Abstract:
A reduced stiction hydrodynamic spindle motor is provided for reducing the starting torque or power of disc drive motors. The reduced stiction motor includes a low surface energy coating having a surface energy less than the surface tension of the motor lubricant.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from U.S. Provisional Application No. 60/119,774, filed Feb. 11, 1999, the entire contents of which are herein incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to a reduction in the stiction for spindle motors. 
     Disc drive data storage devices use rigid discs coated with a medium, e.g. a magnetizable medium, for storage of digital information in a plurality of data tracks. The information is written to and read from the discs using a transducing head mounted on an actuator mechanism which moves the head from track to track across a surface of the disc under control of electronic circuitry. The discs are mounted for rotation on a spindle motor which causes the discs to spin and the surfaces of the disc to pass under the heads. 
     Spindle motors typically include a rotor that rotates about a fixed shaft. During rotation, the radial pressure of the fluid, e.g., gas (air) or liquid, between the rotor and shaft acts as a hydrodynamic bearing to keep these components apart. For example, in a hydrodynamic gas bearing, the axial and/or radial pressure distribution of air is increased and the rotor rotates on a bearing of air about the fixed shaft. In many applications, a lubricant is located between the shaft and the rotor to reduce wear of the motor&#39;s surfaces. 
     SUMMARY OF THE INVENTION 
     In one aspect, the invention features a spindle drive motor including a drive mechanism means and a means for reducing stiction. 
     In another aspect, the invention features a reduced stiction drive mechanism for a spindle drive motor including a shaft and a rotor arranged for relative motion, a lubricant between the shaft and rotor, and a surface energy modifier between the lubricant and at least one of the shaft and rotor. The surface energy modifier has a surface energy effective to reduce stiction. 
     In another aspect, the invention features a computer disc drive including a spindle drive motor for rotating storage media. The spindle drive motor includes a shaft and a rotor arranged for relative motion, a lubricant between the shaft and rotor, and a surface energy modifier between the lubricant and at least one of the shaft and rotor. The surface energy modifier has a surface energy effective to reduce stiction. 
     Embodiments may include one or more of the following features. The shaft and the rotor can be spaced apart by about 0.5 to about 20 microns. The surface energy modifier can have a surface energy lower than the surface tension of the lubricant. Preferably, the surface energy modifier can have a surface energy about 2% lower than the surface tension of the lubricant. More preferably, the surface energy modifier can have a surface energy, about 20% to about 50% or more, lower than the surface tension of the lubricant. For example, the surface energy modifier has a surface energy of about 6 to about 14 dynes/cm and the lubricant has a surface tension of about 12 to about 80 dynes/cm. Each of the surface energy modifier and the lubricant, independently, can have a thickness of about 1 nm to about 2000 nm. 
     The surface energy modifier can be a fluorochemical polymer in a fluorocarbon solvent, e.g., Fluorad or Nye Bar-Type K, isosteric acid, or mixtures thereof. The lubricant can be a fluorinated polyether, a hydrocarbon, an ester, atmosphere moisture, or mixtures thereof. 
     Liquid lubricants include, but are not limited to, motor lubricants, e.g., fluorinated polyether, hydrocarbons, and esters; motor lubricant contaminants, e.g., hydrocarbon contaminants; atmospheric moisture; or mixtures thereof. 
     An advantage of embodiments of the invention is that stiction is reduced, thus lowering the starting power requirements of the motor. Stiction is a type of friction that occurs in the null position, i.e., “touch down,” between two moving members, e.g., a motor journal (shaft) and sleeve (rotor). For instance, when a journal is in contact with a sleeve, liquids in the interface, such as lubricants, redistribute themselves, e.g., via capillary action between the sleeve and journal to form menisci around the contacting areas. The pressure inside a meniscus is lower than the pressure outside the meniscus, thereby leading to an additional force, namely, meniscus force, causing the two mating surfaces to be pulled closer together. Thus, in order to restart the motor, the stiction force between the journal and sleeve must be overcome. 
     Starting torque or power in a spindle motor is governed by many factors, such as the materials used, surface finish, friction coefficient of the surface, environmental condition, etc. In some instances, e.g., humid environments, the starting force or torque will be governed by the stiction between the touch down surfaces and any other materials on the surface, e.g., absorbed moisture and organic materials. Stiction can also result when one or both surfaces are coated with wear-resistant thin films or lubricants. 
