Figure 4.45: The spindle motor with disks
As a consequence of the rapid growth in areal density in magnetic recording,
many manufacturers nowadays produce hard disk drives with only one disk or
even only one surface of a disk to get the required capacity. For disks with same
dimensions, reduction in number of disks can reduce the load of the spindle
motor as both windage loss and loss due to friction between slider and disk are
reduced. Use of fewer disk also lowers the cost of an HDD by reducing costs
of recording media, magnetic heads, and related electronic components.
Though higher speed of spindle rotation increases the loss in the spindle
system, high RPM is a desirable feature as it reduces the latency time of data
access and hence increases the rate of data transfer between the HDD and the
host computer .
4.3.4 Motor Con fi guration
The basic structure of the spindle motor is shown in Figure 4.45. The rotating
parts of the motor include the bearing cover, the rotor shell, the outer rings
and related parts of the bearing, the rotor yoke and the magnet. The inner
ring of the bearing and stator core are rigidly fi xed to the base of the motor.
Rotor yoke, made of steel, is necessary for the PMACM as it increases the
airgap fi eld produced by the permanent magnet. Yoke has another function in
motors with the outer rotor structure shown in Figure 4.46. The rated speed of
the spindle motor is very high which creates a strong centrifugal force acting on
the permanent magnet. The permanent magnet ring is made from the bonded
NdFeB material (see section 4.3.5) which is very weak in mechanical strength.
The rotor yoke protects the magnetic ring from the strong centrifugal force
during high speed operation.
Depending on the location of the electromagnetic components in the motor,
the spindle motors are classi fi ed into two types: Underslung motors and In-hub
motors. The one shown in Figure 4.45 is an underslung spindle motor, where
the EM part of the motor is located under the disks. Key components of the
motor are shown in Figure 4.46.