Abstract:
A differential device is provided with a case rotatable about an axis; an input member configured to receive the torque from the input shaft, the input member being housed in the case and including an engaging outer periphery drivingly engaged with the case; first and second output gears configured to respectively drivingly link with the output shafts, the first and second output gears being rotatably housed in the case to form a row with the input member along the axis; one or more first pinions being rotatably housed in the case in parallel with the axis and meshing with the first output gear, the first pinions having no overlap with the engaging outer periphery of the input member and the second output gears; and one or more second pinions being rotatably housed in the case in parallel with the axis and meshing with the second output gears and the first pinions, the second pinions extending beyond the input member to reach the first pinions.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 2006-181936 (filed Jun. 30, 2006); the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a differential device applied to conveyance means such as automobiles. 
     2. Description of the Related Art 
     As is known, a driveline of an automobile requires one or more differential assemblies for differentially distributing torque generated by an engine to a plurality of shafts and axles, each of which facilitates relative rotation between a pair of output shafts. One type of differentials is known as a parallel axis helical gear differential, which is provided with a planetary gear set typically having helical output gears and paired sets of helical pinion gears. 
     Japanese Patent Application Laid-open No. H07-77263 (in particular, FIG. 3) discloses an art of a parallel axis helical gear differential, in which a central piece receives torque from an engine and the received torque is distributed to left and right follower gears via a differential case and paired sets of pinion gears. In each pair of the pinions, one pinion meshes with the left follower gear and another pinion meshes with the right follower gear. For mutual meshing of the pinion gears, both the pinions are extended toward a center in the axial direction of the differential case and have overlaps around the central piece. 
     SUMMARY OF THE INVENTION 
     The central piece needs to have rigid engagement with the differential case so as to transmit the torque to the differential case. The overlaps of the pinion gears limit a structure and dimensions of the engagement between the central piece and the differential case. Engaging portions of the central piece and the differential case must be so designed as to avoid the overlaps and are therefore difficult to be formed all around the central piece. In other words, as the engaging portions are limited, this structural and dimensional restriction leads to a difficulty in transmission of relatively great torque. 
     The present invention has been achieved to overcome the aforementioned problem. 
     According to a first aspect of the present invention, a differential device is used for differentially transmitting torque from an input shaft to a pair of output shafts. The differential device is provided with: a case rotatable about an axis; an input member configured to receive the torque from the input shaft, the input member being housed in the case and including an engaging outer periphery drivingly engaged with the case; first and second output gears configured to respectively drivingly link with the output shafts, the first and second output gears being rotatably housed in the case to form a row with the input member along the axis; one or more first pinions being rotatably housed in the case in parallel with the axis and meshing with the first output gear, the first pinions having no overlap with the engaging outer periphery of the input member and the second output gears; and one or more second pinions being rotatably housed in the case in parallel with the axis and meshing with the second output gears and the first pinions, the second pinions extending beyond the input member to reach the first pinions. 
     Preferably, the case is provided with a first end wall, which the input shaft and one of the output shafts penetrate, and a second end wall axially opposite to the first end wall, which another of the output shafts penetrates. More preferably, the input member is interposed between and supported by the output gears supported by the first and second end walls. 
     Preferably, the case is provided with a case body configured to rotatably support outer peripheries of the first and second pinions, a first end wall, a second end wall axially opposite to the first end wall, and one or more bolts fixing the first and second end walls to the case body, the pinions are arranged around outer peripheries of the output gears, and the bolts are disposed in a circumferential direction of the pinions. 
