Abstract:
An oblique contact ball bearing adopted to support a pinion shaft, where a sufficient range of rotation torque is provided for the oblique contact ball bearing to facilitate highly accurate setting, adjustment, and management of preload. To achieve the above, in the oblique contact ball bearing, a rust preventive oil having kinematic viscosity at 20° C. of 1-30 mm 2 /s is provided at the portions where raceways of inner and outer rings and balls are in contact with each other. This increases rotation torque of the bearing and preload setting is made in this state. As a result, when a predetermined pressure-contact force (thrust load) is applied to the balls and to the raceways, an oil-less state is relatively easily obtained and only an oil amount necessary for rust prevention stays on the raceways etc.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to an oblique contact ball bearing and a bearing device for supporting a pinion shaft wherein the pinion shaft is supported in a case of a differential device equipped with a vehicle. 
       BACKGROUND TECHNOLOGY 
       [0002]    There is a bearing device available for supporting a pinion shaft in which a tapered roller bearing is used as a roller bearing for supporting the pinion shaft (see the Patent Document 1). The tapered roller bearing for supporting the pinion shaft is advantageous in its large load capacity, however, a rotation torque thereof (rotation resistance with respect to rotation of the pinion shaft) is increased because an area where inner and outer rings contact the tapered rollers is large, and a sliding action occurs in a flange part. There is an oblique contact ball bearing (angular ball bearing) as a bearing usable for supporting the pinion shaft and capable of reducing the rotation torque. The oblique contact ball bearing can reduce the rotation torque since its inner and outer rings contact the balls in a small area.
   Patent Document 1: No. 2003-156128 of the Japanese Patent Application Laid-Open   
 
       DISCLOSURE OF THE INVENTION 
     Problem to be Solved by the Invention 
       [0004]    The oblique contact ball bearing has a feature that a preload is adjusted and set based on the rotation torque, while a range where the rotation torque is set is narrow in comparison to a range where the preload is adjusted. Due to the characteristic, in the case where the oblique contact ball bearing is used for supporting the pinion shaft, it may not be easy to attain a high accuracy in controlling the preload. 
         [0005]    A main object of the present invention is to secure a sufficient range of the rotation torque of the oblique contact ball bearing in the case of adopting the oblique contact ball bearing for supporting the pinion shaft so that the preload with respect to the oblique contact ball bearing can be easily set, adjusted and controlled with a high accuracy. 
       Means for Solving the Problem 
       [0006]    An oblique contact ball bearing according to the present invention is an oblique contact ball bearing for supporting a shaft body so as to freely rotate relative to a case, wherein oil having a kinematic viscosity in the range of 1-30 mm 2 /s at 20° C. is applied to a part where raceways of inner and outer rings contact the balls in the oblique contact ball bearing. 
         [0007]    A bearing device for supporting a pinion shaft according to the present invention comprises an oblique contact ball bearing for supporting the pinion shaft in a case of a differential device, wherein a preload of the ball bearing is set, adjusted and controlled based on a rotation torque, wherein the oil having the kinematic viscosity in the range of 1-30 mm 2 /s at 20° C. is accreted to the a part where the raceways of the inner and outer rings contact the balls in the oblique contact ball bearing. 
         [0008]    In coordinates wherein a horizontal axis represents a preload S and a vertical axis represents a rotation torque T, a relationship between the preload and the rotation torque can be generally set to such a relational expression as T=k·S. In that case, a gradient k relates to a range where the preload is set, adjusted and controlled, and an adjustment range of the rotation torque is increased as the gradient k is larger. As a result, the preload can be set, adjusted and controlled with a high accuracy based on the rotation torque. 
         [0009]    In the present invention, in terms of the foregoing relationship between the rotation torque and the preload, the oil having the before-mentioned kinematic viscosity is intentionally applied to the inner part of the bearing so that the gradient k in the foregoing relational expression (T=k·S) is increased in comparison to that of the conventional technology. As a result, the adjustment range of the rotation torque is increased with respect to the range where the same preload is set, adjusted and controlled, and the preload can be thereby accurately set, adjusted and controlled based on the rotation torque. 
