Abstract:
A lower cost differential gear assembly capable of inputting driving force through a differential gear, whose teeth have sufficient strength is provided. To achieve the object, the differential gear is constructed by engaging a gear part provided with teeth at equal intervals on one of its perimeter surfaces, and a support part pivotally supported by right and left axles through engagement of a spline. Accordingly, the gear part can be formed from a high strength material and the support part can be formed from a lower cost material.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   The present application is a Continuation of application Ser. No. 10/725,598, filed Dec. 3, 2003, which is incorporated in its entirety herein by reference thereto. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a bull gear of a differential gear assembly (a differential bull gear). 
   2. Related Art 
   Conventionally, a differential bull gear is made of high-strength metal such as steel or powder metal sintered in high density because its teeth especially require high strength. However, the bull gear entirely made of such high-strength material is expensive. 
   Therefore, as disclosed in U.S. Pat. Nos. 6,338,690 and 2,608,261, for example, a well-known conventional economical differential bull gear comprises integrally coupled different outer and inner members, wherein an outer member serving as a ring gear is made of high-strength material, and an inner member, a plate which supports a differential pinion, does not require such high strength and may be made of inexpensive material. 
   However, in this type of bull gear, a problem arises because the two different members coupled together may axially shift from each other so as to hinder proper movement of the differential gear assembly. 
   The differential bull gear disclosed in U.S. Pat. No. 6,338,690 avoids the axially positional difference between the inner and outer members by inserting corners of the square thin inner member into corresponding recesses formed in the inner peripheral portion of the outer member serving as a ring gear. However, the thin inner member supporting just a differential pinion is so separated from a pair of axles as to make it difficult to fix the axles in axial location when differential side gears fixed on the respective axles mesh with the differential pinion. 
   Therefore, a differential housing is divided into halves along the bull gear so that the differential housing halves sandwich the bull gear. Each of the differential housing halves is partly extended to fill gaps between the square inner plate and the ring gear so as to reinforce the bull gear and enhance efficiency of torque transmission between the ring gear and the inner plate. However, each of the differential housing halves having such a complicated shape and requiring high-dimensional accuracy may be expensive, and the differential gear assembly is complicated in its assembly. 
   In the differential bull gear disclosed in the U.S. Pat. No. 2,608,261, the inner member supporting the differential pinion and supported on axles has a circular contour, when axially viewed, so that the peripheral surface thereof entirely contacts a inner peripheral surface of the outer member serving as a ring gear. Both members are prevented from relative rotation by tightening some screws, and both contact heads of the screws so as to be prevented from axially relative shifting. 
   However, each of female screws for the screws is divided into halves. The female screw halves are formed on the inner peripheral surface of the outer member and the outer peripheral member of the inner member, respectively. The inner and outer members should be accurately located in their relatively rotational and axial directions so as to form the female screws. Since the half-divided female screws require complicated processing and location, and high accuracy, the resultant bull gear may be expensive. If the bull gear is simplified by decreasing the screws and female screws, efficiency of torque transmission between the inner and outer members is reduced, and the screws are further stressed so as to be damaged, causing rotational and axial moving of the inner and outer members. 
   BRIEF SUMMARY OF THE INVENTION 
   A main object of the present invention is to provide an economical bull gear of a differential gear assembly (a differential bull gear) simplified in assembly and location while keeping sufficient strength of its toothed portion as an input gear for receiving torque from a transmission. 
   To achieve the object, a differential bull gear according to the present invention comprises a ring gear for inputting torque from a transmission, and a support member supporting a differential pinion and supported on an axle. The support member transmits the torque from the ring gear to the axle through the differential pinion. The ring gear and the support member are toothed to be coupled together so as to be relatively axially shiftable and not-relatively rotatable. The ring gear made of high-strength material such as steel or sintered powder metal is sufficiently strong to act as the input gear for receiving torque from a transmission. The support member allowed to be lower in strength than the ring gear is made of sintered powder metal, plastics, die-cast metal, or the like, thereby being inexpensive. 
   In one aspect, the ring gear preferably has a toothed outer periphery and a toothed inner periphery. One of the toothed outer and inner peripheries serves as an input gear for receiving torque from the transmission. The other of the toothed outer or inner periphery meshes with the toothed portion of the support member so as to couple the ring gear with the support member. 
   Alternatively, in another aspect, the ring gear preferably has a toothed outer periphery and a toothed inner periphery. Either the toothed outer or inner periphery serves as an input gear for receiving torque from the transmission. Both toothed outer and inner peripheries mesh with the toothed portion of the support member so as to couple the ring gear with the support member. 
   In this aspect, the support members has an annular recess into which the ring gear is fitted. The recess has a toothed outer periphery and a toothed inner periphery for meshing with the toothed outer and inner peripheries of the ring gear, respectively. 
   A second object of the present invention is to provide the bull gear with the support member and the ring gear prevented from relatively axially moving by simple configuration. 