     According to a second aspect of the present invention, a differential device is used for differentially transmitting torque from an input shaft to a pair of output shafts. The differential device is provided with: a case being rotatable about an axis and having a first end perpendicular to the axis and a second end axially opposite to the first end; a central bore defined in the case along the axis, the central bore including an engaging internal periphery; one or more first bores extending from the first end in parallel with the axis and being short of the engaging internal periphery; one or more second bores extending from the first end to the second end in parallel with the axis; an input member configured to receive the torque from the input shaft, the input member being housed in the central bore and including an engaging outer periphery engaged with the engaging inner periphery of the central bore; first and second output gears configured to respectively drivingly link with the output shafts, the first and second output gears being rotatably housed in the central bore to form a row with the input member along the axis; one or more first pinions being respectively rotatably housed in the first bores and meshing with the first output gear; and one or more second pinions being respectively rotatably housed in the second bores and meshing with both the first pinions and the second output gear. 
     Preferably, the case is provided with a first end wall to cover the first end, which the input shaft and one of the output shafts penetrate, and a second end wall to cover the second end, which another of the output shafts penetrates. More preferably, the first and second end walls support the row of the first output gear, the input member and the second output gear. 
     Preferably, the case is provided with a case body configured to rotatably support outer peripheries of the first and second pinions, a first end wall, a second end wall axially opposite to the first end wall, and one or more bolts fixing the first and second end walls to the case body, the pinions are arranged around outer peripheries of the output gears, and the bolts are disposed in a circumferential direction of the pinions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross sectional view of a differential device according to a first embodiment of the present invention, which is taken from a line I-I of  FIG. 2 ; 
         FIG. 2  is a side view of the differential device shown from an arrow II of  FIG. 1 ; 
         FIG. 3  is a partial sectional view of the differential device, which is taken from a line III-III of  FIG. 2 ; 
         FIG. 4  is a side view of the differential device shown from an arrow IV of  FIG. 1 ; 
         FIG. 5  is a partial sectional view of the differential device, which is taken from a line V-V of  FIG. 4 ; 
         FIG. 6  is a side view of an input member of the differential device for receiving torque; 
         FIG. 7  is a cross sectional view of the input member; 
         FIG. 8  is a side view of a case body of the differential device, which is taken from an arrow VIII of  FIG. 9 ; 
         FIG. 9  is a cross sectional view of the case body, which is taken from a line IX-IX of  FIG. 8 ; 
         FIG. 10  is a side view of the case body, which is taken from an arrow X of  FIG. 9 ; 
         FIG. 11  is a partial sectional view of the case body, which is taken from a line XI-XI of  FIG. 8 ; 
         FIG. 12  is an elevational perspective view of the differential device; 
         FIG. 13  is an elevational perspective view of the case body; 
         FIG. 14  is a schematic view of a drive train of an exemplary automobile to which the differential device in accordance with the first embodiment is applied as a center differential; and 
         FIG. 15  is a schematic view of a drive train to which a differential device in accordance with a second embodiment is applied. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Certain embodiments of the present invention will be described hereinafter with reference to the appended drawings. 
     In the following description, a differential device is employed as, but not limited to, a center differential of a drive train of an exemplary 4WD automobile.  FIG. 14  schematically shows such a drive train. 
     The drive train is provided with a transmission  103  for transmission of torque generated by an engine  101  of the automobile to a transfer  105  via an output shaft  139  of the transmission  103 . The transfer  105  is coupled with a front propeller shaft  109  and a rear propeller shaft  121  to distribute the torque to the propeller shafts  109  and  121 . The torque distributed to the front propeller shaft  109  drives a front differential  111  to drive left and right front axles  113  and  115  respectively coupled with left and right front wheels  117  and  119 . Similarly, the torque distributed to the rear propeller shaft  121  drives a rear differential  123  to drive left and right rear axles  125  and  127  respectively coupled with left and right rear wheels  129  and  131 . 
     The transfer  105  is provided with an input shaft  21  linked with the output shaft  139  of the transmission  103 , a center differential, to which the differential device  1  of the first embodiment is applied, and a chain link  107  both housed in a transfer case  141 . The differential device  1  is disposed so as to have its axis directed to a longitudinal direction of the automobile and supported by the transfer case  141  to be rotatable about the axis. The differential device  1  is to allow differential in rotation speeds between the front propeller shaft  109  and the rear propeller shaft  121  while the differential device  1  is distributing the torque thereto. The differential device  1  is drivingly coupled with the output shaft  139  of the transmission  103  via an input shaft. The differential device  1  is coupled with a pair of output shafts  23  and  25 . The output shaft  23  is made to be a hollow cylinder so as to allow coaxial arrangement with the input shaft  21 . The output shaft  23  is provided with a sprocket  133  to link with a sprocket  135  via a chain  137  of the chain link  107 . The output shaft  25  is linked with the propeller shaft  121 . 
