Abstract:
The present invention relates to a driving unit comprising a multiple-stage planetary gear type reducer used to reduce rotation speed of a hydraulic motor and output the reduced rotation speed, which is used as a driving device for a traveling apparatus. The driving unit of the present invention is so structured that a trunnion boss for rotatably supporting a planetary gear train of a final state is supported at opposite ends thereof. This structure enables the load applied to the trunnion boss to be dispersed to the both ends, and as such can allow the trannion boss to be reduced in diameter or can allow the fixed casing to be reduced in circumferential dimension. This can produce a downsized driving unit. The present invention has additional features, such as the feature that an output shaft of the hydraulic motor and an input shaft of the reducer are formed in the form of a single rotating shaft. This can provide a driving unit structurally optimized for every principal part, to provide downsizing and improved durability.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Technical Field of the Invention  
           [0002]    The present invention relates to a driving unit used to reduce rotation speed of a hydraulic motor and output the reduced rotation speed, which is used as a driving device for a traveling apparatus.  
           [0003]    2. Description of the Prior Art  
           [0004]    A driving unit is used as a driving device of a construction machine traveled by a crawler as typified particularly by a driving device of a hydraulic shovel among construction machines. In the driving unit, a hydraulic motor is disposed in an interior of a fixed casing fixed to a vehicle body, so that the rotation as output is transmitted to a rotating casing concentrically fitted to the fixed casing to freely rotate through a planetary gear mechanism, so as to drive the crawler by means of a sprocket disposed around a periphery of the rotating casing. Because of the constraint that the driving unit is located in the interior of the crawler, there is a restriction on the entire inner configuration space, for the reason of which the driving unit is required to have small size and high power.  
           [0005]    The driving units of this type are known by publications such as Japanese Laid-open (Unexamined) Patent Publications No. Hei 4-140538, No. Hei 6-249297, Hei 8-247223 and No. Hei 9-240525.  
           [0006]    However, the driving units of this conventional type are all being demanded to be further downsized.  
           [0007]    It is the object of the present invention to provide a driving unit structurally optimized for every principal part, to provide downsizing and improved durability.  
         SUMMARY OF THE INVENTION  
         [0008]    In accordance with a 1st aspect of the invention, there is provided a driving unit comprising a fixed casing having a hydraulic motor therein; a rotating casing rotatably supported around a periphery of the fixed casing via a bearing inserted from one end portion of the fixed casing and having an internal gear around an inside thereof; a sun gear mounted on an output shaft projected from the hydraulic motor toward the one end portion of the fixed casing; a planetary gear train disposed between the sun gear and the internal gear to reduce speed in two or more stages; a trunnion boss, integrally projected from the one end portion of the fixed casing, for rotatably supporting the planetary gear train of a final stage engaging with the internal gear; a holder in which a front end portion of the trunnion boss is inserted; support pillars projecting from the holder toward the fixed casing; and fastening means for fixing the support pillars and the fixed casing.  
           [0009]    Known as a conventional driving unit is the one disclosed by Japanese Laid-open (Unexamined) Patent Publication No. Hei 4(1992)-140538, for example. A typical conventional driving unit  101  is shown in FIG. 25. The driving unit  101  has a cylindrical fixed casing  102  in which a hydraulic motor  103  is disposed. An output shaft  104   a  of the hydraulic motor  103  is coupled with an input shaft  104   b  via a spline coupling  117 , and a sun gear  105  is mounted on a front end portion of the input shaft  104   b.  A rotating casing  107  is rotatably supported around a periphery of the fixed casing  102  via a bearing  106 , and an internal gear  108  is formed around an inside of the rotating casing  107 . The rotation of the sun gear  105  is transmitted to the internal gear  108  through a planetary gear  109 , a second sun gear  111  engaged with a planetary gear frame  110  of the planetary gear  109 , and a second planetary gear  113  supported on a trunnion boss  112  projected from the front end portion of the fixed casing  102 , to rotate the rotating casing  107  at a reduced speed. A flange  114  of the fixed casing  102  is bolted to the body (not shown), and a flange  115  of the rotating casing  107  is bolted to a crawler sprocket (not shown).  
           [0010]    The driving torque of the hydraulic motor  103  fixedly mounted in the fixed casing  102  is reduced via a planetary gear train of the first stage comprising the sun gear  105  and the planetary gears  109  and a planetary gear train of the second stage comprising a second sun gear  111  and second planetary gears  113  and is transmitted to the rotation of the rotating casing  107 .  
           [0011]    However, since the trunnion boss  112  for rotatably supporting the second planetary gears  113  is projected from the end of the fixed casing  102  in a cantilever fashion, a bending stress is generated at the basal end of the trunnion boss  112  when a load is applied thereto through the second planetary gears  113 . For this reason, the trunnion boss  112  is required to have a large thickness. As a result of this, the bearing  106  and the floating seal  116  inserted from the trunnion boss  116  side of the fixed casing  102  are increased in size, which causes the rotating casing  107  to increase in size and in turn causes the entire driving unit  101  to increase in radial dimension.  
           [0012]    According to the construction of the 1st aspect of the invention, the trunnion boss is allowed to be supported at opposite ends thereof by the holder fixed to the fixed casing through the support pillars. This enables the load applied to the trunnion boss to be dispersed to the holder and the fixed casing, and as such can allow the trannion boss to be reduced in diameter or can allow the fixed casing to be reduced in circumferential dimension. This produces the result that the rotating casing supported around the periphery of the fixed casing by the bearing inserted thereon is also reduced in outer diameter. Also, the support pillars projected from the holder have thickness such that even when the holder body is small in thickness, the fastening means to be fixed to the fixed casing applies a sufficient fastening force at the support pillars. This enables the support of the planetary gear train for rotation, without any axial elongation and with good durability. Thus, the downsizing of the driving unit can be achieved and improved durability can also be provided.  
           [0013]    In accordance with a 2nd aspect of the invention, there is provided a driving unit according to 1st aspect of the invention, wherein the support pillars are in abutment with support pillars projected from the fixed casing at their abutment surfaces, which are located within a width of the planetary gear of the final stage.  
           [0014]    This construction enables the abutment surfaces to be away from the basal ends of the support pillars to which a maximum bending moment is applied, by projecting the support pillars from the fixed casing side as well.  
           [0015]    In accordance with a 3rd aspect of the invention, there is provided a driving unit according to 1st aspect of the invention, wherein the trunnion boss is projected along a periphery of the fixed casing and a rounded portion is formed at a basal end of the trunnion boss except an area close to the periphery of the fixed casing.  
           [0016]    According to this construction, since the direction of the load acting on the trunnion boss is a tangent direction to the fixed casing, the fixed casing can be reduced in circumferential diameter by forming no rounded portion for relaxing the bending stress at the basal end of the trunnion boss located around the periphery of the fixed casing.  
           [0017]    In accordance with a 4th aspect of the invention, there is provided a driving unit according to 1st aspect of the invention, wherein the abutment surfaces are located at an approximately widthwise center portion of the planetary gear of the final stage.  
           [0018]    This construction can allow the abutment surfaces to be located at an approximately axial center of the support pillar at which a bending moment is minimized. Also, this construction can ensure a dimension from underhead of the bolt used as the fastening means to the abutment surfaces.  
           [0019]    In accordance with a 5th aspect of the invention, there is provided a driving unit comprising a fixed casing having a hydraulic motor therein; a rotating casing rotatably supported around a periphery of the fixed casing via a bearing inserted from one end portion of the fixed casing and having an internal gear around an inside thereof; a sun gear mounted on an output shaft projected from the hydraulic motor toward the one end portion of the fixed casing; a planetary gear train disposed between the sun gear and the internal gear to reduce speed in two or more stages; a trunnion boss, supported at the one end portion of the fixed casing, for rotatably supporting the planetary gear train of a final stage engaging with the internal gear; a holder in which a front end portion of the trunnion boss is inserted; support pillars projecting from the holder toward the fixed casing; and fastening means for fastening the support pillars and the fixed casing, wherein the support pillars are in abutment with support pillars projected from the fixed casing at their abutment surfaces, which are located within a width of the planetary gear of the final stage.  
