Magnets

[RogerP]

I stripped down an old hard drive to recover the magnet. The magnet remains bonded to a 3mm thick piece of the original HD metal framing .

The face of the magnet has extremely strong magnetic force (as you'd expect) BUT the rear of the 3mm thick metal the magnet is fixed to has almost no detectable force. Yet if I place a piece of 3mm metal on the front of the magnet (to mimic the one on the rear) the magnetic force is still very strong through that extra thickness of metal.

So, how is the 3mm metal plate to which the magnet is bonded manage to shield virtually all signs of the magnet's force?

 

[Zeddedhed]

Not being remotely scientific I can only offer you my laymans views:

a.) The Force
b.) Witchcraft
c.) Something to do with the Large Hadron Collider - you've created a black hole or summat.

After that I'm out of ideas.

 

[Eric The Viking]

Assuming you strip it down per normal, the ones I've stripped down have been actually two magnets on each side of the head actuator coil, and four magnets altogether in the whole assembly:

   [--s--] [plate] [--n--]
   [--n--]         [--s--]  

      O--[flat coil]--O

   [--s--]         [--n--]  
   [--n--] [plate] [--s--]

The head actuator coil is shaped as a segment of a ring - there are two straight sides that are on radii from the pivot point. Those sides of the coil ("O" above) are between the pole pieces. One is in the N-S field and the other side in the S-N field. The fields are supposed to be as even as possible.

They're very strong magnets because the arrangement isn't very efficient, yet there needs to be a strong force to move the heads fast.

They're like two horseshoe magnets: with a strong field across the poles, but no field at the back of the magnet.

The difference is that because there are matching N and S poles held close to each other by the assembly, the field is forced mainly to go up/down (in my diagram) rather than across between the poles on each side.

Hope that helps.

E.

 

[RogerP]

Thanks Eric, I hadn't stripped the magnet completely off.

 

[duncanh]

Assuming you strip it down per normal, the ones I've stripped down have been actually two magnets on each side of the head actuator coil, and four magnets altogether in the whole assembly:

E.

Odd. Out of the 20+ hard drives I've stripped down to salvage magnets they've all had just a single pair of magnets except for a very odd design which only 1 magnet. This is with regular 3.5" and laptop drives.