Disc drive memory systems have been used in computers for many years for storage of digital information. Information is recorded on concentric memory tracks of a magnetic disc medium, the actual information being stored in the form of magnetic transitions within the medium. The discs themselves are mounted on a hub that is rotatably mounted on a fixed spindle. The information is accessed by means of read/write heads generally located on a pivoting arm that moves radially over the surface of the disc. The read/write heads or transducers must be accurately aligned with the storage tracks on the disc to ensure proper reading and writing of information.
During operation, the discs are rotated at very high speeds within an enclosed housing by means of an electric motor generally located inside the hub that supports the discs. One type of motor in common use is known as an in-hub or in-spindle motor. Such in-spindle motors typically have a spindle mounted by means of two ball or fluid dynamic bearing systems to a motor shaft disposed in the center of the hub. Generally, such motors include a stator comprising a plurality of teeth arranged in a circle. Each of the teeth support a plurality of coils or windings that may be sequentially energized to polarize the stator. A plurality of permanent magnets are disposed in alternating polarity adjacent the stators. As the coils disposed on the stators are sequentially energized in alternating polarity, the magnetic attraction and repulsion of each stator to the adjacent magnets cause the hub to rotate, thereby rotating the disc and passing the information storage tracks beneath the head.
The use of fluid dynamic bearing assemblies in such drive systems has become preferred due to desirable reductions in drive size and noise generation as compared to conventional ball bearing drive systems. In fluid dynamic bearings, a lubricating fluid functions as the bearing surface between a spindle and a hub. Such bearings are of the journal and thrust types. Journal bearings fix the radial position of a hub as it rotates around a spindle. Thrust bearings constrain the axial position of the hub as it rotates.
One, or the other, or both mating hub and spindle surfaces can be patterned with grooves and lands in various patterns to make lubricant fluid pumps that are actuated by the rotation of the hub relative to the spindle. Such pumps can maintain lubricant fluid pressure gradients while the hub is rotating, providing thrust and journal bearing functions. When the hub is not rotating, lubricant fluids are maintained in place in the hub to spindle gap by capillary forces.
For disc drives having first and second covers mounted to the spindle for improved mechanical stability, lubricant fluid loss is inevitable at both termini of the spindle, and is an operational lifetime limiting factor for such disc drives. Sealing techniques include capillary seals and labyrinth seals. Capillary seals are flared channels that rely on the surface tension of the lubricant fluid to form a meniscus as the walls of a channel flare apart. Capillary seals can also serve as reservoirs for lubricant fluid, but they are prone to lubricant loss through evaporation at the surface of the meniscus. Labyrinth seals can be used with capillary seals to further reduce lubricant evaporation by providing an elongated pathway for lubricant vapor to escape. Unfortunately, effective labyrinth seals tend to consume a fair amount of space, and are therefore difficult to use at both ends of a spindle. Different seal designs can be used at each end of a spindle, but is important for the lubricant fluid pressures at the first and second seals to be at nearly the same pressure to reduce the loss of lubricant fluid from the seal with the lower pressure. Recirculation paths for lubricant fluid can be formed in the hub assembly to equalize pressures. Examples of such recirculation paths are shown in U.S. patent application Ser. No. 11/166823 filed Jun. 24, 2004 by LeBlanc et al., incorporated herein by reference, in its entirety.
Recirculation paths for fluid dynamic bearing motors have previously been drilled directly into hubs, using small diameter carbide drill bits. Because of the small diameter (typically, 0.3 mm), high drill bit rotation speeds and slow drill bit feed rates are generally required. Center drilling is also required to minimize walking of the drill bit. Multiple live tool spindles and drills are typically required, as well as subsequent deburring, which tends to be difficult.