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From: amr@rti-sel.UUCP (Alan Roberts)
Newsgroups: net.micro
Subject: Re: CDC Wren drives, was Re: Hard Disk Noise
Message-ID: <837@rti-sel.UUCP>
Date: Mon, 2-Jun-86 09:56:26 EDT
Article-I.D.: rti-sel.837
Posted: Mon Jun  2 09:56:26 1986
Date-Received: Wed, 4-Jun-86 00:35:26 EDT
References: <1318@ihlpa.UUCP> <2413@islenet.UUCP>
Reply-To: amr@rti-sel.UUCP (Alan Roberts)
Distribution: net
Organization: Research Triangle Institute, NC
Lines: 41
Keywords: CDC, Hard Disk Drives

In article <191@ranhome.UUCP> randy@ranhome.UUCP (Randy Horton) writes:
>[deleted text]
>be very reliable. There does seem to be one small problem that is particular
>to this model however.  It seems that when you turn them off, sometimes the
>head gets stuck to the platter (that is the explanation an engineer gave me).
>When you turn them back on, they do not spin up, unless you give the computer
>a good whack!

This problem is known as "striction," I believe, and is a common problem
with all brands of small hard disks, not just Wrens.  It is more likely
to happen as the drives get older, and is ultimately one of the factors
that limits lifetime of the drive (assuming nothing else kills it first).

As I understand it, when the head(s) on a hard disk land during power-off,
they tend to polish the landing area.  After some time, the landing surface
fits so smoothly against the head that NO air remains between the two
surfaces.  The head then "sticks" to the surface with whatever force the
ambient air pressure exerts.  Its the same effect that makes gauge blocks
stick.  What happens when you next power up depends on the specifics of 
the spindle motor and head assembly.  Either it holds the platter and 
keeps it from spinning, or the torque developed tears the head off the
flexure!  If its the former, then the "whack" will often dislodge the
two.  You'll know if its the latter, the sound of the loose head bouncing
around will tip you.

This "feature" of hard disks is the counter argument to always parking
the heads over a landing zone and powering off the drive, a policy that
I have heard recommended by most computer sales types.  Always landing 
the heads, and in the same place, is probably going to hasten the day
when they stick.  I believe the best policy for your hard disk is to
move the heads over a landing zone (in case of power failure), but leave
the drive up, assuming that your environment is sufficiently cooled and
your power is stable.
-- 


					Cheers,

					Alan Roberts
					Research Triangle Institute
					(decvax!mcnc!rti-sel!amr)