Patent Publication Number: US-2005130785-A1

Title: Bevel gear

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a bevel gear to be used in a differential gear or the like for transmitting a power of an automobile, for example.  
      2. Background Art  
      In a vehicle such as an automobile, there has conventionally been disposed a differential gear  121  shown in  FIG. 6  that distributes a power transmitted from a driving shaft to both wheels (not shown) through right and left axles  161  and  171 . The differential gear  121  includes a differential case  141  rotatably supported by a transmission case  122  of a transmission  110 , and a bevel gear  111  (a pair of side gears  111   a  and  111   a , and a pair of pinion gears  111   b  and  111   b ) arranged in the differential case  141  such that axial centers X and Y of the bevel gear  111  are perpendicular to each other. The pinion gears  111   b  and  111   b  are mounted on a pinion shaft  157  positioned in the differential case  141 . The side gears  111   a  and  111   a  are mounted on external splines  162  and  172  of the right and left axles  161  and  171  which are arranged such that axial centers thereof correspond to a rotation axis of the differential case  141 .  
      An operation of the differential gear  121  is stated below. A driving force of an engine (not shown) is transmitted to a driving shaft  131 , and is input to a ring gear  143  integrally fabricated with the differential case  141 , so that the differential case  141  rotates about its rotation axis. While a rotational speed difference between both the axles  161  and  171  is adjusted, both the axles  161  and  171  can be rotated by the driving force in a balanced manner through the pinion gears  111   b  and  111   b , and the side gears  111   a  and  111   a.    
      Taking one of the side gears  111   a  as an example, a constitution of the bevel gear  111  which transmits a driving force is described below, with reference to the  FIGS. 7A and 7B . The side gear  111   a  has a plurality of teeth  112   a  formed on an outside periphery thereof. A generating line P of a standard pitch cone of each of the teeth  112   a  inclines with respect to the axial center X. Thus, each of the teeth  112   a  has a larger diameter-side end surface  115  whose tooth depth is larger, on the side of a larger diameter of the standard pitch cone. On the other hand, each of the teeth  112   a  has a smaller diameter-side end surface  114  whose tooth depth is smaller, on the side of a smaller diameter of the standard pitch cone. Each of both tooth flanks  113   a  and  113   a  of each of the plurality of teeth  112   a  generally forms an elliptic crowning way with its longer sides being formed of an elliptically arc shape, with each of central portions  120  and  120  being bulged in a tooth trace direction. Similarly in each of the pinion gears  111   b,  a generating line of a standard pitch cone inclines with respect to the axial center Y, so that each of both tooth flanks  113   b  and  113   b  of each of teeth  112   b  forms an elliptic crowning way with its longer sides being formed of an elliptically arc shape, although the shape of the crowning way may be somewhat different from that of each of the teeth  112   a.    
      As shown in  FIG. 6 , the side gears  111   a  and the pinion gears  111   b  are fabricated such that their axial centers X and Y are perpendicular to each other, so as to form the differential gear  121 . By an engagement of the teeth  112   a  of the side gears  111   a  and the teeth  112   b  of the pinion gears  111   b,  the side gears  111   a  and the pinion gears  111   b  can transmit a driving force. The teeth  112   a  and  112   b  are engaged as described below. When the side gears  111   a  and the pinion gears  111   b  are disposed in position, top areas of the tooth flanks  113   a  and  113   b  each forming the crowning way come into contact with each other. Since the contacting portion is subjected to a driving force (load), a position of the contacting portion is designed to be the top areas of the tooth flanks  113   a  and  113   b,  the top areas having an excellent strength.  
