Hardware Reference
In-Depth Information
An HDD usually has one read/write head for each platter surface (meaning that each platter has two
sets of read/write heads—one for the top side and one for the bottom side). These heads are
connected, or ganged , on a single movement mechanism. The heads, therefore, move across the
platters in unison.
Mechanically, read/write heads are simple. Each head is on an actuator arm that is spring-loaded to
force the head into contact with a platter. Few people realize that each platter actually is “squeezed”
by the heads above and below it. If you could open a drive safely and lift the top head with your
finger, the head would snap back down into the platter when you released it. If you could pull down
on one of the heads below a platter, the spring tension would cause it to snap back up into the platter
when you released it.
Figure 9.11 shows a typical hard disk head-actuator assembly from a voice coil drive.
Figure 9.11. Read/write heads and rotary voice coil actuator assembly.
When the drive is at rest, the heads are forced into direct contact with the platters by spring tension,
but when the drive is spinning at full speed, air pressure develops below the heads and lifts them off
the surface of the platter. On a drive spinning at full speed, the distance between the heads and the
platter can be anywhere from 0.5 μ-inches to 5 μ-inches or more in a modern drive.
In the early 1960s, HDD recording heads operated at floating heights as large as 200 μ-inches to 300
μ-inches; today's drive heads are designed to float as low as 10nm (nanometers) or 0.4μμ-inches
above the surface of the disk. To support higher densities in future drives, the physical separation
between the head and disk is expected to drop even further, such that on some drives there will even
be contact with the platter surface. New media and head designs will be required to make full or
partial contact recording possible.
Caution
The small size of the gap between the platters and the heads is why you should never open the
disk drive's HDA except in a clean-room environment. Any particle of dust or dirt that gets
 
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