Direct drive, brushless DC spindle motors are known in the art for mounting rigid data storage disks for rotation. The disks are mounted around a hub or external rotor which is, in turn, mounted on the spindle of the motor. The motor components may be external to the spindle so that belts or other drive methods are used to rotate the spindle; or, the motor may be formed entirely within the spindle and hub assembly, as in a high capacity micro-Winchester disk drive.
The micro-Winchester disk drives have to work over a range of temperatures (typically 2 degrees C. to 55 degrees C.). This temperature variation, in addition to the heat generated internally, causes the assembly to expand according to the coefficients of expansion of the various materials forming the assembly components. As an example, the motor typically requires that a magnetic flux return path, also known as a yoke, be provided. Such a path is usually provided by a magnetic flux-carrying material, such as a rare earth ferroceramic element, or low carbon steel. In contrast, the hub or rotor, the bearing mounts for the spindle, and the base support for the motor are typically constructed from an aluminum alloy. The rigid storage disks mounted around the hub are generally made of aluminum and are coated with a suitable data storage medium.
In assembly of the motor, the yoke is typically attached to the inner surface of the aluminum alloy hub by bonding or gluing. Starting at the outer portion of the disk drive motor and progressing inwardly, the following contiguous materials are encountered: aluminum disks, the aluminum alloy hub, the yoke e.g. steel alloy flux return ring bonded to a portion of the aluminum alloy hub, aluminum bearing holder, bearing outer race, and the steel shaft. The coefficient of thermal expansion of low carbon steel is approximately six to nine microinches per degree Fahrenheit while that of an aluminum alloy hub having a low carbon steel yoke bonded to it is approximately 9 microinches per degree Fahrenheit. The aluminum disk coefficient of thermal expansion is approximately 13 microinches per degree Fahrenheit.
Changes in temperature within the motor during operation of the disk drive cause expansion and contraction of the hub and the magnetic flux return material. The hub tends to expand axially and radially outwardly with heat. The differences in the thermal expansion coefficients will result in more expansion and contraction of the aluminum alloy hub than that of the yoke material. Expansion and/or warpage may result at the interface between the different materials thereby interfering with optimal motor and disk drive performance e.g. causing change in the repeatable run out over temperature.
One prior approach to compensate for warping at the interface e.g. between the yoke and the hub has been to decrease the area of the bond in order to leave a gap between the aluminum and the steel to accommodate the disparate expansion characteristics. Another approach has been to shrink fit the aluminum over steel over a small portion thereby creating a connecting bead. One example of the second prior art approach is found in U.S. Pat. No. 4,717,977 to Brown.