           [0020]    According to this construction, the trunnion boss is allowed to be supported at opposite ends thereof by the fixed casing and the holder fixed to the fixed casing through the support pillars. This enables the load applied to the trunnion boss to be dispersed to the holder and the fixed casing, and as such can allow the trannion boss to be reduced in diameter or can allow the fixed casing to be reduced in circumferential dimension. This produces the result that the rotating casing supported around the periphery of the fixed casing by the bearing inserted thereon is also reduced in outer diameter. Also, since the abutment surfaces at which the support pillars projected from the holder and the support pillars projected from the fixed casing are in abutment are within the width of the planetary gear of the final stage, the support pillars projected from the holder have thickness such that even when the holder body is small in thickness, the fastening means to be fixed to the fixed casing applies a sufficient fastening force at the support pillars. This enables the support of the planetary gear train for rotation, without any axial elongation and with good durability. Also, by projecting the support pillars from the fixed casing side as well, the abutment surfaces can be allowed to be away from the basal ends of the support pillars to which a maximum bending moment is applied. Thus, the downsizing and improved durability of the driving unit can be achieved.  
           [0021]    In accordance with a 6th aspect of the invention, there is provided a driving unit according to 5th aspect of the invention, wherein the abutment surfaces are located at an approximately widthwise center portion of the planetary gear of the final stage.  
           [0022]    This construction can allow the abutment surfaces to be located at an approximately axial center of the support pillar at which a bending stress is minimized. Also, this construction can ensure a dimension from underhead of the bolt used as the fastening means to the abutment surfaces.  
           [0023]    In accordance with a 7th aspect of the invention, there is provided a driving unit comprising a hydraulic motor and a planetary gear type of reducer to reduce an output of the hydraulic motor and transmit the reduced output to a driving portion, wherein an output shaft portion of the hydraulic motor and an input shaft portion of the reducer are integrally formed in the form of a single rotating shaft; wherein a sun gear of the reducer is put in spline engagement with a front end portion of the rotating shaft; and wherein the spline is so formed that a clearance therebetween can gradually broaden toward the end thereof.  
           [0024]    With the conventional type of driving unit  101 , since the output shaft  104   a  and the input shaft  104   b  are coupled with the coupling  117  using the spline engagement, the driving unit is increased in radial dimension as well as in axial length at that coupling part.  
           [0025]    In contrast to this, with the construction of the 7th aspect of the invention, since the rotating shaft can be used both as the input shaft and as the output shaft by projecting the output shaft of the hydraulic motor beyond the center of the reducer, no intermediate coupling is required, thus enabling the radial thickness of the rotating shaft to be optimized. The rotating shaft is journaled by two bearings in the hydraulic motor at two lengthwise locations thereof. When pressure is introduced into the cylinder block, the rotating shaft is subject to a bending load at its portion between the two bearings, so that the front end portion of the rotating shaft is inclined. However, since the spline cogs of the rotating shaft are formed in a crowning fashion or a like fashion so that it can gradually narrow toward the front end to produce a clearance gradually broadened toward the front end, so as to ensure the clearance corresponding to the inclination of the rotating shaft. This can allow the rotating shaft to surface-contact with the sun gear to transmit the running torque of the rotating shaft to the sun gear smoothly.  
           [0026]    In accordance with a 8th aspect of the invention, there is provided a driving unit according to 7th aspect of the invention, wherein spline grooves are formed around an inside of the sun gear so that they are each located at an approximately circumferential center between adjacent spaces between cogs formed around a periphery of the sun gear.  
           [0027]    According to this construction, since the spaces between the cogs of the sun gear and the spline grooves at the fitting portions between the sun gear and the rotating shaft are out of position from each other with respect to the circumferential direction, even when the sun gear is reduced in diameter, the wall thickness of the sun gear can be ensured.  
           [0028]    In accordance with a 9th aspect of the invention, there is provided a driving unit comprising a hydraulic motor and a planetary gear type of reducer to reduce an output of the hydraulic motor and transmit the reduced output to a driving portion, wherein an output shaft portion of the hydraulic motor and an input shaft portion of the reducer are integrally formed in the form of a single rotating shaft; wherein a sun gear of the reducer is mounted on a front end portion of the rotating shaft; and wherein at least one of a planetary gear engaging with the sun gear and the sun gear have cogs which are so formed that a clearance therebetween can gradually broaden toward the end thereof.  
           [0029]    According to this construction, since the rotating shaft can be used both as the input shaft and as the output shaft by projecting the output shaft of the hydraulic motor beyond the center of the reducer, no intermediate coupling is required, thus enabling the radial thickness of the rotating shaft to be optimized. Also, the sun gear and/or the planetary gears allow for the clearance corresponding to the inclination of the rotating shaft, so that the surface-contact between these gears is ensured.  
           [0030]    In accordance with a 10th aspect of the invention, there is provided a driving unit according to 8th aspect of the invention, wherein a distance between P and a tangent line touching one tooth flank of the sun gear at a point and extending perpendicularly to an axis of the sun gear is set at a value of not less than and asymptotic to 2         sin θ when the reducer is in an unloaded state: where          is a maximum radial variation of the sun gear caused by inclination of the rotating shaft; θ is an angle formed by the tangent line and a moving direction of the sun gear in such a positional relationship that when the rotation shaft is inclined, the one tooth flank of the sun gear which is on the opposite side to the other tooth flank of the sun gear which is put into engagement with the planetary gear comes nearest to a confronting tooth flake of the planetary gear; and P is a point on the tooth flank of the planetary gear closest to the sun gear.  
           [0031]    According to this construction, even when the rotating shaft is inclined, the sun gear and the planetary gears can be prevented from colliding with each other at a tooth flake on the opposite side to a tooth flake at which they are engaged with each other.  
           [0032]    In accordance with a 11th aspect of the invention, there is provided a driving unit comprising a hydraulic motor and a planetary gear type of reducer to reduce an output of the hydraulic motor and transmit the reduced output to a driving portion, the driving unit comprising a sun gear coupled with an output shaft portion of the hydraulic motor, planetary gears engaging with the sun gear, and an internal gear engaging with the planetary gears and formed around an inside of a rotating casing of the reducer, wherein a length of pass of contact of the internal gear is shortened so that an engaging area between the planetary gears and the sun gear can be equal in durable period to that between the internal gear and the sun gear.  
           [0033]    This construction can allow the internal gear to reduce in length, and as such can allow the rotating casing to be reduced in size. Thus, the internal gear and the casing can be reduced in weight and further the costs for hardening treatment of the internal gear can be cut.  
           [0034]    In accordance with a 12th aspect of the invention, there is provided a driving unit comprising a fixed casing having a hydraulic motor therein; a rotating casing rotatably supported around a periphery of the fixed casing via a bearing inserted from one end portion of the fixed casing and having an internal gear around an inside thereof; a sun gear mounted on an output shaft projected from the hydraulic motor toward the one end portion of the fixed casing; and a planetary gear train disposed between the sun gear and the internal gear to reduce speed, wherein at least one stage of the planetary gear train has two planetary gears symmetrically disposed about the output shaft and a planetary gear frame for rotatably supporting the two planetary gears at both axial ends thereof in sandwich relation, the planetary gear frame having a pair of flat plate portions for supporting the two planetary gears in sandwich relation and support pillars for connecting between the pair of flat plate portions, the support pillars being partially extended along a periphery of the flat plate portions and disposed near the planetary gears.  
           [0035]    In general, the driving unit having three planetary gears arranged in regular triangle, as disclosed by Japanese Laid-open (Unexamined) Patent Publication No. Hei 8(1996)-247223, for example, is in wise use, in term of the stable support configuration. At present, it can be said that it has reached a critical limit for the structure having the three planetary gears to further reduce parts count and downsizing of the components.  