         [0010]    The reason is described below why the favorable preload can be obtained in the case of selecting the oil having the foregoing kinematic viscosity. The oil having the foregoing kinematic viscosity is relatively superior in its fluidity and tends to easily run down from the accreted part such as the raceway. In the case of accreting the oil having a feature like this to the oblique contact ball bearing, the following outcome can be obtained. When the ball as a rolling body is pressed onto the raceway in order to impart a specified preload to the bearing, a force generated from the pressure-contact of the ball with respect to the raceway (pressure-contact force) pushes the oil away from the contact part between the ball and the raceway, as a result of which an oil is run out at the contact part, and the metal (ball) and the metal (raceway) are substantially directly in contact with each other (metal contact state). The oil-less state can be thus relatively easily generated when the preload (thrust load) of a certain degree is imparted to a part between the ball and the raceway. The rust preventive oil is often accreted to the inner and outer rings and the entire ball. 
         [0011]    Because the preload corresponds to the measurement result of the rotation torque, the preload can be, for example, more easily adjusted by adjusting the rotation torque. In terms of the foregoing fact, the oil having the foregoing kinematic viscosity is used so that the oil-less state intentionally generated between the ball and the raceway in the present invention. 
         [0012]    In the oblique contact ball bearing according to the present invention, the ball and the raceway are in the metal contact state when the rotation torque (activation torque) is measured, which easily generates the oil-less state between them. As a result, the measured value of the rotation torque is increased in comparison to the conventional case where there is an ordinary amount of oil between the ball and the raceway. 
         [0013]    Provided that the adjustment range of the rotation torque T of the conventional oblique contact ball bearing in a state where a thrust load “S 2 ” is imparted is “T 1 ”, and the adjustment range of the rotation torque T of the oblique contact ball bearing according to the present invention in a state where the same load “S 2 ” is imparted is “T 2 ”, T 2 &gt;T 1  is obtained. Therefore, when the same thrust load “S 2 ” is imparted, the oblique contact ball bearing according to the present invention is capable of adjusting the preload in a wider adjustment range of the rotation torque than the conventional oblique contact ball bearing. As is clear from the foregoing description, the preload can be easily and accurately imparted in the oblique contact ball bearing according to the present invention. 
         [0014]    A case is thought where the thrust load “S 2 ” in order to impart a target preload is adjusted in the range from “S 1 ” through “S 3 ” in terms of tolerance. In this case, when the adjustment range of the rotation torque T of the conventional oblique contact ball bearing is “T 3 ”, and the adjustment range of the rotation torque T of the oblique contact ball bearing according to the present invention is “T 4 ”, T 4 &gt;T 3  is obtained. Thus, the oblique contact ball bearing according to the present invention can achieve a wider adjustment range than the conventional oblique contact ball bearing even when the same preload is desirably obtained. The wider range can improve the accuracy and facility in imparting the preload. 
         [0015]    In order to relatively easily generate the oil-less state when the pressure-contact force of a certain degree (thrust load) is imparted to the ball and raceway, oil preferably has a kinematic viscosity in the range of 5-27 mm 2 /s at 20° C., and more preferably has a kinematic viscosity in the range of 5-12 mm 2 /s at 20° C. 
       EFFECT OF THE INVENTION 
       [0016]    According to the oblique contact ball bearing according of the present invention, in the case where the rotation torque for the confirmation of the preload is increased so that the same thrust load is desirably obtained, the preload can be adjusted in the adjustment range wider than that of the conventional oblique contact ball bearing. As a result, the preload can be accurately and easily imparted. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  is a sectional view illustrating a schematic constitution of a differential device according to a preferred embodiment of the present invention. 
           [0018]      FIG. 2  is an enlarged sectional view of a double row ball bearing part of the differential device. 
           [0019]      FIG. 3  is a sectional view showing a state where the double row ball bearing is being built up. 
           [0020]      FIG. 4  is a graph showing a relationship between a thrust load and a rotation torque. 
       
    
    
     DESCRIPTION OF REFERENCE SYMBOLS 
       [0000]    
       
         
           
               1  differential device 
               2  differential case 
               6  pinion gear 
               7  pinion shaft 
               10  first double row ball bearing 
               25  second double row ball bearing 
               11  first outer ring 
               21  first assembly component 
               13  first inner ring 
               12  second outer ring 
               22  second assembly component 
               14  second inner ring 
               28 , 29  row of balls 
               30 , 31  balls 
           
         
       
     
       DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0035]    Hereinafter, preferred embodiments of a bearing device for supporting a pinion shaft according to the present invention are described referring to the drawing.  FIG. 1  is a sectional view illustrating a schematic constitution of a differential device.  FIG. 2  is an enlarged sectional view of a double row ball bearing part.  FIG. 3  is a sectional view showing a state where an oblique contact double row ball bearing is being built up. 