      Since the side gears  111   a  and the pinion gears  111   b  are disposed in position by an engagement of the teeth  112   a  and  112   b  of the respective gears  111   a  and  111   b  in the differential case  141 , an assembly error of the side gears  111   a  and the pinion gears  111   b  is prone to occur. As shown in  FIG. 8 , it may be possible that the axial center X of the side gear  111   a  and the axial center Y of the pinion gear  111   b  are positioned in a slightly inclined manner (not perpendicular to each other), because of an assembly error and/or a process error. In this state, a gap between the tooth flanks  113   a  and  113   b  which are opposed near the central portions  120  is relatively small. This is because the central portion  120  in the tooth trace direction of each of the tooth flanks  113   a  and  113   b  forming an elliptical crowning way with its longer sides being formed of an elliptically arc shape is smoothened (with a smaller curvature) as compared with ends of the tooth flanks  113   a  and  113   b.  Accordingly, because of the slight inclination of the axial center Y, the tooth flanks  113   a  and  113   b  are engaged in contact with each other at a position considerably far (distance S 2 ) from a predetermined contacting portion K in the tooth trace direction.  
      A predetermined strength and endurance of the teeth  112  of the bevel gear  111  can be anticipated when the tooth flanks  113   a  and  113   b  are engaged in contact with each other at the predetermined contacting portion K which is previously designed. Thus, when an actual contacting portion K 2  where the tooth flanks  113   a  and  113   b  are actually engaged in contact with each other is considerably far from the predetermined contacting portion K, the teeth  112  are engaged in contact with each other at a portion of lower strength. As a result, the endurance of the teeth  112  may be deteriorated.  
     SUMMARY OF THE INVENTION  
      An object of the present invention is to bring an actual contacting portion of an engagement of a bevel gear close to a predetermined contacting portion which is previously designed, only by changing a form of a tooth flank, with a basic structure of the bevel gear being maintained.  
      The present invention is a bevel gear having a plurality of teeth on an outside periphery thereof, each of both tooth flanks of each of the plurality of teeth forming a crowning way with a bulge in a tooth trace direction, a generating line of a standard pitch cone of each of the plurality of teeth inclining with respect to an axial center, wherein each of the both tooth flanks of each of the plurality of teeth is formed into an arc shape, at least at a central portion thereof in the tooth trace direction.  
      In the bevel gear according to the present invention, each of the tooth flanks is formed into an arc shape at the central portion in the tooth trace direction, so that a gap between the opposed teeth can be gradually enlarged from a predetermined contacting portion of the tooth flanks of the teeth to be engaged toward ends thereof. Thus, even when an axial center of the bevel gear to be engaged is inclined because of a permissible assembly error and/or a process error, a position of an actual contacting portion can come near the predetermined contacting portion. Consequently, a deterioration of a strength and an endurance of the teeth can be advantageously prevented.  
      For example, each of the both tooth flanks of each of the plurality of teeth may be formed into an arc shape in whole. Alternatively, each of the both tooth flanks of each of the plurality of teeth may be formed into an arc shape, only at a central portion thereof in the tooth trace direction. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a partial cross-sectional front view showing a differential gear including a bevel gear in an embodiment of the present invention;  
       FIG. 2A  is a cross-sectional front view of the bevel gear in the embodiment of the present invention;  
       FIG. 2B  is a cross-sectional view of a tooth of the bevel gear shown in  FIG. 2A  taken along a standard pitch cone;  
       FIG. 3  is an illustrational view showing an engagement of the bevel gear with two axial centers thereof being perpendicular to each other, wherein the solid line indicating an engagement of the bevel gear in the embodiment, while the two-dot chain line indicating an engagement of a conventional bevel gear;  
       FIG. 4  is an illustrational view showing an engagement of the bevel gear of the embodiment, with two axial centers thereof being slightly inclined;  
       FIG. 5A  is a cross-sectional front view of a bevel gear of another embodiment of the present invention;  
       FIG. 5B  is a cross-sectional view of a tooth of the bevel gear shown in  FIG. 5A  taken along a standard pitch cone;  
       FIG. 6  is a partial cross-sectional front view showing a differential gear including a conventional bevel gear;  
       FIG. 7A  is a cross-sectional front view of the conventional bevel gear;  
       FIG. 7B  is a cross-sectional view of a tooth of the bevel gear shown in  FIG. 7A  taken along a standard pitch cone; and  
       FIG. 8  is an illustrational view showing an engagement of the conventional bevel gear, with two axial centers thereof being slightly inclined. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      An embodiment of a bevel gear according to the present invention is described in detail below, with reference to FIGS.  1  to  5 .  
