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Hard disk drive (HDD) designs
have continued to evolve over time to meet the ever-changing
requirements of applications, performance and cost. Ball
bearing spindle motors have been the common design-in for HDD's
for many years; however, the market is shifting toward a different
design known as Fluid Dynamic Bearings (FDB).
Areal densities of HDDs have increased at significant rates of
more than 100 percent. As this trend continues newer
technology is required to keep up with the higher densities,
faster speeds and lower acoustics. The latest technology;
Fluid Dynamic Bearing Spindle Motors, this technology is
attractive for minimizing Non Repeatable Runout (NRRO), lowering
acoustical noise and improving reliability.
Ball Bearing (BB) spindle motors
comprise the majority of shipments in HDDs today. A
transition to FDB is starting to occur today, especially in the
mobile HDD market. NRRO is the highest contributor to track
mis-registration and track following, thus impacting HDD
performance. NRRO is also considered an inhibitor in
achieving higher densities. BB motors produce larger NRRO
due to mechanical contact with the inherent defects found in the
geometry of the race ball interface and the layer of the lubricant
film. BB spindle motors have minimized this issue with
tighter tolerances and closer inspections. There is an upper
limit at which the BB design can no longer overcome the NRRO
problem at higher densities. Currently with BB, NRRO has
settled in the 0.1 microinch range.
By contrast, FDB generates less
NRRO due to the higher viscosity of lubrication oil between the
sleeve and stator. FDB designs are expected to limit the
NRRO in the range of .01 microinch, thus making the HDD operation
faster. Other inherent properties of the FDB design are
higher damping, greater speed control, reduced frequency
resonance, better non-operational shock resistance and improved
acoustics since there is no contributing noise from ball
bearings.
While FDBs are being improved to
overcome the issues associated with temperature characteristics
related to power consumption, their overall advantage to achieving
high areal density, minimized NRRO, and better acoustics makes
them attractive for future HDD designs. FDB motors will
continue to be used more frequently in HDD designs, particularly
in applications requiring very high spindle speeds, high areal
densities, or lower acoustic noise. FDB motors will be the
standard design-in for future HDDs incorporating these advanced
technologies.
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