Abstract:
A differential device for differentially distributing a driving force to axles along an axis is disclosed. The differential device has a case being capable of rotation about the axis, which includes a flange configured to receive the driving force and a shaft crossing the case perpendicularly to the axis; an opening defined by a peripheral border on an outer periphery of the case so as to allow access into the case, lateral extremities of which is deviated from a center of the shaft toward a direction opposite to the flange along the axis; and a differential gear set housed in and drivingly coupled to the case, the differential gear set including an input gear rotatable around the shaft and output gears so combined with the input gear as to differentially distribute the driving force to the output gears, the output gears being drivingly coupled to the axles.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 2006-181927 (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, an automobile is equipped with a differential to distribute a driving force of an engine to right and left output axles. The differential allows differential motion between the axles and hence maintains traction of wheels with the road while the automobile is turning. 
     The differential is provided with a complex gear set for enabling the differential motion. In general, the gear set is housed in a differential case to which the engine inputs the driving force. For installation of the gear set in the differential case, some differential cases are capable of being divided into two pieces. Other differential cases are incapable of being divided and instead have openings so as to allow passage of the gear set therethrough. A structure of the latter is often referred to as “one-piece structure”. 
     SUMMARY OF THE INVENTION 
     The present inventors have found that the one-piece structure may reduce stiffness and strength of the differential case because of existence of the openings. An object of the present invention is intended for overcoming this problem. 
     According to an aspect of the present invention, a differential device differentially distributes a driving force to axles along an axis. The differential device has a case being capable of rotation about the axis, which includes a flange configured to receive the driving force and a shaft crossing the case perpendicularly to the axis; an opening defined by a peripheral border on an outer periphery of the case so as to allow access into the case, lateral extremities of which is deviated from a center of the shaft toward a direction opposite to the flange along the axis; and a differential gear set housed in and drivingly coupled to the case, the differential gear set including an input gear rotatable around the shaft and output gears so combined with the input gear as to differentially distribute the driving force to the output gears, the output gears being drivingly coupled to the axles. 
     Preferably, the opening is so opened as to leave a periphery opposite to the flange, which radially projects over the opening. 
     Preferably, the opening is so dimensioned as to allow passage of any member to be housed in the case. 
     Preferably, the lateral extremities of the peripheral border define a widest portion of the opening, the widest portion being the widest among any portions of the opening along a direction perpendicular to the axis. More preferably, the widest portion comprises an enough width to allow passage of any member to be housed in the case. Still preferably, the differential device further has a through hole configured to fix the shaft, which is defined on the case and so positioned that a circular plane realized by rotating the center of the shaft about the axis does not cross the lateral extremities, or crosses the peripheral border on a side closer to the flange than the lateral extremities. 
     Preferably, the case further has a second opening so dimensioned as to allow passage of any member of the differential gear set. 
     Preferably, the case further has a second opening symmetrical to the opening with respect to the axis. 
     Preferably, the differential device further has a clutch assembly configured to lock differential motion between the output gears, the clutch assembly including a clutch member housed in the case, wherein the widest portion is so dimensioned as to allow passage of the clutch member. 
     Preferably, the clutch member is disposed between the differential gear set and the flange. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view of a differential device in accordance with a first embodiment of the present invention, illustrating a differential-lock state; 
         FIG. 2  is a magnified sectional view of the differential device with respect to a solenoid and its proximity; 
         FIG. 3  is a plan view of a differential case of the differential device; 
         FIG. 4  is a partial front view of the differential case, viewed along an arrow IV of  FIG. 3 ; 
         FIG. 5  is cross sectional view of the differential case, taken from a line V-V of  FIG. 4 ; 
         FIG. 6  is a perspective view of the differential case; 
         FIG. 7  is another perspective view of the differential case; 
         FIG. 8  is a plan view of a differential case of a differential device in accordance with a second embodiment of the present invention; 
         FIG. 9  is a partial front view of the differential case, viewed along an arrow IX of  FIG. 8 ; and 
         FIG. 10  is a cross sectional view of the differential case, taken from a line X-X of  FIG. 9 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Certain embodiments of the present invention will be described hereinafter with reference to the appended drawings. Throughout the specification, claims and the drawings, an axial direction is defined as a direction along an axis of a differential device unless any other particular explanation is given, and a lateral direction is defined as a direction perpendicular to the axial direction. The axial direction is drawn horizontally in  FIGS. 1 ,  3 ,  4 ,  8  and  9 . 