     As stiction increases, the power consumption, i.e., starting torque or starting power, of the spindle motor must also increase so that the spindle motor can overcome the menisci forces. In situations of low power disc drive applications, e.g., portable battery powered laptop type computers, power consumption of the spindle motor cannot be increased and the available starting power may not be sufficient to overcome stiction. This situation renders the spindle drive motor inoperable. The reduced stiction spindle motor decreases the starting power consumption of the spindle motor and lowers the probability that the spindle motor will be inoperable. Reducing power consumption is particularly important in low power disc drive applications in which power conservation is extremely desirable. 
     Other features of the invention will be apparent from the following description of the preferred embodiments and from the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A and 1B are cross-sectional partial side views of a drive mechanism for a spindle motor. 
     FIG. 2 is a cross-sectional partial side view of a drive mechanism in the null position including a surface energy modifier. 
     FIGS. 3A and 3B illustrate additional drive mechanism geometries. 
     FIG. 4 is a diagrammatic view of a disc drive data storage device. 
     FIG. 5 is cross-sectional side view of a disc drive hydrodynamic spindle motor. 
     FIG. 6 is an expanded view of the disc drive hydrodynamic spindle motor about area B. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1A shows a schematic view of the drive components  11  of a motor including a rotor  15  and a shaft  13 , both of which are typically formed of metal or ceramic. Shaft  13  is received within a bore  16  of rotor  15 . During operation, either shaft  13  or rotor  15  can be rotated about longitudinal axis  17 . In either case, shaft  13  is approximately centered within bore  16  due to the increased radial pressure distribution of the fluid, e.g., air, between the shaft and the rotor forming gap  19 . Gap  19  is located between shaft  13  and rotor  15  and has a width, w, typically of about 0.5 to about 20 microns. 
     Drive Components  11  includes a lubricant thin film  12  for lubricating either an inner surface  20  of rotor  15  or a radial surface  22  of shaft  13  during rotation. The thickness of lubricating thin film  12  can vary depending upon the design of the motor. The thickness of lubricating thin film is typically between about 1 nm to about 2000 nm. Typically, the thickness of lubricant thin film  12  is about an order of magnitude smaller than gap  19 . For example, if gap  19  is 5 microns the thickness of lubricant thin film typically is about 500 nm or less. The lubricant has a surface tension lower than the surface energy of the shaft or rotor so that surface is wet. Examples of lubricants include, but are not limited to, fluorinated polyethers, hydrocarbons, and esters. Examples of fluorinated polyethers include Z-Dol 1000, Z-Dol 2000, Z-Dol 3000, AM2001, and Z-Tetraol. These fluorinated polyethers may be obtained froem Ausimont USA, Inc. located in Thorofare, N.J., and Nye Lubricants located in Dublin, Calif. Examples of hydrocarbons and esters include poly-alpha olefins and phosphate esters, respectively. An example of a phosphate ester is triphenyl phosphate available from Aldrich Chemical Company, Inc. located in Milwaukee, Wis. An example of a poly-alpha olefin is polypropylene available from Aldrich Chemical Company, Inc. located in Milwaukee, Wis. The lubricant may also be provided by making the shaft or rotor or both out of or coated with a low friction solid material, e.g., a fluoropolymer such as teflon, or by coating metals and other substrates with a low friction hard coating (e.g., silicon carbide and diamond like carbon). 
     Referring now to FIG. 1B, when the motor is turned off, the rotating element, either rotor  15  or shaft  13 , slows to a stop. Eventually, shaft  13  and rotor  15  touch down into a null position, i.e., the resting position of the shaft and rotor. In the null position, a side  45  of shaft  13  comes into contact with lubricant thin film  12 , i.e., shaft  13  is no longer approximately centered within bore  16 . In this configuration, lubricating thin film  12  will redistribute itself such that it wets both the radial surface of the shaft and the inner surface of the rotor near the null position. When the lubricant wets the surfaces, menisci 5 are formed near the null position resulting in an increase in the stiction between shaft  13  and rotor  15 . 
     Surface wetting and menisci formation are governed by the surface energy effect. As shown in FIG. 1B, the meniscus force (F) is directly proportional to the total surface area (A) of menisci 5 interaction and inversely proportional to the distance (H) between shaft  13  and rotor  15 . The total surface area of menisci interaction is determined by calculating the area of contact between the lubricant and the two surfaces of the rotor and shaft, i.e., proportional to the length of interaction (L) multiplied by the inner diameter of the rotor plus the length of interaction (L) multiplied by the outer diameter of the shaft. Thus, meniscus force (F) can be reduced by decreasing the total surface area (A) of menisci interaction or by increasing the distance (H) between the shaft and rotor. 
     Referring now to FIG. 2, the menisci forces and surface wetting can be reduced without increasing the distance (H) between the rotor and shaft by applying a low surface energy coating  30  to radial surface  22  of shaft  13 . 
