Abstract:
A transmission case housing a transmission mechanism for transmitting power to dirigible wheels includes an axle case having a main body and vertical bosses extending in a bent way from opposite ends of the main body, and pivoting cases supporting axles of the dirigible wheels. The axle case ( 8 ) is bisected into a first case half ( 8 F) and a second case half ( 8 R) at a split plane parallel to a direction of extension of the transmission mechanism (S) housed therein. The first case half ( 8 F) and second case half ( 8 R) are connected by connecting bolt ( 38 ). Each vertical boss ( 8   b ) has bearings ( 26 ), and each pivoting case ( 25 ) is attached to the bearings ( 26 ) to be pivotable relative to the vertical boss ( 8   b ).

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to a transmission case housing a transmission mechanism for transmitting power to dirigible wheels of a relatively small working vehicle such as a lawn mower, agricultural tractor or rice planter. 
   2. Description of the Related Art 
   Generally, the transmission case noted above includes an axle case having a main body and vertical bosses extending in a bent way from opposite ends of the main body, and pivoting cases supporting axles of the dirigible wheels. In the case of a tractor, as disclosed in Japanese Unexamined Patent Publication H10-250391, and in the case of a riding rice planter, as disclosed in Japanese Unexamined Patent Publication H7-17203, the pivoting cases supporting the axles of dirigible front wheels are attached to the vertical bosses at the right and left ends of the axle case to be pivotable about vertical axes. Power is transmitted from a cross transmission shaft extending through the axle case to the axles in the pivoting cases through vertical transmission shafts extending along the vertical axes. 
   In the conventional construction noted above, the main body and the vertical bosses of the axle case are separate components interconnected through flanges to allow assembly into the case of transmission shafts and bevel gears. However, the main body of the axle case is formed as a linear tube, and the vertical bosses are formed as short tubes bent at approximately right angles (in the shape of elbows). The components assembled into these tubular members are subject to restrictions as to outside diameter and assembled position. This leads to a disadvantage of requiring an excessively large inside diameter of the transmission case. 
   SUMMARY OF THE INVENTION 
   Having regard to the disadvantage of the prior art noted above, an object of this invention is to provide a transmission case housing a transmission mechanism for transmitting power to dirigible wheels, which transmission case allows interior components of appropriate outside diameters to be incorporated into appropriate positions. 
   The above object is fulfilled, according to this invention, by an axle case bisectable into a first case half and a second case half at a split plane parallel to a direction of extension of a transmission mechanism housed therein, wherein the first case half and second case half are connected by connecting bolts 
   With the above construction, the axle case including vertical bosses may be split into halves to open up the entire axle case. After assembling components such as bearings, gears, and transmission shafts into predetermined positions within this opened case, the two split cases may be joined and connected with bolts. In this case, components larger than the diameter of end openings of the axle case assembled may be incorporated. There is no restriction such as of having to place the smaller components in positions the deeper away from the end openings. Thus, it is now easy to install interior parts of desired sizes in desired positions, which is advantageous in improving assembling efficiency. 
   Furthermore, in the transmission case according to this invention, a main body of the axle case houses transverse transmission shafts of the transmission mechanism, the vertical bosses of the axle case house vertical transmission shafts of the transmission mechanism, and pivoting cases house gears for transmitting power from the vertical transmission shafts to the axles. Since the pivoting cases are mounted on the vertical bosses split into halves, it is difficult to connect the split vertical bosses with bolts. In the above construction, however, the bearings which support each pivoting case is mounted on each vertical boss, and this reliably prevents the split vertical boss from spreading loose from each other. That is, the bearings which support the pivoting case are mounted on the vertical boss, to secure strength for the vertical boss which cannot be fastened tight by bolt connection. Thus, despite the split construction, the vertical boss can fully function as such. 
   Regarding the arrangement of the transmission mechanism in this transmission case, the transverse transmission shafts and the vertical transmission shafts may center lines thereof located on the split plane. This provides optimal stability. 
   In a preferred embodiment of this invention, each of the vertical bosses has vertical ribs formed on an inner surface thereof and extending along a vertical axis. Then, the vertical bosses, despite the split construction, have excellent bending strength. That is, the split vertical bosses tending to lack in strength may be reinforced by forming the vertical ribs therein. Such vertical bosses can reliably transmit a propelling force to the axle cases. 
   In a further preferred embodiment, each of the vertical bosses has a sleeve mounted therein. In this construction, each split vertical boss is supported by the sleeve in contact with the inner peripheral surface, and is closed by the bearings mounted on the outer peripheral surface. These components assure bending strength of the vertical boss. 