     A lock-up differential of a bevel gear type is exemplified in the following description, however, needless to say, the present invention is not limited thereto. 
     First Embodiment 
       FIGS. 1-7  illustrate a differential device  1  in accordance with a first embodiment of the present invention. In the following description, the right and the left are correspondent to those of  FIGS. 1 and 2 . 
     Referring to  FIG. 1 , the differential device  1  is provided with a differential case  5  which receives a driving force of an engine, a differential gear set  15  of a bevel gear type so as to differentially transmit the driving force to right and left side gears  11  and  13  respectively linked with right and left axles, a clutch assembly  17  for locking (or limiting) differential motion between the side gears  11  and  13 , and a controller (not shown) for controlling the clutch assembly  17 . 
     Referring to  FIG. 3  in combination with  FIG. 1 , the differential case  5  has a drum portion having a substantially cylindrical outer periphery, a flange portion  3  radially extending from the drum portion, and a pair of axially projecting boss portions  27  and  29 . The flange portion  3 , to which a ring gear is fixed, is to receive the driving force of the engine. The boss portions  27  and  29  of the differential case  5  are rotatably supported by a differential carrier (not shown). Bearings respectively intervene between each boss portion and the differential case for smooth rotation of the differential case  5  but are not shown in the drawings. 
     The differential case  5  has through holes  31  for supporting a pinion shaft  7  described later, which penetrate the outer periphery from the exterior to the interior, and a bolt hole  33  crossing one of the through holes  31 . 
     The outer periphery of the differential case has other openings  19  through which the interior of the differential case  5  is accessible. The openings  19  are respectively formed in an irregular oval shape and are symmetrical with each other with respect to the axis of the differential case  5 . In the plan view as shown in  FIG. 3 , the openings  19  are in the shape of a pair of concavities and, in the front view as shown in  FIGS. 1 and 4 , the openings  19  coincide with each other and therefore appear to be a single irregular oval opening. Peripheral borders  20  of the openings  19  are shown as a closed broken line in  FIG. 1  and shown as a closed solid line in  FIG. 4 . 
     Lateral extremities  21  of each peripheral border  20 , which are the utmost portions in a lateral direction, define a widest portion  22  of each opening  19 . More specifically, the extremities  21  are the most distant portions from each other among any portions on the periphery in a direction lateral to the axis of the differential case  5  and the widest portion  22  defined thereby is the widest among any portions of the opening  19  along the lateral direction. The extremities  21  may have certain lengths along the axial direction as shown in  FIGS. 1 and 4 . The widest portion  22  has an enough width to allow passage of any components to be installed in the differential case  5 , particularly a clutch ring  45  described later. 
     A disposition of the lateral extremities  21 , or more specifically the widest portion  22 , in the axial direction is deviated from an axis of the pinion shaft  7  toward a direction opposite to the flange portion  3 . If a circular plane realized by rotating the axis of the pinion shaft  7  is supposed (illustrated as a line C in  FIG. 3 ), the circular plane is deviated from the lateral extremities  21  toward the flange portion  3 . In other words, the circular plane does not cross the lateral extremities  21  but crosses the peripheral border  20  at a side closer to the flange portion  3 . In the plan view of  FIG. 3 , the lateral extremities  21  are shown as a bottom of each concavity and the disposition of the widest portions  22  have an offset D from the axis of the pinion shaft  7 . The widest portion  22  is disposed such that the lateral extremities  21  have a small overlap or no overlap with the through hole  31  along the axis of the case  5 . 
     The peripheral border  20  of each opening  19  at the side of the flange portion  3  is like a slope from the widest portion  21  toward the flange portion  3 . Another side  25  of the peripheral border  20  opposite to the aforementioned side with respect to the widest portions  21  is relatively far from the flange portion  3 . A portion  23  of the outer periphery of the differential case  5 , which is farther from the flange portion  3  than the side  25 , projects radially outward. 
     The differential gear set  15  generally consists of the pinion shaft  7 , pinion gears  9 , and a pair of side gears  11  and  13 . The pinion shaft  7  radially crosses the differential case  5 . The pinion gears  9  are rotatably supported by the pinion shaft  7 . The side gears  11  and  13  engage with the pinion gears  9  from respectively left and right sides. Internal surfaces of the side gears  11  and  13  are splined so as to drivingly engage with left and right axles. Thereby, when the engine of the automobile drives the differential case  5 , the driving force is differentially distributed to the left and right axles via the side gears  11  and  13 . 
     The differential case  5  is further provided with a support portion  57  formed at a side thereof, from which the flange portion  3  extends. The support portion  57  is formed to be a circular internal periphery in such a way as to slidably fit on and support a coil housing  55  of a solenoid  39  in a radial direction. A plurality (three in this example) of plates  61  slidably engage with the solenoid  39  and pairs of bolts  63  respectively fix the plates  61  to the differential case  5 . The coil housing  55  of the solenoid  39  is anti-rotated whereas the differential case  5  is capable of rotating. Therefore relative rotation occurs between the coil housing  55  and the differential case  5 . 
     The clutch assembly  17  generally consists of a dog clutch  37  and an actuator for driving the dog clutch  37 . The dog clutch  37  locks the side gears  11  and  13  relative to the differential case  5  and therefore prevents differential motion therebetween when the dog clutch  37  is made engaged by the plunger  41 . 
     Referring to  FIG. 2 , the actuator in the present embodiment employs an electromagnetic actuator but not limited thereto. The actuator generally consists of a solenoid  39 , a plunger  41  operated by the solenoid  39  to engage the dog clutch  37 , a return spring  43  and a controller (not shown). The return spring  43  urges the dog clutch  37  into a disengaging state. The coil housing  55  in combination with a portion  4  of the differential case  5 , where the solenoid  39  adjoins, encloses a winding of the solenoid  39  but leaves a gap at an internal periphery thereof. 
     The plunger  41  generally consists of a moving yoke  67  and a ring  69  fitting with the moving yoke  67 . The moving yoke  67  spans the aforementioned gap left between the coil housing  55  and the differential case  5 . The coil housing  55 , the differential housing  5 , and the moving yoke  67  are made of a magnetic material such as, but not limited to, AISI SAE1010 (JIS S10C). Thereby, a magnetic flux  71  generated by the solenoid  39  takes a form of a loop via the coil housing  55 , the differential case  5 , and the moving yoke  67 , as shown in  FIG. 2 . In the strict sense, where a radially outer end  59  of the coil housing  55  adjoins the support portion  57 , the magnetic flux  71  branches into two flux paths, namely a first flux path  73  via the plate  61  and a second flux path directly going into the differential case  5 , and merge into a single flux. A lead line  65  is led out of the solenoid  39  and further conducted out of the differential carrier to link with a battery via the controller so that the controller controls excitation of the solenoid  39 . 
     A ring  69  made of a non-magnetic material drivingly fits in the moving yoke  67  and slidably fits on the left boss portion  27 . Thereby the moving yoke  67  and the ring  69  are unitarily movable along the left boss portion  27 . By non-magnetism of the ring  69 , the magnetic flux  71  is prevented from leaking to the left boss portion  27 . 
     The ring  69  has projections projecting in the axial direction toward the differential gear set  15 . As being correspondent to the projections, the differential case  5  has openings  53 , to which the projections are loosely and slidably inserted. Side faces in the rotational direction of the projections abut on peripheries of the openings  53  so that the ring  69  is rotated unitarily with the differential case  5 . As the coil housing  55  is anti-rotated, relative rotation occurs between the coil housing  55  and the ring  69 . 
     The dog clutch  37  generally consists of teeth  47  formed on a right side of a clutch ring  45  and teeth  49  formed on a left side of the left side gear  11 . The teeth  47  and the teeth  49  are opposed to each other and therefore capable of engaging with each other. 
     The clutch ring  45  is supported by the internal periphery of the differential case  5  to be axially movable. The clutch ring  45  is provided with projections  51  as facing to the projections of the ring  69 . As the projections  51  respectively face to the projections of the ring  69 , motion of the moving yoke  67  toward the dog clutch  37  (rightward in  FIGS. 1 and 2 ) is transmitted to the dog clutch  37  via the butted projections so that the dog clutch  37  is made engaged. When the dog clutch  37  is in the engaging state, the differential motion between the left and right side gears  11  and  13  is locked. 
     The projections  51  respectively have side faces which are respectively oblique to the rotation direction. The openings  53  also have oblique cam faces as correspondent to these oblique faces. A combination of the side faces of the projections  51  and the cam faces of the openings  53  compose a cam for converting torque of the differential case  5  into an axial force on the clutch ring  45  to assist the engagement of the dog clutch  37 . With a help of this assistance, the solenoid  39  do not have to generate relatively large magnetic force to maintain the engaging state of the dog clutch  37 . As opposed to the engagement force on the dog clutch  37 , the return spring  43  urges the dog clutch  37  into the disengaging state unless the plunger  41  gives force to the dog clutch  37 . 
