Seal for a joint or juncture – Seal between relatively movable parts – Close proximity seal
Reexamination Certificate
1999-11-12
2002-04-30
Knight, Anthony (Department: 3626)
Seal for a joint or juncture
Seal between relatively movable parts
Close proximity seal
C277S409000, C277S411000
Reexamination Certificate
active
06378874
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to the field of disc drives, and more particularly to an apparatus and method for providing a reliable, low resistance electrical pathway through a ferrofluidic seal between a hub and a shaft of a spindle motor used in a disc drive.
BACKGROUND OF THE INVENTION
Disc drives, including magnetic disc drives, optical disc drives and magneto-optical disc drives, are widely used for storing information. A typical disc drive has one or more discs for storing information in a plurality of concentric circular tracks. This information is written to and read from the discs using read/write heads mounted on actuator arms which are moved from track to track across surfaces of the discs by an actuator mechanism. The discs are mounted on a spindle which is turned by a spindle motor to pass the surfaces of the discs under the read/write heads. The spindle motor generally includes a shaft fixed to a baseplate and a hub, to which the spindle is attached, having a sleeve into which the shaft is inserted. Permanent magnets attached to the hub interact with a stator winding on the baseplate to rotate the hub relative to the shaft One or more bearings between the hub and the shaft facilitate rotation of the hub.
The spindle motor also typically includes an exclusion seal to seal interfacial spaces between the hub and the shaft. This is necessary, because lubricating fluids or greases used in the bearings tend to give off aerosols or vaporous components that migrate or diffuse out of the spindle motor and into a disc chamber in which the discs are maintained. This vapor often transports other particles, such as material abraded from the bearings or other components of the spindle motor, into the disc chamber. These vapors and particles deposit on the read/write heads and the surfaces of the discs, causing damage to the discs and the read/write heads as they pass over the discs. Thus, the migration of these contaminants into the disc chamber must be prevented.
To prevent the migration of these contaminants into the disc chamber, the latest generation of spindle motors utilize a ferrofluidic seal between the shaft and the hub. Ferrofluidic seals are described in, for example, U.S. Pat. No. 5,473,484, which is incorporated herein by reference. A typical ferrofluidic seal consists of a ferrofluid, an axially polarized annular magnet and two magnetically permeable annular pole pieces attached to opposing faces of the magnet. The ferrofluid is conventionally composed of a suspension of magnetically permeable particles suspended in a fluid carrier. Generally, the magnet and the pole pieces are fixed to the hub and extend close to but do not touch the shaft. Magnetic flux generated by the magnet passes through the pole pieces and the shaft, which is also magnetically permeable, to magnetically hold the ferrofluid in magnetic gaps between the pole pieces and the shaft, thereby forming a seal.
One shortcoming of this design is that because the hub is separated from the shaft by grease or lubricant in the bearings and by the ferrofluid in the seal, the hub, spindle and discs build-up a considerable static electric charge while rotating. This leads to electrical arcs or sparks between the discs and the read/write heads, which are grounded, and results in the loss of information and/or permanent damage to the disc drive. This is particularly a problem for magnetic disc drives that typically use inductive or magnetoresistive heads, which are easily damaged by such an electrical discharge. Thus, a reliable, low resistance electrical pathway must be established between the spindle and electrical ground to discharge or eliminate the static electrical charge.
Several approaches have been attempted to provide a reliable, low resistance electrical pathway across the ferrofluidic seal. One approach is described in U.S. Pat. No. 4,604,229, to Raj et al. (RAJ), hereby incorporated by reference. RAJ teaches providing an electrically conducting ferrofluid, which electrically couples a rotating shaft to a housing through the pole pieces.
One problem with the approach taught in RAJ is the high resistance and often unreliable electrical connection between the pole pieces and the housing. This poor electrical connection is due to the small surface area at exterior circumferences of the pole pieces through which they contact the housing. These pole pieces typically have a thickness of less than 0.1 inches and often as little as 0.03 inches. Moreover, due to dimensions selected to facilitate the insertion of the ferrofluidic seal between the housing and the shaft, as well as manufacturing imperfections, the pole pieces generally are not in contact with the housing all the way around their circumference. In recognition of this, RAJ teaches that a snap ring, which primarily serves to hold the ferrofluidic seal in place, may be made of an electrically conducting material to increase electrical contact between the housing and one of the pole pieces (hereinafter the top pole piece). However, any increase in electrical conductivity between the top pole piece and the housing is more than offset by the teaching in RAJ of an o-ring seal between the other, lower pole piece and an inwardly projecting annular portion of the housing. The o-ring lifts the lower pole piece away from the housing, thereby reducing or eliminating contact therebetween and increasing the resistance of the electrical connection between the lower pole piece and the housing. In addition, if a grease, such as is frequently applied to o-rings, is used it migrates into interfacial spaces between the lower pole piece and the housing, further increasing the electrical resistance.
Another generally known approach for providing an electrical pathway across a ferrofluidic seal, which avoids some of the problems of the approach taught in RAJ, is described in U.S. Pat. No. 5,238,254, to Takii et al. (TAKII), hereby incorporated by reference. TAKII teaches using a conductive adhesive, such as a silver epoxy, to couple the pole pieces to the hub or housing. This approach has the additional advantage of being able to seal the lower pole piece to an inwardly projecting annular portion of the hub or housing without an o-ring, which as explained above can increase electrical resistance between the lower pole piece and the housing. However, while a significant improvement over RAJ, this approach is also not wholly satisfactory.
A fundamental problem with this approach is the increased manufacturing time and costs associated with applying the conducting adhesive, distributing it on surfaces to joined and baking the assembled pieces to cure the adhesive. Moreover, because it is necessary to spin test the spindle motor prior to final assembly, the conducting adhesive is generally applied to the upper and lower pole pieces in two separate steps. The lower pole piece is adhered to the hub in a first step prior to spin testing, and the top is adhered to the hub in a second step following a successful test. Thus, the assembled pieces must be baked twice, thereby further increasing the manufacturing time associated with this approach. Furthermore, should the spindle motor fail the spin test, the conducting adhesive generally prevents disassembly and repair of the spindle motor, thereby lower the yield of the manufacturing process. All of the above, i.e., increased manufacturing time and costs, and lowered yields, is extremely undesirable in an industry such as the disc drive industry in which competition has reduced profits to a thin margin. Finally, the use of electrically conductive adhesive is especially problematic in spindle motors used for disc drives because of the possibility of mis-applied or excess adhesive coming loose and contaminating the disc chamber or interfering with the bearings or the ferrofluidic seal itself.
Yet another generally known approach for providing an electrical pathway across a sealing member is described in U.S. Pat. No. 5,050,891, to Ishikawa (ISHIKAWA), hereby incorporated by reference. ISHIKAWA t
Knight Anthony
Moser Patterson & Sheridan LLP
Peavey Enoch E.
Seagate Technology LLC
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