Abstract:
A variable displacement oil hydraulic motor designed to have its axial size minimized is disclosed, which comprises: a cylinder block receiving a plurality of pistons in pressure contact with a slanting surface of a swash plate variable in its angle of inclination, a motor housing accommodating the cylinder block therein; and at least one swash plate inclination changing piston which is disposed at a given circumferential position in an annular space defined between an outer periphery of the cylinder block and an inner periphery of the motor housing and which is movable bidirectionally in a direction parallel to the abovementioned pistons to alter the angle of inclination of the swash plate.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a variable displacement oil hydraulic motor of axial piston type in which the rotational speed of its output shaft is varied by varying the angle of inclination of its swash plate to vary its piston stroke. The invention also relates to a crawler vehicle drive speed reducer using such a variable displacement oil hydraulic motor.  
           [0003]    2. Description of the Prior Art  
           [0004]    The prior art for a variable displacement oil hydraulic motor of this type and a crawler vehicle drive speed reducer using the same is exemplified by one shown in FIG. 7. As shown in the Figure, the variable displacement oil hydraulic motor indicated by reference character  1  has its output shaft  3  to which a reducer  2  is coupled. It is shown that the variable displacement oil hydraulic motor  1  and the reducer  2  together make up a crawler vehicle drive speed reducer. The reducer  2  has an out member  4  rotatably fitted over the outer periphery of a motor housing  6  of the variable displacement oil hydraulic motor  1  and having a driving wheel  5  of the crawler travel device fastened thereto.  
           [0005]    The variable displacement oil hydraulic motor  1  also includes a cover member  7  that closes an opening end of the motor housing  6 . The output shaft  3  is rotatably supported between an end wall  16  of the motor housing  6  and the cover member  7 . A cylinder block  8  is fitted over the output shaft  3  rotatably therewith in the motor housing  6 . A plurality of cylinders  9  are arranged in the cylinder block  8  about its axis, and a like plurality of pistons  10  are slidably fetted through these cylinders  9 , respectively. A swash plate  11  has a slanting surface contacted by the leading ends of these pistons  10  for controlling the piston stroke for the variable displacement oil hydraulic motor  1 . A pressure oil supply circuit  12  is included in the cover member  7  for supplying each of the cylinders  9  with pressure oil. A parking braking unit  13  is provided in a space between the inner peripheral surface of the motor housing  6  and the outer peripheral surface of the cylinder block  8 .  
           [0006]    The swash plate  11  is supported by a trunnion  14  so as to be inclined in a plane containing the axis of the output shaft  3  and has its angle of inclination variable from small to large. A swash plate inclination changing piston  15  for changing the angle of inclination of the swash plate  11  is positioned within the end wall  16  of the motor housing  6 .  
           [0007]    The output shaft  3  is supported at its one end by the cover member  7  and at its other end by the end wall  16  of the motor housing  6 , via bearings  17  and  18 , respectively. Further, because of different types of oil used as the operating oil for the variable displacement oil hydraulic motor  1  and the lubricating oil for the reducer  2 , the output shaft  3  and the end wall  16  of the motor housing  6  are isolated from each other by an oil seal  18  so that these different types of oil may not be mixed together.  
           [0008]    The reducer  2  is of a planetary gear double speed reduction mechanism of ring gear output type whereby the rotation of the output shaft  3  of the oil hydraulic motor  1  is reduced in speed and then transmitted to the output member  4  supported by the motor housing  6  via bearings  27 . The reducer  2  includes a sun gear shaft  19  having its one end in splined mesh with the output shaft  3  of the variable displacement oil hydraulic motor  1  and its other end supported by the output member  4 . The sun gear shaft  19  has at its leading end a first sun gear  20  in mesh with a first planetary gear  21  which in turn is meshed with a ring gear  22  formed on an inner periphery of the output member  4 . Supporting the first planetary gear  21  is a gear holder  23  having a ring gear  23   a  formed on an inner periphery thereof. The ring gear  23   a  of the gear holder  23  is in mesh with a second sun gear  24  through which a mid region of the sun gear shaft  19  freely passes. A second planetary gear  25  is meshed with both the second sun gear  24  and the ring gear  22  of the output member  4  and is supported by a second planetary gear shaft  26 . The gear shaft  26  is in turn supported by a carrier  28  which is in splined mesh with an outer periphery of the end wall  16  of the motor housing  6  and is thereby coupled to the latter.  
