Patent Publication Number: US-2023160428-A1

Title: Bearing device for vehicle wheel

Description:
TECHNICAL FIELD 
     The present invention relates to a bearing device for a vehicle wheel. 
     BACKGROUND ART 
     Conventionally, as a bearing device mainly used for rotatably supporting each wheel of a long-distance transport vehicle with a heavy body weight, such as a truck, a bus, or a pickup truck, a bearing device for a vehicle wheel including a multi-row tapered roller bearing is known. 
     In the bearing device for a vehicle wheel, for example, as disclosed in Patent Literature 1, a plurality of tapered rollers are rollably accommodated between a multi-row outer rolling surface (outer raceway surface) formed on an inner periphery of an outer member and a plurality of inner rolling surfaces (inner raceway surface) formed on an outer periphery of an inner member, and the inner raceway surface (or the outer raceway surface) has a large flange part integrally formed with the inner raceway surface, the large flange part having a guide surface with which a large end surface (large diameter-side end surface) of each of the tapered rollers formed in a convex shape is in sliding contact. 
     Further, in the large flange part, a chamfered part continuous with an outside surface (or a bore surface) of the large flange part, and a grinding cutout (large flange part-side cutout part) provided at a base of the large flange part are formed, and a rounded part having an arc shape in a cross-sectional view is formed in at least one of the chamfered part or the large flange part-side cutout part. 
     When the vehicle travels, lubricating oil enters a gap formed between the large diameter-side end surface of each of the tapered rollers and the guide surface of the large flange part through the rounded part to form an oil film due to a wedge effect, and the lubricating oil is easily drawn into a contact portion between the large diameter-side end surface and the guide surface to increase lubricating performance and durability. 
     CITATION LIST 
     Patent Literature 
     
         
         Patent Literature 1: JP-A 2011-163454 Gazette 
       
    
     SUMMARY OF INVENTION 
     Technical Problems 
     Meanwhile, an axial load applied to the bearing device for a vehicle wheel including such a multi-row tapered roller bearing is borne mainly between the large diameter-side end surface of each of the tapered rollers and the guide surface of the large flange part, and a contact ellipse with a tangential direction of a circular locus when the tapered roller rolls set as a major axis direction is formed at the contact portion between the large diameter-side end surface and the guide surface on the basis of Hertz elastic contact theory. 
     Therefore, the contact portion between the large diameter-side end surface of each of the tapered rollers and the guide surface of the large flange part is in rolling contact with each other with large slippage due to the structure of the bearing, so that wear easily occurs between the large diameter-side end and the guide surface. 
     In the bearing device for a vehicle wheel disclosed in Patent Literature 1, however, an allowable value of the amount of wear between the large diameter-side end of each of the tapered rollers and the guide surface of the large flange part is not specifically set in advance, so that there is a possibility that the gap between the large diameter-side end and the guide surface (clearance of the large diameter-side end as viewed from the guide surface) is not sufficiently secured and becomes minimal when a travel distance of the vehicle reaches a long distance of about 1 million km, for example. 
     As a result, it becomes difficult to sufficiently draw the lubricating oil into the contact portion between the large diameter-side end surface of each of the tapered rollers and the guide surface of the large flange part, and the oil film forming ability decreases due to a failure in producing the wedge effect, which may cause, for example, abnormal heat generation due to metal contact. 
     It is therefore an object of the present invention to provide a bearing device for a vehicle wheel for use in, for example, a long-distance transport vehicle with a heavy body weight, the bearing device for a vehicle wheel reducing the occurrence of, for example, abnormal heat generation caused by metal contact and enhancing anti-seizing properties. 
     Solutions to Problems 
     The problems to be solved by the present invention are as described above, and solutions to the problems will be described below. 
     Specifically, a bearing device for a vehicle wheel includes an outer member having a multi-row outer raceway surface on an inner periphery, an inner member having, on an outer periphery, a multi-row inner raceway surface facing the multi-row outer raceway surface, and a plurality of tapered rollers rollably accommodated between the outer raceway surface and the inner raceway surface, each of the tapered rollers having a large diameter-side end surface formed in a convex shape having a predetermined radius of curvature, the inner raceway surface having a large flange part integrally formed with the inner raceway surface, the large flange part having a guide surface having a conical surface shape, the guide surface being in point contact with the large diameter-side end surface in a slidable manner to guide each of the tapered rollers. The large flange part includes a chamfered part continuous with an outside surface of the large flange part, and a rounded part formed in an arc shape having a predetermined radius of curvature in a cross-sectional view, the rounded part being continuous with the chamfered part on one side and continuous with the guide surface on another side, the large flange part has a large flange part-side cutout part formed at a base of the large flange part, the large flange part-side cutout part being continuous with the guide surface, the large diameter-side end surface of each of the tapered rollers has a tapered roller-side cutout part having a circular shape formed coaxially with the large diameter-side end surface, and a clearance of the large diameter-side end surface as viewed from a contact point between the large diameter-side end surface and the guide surface in an unused initial state is greater than or equal to 15 μm. 
     Advantageous Effects of Invention 
     The effects of the present invention are as follows. 
     That is, with the bearing device for a vehicle wheel according to the present invention, it is possible to sufficiently secure the gap between the large diameter-side end surface of each of the tapered rollers and the guide surface of the large flange part to maintain the oil film forming ability due to the wedge effect, to reduce the occurrence of, for example, abnormal heat generation caused by metal contact, and to enhance anti-seizing properties. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a cross-sectional view showing an overall structure of a bearing device for a vehicle wheel according to an embodiment of the present invention. 
         FIG.  2    is a diagram showing details of a contact portion between a large end surface of a tapered roller and a large flange part of an inner ring and is an enlarged cross-sectional view of a portion indicated by a region II in  FIG.  1   . 
         FIG.  3 A  is a diagram showing a degree of wear in the large end surface of the tapered roller over time and is an enlarged cross-sectional view of the large end surface of the tapered roller in an initial state where wear has not occurred yet. 
         FIG.  3 B  is a diagram showing the degree of wear in the large end surface of the tapered roller over time and is an enlarged cross-sectional view of the large end surface of the tapered roller in a state where wear has occurred to a moderate degree relative to a predetermined wear limit. 
         FIG.  3 C  is a diagram showing the degree of wear in the large end surface of the tapered roller over time and is an enlarged cross-sectional view of the large end surface of the tapered roller in a state where wear has occurred to the predetermined wear limit. 
         FIG.  4 A  is a diagram showing a relationship between the tapered roller and the inner ring in the bearing device for a vehicle wheel in operation and is a diagram schematically showing a state where the tapered roller is skewed as viewed from an outer diameter side in a radial direction. 
         FIG.  4 B  is a diagram showing a relationship between the tapered roller and the inner ring in the bearing device for a vehicle wheel in operation and is a diagram schematically showing a state where a contact ellipse between the tapered roller and the large flange part of the inner ring moves in response to the skew as viewed in a direction of an arrow IV B in  FIG.  4 A . 
         FIG.  5    is a diagram schematically showing a state where the contact ellipse between the tapered roller and the large flange part of the inner ring moves in response to the skew as viewed in a circumferential direction. 
     
