Abstract:
A shaft bearing structure of spindle motor, which holds a rotor rotationally by a shaft bearing and a bearing holder fixed on a motor base of a stator, the rotor is fixed with a spindle shaft and confronted with the stator, an oil cycle material retains and re-flows lubricating oil circulating around the spindle shaft in the shaft bearing, wherein the oil cycle material is placed in the gap between the inner circumference of the bearing holder and the periphery of the spindle shaft, and placed at least contiguous to one edge of the shaft bearing in axial direction of the spindle shaft.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a shaft bearing structure of spindle motor capable for a laser beam printer (hereinafter referred to as LBP) or an optical disc drive.  
           [0003]    2. Description of the Related Art  
           [0004]    A laser beam printer (hereinafter referred to as LBP) is capable of high speed printing, so that the use of such an LBP is increasing especially in business field.  
           [0005]    As the recent LBP complies with color printing, a spindle motor for LBP is required to stably rotate in high speed. In other words, the spindle motor for LBP is required to have a shaft bearing which provides stable rotation in high speed.  
           [0006]    [0006]FIG. 1 is a diagram for explaining a structure and operation of a LBP. In FIG. 1, a LBP “A” is composed of a semiconductor laser driver  100  controlled by an external equipment (such as a computer)  101  through a controller  102  and a DC controller  103 , a scanner motor driver  104  controlled by the DC controller  103 , a cylindrical lens  105  to pass a laser beam emitted from a semiconductor laser controlled by the semiconductor laser driver  100 , a polygon mirror  106  attached to a spindle motor (not shown) of which rotation is controlled by the motor driver  104  by the controller  103  to reflect the laser beam emitted through the cylindrical lens  105 , a spherical lens  107  and a toric lens  108  for spreading the laser beam onto a photosensitive unit  109  for printing information onto a sheet of paper not shown, a horizontal synchronizing mirror  110  and a BD lens  111  for monitoring the laser beam through an optical fiber  112  for feedback control.  
           [0007]    The polygon mirror  106  has four to six sides and is maintained at the rotational velocity of 20,000 to 30,000 rpm. As the polygon mirror  106  reflects the laser beam onto the photosensitive unit  109 , a slight inclination of the polygon mirror  106  (which may be caused by wobbling of rotational axis of the spindle shaft) may not reflect the laser beam properly on the photosensitive unit  109  for scanning.  
           [0008]    In this sense, the spindle motor for rotating the polygon mirror  106  should rotate very fast and stable in high-degree of accuracy.  
           [0009]    However, the spindle motor used for the LBP has a problem of leakage of lubricating oil in the rotational shaft as it rotate very fast in high revolution. The leakage of lubricating oil of the spindle motor may affect the duration of the motor. Further, the leakage of lubricating oil may damage the inside mechanism of the LBP or spoil the printing paper.  
           [0010]    [0010]FIG. 2 is a cross sectional view of half-side of a spindle motor. The cross sectional view of the spindle motor is symmetry to the half-side view. The spindle motor is basically composed of a rotor  201  and a stator  202 . The rotor  201  has a bush  203  pressed to a spindle shaft  204  and fixed with the polygon mirror  106 . The rotor  201  is attached to the spindle shaft  204  and the spindle shaft  204  is rotationally attached to the stator  202  by a shaft bearing  205  and a bearing holder  206 .  
           [0011]    [0011]FIG. 3 is a cross sectional enlarged view of inside portion of the surrounding of the spindle shaft  204  and the bush  203  and the bearing holder  206  and the shaft bearing  205 . In FIG. 3, a hydraulic pressure  31  presses lubricating oil  30  by the rotation of the spindle shaft  204  and the lubricating oil  30  moves along the surface of the spindle shaft  204  and comes out on the surface of the bush  203 . Then the lubricating oil  30  leaks out from a gap portion  32  formed between the bearing holder  206  and the bush  203 .  
           [0012]    The height of the spindle motor may be shortened by mechanical design of an LBP, which makes the gap portion  32  narrower. In the case that the gap portion becomes narrower, the lubricating oil  30  tends to leak out even more.  
         SUMMARY OF THE INVENTION  
         [0013]    Accordingly, in consideration of the above mentioned problems of the related art, an object of the present invention is to provide a shaft bearing structure of spindle motor, including, a rotor rotationally attached to a shaft bearing and a bearing holder fixed on a motor base of a stator, wherein the rotor is fixed with a spindle shaft and confronted with the stator, and the shaft bearing structure is characterized in that an oil cycle material ( 40 ,  41 ) retains and re-flows lubricating oil circulating around the spindle shaft in the shaft bearing, wherein the oil cycle material ( 40 ,  41 ) is placed in the gap between the inner circumference of the bearing holder and the periphery of the spindle shaft, and placed at least contiguous to one edge of the shaft bearing in axial direction of the spindle shaft.  