     As long as the surface energy (dynes/cm) of the low surface energy coating is lower than the surface tension of the lubricant or contaminants in the lubricating thin film, the lubricant or contaminant will not wet the low surface energy coating. Rather, the low surface energy coating causes the lubricant or contaminant to bead or ball. The surface energy of low surface energy coating 30 is lower than the surface tension of lubricating thin film  12  creating a higher contact angle (θ), e.g., greater than 90 degrees, between lubricating thin film and the radial surface of the shaft, i.e., the lubricant will not efficiently wet the shaft. As a result of the lubricant higher contact angle, i.e., beading or balling, the total surface area (A) of lubricant menisci interaction is decreased. Thus, the stiction forces will be reduced. 
     Preferably, the low surface energy coating has a surface energy about 2% lower than the surface tension of the lubricant. More preferably, the low surface energy coating has a surface energy about 20% to about 50% lower or more, than the surface tension of the lubricant. 
     Examples of low surface energy coatings include but are not limited to isosteric acid and fluorochemical polymers suspended in fluorocarbon solvents. Examples of fluorochemical polymers in fluorocarbon solvents include Nye-Bar-Type K available from Nye Lubricants located in Dublin, Calif., and Fluorad™, available from 3M Corporation located in Minn. (surface energy of about 11-12 dynes/cm). Mixtures of such coatings can also be used. The coating can be applied by brushing, wiping, or spraying. The thickness of the low surface energy coating is similar to the thickness of lubricating thin film, e.g., about 1 nm to about 2000 nm. 
     Typically, the surface energies of the fluorochemical polymers (measured in dry films) are between about 6 to about 14 dynes/cm. The surface tension values of commonly used lubricants are between about 12 to about 80 dynes/cm. (Surface energy is used when referring to solids or films. Surface tension refers to liquids. Both are in units of dynes/cm.) 
     Referring to FIGS. 3A and 3B, other spindle motor mechanism geometries are illustrated. In the mechanism 60 of FIG. 3A, a T-shaped shaft  61  is arranged for relative rotation with respect to rotor  62  within bore  63 . In the mechanism 70 of FIG. 3B, a narrow-waisted shaft  71  is arranged for relative rotation with respect to rotor  72  within bore  73 . In both mechanisms of FIGS. 3A and 3B, the bore has a shape complimentary to the shaft. The high surface area of these arrangements can lead to substantial stiction. The surface of either the shaft or the rotor may include a low surface energy coating to reduce stiction. The coating may be applied to the entire surface of the shaft or the rotor or to only portions of those components, such as, a generally planar surface  65  of the T-shaped shaft shown in FIG.  3 A. 
     FIG. 4 is schematic view of a disc drive  102  for use with the present invention. Disc drive  102  includes a base member  104  to which internal components of the unit are mounted. Base member  104  couples to a top cover  106  which forms a sealed environment (cavity) for certain parts of disc drive  102 . Disc drive  102  includes a plurality of discs  108  which are mounted for rotation on a spindle hydrodynamic motor  110 . Examples of spindle motors employing hydrodynamic bearings can be found in U.S. Pat. No. 5,678,929 the entire contents of which are herein incorporated by reference. 
     FIG. 5 shows a cross section through a reduced stiction hydrodynamic spindle drive motor. Spindle drive motor  110  is mounted to base  112  and includes a fixed shaft  114  which is screwed into base  112 . Rotor hub assembly  113  includes hub sleeve  150  and rotor  126  which rotate about fixed shaft  114 . 
     The outer surface of shaft  114  and the adjacent bore of rotor  126  together form hydrodynamic fluid bearing  128 . Bearing gap G at hydrodynamic bearing  128  is typically about 0.5 microns to about 20 microns. Lubricating thin film  129  can be applied to the outer surface of shaft  114  to help alleviate wear between the shaft and rotor. Additionally, low surface energy film  131  can be applied to the bore surface of rotor  126  to reduce menisci forces in the null position. 
     During operation electrical signals supplied to windings  148  of strator assembly  152  create a magnetic field which interacts with permanent magnets  154  to cause rotor hub assembly  113  to rotate. 
     Referring to FIG. 6, an expanded view of box B of FIG. 5 illustrates motor  110  in the null condition. In this configuration, rotor  126 , shaft  114 , lubricating thin film  129 , and low surface energy film  131  are in contact (arrow). However, the lubricant does not wet the shaft. This scenario is true as long as the surface energy of the low surface energy film is lower than the surface tension of the lubricant and any lubricant contaminants. 
     Still further embodiments are in the following claims.