           [0036]    The construction according to the 12th aspect of the invention can produce the driving unit with two planetary gears having a structural stability. Hence, as compared with the conventional type of driving unit having three planetary gears, parts count can be reduced to a large extent and also the structure can be simplified. Hence, the driving unit having an advantage in cost can be produced.  
           [0037]    In accordance with a 13th aspect of the invention, there is provided a driving unit according to 12th aspect of the invention, wherein the flat plate portions are formed into a generally ellipse-like shape.  
           [0038]    This construction enables the components of the driving unit comprising the two planetary gears to be further reduced in size and weight by forming the planetary gear frame into an ellipse-like shape.  
           [0039]    In accordance with a 14th aspect of the invention, there is provided a driving unit comprising a fixed casing having a hydraulic motor therein; a rotating casing rotatably supported around a periphery of the fixed casing via a bearing inserted from one end portion of the fixed casing and having an internal gear around an inside thereof; a sun gear mounted on an output shaft projected from the hydraulic motor toward the one end portion of the fixed casing; a planetary gear train disposed between the sun gear and the internal gear to reduce speed in two or more stages; a trunnion boss, disposed at the one end portion of the fixed casing, for rotatably supporting the planetary gear train of a final stage engaging with the internal gear; a holder in which a front end portion of the trunnion boss is inserted and which is mounted on the fixed casing; a nut threadedly engaged with the periphery of the fixed casing to position the bearing with respect to an axial direction of the fixed casing; and a key plate for locking the nut against rotation, wherein the key plate is fixed at a position corresponding to an end face of the fixed casing from which the trunnion boss is projected.  
           [0040]    In the driving unit, the bearing for rotatably supporting the rotating casing around the periphery of the fixed casing is generally positioned by the nut, to which a lock means is given. Known as this type of conventional driving unit is the one disclosed by Japanese Laid-open (Unexamined) Patent Publication No. Hei 6(1994)-249297, which is shown in FIG. 26. The driving unit  118  has a cylindrical fixed casing  119  in which a hydraulic motor  120  is disposed. A first sun gear  122  is mounted on a front end portion of an output shaft  121  of the hydraulic motor  120 . A rotating casing  124  is rotatably supported around a periphery of the fixed casing  119  via a bearing  123 , and an internal gear  125  is formed around an inside of the rotating casing  124 . The rotation of the first sun gear  122  is transmitted to the internal gear  125  through a first planetary gear  126 , a second sun gear  128  engaged with a planetary gear frame  127  of the first planetary gear  126 , a third sun gear  130  engaged with the planetary gear flame  129  of the second planetary gear  128 , and a third planetary gear  132  rotatably supported on a carrier  131  threadedly engaged with the fixed casing  119 , to rotate the rotating casing  124  at a reduced speed. A flange  133  of the fixed casing  119  is bolted to the body (not shown), and a flange  134  of the rotating casing  124  is bolted to a crawler sprocket  135 .  
           [0041]    The rotating casing  124  is rotatably supported to the fixed casing  119  via the bearing  123 , for which a conical roller bearing is used, and a preload is applied to the bearing  123  by screwing the nut  136  with an adequate torque. In order to keep the bearing  123  in the state in which the preload is applied thereto, the nut  136  must be locked against rotation. For this reason, the structure shown in FIGS.  27 ( a ),  27 ( b ) is adopted, wherein a key plate  137  having a key  137   a  to be fitted in a key slot  119   a  of the fixed casing  119  and the nut  136  are fixed by bolts  138 . A number of threaded holes  136   a  are formed in the side of the nut  136  at regular intervals so that the bolts  138  can be screwed in the related threaded holes  136   a  by only a slight turning of the nut  136  which is in an adequate fastened state.  
           [0042]    However, due to the structure that the key slot  119   a  is formed in the fixed casing  119  and, in addition to the nut  136 , the key plate  137  and bolt heads  138  are interposed between the bearing  123  and the carrier  131 , a distance d between the nut  136  and the end portion of the fixed casing  119  is disadvantageously elongated.  
           [0043]    With the construction according to the 14th aspect of the invention, since the planetary gear of the final stage and the key plate are so disposed as to be partially overlapped, the driving unit can be reduced in axial dimension to the extent corresponding to the overlapped portion.  
           [0044]    In accordance with a 15th aspect of the invention, there is provided a driving unit according to 14th aspect of the invention, wherein support pillars projected from the fixed casing and support pillars projected from the holder are fixed in abutment with each other, and the key plate is fixed to the end face of the fixed casing in the state of being partially engaged in a cutout portion of the support pillar on the fixed casing side.  
           [0045]    According to this construction, since the key plate is disposed in place by means of the cutout portions provided in the support pillars between the holder for supporting the front end portion of the trunnion boss and the fixed casing, it can be prevented from interfering with the planetary gears.  
           [0046]    In accordance with a 16th aspect of the invention, there is provided a driving unit according to 14th aspect of the invention, wherein the trunnion bosses are disposed along the periphery of the fixed casing.  
           [0047]    This construction enables the fixed casing to be reduced in circumferential dimension by extending the fixed casing along a circumscribed circle of the trunnion bosses. This enables the driving unit to be reduced in radial dimension as well as in axial dimension.  
           [0048]    In accordance with a 17th aspect of the invention, there is provided a driving unit comprising a fixed casing having a hydraulic motor therein; a rotating casing rotatably supported around a periphery of the fixed casing via a bearing inserted from one end portion of the fixed casing and having an internal gear around an inside thereof; a sun gear mounted on an output shaft projected from the hydraulic motor toward the one end portion of the fixed casing; a planetary gear train disposed between the sun gear and the internal gear to reduce speed in two or more stages; a trunnion boss, disposed at the one end portion of the fixed casing, for rotatably supporting the planetary gear train of a final stage engaging with the internal gear; a holder in which a front end portion of the trunnion boss is inserted and which is mounted on the fixed casing; and a nut threadedly engaged with the periphery of the fixed casing to position the bearing with respect to an axial direction of the fixed casing; and a pin, disposed between the nut and the holder, for locking the nut against rotation.  
           [0049]    According to this construction, since the nut and the holder are connected by the pin without any use of the key plate, the nut can be locked against rotation without any elongation of the axial dimension of the fixed casing.  
           [0050]    In accordance with a 18th aspect of the invention, there is provided a driving unit according to 17th aspect of the invention, wherein support pillars projected from the fixed casing and support pillars projected from the holder are fixed in abutment with each other, a projection projecting from the holder along a periphery of the support pillar, and the pin is disposed between the projection and the nut.  