         [0036]    As shown in  FIG. 1 , a differential device  1  comprises a differential case  2 . The differential case  2  comprises a front case  3  and a rear case  4 . The cases  3  and  4  are coupled with a bolt/nut  2   a  and thereby integrated. In the front case  3 , annular walls  27 A and  27 B for mounting the ball bearing are formed. 
         [0037]    The differential case  2  comprises internally a differential speed-change mechanism  5  for differentially gearing right and left wheels, and a pinion shaft (drive pinion)  7  having a pinion gear  6  on one side thereof. The pinion gear  6  is meshed with a ring gear  8  of the differential speed-change mechanism  5 . A shaft part  9  of the pinion shaft  7  is formed in a stepwise shape so that a diameter is reduced gradually from one side to the other side. 
         [0038]    The one side of the shaft part  9  of the pinion shaft  7  is supported with respect to the annular wall  27 A of the front case  3  so as to freely rotate around an axial center via a first double row ball bearing  10 . The other side of the shaft part  9  of the pinion shaft  7  is supported with respect to the annular wall  27 B of the front case  3  so as to freely rotate around the axial center via a second double row ball bearing  25 . 
         [0039]    As shown in  FIG. 2 , the first double row ball bearing  10  is an oblique contact double row ball bearing, and comprises a single first outer ring  11  fitted to an inner peripheral surface of the annular wall  27 A and a first assembly component  21 . The first assembly component  21  is incorporated into the first outer ring  11  from the pinion-gear side toward the opposite side of the pinion gear  6  (hereinafter, referred to as counter-pinion-gear side) along the axial-center direction, which constitutes the first double row ball bearing  10 . 
         [0040]    The first outer ring  11  has a structure of a counterbored outer ring. More specifically, the first outer ring  11  comprises a large diameter outer ring raceway  11   a  on the pinion-gear side and a small diameter outer ring raceway  11   b  on the counter-pinion-gear side, and a planar part  11   c  is formed between the large diameter outer ring raceway  11   a  and the small diameter outer ring raceway  11   b . The planar part  11   c  has a diameter larger than that of the small diameter outer ring raceway  11   b  and continuous to the large diameter outer ring raceway  11   a . An inner peripheral surface of the first outer ring  11  is thus formed in the stepwise shape. 
         [0041]    The first assembly component  21  comprises a single first inner ring  13 , a large-diameter-side row of balls  15 , a small-diameter-side row of balls  16 , and retainers  19  and  20 . The first inner ring  13  has a structure of a counterbored inner ring. More specifically, the first inner ring  13  comprises a large diameter inner ring raceway  13   a  and a small diameter inner ring raceway  13   b . The large diameter inner ring raceway  13   a  is opposed in a radial direction to the large diameter outer ring raceway  11   a . The small diameter inner ring raceway  13   b  is opposed in a radial direction to the small diameter outer ring raceway  11   b . A planar part  13   c  is formed between the large diameter inner ring raceway  13   a  and the small diameter inner ring raceway  13   b . The planar part  13   c  has a diameter larger than that of the small diameter inner ring raceway  13   b  and continuous to the large diameter inner ring raceway  13   a . An outer peripheral surface of the first inner ring  13  is thus formed in the stepwise shape. 
         [0042]    The large-diameter-side row of balls  15  are fit to place on the pinion-gear side, in other words, between the large diameter outer ring raceway  11   a  and the large diameter inner ring raceway  13   a . The small-diameter-side row of balls  16  are fit to place on the counter-pinion-gear side, in other words, between the small diameter outer ring raceway  11   b  and the small diameter inner ring raceway  13   b . In the first double row ball bearing  10 , a contact angle of the row of balls  15  and a contact angle of the row of balls  16  have a same direction. In other words, a line of action γ 1  in accordance with the contact angle of the row of balls  15  and a line of action γ 2  in accordance with the contact angle of the row of balls  16  face each other in a such a direction that an angle θ 1  (not shown) made by the lines of action γ 1  and γ 2  is 0° or an acute angle (0°≦θ 1 &lt;90°). Such a constitution is adopted so that a preload is imparted to the both rows of balls  15  and  16  in a same direction (in the present case, direction from the pinion-gear side toward the counter-pinion-gear side). Further, the lines of action γ 1  and γ 2  are tilted in such a direction that outer-diameter sides thereof are on the counter-pinion-gear side and inner-diameter sides thereof are on the pinion-gear side with respect to a thrust surface. To be brief, the lines of action γ 1  and γ 2  are tilted in the upper-right direction in  FIG. 2 . The retainers  19  and  20  retain balls  17  and  18  respectively constituting the rows of balls  15  and  16  at circumferentially equal intervals. 
