Abstract:
A hydrodynamic bearing device includes a gap between a shaft and shaft holder. One of the shaft and shaft holder has a recess for separating by capillary action lubricant at an upper portion of the gap above the recess from lubricant at a lower portion of the gap below the recess. The separated lubricant at the recess forms a gas interposition in order to reduce lubricant resistance. A hole formed in the shaft or shaft holder communicates the air interposition to the external atmosphere to prevent air expansion and the build-up of bubbles in the lubricant which could otherwise cause contamination and leakage. An axially extending storage groove supplies by capillary action lubricant from either the upper or lower portion to the other portion if lubricant in the other portion decreases due to vaporization or leakage, whereby the loss of lubricant is minimized and the operating life of the surrounding components are increased.

Description:
The present invention relates to motor bearings, and more particularly, to hydrodynamic fluid pressure bearings or liquid bearings for an electric motor. 
     BACKGROUND OF THE INVENTION 
     In an electric motor provided with a hydrodynamic bearing which makes use of the dynamic pressure of a lubricant or bearing liquid held in a gap or clearance between a shaft member such as a rotational shaft and a shaft holder such as a sleeve member, either of the shaft member and the shaft holder is rotatably coupled with a rotor of the motor and the other is coupled with a stator. 
     The gap between the shaft member and the shaft holder need not be completely filled with a lubricant, and therefore an air interposition extending circumferentially of the shaft member may be provided in a specific axial region of the gap between the shaft member and the shaft holder for reducing the frictional loss caused by viscosity resistance of the lubricant during the relative rotation of the shaft member and the shaft holder. 
     The air trapped in the air interposition is, however, likely to expand when the rotation of the motor raises the temperature. The expanded air exerts pressure on the lubricant causing it to escape from an outer end of the gap, thereby decreasing the operating life of a bearing means or contaminating the outside of the motor. 
     Japanese Patent Publication 62-4565 (1987), and U.S. Pat. Nos. 4,445,793; 4,892,418 and 5,112,142 provide an air interposition separating lubricants respectively held at upper and lower portions of the device. The air interposition communicates with the atmosphere externally of the motor via a communication hole such that any expanded air within the device is released to the atmosphere. However, in the prior art devices, if the amount of lubricant fluid at either the upper or lower portion is decreased because of vaporization and/or leakage between the outward end of the space and the air interposition, the operating life of the bearing means will still decrease unless the lubricant fluid is periodically supplemented. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a hydrodynamic bearing device having an air interposition communicating with the atmosphere and arranged to minimize the frictional loss of a lubricant or bearing liquid held in a gap between the shaft and an opposing surface, and to prevent escape of the lubricant caused by temperature increase as well as to supply additional lubricant without requiring maintenance service when the amount of lubricant held between the air interposition and an outer end of the gap is decreased, whereby such a decrease in the operating life will be minimized. 
     According to the present invention, the sleeve also has at least one storage groove for communicating the bearing liquid separated by capillary action at the air interposition. The bearing liquid is fed by the capillary action through the groove. When the bearing liquid which is filled in the upper side of the air interposition is decreased due to evaporation, an amount of the bearing liquid is supplemented from the lower side of the air interposition through the groove by the capillary action. 
    
    
     The foregoing and other objectives, features and advantages of the invention will be more readily understood upon consideration of the following detailed description of the invention taken in conjunction with the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic cross-sectional view of an electric motor employing the hydrodynamic bearing device according to a first embodiment of the present invention. 
     FIG. 2 is an enlarged partial view of the corner where the inner surface of the sleeve joins the annular shoulder of the motor of FIG. 1. 
     FIG. 3 is a top plan view showing an outer groove on a sleeve. 
     FIG. 4 illustrates an inner groove of the sleeve of FIG. 3. 
     FIG. 5 illustrates a cross section of the groove of FIG. 4. 
     FIG. 6 is a schematic cross-sectional view of an electric motor employing the hydrodynamic bearing device according to a second embodiment of the present invention. 
