| Hard disk drives (HDDs) are now the most important means of information storage. They continue to be made smaller in size, higher in capacity, and lower in cost. Recently, the minimum spacing under the slider is within the order of several nanometers, the gas dynamics cannot be described from the continuum transport theory directly, and the convectional lubrication theory developed for gas bearings cannot be applied to these kinds of bearing, and the gaseous rarefaction effects must be taken into account in this case.To more accurately simulate the pressure distribution of the ultra-thin air bearing film, Reynolds equation must be modified to account for the gas rarefaction effect and surface roughness et al. Started from a widely used FK model of Reynolds equation, four new simplified models, called continuous FK (CFK) model, second order FK (SOFK) model, simplified FK (PFK) model and precise second order (PSO) model, are proposed to simulate the ultra-thin air bearing film in HDDs in this paper. Those new models are solved using a least square finite difference (LSFD) method and the resultant numerical results are compared with those of FK model. Numerical results Shows that CFK model has modified the defects at the junctions of FK model on the basis of higher accuracy in simulating the air bearing film, and SOFK model and PFK model have higher computational efficiency and higher accuracy than those of FK model, and PSO model has less computational time and high accuracy than those of FK model. In addition, pressure distributions of several typical two-dimensional sliders have been simulated based on the four new simplified models.Generalized non-dimensional Reynolds equation including pressure flow factor and shear flow factor based on PSO model is solved by using LSFD method. Effects of the surface roughness on static characteristics of the air bearing film in HDDs with ultra-low flying heights (1-2 nm) are investigated. Numerical results show that effects of the surface roughness on the pressure distributions and the load carrying capacity are obvious, while effects of the surface roughness on the pressure center are small. Lower flying height and/or greater value of surface roughness will lead to higher influence on the pressure distribution and the load carrying capacity. With the same value of surface roughness, the rough head with transverse roughness leads to the maximum pressure distribution and load carrying capacity of the system. The surface roughness do not have much influence on the pressure center, which indicates that the head/slider has good flying stability. Moreover, based on PSO model to investigate the effects of the surface roughness on static characteristics of air bearing films in HDDs, we can not only obtain good accuracy in numerical results but also achieve very high computational efficiency.The influence of surface accommodation coefficient (AC) is an important factor to govern the static characteristics of the head/slider. Started from the polynomial logarithm fitting equations of Poisueille flow rate and Couette flow rate, a new simplified molecular gas film lubrication (MGL) equation is proposed to simulate the ultra-thin air bearing film in HDDs. The new simplified MGL equation is solved by using LSFD method. Effects of ACs on the static characteristics of air bearing films in HDDs with ultra-low flying height (1nm) are investigated. Numerical results show that effects of ACs on the static characteristics are obvious for the case of symmetric molecular interaction. effects of ACs at the disk surface on the static characteristics are obvious for the case of non-symmetric molecular interaction, while effects of ACs at the slider surface on the static characteristics are weak. On the other hand, effects of ACs on the static characteristics of air bearing films in the free molecular flow region are investigated. Numerical results show that we can change the distributions of the shear stress while not affect the pressure distributions by adjusting the surface AC. |
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