Handbook for Sound Engineers

Magnetic Recording and Playback 1085

the read/write/erase head assembly, and process the data
to and from the disk.
The spinning disk is an aluminum or glass disk
covered with a magnetic layer. Since smaller disks are
flatter and more rigid, the trend has been downward in
disk size from 14 inch diameter disks in 1960 to disks
ranging from 3.5 inch down to 1.8 inch diameter today.
Smaller disks can spin faster and rotation rates have
risen from about 3000 rpm for the 14 inch disks to
targeted speeds of 22,000 rpm for high-performance
small disks.

These higher rates and tighter tolerances are
exceeding the capabilities of the ball bearings that
support the spinning disk, requiring new types of bear-
ings. Fluid dynamic bearings replace the rolling balls in
a ball bearing with a film of oil that is less than
one-tenth the thickness of a human hair. In addition to
providing tighter tolerance, the fluid dynamic bearing is
quieter, longer lasting, and more rugged.

The spindle assembly includes an integral motor for
spinning the disk. The power required from the spindle
motor due to aerodynamic drag of the spinning disks is

(28-11)

where,

n is the number of platters,

Z is the angular velocity of rotation,

r is the disk radius.

Additional power is required to overcome the fric-

tion and viscous losses of the bearings.

The magnetic data on the disk is accessed by

magnetic heads flying over the surface of the disk. The

very close spacing between the head and disk is main-

tained by a cushion of air generated by the aerodynamic

design of the head. For our example drive, the

head-to-disk spacing is 0.6μin (15 nm) or the wave-

length of orange light.

Since the head cannot fly when the disk rotation

stops, provisions must be included to transition from

flying to nonflying status. Some drives land the heads

on a dedicated portion of the disk periphery appropri-

ately known as the landing zone. Other drives move the

head to an extreme position to engage a parking ramp

that holds the head away from the disk. The parking

ramp also provides protection from shock and vibration

incurred during shipping or handling of the computer.

Some of today’s disk surfaces are so smooth that the

head will literally stick to the surface after landing. To

overcome this stiction, the surface of the disk may be

textured with microscopic bumps. The bumps may

require an increase in the flying height, thereby

reducing the maximum storage density of the disk.

To avoid these problems, the example drive uses a

parking ramp at the center of the disk. The resulting

ability to use an untextured disk surface is a major

reason for this drive’s very high packing density.

The flying head is mounted on metallic spring matrix

called a gimbal that allows the head to assume the

proper flying attitude parallel to the disk surface. The

gimbal is at the end of a long support arm called a

flexure that cantilevers the head above the disk surface.

The flexure lightly presses the head onto the disk

surface to overcome the aerodynamic lift generated by

the head.

The flexure is attached to an actuator that moves the

head to the appropriate track of magnetic data on the

disk surface using either linear or rotary motion. The

linear actuator is very similar to the voice coil and

magnet of a loudspeaker. A current in the coil produces

a magnetic field that interacts with the field of the

permanent magnet to create a linear force along the axis

of the coil. A sled assembly with ball bearing wheels

Figure 28-54. Interior of a hard disk drive.

R/W head

Actuator

Spindle motor

Mechanical base

Data disk

Data disk

Spindle motor

R/W head

Mechanical base

Magnetic return plate

Actuator

Power/

interface

connectors

A. Current technology hard drive.

B. New technology hard drive.

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