           [0009]    In such an arrangement, feeding the pressure oil supply circuit  12  with pressure oil causes the cylinder block  8  and the output shaft  3  to rotate, and then a resulting rotation of the output shaft  3  is transmitted to the input shaft (sun gear shaft)  19  of the reducer  2 , reduced by the reducer  2  and then transmitted to the output member  4 , thereby rotating the drive wheels  5 .  
           [0010]    In that arrangement, causing the swash plate inclination changing piston  15  under oil pressure to protrude changes the angle of inclination of the swash plate  11  so it becomes smaller. Then, the piston  10  stroke is reduced, and the speed of rotation of the variable displacement oil hydraulic motor  1  is changed so it becomes quicker.  
           [0011]    In such a variable displacement oil hydraulic motor or a crawler vehicle drive speed reducer using the same, an apparatus to be driven thereby imposes the requirement that its axial size be made as small as possible.  
           [0012]    In such a variable displacement oil hydraulic motor or a crawler vehicle drive speed reducer in the prior art, however, it has been the common practice that the swash plate inclination changing piston  15  for changing the angle of inclination of the swash plate  11  is received in the end wall  16  of the motor housing  6 , namely disposed in a region of the axial extension of the variable displacement oil hydraulic motor  1 . As a result, the end wall  16  of the motor housing  6  must be made large (thick) in size of axial direction and so must be the oil hydraulic motor  1  at least by that extent.  
           [0013]    JP 7-76548 B discloses a variable displacement oil hydraulic motor in which the swash plate inclination changing piston for changing the angle of inclination of the swash plate is disposed internally of the motor housing. In this known variable displacement oil hydraulic motor, the swash plate inclination changing piston is an annular member disposed between an inner peripheral surface of the motor housing and an outer peripheral surface of the cylinder block so as to encircle the cylinder block. This arrangement presents the problem that the swash plate inclination changing piston itself must be made large and calls for a large space for accommodating the same.  
           [0014]    Also in the prior art shown in and described in connection with FIG. 7, the carrier  28  supporting the second planetary gear shaft  26  in the planetary gear double speed reduction mechanism is coupled to a outer periphery of the end of the motor housing  6 , posing the requiring for a considerable length of coupling which becomes one significant factor of lengthening the axial size of the conventional variable displacement oil hydraulic motor.  
           [0015]    Furthermore, the use of a bearing  27  of angular type in supporting the output member  4  by means of the motor housing  6  requires two such bearings facing each other in an axial direction of the motor housing  6 . This requirement further makes it inevitable that the variable displacement oil hydraulic motor must become large in axial size and even in diametric size, too.  
         BRIEF SUMMARY OF THE INVENTION  
         [0016]    With the view to overcoming the problems in the prior art mentioned above, the present invention has an object aimed at providing improvements in the variable displacement oil hydraulic motor and the crawler vehicle drive speed reducer using the same, which having their internal structures improved are reduced to minimum in axial size.  
           [0017]    In order to achieve the object mentioned above there is provided in accordance with the present invention in a primary feature thereof a variable displacement oil hydraulic motor, which comprises: a cylinder block receiving a plurality of pistons in pressure contact with a slanting surface of a swash plate variable in its angle of inclination; a motor housing accommodating the cylinder block therein; and at least one swash plate inclination changing piston which is disposed at a given circumferential position (at a given position on a circle) in an annular space defined between an outer periphery of the cylinder block and an inner periphery of the motor housing and which is movable bidirectionally in a direction parallel to the pistons to alter the in a direction parallel to the pistons to alter the angle of inclination of the swash plate.  
           [0018]    A variable displacement oil hydraulic motor according to this feature of the present invention makes it unnecessary for any swash plate inclination changing piston for changing the angle of inclination of the swash plate to be provided in an end wall of the motor housing, thereby making it possible to make the end wall of the motor housing thinner and thus to make the variable displacement oil hydraulic motor shorter in its axial length by that extent.  
           [0019]    And, the simple need for an aforementioned swash plate inclination changing piston supposed to be disposed only at a given circumferential position in an annular space formed between an inner periphery of the motor housing and an outer periphery of the cylinder block is sufficient to permit making the swash plate inclination changing piston itself smaller in size.  
