Magnetic circuit structure for a rotary actuator in a disk drive

A voice coil motor having a magnetic circuit structure in a disk unit, comprising:

• a yoke formed of a plurality of steel plates stacked along a first direction, said plurality of steel plates having a lowermost steel plate and an uppermost steel plate a side edge of each of said plurality of plates forming a circumferential surface along a second direction orthogonal to the first direction;
• a magnet mounted on said circumferential surface of said yoke, said magnet having two circumferential ends and having upper and lower surfaces, said upper and lower surfaces of said magnet being formed in the second direction at a substantially central portion of said magnet with first and second shallow grooves, respectively;
• said uppermost steel plate of said plurality of steel plates having a first projection seated in said first shallow groove formed on said upper surface of said inner magnet;
• said lowermost steel plate of said plurality of steel plates having a second projection seated in said second shallow groove formed on said lower surface of said magnet;
• a side yoke attached to said yoke by magnetic forces of said magnet;
• said circumferential surface of said yoke having a first pair of shoulders against which both circumferential ends of said magnet abut, whereby said first pair of shoulders prevent said magnet from moving in a circumferential direction with respect to said yoke,
• wherein said circumferential surface of said yoke and said first surface of said magnet are secured exclusively by magnetic attraction, without any adhesive between said circumferential surface and said first surface.

A magnetic disk unit comprising:

• a housing having a base and a cover;
• a magnetic disk rotatably mounted in said housing, said magnetic disk having a plurality of tracks;
• a magnetic head for writing and reading data to and from said magnetic disk;
• an actuator arm assembly rotatably mounted on said base, said actuator arm assembly having one end to which said magnetic head is mounted and another end to which a coil is mounted;
• and
• a voice coil motor including said coil and a magnetic circuit fixed to said base, said coil being operatively inserted in said magnetic circuit;
• said magnetic circuit including a yoke formed of a plurality of steel plates;
• a magnet mounted on a circumferential surface of said yoke and having two circumferential ends, said magnet further having upper and lower surfaces, and being formed with first and second shallow grooves, respectively, at a substantially center portion of said upper and lower surfaces, said circumferential surface of said yoke having a first pair of shoulders against which both circumferential ends of said magnet abut, whereby said first pair of shoulders prevent said magnet from moving in a circumferential direction with respect to said yoke;
• a first projection on an uppermost steel plate of said plurality of steel plates, projecting from said yoke and seated in said first shallow groove formed on said upper surface of said magnet;
• a second projection on a lowermost steel plate of said plurality of steel plates, projecting from said yoke and seated in said second shallow groove formed on said lower surface of said inner magnet;
• and
• a side yoke attached to a side of said yoke by magnetic forces of said magnet
• wherein said circumferential surface of said yoke and said first surface of said magnet are secured exclusively by magnetic attraction, without any adhesive between said circumferential surface and said first surface.