    
    
     DESCRIPTION OF EMBODIMENT 
     Next, an embodiment embodying a bearing device for a vehicle wheel according to the present invention will be described with reference to  FIGS.  1  to  5   . 
     Herein, for the sake of convenience, a direction parallel to a rotation axis G (see  FIG.  1   ) of a bearing device for a vehicle wheel  1  is defined as an “axial direction”, a direction orthogonal to the rotation axis G is defined as a “radial direction”, and a direction along an arc centered on the rotation axis G is defined as a “circumferential direction”. 
     Further, a side adjacent to the rotation axis G in the radial direction is defined as an “inner diameter side”, and a side opposite from the inner diameter side in the radial direction is defined as an “outer diameter side”. 
     [Overall Structure of Bearing Device for Vehicle Wheel  1 ] 
     First, an overall structure of the bearing device for a vehicle wheel  1  according to the present embodiment will be described with reference to  FIGS.  1  and  2   . 
     As shown in  FIG.  1   , the bearing device for a vehicle wheel  1  has a structure called the first generation, and primarily includes an outer ring  2  that is an example of an outer member and has, on an inner periphery, outer raceway surfaces  21 ,  21  of a multi-row type (double-row type in the present embodiment), a pair of inner rings  3 ,  3  that is an example of an inner member and has, on an outer periphery, inner raceway surfaces  31 ,  31  of a multi-row type (double-row type in the present embodiment) facing the outer raceway surfaces  21 ,  21 , a plurality of tapered rollers  4 ,  4 , . . . rollably accommodated between the outer raceway surfaces  21 ,  21  and the inner raceway surfaces  31 ,  31 , a pair of cages  5 ,  5  that hold the plurality of tapered rollers  4 ,  4  . . . so as to make the plurality of tapered rollers  4 ,  4  . . . rollable, seals  6 ,  6  attached to both ends of the outer ring  2  in the axial direction, and the like. 
     In the outer ring  2 , the pair of outer raceway surfaces  21 ,  21  is formed in a mortar shape whose a diameter increases from a center to both ends in the axial direction. Further, the inner raceway surface  31  of each inner ring  3  is formed in a conical shape whose diameter increases from a center to an end of the bearing device for a vehicle wheel  1  in the axial direction. 
     The pair of inner rings  3 ,  3  is disposed, on the inner diameter side of the outer ring  2 , coaxially with the outer ring  2  with the pair of inner rings  3 ,  3  facing the outer ring  2 , and the bearing device for a vehicle wheel  1  is structured as a multi-row tapered roller bearing of a back-to-back arrangement type. 
     In the inner ring  3 , a large flange part  32  extending toward the outer diameter side is integrally formed at an end adjacent to a large diameter side of the inner raceway surface  31 . Further, a small flange part  33  extending toward the outer diameter side and positioned on the inner diameter side relative to the large flange part  32  is integrally formed at an end on the small diameter side of the inner raceway surface  31 . 
     Here, the “large diameter side” means an enlarged diameter side of the inner raceway surface  31 . The “small diameter side” means a reduced diameter side of the inner raceway surface  31 . 
     The plurality of tapered rollers  4 ,  4 , . . . held by the cage  5  are disposed on the inner raceway surface  31 . 
     As a result, each tapered roller  4  slidably comes into contact with the large flange part  32  to be guided to move in the radial direction and restricted from moving toward the large diameter side (that is, an inner ring large flange part side) in the axial direction by the large flange part  32 . 
     Further, when slightly moving toward the small diameter side in the axial direction, each tapered roller  4  comes into contact with the small flange part  33  to be restricted from moving toward the small diameter side (that is, an inner ring small flange part side) in the axial direction by the small flange part  33 . 
     The outer ring  2 , the inner ring  3 , and the tapered roller  4  are made of, for example, high-carbon chromium bearing steel such as SUJ2, and are hardened in a range of 58 to 64 HRC up to a core part by through hardening, but are not limited to such a material, and the outer ring  2 , the inner ring  3 , and the tapered roller  4  may be made of, for example, structural alloy steel such as SCr420, and their surfaces may be hardened in a range of 58 to 64 HRC by carburizing and quenching. 
     Further, the cage  5  is formed by, for example, injection molding an engineering plastic such as polyamide (PA)  66 , a super engineering plastic such as polyphenylene sulfide (PPS), or a material containing, as a base, thermoplastic synthetic resin of such a plastic, and an appropriate amount of a reinforcing material such as glass fiber (GF). 
     As shown in  FIG.  2   , a large diameter-side end surface  41  of the tapered roller  4  is formed in a convex shape having a predetermined radius of curvature R, and a chamfered part  42  is formed at a corner (peripheral part) on an outer peripheral side of the large diameter-side end surface  41 . 
     Further, on an inner peripheral side of the large diameter-side end surface  41 , a tapered roller-side cutout part  43  having a circular shape is formed coaxially with the large diameter-side end surface  41  by, for example, header processing. 
     The radius of curvature R of the large diameter-side end surface  41  is set within a range of 0.75 to 0.95 times a distance RO (not shown) from an apex of a conical surface that is the inner raceway surface  31  of the inner ring  3  to a contact point P between the large diameter-side end surface  41  of the tapered roller  4  and the large flange part  32  (more specifically, a guide surface  32   a  to be described later) of the inner ring  3  (R=0.75 to 0.95 RO). 