   As an aspect, the above-mentioned support member, is preferably divisible into halves, each half having an annular recess. The halves are joined to each other so as to fit the ring gear in the mutually facing recesses, thereby forming the support member holding the ring gear. 
   As an aspect, while the ring gear has a first surface perpendicular to the axle and the support member has a second surface to be leveled with the first surface of the ring gear, a retaining member abuts against the first and second surfaces so as to prevent the ring gear and the support member from relatively axial moving. The retaining member may be provided on a pinion shaft supporting a pinion for transmitting torque from the transmission to the ring gear. Alternatively, the retaining member may be a washer provided on a screw screwed into either the ring gear or the support member. If the outer member is divided into a plurality of pieces, a plurality of the retaining members may be provided to the respective pieces of the outer member. 
   These, other and further objects, features and advantages of the invention will appear more fully from the following description taken in connection with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES 
       FIG. 1  is a side view of a mower tractor equipped with a transaxle. 
       FIG. 2  is a sectional plan view of the transaxle incorporating a differential gear assembly including a differential bull gear according to a first embodiment of the present invention. 
       FIG. 3  is a sectional rear view of the transaxle. 
       FIG. 4  is a side view of the differential bull gear according to the first embodiment. 
       FIG. 5  is a side view of a differential bull gear according to a second embodiment of the present invention. 
       FIG. 6  is a side view of a differential bull gear according to a third embodiment of the present invention. 
       FIG. 7  is a sectional plan view of a differential gear assembly including the differential bull gear according to the third embodiment. 
       FIG. 8  is a side view of a differential bull gear according to a fourth embodiment of the present invention. 
       FIG. 9  is a sectional plan view of a differential gear assembly including the differential bull gear according to the fourth embodiment. 
       FIG. 10  is a side view of a differential bull gear according to a fifth embodiment of the present invention. 
       FIG. 11  is a sectional plan view of a differential gear assembly including the differential bull gear of the fifth embodiment. 
       FIG. 12  is a side view of a differential bull gear according to a sixth embodiment of the present invention. 
       FIG. 13  is a side view of a differential bull gear according to a seventh embodiment of the present invention. 
       FIG. 14  is a sectional plan view of a differential gear assembly including the differential bull gear according to the seventh embodiment. 
       FIG. 15  is a perspective view of a center section for a hydraulic stepless transmission in the transaxle shown in  FIGS. 2 and 3 . 
       FIG. 16  is a perspective view of a movable swash plate for the hydraulic stepless transmission in the transaxle shown in  FIGS. 2 and 3 . 
       FIGS. 17 to 23  are sectional side views of some types of breathers for the transaxle shown in  FIG. 3 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   As an example of a vehicle equipped with a differential gear assembly, as shown in  FIG. 1 , a mower tractor  10  is provided at its rear portion with a transaxle  1 , incorporating a differential gear assembly differentially supporting right and left axles  35 . Right and left rear wheels  16  are fixed onto outer ends of the axles  35 , and right and left front wheels  15  are suspended at a front portion of the mower tractor  10 . An engine  11  is mounted above the front wheels  15 . The engine  11  has a downwardly vertical output shaft  12  with a pulley  13  fixed thereon. The transaxle  1  has an upwardly vertical input shaft  24  with a pulley  23  fixed thereon. 
   A drive belt  17  is wound around the pulleys  13  and  23  so as to transmit driving force from the engine  11  to the input shaft  24  of the transaxle  1 . As shown in  FIGS. 2 and 3 , the transaxle  1  incorporates a hydraulic stepless transmission (hereafter, a HST)  25 , a reduction gear assembly  81  and a differential gear assembly  82 . The driving force of the input shaft  24  is transmitted to the right and left axles  35  through the HST  25 , the reduction gear assembly  81 , and the differential gear assembly  82 , thereby rotating the rear wheels  16 . 
   The mower tractor  10  is provided with an accelerator  26  comprising a front pedal  26   a  and a rear pedal  26   b , which are connected integrally so as to be rotatable like a seesaw centered on a horizontal pivot shaft  27 . The mower tractor  10  travels forward by pressing the pedal  26   a  and travels rearward by pressing the pedal  26   b.    
   An arm  28  is fixed to the accelerator  26  so as to be rotatable around the pivot shaft  27 . The arm  28  is pivotally connected to a speed change lever  30  through a rod  29 . The speed change lever  30  is fixed onto a horizontal operation shaft  31  extended outward from the HST  25 . Therefore, depression of either the front or rear pedal  26   a  or  26   b  is transmitted to the operation shaft  31  of the HST  25 . 
   A mower deck  22  incorporating a rotary blade  21  for cutting grass is disposed below the center portion of the vehicle body. A pulley  14  is also fixed on the output shaft  12  of the engine  11 , and a pulley  19  is fixed on an input shaft  20  extended upward from the mower deck  22 . A drive belt  18  is wound around the pulleys  14  and  19  so as to transfer driving force from the engine  11  into the mower deck  22 , thereby rotating the rotary blade  21 . 
   The transaxle  1  will be explained with reference to  FIGS. 2 and 3 . The transaxle  1  comprises a housing  33 , which is dividable into an upper housing member  33   a  and a lower housing member  33   b . The housing  33  incorporates the HST  25 , a brake assembly  80 , the reduction gear assembly  81  and the differential gear assembly  82 . The HST  25  comprises a hydraulic motor  36 , a hydraulic pump  37 , a center section  38  and others. 
   The vertical input shaft  24  is extended above the housing  33 . The pulley  23  and a cooling fan  32  are fixed on the upward extended portion of the input shaft  24  above the housing  33 . 
   The input shaft  24  serves as a pump shaft of the hydraulic pump  37  so that, by rotating the input shaft  24 , the hydraulic pump  37  supplies hydraulic oil to the hydraulic motor  36 . 
   The variable displacement hydraulic pump  37  comprises a cylinder block  44  and a movable swash plate  45 . In the cylinder block  44  is disposed the axial pump shaft  24  and vertically slidable pistons  43 . The movable swash plate  45  is integrally provided with a thrust bearing  46  abutting against the pistons  43 . The movable swash plate  45  has an arcuately curved surface  45   a  (see  FIG. 16 ) slidably fitted to the housing  33 . 
   In the housing  30 , a connection arm  50  with a horizontal projection  50   a  inserted into the movable swash plate  45  is fixed on the operation shaft  31 . By rotating the speed change lever  30 , the operation shaft  31  and the connection arm  50  are rotated integrally with the lever  30 , thereby changing a tilt angle (and direction) of the movable swash plate  45  so as to change the amount and direction of oil discharged from the hydraulic pump  37  to the hydraulic motor  36 . 
   An adjuster  51  for adjusting the neutral position of the connection arm  50  is attached to the housing  33  so as to correct the neutral position of the movable swash plate  45 . 
   The center section  38  is fastened to the housing  33  by bolts so as to be apart from the bottom of the housing  33 . An oil filter  47  is disposed between the center section  38  and the bottom of the housing  33 . The center section  38  having a horizontal pump mounting surface  38   b  and a vertical motor mounting surface  38   a  (see  FIG. 15 ) is arranged in the housing  33  so as to place the pump mounting surface  38   b  between the axle  35  and the motor mounting surface  38   a . The vertically axial cylinder block  44  of the hydraulic pump  37  is slidably and rotatably fitted onto the pump mounting surface  38   b , and a horizontally axial cylinder block  40  of the hydraulic motor  36  is slidably and rotatably fitted onto the motor mounting surface  38   a.    
   The fixed displacement hydraulic motor  36  comprises the cylinder block  40  with horizontal pistons  39  inserted therein, and a fixed swash plate  41 . A thrust bearing  42  is integrally provided in the swash plate  41  so as to abut against the pistons  39 . 
   The hydraulic motor  36  has a horizontal motor shaft  74  disposed parallel to the axles  35 . The motor shaft  74  axially and not-relatively rotatably penetrates the cylinder block  40 . The motor shaft  74  is extended from the cylinder block  40  and freely passed through the swash plate  41 , and a gear  75  is fixedly provided thereon. Accordingly, output from the hydraulic motor  36  is transmitted to the gear  75 . The brake assembly  80  is provided around the gear  75  so as to apply braking force onto the motor shaft  74 . 
   A counter shaft  79  is supported between an outer wall and a partition of the housing  33 . An axially long and diametrically small pinion  77  rotatably fitted on the shaft  79 , and a diametrically large gear  76  not-relatively rotatably fitted on the pinion  77  constitute the reduction gear assembly  81 . The gear  76  meshes with the gear  75 , and the pinion  77  meshes with a differential bull gear  78  of the differential gear assembly  82 . Thus, the reduction gear assembly  81  transmits output of the hydraulic motor  36  to the differential gear assembly  82 . 
   The differential gear assembly  82  differentially rotates the right and left axles  35 . The right and left axles  35  are rotatably supported by the rear portion of the housing  33 , and extended outward therefrom. The rear wheels  16  are fixed onto distal ends of the axles  35 . 
   As described above, output of the engine  11  is transmitted to the right and left axles  35  through the HST  25 , the reduction gear assembly  81 , and the differential gear assembly  82 , thereby rotating the rear wheels  16 . 
   As shown in  FIG. 3 , in the housing  33 , a vacant space above the gear on the counter shaft  79  serves as an oil tank  100 . A breather  101  is disposed on an upper surface of the housing  33  above the oil tank  100 , as described below. 
   The differential gear assembly  82  will now be explained. As shown in  FIG. 2 , bevel side gears  34  are fixed on proximal portions of the right and left coaxial axles  35 . The bull gear  78  disposed between the bevel side gears  34  meshes with the pinion  77  so as to serve as an input gear of the differential gear assembly  82 . The bull gear  78  has an axial center hole  91   a  into which proximal ends of the axles  35  are relatively rotatably inserted so as to support the bull gear  78 . 
   At least one differential bevel pinion  93  is rotatably supported in the bull gear  78  through a pinion shaft  92 , and meshes with the right and left side gears  34  so as to differentially transmit torque of the bull gear  78  to the axles  35 . 
   The differential bull gear  78  will be explained. As shown in  FIGS. 4 and 5 , the differential bull gear  78  comprises an outer ring gear part  90  and an inner support part  91 . The ring gear part  90  has an outer peripheral gear to mesh with the pinion  77 , and also has an inner peripheral gear. The support part  91  has an outer peripheral gear, which meshes with the inner peripheral gear of the ring gear part  90 , so that the ring gear part  90  and the support part  91  are not-relatively rotatably but they are relatively axially slidably joined with each other. In other words, the ring gear part  90  and the support part  91  are relatively axially slidable so as to be separated from each other. 
   The axial center hole  91   a  into which the proximal ends of the axles  35  are inserted is formed in the support part  91 . The support part  91  also has at least one hole  91   b  in which the pinion shaft  92  is supported and the pinion  93  on the shaft  92  is allowed to rotate. 
   The support part  91  of the bull gear  78  with two pinions  93  as a first embodiment shown in  FIG. 4  is provided with two holes  91   b  for the respective pinions  93  and shafts  92 . The two holes  91   b , and the shafts  92  with pinions  93  therein are symmetrical with respect to the axial center hole  91   a  (the axles  35 ). The support part  91  of the bull gear  78  with three pinions  93  as a second embodiment shown in  FIG. 5  is provided with three holes  91   b  for the respective pinions  93  and shafts  92 . The three holes  91   b , and the shafts  92  with the pinions  93  therein are arranged regular-triangularly around the axial center hole  91   a  (the axles  35 ). 
   As shown in  FIGS. 4 and 5 , vacant holes  91   c  for weight-reduction may be suitably provided in the support part  91  in addition to the axial center hole  91   a  and the holes  91   b.    
   As shown in  FIG. 2 , on the counter shaft  79  are axially-immovably fitted right and left radial retaining rings  94  through the pinion  77  so as to abut against right and left side surfaces of the ring gear part  90  and the support part  91 , respectively. The right and left side surfaces of the ring gear part  90  are leveled with the right and left side surfaces of the support part  91 , respectively, so as to surely abut against the respective retaining rings  94 . Therefore, the right and left retaining rings  94  sandwich the bull gear  78  while preventing the ring gear part  90  and the support part  91  from relatively axially shifting. 
   The ring gear part  90  is made from high-strength metal material, such as forged steel or high-density sintered powder metal. The support part  91  may be made from economical material, such as plastics, die-cast metal, or low-density sintered powder metal, having lower strength than the ring gear part  90 . Accordingly, the differential bull gear  78  may be reduced in cost while keeping the required strength for its teeth to perform as an input gear receiving torque from the reduction gear assembly  81 . Further, the ring gear part  90  and the support part  91  are joined together through their meshing teeth so as to ensure sufficient transmission of torque therebetween. 
   A third embodiment of the differential bull gear  78  shown in  FIGS. 6 and 7  will be explained. While the inner support part  91  with the pinions  93  is identical or similar to that of  FIG. 4  or  5 , a reshaped outer ring gear part  90   a  is axially longer than the inner support part  91 . The ring gear part  90   a  has no outer peripheral gear but an inner peripheral gear, which meshes with the outer peripheral gear of the support part  91  and a pinion  77   a , as shown in  FIG. 7 . A tip of a pinion shaft on which the pinion  77   a  is provided (integrally formed) is disposed in parallel to the axles  35  and laterally sidewise from the support part  91  so that the pinion  77   a  meshes with the inner peripheral gear of the ring gear part  90   a  so as to transmit output torque of the hydraulic motor  36  to the ring gear part  90   a . The transaxle  1  needs to be modified corresponding to this side-by-side arrangement of the differential bull gear  78  and pinion  77   a . The suitably modified transaxle  1  may have a reduced distance between the motor shaft  74  and the differential bull gear  78  so its dimensions perpendicular to axles  35  may be minimized. 
   A pair of left and right retaining plates  94   a  are fixed to an inner wall of the housing  33  or the like and contact the right and left surfaces of the outer ring gear part  90 . The inner support part  91  is axially restricted when it is supported on the axles  35 , and the left and right retaining plates  94   a  prevent the outer ring gear part  90  from axially moving relative to the inner support part  91 . 