           [0014]    Other object and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings.  
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0015]    [0015]FIG. 1 is a diagram for explaining a structure and operation of a conventional LBP.  
         [0016]    [0016]FIG. 2 is a cross sectional view of half-side of a spindle motor in accordance with the related art.  
         [0017]    [0017]FIG. 3 is a cross sectional view of the spindle motor shown in FIG. 2 partially enlarged.  
         [0018]    [0018]FIG. 4 is a cross sectional view of half-side of a spindle motor in accordance with a first embodiment of the present invention.  
         [0019]    [0019]FIG. 5 is a cross sectional enlarged view of the substantial part of the spindle motor shown in FIG. 4.  
         [0020]    FIGS.  6 (A) to  6 (C) are plan views of a ring magnet to be inserted between a rotor and a stator.  
         [0021]    [0021]FIG. 7 is a diagram for explaining the relation between the reliability and the clearance of the spindle shaft and the shaft bearing of the spindle motor in accordance with the first embodiment of the present invention.  
         [0022]    [0022]FIG. 8 is a cross sectional view of half-side of the spindle motor in accordance with a second embodiment of the present invention.  
         [0023]    [0023]FIG. 9 is a cross sectional enlarged view of the substantial part of the spindle motor shown in FIG. 8.  
         [0024]    [0024]FIG. 10 is a diagram for explaining the relation between the reliability and the clearance of the spindle shaft and the shaft bearing of the spindle motor in accordance with the second embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0025]    [First embodiment] 
         [0026]    [0026]FIG. 4 shows a cross sectional view of half-side of a spindle motor in accordance with a first embodiment of the present invention. The cross sectional view of the spindle motor is symmetry to the half-side view. The spindle motor is basically composed of a rotor  1  and a stator  10 . The rotor  1  is composed of a bush  2  pressed with a spindle shaft  3 , a rotor yoke  4  fixed with the bush  2 , the polygon mirror  106 , a spring  5  for fixing the mirror  106 , a ring magnet  6  fixed at the periphery of the rotor yoke  4 .  
         [0027]    The stator  10  is composed of a motor base  11 , a shaft bearing  12 , a thrust plate  13 , a thrust cover  14 , an oil seal  15 , a bearing holder  16 , a core  17 , and a coil  18 . The shaft bearing  12  is made of copper or iron sintered alloy, and has a collar bearing portion  19  which is fixed with motor base  11  by a screw  20 . The shaft bearing  12  is attached to the bearing holder  16  and the core  17  is attached to the periphery of the bearing holder  16 . The core  17  is made of silicon steel plate and wound with the coil  18 .  
         [0028]    The spindle shaft  3  is rotatably held by the shaft bearing  12 . The bottom side of the spindle shaft  3  is covered by the thrust plate  13  and the thrust cover  14 . A ring shaped oil seal  15  made of flexible material is inserted between the periphery of the thrust cover  14  and the motor base  11  to prevent the leakage of lubricating oil described below. A screw  21  holds the rotor yoke  4  stably to the motor base  11 . The rotor  1  is rotationally attached to the stator  10 .  
         [0029]    The mirror  106  is placed on the top of the rotor  1  and fixed with the bush  2  by the spring  5 . When an electric current of predetermined value is provided to the coil  18 , the rotor  1  starts to rotate with the mirror  106  fixed thereon. The mirror  106  is a polygon mirror having four to six sides. As described above, the rotation of the mirror  106  reflects the laser beam onto the photosensitive unit  109  shown in FIG. 1 for scanning.  
         [0030]    The spindle motor is further composed of an oil deflector  22 , a bias magnet  23  and an oil cycle material  40 . The oil deflector  22  is provided at the bearing portion of the bush  2  to prevent the lubricating oil  34  from leaking out of the bush  2 .  
         [0031]    The bias magnet  23  is inserted on the periphery of the bearing holder  16  opposing to an inner edge  24  of the rotor yoke  4 . In FIG. 4, the oil cycle material  40  is incorporated on the top and also bottom portions of the shaft bearing  12 .  