     FIG. 7 shows an alternative air passageway for the shaft shown in FIG. 6. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a schematic cross-sectional view of an electric motor for driving a data storage medium, such as a hard disk, which is assembled into a driving unit, in a first embodiment of the present invention. With reference to FIG. 1 showing an electric motor employing the hydrodynamic bearing device, a shaft holder 12 is fixedly fitted at the lowermost outer side of its mounting portion 12a into a circular fitting hole 10a provided in a base member 10. 
     The shaft holder 12 has a cylindrical sleeve portion 12c coaxially connected to a mounting portion 12a through an annular shoulder portion 12b . The mounting portion 12a has a diameter larger than the diameter of the sleeve portion 12c. A thrust cover plate 13 is fitted to the lowermost portion of the inner side of the mounting portion 12a for closing an opening defined by an inner surface of the mounting portion 12a so as to cooperate with the shaft holder 12 to form an internal dead end or closed bottom surface of the motor. In this embodiment, the shaft holder 12 and the thrust cover plate 13 constitute a shaft holding assembly. 
     At least one axially extending communicating groove 12d is formed on an outer side surface of the sleeve portion 12a. At least one air passageway 12e is provided at a substantially axially intermediate region of the communicating groove 12d for communicating between the groove 12d and the interior of the sleeve portion 12c in order to permit gas to escape to the external atmosphere. In this embodiment, the communicating groove 12d and the air passageway 12e constitute a communicating means. A tapered portion 12f is formed on the uppermost inner side of the sleeve portion 12c which thus increases the inner diameter of the sleeve portion 12c toward its upper end. FIG. 3 is a top plan view of a part of the sleeve portion 12c showing the cross-sectional shape of the groove 12d. The tapered portion 12f cooperates with the outer surface of the shaft 14 such that lubricant oil forms a capillary seal by surface tension. 
     Referring to FIG. 1, a shaft 14 has an annular thrust plate 14b fitted coaxially onto the lower end of a cylindrical shaft member 14a and is rotatably supported by the shaft holder 12 and the thrust cover plate 13. The shaft member 14a extends through the sleeve portion 12c. The thrust plate 14b is located in a circular space defined between the thrust cover plate 13 and the annular shoulder portion 12b. 
     An annular recess 15 is provided on the shaft member 14a at a substantially axially intermediate region of the shaft member 14a. The annular recess 15 is provided with an upper tapered surface 15a which is inwardly tapered toward the lower side of the shaft member in its axial direction and a lower tapered surface 15b which is inwardly tapered toward the upper side of the shaft member in its axial direction. The tapered surface 15b is rather moderately tapered relative to the upper tapered surface 15a. The inner end of the air passageway 12e opens at a portion of the inside of the sleeve portion 12c which is somewhat axially and upwardly displaced from the axial center portion of the recess and corresponds to the deepest portion of the annular recess 15. 
     The gap which is formed between the shaft 14, the shaft holder 12 and the thrust cover plate 13 is filled with a lubricant oil 16 as the bearing liquid. The lubricant 16 is held at a lower portion of the gap by capillary action at the lubricant&#39;s upper level adjacent to the lower end of the annular recess 15. Also, the gap which is formed between the shaft member 14a and the sleeve portion 12c is filled with the lubricant 16 which is held by capillary action at an upper portion of the gap at the lubricant&#39;s upper level adjacent to the lower end of the tapered portion 12f and at its lower level adjacent to the upper end of the annular recess 15. The tapered portion 12f prevents the lubricant from leaking out by surface tension. The gap confronting surfaces of the shaft member 14a and the sleeve portion 12c and the lubricant 16 filled therebetween constitute a radial hydrodynamic bearing. The confronting surface of the thrust plate 14b, the annular shoulder portion 12b, the thrust cover plate 13 and the lubricant 16 filled therebetween constitute a thrust hydrodynamic bearing. 
     The gap between the annular recess 15 and the inner side of the sleeve portion 12c contains an amount of gas (ordinarily air) thus forming an air or gas interposition 17 which separates by means of capillary action the lubricant at the upper portion of the gap from the lubricant at the lower portion of the gap, and which is communicated through the air passageway 12e and the communicating groove 12d to the outside atmosphere. 
     As best shown in FIG. 2, a corner 12h where the inner surface of the sleeve portion 12c joins the annular shoulder portion 12b is chamfered to facilitate the collection of bubbles which are generated in the lubricant 16 between the thrust plate 14b and the air interposition 17. 
     A plurality of herringbone shaped bearing grooves 18, 20, 22 and 24 (shown by broken lines) is provided on the upper and lower surface of the thrust plate 14b and the inner surface of the sleeve portion 12c for generating in the lubricant 16 a thrust load and a radial load during the rotation of the shaft 14 in a forward direction. The herringbone shaped grooves may be provided on the thrust cover plate 13, the annular shoulder portion 12b and the shaft member 14a. A groove pattern may be modified to a V-shape, X-shape or spiral shape or a combination thereof. 
     A storage groove 12g is formed on the inner surface of the sleeve portion 12c at a position opposing the annular recess 15 for storing an amount of the lubricant 16 by capillary action. For the storage groove 12g, consideration is made as to its cross-sectional configuration, material, surface roughness and characteristics of the lubricant 16 to enable proper holding of the lubricant 16 in the storage groove 12g by capillary action. The lubricant 16 is stored in the storage groove 12g for communicating the lubricant 16 by capillary action from one of the lower and upper portions of the gap to the other portion of the gap when the lubricant in the other portion of the gap is reduced due to vaporization or other means, whereby the reduced lubricant in one portion of the gap is supplemented by the lubricant in the other portion of the gap via the storage groove in order to minimize any reduction in operating life because of a loss of lubricant. In other words, the storage groove 12g communicates the lubricant 16 on both sides of the annular recess 15 to supplement reduced lubricant which would otherwise shorten the operating life of the surrounding components. The viscosity of the lubricant 16 is preferably 5 to 100 cp. 
     The width of the storage groove 12g may generally be less than 200 micrometers or preferably about 100 micrometers. The storage groove 12g preferably has a width greater than 5 to 10 micrometers such that the lubricant can be stored therein. The depth of the storage groove 12g is preferably as deep as possible so long as the physical strength of the sleeve portion 12c is not affected. Although the storage groove 12g of this embodiment is square in cross section and extends axially of the shaft 14 as shown in FIGS. 4 and 5, it is not limited to any particular shape. The storage groove 12g is circumferentially spaced on the shaft 14, one hundred eighty degrees (180) from the air aperture 12e,but may be located at any angular position on the shaft so long as the storage groove 12g is not directly opposing the air passageway 12e. Also, two or more grooves 12g may be provided and its cross-sectional shape may be modified to a triangular or semicircular shape. FIG. 4 illustrates a part of the inner side of the sleeve portion 12c where the storage groove 12g is formed, and FIG. 5 is a cross-sectional view of the storage groove 12g. 
     Although a stator 26 with stator coils wound on stator cores is mounted on the outside of the sleeve portion 12c, the air interposition 17 communicates to the outside atmosphere through the air passageway 12e and the communicating groove 12d. The communicating groove 12d need not extend from the uppermost portion to the lowermost portion of the sleeve portion 12c so long as a sufficient amount of air is permitted to move in. 
     A cup-like rotor 28 is mounted on the upper end of the shaft body 14a. A rotor magnet 30 is attached to the inner circumferential portion of the side wall portion 28a of the rotor hub 28 so as to radially oppose the stator 26, thereby constituting a rotary structure. If the rotor hub 28 is not made of a ferromagnetic material, it is desirable to install a ferromagnetic cylindrical yoke between the rotor magnet 30 and the outer wall portion 28a. 
     When the shaft 14 rotates relative to the shaft holder 12 and the thrust cover plate 13, the shaft member 14a and the sleeve portion 12c remain free from viscosity resistance of the lubricant 16 at the air interposition 17, thus the frictional loss caused by the lubricant 16 is reduced. As the air interposition 17 communicates through the air passageway 12e and the communicating groove 12d with the outside atmosphere, this structure vents gas to the external atmosphere in order to prevent the lubricant 16 from escaping from the upper end of the gap between the shaft member 14a and the sleeve portion 12c even if a temperature rise during the rotation causes expansion of the air in the air interposition 17 or the creation of bubbles in the lubricant 16. 
     The lubricant 16 is reserved or stored by capillary action in the storage groove 12g which communicates the lubricant 16 on both sides of the air interposition 17 (i.e. the upper and lower portions of the gap). If the portion of the lubricant 16 which is filled in the upper portion of the gap above the air interposition 17 is decreased due to evaporation, an amount of the lubricant 16 in the upper portion of the gap is supplemented through the supply groove 12g from the lubricant in the lower portion of the gap below the air interposition. Conversely, if the portion of the lubricant 16 which is filled in the lower portion of the gap below the air interposition 17 is decreased due to evaporation, an amount of the lubricant 16 in the lower portion of the gap is supplemented through the supply groove 12g from the lubricant in the upper portion of the gap above the air interposition. Hence, reduction in operating life caused by shortage of the lubricant 16 will be minimized. 
     The thrust hydrodynamic bearing portion generates a substantial number of bubbles in construction. Therefore, the tapered surface 15b of the annular recess 15 is milled to an elongated and moderate tapered form for compensating the upper level of the lubricant 16 in the thermal expansion of the bubbles. 
     FIG. 6 shows an alternative embodiment of the present invention. In this embodiment, a stationary shaft 32 is fixed to a base member 10. A sleeve member 34 has a cylindrical sleeve portion 34c coaxially coupled to an annular shoulder portion 34b which has a diameter larger than that of the sleeve portion 34c. The sleeve member 34 also has a rotor hub 36 connected to the outer end portion of the annular shoulder portion 34b. A thrust cover plate 13 is fitted to the uppermost inner side of the annular shoulder portion 34b for closing its opening and forming an internal dead end of the sleeve member 34. A rotor magnet 30 is attached to an inner circumferential surface of the rotor hub 36. A stator 26 located to oppose the rotor magnet 30 constitutes a rotary structure. If the rotor hub 36 is not made of a ferromagnetic material, it is desirable to install a ferromagnetic cylindrical yoke between the rotor magnet 30 and the inner circumferential surface of the rotor hub 36. An air passageway 40 is formed through the shaft 32. In this embodiment, the hole serves as the air passageway instead of the air passageway 12e and the communicating groove 12d as employed in the first embodiment. An annular recess 38 is provided on the sleeve portion 32c at a substantially axially intermediate region of the sleeve portion 34c. The annular recess 38 is provided with an upper tapered surface 38a which is inwardly tapered toward the upper side of the shaft member in its axial direction and a lower tapered surface 38b which is inwardly tapered toward the lower side of the shaft member in its axial direction. The tapered surface 38b is rather moderately tapered relative to the upper tapered surface 38a. The inner end of the air passageway 40 opens at a portion of the inside of the shaft 32 which is somewhat axially and downwardly displaced from the axial center portion of the recess and corresponds to the deepest portion of the annular recess 38. A groove 32a is formed on the inner surface of the shaft 32 at a position opposing the annular recess 38 for storing an amount of the lubricant 16 by capillary action. 
     An alternative embodiment of the air passageway 40 is shown in FIG. 7. The air passageway 40 diametrically passes through the stationary shaft between the air interposition, but does not open at the location of the groove 32a (as denoted by the phantom lines). 
     As has been described in the specification and shown in the drawings, the present invention employs an air interposition communicating through a communication means with the outside atmosphere in order to minimize the frictional loss of the lubricant and to prevent the lubricant from leaking out during a motor temperature increase. In case any of the two portions of the lubricant provided at opposite sides of the air interposition is decreased, the lubricant is supplemented by the other portion through a storage groove by capillary action. Accordingly, the lubricant requires no external replenishment and will stay for a relatively long period in the hydrodynamic bearing, thereby increasing the operating life of the motor. 
     The terms and expressions which have been employed in the foregoing specification are used therein as terms of description and not limitation, and there is no intention in the use of such terms and expressions of excluding equivalents of the features shown and described or portions thereof, it being recognized that the scope of the invention is defined and limited only by the claims which follow.