         [0043]    The pinion shaft  17  is inserted through the first inner ring  13 , and an end surface of the first inner ring  13  abuts an end surface of the pinion gear  6  from the axial-center direction. The first inner ring  13  is sandwiched between the end surface of the pinion gear  6  and a plastic spacer  23  for setting the preload externally mounted on the shaft part  9  of the pinion shaft  7  at an intermediate position thereof from the axial-center direction. 
         [0044]    In the first double row ball bearing  10 , a diameter of the ball  17  in the large-diameter-side row of balls  15  and a diameter of the ball  18  in the small-diameter-side row of balls  16  are equal to each other, while pitch circle diameters D 1  and D 2  of the respective rows of balls  15  and  16  are different to each other. More specifically, the pitch circle diameter D 1  of the large-diameter-side row of balls  15  is set to a value larger than that of the pitch circle diameter D 2  of the small-diameter-side row of balls  16 . As described, the first double row ball bearing  10  has a double row structure (rows of balls  15  and  16 ) in which the two rows of balls have the pitch circle diameters D 1  and D 2  different to each other. 
         [0045]    The second double row ball bearing  25  is an oblique contact double row ball bearing, and comprises a single second outer ring  12  fitted to an inner peripheral surface of the annular wall  27 B and a second assembly component  22 . The second assembly component  22  is built up into the second outer ring  12  from the counter-pinion-gear side toward the pinion-gear side along the axial-center direction. 
         [0046]    The second outer ring  12  has a structure of a counterbored outer ring. More specifically, the second outer ring  12  comprises a small diameter outer ring raceway  12   a  on the pinion-gear side and a large diameter outer ring raceway  12   b  on the counter-pinion-gear side, and a planar part  12   c  is formed between the small diameter outer ring raceway  12   a  and the large diameter outer ring raceway  12   b . The planar part  12   c  has a diameter larger than that of the small diameter outer ring raceway  12   b  and continuous to the large diameter outer ring raceway  12   a . Accordingly, an inner peripheral surface of the second outer ring  12  is thus formed in the stepwise shape. 
         [0047]    The second assembly component  22  comprises a single second inner ring  14 , a small-diameter-side row of balls  28 , a large-diameter-side row of balls  29 , and retainers  32  and  33 . The second inner ring  14  has a structure of a counterbored inner ring. More specifically, the second inner ring  14  comprises a small diameter outer ring raceway  12   a  and a large diameter inner ring raceway  14   b . The small diameter inner ring raceway  14   a  is opposed in a radial direction to the small diameter outer ring raceway  12   a . The large diameter inner ring raceway  14   b  is opposed in a radial direction to the large diameter outer ring raceway  12   b . A planar part  14   c  is formed between the small diameter inner ring raceway  14   a  and the large diameter inner ring raceway  14   b . The planar part  14   c  has a diameter smaller than that of the large diameter inner ring raceway  14   b  and continuous to the small diameter inner ring raceway  14   a . Accordingly, an outer peripheral surface of the first inner ring  14  is thus formed in the stepwise shape. 
         [0048]    The pinion shaft  7  is inserted through the second inner ring  14 . The second inner ring  14  is sandwiched between the plastic spacer  23  for setting the preload and a closure plate  37  from the axial-center direction. 