   A fourth embodiment of the differential bull gear  78  shown in  FIGS. 8 and 9  will be explained. The ring gear part  90  has an outer peripheral gear to mesh with the pinion  77 , and an inner peripheral gear meshing with an outer peripheral gear of the support part  91 . As shown in  FIG. 8 , the ring gear part  90  is divisible into a plurality of arcuate pieces (in this embodiment, four equal pieces)  90   b  in its peripheral direction. The dividable ring gear part  90 , while being made of expensive high-strength material, is advantageous in maintenance and cost-saving because only the damaged piece  90   b  can be exchanged or repaired. The support part  91  supports the pinions  93  and is supported on the axles  35  similar to the above description. 
   As shown in  FIG. 9 , from the right surface of the support part  91  is extended a retaining portion  91   d  along the right surface of the ring gear part  90 , thereby preventing the ring gear part  90  (i.e., all of the pieces  90   b ) from axial rightward movement relative to the support part  91 . The retaining portion  91   d  is toothed to mesh with the right end of the outer peripheral gear of the ring gear part  90 , thereby further surely retaining the ring gear part  90 . 
   Screws  95  with washers  96  are screwed into the support part  91  so that heads of the screws  95  and the washers  96  are disposed on the left surface of the support part  91 . The screws  95  with the washers  96  are as many as the divisional pieces  90   b  so that each of the washers  96  contacting the left surface of the support part  91  also contacts the left surface of each piece  91   b  so as to prevent each piece  91   b  from axial leftward movement. 
   Accordingly, the washers  96  and the retaining portion  91   d  of the support part  91  prevent all of the pieces  90   b  of the ring gear part  90  from axial movement relative to the support part  91 . The cross-section of the retaining portion  91   d  of the support part  91  and the screws  95  with the washers  96  in this embodiment are shown for convenience. It is only important that the washer  96  and the retaining portion  91   d  are axially opposed with the bull gear  78  therebetween. 
   A fifth embodiment of the different bull gear  78  shown in  FIGS. 10 and 11  will be explained. The support part  91  in this embodiment is radially extended so that the major radius (the distance from the axis of the axles  35 ) of the support part  91  is longer than the major radius of the ring gear part  90 . The ring gear part  90  is divisible into a plurality of pieces  90   c , similar to that of the fourth embodiment shown in  FIGS. 8 and 9 . The support part  91  has a main portion supporting the pinions  93  and supported on the axles  35 . The support part  91  is provided on one side surface thereof with an annular recess  91   e  surrounding the main portion. The ring gear part  90  is (the pieces  90   c  are) inserted at one axial end thereof into the annular recess  91   e . The support part  91  has an outer peripheral portion  91   f  axially extended so as to entirely cover the outer peripheral surface of the ring gear part  90 , as shown in  FIG. 11 . The axial end surface of the ring gear part  90  (the pieces  90   c ) located opposite to the recess  91   e  is leveled with the extended axial end surface of the outer peripheral portion  91   f.    
   In the support part  91 , a step between the main portion and the recess  91   e  and a step between the recess  91   e  and the outer peripheral portion  91   f  are toothed to mesh with inner and outer peripheral gears of the ring gear part  90  (pieces  90   c ), respectively. The axial end side surface of the support part  91  in the recess  91   e  restricts further axial movement of the ring gear part  90  toward the support part  91 . The inner peripheral gear teeth of the ring gear part  90  (pieces  90   c ) also mesh with the pinion  77   a  similar to that of the third embodiment shown in  FIG. 7 . 
   Screws  95  with washers  96  are axially screwed into the outer peripheral portion  91   f  of the support part  91 . The screws  95  with the washers  96  are as many as the divisional pieces  90   c  so that each of the washers  96  contacts both the leveled axial end surfaces of the outer peripheral portion  91   f  and each piece  90   c  so as to prevent each piece  90   c  from axial movement apart from the support part  91 . 
   Accordingly, the support part  91  and the washers  96  prevent all of the pieces  90   c  of the ring gear part  90  from axial movement relative to the support part  91 . 
   A sixth embodiment of the differential bull gear  78  shown in  FIG. 12  will be explained. This differential bull gear  78  provided with two pinions  93  is similar to that of the first embodiment except for the reshaped support part  91 . The support part  91 , which is not a circular disc, consists of only the minimum necessary portions for being supported on the axles  35 , supporting the pinions  93 , and being joined with the ring gear part  90 , thereby being lightweight and saving costs. 
   Opposite toothed narrow edges  91   g  of the support part  91 , which are symmetric with respect to the axial center hole  91   a , mesh with the inner peripheral gear of the ring gear part  90 . The center portion of the support part  91  having the axial center hole  91   a  is extremely narrowed. The portion between the axial center hole  91   a  and each toothed edge  91   g  is considerably expanded so as to have the hole  91   b  for supporting the pinion  93 . Corresponding to the number or arrangement of the pinions  93  or for another reason, the edges  91   g  may be increased, decreased or changed in location. Preferably, the narrowed support part  91  is so elastic as to cause centrifugal biasing force toward the ring gear part  90  for keeping the circular shape of the ring gear part  90  when the edges  91   g  mesh with the inner peripheral gear of the ring gear part  90 . 