     Note that, in the present embodiment, as will be described later, the radius of curvature R of the large diameter-side end surface  41  is set so as to make a clearance between the large diameter-side end surface  41  and the guide surface  32   a  equal to a predetermined value. 
     On the other hand, the large flange part  32  of the inner ring  3  has the guide surface  32   a  having a conical surface shape for guiding the tapered roller  4 , and each tapered roller  4  is disposed on the inner raceway surface  31  with the large diameter-side end surface  41  in point contact with the guide surface  32   a  in a slidable manner. 
     Further, a chamfered part  32   c  and a rounded part  32   d  are sequentially and continuously formed from an outside surface  32   b  to the guide surface  32   a  of the large flange part  32 , and in a circumferential cross-sectional view, the chamfered part  32   c  is formed in a gently curved arc shape having a predetermined radius of curvature r 1 , and the rounded part  32   d  is formed in an arc shape having a predetermined radius of curvature r 2  sufficiently smaller than the radius of curvature r 1  of the chamfered part  32   c.    
     In other words, the large flange part  32  of the inner ring  3  has the chamfered part  32   c  continuous with the outside surface  32   b  thereof (of the large flange part  32 ) and the rounded part  32   d  that is formed in an arc shape having the predetermined radius of curvature r 2  in a cross-sectional view and is continuous with the chamfered part  32   c  on one side and continuous with the guide surface  32   a  on the other side. 
     Note that, in the present embodiment, as will be described later, the shape of the rounded part  32   d  is set so as to make the radius of curvature r 2  and a degree of rounding equal to their respective predetermined values. 
     Further, a start point position of the rounded part  32   d  is set to a predetermined position. 
     Thein, with the tapered roller  4  installed in the inner ring  3 , a start point of the tapered roller-side cutout part  43 , that is, an outer peripheral end  43   a  is set at a position on the inner diameter side relative to the outside surface  32   b  of the large flange part  32 . 
     Further, the outer peripheral end  43   a  of the tapered roller-side cutout part  43  is positioned in the vicinity of a boundary between the guide surface  32   a  and the rounded part  32   d  of the large flange part  32 , and an annular space q that is a wedge-shaped gap opened toward the outside surface  32   b  in the circumferential cross-sectional view is formed between the large diameter-side end surface  41  of the tapered roller  4  and the guide surface  32   a  of the large flange part  32 . 
     As a result, at a contact portion between the large diameter-side end surface  41  of the tapered roller  4  and the large flange part  32  of the inner ring  3 , lubricating oil is drawn into the annular space q by a wedge effect, and an oil film is easily formed, which allows an increase in lubricating performance and an increase in durability of bearing device for a vehicle wheel  1 . 
     Note that, in the present embodiment, as will be described later, the start point position of the tapered roller-side cutout part  43  is set to a predetermined position. 
     In the inner ring  3 , a large flange part-side cutout part  34  continuous with the guide surface  32   a  is formed at a base of the large flange part  32 , that is, a corner between the inner raceway surface  31  and the large flange part  32 . 
     An edge  34   a  of the large flange part-side cutout part  34  adjacent to the guide surface  32   a  is set at a position on the outer diameter side relative to a corner between the large diameter-side end surface  41  and the chamfered part  42  of the tapered roller  4 . 
     As a result, even when the large flange part  32  wears due to contact with the large diameter-side end surface  41  of the tapered roller  4 , the corner of the tapered roller  4  is less prone to come into contact with the guide surface  32   a , and it is thus possible to prevent formation of a harmful protrusion on the edge  34   a  of the large flange part-side cutout part  34 . 
     Note that, at the base of the large flange part  32 , a rounded part formed in an arc shape having a predetermined radius of curvature in a circumferential cross-sectional view, the rounded part circumscribing the guide surface  32   a , may be separately provided from the edge  34   a  of the large flange part-side cutout part  34  toward the outer diameter side. 
     With such a structure, it is possible to prevent, with higher reliability, formation of a harmful protrusion on the edge  34   a  of the large flange part-side cutout part  34  due to wear in the large flange part  32 . 
     The bearing device for a vehicle wheel  1  having the above-described structure is excellent in durability against high loads or impact loads, and is suitable mainly for use as a bearing that rotatably supports each wheel of a long-distance transport vehicle with a heavy body weight, such as a truck, a bus, or a pickup truck. 
     Meanwhile, in a conventional bearing device for a vehicle wheel including a multi-row tapered roller bearing, when the bearing device is used as a bearing of a long-distance transport vehicle, there is no problem under a condition where a travel distance of the vehicle is about several hundreds of thousands km, but when the travel distance reaches a long distance of, for example, about one million km, the degree of wear between the large diameter-side end surface  41  of the tapered roller  4  and the guide surface  32   a  of the large flange part  32  of the inner ring  3  increases, and there is a possibility that lubricating oil is not sufficiently drawn into the above-described annular space q that is a wedge-shaped gap. 
     As a result, at the contact portion (contact point P) between the large diameter-side end surface  41  of the tapered roller  4  and the guide surface  32   a  of the large flange part  32  of the inner ring  3 , oil film forming ability decreases, which may cause, for example, abnormal heat generation due to metal contact. Therefore, there has been a high demand for improvement measures for sufficiently drawing lubricating oil into the annular space q even under a condition where the travel distance of the vehicle has reached a long distance of about 1 million km. 
     Further, a contact ellipse E based on Hertz elastic contact theory appears at the contact portion (contact point P) between the large diameter-side end surface  41  of the tapered roller  4  and the guide surface  32   a  of the large flange part  32  of the inner ring  3 , and it is known that the contact ellipse E increases in size in response to an increase in axial load during the use of the bearing and becomes the maximum, for example, during maximum turning of the vehicle. 
     Therefore, in the conventional bearing device for a vehicle wheel including a multi-row tapered roller bearing, in order to reduce the occurrence of galling between the large diameter-side end surface  41  of the tapered roller  4  and the large flange part  32  of the inner ring  3 , the shape of the rounded part  32   d  is set such that at least the contact ellipse E that has become the maximum does not ride up on the outside surface  32   b  of the large flange part  32 , and a boundary between the guide surface  32   a  of the large flange part  32  and the rounded part  32   d  (the start point of the rounded part  32   d  to be described later, that is, an inner peripheral end a 1  of the rounded part  32   d  adjacent to the guide surface  32   a ) is positioned in the vicinity of an outer edge of the contact ellipse E that has become the maximum. 
     As will be described later, however, in the bearing device for a vehicle wheel during rotation operation, the tapered roller  4  may be inclined and skewed in the circumferential direction relative to a rotation axis g (see  FIG.  4 A ) thereof due to the influence of an oil film such as lubricating oil, and at this time, the contact ellipse E also changes in position in response to the skew of the tapered roller  4 , so that it is not sufficient that only the influence of the contact ellipse E that has become the maximum is taken into account for the setting of the rounded part  32   d.    
     As a result of intensive studies in view of such improvements to the conventional bearing device for a vehicle wheel, various improvements described below allows the bearing device for a vehicle wheel  1  according to the present embodiment to be implemented as a bearing device for a vehicle wheel that reduces the occurrence of, for example, abnormal heat generation caused by metal contact even when the bearing device is used in a long-distance transport vehicle having a heavy body weight and has better anti-seizing properties. 
     Note that, as described above, the bearing device for a vehicle wheel  1  according to the present embodiment is structured as a multi-row tapered roller bearing of a back-to-back arrangement type, but is not limited to such a bearing, and may be structured as a single-row tapered roller bearing. 
     Further, the bearing device for a vehicle wheel  1  according to the present embodiment has a structure called the first generation as described above, but is not limited to such a structure, and for example, the bearing device for a vehicle wheel  1  may have a structure called the second generation in which an outer ring has a flange or the third generation in which an inner raceway surface is directly formed on an outer periphery of a hub ring. 
     [Improvements of Bearing Device for Vehicle Wheel  1 ] 
     Next, improvements of the bearing device for a vehicle wheel  1  according to the present embodiment that have been made to the conventional bearing device for a vehicle wheel will be described with reference to  FIGS.  2  to  5   . 
     &lt;Shape of Large Diameter-Side End Surface  41  of Tapered Roller  4 &gt; 
     In  FIG.  2   , the large diameter-side end surface  41  of the tapered roller  4  is formed in a convex shape having the predetermined radius of curvature R as described above, and is in point contact with the guide surface  32   a  of the large flange part  32  of the inner ring  3  at the contact point P. 
     Therefore, between the large diameter-side end surface  41  and the guide surface  32   a  and in the vicinity of the contact point P, the annular space q that is a wedge-shaped gap opened toward the outside surface  32   b  in the circumferential cross-sectional view is formed. 
     Here, the large diameter-side end surface  41  of the tapered roller  4  and the guide surface  32   a  of the large flange part  32  of the inner ring  3  are in sliding contact with each other and are thus prone to wear, as the total rotation count of the bearing device for a vehicle wheel  1  increases, the degree of wear between the large diameter-side end surface  41  and the guide surface  32   a  gradually increases, and the annular space q gradually decreases and finally disappears accordingly. 
     Specifically, as shown in  FIG.  3 A , in a state where the bearing device for a vehicle wheel  1  has yet to be used or the total rotation count is relatively low, the large diameter-side end surface  41  of the tapered roller  4  and the guide surface  32   a  of the large flange part  32  of the inner ring  3  are in point contact with each other at a predetermined position (contact point P), and the annular space q located between the large diameter-side end surface  41  and the guide surface  32   a  has a sufficient gap having a predetermined shape set in advance. 
     As the total rotation count of the bearing device for a vehicle wheel  1  increases, the degree of wear between the large diameter-side end surface  41  of the tapered roller  4  and the guide surface  32   a  of the large flange part  32  of the inner ring  3  gradually increases, and the outer peripheral end  43   a  of the tapered roller-side cutout part  43  comes closer to the guide surface  32   a  accordingly. 
     As a result, the gap formed by the annular space q gradually decreases, and when the large diameter-side end surface  41  and/or the guide surface  32   a  has worn to a moderate degree as illustrated in  FIG.  3 B , for example, the gap formed by the annular space q becomes the minimum. 
     Then, when the total rotation count of the bearing device for a vehicle wheel  1  further increases and the degree of wear of the large diameter-side end surface  41  of the tapered roller  4  and/or the guide surface  32   a  of the large flange part  32  of the inner ring  3  reaches a preset limit as illustrated in  FIG.  3 C , for example, the outer peripheral end  43   a  of the tapered roller-side cutout part  43  reaches the guide surface  32   a , and the annular space q disappears. 
     When the gap formed by the annular space q decreases to the minimum, it becomes difficult to sufficiently draw the lubricating oil due to the wedge effect, and the oil film forming ability at the contact portion (contact point P) between the large diameter-side end surface  41  of the tapered roller  4  and the guide surface  32   a  of the large flange part  32  of the inner ring  3  decreases accordingly. 
     As a result, as shown in  FIG.  3 B , for example, when the large diameter-side end surface  41  and/or the guide surface  32   a  has worn to a moderate degree and the gap formed by the annular space q becomes the minimum, a sufficient wedge effect cannot be produced any more, and it becomes difficult to form an oil film, which may cause, for example, abnormal heat generation due to metal contact. Further, the tapered roller  4  is easily skewed, and a degree of skew (skew amount) also increases, so that galling easily occurs between the large diameter-side end surface  41  and the large flange part  32 . 
     Therefore, as will be described later, for example, even when the start point position of the rounded part  32   d , that is, the position of the inner peripheral end a 1  of the rounded part  32   d  adjacent to the guide surface  32   a  (see  FIG.  2   ) is set with the state where the tapered roller  4  is skewed taken into account in advance in order to reduce the occurrence of galling between the large diameter-side end surface  41  of the tapered roller  4  and the large flange part  32  of the inner ring  3 , the skew amount of the tapered roller  4  gradually increases as the degree of wear of the large diameter-side end surface  41  and/or the guide surface  32   a  increases, and thus galling is highly likely to occur at any point in time. 
     Accordingly, in order to reduce the occurrence of galling between the large diameter-side end surface  41  and the large flange part  32 , it is important to set the shape of the gap formed by the annular space q, that is, the clearance between the large diameter-side end surface  41  and the guide surface  32   a  (more specifically, a size of a gap between the guide surface  32   a  and the outer peripheral end  43   a  of the tapered roller-side cutout part  43 ) with an amount of wear up to which the oil film forming ability can be kept high enough even when the degree of wear increases at the contact portion (contact point P) between the large diameter-side end surface  41  of the tapered roller  4  and the guide surface  32   a  of the large flange part  32  of the inner ring  3  taken into account in advance. 
     Meanwhile, a long-distance transport vehicle such as a truck tends to be longer in travel distance than a passenger vehicle, and may travel a long distance of, for example, about 1 million km, and even when used in such a long-distance transport vehicle, it is important to set the clearance to an optimum value in advance such that the oil film forming ability can be kept high enough. 
     For the conventional bearing device for a vehicle wheel, however, the above-described clearance is not sufficiently managed, and the optimum value of the clearance that allows the oil film forming ability to be kept high enough even when the travel distance reaches a long distance of about 1 million km is not clear, so that there has been an increasing demand for finding such an optimum value of the clearance in recent years. 
     Then, the present inventor has found, as a result of intensive studies, that it is possible to keep the oil film forming ability high enough even when the travel distance reaches a long distance of about 1 million km by setting in advance the clearance between the large diameter-side end surface  41  of the tapered roller  4  and the guide surface  32   a  of the large flange part  32  of the inner ring  3  greater than or equal to 15 μm. 
     As described above, in the bearing device for a vehicle wheel  1 , it is apparent from the result of the studies based on the following examination results that the oil film forming ability can be kept high enough by setting in advance the clearance of the large diameter-side end surface  41  as viewed from the contact point P between the large diameter-side end surface  41  of the tapered roller  4  and the guide surface  32   a  of the large flange part  32  of the inner ring  3  greater than or equal to 15 μm. 
     That is, for a long-distance transport vehicle provided with a bearing device for a vehicle wheel including a multi-row tapered roller bearing, the amount of wear (unit: μm) in the large flange part  32  of the inner ring  3  and the clearance (unit: μm) of the large diameter-side end surface  41  of the tapered roller  4  at the time when the travel distance reaches 100,000 km, 250,000 km, 500,000 km, 1 million km, and 1.5 million km were examined, and the results shown in the following [Table 1] were obtained. 
     
       
         
           
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 Travel distance 
               
               
                   
                 [ten thousand km] 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 10 
                 25 
                 50 
                 100 
                 150 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Amount of wear in large flange 
                 3 
                 5 
                 10 
                 11 
                 12 
               
               
                 part of inner ring [μm] 
               
               
                 Clearance of large diameter-side 
                 12 
                 10 
                 5 
                 4 
                 3 
               
               
                 end surface of tapered roller [μm] 
               
               
                   
               
            
           
         
       
     
     As shown in Table 1, the amount of wear in the large flange part  32  of the inner ring  3  gradually increased as the travel distance of the long-distance transport vehicle increased up to about 500,000 km. Specifically, the amount of wear increased by 2 μm until the travel distance increased from 100,000 km to 250,000 km, whereas the amount of wear increased by 5 μm until the travel distance increased from 250,000 km to 500,000 km. 
     On the other hand, when the travel distance of the long-distance transport vehicle exceeded 500,000 km, a degree of increase in the amount of wear in the large flange part  32  of the inner ring  3  decreases regardless of the travel distance. Specifically, an increase in the amount of wear was 1 μm in both cases where the travel distance increased from 500,000 km to 1 million km and where the travel distance increased from 1 million km to 1.5 million km. 
     It is considered that the above-described results were obtained on the basis of the fact that when the travel distance of the long-distance transport vehicle reaches about 500,000 km, and the large diameter-side end surface  41  of the tapered roller  4  and/or the guide surface  32   a  of the large flange part  32  of the inner ring  3  wears to some degree, the large diameter-side end surface  41  and the guide surface  32   a  of the large flange part  32  are well-fitted to each other, and “stability” of the large diameter-side end surface  41  with respect to the guide surface  32   a  increases, so that an increase in the degree of wear is suppressed. 
     Accordingly, setting in advance the clearance greater than or equal to 15 μm prevents the gap formed by the annular space q from becoming the minimum as illustrated in  FIG.  3 B , for example, even when the travel distance of the long-distance transport vehicle provided with the bearing device for a vehicle wheel  1  exceeds 1 million km and allows the gap between the large diameter-side end surface  41  of the tapered roller  4  and the guide surface  32   a  of the large flange part  32  of the inner ring  3  to be kept large enough so as to maintain the oil film forming ability due to the wedge effect. 
     It is therefore possible to reduce the occurrence of, for example, abnormal heat generation caused by metal contact, enhance anti-seizing properties, suppress an increase in the skew amount of the tapered roller  4 , and reduce the occurrence of galling between the large diameter-side end surface  41  of the tapered roller  4  and the large flange part  32  of the large flange part  32  of the inner ring  3 . 
     &lt;Radius of Curvature r 2  and Degree of Rounding of Rounded Part  32   d&gt;   
     As described above, in the bearing device for a vehicle wheel  1  during rotation operation, the tapered roller  4  may be inclined and skewed in the circumferential direction relative to the rotation axis g thereof (see  FIG.  4 A ) due to the influence of an oil film such as lubricating oil. 
     Specifically, as shown in  FIG.  4 A , when the tapered roller  4  rolls in one direction (a direction of an arrow A in  FIG.  4 A . Hereinafter referred to as a “rolling direction A” as needed) relative to the inner ring  3 , the tapered roller  4  is skewed so as to cause a moment produced by a thrust force received by the tapered roller  4  and a moment produced by an oil film reaction force to be balanced. 
     As a result, the contact point P between the large diameter-side end surface  41  of the tapered roller  4  and the guide surface  32   a  of the large flange part  32  of the inner ring  3  moves in a direction opposite to the rolling direction A and is positioned in the vicinity of the outside surface  32   b  (a position of a contact point P 1  in  FIG.  4 B ) on the guide surface  32   a  of the large flange part  32  as illustrated in  FIG.  4 B . 
     When the position of the contact point P moves to the position of the contact point P 1  due to the skew of the tapered roller  4 , the position of the contact ellipse E appearing at the contact point P also moves. 
     Specifically, as illustrated in  FIG.  5   , in the circumferential cross-sectional view, the contact ellipse E moves toward the outside surface  32   b  of the large flange part  32  of the inner ring  3  along with the movement of the contact point P, and appears at the position of the contact point P 1  (the position of the contact ellipse E 1  in  FIG.  5   ). 
     Note that, in  FIG.  5   , a region indicated by an arrow S means a region where the contact ellipse E 1  appears. 
     Therefore, in the bearing device for a vehicle wheel  1  according to the present embodiment, in order to reduce the occurrence of galling between the large diameter-side end surface  41  of the tapered roller  4  and the large flange part  32  of the inner ring  3 , the rounded part  32   d  is formed to prevent at least the contact ellipse E 1  that has become the maximum, that is, the contact ellipse E 1  moved to the position of the contact point P 1 , from riding up on the outside surface  32   b  of the large flange part  32 . 
     On the other hand, the degree of rounding of the rounded part  32   d  (a distance between the outer peripheral end a 2  of the rounded part  32   d  adjacent to the chamfered part  32   c  and the large diameter-side end surface  41  of the tapered roller  4 ) is deeply related to the radius of curvature r 2  of the rounded part  32   d , and the larger the set value of the radius of curvature r 2 , the smaller the degree of rounding. 
     With the degree of rounding set too small, when the tapered roller  4  is skewed, the large diameter-side end surface  41  of the tapered roller  4  easily comes into contact with the rounded part  32   d , which makes it difficult to reduce the occurrence of galling between the large diameter-side end surface  41  of the tapered roller  4  and the large flange part  32  of the inner ring  3 . 
     It is therefore important to set the radius of curvature r 2  of the rounded part  32   d  so as to obtain an appropriate degree of rounding. 
     Therefore, each degree of rounding (unit: μm) when the radius of curvature r 2  of the rounded part  32   d  was set to 0.5 mm, 1.0 mm, 2.0 mm, and 5.0 mm was confirmed first, then the presence or absence of galling marks generated on the large diameter-side end surface  41  of the tapered roller  4  and/or the large flange part  32  of the inner ring  3  was confirmed after putting each bearing device for a vehicle wheel into rotation operation with the tapered roller  4  skewed, a quality verification test was conducted, and the results shown in the following [Table 2] were obtained. 
     Note that, for a quality determination, when no galling mark was observed, “Good” was entered, when a slight galling mark was observed but the quality was not significantly affected, “Acceptable” was entered, and when a galling mark affecting the quality was observed, “Poor” was entered. 
     
       
         
           
               
               
             
               
                   
                 TABLE 2 
               
             
            
               
                   
                   
               
               
                   
                 Radius of curvature r2 of 
               
               
                   
                 rounded part [mm] 
               
            
           
           
               
               
               
               
               
            
               
                   
                 0.5 
                 1.0 
                 2.0 
                 5.0 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
            
               
                 Degree of rounding [μm] 
                 56 
                 22 
                 10 
                 4 
               
               
                 Presence of galling mark 
                 No 
                 No 
                 Slight contact 
                 Yes 
               
               
                   
                   
                   
                 mark is present 
               
               
                 Quality determination 
                 Good 
                 Good 
                 Acceptable 
                 Poor 
               
               
                   
               
            
           
         
       
     
     As shown in Table 2, when the radius of curvature r 2  of the rounded part  32   d  was set to 0.5 mm and 1.0 mm, their respective degrees of rounding were 56 μm and 22 μm, and no galling mark was observed on the large diameter-side end surface  41  of the tapered roller  4  and/or the large flange part  32  of the inner ring  3 , so that the quality was determined to be “Good”. 
     Further, when the radius of curvature r 2  of the rounded part  32   d  was set to 2.0 mm, a corresponding degree of rounding was 10 μm, and a slight contact mark was observed on the large diameter-side end surface  41  of the tapered roller  4  and/or the large flange part  32  of the inner ring  3 , but the quality was determined to be “Acceptable”. 
     On the other hand, when the radius of curvature r 2  of the rounded part  32   d  was set to 5.0 mm, a corresponding degree of rounding was 4 μm, and galling marks were observed on the large diameter-side end surface  41  of the tapered roller  4  and/or the large flange part  32  of the inner ring  3 , so that the quality was determined to be “Poor”. 
     From the above-described results, in the bearing device for a vehicle wheel  1  according to the present embodiment, the radius of curvature r 2  of the rounded part  32   d  of the large flange part  32  of the inner ring  3  is set less than 2 mm, and the degree of rounding of the rounded part  32   d  is set greater than or equal to 10 μmm. 
     With such a structure, even with the tapered roller  4  skewed, it is possible to prevent, by the rounded part  32   d , at least the contact ellipse E that has become the maximum from riding up on the outside surface  32   b  of the large flange part  32  with higher reliability, to reduce the occurrence of galling between the large diameter-side end  41  of the tapered roller  4  and the large flange part  32  of the inner ring  3 , and to further enhance the anti-seizing properties against the occurrence of, for example, abnormal heat generation caused by the galling. 
     &lt;Start Point Position of Rounded Part  32   d  of Large Flange Part  32  of Inner Ring  3 &gt; 
     As described above, in order to reduce the occurrence of galling between the large diameter-side end surface  41  of the tapered roller  4  and the large flange part  32  of the inner ring  3 , it is necessary to set the position of the start point (inner peripheral end a 1 ) of the rounded part  32   d  with the state where the tapered roller  4  is skewed taken into account in advance. 
     Specifically, it is necessary to set the position of the start point (inner peripheral end a 1 ) of the rounded part  32   d  so as to prevent at least contact ellipse E 1  that has become the maximum, that is, the contact ellipse E 1  moved to the position of the contact point P 1 , from riding up on the outside surface  32   b  of the large flange part  32 . 
     Here, as shown in  FIG.  2   , the contact ellipse E appearing at the contact point P between the large diameter-side end surface  41  of the tapered roller  4  and the guide surface  32   a  of the large flange part  32  of the inner ring  3  typically has an elliptical shape with a tangential direction of a circular locus (more specifically, a circular locus drawn by the contact point P) when the tapered roller  4  roll set as the major axis direction and a direction orthogonal to the major axis direction on the guide surface  32   a  set as the minor axis direction. 
     Further, the contact ellipse E 1  (see  FIG.  5   ) appearing at the contact point P 1  that has moved toward the outside surface  32   b  of the large-diameter part  32  of the inner ring  3  is also considered to have an elliptical shape nearly the same as the contact ellipse E. 
     Therefore, as a result of intensive studies, although it is difficult to precisely estimate the position of the contact point P 1 , that is, the amount of movement of the contact point P, in advance because the position is complicatedly affected by the overall specifications or load conditions of the bearing device for a vehicle wheel  1 , deformation of peripheral components, or the like, it has become clear that setting the position of the start point (inner peripheral end a 1 ) of the rounded part  32   d  at a position separated by more than twice a minor axis radius Ri of the contact ellipse E when the maximum axial load is applied from the contact point P between the large diameter-side end surface  41  of the tapered roller  4  and the guide surface  32   a  of the large flange part  32  of the inner ring  3  makes it possible to sufficiently prevent the contact ellipse E 1  from riding up on the outside surface  32   b  of the large flange part  32  even with the tapered roller  4  skewed. 
     Note that the position of the contact point P is desirably set at a position separated from the edge  34   a  of the large flange part-side cutout part  34  by more than the minor axis radius Ri of the contact ellipse E when the maximum axial load is applied so as to prevent the contact ellipse E from riding up on the large flange part-side cutout part  34  of the inner ring  3 . 