         [0049]    The small-diameter-side row of balls  28  is fit to place on the pinion-gear side, that is, between the small diameter outer ring raceway  12   a  and the small diameter inner ring raceway  14   a . The large-diameter-side row of balls  29  is fit to place on the counter-pinion-gear side, that is, between the large diameter outer ring raceway  12   b  and the large diameter inner ring raceway  14   b . In the second double row ball bearing  25 , a contact angle of the row of balls  28  and a contact angle of the row of balls  29  face a same direction each other. In other words, a line of action γ 3  in accordance with the contact angle of the row of balls  28  and a line of action γ 4  in accordance with the contact angle of the row of balls  29  face each other in such a direction that an angle θ 2  (not shown) made by the lines of action γ 3  and γ 4  is 0° or an acute angle (0°≦θ 2 &lt;90°). Such a constitution is adopted so that the preload is imparted to the both rows of balls  28  and  29  in a same direction (in the present case, direction from the counter-pinion-gear side toward the pinion-gear side). Further, the lines of action γ 3  and γ 4  are tilted in such a direction that outer-diameter sides thereof are on the pinion-gear side and inner-diameter sides thereof are on the counter-pinion-gear side with respect to the thrust surface. To be brief, the lines of action γ 3  and γ 4  are tilted in the upper-right direction in  FIG. 2 . The retainers  32  and  33  retain balls  30  and  31  respectively so as to constitute the rows of balls  28  and  29  at circumferentially equal intervals. 
         [0050]    Thus, the inner-diameter sides of the lines of action γ 1  and γ 2  of the first double row ball bearing  10  are on the pinion-gear side with respect to the thrust surface, while the outer-diameter sides of the lines of action γ 3  and γ 4  of the second double row ball bearing  25  are on the pinion-gear side with respect to the thrust surface. Accordingly, the gradients of the lines of action in accordance with the contact angles of the bearings  10  and  25  are thereby reverse to each other. Such a constitution is adopted in order to reverse the directions where the preload is imparted in the bearings  10  and  25 . 
         [0051]    In the second double row ball bearing  25 , a diameter of the ball  30  in the small-diameter-side row of balls  28  and a diameter of the ball  31  in the large-diameter-side row of balls  29  are equal to each other, while pitch circle diameters D 3  and D 4  of the respective rows of balls  28  and  29  are different to each other. More specifically, the pitch circle diameter D 3  of the large-diameter-side row of balls  28  is set to a value smaller than that of the pitch circle diameter D 4  of the small-diameter-side row of balls  29 . As described, the second double row ball bearing  25  has a double row structure (rows of balls  28  and  29 ) in which the two rows of balls respectively have the pitch circle diameters D 3  and D 4  different to each other. 
         [0052]    An oil-circulating path  40  is formed between an outer wall of the front case  3  and one side of the annular wall  27 A. An oil inlet  41  of the oil-circulating path  40  is opened toward a ring-gear- 8  side of the oil-circulating path  40 , while an oil outlet  42  of the oil-circulating path  40  is opened toward between the annular walls  27 A and  27 B. 
         [0053]    The differential device  1  comprises a companion flange  43 . The companion flange  43  comprises a barrel part  44  and a flange part  45  integrally formed to the barrel part  44 . 
         [0054]    The barrel part  44  is externally mounted on the other side of the shaft part  9  of the pinion shaft  7 , that is, on a drive-shaft side (not shown). The closure plate  37  is interposed between an end surface of the barrel part  44  and an end surface of the second inner ring  14  of the second double row ball bearing  25 . 
         [0055]    An oil seal  46  is provided between an outer peripheral surface of the barrel part  44  and an inner peripheral surface of an opening of the front case  3  on the other side thereof. A seal protective cap  47  is attached to the opening part of the other side of the front case  3 . The oil seal  46  is covered with the seal protective cap  47 . 
         [0056]    A screw part  48  is formed at an end part of the shaft part  9  on the other side thereof. The screw part  48  is protruded into a central recess part  43   a  of the flange part  45 . A nut  49  is screwed into the screw part  48 . 
         [0057]    The nut  49  is screwed into the screw part  48  so that the first inner ring  13  of the first double row ball bearing  10  and the second inner ring  14  of the second double row ball bearing  25  are sandwiched between the end surface of the pinion gear  6  and an end surface of the companion flange  43  in the axial-center direction, and a predetermined preload is imparted to the first double row ball bearing  10  and the second double row ball bearing  25  via the closure plate  37  and the plastic spacer  23 . 
         [0058]    In the present preferred embodiment, the first double row ball bearing  10  and the second double row ball bearing  25  constitute the bearing device for supporting a pinion shaft. 
