Abstract:
A breather structure for a reduction gear in which a plurality of stop ribs are disposed within a housing. The stop ribs are disposed at nonuniform distances between successive ribs in order to prevent oil from entering a breather chamber of the housing. The relative spacing between successive ribs allows the breather structure to prevent oil traveling at varying speeds from entering the breather chamber. The ribs have an upper surface which is disposed at declining, or obtuse, angles with respect to an interior of the housing, and a lower surface which acts to receive oil rising from a bottom of the housing.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This non-provisional application claims priority under 35 U.S.C. § 119(a) on Patent Application No. Hei-11-249376 filed in Japan on Sep. 2, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a breather structure for a final reduction gear equipped with a differential. In particular, the invention is addressed to a breather structure of reduced cost, reduced noise and durability, and which prevents oil leakage. 
     2. Background Art 
     A conventional gear box exists for storing a gear, a bearing, and lubricant. In the conventional gear box, a communication pipe-like breather structure prevents changes in internal pressure due to increasing temperature, so that the internal pressure is always made equivalent to the atmospheric pressure. This prevents oil leakage from a sealed part due to increasing internal pressure. 
     Such a system is disclosed in Japanese Utility Publication No. Sho. 57-51940 entitled “Final Reduction Gear.” In FIG. 4 of this publication, a final reduction gear includes an air introducing slot  9 A of an air breather  9  in a space surrounded by ribs  8 ,  8 A on the inside of the rear cover  5 . A shroud  10  is located on the rear cover  5  to prevent oil scattered by a rotating hypoid gear from entering the air introducing slot  9 A and the upper surface of the rib  8 A. 
     In FIG. 5 of the publication, another embodiment of a final reduction gear includes a packing  6 , provided between the housing  4  and the rear cover  5 . The packing  6  is formed as partly projecting so that oil is prevented from entering the air introducing slot  9 A and the upper surface of the rib  8 A. 
     According to the above technique, attachment of a shroud  10  to the rear cover  5  results in an increased number of parts. The parts include the shroud  10 , a bolt for attachment, and so on. The increase in number of parts increases costs. 
     Also, when the packing  6  is formed as partly projecting, a thin packing that is less rigid is likely to vibrate, which results in increased noise and decreased durability. 
     Further, as the hypoid gear  1  rotates at a high speed, the amount of scattered oil increases and its velocity increases. Therefore, a larger amount of oil may enter the air introducing slot  9 A or the upper surface of the rib  8 A, through spaces between the rib  8 A and the shroud  10 , or between the rib  8 A and the extension of the packing  6 . 
     In view of the above, objects of the present invention include providing a breather structure for a final reduction gear having a decreased cost, less noise, increased durability, and which may prevent oil leakage. 
     SUMMARY OF THE INVENTION 
     In order to achieve the above objects, a final reduction gear, in which an upper part of a housing is made into a breather chamber, which is communicated to the outside via a breather pipe, and a differential is rotatably stored in a housing. A plurality of oil stop ribs are provided, extending toward the differential, on an inner circumferential surface of the housing, to prevent oil having been scooped by the differential from flowing into the breather chamber. The oil stop ribs may be provided at irregular intervals. 
     When the scooped oil is prevented from flowing into the breather chamber by the plurality of oil stop ribs formed on the inner circumferential surface of the housing, and the adjacent oil stop ribs are provided at irregular intervals, a place where the oil is to be accumulated is shifted from a place between adjacent oil stop ribs with a larger interval to a place between adjacent oil stop ribs with a smaller interval as the differential rotates at a higher rotation frequency. In this manner, an amount of oil to be blocked is gradually reduced. 
     As oil stop ribs are provided to the housing, the need to provide additional oil stopping parts to the housing is eliminated. This reduces the number of parts and suppresses noise or damage due to vibration. 
     Also, when adjacent oil stop ribs are provided at irregular intervals, an amount of oil to be blocked can be gradually reduced as the differential rotates at a higher rotation frequency, whereby the oil can be reliably prevented from flowing into the breather chamber. 
     The oil stopping ribs may be formed having upper and lower surfaces of differing shape. Specifically, the upper surface of the oil stop rib may be formed as declining, and may have a curvature, so that the blocked oil flows downward, and the lower surface thereof is formed as a receiver for directly receiving the rising oil. 
     The blocked oil flows downward along the declining upper surface of the oil stop rib, while the rising oil is received by the lower surface of the oil stop rib, which is formed as a receiving surface. 
     As a result, oil does not accumulate on the upper surface of the oil stop rib, and oil flow is blocked by the lower surface of the oil stop rib, so that the oil can accumulate in the lower part of the housing. 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein: 
     FIG. 1 is a perspective view of a saddled vehicle equipped with a final reduction gear according to the present invention; 
     FIG. 2 is a perspective view of a power transmission mechanism of a saddled vehicle equipped with a final reduction gear according to the present invention; 
     FIG. 3 is an exploded perspective view of a differential according to the present invention; 
     FIG. 4 is an exploded perspective view of a differential case assembly of a differential according to the present invention; 
     FIG. 5 is a cross sectional view of FIG. 2 along the line  5 — 5 ; 
     FIGS.  6 ( a )- 6 ( c ) are views of an input block of a differential according to the present invention; 
     FIGS.  7 ( a ) and  7 ( b ) are views of a breather structure of a front final assembly equipped with a differential according to the present invention; 
     FIGS.  8 ( a )-( d ) are views of an input block and an output cam of a differential according to the present invention, which are developed in a circumferential direction; 
     FIGS.  9 ( a ) and  9 ( b ) are diagrams showing the operation of a differential according to the present invention; 
     FIGS.  10 ( a ) and  10 ( b ) are diagrams showing driving force distribution by a saddled vehicle equipped with a differential according to the present invention, running in a straight direction; 
     FIGS.  11 ( a ) and  11 ( b ) are diagrams showing a steering force of a saddled vehicle equipped with a differential according to the present invention; 
     FIG. 12 is an operation diagram comparing steering forces of a vehicle equipped with a differential; and 
     FIGS.  13 ( a ) and  13 ( b ) are diagrams showing the operation of a breather structure of a front final assembly equipped with a differential according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a perspective view of a saddled vehicle equipped with a final reduction gear according to the present invention. The saddled vehicle  10  includes a handle  12  rotatably attached to a vehicle frame  11 , front wheels  13 ,  14  ( 13  for a left front wheel,  14  for a right front wheel) steerably connected to the handle  12  via a steering device (not shown), and vertically movably attached to the vehicle frame  11  via an arm (not shown). A seat  15  is arranged on the upper part of the vehicle frame  11 , and a power unit  16  comprising an engine and a transmission is arranged below the seat  15 . Rear wheels  17 ,  18  ( 17  for a left rear wheel,  18  for a right rear wheel (not shown)) are driven, together with the front wheels  13 ,  14 , by the power unit  16  via a power transmission mechanism. Reference numeral  22  denotes a front bumper, numeral  23  an under cover, numeral  24  a front fender, numeral  25  a headlamp, numeral  26  a rear fender, and numeral  27  a muffler. 
     Reference numeral  21  denotes a front final assembly as a final reduction gear, or one of the components of the power transmission mechanism. The front final assembly  21  incorporates a differential (described later), and is installed between the left front wheel  13  and the right front wheel  14 . 
     FIG. 2 is a perspective view showing a power transmission mechanism of a saddled vehicle equipped with a final reduction gear according to the present invention. A power transmission mechanism  30  comprises a front propeller shaft  31  extending frontward from the bottom of the power unit  16 , a front final assembly  21  connected to the leading end of the front propeller shaft  31 , front drive shafts  32 ,  33  connected on the left and right sides of the front final assembly  21 , hubs  34 ,  35  connected to the respective ends of the front drive shafts  32 ,  33 , a rear propeller shaft  36  extending backward from the bottom of the power unit  16 , a rear final assembly  37  connected to the trailing end of the rear propeller shaft  36 , a rear drive shaft  38  penetrating the rear final assembly  37  in the left and right direction thereof, and hubs  42 , 43  connected to the respective ends of the rear drive shaft  38 . Reference numerals  45 ,  46 ,  47  denote tubes covering the rear propeller shaft  36  and left and right sides of the rear drive shaft  38 . A supporting member  48  supports the tubes  45 ,  47 . 
     The hubs  34 ,  35 ,  42 ,  43  are for attaching the hubs  34 ,  35 ,  42 ,  43  to the left front wheel  13 , the right front wheel  14 , the left rear wheel  17 , and the right rear wheel  18 , shown in FIG. 1, respectively. 
     FIG. 3 is an exploded perspective view showing a differential according to the present invention. In FIG. 3, the front final assembly  21  comprises a differential case assembly  50  as a differential, a housing  52  for rotatably storing the differential case assembly  50  via the bearings  51 ,  51 , a drive pinion  54  for insertion into the housing  52  from the rear side thereof via the bearing  53 , a bearing  55  for rotatably installing the driving pinion  54  to the housing  52 , a lock nut  56  for preventing displacement of the bearing  55 , and a joint  58  to be installed on an end of the driving pinion  54 . 
     In FIG. 3, reference numeral  52 a denotes a housing body,  52 b a housing cover,  61 ,  61  oil seals,  62 ,  63  bolts,  64 ,  64  spacers,  65  a maintenance hole plug,  66  an  0 -ring,  67  a spacer,  68  an oil seal, and  69  an O-ring. 
     FIG. 4 is an exploded perspective view showing a differential according to the present invention, in which a differential case assembly  50  comprises a differential case  71  and a storage part  72  to be stored in the differential case  71 . 
     The differential case  71  comprises a column-like case body  73 , a ring-geared cap  75 , in which a ring gear  75   a  is integrally formed on a left cap (described later) to be attached to one of the openings of the case body  73 , and a right cap  76  to be attached to the other opening of the case body  73 . 
     The storage part  72  comprises two types of input blocks  77 ,  78 , for rotating integrally with the differential case  71 , left and right output cams  81 ,  82  for sandwiching the input blocks  77 ,  78  so as to allow them to slide relative to each other, and capable of rotating independently due to a frictional force with the respective blocks, thrust bearings  83 ,  83 , arranged adjacent to the left and right output cams  81 , thrust washers  84 ,  84 , and a disk spring  85 . The thrust bearings  83 ,  83  may be omitted. 
     FIG. 5 is a cross sectional view of FIG. 2 along the line  5 — 5 . 
     The front final assembly  21  is a device in which a differential case assembly  50  is assembled by forming a ring-geared cap  75  through integral formation of a ring gear  75   a  to a left cap  74 . A left output cam  81  is provided in the inside of the ring-geared cap  75  via a disk spring  85 , a thrust washer  84 , and a thrust bearing  83 . The case body  73  is installed to the ring-geared cap  75  by a bolt  87 . Input blocks  77 ,  78  are arranged in the case body  73  in the circumferential direction so as to contact the left output cam  81 , and a right output cam  82  is arranged so as to contact the input blocks  77 ,  78 . A right cap  76  is provided adjacent to the right output cam  82  via a thrust bearing  83  and a thrust washer  84 , and the right cap  76  is attached to the case body  73 . 
     The front final assembly  21  is a device in which the housing  52  is assembled by attaching a column part  75   b  of the ring-geared cap  75  to a journal part  52   c  of the housing body  52   a  via a bearing  51 . A cylinder part  76   a  of the right cap  76  is attached to the journal part  52   d  of the housing cover  52   b  via the bearing  51 , and the housing cover  52   b  is attached to the housing body  52   a  by bolts  62  (see FIG.  3 ),  63  (only one is shown). The differential case assembly  50  is rotatably provided inside the housing  52 . 
     The front final assembly  21  is a device in which the end  54 a of a driving pinion  54  is attached in the inside of a rear cylinder part  52   e  of the housing body  52   a  via a bearing  53 . The middle part  54   b  of the driving pinion  54  is attached to the rear cylinder part  52   e  via the bearing  55  to thereby cause the driving pinion  54  to be engaged with the ring gear  75   a . A lock nut  56  is screwed into the inner circumferential part of the rear cylinder part  52   e  to thereby prevent displacement of the bearing  55 . A joint  58  is attached to the trailing end of the driving pinion  54 , and an oil seal  68  is provided between the inner circumferential part of the rear cylinder part  52   e  and the joint  58 . 
     The input blocks  77 ,  78  each have convex parts  77   a ,  78   a , which are fixed to axial slots  73   a ,  73   b , formed on the inner surface of the case body  73 , whereby the input blocks  77 ,  78  can rotate together with the case body  73 . 
     The left and right output cams  81 ,  82  transmit a driving force to the left and right front wheels  13 ,  14  (see FIG. 1) by spline connecting the front drive shafts  32 ,  33  to the cylinder parts  81   a ,  82   a , respectively. 
     The drive pinion  54  transmits a driving force from the power unit  16  (see FIG. 1) to the differential case assembly  50  by spline connecting the front propeller shaft  31  (see FIG. 2) to the joint  58 . 
     As described above, in the differential case assembly  50  of the present invention a ring gear  75   a  is integrally formed on the left cap part  74 , which is a part of the differential case  71 . 
     With the above structure, the ring gear  75   a  is integrally formed on the left cap  74  of the differential case  71  so that the left cap part  74  and the ring gear  75   a  can be formed as a single part. A conventional bolt for connection is therefore unnecessary. In a conventional design, A case and a ring gear are different entities and require an attachment bolt. Thus, the number of parts can be reduced, and molding can be facilitated, as a result of which manufacturing costs can be reduced. 
     FIGS.  6 ( a ) to ( c ) are diagrams explaining an input block of a differential according to the present invention. 
     FIG.  6 ( a ) shows an internal state of the differential case assembly  50  with the ring-geared cap  75  (see FIG. 5) and the left output cam  81  both removed. 
     Input blocks  77 ,  78  are arranged alternately (every two blocks) in the circumferential direction such that convex parts  77   a ,  78   a  are fitted into the axis direction slots  73   a ,  73   b , respectively, formed on the inner surface of the case body  73 . 
     FIG.  6 ( b ) is an enlarged diagram of a selected portion of FIG.  6 ( a ), showing the case body  73  and the input block  77  in engagement with the case body  73 . 
     The axis direction slot  73   a  is a slot having a substantially trapezoidal shape. The convex  77   a  is a part having a shape substantially analogous to the shape of the axis direction slot  73   a . Here, the width of the upper part of the convex  77   a  is defined as L 1 . 
     FIG.  6 ( c ) is an enlarged diagram of selected parts of FIG.  6 ( a ), showing a case body  73  and an input block  78  in engagement with the case body  73 . 
     The axis direction slot  73   b  is a slot having a substantially trapezoidal shape. The convex part  78   a  has a shape substantially analogous to the shape of the axis direction slot  73   b . Here, the width of the upper part of the convex part  78   a  is defined as L 2 . That is, the upper width L 2  differs from the upper width L 1  in FIG.  6 ( b )—L 1 &gt;L 2 . 
     Although L 1 &gt;L 2  is shown in FIG.  6 ( b ) and ( c ), L 1 &lt;L 2  may be possible. 
     Also, the axis direction slot  73   b  has a projection  73   c  at the bottom thereof, and the convex  78   a  has a hollow  78   b  on the upper surface thereof, which corresponds to the above-described projection  73   c.    
     FIG.  7 ( a ), ( b ) are diagrams illustrating a breather structure for a front final assembly equipped with a differential according to the present invention. FIG.  7 ( a ) is a view in the direction of arrow  7  in FIG. 2, while FIG.  7 ( b ) is a cross sectional view of (a) along line b—b. 
     In FIG.  7 ( a ), the front final assembly  21  has a breather joint  91  on the upper part of the housing cover  52   b , as a breather pipe for communicating between inside and outside of the housing  52 . 
     In FIG.  7 ( b ), the housing cover  52   b  has an upper part thereof formed projecting to form a breather chamber  92 , and the breather joint  91  is attached on the wall in the upper part of the breather chamber  92 . 
     A plurality of oil stop ribs  52   g ,  52   h ,  52   j ,  52   k  are formed with irregular intervals and substantially parallel to the rotation axis of the differential case assembly  50 . The oil stop ribs  52   g ,  52   h ,  52   j ,  52   k  are formed on the inner surface of the housing cover  52   b  below the breather chamber  92 . These oil stop ribs  52   g ,  52   h ,  52   j ,  52   k  are set close to the case body  73  of the differential case assembly  50 . 
     The direction in which the differential case assembly  50  rotates when the associated vehicle runs forward is determined as a forward rotation direction, which is the direction of the arrow in the drawing. 
     The oil stop ribs  52   g ,  52   h ,  52   j ,  52   k  are formed such that the lower surfaces thereof, or walls LW, which are further from the breather chamber  92 , rise substantially vertically with respect to the inner surface of the housing cover  52   b  , with corners RA having a small arc radius r 1 . The walls UW, which are closer to the breather chamber  92 , are formed in an arc, having a large arc radius r 2 . In this embodiment, r 2 &gt;r 1 . 
     The oil stop rib  52   g  is formed on the other side of the inner surface of the housing cover  52   b  , with the differential case assembly  50  intervening, from the surface where the oil stop ribs  52   h ,  52   j ,  52   k  are formed. 
     Respective intervals between the oil stop ribs  52   g ,  52   h ,  52   j ,  52   k  correspond to angles C 1 , C 2 , C 3 , respectively, wherein C 1 &gt;C 2 &gt;C 3 . 
     The relationship among the angles C 1 , C 2 , C 3  in terms of degrees, represents a relationship in an amount of oil allowed to accumulate between adjacent oil stop ribs  52   g ,  52   h ,  52   j ,  52   k.    
     The relationship between the respective amounts of oil accumulation may be stated as follows: 
     
       
         amount of oil accumulating between the oil stop ribs  52   g ,  52 h&gt;amount of oil accumulating between the oil stop ribs  52   h ,  52 j&gt;amount of oil accumulating between the oil stop ribs  52   j ,  52   k.   
       
     
     In the present invention, the breather structure of a front final assembly  21  equipped with a differential case assembly  50  is characterized by the fact that the upper part of the housing  52  is made into a breather chamber  92  which communicates with the outside via a breather joint  91 , the differential case assembly  50  is rotatably accommodated in the housing  52 , and a plurality of oil stop ribs  52   g ,  52   h ,  52   j ,  52   k  are formed on the inner circumferential surface of the housing  52 , extending toward the differential case assembly  50 . In this configuration, the oil scooped by the differential case assembly  50  is prevented from entering the breather  92 . In addition, irregular intervals exist between adjacent oil stop ribs  52   g  and  52   h ,  52   h  and  52   j ,  52   j  and  52   k.    
     In the above structure, when oil stop ribs  52   g ,  52   h ,  52   j ,  52   k  are provided on the housing  52 , the need for additional oil stoppers on the housing  52  is eliminated. This reduces the number of parts and decreases cost. Also, as the oil stop ribs  52   g ,  52   h ,  52   j ,  52   k  are integrally formed on the housing  52 , the oil stop ribs  52   g ,  52   h ,  52   j ,  52   k  can be prevented from vibrating, and from being damaged by vibration of the housing  52 . 
     Also, in the breather structure of a front final assembly  21 , the walls UW, LW are formed in different shapes. Specifically, the oil stop ribs  52   g ,  52   h ,  52   j ,  52   k  are formed such that the upper surface walls UW are formed declining so that blocked oil flows downward therealong, and the lower surface walls LW are formed as a receiver for directly receiving rising oil. The upper surface walls UW of the oil stop ribs  52   g ,  52   h ,  52   j ,  52   k  may have a curvature in order to facilitate the flow of oil to the lower surface walls LW. The upper surface walls are may also be at an angle with respect to vertical in order to downward facilitate oil flow. 
     FIGS.  8 ( a ) to ( d ) schematic views of an input block and an output cam of a differential of the present invention, being developed in the circumferential direction. FIGS.  8 ( a ) to ( d ) show chronological steps, in which the left output cam  81  moves leftward in the drawing, relative to the right output cam  82 , as time passes. 
     In FIG.  8 ( a ), the input block  77  has a hexagonal shape, when developed, which is opposite in terms of left and right directions from the shape of the input block  78  being developed. 
     The left output cam  81  has an uneven cam surface  81   b , which alternates connection of a left first inclining surface  81   c  and a left second inclining surface  81   d.    
     The right output cam  82  has an uneven cam surface  82   b , which alternates connection of a right first inclining surface  82   c  and a right second inclining surface  82   d.    
     Here, S 1 , S 2  are reference lines using a part of the right output cam  82  as a reference. 
     FIG.  8 ( b ) shows a state in which, relative to the state shown in (a), upon receipt of a force in the circumferential direction (leftward in the drawing), the input block  77  moves from the right output cam  82  side to the left output cam  81  side by a distance V 1 , and also the right output cam  82  moves relatively in a direction opposite (rightward in the drawing) from the input block  77  by a distance H 1 , and the left output cam  81  moves leftward relative to the right output cam  82  by a distance M 1 . 
     FIG.  8 ( c ) shows a state in which, relative to the state shown in (a), upon receipt of a force in the circumferential direction (leftward in the drawing), the input block  77  moves from the right output cam  82  side to the left output cam  81  side by a distance V 2 . The right output cam  82  moves relatively in a direction opposite from the input block  77  by a distance H 2 , and the left output cam  81  moves leftward relative to the right output cam  82  by a distance M 2 . 
     FIG.  8 ( d ) shows a state in which, relative to the state shown in FIG.  8 ( a ), upon receipt of a force in the circumferential direction (leftward in the drawing), the input block  77  moves from the right output cam  82  side to the left output cam  81  side by a distance V 3 . The right output cam  82  moves relatively in a direction opposite from the input block  77  by a distance H 3 , and the left output cam  81  moves leftward relative to the right output cam  82  by a distance M 3 . 
     As represented by the input block  77 , described above, when the moving speed, or a rotating frequency, is different between the left output cam  81  and the right output cam  82 , the input blocks  77 ,  78  undergo relative movement, or relative rotation, while causing a frictional force between the left and right output cams  81 ,  82 , respectively. 
     When no difference is caused in rotation frequency between the left output cam  81  and the right output cam  82 , the input blocks  77 ,  78  and the left and right output cams  81 ,  82  rotate together, rather than relative to one another. 
     The operation of the above-described differential will be described as follows. 
     FIGS.  9 ( a ) and  9 ( b ) are operation diagrams explaining the operation of a differential according to the present invention. 
     FIG.  9 ( a ) is an enlarged diagram of the input block  77  (the leftmost one) and the left and right output cams  81 ,  82 , shown in FIG.  8 ( a ). In this figure, the inclination angle of the left first inclining surface  81   c  of the left output cam  81  is denoted as θ, that of the right first inclining surface  82   c  of the right output cam  82  is denoted as θ. 
     In FIG.  9 ( b ), an example will be described in which, when a leftward force F is applied to the input block  77 , for example, when the left output cam  81  rotates at a high speed, and the right output cam  82  rotates at a low speed, resulting in a difference in rotation frequency between the left output cam  81  and the right output cam  82 . In this case, assume that if the input block  77  pushes the left first inclining surface  81   c  of the left output cam  81  with a force N perpendicular to the inclining surface  81   c , and the right first inclining surface  82   c  of the right output cam  82  with a force N perpendicular to the inclining surface  82   c , the leftward component of the force N is Nsinθ. 
     Also, when the left output cam  81  moves leftward relative to the input block  77 , a frictional force μN is caused between the input block  77  and the left first inclining surface  81   c , in which a rightward component of the frictional force μN is Ncosθ. A leftward component is −μNcosθ. 
     Therefore, a leftward force applied from the input block  77  to the left output cam  81  is Nsinθ−μNcosθ. 
     On the other hand, when the right output cam  82  moves rightward relatively to the input block  77 , a frictional force μN is caused in the input block  77  and the right first inclining surface  82   c , in which a leftward component of the frictional force μN is μNcosθ. 
     Therefore, a leftward force applied from the input block  77  to the right output cam  82  is Nsinθ+μNcosθ. 
     As described above, when a difference in a rotation frequency is caused between the left output cam  81  and the right output cam  82 , a larger force is caused to the right output cam  82 , which rotates at a lower speed, compared to the force caused to the left output cam  81 , which rotates at a high speed. 
     The ratio of leftward forces applied to the left output cam  81  and the right output cam  82  are denoted as: 
     
       
         (N sin θ−μN cos θ):(N sin θ+μN cos θ)=(sin θ−μcos θ):(sin θ+N cos θ), which varies depending on a friction coefficient μ and an inclination angle θ. 
       
     
     The above ratio is ultimately a ratio at which to distribute driving torque to the left and right front wheels. 
     As described with reference to FIG. 4, in this embodiment, a differential case assembly  50  comprises: a plurality of input blocks  77 ,  78  for moving in a circumferential direction, following the rotating ring gear  75 a; two left and right output cams  81 ,  82  for sandwiching the blocks  77 ,  78  so as to allow them to perform relative movement, and capable of rotating independently by utilizing a frictional force with the respective blocks  77 ,  78 ; and, a differential case  71  for storing the input blocks  77 ,  78  and left and right output cams  81 ,  82 . 
     In the above structure, driving torque can be distributed to the output cams  81 ,  82  variably according to a predetermined ratio, depending on a frictional force direction, which varies due to relative sliding of the input blocks  77 ,  78  and the output cams  81 ,  82  based on the rotation frequency of the two output cams  81 ,  82 . Thus, even though a driving force for some wheels becomes smaller due to a change in a friction coefficient of a road surface, a driving force for other wheels does not decrease, so that total driving force can be ensured and traveling performance can be improved. 
     Next, the distribution of a driving force when a saddled vehicle equipped with a differential runs along a straight path will be described. 
     FIGS.  10 ( a ) and  10 ( b ) are operation diagrams illustrating the distribution of a driving force when a saddled vehicle equipped with a differential of the present invention runs along a straight path. For the purposes of illustration, the length of the black arrows in the drawing indicates the magnitude of a driving force. 
     In FIG.  10 ( a ), when the rear wheels  17 ,  18  traverse a road with a small friction coefficient, such as on mud Mu, for example, the rear wheels  17 ,  18  slip, so that driving forces D 1 , D 1  become smaller, as shown by the arrow. However, if the front wheels  13 ,  14  traverse a road with a large friction coefficient, large driving forces D 2 , D 2 , as shown by the arrow, result. 
     In FIG.  10 ( b ), when the vehicle is running straight, if, e.g., the right front wheel  14  and the rear wheels  17 ,  18  traverse a road with a small friction coefficient, such as mud Mu, and so on, a driving force D 3  of the right front wheel  14  and driving forces D 4 , D 4  of the rear wheels  17 ,  18  become smaller, as shown by the arrows. However, if the left front wheel  13  traverses a road with a large friction coefficient, a difference in a rotation frequency will be caused between the left front wheel  13  and the slipping right front wheel  14 . As a result, a large driving force D 5 , as shown by the arrow, will be caused in the left front wheel  13  due to operation of the front final assembly  21 . 
     Next, a steering force of a saddled vehicle equipped with a differential will be described. 
     FIGS.  11 ( a ) and  11 ( b ) are operating diagrams illustrating a steering force of a saddled vehicle equipped with a differential according to the present invention. The length of black arrows in the drawing corresponds to the magnitude of a driving force, and the length of the outlined arrows corresponds to the magnitude of a resistance force when the vehicle is running. 
     In FIG.  11 ( a ), in a saddled vehicle of the present invention, a rotation frequency for the rear wheels  17 ,  18  is set slightly higher than that for the front wheels  13 ,  14 , for reduction of a steering force. 
     In this arrangement, when the vehicle is running straight, although the driving forces D 6 , D 6  for the rear wheels  17 ,  18  become larger, as shown by the black arrows, driving forces D 7 , D 7  for the front wheels  13 ,  14  become smaller, as shown by the black arrows. The resistance forces R 1 , R 1  are caused to the front wheels  13 ,  14 , as shown by the outlined arrows. 
     In FIG.  11 ( b ), when the front wheels  13 ,  14  are steered while the vehicle is running, a larger resistance force R 2  is caused in the front wheel  13 , which then runs inside, than a resistance force R 3 , caused in the front wheel  14 , which then runs outside. This is due to a slightly higher rotation frequency set for the rear wheels  17 ,  18  than the front wheels  13 ,  14 , and of an operation of the front final assembly  21 . This enables a reduction of a steering force. 
     FIG. 12 is an operation diagram explaining a comparative example of steering forces of a vehicle equipped with a differential, in which the differential distributes equal driving forces to the right and left front wheels to be steered. The length of the black arrows in the drawing corresponds to the magnitude of a driving force, while the length of outlined arrows corresponds to the magnitude of a resistance force. 
     The vehicle  100  transmits a driving force of the power unit  101  to the differential  103  via the front shaft  102 , and further from the differential  103  to the front wheels  106 ,  107  via the front drive shafts  104 ,  105 . 
     The vehicle  100  also transmits a driving force of the power unit  101  to the differential  111  via the rear shaft  108 , and further from the differential  111  to the rear wheels  114 ,  115  via the left and right rear drive shafts  112 ,  113 . 
     In the vehicle  100 , as the front wheels  106 ,  107  and the rear wheels  114 ,  115  rotate at the same rotation frequency, and the differential  103  distributes equal driving forces, driving forces Dr, Dr for the left and right front wheels  106 ,  107  become equal when being steered. Moreover, even if a rotation frequency of the rear wheels  114 ,  115  is set slightly higher than that for the front wheels  106 ,  107 , resistance forces Re, Re applied to the left and right front wheels  106 ,  107 , become equal, and no contribution to reduction of a steering force is thus obtained. 
     In the embodiment as described with reference to FIG.  11 ( b ), in a saddled vehicle  10  equipped with a differential case assembly  50  between the front left and right wheels  13 ,  14 , a device which distributes driving forces differently at a predetermined ratio to the left and right wheels  13 ,  14 , when a difference is caused in rotation frequencies between the left and right wheels  13 ,  14 , is employed as a differential case assembly  50 . 
     In the above structure, when steering, a larger driving torque can be distributed to an inside wheel running at a slower speed than an outside wheel, whereby a larger resistance force is applied to the inside wheel than the outside wheel. As a result, steering performance can be further improved and a steering force can be further reduced 
     The operation of the breather structure of the above-described front final assembly will next be described. 
     FIGS.  13 ( a ) and  13 ( b ) are operating diagrams illustrating the operation of a breather structure of a front final assembly equipped with a differential of the present invention. 
     In FIG.  13 ( a ), when the differential case assembly  50  rotates forward as shown by the arrow, oil in the housing  52  tends to flow counterclockwise in a space between the external circumferential surface of the differential case assembly  50  and the inner surface of the housing cover  52   b , following the rotation of the differential case assembly  50 . However, the oil flow is blocked by the oil stop ribs  52   h ,  52   j ,  52   k.    
     When the differential case assembly  50  rotates at a lower rotation frequency, most of the oil is blocked by the oil stop rib  52   h  provided upstream of the oil flow, so that the blocked oil accumulates in a lower part of the housing cover  52   b , or between the two oil stop ribs  52   g ,  52   h.    
     When the differential case assembly  50  rotates at a higher rotation frequency, a lot of oil passes through the space between the differential case assembly  50  and the oil stop rib  52   h . The passed oil is then blocked by the oil stop rib  52   j , and accumulates between two oil stop ribs  52   h ,  52   j.    