           [0051]    According to this construction, at the same time when the holder is inserted toward the fixed casing so that the end faces of the support pillars at the front ends thereof are put into abutment with each other, the nut is locked against rotation by means of the pin. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0052]    [0052]FIG. 1 is a sectional view of a driving unit of a first embodiment of the present invention;  
         [0053]    [0053]FIG. 2 is a schematic diagram of a holder supporting structure to an end face of a fixed casing of the first embodiment of the present invention;  
         [0054]    [0054]FIG. 3 is a front view of one end surface of the fixed casing of the first embodiment of the present invention;  
         [0055]    [0055]FIG. 4 is a sectional view taken along line A-A of FIG. 3;  
         [0056]    [0056]FIG. 5 is a front view of the holder of the first embodiment of the present invention;  
         [0057]    [0057]FIG. 6 is a sectional view taken along line B-B of FIG. 5;  
         [0058]    [0058]FIG. 7 is a diagram showing stress distribution of the holder supporting structure to the end face of the fixed casing of the first embodiment of the present invention;  
         [0059]    [0059]FIG. 8 is a front view of the fixed casing to which a nut is screwed in and a key plate is mounted in the first embodiment of the present invention;  
         [0060]    [0060]FIG. 9 is a sectional view taken along line C-C of FIG. 8;  
         [0061]    [0061]FIG. 10 is a side view showing a supporting structure of a rotating shaft of the first embodiment of the present invention;  
         [0062]    [0062]FIG. 11 illustrates a structure of a first sun gear of the first embodiment of the present invention,  
         [0063]    [0063]FIG. 11( a ) is a side view of the same and  
         [0064]    [0064]FIG. 11( b ) is a vertically sectioned view of the same;  
         [0065]    [0065]FIG. 12 illustrates a configuration example of spline cogs at an end portion of an input shaft portion of the rotating shaft of the first embodiment of the present invention,  
         [0066]    [0066]FIG. 12( a ) is a front view of the input shaft portion;  
         [0067]    [0067]FIG. 12( b ) is a sectional view of the input shaft portion; and  
         [0068]    [0068]FIG. 12( c ) is a top view of one of the spline cogs;  
         [0069]    [0069]FIG. 13 illustrates another configuration example of the spline cogs at the end of the input shaft portion of the rotating shaft of the first embodiment of the present invention,  
         [0070]    [0070]FIG. 13( a ) is a front view of the input shaft portion;  
         [0071]    [0071]FIG. 13( b ) is a sectional view of the input shaft portion; and  
         [0072]    [0072]FIG. 13( c ) is a top view of one of the spline cogs;  
         [0073]    [0073]FIG. 14 is a sectional view of a driving unit of a second embodiment of the present invention;  
         [0074]    [0074]FIG. 15 illustrates a structure of the first sun gear of the second embodiment of the present invention,  
         [0075]    [0075]FIG. 15( a ) is a side view of the same and  
         [0076]    [0076]FIG. 15( b ) is a vertically sectioned view of the same; and  15 ( c ) is a top view of one of the engaging cogs of the first sun gear;  
         [0077]    [0077]FIG. 16 is a sectional view of a planetary gear frame as viewed from line D-D of FIG. 14;  
         [0078]    [0078]FIG. 17 is a sectional view taken along the arrowed line E-E of FIG. 16, and developed with the hydraulic motor side up;  
         [0079]    [0079]FIG. 18 is a view of the planetary gear frame of the second embodiment of the present invention, as viewed from the opposite side to the hydraulic motor side;  
         [0080]    [0080]FIG. 19 is a view showing engagement of a sun gear, planetary gears and a internal gear of the second embodiment of the present invention;  
         [0081]    [0081]FIG. 20 is an enlarged view of a principal part of FIG. 19;  
         [0082]    [0082]FIG. 21 is a front view of a holder of the second embodiment of the present invention;  
         [0083]    [0083]FIG. 22 is a sectional view taken along line G-G of FIG. 21;  
         [0084]    [0084]FIG. 23 is a front view of one end face of a fixed casing of the second embodiment of the present invention;  
         [0085]    [0085]FIG. 24 is a sectional view taken along line H-H of FIG. 23;  
         [0086]    [0086]FIG. 25 is a sectional view of an example of a conventional driving unit;  
         [0087]    [0087]FIG. 26 is a sectional view of another example of a conventional driving unit; and  
         [0088]    [0088]FIG. 27( a ) is an enlarged sectional view of a principal part showing a structure of a lock nut of FIG. 26 taken along line I-I of FIG. 27( b ); and  
         [0089]    [0089]FIG. 27( b ) is an enlarged view of a side surface of a key plate of FIG. 26. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0090]    In the following, description will be given to the preferred embodiments of the present invention with reference to the accompanying drawings. FIGS.  1 - 13  are illustrations on the first embodiment of the present invention. FIGS.  14 - 24  are illustrations on the second embodiments of the present invention.  
         [0091]    (An Example of First Embodiment)  
         [0092]    First, an example of the first embodiment of the present invention will be described below. FIG. 1 is a sectional view of a driving unit  1   a  according to the first embodiment of the present invention. FIG. 2 is a perspective view schematically showing a holder  24  supporting structure to an end face of a fixed casing  11 .  
         [0093]    In FIG. 1, the driving unit  1   a  comprises a hydraulic motor  3  disposed in an interior of a fixed casing  11 , a rotating casing  12  rotatably fitted to the fixed casing  11 , and a reduction gear mechanism  2  of a two-stage planetary gear train housed in the rotating casing  12 .  
         [0094]    The rotating casing  12  is mounted on the outer of the fixed casing  11 , so as to be freely rotatable and be axially immovable via bearings  13 . The rotating casing  12  and the hydraulic motor  3  are combined with each other, with partially overlapping in the axial direction, at an approximately axial center of which a flange  14  for mounting a sprocket thereon, not shown, is provided. The bearing  13  and a floating seal  32  are fitted to the fixed casing  11  from one end portion thereof at the opposite side to the hydraulic motor  3 . For avoidance of increase of outer diameters of the bearing  13  and floating seal  32 , it is necessary to reduce an outer diameter of the fixed casing  11  on the reduction gear mechanism  2  side.  
         [0095]    The reduction gear mechanism  2  having a two-stage planetary gear train for a large reduction gear ratio with a smallest possible number of gears is disposed in the interior of the rotating casing  12  at the one end portion of the fixed casing  11 . An internal gear  15  is formed around the inner periphery of the rotating casing  12 . A first sun gear  17  is fitted to an end of an output shaft  16  of the hydraulic motor  3 , which serves as an input shaft of the reduction gear mechanism  2 , via a spline  18 . In other words, the hydraulic motor  3  and the first sun gear  17  are coupled with each other via the integrally molded rotating shaft. Three first planetary gears  20  which are rotatably supported on the planetary gear frame  19  are engaged between the first sun gear  17  and the internal gear  15 . An outer periphery of a second sun gear  22  and an inner periphery of the planetary gear frame  19  which operates to transmit a orbital motion of the first planetary gear  20  around the first sun gear  17  are engaged with each other via a spline  21 . The single first sun gear  17 , the three first planetary gears  20 , the internal gear  15  and the planetary gear frame  19 , that functions as the output shaft as well, form a first stage of the planetary gear train.  
         [0096]    Three trunnion bosses  25  are integrally projected from one end portion of the fixed casing  11 , and three second planetary gears  26  engaging between a second sun gear  22  and the internal gear  15  are rotatably supported by the trunnion bosses  25 . There is provided a holder  24  having holes  37  in which end portions of the trunnion bosses  25  are fitted. Support pillars  27  extending from the holder  24  and support pillars  28  extending from the fixed casing  11  side are put in abutment with each other at their abutment surfaces  29  and are fixed by bolts  30  and locating pins  31 . The second sun gear  22 , the three second planetary gears  26 , and the internal gear  15  that functions as the output shaft, form a second stage of the planetary gear train.  
         [0097]    With the reduction gear mechanism  2  thus structured, when the first sun gear  17  is rotated by the drive of the hydraulic motor  3 , the first planetary gears  20  engaging with both of the first sun gear  17  and the internal gear  15  are rotated together with the planetary gear frame  19  at a reduced speed around the first sun gear  17 . The rotation is transmitted to the second sun gear  22  and, further, through the second planetary gear  26 , the rotating casing  12  having the internal gear  15  is rotated at a reduced speed, so that the sprocket (not shown) mounted on the flange  14  or the driving portion is rotationally driven.  
         [0098]    Referring to FIGS.  2 - 6 , the support structure of the second planetary gear  26  of the secondary planetary gear train of the final stage will be described. FIG. 3 is a front view of one end face of the fixed casing  11  and FIG. 4 is a sectional view taken along line A-A of FIG. 3. The three trunnion bosses  25  are projected at the one end portion of the fixed casing  11  at regular circumferential intervals of 120°. The trunnion bosses  25  are rounded to have rounded portions  34  at their basal ends. However, a diameter of a circumscribed circle  35  of the three trunnion bosses  25  is substantially the same as a diameter of the periphery  36  of the fixed casing  11  onto which the bearing  13  (FIG. 1) is fitted. Thus, the rounded portions  34  are not provided at portions thereof at which they interfere with the circumscribed circle  35 .  