         [0059]    In the differential device  1  thus constituted, the lubricating oil  50  is reserved in the differential case  2  at a predetermined level L in a state where the operation is halted. The lubricating oil  50  is raised upward by the rotation of the ring gear  8  under the operation, travels through the oil circulating path  40  in the front case  3 , and is introduced and supplied to upper parts of the first double row ball bearing  10  and the second double row ball bearing  25 . Thereby, the lubricating oil  50  circulates in the differential case  2  so as to lubricate the first double row ball bearing and the second double row ball bearing  25 . 
         [0060]    Next, a method of assembling the differential device  1  thus constituted is described. In order to assemble the differential device  1 , the first double row ball bearing  10  and the second double row ball bearing  25  are assembled in advance. Before the first double row ball bearing  10  is assembled, clearances between the balls  17  of the large-diameter-side row of balls  15  and the raceways  11   a  and  13   a  and clearances between the balls  18  of the small-diameter-side row of balls  16  and the raceways  11   b  and  13   b  are adjusted. More specifically, the respective parts of the first double row ball bearing  10  are formed so that desired clearances can be obtained, and further, shapes of the respective parts are adjusted so that the desired clearances are obtained. 
         [0061]    In assembling the second double row ball bearing  25 , clearances between the balls  30  of the small-diameter-side row of balls  28  and the raceways  12   a  and  14   a  and clearances between the balls  31  of the large-diameter-side row of balls  29  and the raceways  12   b  and  14   b  are adjusted. More specifically, the respective parts of the second double row ball bearing  25  are formed so that desired clearances can be obtained, and further, shapes of the respective parts are adjusted so that the desired clearances are obtained. 
         [0062]    Further, oil, which is a rust-preventive oil  35 , is applied to the raceways, balls and any necessary region including the raceways and balls in order to prevent generation of rust when the bearings  10  and  25  are stored and transported before they are assembled into the differential device  1 . The rust preventive oil  35  has a kinematic viscosity in the range of 1-30 mm 2 /s at 20° C. 
         [0063]    After the foregoing adjustments and preparations are made, the first double row ball bearing  10  is disassembled into the first outer ring  11  and the first assembly component  21 , and the second double row ball bearing  25  is disassembled into the second outer ring  12  and the second assembly component  22 . Then, the first double row ball bearing  10  and the second double row ball bearing  25  are built up into the differential device  1 . More specifically, the first outer ring  11  and the second outer ring  12  are respectively pressed into the annular walls  27 A and  27 B. More specifically, in a state where the front case  3  and the rear case  4  are still separated, the first outer ring  11  is incorporated into the front case  3  and further pressed into from the one-side opening of the front case  3  until it abuts a stepwise part formed on the annular wall  27 A in the axial-center direction. Then, the second outer ring  12  is pressed into from the other-side opening of the front case  3  until it abuts a step part formed on the annular wall  28 B in the axial-center direction. 
         [0064]    The first assembly component  21  (to be specific, first inner ring  13 ) is inserted through the pinion shaft  7 . Then, the first assembly component  21  is built up into the pinion shaft  7  so as to locate on the pinion-gear- 6  side of the shaft part  9  of the pinion shaft  7 . 
         [0065]    The pinion shaft  7  into which the first assembly component  21  is built up is inserted through a one-side opening of the front case  3 . At the time, the pinion shaft  7  is inserted so that the balls  18  of the small-diameter-side row of balls  16  of the first assembly component  21  are fitted into the small-diameter outer ring raceway  11   b  of the first outer ring  11 . Further, the pinion shaft  7  is inserted so that the balls  17  of the large-diameter-side row of balls  15  are fitted into the large-diameter outer ring raceway  11   a  of the first outer ring  11 . In order to realize the assembly process described above, the small-diameter-side row of balls  18  is provided to be closer to a rear side in the direction where the pinion shaft  7  is inserted (the counter-pinion-gear side) than the large-diameter-side row of balls  16 . 
         [0066]    Next, the plastic spacer  23  is externally fit and inserted to the shaft part  9  of the pinion shaft  7  from the other-side opening of the front case  3 . Next, the second assembly component  22  (to be specific, second inner ring  14 ) is externally fit and inserted to the shaft part  9  of the pinion shaft  7  from the other-side opening of the front case  3 . In order to realize the foregoing external mounting and inserting, the small-diameter-side row of balls  28  is provided to be closer to a rear side in the direction where the pinion shaft  7  is inserted (pinion-gear side) than the large-diameter-side row of balls  29 . 
