[Asterisk-Users] UPS for Asterisk

[Peter Svensson]

On Sun, 23 Jan 2005, Andrew Kohlsmith wrote:

> Why would the heads come in contact with the platters on a powerfail?  The 
> arms are very rigid -- the heads only float a few thousandths of an inch over 
> the platters -- something that I don't believe has anything to do with the 
> platters spinning (that may *help* but I don't think the heads will contact 
> the platters if they're not spinning) and besides -- any drive manufactured 
> in the last 5 years will autopark on power fail...  There's an awful lot of 
> energy stored up in the spindle motor that is used to slam the heads into the 
> parking zone...

Actually, the only thing that keeps the heads off the platter is the fact 
that they are spinning. The movement of the platters cause an airstream 
which the heads float on. This airstream is what keeps the heads at just 
the right distance. The arms are not very rigid at all in the axis 
direction of the disks. This has been the standard design in hard disks 
since a very long time.

The comment about autopark is correct. Actually, with the voice coils used 
on modern disks the energy needed to retract the heads is already stored 
in the return spring. The platter energy is sometimes used to complete 
any sector write that is in progress. Some hard disks did not do this and 
those generated bad sectors every time they were powered down in 
mid-write.

Peter