         [0099]    Three support pillars  28 , each having a generally triangular shape, are provided between the adjacent trunnion bosses  25  at the one end portion of the fixed casing  11  so as to integrally project therefrom. The support pillars  28  are positioned at a height H substantially the same as an approximately widthwise center of the second planetary gears  26  supported by the trunnion bosses  25  (See FIG. 1).  
         [0100]    [0100]FIG. 5 is a front view of the holder  24  and FIG. 6 is a sectional view taken along line B-B of FIG. 5. The holder  24  having a disk-like shape has three generally triangular support pillars  27  integrally projected toward the one end of the fixed casing  11 . Three holes  37  in which the end portions of the trunnion bosses  25  (FIG. 4) are fitted are provided between the adjacent support pillars  27 .  
         [0101]    The second planetary gears  26  (FIG. 1) are fitted onto the trunnion bosses  25  of FIG. 4 so as to be freely rotate via needle bearings and the like. In this state, the pins  31  (FIG. 1) are fitted in holes  38  between the support pillars  27 ,  28  and, further, the bolts  30  (FIG. 1) are screwed in bolt holes  39  of the support pillars  27  and threaded holes  40  of the support pillars  28 , so that the support pillars  27 ,  28  are fixed in abutment with each other at their abutment surfaces  29 . The bolts  30  and the pins  31  form fastening means of the support pillars  27 ,  28 .  
         [0102]    Shown in FIG. 2 is the state of the holder  24  fixed at the one end portion of the fixed casing  11  by the fastening means. Portions of the trunnion bosses  25  extending along the circumscribed circle  35  (See FIG. 3) are substantially tangent to the periphery  36  of the fixed casing  11 , and the trunnion bosses  25  are rounded at the basal ends to have the rounded portions  34  thereat. This projected structure of the trunnion bosses  25  can provide reduction in stress concentration to the basal ends, as well as in diameter of the periphery  36  of the portion of the fixed casing  11  onto which the bearing  13  (FIG. 1) is fitted. Also, the support pillars  28  integrally projected from the fixed casing  11  and the support pillars  27  integrally projected from the holder  24  are put in abutment with each other at their abutment surfaces  29  and are fixed together by the bolts  30  through the pins  31 . Thus, the ends of the trunnion bosses  25  are fitted into the holes  37  of the holder  24  and, as a result of this, the trunnion bosses are brought into the state of being supported at both ends.  
         [0103]    Shown in FIG. 7 is a diagram showing stress distribution provided when a bending force acts on the one end portion of the fixed casing  11  of FIG. 2. When a clockwise bending stress acts on an approximately center portion of a trunnion boss  25 , a maximum bending stress (22 kgf/mm 2 ) in the counterclockwise direction is generated at the basal end of the trunnion boss  25 . In addition to this, a large stress (14 kgf/mm 2 ) is also generated in at a counterclockwise corner of the support pillar  28  at the fixed casing  11  side. It is found from these facts that the bending stress caused by the bending force acting on the trunnion boss  25  is dispersed and burdened by the support pillar  28  on the fixed casing  11  side through the support pillar  27  of the holder  24 . It is also found that the bending stress is not substantially generated around the abutment surfaces  29  of the support pillars  27 ,  28 , from which it is found that the abutment surfaces  29  should preferably be located within the limits of an effective length of each trunnion boss  25 , or at an approximately center portion of the same, in particular.  
         [0104]    This specific constitution of the support pillars  27 ,  28  as shown in FIG. 2 enables the bending stress on the basal end of the trunnion bosses  25  to be reduced and also enables the trunnion bosses  25  to be reduced in size. Also, since the circumscribed circle  35  of the trunnion bosses  25  and the periphery  36  of the fixed casing  11  are made substantially equal to each other, the bearing  13  and the floating seal  32  (FIG. 1) which are inserted from the one end portion of the fixed casing  11  can be reduced in outer diameter, and as such can allow the rotating casing  12  to reduce in outer diameter and in turn can allow the radial dimension of the driving unit  1   a  to be minimized.  
         [0105]    In FIG. 1, a tapered roller bearing is used as the bearing  13  to rotatably support the rotating casing  12  to the fixed casing  11 . The bearing  13  is held in place, with an adequate tightening force kept constant, by a nut  61  screwably engaged with a threaded portion  11   b  of the fixed casing  11  formed from an end face  41  toward the bearing  13 .  
         [0106]    Referring to FIGS. 8 and 9, a lock mechanism of the nut  61  will be described. FIG. 8 is a front view of the fixed casing  11  to which the nut  61  is screwed and a key plate  62  is mounted. FIG. 9 is a sectional view taken along line C-C of FIG. 8.  
         [0107]    As shown in FIG. 8, a number of pin holes  61   a  are formed on the side of the nut  61  at regular intervals. As shown in FIGS. 8 and 9, the fixed casing  11  is cut out in arc at the periphery of the support pillar  28  to form a cutout portion  65  extending laterally from the end face  41 . The key plate  62  is fixed to a side surface  65   a  of the cutout portion  65  by two bolts  64 , with its side surface being in abutment with the side surface  65   a.  The two bolts  64  are disposed at different lateral distances from a center line  11   c  therebetween. Also, the key plate  62  has two pin holes  62   a  corresponding in position to the pin holes  61   a  of the nut  61  which are formed symmetrically at equal distances from the center line  11   c.  By screwing the nut  61  slightly, either of the two pin holes  62   a  can be aligned with any one of the pin holes  61   a  of the nut  61 .  
         [0108]    As shown in FIG. 9, a pin  63  is fitted in the aligned pin holes  61   a,    62   a,  so that the nut  61  is locked against rotation by the key plate  62 .  
         [0109]    Although the circular-curved cutout portion  65  is provided at the approximately circumferential center of the one support pillar  28  on the fixed casing  11  side, since a little stress is distributed over the entire support pillar  28 , except the ends of the support pillar  28  at the circumferential side thereof, as shown in FIG. 7, the provision of the cutout  65  does not impair the stress relief function of the support pillar  28 . In addition, since the key plate  62  is disposed outside of the side surface  41  at one end portion of the fixed casing  11 , the fixed casing  11  is prevented from being elongated axially by the key plate  62 .  
         [0110]    In FIG. 1, the fixed casing  11  has an inner cavity  52  which has a bottom  50  at one end portion thereof at the inside and is closed by a lid  51  at the other end portion thereof. The rotating shaft  16  is disposed along an axis of the inner cavity  52 . The rotating shaft  16  is journaled for free rotation by a bearing  53  fitted in the lid  51  at one end of the shaft  16  and by a bearing  54  fitted in the bottom  50  at a mid portion of the other end side of the shaft  16 . A cylinder block  56  in which a plurality of pistons  55  are slidably inserted is splined to the output shaft  16  for non-rotatable and slidable movement. A swash plate  58  swingably supported by means of a steel ball  57  is mounted on the bottom  50  side, and a cylinder  59  for slanting the swash plate  58  is disposed at one end of the swash plate  58 . Front ends of the pistons  55  are in abutment with the swash plate  58  for freely sliding movement. Compressed oil is fed to and discharged from the cylinder block  56  via a counterbalance valve (not shown) provided in the lid  51 .  
         [0111]    As clearly shown in FIG. 10, the rotating shaft  16  journaled by the two bearings  53 ,  54  located on the hydraulic motor  3  side is cantilevered beyond a center of the reduction gear mechanism  2 , passing through it. The rotating shaft  16  is formed by an output shaft portion  16   a  on the hydraulic motor  3  side and an input shaft portion  16   b  on the reduction gear mechanism  2  side being formed into one piece. The first sun gear  17  is fitted to the end of the input shaft portion  16   b  by means of the spline  18 . The spline  18  comprises spline cogs  18   a  on the outer periphery side of the input shaft  16   b  and spline grooves  18   b  on the inner periphery side of the first sun gear  17 .  
