Abstract:
A transmission for speed changing and steering of a vehicle includes a first hydrostatic pump ( 52 ) and motor ( 53 ), a first differential ( 24′ ) for speed changing, a second hydrostatic pump ( 71 ) and motor ( 72 ), and a second differential ( 23′ ) for steering. A prime mover drives the first pump ( 52 ), and the second pump ( 71 ) is driven by the output from the first motor ( 53 ). The second motor ( 72 ) drives the second differential ( 23′ ) for steering the vehicle.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a transmission for driving left and right axles of a vehicle comprising a pair of hydrostatic transmissions (to be referred to as HSTs), each of which is a fluid combination of a hydraulic pump and a hydraulic motor, wherein one of the HSTs is provided for speed change in advancing and reversing and the other is for steering. 
     2. Related Art 
     U.S. Pat. No. 4,782,650, for example, describes a vehicle provided with a pair of HSTs disposed in a lateral row and connected with each other, with left and right axles projecting laterally outward from its respective HST. Driving wheels are fixed onto utmost ends of the axles. The left and right axles are driven by changing the angles of slant of movable swash plates provided for the its respective left or right HST. 
     The left and right axles are driven at the same speed by the pair of HSTs when the vehicle is advancing or reversing in a straight line. They are driven at different speeds when turning. 
     However, the above mentioned conventional vehicle can advance or reverse straight only when the output rotational speeds of the pair of the HSTs are equal. Thus, equalization of the output speed of each of the HSTs is required. This equalization takes a long time during operation of the vehicle. Additionally, accuracy is required when manufacturing and assembling the components of each HST. If there is any difference in volume of the hydraulic pumps or motors between the HSTs, the feeling in turning left and right are different from each other, thereby making the vehicle difficult to control. 
     Furthermore, a speed changing operating tool, such as a speed change pedal, is connected to both output speed changing members, such as a movable swash plates, of the pair of HSTs. A steering operating tool, such as a steering wheel, is also connected to both members. This arrangement requires complicated linkages interposed between the speed changing operating tool and the both speed changing members and between the steering operating tool and both speed changing members, thereby increasing the manufacturing cost of the linkages and the space for disposal thereof in the vehicle. 
     If one HST is provided for speed change in traveling and another is provided for steering, each of the operating tools for speed changing and steering need only be connected with one output speed changing member of either of the HSTs, thereby simplifying the linkages between the operating tools and the speed changing members and reducing the manufacturing cost and the space required for disposal thereof. 
     However, the transmission for speed changing and steering must be capable of turning along a small radius if it is employed by a working vehicle like a mower tractor required to travel among trees. It also must be compact if it is provided for a small vehicle. 
     Furthermore, it is required to be capable of being operated for speed changing and steering as easily and sensitively as a passenger car. 
     It is also desirable that the transmission&#39;s activity in turning corresponds to the traveling speed to improve the sensitivity of the steering operation. Moreover, it is desired for safety and efficiency of work that when the same degree of steering operation is employed, the vehicle provided with the transmission turns along a small radius when traveling slowly and turns along a large radius when traveling fast. 
     SUMMARY OF THE INVENTION 
     A transmission for steering and speed changing of a vehicle in accordance with the present invention comprises a first HST including a hydraulic pump and a hydraulic motor for speed changing for advancing and reversing and a second HST including a hydraulic pump and a hydraulic motor for steering. The first HST is driven by a prime mover. Output power of the first HST is transmitted into a pair of axles through a differential and is also transmitted into the second HST. 
     Since the transmission is so constructed, the pair of HSTs can be driven by the single prime mover, so that only one input means, for example an input shaft, of the first HST is needed to receive the output power of the prime mover. Also, since speed changing is provided only by the single first HST, and steering is provided by only the single second HST, the oil circuits thereof can be simplified and the manufacturing cost can be reduced. The transmission does not need complicated equalization of output power between the two HSTs when advancing or reversing in a straight line as in conventional dual HST arrangements. Moreover, since the wheels connected to the axles are used for both traveling and steering, the vehicle can employ simple follower wheels such as casters, thereby having a small turning radius and reducing the manufacturing cost. 
     For braking the axles, a brake device can be provided on any rotational member in the transmitting system for speed changing in traveling between an input side of the first HST and the axles. This provides flexibility in choosing the optimal position for placement of the brake device in order to provide a compact transmission. 
     An output speed changing member of the first HST is connected with a speed changing operating tool provided on a vehicle, so that the rotary speed of the motor of the first HST is steplessly changeable by operation of the tool, whereby the traveling speed of the vehicle can be steplessly controlled. Also, the connection between the speed changing operating tool and the output speed changing means is simpler in comparison with the conventional connection between a steering operating tool and a pair of output speed changing members of two HSTs. 
