A hard disk
drive with the platters and spindle motor hub removed showing the copper colored
motor coils surrounding a bearing at the center of the spindle
The motor has an external rotor; the stator windings are copper-colored. The
spindle bearing is in the center. To the left of center is the actuator with a
read-write head under the tip of its very end (near center); the orange stripe
along the side of the arm, a thin printed-circuit cable, connects the read-write
head to the hub of the actuator. The flexible, somewhat 'U'-shaped, ribbon cable
barely visible below and to the left of the actuator arm is the flexible
section, one end on the hub, that continues the connection from the head to the
controller board on the opposite side.
The head support arm is very light, but also rigid; in modern drives,
acceleration at the head reaches 250 gs.
The silver-colored structure at the upper left is the top plate of the
permanent-magnet and moving coil "motor" that swings the heads to the desired
position. Beneath this plate is the moving coil, attached to the actuator hub,
and beneath that is a thin neodymium-iron-boron (NIB) high-flux magnet. That
magnet is mounted on the bottom plate of the "motor".
The coil, itself, is shaped rather like an arrowhead, and made of
doubly-coated copper magnet wire. The inner layer is insulation, and the outer
is thermoplastic, which bonds the coil together after it's wound on a form,
making it self-supporting. Much of the coil, sides of the arrowhead, which
points to the actuator bearing center, interacts with the magnetic field to
develop a tangential force to rotate the actuator. Considering that current
flows (at a given time) radially outward along one side of the arrowhead, and
radially inward on the other, the surface of the magnet is half N pole, half S
pole; the dividing line is midway, and radial.