     Referring to  FIG. 1 , the differential device  1  is provided with a case  3 , an input member  11  for receiving the torque from the input shaft  21 , a pair of output gears  5  and  7  for respectively outputting distributed torque to the output shafts  23  and  25 , and plural (four, in this example) pairs of shorter pinions  13  and longer pinions  15  for differentially distributing the torque to the output gear  5  and  7 . The input member  11  and the output gears  5  and  7  form a row along a central axis of the differential device  1  and are rotatable about the axis. 
     The differential case  3  is provided with a case body  27  shown in  FIGS. 8 through 11 . The case body  27  defines a central bore at a center thereof along the axis, and plural (four, in this example) pairs of longer housing bores  31  and shorter housing bores  33  in parallel with the central bore. The case body  27  further defines plural (four, in the example) through holes  47  to which bolts  29  are inserted. 
     Referring to  FIG. 2 , the central bore houses the output gears  5  and  7  and the input member  11  along the axis. The central bore has an engaging internal periphery  45  for engaging with the input member  11 . The engaging internal periphery  45  forms a cylindrical hollow around the axis and is totally splined all around the hollow. Further referring to  FIGS. 1 and 3 , as adjacent to the engaging internal periphery  45 , the central bore includes a housing portion  41  for rotatably housing the output gear  5  and another housing portion  43  for rotatably housing the output gear  7 . The housing portion  43  is larger in diameter than the housing portion  41 . 
     Referring to  FIG. 2 , the longer housing bores  31  respectively house the longer pinions  15  and the shorter housing bores  33  respectively house the shorter pinions  13 . The housing bores  31  and  33  extend in parallel with the axis of the case  3 . Each paired housing bores  31  and  33  communicate with each other so as to allow engagement between the pinion  15  and the pinion  13  housed therein. The shorter housing bores  33  are arranged at even intervals on a circle around the axis. The longer housing bores  31  are also arranged at even intervals on another circle slightly larger than the circle on which the shorter housing bores  33  are arranged. The longer housing bores  31  extend from the front end to the rear end of the case body  27 . Referring to  FIGS. 1 ,  3  and  5 , near the rear end, the longer housing bores  31  communicate with the housing portion  43  so as to allow engagement between the output gear  7  and the longer pinions  15 . The shorter housing bores  33  extend from the front end of the case body  27  toward the rear end but is short of the engaging internal periphery  45 . The shorter housing bores  33  communicate with the housing portion  41  so as to allow engagement between the output gear  5  and the shorter pinions  13 . 
     As the shorter housing bores  33  is short of the engaging internal periphery  45 , the housing bores  33  do not have overlaps with the engaging internal periphery  45 . As the longer housing bores  31  are spaced from the central bore, the longer housing bores  31  are not obstructive to formation of the engaging internal periphery  45 . Therefore, the engaging internal periphery  45  is not required to be cut off by the housing bores  31  and  33  and hence totally contributes to engagement with the input member  11 . 
     The case body  27  further defines openings  65  at side peripheries and the rear cover  19  defines openings  67  as shown in  FIG. 4 . These openings  65  and  67  allow inflow and outflow of transfer oil. The oil lubricates various locations where gears mesh with each other and members slide on other members. 
