Maximizing Minimum Pressure in Fluid Dynamic Bearings of Hard Disk Drives

Engineering design of computer hard disk drives is a continuously evolving area where fierce competition drives innovations in smaller, faster, higher-density designs. Different elements of the overall hard drive design involve various branches of science and engineering including materials science, electromagnetics, control theory, electrical engineering, mechanical engineering and fluid dynamics. The focus of our current work is the central spindle which supports the rotating magnetic platters which hold all of the data, see Fig. 1. The spindle must operate with great precision and stability at high rotational speeds. Design practice has converged on oil-lubricated hydrodynamic journal bearings as the most common choice for spindles. That is, a layer of viscous oil separates a rotating shaft (the bearing) from the fixed outer sleeve (the journal), see cross-section in Fig 1(right). In hard drives, it is very important for the shaft to be centered within the sleeve. Plain journal bearings (i.e. both surfaces are circular cylinders) are unstable to perturbations that push the shaft off-center. It was found that this stability problem can be overcome by cutting diagonal grooves into the journal in a pattern called a herring-bone, see Fig. 2 [4,8,13,14,17,18]. Another consequence of this design is that very high pressures are generated by the grooves as they drive the oil to the middle of the bearing, away from the top/bottom ends of the spindle. This pumping action generally works to oppose leakage out of the bearing [8, 14,17,18]. In this report we examine how choices for the groove pattern can influence the key properties of the bearing. The focus is to understand the effect of the groove geometry on the pumping action. In particular the undesirable behavior caused by the low pressures created near the top/bottom ends of the bearing which, under many conditions, may result in the pressure becoming negative, relative to atmospheric pressure. Negative pressure can result in cavitation or, when it occurs near an air-oil interface, can cause air to be ingested and hence create bubbles. Any bubbles in the oil can corrupt the lubricating layer in the bearing [11, 16] and, as they are created and collapse, can cause significant undesirable vibrations. The negative pressures have therefore been identified