         [0032]    [0032]FIG. 5 is a cross sectional enlarged view of the substantial part of the spindle motor shown in FIG. 4. In FIG. 5, an oil cycle material  40  is placed on the shaft bearing  12  between the bearing holder  16  and the spindle shaft  3 . The clearance between the shaft bearing  12  is G, the clearance between the spindle shaft  3  and the oil cycle material  40  is C, the outer diameter of the spindle shaft  3  is D. Lubricating oil  50  is applied to the surface of the spindle shaft  3 .  
         [0033]    The oil deflector  22  is placed at the bearing portion of the bush  2  to prevent the lubricating oil  50  from leaking out of the bush  2 .  
         [0034]    Upon the rotation of the rotor  1 , the lubricating oil  50  moves to the top and bottom edges of the shaft bearing  12 . In the case of this embodiment, the clearance G is 0.003 mm.  
         [0035]    The oil cycle material  40  has an internal diameter bigger than that of the shaft bearing  12 . The clearance C is bigger than the clearance between the internal diameter of the bearing holder  16  and the outer diameter of the oil cycle material  40 .  
         [0036]    As set above, the oil  50  lubricates the spindle shaft  3  and the shaft bearing  12 . A hydraulic pressure  51  and a surface tension move the lubricating oil  50  along the clearance C. Then the oil  50  moves along the outer diameter of the cycle material  40  towards the shaft bearing  12  so that the oil  50  circulates to lubricate the shaft bearing  12  for rotation of the spindle shaft  3 .  
         [0037]    The oil cycle material  40  has a donut shaped plate, and a plurality of donut shaped plates with equal thickness are laminated. The material  40  is made of stainless plate having outer diameter 5.96 mm, internal diameter 3.24 mm and thickness 0.3 mm. For example, five plates are laminated.  
         [0038]    The bias magnet  23  fixed on the core  17  has a ring shape to adjust the wobbling of the rotor yoke  4  and magnetized in various patterns as shown in FIGS.  6 (A) to  6 (C).  
         [0039]    FIGS.  6 (A) to  6 (C) are plan views of a ring magnet to be inserted between the rotor  1  and the stator  10 .  
         [0040]    For example, in FIG. 6(A), the bias magnet  23  is divided into three portions where two portions are magnetized in S pole and N pole respectively, and a third portion is not magnetized. Such the bias magnet  23  is opposed to the inner edge  24  of the rotor yoke  4  that the inner edge  24  is attracted by the magnetic force of the bias magnet  23 .  
         [0041]    As the bias magnet  23  is not magnetized evenly, the rotor  1  is attracted to one side and consequently, the spindle shaft  3  is also attracted to one side in relation to the rotation of the rotor  4 .  
         [0042]    The clearance between the spindle shaft  3  and the shaft bearing  12  is minimized by the magnetic force of the bias magnet  23  attracting the spindle shaft  3  to one side. In this sense, the vibration caused by oil whirl can be suppressed. The oil whirl is unstable movement of the spindle shaft  3  caused by high-speed rotation of the spindle shaft  3 . As the mirror  106  is fixed with the spindle shaft  3 , the unstable movement or vibration of the spindle shaft  3  causes the wobbling of the reflection surface of the mirror  106 .  
         [0043]    The magnetization pattern of the bias magnet  23  can be smaller as shown in FIG. 6(B). The small magnetization pattern makes the attraction of the inner edge  24  stronger.  
         [0044]    In addition, the whole bias magnet can be magnetized as shown in FIG. 6(C). In this case, the center portion is moved from the spindle shaft  3  to shift the center of the rotor  1  to an appropriate position by the difference of the attraction force of the bias magnet  23  from side to side.  
         [0045]    The reliability of the oil cycle material  40  is explained as follows by reference of FIG. 7 in relation to the clearance C between the inner circumference of the material  40  and the periphery of the spindle shaft  3 .  
         [0046]    [0046]FIG. 7 shows reliability, that is, life (hour) of the motor in relation to the clearance C in different revolution of the motor. The motor used for this has a donut shaped plate for oil cycle material, and has the spindle of diameter D (which is 3 mm), and has the clearance C between the inner circumference of the oil cycle material and the diameter D.  
         [0047]    The revolution of the motor is set to 5,000 rpm, 20,000 rpm and 40,000 rpm respectively. FIG. 7 shows that the reliability of the motor is more influenced by the clearance C as the motor rotates faster.  
         [0048]    When the motor rotates at 40,000 rpm, the reliability drops below 3000 hours if the clearance C becomes smaller than 0.03 mm which is centesimal of the diameter D (D/100). This is caused by an outflow of the lubricating oil pushed by the hydraulic pressure of the oil cycle material when the clearance C becomes smaller than 0.03 mm. Consequently, the lubrication is depressed and the reliability is deteriorated.  