         [0112]    Shown in FIG. 11 is the structure of the first sun gear  17 . FIG. 11( a ) is a side view of the first sun gear and FIG. 11( b ) is a vertically sectioned view of the same. In FIG. 11( b ), the first sun gear  17  has engaging cogs  17   a  engageable with the first planetary gear  20 , not shown, formed around the periphery of the first sun gear  17 , and the spline grooves  18   b  engageable with the spline cogs  18   a  of the input shaft  16   b,  not shown, formed around the inside of the first sun gear  17 . In FIG. 11( a ), the engaging cogs  17   a  formed around the periphery of the first sun gear  17  are equal in number to the spline grooves  18   b  formed around the inside thereof. The spline cogs  18   a  are arranged so that the spline grooves  18   b  can be positioned between spaces  17   b  between the engaging cogs  17   a.  This arrangement can prevent the spaces  17   b  between the engaging cogs  17   a  and the spline cogs  18   a  from being overlapped with each other to ensure the wall thickness t of the first sun gear  17 , and as such can allow the first sun gear  17  to have a reduced outer diameter.  
         [0113]    Shown in FIG. 12 is the structure of the spline cogs  18   a  at the end of the input shaft portion  16   b  of the rotating shaft  16 . FIG. 12( a ) is a front view of the input shaft  16   b;  FIG. 12( b ) is a sectional view of the input shaft portion  16   b;  and FIG. 12( c ) is a top view of a single spline cog. The spline cogs  18   a  extend in an axial direction of the input shaft portion  16   b,  as shown in FIGS.  12 ( a ) and  12 ( b ). A groove  16   c  is used for fitting therein a lock ring for locking the first sun gear  17  to the input shaft portion  16   b.  Opposite slanted surfaces of each spline cog  18   a  have a curved surface extending along an arcuate line of a radium R, such that the each spline cog  18   a  has a crown shape, protruding at an axial center thereof and gradually narrowing toward the opposite ends. The angle of inclination at the both ends of the spline cog  18   a  is a.  
         [0114]    Turning to FIG. 10, a reaction force to a force of the piston  55  to press the swash plate  58  acts on the rotating shaft  16 , and the load F is applied thereto. The input shaft portion  16   b  is rotated, with its front end inclined at an angle of a by the load F. The angle of inclination a at the front end of the rotating shaft  16  and the angle of inclinations a at the opposite ends of the spline cog  18   a  are generally identical with each other. As shown in FIG. 12, the spline cog  18   a  at the front end of the rotating shaft  16  has a widthwise crowned portion, so that even when inclination is caused at the end of the rotating shaft  16 , the spline cog  18   a  is brought into abutment with the spline groove  18   b  (FIG. 11) at an approximately lengthwise center thereof.  
         [0115]    Shown in FIG. 13 is the structure of another spline cog  181   a  at the front end of the input shaft portion  16   b  of the rotating shaft  16 . FIG. 13( a ) is a front view of the input shaft portion  16   b,  FIG. 13( b ) is a sectional view of the input shaft portion  16   b,  and FIG. 13( c ) is a top view of a single spline cog. As shown in FIG. 13( c ), the spline cog  181   a  has a tapered shape to be gradually narrowed toward the front end. The degree to which the spline cog is narrowed corresponds to the degree to which the angle of inclination of the side surfaces becomes a. Other respects are the same as those of FIG. 12.  
         [0116]    As shown in FIGS. 12 and 13, the spline cog is preferably crowned or inclined to be gradually narrowed toward the end thereof, in terms of machinability and function. Alternatively, the spline groove  18   b  on the first sun gear  17  side may be crowned so that an axial center of the spline groove  18   b  is gradually narrowed in width or may be inclined so that the spline groove  18   b  is gradually widened toward the axial front end thereof. Further, both of the spline cog  18   a  and the spline groove  18   b  may be provided with a crowned portion or an inclined portion corresponding to clearance.  
         [0117]    As mentioned above, the front end portion of the rotating shaft  16  is inclined by the application of the reaction force of the hydraulic motor  3 . To allow for this inclination, either or both of the spline cog and the spline groove of the spline  18  are provided with the crowned portion or inclined portion so that the clearance therebetween can be gradually widened toward the front end of the spline  18 . This clearance can prevent generation of collision between the spline cog and the spline groove even when the front end portion of the rotating shaft  16  is bent. Thus, in contrast to the prior art shown in FIG. 25 which is so constituted that the inclination of the rotating shaft is absorbed by a coupling  117  for connecting between an output shaft  104   a  of the motor and an input shaft  104   b  of the reduction gear, the embodiment of the present invention is so constituted that the inclination can be absorbed by the first sun gear  17 . Hence, the subsequent gears are prevented from being adversely affected by the inclination of the rotating shaft  16 .  
         [0118]    Also, as shown in FIG. 1, the output shaft portion  16   a  and the input shaft portion  16   b  of the rotating shaft  16  are formed in one piece without any coupling provided therebetween, so that the rotating shaft  16  involves no large diameter portion at any location throughout the rotating shaft  16 . This enables the second sun gear  22  disposed around the input shaft portion  16   b  of the rotating shaft  16  to be reduced in diameter, thus enabling the number of teeth of the second sun gear  22  to be reduced. As a result of this, if the reduction gear ratio is kept unchanged, the number of teeth of the internal gear  15  can also be reduced, and as such can reduce the diameter or size of the rotating casing  12 . In addition, the distance between a center of the second sun gear  22  and a center of the second planetary gear  26  is shortened and, as a result of this, outward protrusion of the second sun gear  26  can be reduced. Therefore, the radial dimension or size of the driving unit  1   a  can be minimized.  
         [0119]    Further, as shown in FIG. 11, the spaces  17   b  between the engaging cogs  17   a  and the spline cogs  18   a  are prevented from being overlapped with each other so that the first sun gear  17  can be allowed to have a reduced outer diameter. This enables the number of teeth of the first sun gear  17  to be reduced. As a result of this, if the reduction gear ratio is kept unchanged, the number of teeth of the internal gear  15  can also be reduced, and as such can reduce the diameter or size of the rotating casing  12 . Consequently, outward protrusion of the first planetary gear  20  can be reduced. Therefore, the first planetary gear train and the second planetary gear train can both be reduced in size.  
         [0120]    The example of the first embodiment of the invention as described above may be modified as follows, for practical use of the invention.  
         [0121]    (1) While the reduction gear mechanism  2  having the two-stage planetary gear train was illustrated, the supporting structure of the embodiment of the present invention can be applied to a three-stage planetary gear train as well by the application to the final stage planetary gear train.  
         [0122]    (2) The planetary gears revolving around the sun gear of the planetary gear train is not limited in number to three. For example, for four planetary gears, the supporting structure of the embodiment of the present invention can be applied thereto by increasing the trunnion bosses and the support pillars in number to four.  
         [0123]    (3) In FIGS. 12 and 13, the spline  18  is not limited to the straight spline extending in parallel to the axis of the rotating shaft. The spline formed to extend obliquely with respect to the axial direction may be used.  
         [0124]    (An Example of Second Embodiment)  
         [0125]    Then, an example of the second embodiment of the present invention will be described below. To avoid repetition of description of corresponding construction to that of the example of the first embodiment, like numerals are labeled to corresponding parts throughout the drawings. FIG. 14 is a sectional view of the driving unit  1   b  according to an example of the second embodiment. The driving unit  1   b  of the example of the second embodiment is different from the driving unit  1   a  of the example of the first embodiment shown in FIG. 1 in the following points.  
