Spindle Motor Control - Hard Disk Drive: Mechatronics and Control - page 257

Figure 4.52: Structure of the ball bearing used in spindle motor
These bearings are small in size and, therefore, sensitive to the ingress of
foreign particles. It is important to take measures to prevent external particles
from entering the inside of bearing races. Rolling balls made of steel are usually
used in ball bearings. Many manufacturers use ceramic ball bearings in order
to obtain improved performance of the motors operating at high speed. Rolling
balls of these bearings are made of ceramic materials like silicon nitride and
zirconia, which possess properties such as good heat resistance and low thermal
expansion, and have longer lifespan.
4.3.7.1 Spindle Motor Runout
The runout induced during the motor operation is an important issue in the
selection of bearing used in the spindle motors. The concept of runout was
explained in chapter 2 and in section 4.3.1 of this chapter. Runout is the
lateral and vertical movement of the spindle away from its nominal rotating
motion. Some of these motions are periodic and repeat themselves at regular
intervals; these are called repeatable runout (RRO). There are other types of
eccentricity in the rotation of spindle which do not repeat, and are known as
non-repeatable runout (NRRO).
Factors affecting the RRO of the spindle motor are mostly related to the
defects found in motor components such as bending of rotor shaft, misalign-
ment of the rotor etc. It may also be caused by the problems in bearing, e.g.,
misalignment of bearing components. Factors contributing to NRRO, on the
other hand, are complex and difficult to be attributed to any speci fi cfeature
of the spindle components. In the ball bearing operation, all the balls run in
the raceways between the inner and outer rings, and also rotate about their
own axes. The speeds of rotation are different for different balls and they vary
with time. The direction of rotational axis of a ball changes randomly with
time (see Figure 4.53). When the inner ring of the bearing is fi xed and outer
ring rotates from position 1 to 2 and 3, all the balls rotate with asynchronous
speed, and the axis of each ball may rotate in different directions.