         [0049]    Similarly, the reliability drops below 3000 hours if the clearance C becomes bigger than 0.3 mm which is tenth part of the diameter D (D/10). This is caused by capillary phenomenon of the lubricating oil that the suction power of the oil is depressed and the reliability is deteriorated.  
         [0050]    The spindle motor requires at least 3000 hours of reliability which is preferable for a LBP motor. In this sense, the clearance C should be determined as follows. 
         1/100 ≦C/D≦ 1/10 
         [0051]    Accordingly, the lubricating oil  50  flowed along the spindle shaft  3  re-flows to the shaft bearing  12  by the donut plate  40  when the clearance C between the inner circumference of the donut plate  40  and diameter D of the spindle shaft  3  is more than or equal to D/100 and less than or equal to D/10. Therefore, the lubricating oil  50  does not outflow to the outside of the shaft bearing  12 .  
         [0052]    [Second embodiment] 
         [0053]    [0053]FIG. 8 is a cross-sectional view of the spindle motor in accordance with a second embodiment of the present invention. The spindle motor shown in FIG. 8 is identical to the spindle motor shown in FIG. 4 except for a ring plate  41  substituted from the oil cycle material  40 .  
         [0054]    The ring plate  41  is made of leachy sintered metal. The inner circumference of the ring plate  41  is bigger than the inner circumference of the spindle shaft  3 . The periphery of the ring plate  41  is held by the inner periphery of the bearing holder  16 .  
         [0055]    The ring plate  41  may be impregnated with the lubricating oil which increases the amount of the lubricating oil in the bearing shaft and extends the lubrication of the bearing shaft.  
         [0056]    [0056]FIG. 9 is a cross sectional enlarged view of the substantial part of the spindle motor shown in FIG. 8. In FIG. 9, the ring plate  41  has an internal diameter bigger than that of the shaft bearing  12 . The clearance between the ring plate  41  and the diameter D of the spindle shaft is bigger than the clearance between the internal diameter of the bearing holder  16  and the outer diameter of the oil cycle material  41 .  
         [0057]    As set above, the oil applied to the surface of spindle shaft  3  lubricates the spindle shaft  3  and the shaft bearing  12 . A hydraulic pressure  51  and a surface tension move the lubricating oil along the clearance between the spindle shaft  3  and the oil cycle material  41 . Then the oil moves along the outer diameter of the ring plate  41  towards the shaft bearing  12  so that the oil circulates and lubricates the shaft bearing  12  for rotation of the spindle shaft  3 .  
         [0058]    [0058]FIG. 10 shows reliability, life (hour) of the motor in relation to clearance C in different revolution of the motor. The motor used for this has the ring plate  41  for oil cycle material, and has the spindle shaft of diameter D (which is 3 mm), and has the clearance C between the inner circumference of the oil cycle material and the diameter D.  
         [0059]    As shown in FIG. 10, the clearance C between the inner circumference of the ring plate  41  and the diameter D of the spindle will be determined as follows. 
         1/100≦ C/D≦ 1/10 
         [0060]    Consequently, the reliability of the motor can be extensively improved by setting the clearance C within the above range.  
         [0061]    As described above, the spindle motor in accordance with the present invention provides the oil cycle material having a donut shaped plate which sucks in the lubricating oil flowing out from the bearing shaft by capillary phenomenon. Further, the lubricating oil can re-flow from the gap in the periphery of the donut shaped plate and the inner circumference of the bearing holder, so that the oil is prevented from flowing out from the bearing shaft. As the lubricating oil is circulated in the bearing shaft to prevent the spindle of the motor from burned and stuck, and the reliability of the motor will extensively improved.  
         [0062]    Further, as described above, the present invention provides the oil cycle material having donut shaped flat washer, that the lubricating oil is retained in the gap of the flat washer, and the oil is prevented from leaking out from the bearing shaft even the motor rotates in high speed.  
         [0063]    Furthermore, as described above, the present invention provides the oil cycle material which is made of leachy sintered alloy and formed in a ring shape, the lubricating oil is prevented from leaking out from the bearing shaft. Consequently, the reliability of the motor will be extensively improved.  
         [0064]    It is to be understood that the invention is not limited in its application to the details of construction and arrangement of parts illustrated in the accompanying drawings, since the invention is capable of other embodiments and of being practiced or carried out in various ways. Also it is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation.