Abstract:
A differential gear set includes a pinion gear that at least a groove having a inner planar surface, at least a gear is formed in an exterior circumference thereof and at least a pinion shaft in which an end portion is inserted into the groove such that an end surface faces the inner surface of the groove and a planar portion is formed by cutting off in one side of an exterior circumference thereof corresponding to the interior circumference of the groove.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to and the benefit of Korean Patent Application No. 10-2007-0124929 filed in the Korean Intellectual Property Office on Dec. 4, 2007, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     (a) Field of the Invention 
     The present invention relates to a differential gear set, more particularly, the differential gear set in which a lubrication structure thereof is improved, so durability and safety are enhanced. 
     (b) Description of the Related Art 
     Generally, a differential apparatus transfers power of an engine from a drive shaft to a driving axle. 
     The differential apparatus prevents slipping of a wheel when a vehicle turns. 
     Typical differential gear includes a pinion gear, a side gear, a ring gear, and a case. The pinion gear is coupled to the side gear that is splined to a rear axle shaft. 
       FIG. 1  is a partial cross-sectional view of a general differential gear set. 
     As shown in  FIG. 1 , the differential gear set includes a housing  100 , a pinion shaft  105 , a pinion gear  110 , a side gear  115 , and an axle shaft  120 . 
     However, lubrication efficiency can be decreased between an interior circumference of the pinion gear  110  and an exterior circumference of the pinion shaft  105  when the pinion gear  110  rotates on the pinion shaft  105 . Further, there is a problem that the elements  105  and  110  stick to each other in a particular case. 
     The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in an effort to provide a differential gear set having advantages of improving a lubrication structure such that a pinion gear rotates well on a pinion shaft, and such that durability thereof is enhanced and friction is decreased. 
     A differential gear set in which a lubrication structure is improved may include: at least a pinion gear including a groove that is formed on at least one side of the pinion gear and has an inner planar bottom surface and a circular interior circumference formed substantially on a rotation shaft, wherein a gear is formed at an exterior circumference of the pinion gear; and at least a pinion shaft in which at least an end portion is inserted into the groove such that a bottom surface of end portion of the pinion shaft faces the inner planar bottom surface of the groove and at least a planar portion is formed at the end portion by cutting off at least one side of an exterior circumference thereof. 
     Further, at least an oil hole may be formed across from the planar portion to an opposite outside surface thereof at least at the end portion of the pinion shaft and two planar portions may be formed at respective sides of the pinion shaft. 
     A center portion of the bottom surface of the pinion shaft may contact the inner planar bottom surface of the groove and an edge portion of the pinion shaft has a gap from a inner planar bottom surface of the groove. The gap from the inner planar bottom surface of the groove to the bottom surface of the pinion shaft become increased from a center portion of a rotation shaft toward an edge of an exterior circumference direction thereof. 
     The groove that has a circular shape substantially on a center of a rotation shaft of the pinion gear may be formed at both sides of the pinion gear, and pinion shafts are inserted into the two grooves. 
     An arc-shaped space may be formed between the planar portion and an interior circumference of the groove. 
     As an another exemplary embodiment of the present invention, the differential gear set in which a lubrication structure is improved, may comprise: a first pinion gear including a first groove formed at upper portion thereof and a second groove formed at lower portion thereof; a second pinion gear including a third groove at upper portion thereof; a first pinion shaft wherein one portion of the first pinion shaft is coupled to a housing and lower portion of the first pinion gear is inserted into the first groove of the first pinion gear and a first space is formed between the lower portion of the first pinion shaft and the first groove; and a second pinion shaft wherein upper portion of the second pinion shaft is inserted into the second groove of the first pinion gear and forms a second space between the upper portion of the second pinion shaft and the second groove, and wherein lower portion of the second pinion shaft is inserted into the third groove of the second pinion gear and forms a third space between the lower portion of the second pinion shaft and the third groove. 
     Further the upper portion and/or lower portion of the second pinion shaft may be formed of at least a planar portion in the longitudinal direction of the second pinion shaft and at least a fourth space is formed between interior circumference of the second and/or third groove and an exterior circumference of the second pinion shaft. 
     The first, second and third grooves may have inner planar bottom surfaces and a center portion of the lower portion of the first pinion shaft is placed on the inner planar bottom surface of the first groove, a center portion of the upper portion of the second pinion shaft is placed on the inner planar bottom surface of the second groove, and a center portion of the lower portion of the second pinion shaft is placed on the inner planar bottom surface of the third groove. 
     At least an oil hole may be formed between the planar portions therethrough. 
     An oil hole formed at upper portion of the second pinion shaft may be positioned in the second groove. 
     An oil hole formed at lower portion of the second pinion shaft may be positioned in the third groove. 
     A gap between the inner planar bottom surface of the first groove and the lower end surface of the first pinion shaft become larger from the center portion toward an edge of an exterior circumference direction thereof. 
     A gap between the inner planar upper surface of the second groove and the upper end surface of the second pinion shaft become larger from the center portion toward an edge of an exterior circumference direction thereof. 
     A gap between the inner planar bottom surface of the second groove and the lower end surface of the second pinion shaft become larger from the center portion toward an edge of an exterior circumference direction thereof. 
     The first pinion shaft, the second pinion shaft, the first pinion gear and the second pinion gear may be aligned coaxially. 
     Accordingly, from this configuration, the oil hole through which oil flows is formed in the pinion shaft, so lubrication efficiency is improved in a section at which the pinion gear and the pinion shaft slides. 
     In addition, the pinion shaft is inserted into the groove that is formed on the pinion gear and a contact area between the end surface of the pinion shaft and the inner surface of the groove is minimized, so friction/abrasion is reduced and lubrication efficiency is enhanced. 
     Further, the space is formed between the end surface of the pinion shaft and the inner surface of the groove that is formed in the pinion gear, so lubrication efficiency is improved. 
     The above features and advantages of the present invention will be apparent from or are set forth in more detail in the accompanying drawings, which are incorporated in and form a part of this specification, and the following Detailed Description of the Invention, which together serve to explain by way of example the principles of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other features of the present invention will now be described in detail with reference to certain exemplary embodiments thereof illustrated the accompanying drawings which are given hereinbelow by way of illustration only, and thus are not limitative of the present invention, and wherein: 
         FIG. 1  is a partial cross-sectional view of a general differential gear set; 
         FIG. 2  is a partial cross-sectional view of a differential gear set according to an exemplary embodiment of the present invention; 
         FIG. 3  is a partial detailed cross-sectional view of a differential gear set according to an exemplary embodiment of the present invention; 
         FIG. 4  is a cross-sectional top view according to an I-I line of  FIG. 3 ; and 
         FIG. 5  is a detailed cross-sectional front view of a section II of  FIG. 3 . 
     
    
    
     DESCRIPTION OF REFERENCE NUMERALS INDICATING PRIMARY ELEMENTS IN THE DRAWINGS 
     
       
         
               
               
               
             
           
               
                   
               
             
             
               
                 200: housing 
                   
                   
               
               
                 205a: first pinion shaft 
                 205b: second pinion shaft 
               
               
                 210a: first pinion gear 
                 210b: second pinion gear 
               
               
                 215: side gear 
                 220: axle shaft 
               
               
                 300a: first space 
                 300b: second space 
                 300c: third space 
               
               
                 305a: first groove 
                 305b: second groove 
                 305c: third groove 
               
               
                 310: rotation shaft 
               
               
                 400, 415: planar portion 
               
               
                 401: first oil hole, 
                 405: second oil hole 
               
               
                 410: third space 
                 505: inner bottom surface 
               
               
                   
               
             
          
         
       
     
     It should be understood that the appended drawings are not necessarily to scale, presenting a somewhat simplified representation of various preferred features illustrative of the basic principles of the invention. The specific design features of the present invention as disclosed herein, including, for example, specific dimensions, orientations, locations, and shapes will be determined in part by the particular intended application and use environment. 
     In the figures, reference numbers refer to the same or equivalent parts of the present invention throughout the several figures of the drawing. 
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter reference will now be made in detail to various embodiments of the present invention, examples of which are illustrated in the accompanying drawings and described below. While the invention will be described in conjunction with exemplary embodiments, it will be understood that present description is not intended to limit the invention to those exemplary embodiments. On the contrary, the invention is intended to cover not only the exemplary embodiments, but also various alternatives, modifications, equivalents and other embodiments, which may be included within the spirit and scope of the invention as defined by the appended claims. 
     A differential gear set is explained in the following according to an exemplary embodiment of the present invention, referring to the accompanying drawings. 
       FIG. 2  is a partial cross-sectional view of a differential gear set according to an exemplary embodiment of the present invention. 
     As shown in  FIGS. 2 and 3 , a differential gear set includes a housing  200 , a ring gear (not shown), a pinion shaft  205 , first and second pinion gears  210   a  and  210   b , side gears  215 , and axle shafts  220 . 
     The ring gear is disposed outside the housing  200 . The ring gear is rotated by a pinion gear that is formed in an end portion of a drive shaft (not shown). Accordingly, the ring gear and the housing  200  rotate. 
     Axle shafts  220  are inserted into the right and left sides of the housing  200  respectively in the drawing, and the side gears  215  are splined to distal end portions of the axle shafts  220 . Each side gear  215  of the axle shaft  220  is coupled to the first and second pinion gears  210   a  and  210   b.    
     Pinion shaft  205  is configured to support the first pinion gear  210   a  and the second pinion gear  210   b . The pinion shaft  205  includes a first pinion shaft  205   a  and a second pinion shaft  205   b  in the present exemplary embodiment. 
     Upper portion of the first pinion shaft  205   a  is coupled to the housing  200 . Lower portion of the first pinion shaft  205   a  and upper portion of the second pinion shaft  205   b  supports the first pinion gear  210   a  downwards and upwards respectively. 
     A first groove  305   a  is formed in upper side of the first pinion gear  210   a  and a second groove  305   b  is formed on lower side of the first pinion gear  210   a  along a rotation center of a rotation shaft  310  for the pinion shaft  205 . 
     The lower portion of the first pinion shaft  205   a  is complimentarily inserted into the first groove  305   a  and the upper portion of the second pinion shaft  205   b  is complimentarily inserted into the second groove  305   b.    
     Also, third grooves  305   c  is formed in upper side of the second pinion gear  210   b  along a rotation center of a rotation shaft  310  for the pinion shaft  205 , and a lower portion of the second pinion shaft  205   b  is complimentarily inserted into the third groove  305   c.    
     Coupling structures of the first pinion gear  210   a  and the first pinion shaft  205   a  and the second pinion shaft  205   b  are explained in detail referring to  FIG. 3 . 
       FIG. 3  is a partial detailed cross-sectional view of a differential gear set according to an exemplary embodiment of the present invention. 
     As shown in  FIG. 3 , the first pinion gear  210   a  is externally meshed with the side gear  215  and rotates with respect to the rotation center of rotation shaft  310 . 
     The first groove  305   a  is formed on upper side of the first pinion gear  210   a  and the second groove  305   b  is formed on lower side thereof. A lower portion of the first pinion shaft  205   a  is inserted into the first groove  305   a  complimentarily and an upper portion of the second pinion shaft  205   b  is inserted into the second groove  305   b  complimentarily. 
     The first groove  305   a  and the second groove  305   b  have a circular interior circumference, and the first and second pinion shafts  205   a  and  205   b  are inserted complimentarily thereto. Further, inner bottom surfaces of the first groove  305   a  and the second groove  305   b  are planar. 
     A lower portion of the second pinion shaft  205   b  is inserted into the third groove  305   c  complimentarily as shown in  FIG. 2 , and a lower end surface of the second pinion shaft  205   b  contacts on an inner bottom surface of the third groove  305   c.    
     Particularly, there is a characteristic shape of an end surface of the first pinion shaft  205   a  in the present exemplary embodiment as explained hereinafter, referring to  FIG. 3 . 
     A center portion in the lower end surface of the first pinion shaft  205   a  contacts onto an inner bottom surface of the first groove  305   a . From this configuration, the first pinion shaft  205   a  and the first pinion gear  210   a  may be coaxially aligned. 
     Further, a distance between the inner bottom surface of the first groove  305   a  and the lower surface of the first pinion shaft  205   a  become increased toward an inner circumference of first groove  305   a  from the rotation shaft  310  of the first pinion shaft  205   a  radially. 
     From this configuration, a first space  300   a  is formed between the lower end surface of the first pinion shaft  205   a  and the inner bottom surface of the first groove  305   a , and thus lubricant is interposed in the first space  300   a . Accordingly, lubrication efficiency is improved when the first pinion gear  210   a  rotates between the first pinion shaft  205   a  and the second pinion shaft  205   b  about the rotation shaft  310 . 
     The center portion in the upper end surface of the second pinion shaft  205   b  contacts onto the inner upper surface of the second groove  305   b , and the gap between the upper end surface of the second pinion shaft  205   b  and the inner upper surface of the second groove  305   b  become increased toward an inner circumference of second groove  305   b  from the center line of the second pinion shaft  205   b  radially. 
     Accordingly, a second space  300   b  is formed between the upper end surface of the second pinion shaft  205   b  and the inner upper surface of the second groove  305   b , and thus lubricant is interposed in the second space  300   b . Accordingly, lubrication efficiency is improved when the first pinion gear  210   a  rotates on the second pinion shaft  205   b.    
     Further, in an exemplary embodiment of the present invention, a third space  300   c  is formed between the lower end surface of the second pinion shaft  205   b  and the inner lower surface of the third groove  305   c , and thus lubricant is interposed in the third space  300   c . Accordingly, lubrication efficiency is improved when the second pinion gear  210   b  rotates on the second pinion shaft  205   b.    
     Particularly, the pinion shaft  205  does not penetrate the first and second pinion gears  210   a  and  210   b  in an exemplary embodiment of the present invention. Generally, in a case in which the pinion shaft penetrates, lubricant can pass through a penetration hole, but a penetration hole is not formed in the present exemplary embodiment. 
     Referring to  FIG. 2  again, when the first and second pinion gears  210   a  and  210   b  and the pinion shaft  205  rotate on a rotation center line of the axle shaft  220 , lubricant placed in the second groove  305   b  and the third groove  305   c  cannot move to an inner surface of the housing  200  through the first and second pinion gear  210   a  and  210   b.    
     In contrast, since the pinion shaft  105  of general differential gear set completely penetrates the pinion gears  110 , lubricant that is interposed between the pinion gears  110  and the pinion shaft  105  may flow to the inner side of the housing, so lubrication efficiency can be decreased. 
     However, since the pinion shaft  205  does not penetrate the first and second pinion gears  210   a  and  210   b  in an exemplary embodiment of the present invention, the problems stated above are improved. 
       FIG. 4  is a cross-sectional view according to I-I line of  FIG. 3 . 
     As shown in  FIG. 4 , a lower side of the first pinion gear  210   a  is provided to receive an exterior circumference of an upper portion of a second pinion shaft  205   b . Planar portions  400  and  415  are formed at both lateral sides of an exterior circumference of the second pinion shaft  205   b . The planar portions  400  and  415  can be formed by grinding/cutting the exterior circumference of the second pinion shaft  205   b.    
     A third space  410  is formed between the interior circumference of the first pinion gear  210   a  and the planar portions  400  and  415  of the second pinion shaft  205   b , and lubricant is interposed in the third space  410 . 
     Accordingly, a lubrication film can be formed easily between the interior circumference of the first pinion gear  210   a  and the exterior circumference of the second pinion shaft  205   b.    
     As shown in  FIG. 4 , an oil hole  405  is formed across end portion of the second pinion shaft  205   b.    
     The oil hole  405  is penetrated between the planar portions  400  and  415  that are formed on the second pinion shaft  205   b.    
       FIG. 5  is a detailed cross-sectional view of a section II of  FIG. 3 . 
     As shown in  FIG. 5 , the first groove  305   a  is formed in an upper portion of the first pinion gear  210   a , and a lower portion of the first pinion shaft  205   a  is inserted into the first groove  305   a.    
     Lower portion of the first pinion shaft  205   a  is rotatably disposed in the first groove  305   a . Further, a portion of the lower end surface of the first pinion shaft  205   a  contacts the inner bottom surface  505  of the first groove  305   a  on the a rotation shaft  310  and an edge portion of the first pinion shaft  205   a  has a gap from the inner bottom surface  505  of the first groove  305   a.    
     Particularly, a distance between the lower end surface  510  of the first pinion shaft  205   a  and the inner bottom surface  505  of the first groove  305   a  becomes larger further from the rotation shaft  310  of the first pinion shaft  205   a  to the exterior circumference direction radially. As described above, the inner bottom surface  505  of the first groove  305   a  is planar. 
     Accordingly, a first space  300   a  is formed between the lower end surface  510  of the first pinion shaft  205   a  and the inner bottom surface  505  of the first groove  305   a , so lubricant is easily interposed in the first space  300   a.    
     Explaining with regard to elastic fluid lubrication, when the rotation bodies  205   a  and  210   a  rotate, a lubricant  500  therearound comes together, and density and viscosity of the lubricant that is compressed in a narrow space is increased. The lubricant  500  of which a viscosity thereof is increased has a semisolid state, and functions as a wedge that separates the rotation bodies  205   a  and  210   a.    
     Lubrication efficiency is improved in the present exemplary embodiment by making a structure in which the lubricant functions as a wedge between the first pinion shaft  205   a  and the first pinion gear  210   a.    
     While this invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.