   Next, a seventh embodiment of the differential bull gear  78  shown in  FIGS. 13 and 14  will be explained. The illustrated ring gear part  90  is divisible into pieces  90   b  similar to that of  FIG. 8 . The support part  91  is axially divisible into right and left plates  91   h . The right and left plates  91   h  are fastened together by bolts  97  so as to constitute the support part  91 . The support part  91  supports the pinions  93  and is supported on the axles  35 , similar to the above description. 
   The support part  91  has an outer peripheral gear to mesh with the inner peripheral gear of the ring gear part  90 . Similar to the retaining portion  91   d  of the support part  91  shown in  FIG. 9 , each plate  91   h  has a retaining portion  91   i  extended from its axial outer end surface along the corresponding axial outer surface of the ring gear part  90 . The retaining portion  91   i  is toothed to mesh with the outer peripheral gear of the ring gear part  90 . By joining the plates  91   h  for making the support part  91 , the opposite retaining portions  91   i  of the plates  91   h  form an annular hole therebetween so as to incorporate the ring gear part  90 . The pinion  77  is inserted into a gap between outer peripheral surfaces of the retaining portions  91   i  of the joined plates  91   h  so as to mesh with the outer peripheral gear of the ring gear part  90 . 
   For coupling the ring gear part  90  and the support part  91 , the ring gear part  90  is engaged into a recess formed by the retaining portion  91   i  of one plate  91   h . Then, the other plate  91   h  is joined to the plate  91   h  engaging with the ring gear part  90 , whereby the two retaining portions  91   i  fittingly cover the opposite axial end surfaces of the ring gear part  90  so as to prevent the ring gear part  90  from axial movement. 
   Incidentally, the dividable ring gear part  90  according to each of the third to seventh embodiments may alternatively be a single undividable member. 
   The concept underlying the differential bull gear  78  constituted by the joined ring gear part  90  and the support part  91  is to make a member from inexpensive material while keeping high strength in its frictional (contacting) portion. The same concept is applicable for making HST members having sliding portions, such as a center section, pistons and a movable swash plate. In this regard, as for the HST  25 , the center section  38  shown in  FIG. 15  and the movable swash plate  45  shown in  FIG. 16  will be explained. 
   As mentioned above, the center section  38  has the surfaces  38   a  and  38   b , onto which the cylinder block  40  of the hydraulic motor  36  and the cylinder block  44  of the hydraulic pump  37  are slidably rotatably mounted. While the center section  38  is economically made from low-density sintered powder metal or the like, the surfaces  38   a  and  38   b  are coated with material having a low coefficient of friction and high abrasive resistance so as to enhance durability of the center section  38  and ensure smooth sliding of the cylinder blocks  40  and  44  on the surfaces  38   a  and  38   b . The center section  38 , if it is made from sintered powder metal, is so porous as to cause insufficient lubrication on the surfaces  38   a  and  38   b . By coating the surfaces  38   a  and  38   b , the pores opening on the surfaces  38   a  and  38   b  are filled up so as to ensure sufficient lubrication thereon. For example, ceramic material and PVD (physical vapor deposition) can be used as a material and method for coating the surfaces  38   a  and  38   b.    
   While the movable swash plate  45 , pistons  39  and  43 , and cylinder blocks  40  and  44  are made of economic material, the slide surface  45   a  of the movable swash plate  45  shown in  FIG. 16  may be coated with suitable material as mentioned above, and the pistons  39  and  43  and the cylinder blocks  40  and  44  may be coated on their surfaces similarly. 
   Next, some embodiments of the breather  101  will be explained with reference to  FIGS. 17 to 23 . A vertical hole  33   c  penetrates an upright portion  33   d  of the upper housing member  33   a  of the housing  33  just above the oil tank  100  shown in  FIG. 3 . Referring to a breather  101  shown in  FIG. 17 , an elastic valve housing  103  is inserted downward into the hole  33   c  so as to be elastically filling the hole  33   c . A cover  104  is placed on the top of the valve housing  103  upwardly projecting from the upright portion  33   d  and surrounding the upright portion  33   d  with a horizontal annular gap between the vertical outer side surface of the upright portion  33   d  and the bottom edge of the cover  104 . Therefore, the cover  104  covering the valve housing  103  and the upright portion  33   d  of the upper housing member  33   a  is open downward to the open air so as to prevent dust from entering the oil tank  100 . 
