One distinguishing characteristic of hard disk technology that makes it different from
how floppy disks, VCRs and tape decks work, is that the read/write heads do not make
contact with the media. The reason for this is that due to the high speed that the hard
disk spins, and the need for the heads to frequently scan from side to side to different
tracks, allowing the heads to contact the disk would result in unacceptable wear to both
the delicate heads and the media. In fact the earliest hard
disks did have their heads in contact with the media, and this design was changed due
to the wear that contact caused.
Modern drive heads float over the surface of the disk and do all of their work without
ever physically touching the platters they are magnetizing. The amount of space between
the heads and the platters is called the floating height or flying height.
It is also sometimes called the head gap, and some hard disk manufacturers refer to
the heads as riding on an "air bearing". The read/write head assemblies are
spring-loaded--using the spring steel of the head arms--which
causes the sliders to press against the platters when the
disk is stationary. (This is done to ensure that the heads don't drift away from the
platters; maintaining an exact floating height is essential for correct operation.) When
the disk spins up to operating speed, the high speed causes air to flow under the sliders
and lift them off the surface of the disk--the same principle of lift that operates on
aircraft wings and enables them to fly.
|
A pair of mated head sliders with their platter
removed.
You can see that the tension of the head arms has caused
them to press against each other. |
Due to the very small distance from the heads to the platters--normally measured in
millionths of an inch--the hard disk is assembled in a clean room containing air
specially filtered to remove all but the tiniest particles. Air however is required
for the heads to function. Whenever someone suggests that the inside of a hard disk is
maintained under a vacuum--and it always happens--just ask them how exactly the heads can
float on the surface of the disk if there is no air. 
You will also hear people say
that the drive's interior is "sealed" (including, I must admit, myself at one
point). This is also generally untrue: while the disk's internal environment is separate
from the outside air to keep it clean, air exchange is permitted between the outside and
inside of the drive to allow the drive to adjust to changes in air pressure. A special
"breather" filter is installed to prevent foreign matter from contaminating the
drive; see here for more details.
Note: If a drive is used
at too high an altitude, the air will become too thin to support the heads at their proper
operating height and failure will result; special industrial drives that truly are
sealed from the outside are made for these special applications.
The distance from the platters to the heads is a specific design parameter that is
tightly controlled by the engineers that create the drive. By adjusting the strength of
the springs to match the other drive parameters (such as the speed the disks are spinning
and the size and shape of the heads) the float height can be precisely maintained. If the
height is too great, the heads can't properly read and write the platter. If it is too
small, there is increased chance of a head crash
(ouch.) As mentioned in the section on operation,
increasing areal density means that weaker magnetic fields must be used in storing data on
the disks. When this is done the heads must be allowed to ride closer and closer to the
platter surface to pick up the weaker signals, which requires other quality improvements
to the drive to make sure that there is no chance of a head crash (ouch. )
Tip: Some modern
drives include sensors that monitor the flying height of the heads and signal a warning if the parameter falls out of the acceptable
range.
It's actually quite amazing how close to the surface of the disks the heads fly without
touching. To put it into perspective, a modern hard disk has a floating height of an
amazing 0.5 microinches. A human hair has a thickness of over 2,000 microinches! You can
see why keeping dirt out of the hard disk is so important! In fact, the floating height of
a hard disk is smaller than the circuit size
of a microprocessor. What's even more amazing is how much abuse these hard disks can
take when they are placed in laptop PCs, for example, given these facts, and how many
people take this technology for granted every day...
|
This illustration gives you some idea of just how small
the flying
height of a modern hard disk is (and today's hard disks have
flying heights significantly lower than 3-7 millionths of an inch! |
Image © Quantum
Corporation
Image used with permission. |
When the areal density of a drive is increased to
improve capacity and performance, the magnetic fields are made smaller and weaker. To
compensate, either the heads must be made more sensitive, or the floating height must be
decreased. Each time the floating height is decreased, the mechanical aspects of the disk
must be adjusted to make sure that the platters are flatter, the alignment of the platter
assembly and the read/write heads is perfect, and there is no dust or dirt on the surface
of the platters. Vibration and shock also become more of a concern, and must be
compensated for. This is one reason why manufacturers are turning to smaller platters, as well as the use of glass platter substrates. Newer heads such as GMR are preferred because they allow a higher flying height than
older, less sensitive heads, all else being equal.
As the flying height of drives continues to decrease, hard disk engineers are
recognizing that we may soon reach the point where it cannot be made any smaller without
touching the surfaces of the platters. There is actually talk about the possibility of
going back to the concept of contact disks, where the head gap is intentionally
made zero. This would allow even smaller magnetic fields than is possible in today's
drives. Of course, this brings us full circle to the first hard disk experiments in the
1950s! The difference of course is almost 50 years of advances in technology. For example,
thin film media is much tougher than the oxide media used
on contact disks half a century ago, and lubricating agents are much more advanced as
well. Even so, it will probably be several years before we know if this technology will be
feasible from both an engineering and manufacturing standpoint.
Next: Head Crashes