         [0067]    Thereafter, the closure plate  37  is inserted through the shaft part  9  of the pinion shaft  7  from the other-side opening of the front case  3 . Further, the oil seal  46  is mounted on the shaft part  9  of the pinion shaft  7  from the other-side opening of the front case  3 . The seal protective cap  47  is mounted on the other-side opening of the front case  3 . The barrel part  44  of the companion flange  43  is inserted through the seal protective cap  47  so that the end surface of the barrel part  44  abuts the closure plate  37 . Then, the nut  49  is screwed into the screw part  48 . Thereby, a thrust load is imparted to the first double row ball bearing  10  and the second double row ball bearing  25 , and a predetermined preload is imparted thereto. The direction where the preload is imparted is as follows. The preload is imparted to the first double row ball bearing  10  along the direction from the pinion-gear side toward the counter-pinion-gear side, while the preload is imparted to the second double row ball bearing  25  along the direction from the counter-pinion-gear side toward the pinion-gear side. Thus, the preload is imparted to the first and second double row ball bearings  10  and  25  in the reverse directions. 
         [0068]    In the first and second double row ball bearings  10  and  25  according to the present preferred embodiment, the rust preventive oil  35  having such a relatively good fluidity as the kinematic viscosity in the range of 1-30 mm 2 /s at 20° C., preferably 5-27 mm 2 /s, or more preferably 5-12 mm 2 /s is incorporated as one of the constituents. The rust preventive oil  35  having the above feature tends to easily run down from the adherent part such as the raceway. 
         [0069]    Below is described a reason why the rust preventive oil  35  having the above feature is used. When the nut  49  is screwed into the screw part  48  and the predetermined preload is imparted to the first and second double row ball bearings  10  and  25 , the balls  17  and  18  are respectively fit into the raceways  11   a ,  11   b ,  13   a  and  13   b , while the balls  28  and  29  are respectively fit into the raceways  12   a ,  12   b ,  14   a  and  14   b . When the rust preventive oil  35  having the kinematic viscosity in the range of 1-30 mm 2 /s at 20° C. is applied to the inside of the respective bearings at the time, the rust preventive oil  35  is pushed out from the a part where the balls are fit into the raceways by a pressure-contact force between the balls and the raceways, which easily generates an oil-less state. The balls and the raceways thereby easily generate a metal contact state where metal (balls) and metal (raceways) are substantially in contact with each other. 
         [0070]    Then, the preload of a certain degree (thrust load) is imparted to between the balls and the raceways in the oblique contact ball bearing having the above feature according to the present preferred embodiment, the oil-less state can be relatively easily generated. As a result, a measured value of a rotation torque in the oblique contact ball bearing according to the present preferred embodiment becomes larger in comparison to the conventional structure where an ordinary amount of oil is present between the balls and the raceways in the state where the preload is applied. As described, the oblique contact ball bearing according to the present preferred embodiment is capable of adjusting the preload in an adjustment range wider than that of the conventional oblique contact ball bearing. 
         [0071]    It is not preferable for the kinematic viscosity at 20° C. of the rust preventive oil  35  to exceed 30 mm 2 /s because the generation of the oil-less state becomes difficult, and even slight changes in the rotation and temperature make the rotation torque value variable in the case of measuring the rotation torque at a low speed, which destabilizes a range between a maximum value and a minimum value. When the kinematic viscosity at 20° C. is below 1 mm 2 /s, the oil-less state is easily generated, while it is not favorable because it becomes difficult for the oil to be retained in the raceways. Based on the foregoing reason, the rust preventive oil  35  having the kinematic viscosity in the range of 1-30 mm 2 /s at 20° C. is adopted in the oblique contact ball bearing according to the present preferred embodiment. 
