Types Of Contaminants And Critical Elements - HP 7925D Service Manual

SMOKE PARTICLE
6.35 MICRONS
(250 MICROINCHES)
¢
DIRECTION OF ROTATION IS COUNTERCLOCKWISE.
SURFACE SPEED AT 2700 RPM IS 112 MILES PER
HOUR AT THE OUTER CIRCUMFERENCE.
7925
the fins of the heat sink on power and motor regulator
PCA-A9 and the remaining path is distributed over the
components mounted on the PCA. The flow of air from
the heat sink exits through the ducting and vent open-
ings provided at the rear of the enclosure.
The remammg half of the developed air flow passes
through the filtration element in the absolute filter where
99 percent of all contaminants 0.3 micron or larger are
trapped. After the air is thoroughly filtered, it is ducted
into the pack chamber. When a disc pack is installed, all
critical areas will be purged of any foreign matter. Also,
the high positive pressure developed within the pack
chamber tends to reject any foreign matter that may be
airborne.
Figure 1-12 shows the critical elements involved in the
read/write process, i.e., the read/write gap, the flying
height of the heads, and the thickness of the oxide coating
on the disc surfaces. The flying height of the heads is an
average value due to the surface irregularities of both the
READ/WRITE GAP
1.52 MICRONS
(60 MICROINCHES))
TYPICAL FLYING
HEIGHT AT
CYLINDER 822
0.69 MICRONS
~
(27 MICROINCHES)
-1.
I ....
~ n~
l l H ~
OXIDE COATING
J
THICKNESS
1.01 MICRONS
(40 MICROINCHES)
Theory of Operation
heads and discs. Figure 1-12 also shows various types of
contaminants and their size relationships. If a particle
was hard enough and of the right size, it could scratch
either the oxide coating or the head surface. Even if it was
not hard enough to scratch, it may be large enough to
increase the head-to-disc spacing, thereby causing data
errors.
Therefore, to prevent potential damage due to head-to-disc
contact and possible data losses, it becomes extremely
important to maintain the cleanliness of the air supply
within the disc drive. To ensure that clean air will be
present, the disc drive must be operated in the specified
environment and the cleanliness of the prefilter and effi-
ciency of the absolute filter must be checked on a regular
basis. Refer to Section II, Maintenance, for the absolute
filter output air pressure measurement procedure.
Further, the absolute filter must be changed whenever the
air flow through it becomes restricted and the output air
pressure drops below the value specified in paragraph
2-13. Refer to Section V, Removal and Replacement, for
the prefilter and absolute filter replacement procedures.
~
ALUMINUM SUBSTRATE
0.19 CENTIMETRE
(0.075 INCH)
REF 7301-46
Figure 1-12. Types of Contaminants and Critical Elements
1-31