1. Field of the Invention
The present inventions relates to data storage devices and more particularly to a spindle motor assembly for a hard disk drive.
2. Background Information
Hard disk drives contain one or more magnetic heads which magnetize and sense the magnetic field of a rotating disk. The magnetization of the disk typically corresponds to a series of digital bits. The digital bits are typically arranged within concentric tracks that extend radially across the disk. Each track may contain a number of sectors. Each sector may contain a servo field, an identification field, a data field and an error correction code field.
The heads are typically mounted to an actuator arm that is pivotally attached to a base plate of the disk drive. The actuator arm may include a voice coil motor which can move the heads across the surfaces of the disk. The voice coil motor is connected to control circuitry which can move the heads from one track to another track of the disk. The control circuitry utilizes the servo field of each sector to maintain the head at the centerline of the track. A deviation from the centerline may create errors in reading or writing data from the disk.
The disk is rotated by a spindle motor which is mounted to the base plate. The spindle motor typically includes a hub that supports one or more disks. Within the hub is an electric motor which spins the disks. The disks may be separated by an annular spacer(s) and coupled to the hub by a clamp ring.
The spindle motor may have a dynamic imbalance which causes the disks to "wobble" during rotation. The wobble may be created during an assembly process wherein the disk is not concentrically mounted to the hub of the spindle motor. The wobble moves the track relative to the heads. Such a phenomenon is typically referred to as a "run-out" of the disks.
The aerial density of a disk is somewhat defined by the spacing between adjacent tracks. Disk drives with disk run-out require larger track to track spacing to compensate for the relative movement between the heads and the disk. This larger track to track spacing reduces the data storage capacity of the disk, and the hard disk drive. It is therefore desirable to minimize the wobble of the spindle motor and the run-out of the disk.
Some disk drive manufacturers attempt to dynamically balance the spindle motor by placing each motor on a spin stand and then add counterweights to the hub until the assembly is in balance. The counterweights may be set screws that are attached to the hub. Such a procedure is time consuming and adds to the complexity and cost of mass producing hard disk drives. It would be desirable to provide a hard disk drive that is dynamically balanced without the use of rigid counterweights.