         [0126]    {circle over (1)} Rather than being integrally projected from the bottom  50  of the fixed casing  11 , a trunnion boss  75  is formed as a single part and journaled at its opposite ends between the bottom  50  of the fixed casing  11  and the holder  24 ;  
         [0127]    {circle over (2)} The crowned portion is formed in the engaging cog  17   c  of the first sun gear  17 , rather than being formed in the spline cog  18   a  of the rotating shaft  16  as in the example of the first embodiment;  
         [0128]    {circle over (3)} While in the example of the first embodiment, the first-stage planetary gear train comprises three first planetary gears  20 , the first planetary gears  20  in the example of the second embodiment are reduced in number to two;  
         [0129]    {circle over (4)} The internal gear  15  is formed to have a reduced length, as compared with the example of the first embodiment; and  
         [0130]    {circle over (5)} While in the example of the first embodiment, the nut  61  for supporting the bearing  13  is locked against rotation by the key plate  62 , the nut is locked against rotation by a pin, instead of the key plate.  
         [0131]    In the following, description on the different points mentioned above will be given. First, reference is given to the first difference that the trunnion boss  75  is formed as a single part and journaled at its opposite ends.  
         [0132]    In FIG. 14, the trunnion boss  75  is formed as a single part, comprising a large diameter body  75   a  and two small diameter shafts  75   b  projecting from the opposite ends of the large diameter body  75   a.  A hole  76  is formed in the bottom  50  of the fixed casing  11 , and a hole  37  is formed in the holder  24  in such a manner as to confront the hole  76 . The one shaft  75   b  of the trunnion boss  75  is fitted in the hole  76  and the other shaft  75   b  of the same is fitted in the hole  37 , whereby the trunnion boss  75  is journaled at its opposite ends between the bottom  50  of the fixed casing  11  and the holder  24 . Three trunnion bosses  75  are arranged circumferentially and three second planetary gears  26  engageable between the second sun gear  22  and the internal gear  15  are rotationally supported on the bodies  75   a  of the three trunnion bosses  75 , respectively.  
         [0133]    Three support pillars  27  are integrally projected from a portion of the holder  24  between the trunnion bosses  75 , and three support pillars  28  are integrally projected from a portion of the fixed casing  11  between the trunnion bosses  75 . The support pillars  27  on the holder  24  side and the support pillars  28  on the fixed casing  11  side are put in abutment with each other at their abutment surfaces  29  and are fixed by bolts  30  and locating pins  31 . The abutment surfaces  29  are preferably within the width of the second planetary gear  26 , or preferably at an approximately center thereof.  
         [0134]    By virtue of this supporting structure wherein the trunnion bosses  75  are journaled at the opposite ends between the fixed casing  11  and the holder  24 , the trunnion bosses  75  are replaceable with new ones and are supported with little bending. Also, since the abutment surfaces  29  of the support pillars  27  of the holder  24  and those of the support pillars  28  of the fixed casing  11  are located within the width of the second planetary gear  26  and are located at an approximately center thereof, the support pillars  27 ,  28  can be tightened firmly by the bolts  30 . In addition, since the basal ends of the support pillars  27 ,  28  are integral with the holder  24  or the fixed casing  11 , the support pillars can withstand a stress concentration. By virtue of these specific designs, the radial and axial dimension of the fixed casing  11  can be reduced, thus providing a reduced size and weight of the device.  
         [0135]    Second, reference is given to the second difference that the crowned portion is formed in the engaging cog  17   c  of the first sun gear  17 , rather than being formed in the spline cog  18   a  of the rotating shaft  16 . FIG. 15( a ) is a side view of the first sun gear  17 , FIG. 15( b ) is a vertically sectioned view, and FIG. 15( c ) is a top view showing one of the engaging cogs of the first sun gear. In FIG. 15( c ), the opposite slanted surfaces of each engaging cog  17   c  have a curved surface extending along an arcuate line of a radium R, such that the each engaging cog  17   c  has a crown shape, protruding at an axial center thereof and gradually narrowing toward the opposite ends. The angle of inclination at the both ends of the engaging cog  17   c  is a. As is the case with the example of the first embodiment of FIG. 10, the rotating shaft is rotated in the state in which the input shaft portion  16   b  of the rotating shaft  16  is inclined at an angle α at the front end portion thereof by the load F. This inclination of the rotating shaft  16  causes the first sun gear  17  to be inclined. But, since the engaging cogs  17   c  of the first sun gear  17  are provided with the widthwise crowned portions, the engaging cogs  17   c  come into abutment with the first planetary gears  20  at their approximately lengthwise center portions thereof.  
         [0136]    Thus, all the spline cogs of the rotating shaft  16  are brought into abutment with the first sun gear  17  at the splined connection therebetween. This can prevent a running torque of the rotating shaft from being transmitted by only some spline cogs, and as such can provide improved durability of the rotating shaft  16  and the first sun gear  17 . It is to be noted that the cogs of the first sun gear  17  may be tapered as is the case with the front end portion of the rotating shaft  16  of FIG. 13.  
         [0137]    Third, reference is given to the third difference that the first planetary gears  20  are reduced in number to two. As shown in FIG. 14, the two first planetary gears  20  are rotatably supported on the planetary gear frame  19  and are engaged between the first sun gear  17  and the internal gear  15 .  
         [0138]    Shown in FIGS.  16 - 18  is the constitution of the planetary gear frame  19 . FIG. 16 is a sectional view of the planetary gear frame  19  as viewed from line D-D of FIG. 14. FIG. 17 is a sectional view taken along the arrowed line E-E of FIG. 16 and developed with the hydraulic motor  3  side up. FIG. 18 is a view of the planetary gear frame  19  as viewed from the opposite side to the hydraulic motor  3  side. As best shown in these diagrams, the planetary gear frame  19  has a pair of generally ellipse-like flat plate portions  19   a,    19   b.  As best shown in FIG. 16, the flat plate portion  19   a  has an insertion hole  19   c  for inserting the rotating shaft  16  therein. As best shown in FIG. 18, the flat plate portion  19   b  has an opening  19   d  from which the first sun gear  17  can be fitted onto the rotating shaft  16 . The insertion hole  19   c  has, around its inside, grooves engageable with the periphery of the second sun gear  22  which form the spline  21  (See FIG. 17, not shown in FIG. 16). The opening  19   d  is closed by a lid  23  after the first sun gear  17  is fitted onto the rotating shaft  16 , as shown in FIG. 14.  
         [0139]    As best shown in FIGS. 16 and 18, the flat plate portions  19   a,    19   b  have two supporting hole  19   e  for the two first planetary gears  20  to be supported in such a manner as to be symmetrically disposed about the rotating shaft  16 . As shown in FIG. 14, the first planetary gears  20  are mounted on shaft members fitted into the supporting holes  19   e.  In other words, the two first planetary gears  20  are rotatably supported at the axially opposite ends thereof in sandwich relation between the two flat plate portions  19   a,    19   b.    
         [0140]    When the two first planetary gears  20  are rotated around the first sun gear  17 , the reaction forces are applied to the first sun gear  17  from the two first planetary gears  20 , respectively. Since the two first planetary gears  20  are symmetrically disposed about the rotating shaft  16 , the reaction forces are balanced each other out, and as such can prevent the first sun gear  17  from being moved in the radial direction by the reaction forces. Thus, an undesired partial abutment between the first planetary gears  20  and the first sun gear  17  can be restricted, and as such can provide improved durability of these gears.  
         [0141]    The planetary gear frame  19  has paired support pillars  19   f  for the pair of flat plate portions  19   a,    19   b  to be fixedly held at positions symmetrical with respect to the rotating shaft  16 . The two pairs of support pillars  19   f  extend partially along a generally ellipse-like circumference of the flat plate portions  19   a,    19   b  and are disposed at positions in the vicinity of the first planetary gears  20 . The positions of the support pillars  19   f  enable the support pillars, to which the reaction forces generated when the first planetary gears  20  are driven are applied, to be slenderized. This can produce the driving unit comprising the two first planetary gears combining structural stability with weight reduction.  
         [0142]    Thus, the driving unit thus constructed can be reduced in size to a large extent, as compared with the conventional driving unit having three first planetary gears. Further, parts count can also be reduced to a large extent and also the structure can be simplified. Thus, the driving unit thus produced is also excellent in cost. Also, the ellipse-like shape of the planetary gear frame  19  contributes to the downsizing and lightweight of the driving unit.  