   The valve housing  103  is formed with a top flange portion  103   h . In the valve housing  103  is bored a downwardly open vertical air passage  103   a  in which a ball valve  106  is vertically movably disposed. The top flange portion  103   h  of the valve housing  103  is partly cut away so as to form a substantially horizontal and upwardly open air passage  103   b  between the cover  104  and the valve housing  103 . The air passage  103   b  is connected to the air passage  103   a  through an upper orifice  103   f , covered at its open top with the cover  104 , and open sideward toward the downwardly open space in the cover  104 . 
   The top of the cover  104  is so high apart from a top surface of the upright portion  33   d  so as to ensure an air passage in the cover  104  between the sideward opening of the air passage  103   b  and the downward opening of the cover  104 . A portion of the cover  104  opposite to the sideward opening of the air passage  103   b  is downwardly stepped so as to engage with the flange portion  103   h  of the valve housing  103  and abut against the top surface of the upright portion  33   d , thereby firmly engaging with the valve housing  103  and the upper housing member  33   a  and preventing air leakage. 
   The valve housing  103  and the cover  104  are made of elastic material, such as rubber or synthetic resin, so that the valve housing  103  is tightly fitted to the upper housing member  33   a  in the hole  33   c , and the cover  104  is tightly fitted to the valve housing  103  and the upper housing member  33   a , thereby sealing oil in the oil tank  100  and preventing air leakage. A vertically cylindrical reinforcing member  105  made of metal or the like is buried in the valve housing  103  surrounding the air passage  103   a  so as to compensate for softness of the valve housing  103 . The ball valve  106  may be made of either hard material such as steel or soft material such as rubber corresponding to different situations. The ball valve  106 , if made of soft material, may be further tightly fitted on each of the valve seats  103   c  and  103   d , so that, particularly when the upper housing member  33   a  is vertically reversed, the ball valve  106  fitted on the valve seat  103   c  surely shut out the orifice  103   f  from the valve chamber so as to enhance the certainty of prevention of oil leakage. 
   The vertically middle portion of the air passage  103   a  is diametrically larger than the upper and lower portions thereof, thereby forming an upper step  103   c  between the upper and middle portions, and a lower step  103   d  between the lower and middle portions. The vertically middle portion of the air passage  103   a  is provided as a valve chamber in which the ball valve  106  is vertically movably disposed. The upper and lower steps  103   c  and  103   d  serve as valve seats for the ball valve  106 . The lower step  103   d  is partly notched to form a lower orifice  103   e  opening into the housing  33 . 
   When the housing  33  is normally arranged in its vertical direction, the ball valve  106  abuts against the lower step  103   d  so that the valve chamber in the valve housing  103  communicates with the air above oil level of the oil tank  100  in the housing  33  through only the orifice  103   e , thereby breathing the air from the housing  33 . If the air pressure in the housing  33  excessively arises, the ball valve  106  is pushed up apart from the step  103   d  so as to increase the outward airflow. 
   If the housing  33  is vertically reversed for some reason, such as arranging parts into the housing  33 , the ball valve  106  falls to abut against the step  103   c  so as to completely block the valve chamber from the orifice  103   f  below the valve chamber, thereby preventing oil from leaking out from the housing  33 . The breather  101  prevents oil leakage in assembly facilities and during maintenance of the transaxle  1 . 
     FIGS. 18 to 23  illustrate various modified breathers  101 . Each of the modified breathers  101  has some common elements, i.e., the air passage  103   a  serving as a valve chamber, the valve  106  in the valve chamber, the upper and lower valve seats  103   c  and  103   d  with the valve  106  therebetween, and the upper and lower orifices  103   f  and  103   e  with the upper and lower valve seats  103   c  and  103   d  therebetween. The common important point ensured by all the modified breathers  101  is that, while under normal conditions, the lower orifice  103   e  is constantly open into the housing  33 , the upper orifice  103   f  is constantly open to the open air, and the valve  106  is fitted onto the lower valve seat  103   d  opening the orifice  103   e  to the valve chamber, but when housing  33  is flipped vertically, valve  106  becomes fitted to the valve seat  103   c  to completely shut the orifice  103   f  from the valve chamber thereby preventing oil in the housing  33  from leaking out through the breather  101 . 
   A breather  101  shown in  FIG. 18  has a plastic valve housing  103  having required rigidity and strength without the reinforcing member  105 . Similar to the valve housing  103  of  FIG. 17 , the valve housing  103  of  FIG. 18  is provided with the vertical air passage  103   a  opening into the housing  33 , the sideward air passage  103   b  opening to the open air, the orifice  103   f  connecting the air passages  103   a  and  103   b , the ball valve  106  in the air passage  103   a , and the top flange portion  103   h.    