       FIG. 1  is a partial cross-sectional front view showing a differential gear including a bevel gear in an embodiment of the present invention.  FIG. 2A  is a cross-sectional front view of the bevel gear in the embodiment of the present invention.  FIG. 2B  is a cross-sectional view of a tooth of the bevel gear shown in  FIG. 2A  taken along a standard pitch cone.  FIG. 3  is an illustrational view showing an engagement of the bevel gear with two axial centers thereof being perpendicular to each other, wherein the solid line indicating an engagement of the bevel gear in the embodiment, while the two-dot chain line indicating an engagement of a conventional bevel gear.  FIG. 4  is an illustrational view showing an engagement of the bevel gear of the embodiment, with two axial centers thereof being slightly inclined.  FIG. 5A  is a cross-sectional front view of a bevel gear of another embodiment of the present invention.  FIG. 5B  is a cross-sectional view of a tooth of the bevel gear shown in  FIG. 5A  taken along a standard pitch cone.  
      As shown in  FIG. 1 , a bevel gear  11  in the embodiment is installed in a differential gear  21 . The bevel gear  11  is formed of side gears  11   a  and pinion gears  11   b.  The differential gear  21  is rotatably supported in a transmission case  22  through ball bearings  55  and  56 .  
      The differential gear  21  includes a case semi-body  42  of a half shell shape having a case opening  45  at a center thereof, and a ring gear  43  of a disk shape having. a ring opening  46  at a center thereof. The case semi-body  42  and the ring gear  43  are integrally fabricated by means of bolts  71  to form a differential case  41 . A diameter of the ring opening  46  is the same as that of the case opening  45 . A beveled ring tooth  44  is disposed on an outside peripheral surface of the ring gear  43 .  
      Axles  71  and  61  project respectively from the case opening  45  and the ring opening  46  of the differential case  41  in opposite directions. The side gears  11   a  and  11   a  are respectively mounted on splines  72  and  62  of the axles  71  and  61  disposed in the differential case  41 . A pinion shaft  57  is positioned between the axles  71  and  61  such that the pinion shaft  57  is perpendicular to axial centers of the axles  71  and  61 . The pinion gears  11   b , each of which is engaged with a tooth  12  of each of the side gears  11   a,  are mounted on the pinion shaft  57 .  
      The ring tooth  44  of the ring gear  43  constituting the differential case  41  is engaged with a driving gear  32  of a driving shaft  31  which is positioned so as to be perpendicular to the axial centers of the axles  61  and  71 . The driving shaft  31  is rotatably supported in the transmission case  22  through a ball bearing  33  and a needle bearing  34 .  
      An operation of the differential gear  21  is stated below. A driving force of an engine (not shown) is transmitted to the ring gear  43  constituting the differential case  41  through the driving shaft  31 , so that the differential case  41  rotates about its rotation axis. While a rotational speed difference between both the axles  61  and  71  is adjusted, both the axles  61  and  71  can be rotated by the driving force in a balanced manner through the pinion gears  11   b  and  11   b  and the side gears  11   a  and  11   a.    
