1. Field of the Invention
The present invention relates to a method of centering a single disk or multiple disks for a disk drive on the hub of a disk drive motor and an apparatus for performing disk centering.
2. Description of the Related Art
Motors in disk drives usually spin the disk mounted on the motor at approximately 3600 rpm. The sensitivity of disk drives to vibration requires that the disk and motor assembly be balanced so that the motor assembly does not vibrate beyond a specified degree during operation of the disk drive. The disk and motor are usually balanced with weighted screws threaded into the motor or by using other weighing methods.
FIG. 1A illustrates a disk drive motor 10 having a first type of hub 12. Hub 12 has a first hub portion 13.sub.1 which protrudes into the center hole of disk 14 and a second hub portion 13.sub.2 which protrudes through the center hole of disk 14 and through disk clamp 16. Disk 14 is secured to motor 10 by disk clamp 16 and screws 18.sub.1-2. Disk 14 has a ring-like shape with an outer diameter 20 and an inner diameter 22; inner diameter 22 defines the center hole of disk 14.
If weighted screws are to be added for balancing, more holes than are necessary to hold disk 14 in place are provided in the hub 12 of motor 10 and the weighted screws are threaded into the extra holes to balance disk 14. In addition, screws 18, which are usually all of the same weight, may be replaced with screws of varying weights during the balancing process.
As used herein, "hub" means the rotating portion of a disk drive motor, or a rotating spindle or shaft attached to a motor.
The balancing procedure is conventionally performed by spinning the disk, detecting an out-of-balance condition, providing screw(s) of the appropriate weight in the appropriate hole(s) in the hub, and repeating the procedure until the disk is balanced. Alternatively, weight can be added in different manners (e.g., by placing lead tape on the hub 12 or disk 14).
Several problems are associated with the weighted balancing procedure. First, the balancing procedure is time and labor intensive, and extremely difficult to automate. Second, the efforts associated with the balancing procedure are often wasted because of poor disk placement relative to the hub, particularly first hub portion 13.sub.1 which protrudes into the center hole in disk 14. If the disk 14 is not centered on the hub 12, a portion of inner diameter 22 of disk 14 is closer to first hub portion 13.sub.1 than the remaining portions of inner diameter 22; in some cases a portion of inner diameter 22 may even contact first hub portion 13.sub.1.
Further, the clearance between inner diameter 22 of disk 14 and first hub portion 13.sub.1 is on the order of 0.0004 to 0.008 inches, and therefore manually locating disk 14 to prevent contact with first hub portion 13.sub.1 is difficult if not impossible. The problems associated with centering the outer diameter 20 of the disk 14 relative to the axis 11 of motor 10 are compounded by runout of motor 10 and non-concentricity of outer diameter 20 and inner diameter 22 of disk 14.
Thermal expansion of hub 12 and/or thermal contraction of disk 14 will cause a portion of hub 12 and disk 14 to contact one another, if they are not already in contact. Contact between disk 14 and first hub portion 13.sub.1 causes disk 14 to move relative to hub 12, placing disk 14 in an out-of-balance condition. This thermal expansion/contraction problem is enhanced by the different coefficients of thermal expansion of the disk material and the hub material, e.g., the aluminum-based disk and steel-based motor hub, and the difficulty in placing disk 14 on hub 12 without contact between first hub portion 13.sub.1 and disk 14.
One solution to this problem, conceived by one of the inventors of the present invention, is disclosed in U.S. Pat. No. 4,933,927, ("the '927 patent") inventor John P. Ross. The method and apparatus disclosed in the '927 patent include placing an element in contact with outer diameter 20 of the disk 14, moving the element towards the rotational axis of the motor until a portion of the inner diameter contacts hub 12, and simultaneously rotating disk 14 and hub 12 while moving the contact element away from the rotational axis of the motor until the contact element no longer contacts the disk.
In the method and apparatus of the '927 patent, a drive head having a number of pins is lowered onto disk clamp 16, which includes a number of holes for receiving the drive head pins. The method and apparatus disclosed in the '927 patent thus cannot easily be adapted to a single screw disk clamp such as shown in FIG. 1B, wherein single screw disk clamp 17 is attached to a second type of hub 15 by a single center screw 19. Single screw disk clamp 17 substantially reduces time and labor involved in attaching disk 14 to drive motor 10.
With the method disclosed in the '927 application, there is some difficulty in centering disks on drives utilizing more than one disk. Generally, multiple disks are arranged in a stacked configuration about the same rotational axis of the drive motor. In such a configuration, the disks are spaced apart by spacing elements that are themselves in contact with the disk. In such drives, difficulty has arisen in centering a number of disks at the same time because movement of any one of the disks causes the spacer element to move, thereby shifting the position of one or more of the other disks.