Disk drive with reduced thermal expansion induced disk slip

A disk drive comprising:

• a base;
• a spindle motor attached to the base, the spindle motor comprising:
• a shaft defining a longitudinal axis;
• an aluminum hub concentrically positioned about the shaft, the hub including an axial wall having an inner surface extending along a direction of the axis and including a flange extending radially from a bottom of the axial wall, the flange defining an upper disk receiving surface and an opposing lower surface;
• a bearing positioned between the hub and the shaft;
• a magnetic steel back iron including an upper portion and a lower portion, the upper portion abutting the inner surface, and the lower portion extending below the disk receiving surface, the lower portion being spaced-apart radially from the inner surface thereby forming a single gap between the back iron and the hub, the single gap extending from a location above the disk receiving surface to the opposing lower surface, the magnetic steel back iron further terminating at an end that is below the opposing lower surface;
• a magnet attached to the back iron, the magnet having an axial length which is substantially coextensive with an axial length of the back iron;
• and
• a disk mounted on the hub.

A disk drive comprising:

• a base;
• a spindle motor attached to the base, the spindle motor comprising:
• a shaft defining a longitudinal axis;
• a hub concentrically positioned about the shaft and made from a first material having a first thermal coefficient of expansion, the hub including an axial wall having an inner surface extending along a direction of the axis and including a flange extending radially from a bottom of the axial wall, the flange defining an upper disk receiving surface;
• a bearing positioned between the hub and the shaft;
• a magnetic steel back iron including an upper portion and a lower portion terminating at an end, the upper portion abutting the inner surface, and the lower portion extending below the disk receiving surface, the lower portion being spaced-apart radially from the inner surface thereby forming a single gap between the back iron and the hub, the single gap extending from a location above the disk receiving surface;
• a magnet attached to the back iron, the magnet having an axial length which is substantially coextensive with an axial length of the back iron, the magnet further terminating at an end that is aligned with the end of the back iron;
• and
• a disk mounted on the hub, the disk having a substrate made from a second material having a second thermal coefficient of expansion which is substantially the same as the first thermal coefficient of expansion.