     Thereby, when the solenoid  39  is excited, the dog clutch  37  is driven into the engaging state. Then the differential motion of the differential gear set  15  is locked. When the excitation is cut off, the return spring  43  urges the dog clutch  37  into the disengaging state. Then the differential motion of the differential gear set  15  is allowed. 
     Most components of the differential gear set  15  and the clutch ring  37  are inserted through the openings  19  into the differential case  5  and then installed. The pinion shaft  7  is inserted into the through holes  31  and prevented from displacing by a bolt  35  tightened in the bolt hole  33 . Further lubrication oil flows in and out of the openings  19 . 
     In the cylindrical outer periphery of the differential case  5 , portions  75  close to the flange portion  3  must bear far larger twisting moment than other portions (see  FIG. 3 ) as the force is input into the flange portion  3  and output to the pinion shaft  7  mainly via the portions  75 . In contrast, existence of the openings  19  may reduce stiffness and strength of the differential case  5  around the openings  19 , particularly the widest portions  22  thereof. As the disposition of the widest portions  22  is deviated from the axis of the pinion shaft  7  toward the direction opposite to the flange portion  3  by the length of the offset D, the portions  75  have sufficient widths to have enough stiffness and strength to bear the force input to the flange portion  3 . Further, as the circular plane realized by rotating the axis of the pinion shaft  7  does not cross the lateral extremities  21  but crosses the peripheral border  20  at the side closer to the flange portion  3 , a sufficient width from the through hole  31  to the peripheral border  20  along this circular plane can be held. This leads to sufficient stiffness and strength of the differential case  5  in the circumferential direction. 
     Second Embodiment 
       FIGS. 8-10  illustrate a differential device  101  in accordance with a second embodiment of the present invention. In the following description, the right and the left are correspondent to those of  FIGS. 8 and 9 . Substantially the same elements as any of the aforementioned elements are referred to as the same reference numerals and detailed descriptions thereof will be omitted. The following description will be mainly given to differences from the aforementioned first embodiment. 
     The differential device  101  is provided with a differential case  103  having an opening  19  and an opening  105  for allowing passage of internal components therethrough. As with the opening  19  in accordance with the aforementioned first embodiment, the opening  19  in accordance with the present second embodiment is formed in an irregular oval shape and has a widest portion  22  where a width of the opening  19  along the circumferential direction of the outer periphery of the differential case  103  is largest. The widest portion  22  has an enough width to allow passage of any components to be installed in the differential case  103 . The disposition of the widest portion  22  in the axial direction is deviated from a center of a pinion shaft  7  toward a direction opposite to a flange portion  3 . A circular plane (shown as a line C in  FIG. 8 ) realized by rotating the axis of the pinion shaft  7  does not cross the lateral extremities  21  but crosses the peripheral border  20  at a side closer to the flange portion  3 . 
     The opening  105  has different dimensions from those of the opening  19 . The opening  105  is a substantially round opening and has enough dimensions to allow passage of the pinion gears  9 . The opening  105  and the opening  19  are opposed to each other with respect to the axis of the differential case  101 . The opening  105  is generally smaller than the opening  19 . This may be understood from a comparison between a solid line of the opening  105  and a two-dot chain line showing a hypothetical curve as if the opening  19  exists around the opening  105 . As a result of smallness of the opening  105 , a portion  107  larger than the portion  23  is left at a periphery of the opening  105  opposed to the flange portion  3 . 
     As the disposition of the widest portion  22  is deviated from the center of the pinion shaft  7  toward the direction opposite to the flange portion  3  by the length of the offset D and the opening  105  is smaller than the opening  19 , portions  109  and  112  have sufficient widths to have enough stiffness and strength to bear the force input to the flange portion  3 . Further, as the relatively large portion  107  is left, the differential case  103  has a large stiffness particularly to tension stress induced by a thrust load on the flange portion  3 . 
     In the above description, the invention is applied to a lock-up differential of a bevel gear type. However, the invention is also applied to any other differentials such as a free-running differential in which transmission of power to a differential can be intermitted, and an axle disconnect device in which transmission to both axles can be intermitted. Further, applicable types are not limited to the bevel gear type but may range over any types. Moreover, a differential in accordance with any embodiments of the present invention can be applied to a front differential, a center differential and a rear differential. 
     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.