     The differential case  3  is further provided with a front cover  17  and a rear cover  19  for covering the front end and the rear end of the case body  27 , respectively. The front cover  17  and the rear cover  19  respectively fit on the case body  27  at fitting portions  51  and  53 . The input shaft  21  and the output shaft  23  coaxial therewith penetrate the front cover  17  to respectively link with the input member  11  and the output gear  5 . The output shaft  25  penetrates the rear cover  19  to link with the output gear  7 . The rear cover  19  defines plural screw holes  49  by which the bolts  29  are fixed. The rear cover  19  is provided with a boss portion  61  for being rotatably supported by the transfer case  141  with intervening bearings. 
     Referring to  FIGS. 6 and 7 , the input member  11  has a splined outer periphery  9  for engaging with the engaging internal periphery  45  of the case body  27 . An inner periphery of the input member  11  is also splined so as to engage with and receive torque from the input shaft  21 . Referring again to  FIGS. 1 through 5 , the input member  11  is inserted in the central bore and disposed substantially at the center of the case body  27 , where the input member  11  is made engaged with the case body  27 . 
     The output gear  5  and  7  also respectively include splined inner peripheries so as to link with the output shafts  23  and  25 . The output gear  7  is made relatively larger so as to mesh with the longer pinions  15  disposed apart from the central bore. The output gears  5  and  7  are also inserted in the central bore and disposed in the housing portions  41  and  43  of the central bore, respectively. The output gears  5  and  7  are made rotatable in the housing portions  41  and  43 . 
     The output gear  5  has a support portion  55  for rotatably supporting the input member  11 , which fits on a shoulder of the input member  11 . Similarly, the output gear  7  also has a support portion  57 . The rear cover  19  has a support portion  59  fitting on a shoulder of the output gear  7 . Thrust washers  63  are interposed respectively among the output gears  5  and  7 , the input member  11  and the covers  17  and  19  and receive thrust force acting on the input member  11  and the output gears  5  and  7 . The input member  11  is thus supported from both ends by the covers  17  and  19  so as not to be displaced in the axial direction. 
     The shorter pinions  13  are rotatably inserted in the shorter housing bores  33  and the longer pinions  15  are rotatably inserted in the longer housing bores  31 . The length of the shorter pinions  13  is substantially limited within a range of a teeth width W of the output gear  5  as shown in  FIG. 5 . Each pinion  13  is made to mesh with both each pinion  15  and the output gear  7 . Each pinion  15  extends from the rear end to the front end of the case body  27 . A proximal portion  35  of each pinion  15  is made to mesh with the output gear  5 , and a distal portion  37  thereof beyond a mid portion  39  is made to mesh with each pinion  13 . As the pinions  13  and  15  are made rotated along with the differential case  3 , the torque input to the input member  11  is transmitted to the output gears  5  and  7  via the pinions  13  and  15  meshing therewith. Further, as the output gear  5  and  7  are coupled with each other by the pairs of the pinions  13  and  15 , differential in rotation speeds between the output gears  5  and  7  is allowed. 
     All of the gears  5  and  7  and the pinions  13  and  15  are profiled to have helical gear teeth meshing with each other. During transmission of the torque, the pinions  13  and  15  respectively receive thrust forces generated by the meshing helical gear teeth and centrifugal forces generated by rotation of the differential case  3 . These forces urge the pinions  13  and  15  toward wall faces of the cover  17  and  19  and the housing bores  31  and  33  and hence lead to frictional resistance to rotation of the pinions  13  and  15 . As the frictional resistance limits differential motion, the differential device  1  functions as a torque-sensitive limited slip differential, which assures all-wheel traction when the automobile requires relatively large torque, for example at a time of start or acceleration. 
     As the shorter pinions  13  do not have overlaps with the splined outer periphery  9  of the input member  11  and the longer pinions  15  are disposed apart in the radial direction from the splined outer periphery  9 , the engagement of the splined outer periphery  9  with the engaging internal periphery  45  is not structurally or dimensionally limited. The whole length of the splined outer periphery  9  can contribute to engagement with the differential case  3  and is therefore enabled to transmit relatively large torque. 
