The spin-axis motion has a component that is in phase and at the same frequency as the spindle rotation. This is repeatable runout. There is also a component of spin-axis motion that is random. Improved NRRO improves the seek time and ability to track follow for a given track pitch and servo mechanism. A disk drive with a low NRRO spindle bearing accommodates higher track density, allowing higher areal density.
Ball bearing spindle motors comprise the majority of disk drives on the market today.
The rolling elements in ball bearings produce a large NRRO because the boundary lubricant film allows mechanical contact with geometric defects in the ball-race interface. Consequently, ball bearings cause random vibrations in the disk pack. The FDB achieves lower levels of NRRO because a relatively thick film of lubricant separates the sleeve and the stator. The film provides viscous damping which attenuates NRRO below that achievable in ball bearings. The forces generated on the sleeve by the FDB are of a lower frequency and amplitude than with ball bearings.