Simulation and Experimental Verification to Investigate the Oil Behavior for the Fluid Dynamic Bearings with Double Seals

Abstract

Fluid dynamic bearings (FDBs) have used to support hard disk drives (HDD) since FDBs in HDDs generate a flow pressure through the fluid lubricant and it prohibits the solid contact between a shaft and sleeve that are rotating and stationary part. The FDBs can be classified into two types according to rotating or tied shafts. The FDBs with the rotating shaft have a seal which is located to upward part of upper thrust bearing and the shaft combined with rotating parts of HDD rotates during operation. On the other hand, the FDBs with the tied shaft have double seals which are upper and lower seals and the sleeve combined with rotating parts of HDD rotates during operation. HDDs with the tied shaft are more robust under external shock than those with the rotating shaft because both top and bottom of the shaft are fixed to cover and base of HDDs, respectively. However, the manufacturing and assembling processes of the HDD spindle motor with the tied shaft are more complicated and it takes a longer time to inject oil lubricant into the FDBs compared to the rotating shaft. Also, the FDBs with the tied shaft are more vulnerable to oil leakage than those in the rotating shaft because the FDBs in the tied shaft have double seals located both at the upper and lower air-oil interfaces. Oil may leak out if the air-oil interfaces of upper or lower seals may break with the increase of rotating speed because increase of rotating speed generated high flow pressure inside FDBs and it can occur oil leakage on FDBs with tied shaft. Firstly, this dissertation proposes the method to predict the oil injection time of FDBs with tied shaft. Since prediction of oil injection time is important to reduce manufacturing and assembling processes of the HDD spindle motor with the tied shaft, Kirchhoff’s pressure law was used to formulate equation of oil injection time. The proposed method was verified by measurement of oil injection time. Secondly, this dissertation develops the method to anticipate the height of upper and lower air-oil interfaces of the FDBs with tied shaft in non-operating. To predict the height of upper and lower air-oil interfaces during non-operating condition, linear pressure and volume equations were formulated the in cases of non-operating. The initial heights of upper and lower air-oil interfaces are determined by pressure and volume equations during non-operating condition. Pressure equation consists of capillary pressure, atmospheric pressure, hydrostatic pressure and applied pressure, and volume equation consists of the volumes of the clearance and the injected oil. The linear pressure and volume equations are solved simultaneously to calculate the heights of upper and lower air-oil interfaces in non-operating. Proposed method was verified by measuring the height of upper air-oil interface in the non-operating condition. Finally, this dissertation develops a method to predict the height of upper and lower air-oil interfaces of the FDBs with tied shaft in operating. To calculate the height of upper and lower air-oil interfaces of the FDBs with tied shaft during operating condition, hydrodynamic pressure generated by grooved bearings is additionally considered in the pressure equation, because heights of air-oil interfaces vary due to hydrodynamic pressure during operating condition. The linear pressure and volume equations are also solved simultaneously to calculate the heights of upper and lower air-oil interfaces in operating. The hydrodynamic pressure generated by grooved bearings was calculated, and solved the linear equations repeatedly with hydrodynamic pressure to calculate the heights of upper and lower air-oil interfaces. The results are verified by measuring the height of upper air-oil interface in the operating condition.