   A cup-like elastic seal member  107  made of elastic material such as rubber or synthetic resin is tightly filled in the vertically cylindrical gap between the upright portion  33   d  and the valve housing  103  so as to seal the gap. The seal member  107  may be bonded to the valve housing  103  so as to be integrated with the valve housing  103 . The horizontal bottom portion of the seal member  107  is fitted to the bottom surface of the valve housing  103 . The center bottom portion of the seal member  107  is vertically bored throughout so as to form the lower step (lower valve seat)  103   d  and orifice  103   e  at the bottom opening of the air passage  103   a . The cover  104  made of elastic material is similar to that of  FIG. 17 , and tightly fitted to the valve housing  103  and the upper housing member  33   a , similarly. 
   A breather  101  of  FIG. 19  is modification of the breather  101  of  FIG. 18 , wherein the elastic cover  104  is further integrally formed with an expanding portion functioning similar to the seal member  107  shown in  FIG. 18 . That is, the expanding portion of the cover  104  is vertically cylindrical so as to be tightly filled in the vertical gap between the valve housing  103  and the upright portion  33   d . A bottom edge of the expanding portion of the cover  104  is partly extended horizontally so as to prevent the ball valve  106  from falling out from the valve housing  103 . This extended bottom edge of the cover  104  also serves as the lower valve seat  103   d . The valve chamber of the air passage  103   a  is widely open downward into the housing  33 , however, the ball valve  106  is normally placed on the valve seat  103   d  while ensuring a small gap between the ball valve  106  and the open bottom peripheral edge of the air passage  103   a . This small gap serves as the lower orifice  103   e.    
   Each of breathers  101  shown in  FIGS. 20 to 23  uses elastic valve housings  103  in which the reinforcing member  105  is buried, similar to that of  FIG. 17 . However, instead of the elastic cover  14 , each of the elastic valve housings  103  is formed with a flange portion  103   i  horizontally extended from its upper portion above the upright portion  33   d . An outer peripheral end of the flange portion  103   i  is extended downward so as to elastically clamp the upright portion  103   d  with a main central body of the valve housing  103  in the hole  33   c , whereby the valve housing  103  firmly engages with the upper housing member  33   a.    
   The valve housing  103  used for both the breathers  101  shown in  FIGS. 20 and 21  is downwardly recessed at its top. A plastic lid  108  is filled in the top recess of the valve housing  103 . A top portion of the lid  108  shown in  FIG. 20  is centrifugally horizontally extended over the top peripheral edge of the valve housing  103  surrounding the top recess and extended downwardly at its outer peripheral end so as to clamp the top peripheral edge of the valve housing  103  with the main body of the lid  108  filled in the top recess of the valve housing  103 . Alternatively, the lid  108  shown in  FIG. 21 , fully fitted in the valve housing  103 , is leveled at its top surface with the top surface of the valve housing  103 . 
   Each of the valve housings  103  of  FIGS. 20 and 21  is formed therein with the vertical air passage  103   a  with the upper and lower orifices  103   f  and  103   e , similar to the foregoing embodiments. The valve housing  103  is further formed with a groove  103   g  extended outward from the upper orifice  103   f . A horizontal part of the groove  103   g  directly connected to the orifice  103   f  is open upward to the top recess of the valve housing  103  and covered at the top opening with the lid  108  filled in the recess. The groove  103   g  is downwardly stepped from the upwardly open horizontal part thereof and downwardly open at the inner surface of the bent flange portion  103   i  of the valve housing  103 . A downwardly outward opening of the groove  103   g  is disposed at the bottom edge of the bent flange portion  103   i  along the outer peripheral surface of the upright portion  33   d.    
   Each of the breathers  101  of  FIGS. 22 and 23  is provided with an upwardly exposed valve housing  103  without a covering member such as the cover  104  or the lid  108 . The valve housing  103  is formed with a downwardly outward open vertical groove  103   j  on the inner surface of the vertical peripheral edge of the flange portion  103   i  along the upright portion  33   d . The vertical groove  103   j  is inwardly open to a space between the horizontal bottom surface of the flange portion  103   i  and the top surface of the upright portion  33   d.    
   The valve housing  103  shown in  FIG. 22  is bored therein with a slanting sideward air passage  103   k  connected to the top end of the vertical air passage  103   a  through the upper orifice  103   f . The air passage  103   k  is open to the space between the flange portion  103   i  and the upright portion  33   d , thereby communicating with the outwardly open vertical groove  103   j.    
   The valve housing  103  shown in  FIG. 23  is bored therein with a horizontal air passage  103   m  directly connected to the top end of the vertical air passage  103   a . A plug  110  plugs an outward opening of the horizontal air passage  103   m . The valve housing  103  is bored therein with the upper orifice  103   f , which is vertically downwardly extended from an intermediate portion of the horizontal air passage  103   m  and open at its bottom end to the space between the flange portion  103   i  and the upright portion  33   d  so as to communicate with the outwardly open vertical groove  103   j.    
   While the invention has been described in its preferred embodiments, it is to be understood that the words which have been used are words of description rather than limitation and that changes within the purview of the appended claims may be made without departing from the true scope and spirit of the invention in its broader aspects.