         [0143]    In addition, the output shaft of the motor is doubled as the input shaft by forming the rotating shaft in one piece and projecting it to extend through a center portion of the reduction gear. This enables the radial movement of the rotating shaft to be reduced, as compared with the rotating shaft comprising the output shaft and the input shaft coupled with each other through an intermediate coupling. As a result of this, undesired partial abutment between the planetary gears and the sun gear can be restricted, and as such can maintain the durability of the sun gear and the rotating shaft.  
         [0144]    Further, since the spaces between the cogs of the sun gear and the spline grooves at the fitting portions of the sun gear and the rotating shaft are out of position from each other with respect to the circumferential direction, even when the sun gear is reduced in diameter, the wall thickness of the sun gear can be ensured. As a result of this, deformation of the sun gear produced when it transmits the output can be reduced, so that the noise emitted when the sun gear and the planetary gears are engaged can be suppressed.  
         [0145]    Further, as is the case with the example of the first embodiment, the first sun gear can be reduced in radial dimension without the distances between the spline grooves and the spaces between the cogs of the first sun gear being shortened and, as a result, the second sun gear can also be reduced in diameter to such an extent that when the rotating shaft is inclined, the second gear does not interfere with it. This enables the reduction gear ratio of the reduction gear to be increased. As a result of this, a compact, low-torque, high-revolution, hydraulic motor can be applied to the driving unit, then enabling the driving unit to be reduced in size.  
         [0146]    Here, detailed description will be given on the engagement structure between the first sun gear  17  and the first planetary gears  20  of the driving unit  1   b  of the example of the second embodiment. FIG. 19 is a view showing the engaging state of the sun gear  17 , the first planetary gears  20  and the internal gear  79  having internal cogs  15 . FIG. 20 is an enlarged view of a principal part F surrounded by a dotted line of FIG. 19. In FIG. 20, P is a point on a tooth flank  20   b  of the first planetary gear  20  which is in the opposite side to a tooth flank  20   a  where the first sun gear  17  and the first planetary gear  20  are put in engagement with each other and which comes nearest the first sun gear  17  when the rotation shaft  16  is inclined, and Q is a point on a tooth flank  17   d  of the confronting first sun gear  17 . A straight line connecting between P and Q is parallel to a connecting line between the axes of the two first planetary gears  20 . θ is an angle formed by a tangent line j extending perpendicularly to the axis of the first sun gear  17  and a moving direction R of the first sun gear  17  (the direction of the connecting line P-Q).          is a distance of the first sun gear  17  in the moving direction R. A distance between the point P and the tangent line, in other words, a clearance  1  between the tooth flank  20   b  and the tooth flank  17   d  which is a length of a perpendicular dropped from the point P to the tangent line j is set to 2         sin θ.  
         [0147]    This can produce the result that even when the rotating shaft  16  is inclined, the tooth flank  17   d  of the first sun gear  17  and the tooth flank  20   b  of the first planetary gear  20  are prevented from colliding with each other, thus providing improved durability. Also, since the inclination of the rotating shaft  16  is absorbed between the first sun gear  17  and the first planetary gears  20 , inclination of the first planetary gears  20  or second planetary gears  26 , partial abutment between the respective gears, and the like adverse effect can be prevented. Further, since a value of the clearance  1  (2         sin θ) is a minimum value to prevent the collision between the tooth flake  17   d  and the tooth flake  20   b,  the backlash of the first planetary gears  20  resulting from the clearance  1  can be minimized. Thus, undesirable movement of a construction machine using the driving unit of the example of this embodiment resulting from the clearance can be suppressed, so that the construction machine can be prevented from swinging back or slipping down a sloping road.  
         [0148]    Fourth, reference is given to the fourth difference that the internal gear  15  is formed to have a reduced length. In FIG. 14, a length of pass of contact n between the first sun gear  17  and the first planetary gears  20  is set to a bending stress calculated to obtain a desired durable period. The bending stress is small in the engaging area between the internal gear  15  and the first planetary gears  20 , because tooth thickness of dedendum of the internal gear  15  is formed to be larger than that of dedendum of the first sun gear  17 , as shown in FIG. 19. Further, the number of times the internal gear  15  engages with the first planetary gears  20  is smaller than the number of times the first sun gear  17  engages with the first planetary gear  20 . Thus, a length of pass of contact m between the internal gear  15  and the first planetary gears  20  can be formed to be smaller than the length of pass of contact n between the first sun gear  17  and the first planetary gears  20 . Preferably, the length of pass of contact m should be determined so that the engaging area between the first sun gear  17  and the first planetary gears  20  are equal in durable period to that between the internal gear  15  and the first planetary gear  20 .  
         [0149]    This can allow the internal gear  15  to be shortened by making the engaging area between the first sun gear  17  and the first planetary gears  20  equal in durable period to that between the internal gear  15  and the first planetary gears  20 . Further, the casing can be reduced in size. Thus, the internal gear and the casing can be reduced in weight and, as a result of this, the hardening treatment of the internal gear can be cut.  
         [0150]    Finally, reference is given to the fifth difference that the nut  61  for supporting the bearing  13  is locked against rotation by use of a pin  78 , instead of the key plate. In FIG. 14, a projection  77  projecting toward the periphery of the support pillars  28  of the fixed casing  11  is disposed at an end face of the support pillar  27  of the holder  24  at the periphery side thereof, and a lock pin  78  is disposed between the projection  77  and the nut  61  pressing the bearing  13 .  
         [0151]    As shown in FIGS. 21 and 22, the projection  77  projected from the holder  24  is integrally projected toward the periphery of the support pillar  27  of the holder  24 . The projection  77  has a hole  77   a  for the pin  78  to be axially inserted. Fitted in the hole  77   a  is a spring  78   a  to prevent the pin  78  from falling out. As shown in FIG. 23, pin holes  61   a  for fitting the pins  78  therein are formed in the side wall of the nut  61 , with higher density than those in the example of the first embodiment of FIG. 8.  
         [0152]    As shown in FIG. 24, the nut  61  is screwed into the threaded portion  11   b  of the fixed casing  11  to press the bearing  13  to a predetermined position. The nut  61  is stopped screwing with the pin hole  61   a  up, as shown in FIG. 23. Then, the holder  24  is pressed in on the basis of the locating pin  31 . At that time, when the pin  78  is previously fitted in either of the holes  77   a  and  61   a,  the pin  78  is put into the fitted state shown in the diagram to lock the nut  61  against rotation.  
         [0153]    Since no cutout is provided for the support pillar  28  on the fixed casing  11  side, the strength of the support pillar  28  is maintained. In addition, since the key plate  62  is not used in the driving unit of the second embodiment, differently from the driving unit  1   a  of the example of the first embodiment, the parts count is further reduced and also the axial dimension of the fixed casing  11  is not increased to that extent.  
         [0154]    The example of the second embodiment of the invention as described above may be modified as follows, for practical use of the invention.  
         [0155]    (1) While in this embodiment, it is only the first planetary gear that comprises two planetary gears, the second planetary gear may also comprise the two planetary gears;  
         [0156]    (2) The reduction gear mechanisms that may be used include the one comprising at least a two-stage planetary gear train (e.g. a three-stage or more planetary gear train). Also, the third stage or subsequent stage of planetary gear train that may be used include the one comprising two or three planetary gears.  
         [0157]    (3) The second planetary gears revolving around the second sun gear is not limited in number to three. For example, for four planetary gears, the supporting structure of the embodiment of the present invention can be applied thereto by increasing the trunnion bosses and the support pillars in number to four.  
         [0158]    (4) The arrangement of the support pillars for supporting the planetary gear frame is not necessarily limited to the illustrated arrangement wherein two pairs of support pillars are arranged to partially extend along the circumferential direction of the generally ellipse-shaped flat plate portion. For example, a pair of or three pairs of support pillars may be used. Also, the support pillars may be formed into a wall-like configuration arranged to partially along the circumferential direction.