      Taking one of the side gears  11   a  as an example, a constitution of the bevel gear  11  which transmits a driving force is described below, with reference to  FIGS. 2A and 2B . As shown in  FIG. 2A , the side gear  11   a  has a through-hole  18  which is in parallel with an axial center X. An internal spline  19  is formed on an inside periphery of the through-hole  18 . The internal spline  19  is engaged with one of the external splines  62  and  72  of the axles  61  and  71 . The side gear  11   a  has a plurality of teeth  12   a  formed on an outside periphery thereof. A generating line P of a standard pitch cone of each of the teeth  12   a  inclines with respect to the axial center X. Thus, each of the teeth  12   a  has a larger diameter-side end surface  15  whose tooth depth is larger, on the side of a larger diameter of the standard pitch cone. On the other hand, each of the teeth  12   a  has a smaller diameter-side end surface  14  whose tooth depth is smaller, on the side of a smaller diameter of the standard pitch cone. Each of both tooth flanks  13   a  and  13   a  of each of the plurality of teeth  12   a  forms an arcuate (not elliptic) crowning way, which is a characteristic feature of the present invention, with each of central portions  20  and  20  being bulged in a tooth trace direction. Similarly in each of the pinion gears  11   b , a generating line of a standard pitch cone inclines with respect to the axial center Y, so that each of both tooth flanks  13   b  and  13   b  of each of teeth  12   b  forms an arcuate crowning way, although the shape of the crowning way may be somewhat different from that of each of the teeth  12   a.    
      As shown in  FIG. 1 , the side gears  11   a  and the pinion gears  11   b  are fabricated such that their axial centers X and Y are perpendicular to each other, so as to form the differential gear  21 . By an engagement of the teeth  12   a  of the side gears  11   a  and the teeth  12   b  of the pinion gears  11   b , the side gears  11   a  and the pinion gears  11   b  can transmit a driving force. Referring to  FIG. 3 , the engagement of the teeth  12   a  and  12   b  is explained below. When the side gears  11   a  and pinion gears  11   b  are disposed in position, top areas of the tooth flanks  13   a  and  13   b  (indicated by the solid line) each forming the crowning way come into contact with each other. Since a contacting portion K is subjected to a driving force (load), a position of the contacting portion K is designed to be the top areas of the tooth flanks  13   a  and  13   b,  the top areas having an excellent strength. Near the central portions  20  where the tooth flanks  13   a  and  13   b  are in contact with each other, a relatively larger gap is formed between the opposed tooth flanks  13   a  and  13   b  toward ends thereof, as compared with a gap between conventional tooth flanks (indicated by the two-dot chain line) each having an elliptic shape with its longer sides being formed of an elliptically arc shape.  
      Since the side gears  11   a  and the pinion gears  11   b  are disposed in position by the engagement of the teeth  12   a  of the side gears  11   a  and the teeth  12   b  of the pinion gears  11   b  in the differential case  41 , an assembly error of the side gears  11   a  and the pinion gears  11   b  is prone to occur. As shown in  FIG. 4 , it may be possible that the axial center X of the side gear  11   a  and the axial center Y of the pinion gear  11   b  are positioned in a slightly inclined manner (not perpendicular to each other), because of an assembly error and/or a process error. In this state, near the central portions  20  where the tooth flanks  13   a  and  13   b  each forming the arcuate crowning way are in contact with each other, a relatively larger gap is formed between the opposed tooth flanks  13   a  and  13   b  toward the ends thereof, as compared with a gap between conventional tooth flanks each having an elliptic shape with its longer sides being formed of an elliptically arc shape. Thus, even when the axial center Y is slightly inclined, an actual contacting portion K 1  where the tooth flanks  13   a  and  13   b  are engaged in contact with each other can be prevented from being considerably drawing away from a predetermined contacting portion K which has been previously designed (deviation is only a distance S 1 ). That is, the actual contacting portion K 1  can be brought closer to the predetermined contacting portion K.  
      The preferred embodiment of the present invention has been described as above. However, not limited thereto, various modifications and changes can be made, without departing from the scope of the present invention. In the above embodiment, each of the tooth flanks  13   a  and  13   b  of the bevel gear  11  is formed into an arc shape in whole. However, as shown in  FIGS. 5A and 5B  for example, each of the tooth flanks  13   a  and  13   b  may be formed into an arc shape only at a central portion  20  thereof in the tooth trace direction. In this case, each of rest parts of the tooth flanks extending continuously from the central portions  20  to the respective ends  14  and  15  may be formed into any shape.