[comp.arch] Why not evacuate Disk drives.

deweeset@turing.cs.rpi.edu (Thomas DeWeese) (07/31/90)

The problem with evacuating disk drives is that the fact that they are not
evacuated is the major reason why the heads are able to fly so close to the
disk media.  Yes fluid mechanics is a *bitch* to deal with but it does
several nice things for you.  The air over a platter is moving with the platter
(at a relatively high speed) also the speed of the air also drops off quickly
as one moves away from the platter.  So what is done it that the head is 
shaped so that it will be blown upward by the air
eg.      |   |
         \   |
    ->    \  |
->  ->  -> \_|<- read-write head
->->->->->________________________Platter
^^^^^^^Air

So that if it tries to come in contact with the platter it is blown upward
but if it goes to high there will not be enough lift to hold it up and it will
come back down.  Without the air to act as a sperator you would have to do
a lot of fancy foot work like 
  1. Guess where the platter is (impossable to do with reasonable accuracy).
  2. Use range finding  hardware (expensive if possable).

and with both of these meathods you loose head crash protection.

Thomas DeWeese

eugene@wilbur.nas.nasa.gov (Eugene N. Miya) (07/31/90)

There are other alternatives to evacuation, long researched many years
ago by IBM and others (I worked at Information Magnetics 2 summers).
The easiest probably used sealed Helium.  Advantages and disadvantages
can be found in the literature.

--e. nobuo miya, NASA Ames Research Center, eugene@orville.nas.nasa.gov
  {uunet,mailrus,other gateways}!ames!eugene

sysmgr@KING.ENG.UMD.EDU (Doug Mohney) (08/01/90)

In article <7592@amelia.nas.nasa.gov>, eugene@wilbur.nas.nasa.gov (Eugene N. Miya) writes:

>The easiest probably used sealed Helium.  Advantages and disadvantages
>can be found in the literature.

Why helium? That's pretty expensive, I'd think. Why not something "common" 
like nitrogen?

lindsay@MATHOM.GANDALF.CS.CMU.EDU (Donald Lindsay) (08/01/90)

In article <0093A806.2A4E2160@KING.ENG.UMD.EDU> sysmgr@KING.ENG.UMD.EDU 
	(Doug Mohney) writes:
>Why helium? That's pretty expensive, I'd think. Why not something "common" 
>like nitrogen?

The speed of sound is higher in helium.  Actually, hydrogen is even
faster, but it's not chemically inert.
-- 
Don		D.C.Lindsay

skeeve@pawl.rpi.edu (Sean C. Cox) (08/02/90)

lindsay@MATHOM.GANDALF.CS.CMU.EDU (Donald Lindsay) writes:

>In article <0093A806.2A4E2160@KING.ENG.UMD.EDU> sysmgr@KING.ENG.UMD.EDU 
>	(Doug Mohney) writes:
>>Why helium? That's pretty expensive, I'd think. Why not something "common" 
>>like nitrogen?

>The speed of sound is higher in helium.  Actually, hydrogen is even
>faster, but it's not chemically inert.

       The speed of sound?? I thought that the lower viscosity would be the
  primary reason, lower viscosity=lower drag on the surface of the disk,
  as well as a smaller boundary layer (the layer of fluid the heads "float"
  on to avoid contact with the surface of the disk). The lesser drag helping
  rotation, but the smaller boundary layer being making it more difficult to
  "float" the heads.

                                          -Sean

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sritacco@hpdmd48.boi.hp.com (Steve Ritacco) (08/02/90)

/ hpdmd48:comp.arch / sysmgr@KING.ENG.UMD.EDU (Doug Mohney) /  1:56 pm  Jul 31, 1990 /
In article <7592@amelia.nas.nasa.gov>, eugene@wilbur.nas.nasa.gov (Eugene N. Miya) writes:

>The easiest probably used sealed Helium.  Advantages and disadvantages
>can be found in the literature.

Why helium? That's pretty expensive, I'd think. Why not something "common" 
like nitrogen?
----------

Maybe because the atoms are smaller!

davin@me.utoronto.ca (Davin Yap) (08/03/90)

In article <14900013@hpdmd48.boi.hp.com> sritacco@hpdmd48.boi.hp.com (Steve Ritacco) writes:
>
>>The easiest probably used sealed Helium.  Advantages and disadvantages
>>can be found in the literature.
>
>Why helium? That's pretty expensive, I'd think. Why not something "common" 
>like nitrogen?

Air is mostly (70%?) nitrogen.