Hardware Reference
In-Depth Information
Acoustic noise of HDD should be as low as possible, especially for applica-
tions in consumer electronics. Industrial and academic research over the past
years has been achieving continuous improvement in the NRRO and acoustic
performances of spindle motors. However, such improvements have reached
near saturation and these achievements are not enough to meet the require-
ments demanded by the rapid development of high density data recording.
Figure 4.54: Acoustic noise spectrum of a spindle motor with ball bearing
(5,400 rpm, PWM drive).
4.3.7.2 Fluid Dynamic Bearing Motors
Because of its excellent performances in reducing NRRO and acoustic noise,
fl
uid dynamic bearing (FDB) is gradually replacing ball bearings used in the
HDD spindles. In FDB, the function of the balls of the ball-bearing is realized
using an oil
fi
lm
fi
lled in the gap between the sleeve and the shaft. Grooves are
created on the shaft or sleeve as shown in Figure 4.55. When the shaft rotates,
pumping effect produced by the groves enhance the squeeze
fi
lm action [75]
generating hydrodynamic pressure wave in the oil
fi
lm and makes the rotating
surface
fl
oat on the oil
fi
lm. Because of this effect of levitation, the sleeve and
the shaft are not in contact. This helps to avoid local overheating. As there
is no solid connection between rotating and stationary parts, the problems of
horizontal and vertical movements caused by the contact defects or deformation
due to shock etc found in the rolling balls are eliminated.
FDB motors used in HDD spindle can be classi
fi
ed into two types according
to the operational state of the motor shaft - moving shaft and
fi
xed shaft -
depending on whether the shaft rotates or not. Figure 4.55 shows the basic
structure of a moving shaft FDB. The shaft of this kind of FDB is
fi
xed with
the rotor (Figure 4.56). For a spindle motor with
fi
xed shaft, the sleeve is
fi
xed
with the rotor, shown in Figure 4.57. As one end of the shaft is
fi
xedonthe
motorbaseandtheotherscrewedontoHDDcover,the
fi
xed shaft FDB motor
has better mechanical performance than the moving shaft FDB. However, the
construction is simpler for moving shaft FDB motor making it less costly.