           [0020]    According to a second, more specific feature of the present invention, in the variable displacement oil hydraulic motor of the first form of implementation a cylinder having the swash plate inclination changing piston received therein is mounted at the given circumferential position that is displaced radially inwards from an inner peripheral surface of the motor housing into the annular space. This feature allows the swash plate inclination changing piston and the cylinder in which the piston is received to be placed in a narrow space.  
           [0021]    According to a third feature of the present invention, there is provided a crawler vehicle drive speed reducer using a variable displacement oil hydraulic motor, which comprises the variable displacement oil hydraulic motor according to the first or second form of implementation of the invention, a planetary gear type speed reduction mechanism to which the motor housing and a motor output shaft are coupled combinatorially, and a drive wheel rotatably supported by an outer periphery of the motor housing and adapted to be rotationally driven by the variable displacement oil hydraulic motor with the intermediary of the planetary gear type speed reduction mechanism. This feature that requires the axially shortened variable displacement oil hydraulic motor to be combinatorially coupled to a planetary gear type speed reduction mechanism to form a crawler vehicle drive speed reducer allows the latter to be made smaller in axial length than the conventional one and thus to be made to stay well within the narrow width of the crawler, which has hitherto been hard to achieve.  
           [0022]    According to a fourth feature of the present invention, the planetary gear type speed reduction mechanism according to the third feature of the invention is a planetary gear double speed reduction mechanism of ring gear output type. And, according to the fifth feature of the present invention, the planetary gear type speed reduction mechanism according to the third feature of the invention is a planetary gear double speed reduction mechanism of carrier output type. These features which require the planetary gear type speed reduction mechanism to be a planetary gear double speed reduction mechanism allow achieving an increased reduction ratio.  
           [0023]    According to a sixth feature of the present invention, there is provided a crawler vehicle drive speed reducer using a variable displacement oil hydraulic motor according to the fourth or fifth feature of the invention, which further comprises a roller bearing for supporting the drive wheel rotatably by the outer periphery of the motor housing, wherein the roller bearing is a cross roller bearing. This feature which in contrast to an angular type bearing requires no more than one bearing allows the bearing assembly for supporting an output member of the reducer to be made shorter in axial length and makes it possible to shorten the axial length of the reducer by that extent.  
           [0024]    According to a seventh feature of the present invention, in the crawler vehicle drive speed reducer using a variable displacement oil hydraulic motor according to the fourth feature of the invention the planetary gear type double speed reduction mechanism of gear output type includes a gear shaft for supporting a planetary gear rotatably, wherein the gear shaft is an axial extension of the end wall of the motor housing and formed monolithically therewith and of a same material of which it is formed. This feature that requires the planetary gear shaft to be formed of a material of which the motor housing is formed makes the number of component parts smaller than the conventional makeup in which the planetary gear shaft is formed separately of the motor housing while shortening the axial length of the crawler vehicle drive speed reducer.  
           [0025]    According to an eighth feature of the present invention, in the crawler vehicle drive speed reducer using a variable displacement oil hydraulic motor according to the fourth feature of the invention a sun gear in the planetary gear double speed reduction mechanism of ring gear output type is formed monolithically with the output shaft of the variable displacement oil hydraulic motor. This feature which requires the output shaft in the motor and the sun gear shaft in the reducer to be made of a single member reduces the number of component parts by that extent and allows simplifying the support structure in a region of the sun gear in the reducer.  