     Accordingly, in the bearing device for a vehicle wheel  1  according to the present embodiment, assuming that Ri denotes the minor axis radius of the maximum contact ellipse E obtained by the point contact between the large-diameter-side end surface  41  of the tapered roller  4  and the guide surface  32   a  of the large flange part  32  of the inner ring  3  under the allowable maximum axial load, setting is made such that X1≥2×Ri is satisfied where X1 denotes the shortest length from the contact point P between the large diameter-side end surface  41  and the guide surface  32   a  to the boundary position between the rounded part  32   d  and the guide surface  32   a  (that is, the position of the inner peripheral end a 1  that is the start point of the rounded part  32   d ), and X2&gt;Ri is satisfied where X2 denotes the shortest length from the contact point P between the large diameter-side end surface  41  and the guide surface  32   a  to the boundary position between the large flange part-side cutout part  34  and the guide surface  32   a  (that is, the position of the edge  34   a  of the large flange part-side cutout part  34 ). 
     With such a structure, even with the tapered roller  4  skewed and the position of the contact ellipse E moved toward the outside surface  32   b  of the large flange part  32 , for example, it is possible to prevent the contact ellipse E 1  after the movement from riding up on the outside surface  32   b  of the large flange part  32 , to reduce the occurrence of galling between the large diameter-side end  41  of the tapered roller  4  and the large flange part  32  of the inner ring  3 , to reduce the occurrence of, for example, abnormal wear or abnormal heat generation caused by the galling, and to further enhance the anti-seizing properties. 
     Further, the contact ellipse E never rides up on the large flange part-side cutout part  34  regardless of the behavior of the tapered roller  4 , so that it is possible to reduce the occurrence of galling between the large diameter-side end  41  of the tapered roller  4  and the large flange part  32  of the inner ring  3 , to reduce the occurrence of, for example, abnormal wear or abnormal heat generation caused by the galling, and to further enhance the anti-seizing properties. 
     &lt;Start Point Position of Tapered Roller-Side Cutout Part  43  of Tapered Roller  4 &gt; 
     As described above, with the tapered roller  4  installed in the inner ring  3 , the start point of the tapered roller-side cutout part  43 , that is, the outer peripheral end  43   a , is set to be positioned on the inner diameter side relative to the outside surface  32   b  of the large flange part  32 . 
     This is mainly intended to prevent stepped uneven wear from occurring on the large diameter-side end surface  41  of the tapered roller  4  as the degree of wear between the large diameter-side end surface  41  and the guide surface  32   a  of the large flange part  32  of the inner ring  3  gradually increases and the large diameter-side end surface  41  protrudes and comes into contact with the guide surface  32   a.    
     However, when the start point (outer peripheral end  43   a ) of the tapered roller-side cutout part  43  is positioned on the outer diameter side relative to the start point (inner peripheral end a 1 ) of the rounded part  32   d , there is a possibility that the large diameter-side end surface  41  protrudes and comes into contact with the guide surface  32   a  due to an increase in the degree of wear, and it is thus difficult to sufficiently prevent the occurrence of stepped uneven wear on the large diameter-side end surface  41 . 
     Therefore, in the bearing device for a vehicle wheel  1  according to the present embodiment, the start point of the tapered roller-side cutout part  43  of the tapered roller  4 , that is, the outer peripheral end  43   a  (outer edge end), is set to be positioned adjacent to the large flange part-side cutout part  34  relative to the start point of the rounded part  32   d  of the large flange part  32  of the inner ring  3 , that is, the inner peripheral end a 1  that is the boundary position between the rounded part  32   d  and the guide surface  32   a.    
     Specifically, when the shortest length from the edge  34   a  of the large flange part-side cutout part  34  to the outer peripheral end  43   a  of the tapered roller-side cutout part  43  is denoted as Y1, and the shortest length from the edge  34   a  of the large flange part-side cutout part  34  to the inner peripheral end a 1  of the rounded part  32   d  is denoted as Y2, Y1&lt;Y2 is always satisfied. 
     With such a structure, even when the degree of wear between the large diameter-side end surface  41  of the tapered roller  4  and the guide surface  32   a  of the large flange part  32  of the inner ring  3  increases, the contact portion between the large diameter-side end  41  and the guide surface  32   a  is always stably positioned on the guide surface  32   a , and it is thus possible to reduce the occurrence of galling between the large diameter-side end surface  41  of the tapered roller  4  and the large flange part  32  due to the occurrence of stepped uneven wear on the large diameter-side end surface  41 , to reduce the occurrence of galling marks or abnormal heat generation caused by the galling, and to further enhance the anti-seizing properties. 
     Although descriptions have been given above of the embodiment of the present invention, the present invention is in no way limited to the embodiment, and the embodiment is merely an example. It is needless to say that various other embodiments can be made without departing from the scope of the present invention, and the scope of the present invention is set forth in the claims and further contains meaning of equivalents set forth in the claims and all modifications within the scope. 
     INDUSTRIAL APPLICABILITY 
     The present invention is applicable to a bearing device for a vehicle wheel including a multi-row tapered roller bearing, which is mainly used as a bearing device for rotatably supporting each wheel of a long-distance transport vehicle with a heavy body weight, such as a truck, a bus, or a pickup truck. 
     REFERENCE SIGNS LIST 
     
         
         
           
               1  bearing device for vehicle wheel 
               2  outer ring (outer member) 
               3  inner ring (inner member) 
               4  tapered roller 
               21  outer raceway surface 
               31  inner raceway surface 
               32  large flange part 
               32   a  guide surface 
               32   b  outside surface of large flange part 
               32   c  chamfered part of large flange part 
               32   d  rounded part 
               34  large flange part-side cutout part 
               34   a  edge of large flange part-side cutout part (boundary position between large flange part-side cutout part and guide surface) 
               41  large diameter-side end surface 
               43  tapered roller-side cutout part 
               43   a  outer peripheral end (outer edge end) of tapered roller-side cutout part 
             a 1  inner peripheral end of rounded part (boundary position between rounded part and guide surface) 
             E contact ellipse 
             P contact point between large diameter-side end surface and guide surface  32   a    
             R radius of curvature of large diameter-side end surface 
             Ri minor axis radius of contact ellipse 
             r 2  radius of curvature of rounded part