How do the motors for read-write heads in HDs work?

[Paul Colby]

I've dissected several hard disk drives to scavenge the "linear" motors which move the read-write heads across the disk platens. At first glance these look simple enough. There is a pair of high energy permanent magnets in a yoke which provide a circular sector shaped high magnetic field in a gap ~1/8" wide. The yoke provides a good magnetic circuit with low leakage. The motor armature consists of a circular sector single coil winding loop that is held in the gap by a bearing such that it is free to rotate. When a current is applied to the winding a force is developed (as expected) from the $I\times B$. Now, from the magnet/yoke design I would expect a uniform $B$-field in the gap. If this is the case then $I\times B$ should sum to zero (provided the field is uniform). If it's not uniform then this is a really poor looking design. Obviously I'm missing something.

 

[Paul Colby]

Ah, I am missing something. The sector magnets flip polarity across the gap. Split one yoke and tested with a bar magnet. Order to my world is restored.