     The differential device  1  in accordance with the present embodiment of the present invention enables input from one end and output to both ends, one of which is coaxial to the input. By coupling the input with the engine and the outputs with front and rear axles, the differential device  1  enables differential distribution of torque generated by the engine to the front and rear axles. The differential device  1  allows differential in rotation speeds between both the outputs. 
     Support of the input member  11  in the axial direction is assured by the front and rear covers  17  and  19  via the interposed output gears  5  and  7  which put the input member  11  therebetween. 
     Helical gear teeth of the gears  5  and  7  and the pinions  13  and  15  generate thrust force which leads to torque-sensitive limitation of differential rotation. 
     Further, the bolts  29  may be disposed at respective spaces between respective pairs of housing bores  31  and  33  and disposed on a slightly larger circle than the circle on which the housing bores  31  are arranged, as shown in  FIG. 2 . Such an arrangement enables a fixation structure in which the bolts  29  can avoid the pinions  13  and  15 . This leads to secure fixation. Further such an arrangement saves dead spaces and therefore the differential case  3  is prevented from being largescale. 
     The aforementioned embodiment may be modified into a second embodiment as will be described hereinafter. A differential device  201  in accordance with the second embodiment of the present invention is employed as a center differential in a drive train shown in  FIG. 15 . 
     The drive train is provided with a transmission  253  for transmission of torque generated by an engine  251  to a transfer  255 . The transmission  253  may include a proper gear set to change rotation speed. The transfer  255  receives the torque after the result of changing speed. The transfer  255  is coupled with a front output shaft  221  and a gear link  257 . The gear link  257  includes a driving gear  265  and a follower gear  267  which meshes with the driving gear  265  and is coupled with a rear propeller shaft  261 . Thereby the torque is distributed to the shafts  221  and  261 . The torque distributed to the front output shaft  221  drives a front differential  259  to drive left and right front axles (not shown). The torque distributed to the rear propeller shaft  261  drives a rear differential  263  to drive left and right rear axles (not shown). 
     The transfer  255  is provided with a center differential, to which a differential device  201  in accordance with the second embodiment is applied. The differential device  201  receives the torque via an input shaft  219  which is coaxial with the output shaft  221 . 
     The differential device  201  is provided with a differential case  203 , an input member  209  for receiving the torque from the input shaft  219 , a pair of output gears  205  and  207  fore respectively outputting distributed torque to the output shafts  221  and  223 , and plural pairs of shorter pinions  211  and longer pinions  213 . 
     The differential case  203  is provided with a case body  225  and covers  215  and  217  respectively covering front and rear ends thereof. The covers  215  and  217  are fixed by means of plural bolts  227 . 
     As with the differential device  1  of the first embodiment, also in the differential device  201  in accordance with the second embodiment, each longer pinion  213  extends from the rear end to the front end of the case body  225  but each shorter pinion  211  extends from the rear end toward the front end but is short of the input member  209 . Each longer pinion  213  meshes with both the output gear  207  and the shorter pinion  211 , and each shorter pinion  211  meshes with both the output gear  205  and the longer pinion  213 . As the pinions  213  and  215  are made rotated along with the differential case, the torque input to the input member  209  is transmitted to the output gears  205  and  207 . 
     All of the gears  205  and  207  and the pinions  211  and  213  are profiled to have helical gear teeth meshing with each other. Thereby, differential in rotation speeds between the output gear  5  and  7  is allowed and also a function of a torque-sensitive limited slip differential is obtained. 
     As the shorter pinions  211  do not have overlaps with the input member  209  and the longer pinions  213  are spaced from the input member  209  in the radial direction, engagement all around the input member  209  can be assured. 
     In contrast with the differential device  1  of the first embodiment, all of the input shaft  219  and the output shafts  221  and  223  are commonly coaxially led out of the front end of the differential case  203 . Therefore, input and output of the torque are commonly executed at the same (front) end. 
     Although the invention has been described above by reference to certain embodiments of the invention, the invention is not limited to the embodiments described above. Modifications and variations of the embodiments described above will occur to those skilled in the art, in light of the above teachings.