   In a different aspect of the invention, a transmission case has a first projection formed integrally with an intermediate region of the first case half, and a second projection formed in a position of the second case half corresponding to the first projection. The first projection and the second projection act as rolling fulcrum shafts. With this construction, the transmission case may be manufactured simply, which is suitable for connection to a vehicle body to be capable of rolling motion. 
   Other features and advantages of this invention will be apparent from the following description of the embodiment to be taken with reference to the drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a side elevation of a riding lawn mower; 
       FIG. 2  is a plan view of a propelling transmission system; 
       FIG. 3  is a plan view of a front wheel support structure; 
       FIG. 4  a front view of the front wheel support structure; 
       FIG. 5  is a front view in vertical section of an axle case; 
       FIG. 6  is a side view in vertical section showing a rolling fulcrum structure of the axle case; 
       FIG. 7  is a development of split case portions forming the axle case; 
       FIG. 8  is a side view in vertical section of the split case portions forming the axle case; 
       FIG. 9  is a side view in vertical section showing a part of the axle case; 
       FIG. 10  is a cross section of a part of the axle case; and 
       FIG. 11  is a front view in vertical section showing a part of an axle case in another embodiment. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  shows a side elevation of a riding lawn mower which is one example of working vehicle having a transmission case according to this invention. This riding lawn mower is the four-wheel drive type having front drive wheels  1  and rear drive wheels  2 . Its vehicle body  3  has a mower unit  5  suspended from a lower position thereof, to be capable of vertical parallel movement, through a four-point link mechanism  4  having front links  4   a  and rear links  4   b . A grass catcher  6  is connected to the rear of the vehicle body  3  for collecting grass clippings cut by the mower unit  5 . 
   The vehicle body  3  includes a pair of right and left main frames  7  having an axle case  8 , which is one of the compositions of the transmission case, connected to forward positions thereof to be capable of rolling motion, with the front wheels  1  dirigibly attached to the right and left sides of the axle case  8 . An engine  10  covered by a hood  9  is mounted on the main frames  7 . A transmission case  11  having the right and left rear wheels  2  is fixed to rear positions of the main frames  7 . A step  12  is mounted on the main frames  7 , and a driver&#39;s seat  13  is disposed between right and left fenders  12   a  formed integrally with the rear of the step  12 . 
   Output of the engine  10  is inputted to the transmission case  11  through a transmission shaft  14 , and branched therein to a propelling system and a working system. Power in the propelling system is inputted to a hydrostatic stepless transmission (HST)  15  connected to the rear end of the transmission case  11 , which is capable of forward and backward drive switching. Variable speed power in forward rotation (for forward drive) or in reversed rotation (for backward drive) from the stepless transmission  14  is transmitted to the right and left rear wheels  2  through side clutches  16 , and to the axle case  8  through transmission shafts  17   a  and  17   b  extending forward along and under the vehicle body. Thus, the front wheels  1  and rear wheels  2  are driven at synchronized speeds. This embodiment provides mainly a transmission case as a case housing a transmission mechanism for transmitting power to the front wheels  1  acting as dirigible wheels. The transmission case is formed of the axle case  8  and pivoting cases  25 . 
   Power for the working system branched within the transmission case is taken from a PTO shaft  18  projecting from the front surface of the transmission case, and is transmitted to the mower unit  5  through a transmission shaft  19 . 
   The transmission case  11  is offset to the left side relative to the vehicle body  3 . A duct  20  for guiding grass clippings from the mower unit  5  to the grass catcher  6  is disposed in a space formed under the vehicle body by the offset location of the transmission case  11 . 
   As shown in  FIGS. 4 and 5 , the axle case  8  includes a sideways elongated axle case main body  8   a  and vertical bosses  8   b  at the right and left ends thereof. The axle case  8  is supported in a middle position in the right and left direction by a pair of front and rear support brackets  21   f  and  21   r  attached to lower front positions of the main frames  7 , to be rollable about a fore-and-aft axis P. Power of the propelling system is transmitted to an input shaft  22  disposed in a position sideways offset from this rolling axis P. The power inputted is branched to right and left transverse transmission shafts  24  through a differential mechanism  23 . The pivoting cases  25  are fitted on the vertical bosses  8   b  formed integrally with the right and left ends of the axle case  8 , each through a pair of upper and lower roller bearings  26 , to be pivotable about vertical axes X. The transverse transmission shafts  24  extending right and left from the differential mechanism  23 , and axles  27  mounted in horizontal posture in the pivoting cases  25  are interlocked for reduction, each through two sets of bevel gears  28   a ,  28   b ,  29   a  and  29   b  and a vertical transmission shaft  30  extending along the vertical axis X. 