     When the differential case assembly  50  rotates at an even higher rotation frequency, a lot of oil passes through the space between the differential case assembly  50  and the oil stop rib  52   j . The passed oil is then blocked by the oil stop rib  52   k , and accumulates between the oil stop ribs  52   j ,  52   k , so that the oil can be prevented from flowing into the breather chamber  92 . 
     As described above, provision of a plurality of oil stop ribs  52   g ,  52   h ,  52   j ,  52   k , enables oil blocking in a broad range of rotation frequency of the differential case assembly  50 . 
     Also, irregular intervals between the adjacent oil stop ribs  52   g ,  52   h ,  52   j ,  52   k  enables blocking and accumulation of a lot of oil upstream of the oil flow, compared to a design having regular intervals. The amount of oil to be blocked downstream of the oil flow can therefore be reduced. In particular, it is difficult for oil to overflow from between the oil stop ribs  52   j ,  52   k , and can be prevented from flowing into the breather chamber  92 . 
     Further, as the walls LW of the oil stop rib  52   h ,  52   j ,  52   k , further from the breather chamber  92 , are formed sharply rising, with corners RA having a small arc radius, oil can be reliably blocked. 
     In FIG.  13 ( b ), when the differential case assembly  50  rotates in a direction opposite from a forward direction, i.e., in a reverse direction, as shown by the arrow, the oil in the housing  52  then flows clockwise, following the rotation of the differential case assembly  50 , in a space between the external circumference surface of the differential case assembly  50  and the inner surface of the housing cover  52   b.    
     When the walls UW of the oil stop ribs  52   h ,  52   j ,  52   k , closer to the breather chamber  92 , are formed declining having a large arc radius, the oil around the respective oil stop ribs  52   h ,  52   j ,  52   k  can be smoothly introduced to the respective spaces between the differential case assembly  50  and the oil stop ribs  52   h ,  52   j ,  52   k . Oil can therefore flow toward the lower part of the housing cover  52   b.    
     As the oil reaching the lower part of the housing cover  52   b  moves from the differential case assembly  50  side to the housing cover  52   b  side due to centrifugal forces, while flowing clockwise, as shown by the arrow, the oil can be efficiently blocked by the wall LW of the oil stop rib  52   g.    
     As described above with reference to FIGS.  13 ( a ) and ( b ), when the differential case assembly  50  rotates forward, an amount of oil to be blocked can be reduced as the differential case assembly  50  rotates at a higher rotation frequency, so that oil can be reliably blocked from flowing into the breather chamber  92 . 
     Therefore, oil leakage to the outside of the front final assembly can be prevented. 
     Also, when the differential case assembly  50  rotates forward, the climbing oil can be received by the walls LW, or receiver surfaces, of the oil stop ribs  52   h ,  52   j ,  52   k , to thereby block the oil flowing. Thus, it is possible to make the oil to accumulate in the lower part of the housing  52 . 
     On the other hand, when the differential case assembly  50  rotates backward, the blocked oil flows downward along the declining walls UW of the oil stop ribs  52   h ,  52   j ,  52   k , without accumulating on the walls UW. The flowing oil is then received by the wall LW of the oil stop rib  52   g , or a receiving surface, to thereby block the oil flow. 
     Therefore, oil does not reach the breather chamber  92  when the differential case assembly  50  rotates either forward or backward, so that oil can be prevented from flowing to the outside of the front final assembly via the breather joint  91 . As a result, the life of the front final assembly can be prolonged. 
     The present invention with the above arrangement can produce the following advantages: 
     A plurality of oil stop ribs are provided on an inner circumferential surface of the housing to prevent oil having been scooped by the differential from flowing into the breather chamber, and adjacent oil stop ribs are provided at irregular intervals. 
     When the oil stop ribs are formed on the housing, the need to provide additional parts for oil stoppage on the housing is eliminated, so that the number of parts, as well as cost, can be reduced, and noise or damage due to vibration can be reduced. 
     When adjacent oil stop ribs are provided at differing intervals, an amount of oil to be blocked can be gradually reduced as the differential rotates at a higher rotation frequency, so that the oil can be reliably prevented from flowing into the breather chamber. Thus, oil leakage to the outside of the final reduction gear can be prevented. 
     The upper surface of the oil stop rib may be formed declining and the lower surface thereof formed as a receiving surface. With this arrangement, oil does not sump on the upper surface of the oil stop rib. Moreover, as an oil flow is blocked by the lower surface of the oil stop rib, the oil can sump in the lower part of the housing. Thus, oil does not flow into the breather chamber, and so the oil can be prevented from flowing to the outside of the final reduction gear via the breather pipe. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.