           [0026]    According to a ninth feature of the present invention, there is provided a crawler vehicle drive speed reducer which comprises in combination: a variable displacement oil hydraulic motor having a swash plate inclination changing piston according to the first or second feature and a motor housing according to the third feature of the invention; a planetary gear double speed reduction mechanism of ring gear output type according to the fourth feature of the invention; and a drive wheel held by means of a cross roller bearing according to the sixth feature of the invention. Also, according to a tenth feature of the present invention there is provided a crawler vehicle drive speed reducer which comprises in combination: a variable displacement oil hydraulic motor having a swash plate inclination changing piston according to the first or second feature and a motor housing according to the third feature of the invention; a planetary gear double speed reduction mechanism of ring gear output type according to the fourth feature of the invention; a shaft made monolithic according to the eighth feature of the invention; and a drive wheel held by means of a cross roller bearing according to the sixth feature of the invention. Further, according to an eleventh feature of the present invention there is provided a crawler vehicle drive speed reducer which comprises in combination: a variable displacement oil hydraulic motor having a swash plate inclination changing piston according to the first or second feature of the invention; a planetary gear double speed reduction mechanism of carrier output type according to the fifth feature of the invention; and a drive wheel held by means of a cross roller bearing according to the sixth feature of the invention. These features allow providing a crawler vehicle drive speed reducer with its axial size minimized. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]    These and other objects, features and advantages of the present invention and other manners of its implementation will be more readily apparent, and the invention itself will also be better understood, from the following detailed description when taken with reference to the drawings attached hereto showing certain illustrative forms of implementation of the present invention. In the drawings:  
         [0028]    [0028]FIG. 1 is a cross sectional view illustrating a structure according to a first form of implementation of the present invention;  
         [0029]    [0029]FIG. 2 is a cross sectional view of the structure of FIG. 1 taken along the line II-II;  
         [0030]    [0030]FIG. 3 is a cross sectional view illustrating a structure according to a second form of implementation of the present invention;  
         [0031]    [0031]FIG. 4 is a cross sectional view illustrating a structure according to a third form of implementation of the present invention;  
         [0032]    [0032]FIG. 5 is a cross sectional view illustrating a structure according to a fourth form of implementation of the present invention;  
         [0033]    [0033]FIG. 6 is a cross sectional view illustrating a structure according to a fifth form of implementation of the present invention; and  
         [0034]    [0034]FIG. 7 is a cross sectional view of the conventional structure for variable displacement oil hydraulic motors.  
     
    
     DETAILED DESCRIPTION  
       [0035]    The present invention as regards its first to fifth forms of implementation will be described hereinafter with reference to FIGS.  1  to  6 . In this connection, it should be noted that like reference characters used in FIG. 7 to designate components in the conventional variable displacement oil hydraulic motor  1  and the conventional crawler vehicle drive speed reducer using the same are used to designate like components in these forms of implementation. For each instance, therefore, the following discussion will omit their repeated descriptions and will be focused on differences in construction.  
         [0036]    Referring now to FIGS. 1 and 2 which show the fist form of implementation of the invention, a variable displacement oil hydraulic motor  31  provides a motor housing  32  with a cylinder  33  that is disposed at a place in a circumferential direction of an annular space formed between an inner periphery of the motor housing  32  and an outer periphery of the cylinder block  8 . The cylinder  33  is open facing an abutting face  11   a  formed as a radial extension of the swash plate  11  on its peripheral edge portion. A swash plate inclination changing piston  34  is slidably received by the cylinder  33  and is movable to bring its leading end into contact with the abutting face  11   a  of the swash plate  11 . With the cylinder  33  mounted to extend parallel to the cylinders  9  in the cylinder block  8 , the swash plate inclination changing piston  34  is movable in the cylinder  33  in a direction parallel to the pistons  10  in those cylinders  9 . The innermost recess of the cylinder  33  has an oil hydraulic circuit  35  connected thereto from a pressure oil supply source via an ON/OFF switching valve (neither shown). Turning the switching valve ON to supply the oil hydraulic circuit  35  with pressure oil causes the swash plate inclination changing piston  34  to move forward to push the radial extension  11   a  of the swash plate  11 , thereby causing the latter to swing in a direction in which the angle of its inclination becomes smaller. A spring  26  is interposed between the swash plate inclination changing piston  34  and the innermost area of the cylinder  33  to bias the swash plate inclination changing piston  34  always against the swash plate  11 , thereby restraining the piston  34  from oscillating. Turning the switching valve OFF to stop supply of pressure oil into the oil hydraulic circuit  35  causes the swash plate  11  to swing back in a direction in which its angle of inclination becomes larger.  
         [0037]    The cylinder  33  in which the swash plate inclination changing piston  34  is received is mounted as shown in FIG. 2 to protrude cylindrically from the inner peripheral surface of the motor housing  32  radially inward to an extent at which it still does not interfere with the cylinder block  8 .  
         [0038]    Also in this form of implementation, an identical type of oil may be used as the operating oil for the variable displacement oil hydraulic motor  31  and the lubricating oil for a reducer  37 . This eliminates the need for the oil seal  18   a  required in the prior art on the end wall  16  of the motor housing  6 . To wit, the end wall  16  of the motor housing  32  here is designed to allow oil to flow from the variable displacement oil hydraulic motor  31  to the reducer  37  and vice versa.  