   As shown in  FIG. 2 , knuckle arms  31  attached to upper ends of the right and left pivoting cases  25  are connected to each other by a tie rod  32 . A pitman arm  34  pivotable about a fulcrum Z by a steering wheel  33  is interlocked to the left knuckle arm  31  through a drag link  35 . These components constitute a front wheel steering system. 
   The axle case  8  has a front and rear split structure formed by aluminum die casting. Its details are shown in  FIGS. 6 through 10 . Specifically, the axle case  8  includes front and rear split case portions  8 F and  8 R with a dividing plane S including the axis Y of the transverse transmission shafts  24  and the axes X of the vertical transmission shafts  30 , and with peripheral surfaces thereof mating through flanges and fixed together by tightening bolts  37  in the fore-and-aft direction. Bolts are not usable for tightening the vertical bosses  8   b  having the pivoting cases  25  fitted thereon. However, rigidity is secured for each boss  8   b  by press-fitting the upper and lower bearings  26 , for pivotably supporting the pivoting case  25 . Each vertical boss  8   b  has vertical ribs  38  formed inside and extending along the vertical axis to increase its bending rigidity. 
   The front and rear split case portions  8 F and  8 R have rolling fulcrum shafts  41   f  and  41   r  formed in transversely middle positions on outer surfaces thereof. The fore and aft fulcrum brackets  21   f  and  21   r  have cup-shaped fulcrum bosses  42   f  and  42   r  rigidly welded thereto as inwardly opposed to each other for rotatably receiving and supporting the rolling fulcrum shafts  41   f  and  41   r . The rolling fulcrum shafts  41   f  and  41   r  are slightly tapered, while the fulcrum bosses  42   f  and  42   r  also are tapered toward the deep ends thereof at the same taper angle. A grease nipple  43  is attached to the end of each of the fulcrum bosses  42   f  and  42   r . The split case portions  8 F and  8 R have annular projections  44  formed on the outer surfaces thereof to surround the rolling fulcrum shafts  41   f  and  41   r  and in tight contact with the inner surfaces of the front and rear fulcrum brackets  21   f  and  21   r , thereby forming fluid-tight oil sumps inside the fulcrum bosses  42   f  and  42   r . A plurality of axial oil grooves  45  are formed in outer peripheral surfaces of the rolling fulcrum shafts  41   f  and  41   r  to assure a sufficient supply of grease to rolling slide positions. 
   In order to allow the axle case  8  to be assembled between the front and rear fulcrum brackets  21   f  and  21   r , the rear fulcrum bracket  21   r  is rigidly welded to the main frames  7 , while the front fulcrum bracket  21   f  is removably connected by bolts. Bolts  46  are mounted between lower ends of the two fulcrums brackets  21   f  and  21   r  to prevent the fulcrum brackets  21   f  and  21   r  from moving away from each other. A spacing between these brackets is fine adjusted by appropriately tightening the bolts  46 , so that the rolling fulcrum shafts  41   f  and  41   r  may be received and supported in the fulcrum bosses  42   f  and  42   r  free of clattering. 
   The axle case  8  has projections  47  formed on the front surface and rear surface thereof at opposite sides of the rolling fulcrum P and having the same height has the annular projections  44 . These projections  47  are in contact with the inner surfaces of the fulcrum brackets  21   f  and  21   r , whereby a yawing load acting on the axle case  8  is borne by a large span between the right and left projections  47 . 
   Furthermore, the axle case  8  has a pair of ridges  48  formed on an upper surface thereof in positions at right and left sides of the rolling fulcrum P. These ridges  48  are movable upward into contact with stoppers  7   a  formed at the lower ends of the main frames  7  to limit rolling motion of the axle case  8 . 
   OTHER EMBODIMENTS 
   This invention may be modified as follows: 
   (1) Where ribs are formed on the inner surfaces of each vertical boss  8   b  for reinforcement, circumferential ribs or criss-cross ribs may be formed besides or as alternative to the vertical ribs  38  extending along the vertical axis noted hereinbefore. The shape and number of ribs may be selected as desired according to load. 
   (2) As shown in  FIG. 11 , a cylindrical sleeve  50  may be tight-fit inside each vertical boss  8 B of split structure. This will further promote the bending strength of the vertical boss  8   b.    
   (3) This invention is applicable also to a drive structure for dirigible wheels of a riding rice planter or a small tractor besides the riding lawn mower. Where the front wheels are non-dirigible and the rear wheels dirigible, the invention may be applied to the drive structure for the rear wheels.