of a magnetic fluid based rough porous rotating circular step bearing. Mainly, the combination of the properties of the surfaces, the lubricant and viscosity of the lubricant are responsible for thin film lubrication between two rough surfaces in relative motion. The effects induced by the transverse roughness and the couple stress cannot be disregarded in the regime while the ordered molecules dominate the fluid field. The random roughness of the surfaces is characterized by a random variable with non zero mean, variance and skewness. The associated Reynolds’ equation is then stochastically averaged and solved with appropriate boundary conditions to obtain the pressure distribution, leading to the calculation of load carrying capacity. It is easily observed that basically, the magnetic fluid lubricant combined with the couple stress effect is responsible for the improved performance of the bearing system. It is clearly seen that the adverse effect of transverse roughness is relatively less when considered with thin film lubrication at nano scale. The increased load carrying capacity due to variance (-ve) gets further increased due to negatively skewed roughness which becomes more pronounced owing to thin film lubrication at the nano scale. It is seen that the existence of couple stress enhances the load carrying capacity. In addition, the characteristic length contributing to the couple stress increases load carrying capacity considerably. Even, size dependent effects are noticed in the lubrication with couple stress while the thinner the lubrication film the more obvious is the effect.

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