         [0039]    Further in this form of implementation, the reducer  37  makes use of a planetary gear double speed reduction mechanism substantially identical to that shown in FIG. 7 except that its second planetary gear shaft  38  is an axial extension of the end of the motor housing  32  and formed monolithically therewith of an identical material of which the motor housing  32  is formed.  
         [0040]    According to this form of implementation of the invention in which the swash plate inclination changing piston  34  for changing the angle of inclination of the swash plate  11  is provided in an annular space formed between an inner periphery of the motor housing  32  and an outer periphery of the cylinder block  8 , it is made no longer necessary for a space for accommodating such a swash plate inclination changing piston to be provided axially adjacent to the swash plate  11 . It thus becomes possible to make the end wall  16  of the motor housing  32  thinner and thus to shorten the axial size of the variable displacement oil hydraulic motor  31  to that extent.  
         [0041]    And, the simple need for an aforementioned swash plate inclination changing piston  34  to be disposed at a given circumferential position (at a given position on a circle) in an annular space formed between an inner periphery of the motor housing  32  and an outer periphery of the cylinder block  8  and at the position facing the abutting face  11   a  formed as a radial extension of the swash plate  11  is sufficient to permit making the swash plate inclination changing piston  34  itself smaller in size.  
         [0042]    It should be noted in this connection that more than one, namely two or three such pistons  34  may be employed insofar as they are positioned to face the abutting face  11   a  as an radial extension of the swash plate  11 .  
         [0043]    Further in this form of implementation of the invention, forming a second planetary gear shaft  38  monolithically with the motor housing  32  and of a material of which the latter is formed makes types of components smaller in number than when the second planetary gear shaft must be prepared separately from the motor housing as in the prior art.  
         [0044]    Referring next to FIG. 3 which shows a second form of implementation of the present invention, mention is made of how it differs from the prior art and the preceding first form of implementation of the invention.  
         [0045]    A variable displacement oil hydraulic motor  40  in this form of implementation has an output shaft  41  extending into the input side of a reducer  42  wherein a part corresponding to the sun gear shaft of the reducer  37 ,  2  shown in FIGS. 1 and 7, respectively, is formed monolithically with this output shaft  41 . The output shaft  41  has its one end supported by the cover member  7  of the variable displacement oil hydraulic motor  40  and its other end supported from the end wall of an output member  50 , via tapered roller bearings  44  and  43 , respectively, which are opposing to each other. The output shaft  41  passes through an end wall  46  of a motor housing  45  where it is supported by a bearing  47 . Thus, the end wall  46  can be made so thin that it can only accommodate the bearing  47  and indeed be much thinner than the corresponding end wall in the aforementioned prior art where the oil seal must be provided.  
         [0046]    Also in this form of implementation, use is made of a reducer  42  having a second planetary gear shaft  48  formed separately of the motor housing  45  and integrally fastened thereto by means of a bolt  49 .  
         [0047]    Further, the output member  50  of the reducer  42  is supported by the motor housing  45  via a cross roller bearing  51 .  
         [0048]    This form of implementation of the invention having the end wall  46  of the motor housing  45  that is only necessary to accommodate the bearing and unnecessary to provide the oil seal makes it sufficient for the end wall to have a wall thickness just enough to accommodate the bearing and hence makes the drive speed reducer using the variable displacement oil hydraulic motor smaller in axial size than when the oil seal must be provided as in the prior art.  
         [0049]    Also, this form of implementation of the invention having the output shaft  41  of the variable displacement oil hydraulic motor  40  that is made monolithic with the sun gear shaft in the reducer  42  makes types of parts used small in number to that extent while simplifying the supporting structure of a region of the sun gear shaft in the reducer  42 .  
         [0050]    Further, this form of implementation of the invention using a cross roller bearing  51  to support the output member  50  of ring gear output type in the reducer  42  by the motor housing  45  makes such a support region smaller in axial size than when the angular axial bearing  27  must be used as in the prior art shown in FIG. 7 and as in the first form of implementation shown in FIG. 1.  
         [0051]    Referring next to FIG. 4 which shows a third form of implementation of the present invention, a variable displacement oil hydraulic motor  52  has a cylinder block  53  supported in the inside of a motor housing  54  via a bearing  55 . Its output shaft  56  has one end coupled to the cylinder block  53  in a spline engagement therewith and the other end extending to form a sun gear shaft in a reducer  57  whose leading end is supported by an output member  58  in the reducer  57  via a bearing  59 . The output shaft  56  passes through an end wall  60  of the motor housing  54  where it is provided with an oil seal  61 . This oil seal  61  is dispensed with if a same type of oil is used as the operating oil for the variable hydraulic motor  52  and the lubricating oil for the reducer  57 .  