         [0072]    The rust preventive oil  35  is obtained in such a manner that lubricating oil is mixed with a rust preventive additive, however, the type of the additive is not particularly limited. General examples of the types of the rust preventive oil  35  are a rust preventive oil of solvent dilution type, a rust preventive oil of lubricating type and the like, and any of them can be used. The rust preventive additive is generally a compound consisting of a polarity group and a lipophilic group in one molecule such as carboxylate, sulfonate, ester, amine, amide, phosphate or the like, which has a strong adsorption to metal and also a favorable solubility to oil. For example, an alkyl succinic acid derivative having an alkyl group such as C12-C18 is often used, and an amount of the additive is approximately 0.05%. Typical examples of the rust preventive additive, other than the foregoing examples, include metal soap such as calcium, zinc or lead salt of lanolin fatty acid, wax oxides or metal soap thereof, or soap of naphthenic acid, ester such as sorbitan monooleate, pentaerythritol monooleate, sulfonate or phosphate, and amine such as rosinamine, N-oleyl sarcosine. 
         [0073]    When the rotation torque is measured in the first and second double row ball bearings  10  and  25  in the substantially oil-less state, the measured values are larger than in the case where the rust preventive oil is present because the metal and the metal are fitted into each other. 
         [0074]    A graph of  FIG. 4  shows a relationship between a thrust load S (preload) imparted to the oblique contact double row ball bearing and a rotation torque T corresponding to the thrust load S. The thrust load S imparted to the oblique contact double row ball bearing can be known through the measurement of the rotation torque T. 
         [0075]    In the drawing, a broken line  60  (T=k 1 ·S) shows a result of the conventional oblique contact double row ball bearing, while a solid line  61  (T=k 2 ·S) shows a result of to the oblique contact double row ball bearings  10  and  25  according to the present invention. Comparing a gradient of the broken line  60  (k 19 ) with that of the solid line  61  (k 2 ) to each other, the gradient of the solid line  61  (k 2 ) is larger than the that of the broken line  60  (k 1 ) (k 2 &gt;k 1 ). It is shown that the double row ball bearings  10  and  25  are in the oil-less state as described above when the preload is applied thereto, and the rotation torque at the time is larger than that of the conventional oblique contact double row ball bearing. 
         [0076]    Below is described a case where, for example, a S 2  value is imparted as the thrust load S in order to impart the preload to the oblique contact double row ball bearing referring to  FIG. 4 . In the broken line  60 , the adjustment range of the rotation torque T corresponding to the S 2  value is T 1 . Correspondingly, the adjustment range of the rotation torque T is T 2  in the double row ball bearings  10  and  25  according to the present invention. The gradient of the solid line  61  is larger than that of the broken line  60  (T 2 &gt;T 1 ). 
         [0077]    In other words, when the same preload is imparted, the rotation torque T can be adjusted in the wider adjustment range in the double row ball bearings  10  and  25  according to the present invention than in the conventional double row ball bearing. As a result, the preload can be accurately and easily imparted. 
         [0078]    Further, a case is thought where the thrust load S 2  to be imparted is made to be a range from S 1  through S 3  in consideration of tolerance range. In this case, the adjustment range of the rotation torque T in the conventional oblique contact double row ball bearing is T 3 , while the adjustment range of the rotation torque T in the double row ball bearings  10  and  25  according to the present invention is T 4 . In this case, it is k 4 &gt;k 3  as shown in  FIG. 4 . Therefore, when the same preload is imparted, the rotation torque T can be adjusted in the wider adjustment range in the double row ball bearings  10  and  25  according to the present invention than in the conventional double row ball bearing. As a result, the thrust load (preload) can be accurately and easily imparted. 
         [0079]    In order to relatively easily generate the oil-less state when the pressure-contact force (thrust load) of a certain degree is imparted to the balls and the raceways, the rust preventive oil  35  having the kinematic viscosity in the range of 5-27 mm 2 /s at 20° C. is preferably used, and the rust preventive oil  35  having the kinematic viscosity in the range of 5-12 mm 2 /s at 20° C. is more preferably used. 
         [0080]    In the foregoing preferred embodiment, the present invention was applied to the oblique contact double row ball bearing (first double row ball bearing  10  and second double row ball bearing  25 ). However, the present invention is not limitedly applied to the double row bearing, and can be applied to an oblique contact ball bearing of other types such as a single row ball bearing in a similar manner. Furthermore, in the foregoing preferred embodiment, the present invention was applied to the structure in which the roller bearings constituting the bearing device for supporting the pinion shaft are both the oblique contact ball bearings. However, the present invention can be applied to a structure in which one of the roller bearings constituting the bearing device for supporting the pinion shaft is the oblique contact ball bearing in a similar manner.