Figure 4.53: Movements of balls in the ball bearing.
As there is clearance between the cage, the rolling balls and raceway rings
and the waviness on the component surfaces, the position and state of each
ball are uncertain in the motor operation, even at constant rotational speed.
These uncertainties make the center of the rotating ring move irregularly as a
function of time causing NRRO.
Increase in both track density and linear density is necessary to meet the re-
quirements for rapidly growing demand for higher areal density. The runout of
spindle motor becomes an impediment to the realization of high density of data
tracks. However, the RRO is repeatable and therefore can be modelled. Its
in fl uence on the tracking error of head positioning servomechanism can be mit-
igated using precision actuator and advanced servo control algorithms. Such
methods are explained in details in chapter 3. However, the motion caused by
NRRO is uncertain and, therefore, cannot be modelled. Its spectrum spans a
signi fi cantly wide range of frequencies making it diﬃcult to be compensated
by the feedback control. Even though the movement due to NRRO of spindle
motor contributes to displacement of quite small magnitude (in the scale of
of 10 −8 m), the NRRO has become the major factor limiting the achievable
track density. This issue therefore drew attention of many researchers over
past years. Besides compensating in the head positioning servo control loop,
killing the source, i.e., reducing effectively the NRRO of the spindle motor
itself has been an important research topic for years.
In ball-bearing spindle motors, all balls are in contact with the raceways
and each of these contacts occurs on a very small area. This makes the spin-
dle very sensitive to mechanical shock. This problem is particularly severe for
hard disk drives used in mobile applications. Ball-bearings are also the major
contributors to the acoustic noise in the hard disk drives. This fact was exper-
imentally veri fi ed by researchers . The noise of the ball-baring is generated
by the mechanical contacts between two solid parts with one moving relative to
the other, e.g., balls to rings, balls to cage, and cage to rings. The regular and
irregular movements of the rolling elements make the spectrum of the acoustic
noise rich in frequencies. Spectrum of the acoustic noise generated by a spindle
motor running at 5,400 RPM is shown in Figure 4.54.