         [0052]    This form of implementation of the invention having the output shaft  56  at its one end connected to the cylinder block  53  supported by the inside of the motor housing  54  via the bearing  55  makes the output shaft itself shorter than when those must be supported by the cover member  7  as in the preceding forms of implementation of the invention while eliminating the need to provide a bearing where the output shaft  56  passes though the end wall  60 .  
         [0053]    In the third form of implementation of the invention, the second planetary gear shaft  38  is formed monolithically with the motor housing  54  and of a material of which the latter is formed, as in the first form of implementation of the invention. Also, the output member  58  of the reducer  57  is supported from the motor housing  54  via a cross roller bearing  51  as in the second form of implementation of the invention.  
         [0054]    The third form of implementation having the output shaft  56  whose one end is connected to the cylinder block  53  supported by the motor housing  54  via the bearing  55  makes it unnecessary to support the one end of the output shaft  56  of the variable displacement oil hydraulic motor  52  via a bearing by the cover member  7  of the variable displacement oil hydraulic motor  52  via a bearing and makes it unnecessary to support the output shaft  56  by the end wall  60  of the motor housing  54  and hence makes it possible to shorten to those extents the length of the output shaft  56  of the variable displacement oil hydraulic motor  52  serving also as the sun gear shaft in the reducer  57 , and further the axial length as well of the drive speed reducer using the variable displacement oil hydraulic motor  52 .  
         [0055]    Referring next to FIG. 5, there is shown a forth form of implementation of the present invention, which is substantially identical to the first form of implementation of the invention shown in FIG. 1, with the exception of the use of a swash plate  62  that is shaped differently and an output member  63  with a different support structure in a reducer  64 . The swash plate  62  has its rear surface formed with a recess in a region opposite the bearing  18 , providing for a structure such that a portion of the bearing  18  enters the recess. The structure thus shortens the axial length of the drive speed reducer using the variable displacement oil hydraulic motor, to an extent corresponding to the size of this entry.  
         [0056]    In this form of implementation, the reducer  64  adopts its support structure substantially identical to that in the third form of implementation shown in FIG. 3, wherein the output member  63  is supported from a motor housing  65  via the cross roller bearing  51 .  
         [0057]    This fourth form of implementation of the invention adopting a structure in which a variable displacement oil hydraulic motor  66  has its output shaft  3  whose other end is supported with the bearing  18  allows this bearing  18  to be located axially overlapping with the swash plate  61  and hence permits making a drive speed reducer using the variable displacement oil hydraulic valve  66  smaller in axial length by that extent.  
         [0058]    Referring next to FIG. 6, there is shown a fifth form of implementation of the present invention, in which a variable displacement oil hydraulic motor  67  that is substantially identical in makeup to that in the second form of implementation shown in FIG. 3 adopts an output shaft  68  having a reducer  69  coupled therewith which is of a planetary gear double speed reduction mechanism of carrier output type  
         [0059]    Coupled to the variable displacement oil hydraulic motor  67  having the output shaft  68 , this reducer  69  includes a first sun gear  70  provided at a leading end of the output shaft  68 , a second sun gear  71  supported coaxially with the first gun gear  70 , a first planetary gear  72  in mesh with the first sun gear  70 , a holder  73  supported by the second sun gear  71  in splined mesh therewith, a first planetary gear shaft  74  formed monolithically with the holder  73  for supporting the first planetary gear  72 , a holder  76  held in splined mesh with a motor housing  75  of the variable displacement oil hydraulic motor  67 , a ring gear  77  formed on an inner surface of the holder  76  and held in mesh with the first planetary gear  72 , a second planetary gear  78  held in mesh with both the second sun gear  71  and the ring gear  77 , and a second planetary gear shaft  79  monolithically formed with an output member  80  for supporting the second planetary gear  78 . A rotation of the output shaft  68  of the variable displacement oil hydraulic motor  67  is transmitted from the second planetary gear shaft  79  to the output member  80  via the reducer  69  using the planetary gear double speed reduction mechanism. Namely, any output whatsoever from the reducer  69  is designed to be output from the second planetary gear shaft (carrier)  79 .