     Regarding the transmitting system for steering, the second HST driven by the first HST has a pair of output means, which rotate in opposite directions by output of the motor of the second HST. The pair of output means are drivingly connected with the pair of axles. An output speed changing member of the second HST is connected with a steering operating tool provided on the vehicle, so that the rotary speed of the motor of the second HST is steplessly changeable by operation of the steering operating tool. The connection between the steering operating tool and the output speed changing means is simpler in comparison with the conventional connection between a steering operating tool and a pair of output speed changing members of two HSTs. 
     In this construction, when the steering operating tool is operated for turning, the motor of the second HST is driven so as to rotate the output means of the second HST in opposite directions. Accordingly, one of the axles is accelerated and the other is decelerated, so that the vehicle turns to the side of the decelerated axle. 
     Since the rotational direction of the pump of the second HST is reversed according to reversing the motor of the first HST when the vehicle travels in reverse, the turning direction of the vehicle can coincide with the same direction of operation of the steering operating tool in both cases of advancing and reversing. This arrangement therefore does not require a mechanism for coinciding the turning direction of the vehicle with the operational direction of the steering operating tool when in reverse. Thus, the vehicle employing the transmission can be steered as easily as a passenger car. 
     Also, since the rotary speed of the pump of the second HST is increased in proportion to that of the motor of the first HST, the replication of turning to the steering operation can be more sensitive in proportion to traveling speed. Thus, the vehicle can be nicely steered because it is prevented from delay in turning in relation to the steering operation. Moreover, the pump of the second HST for steering is stopped when the vehicle is stopped because the second HST is driven by output power of the first HST for speed changing. Thus, the vehicle is safe from unexpected start even if an operator touches the steering operating tool on the stopped vehicle. 
     With regard to the differential connected with the axles, it may constitute a pair of planetary gears. In this case, the output means of the second HST is drivingly connected with the pair of planetary gears. Due to this construction, the pair of planetary of gears as the differential can be narrowed and made more compact because of omission of a differential casing. 
     With regard to the connection between the steering operating tool and the output speed changing member of the second HST, it may be constructed such that the member is moved at a small rate of speed relative to an operational degree of the tool when the member is in vicinity of its neutral position. This construction allows the vehicle to travel straight even if the steering operating tool is slightly moved from its straight traveling position, and it turns gradually on steering of the steering operation for turning. 
     For the purpose of providing a changeable turning circle in correspondence to traveling speed, the transmission employs either of the following two constructions. 
     A first linkage, which has a pair of fixed pivotal points and an intermediate movable pivotal point between the fixed pivotal points, is interposed between the steering operating tool and the output speed changing member of the second HST. A second linkage is interposed between the speed changing operating tool and the movable pivotal point. The first linkage is bendable at the movable pivotal point thereof in correspondence to the steering operation. The second linkage moves the movable pivotal point in correspondence to the speed changing operation so as to change a ratio of a distance between one fixed pivotal point and the movable pivotal point to that between the other fixed pivotal point and the movable pivotal point, whereby the output rotational speed of the second HST is reduced in inverse proportion to the traveling speed. 
     Alternatively, the pump of the first HST is of variable capacity type and the motor thereof is of fixed capacity type and both of the pump and the motor of the second HST are of variably capacity types. A first capacity changing means of the pump of the first HST is connected with a second capacity changing means of the motor of the second HST, so that the rotary speed of the motor is reduced in inverse proportion to that of the pump. 
     In either construction, when the steering operating tool is operated to the same degree, the turning radius of the vehicle is large when traveling fast and is small when traveling slowly. The vehicle is safe from sudden turning when traveling fast, and it can turn rapidly by small steering operation when traveling slowly. 
     With regard to disposal of the first and second HSTs in relation to the housing containing the differential, the axles and the gear trains between the differential and the axles therein, the HSTs may be provided outside the housing, thereby enabling the HSTs to be easily attached and removed or assembled and disassembled independently of the settled housing. 
     Alternatively, the first and second HST may be contained within the housing, thereby providing a more compact, lighter transmission with reduced manufacturing cost in comparison with that provided with other housings for the HSTs. 
     A third option is to provide either of the HSTs outside the housing and the other within the housing, whereby the transmission can be constructed easily and cheaply by using a common axle driving apparatus comprising an HST, a differential and axles contained in a housing and using another common HST for the externally provided HST. 
     These and other objects, features and advantages of the invention will become more apparent in the detailed description and examples which follow. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 is a side view showing a mower tractor as a whole provided with a transmission for steering and speed changing in accordance with the present invention. 
     FIG. 2 is a perspective view of the transmission in accordance with a first embodiment of the present invention, provided on the mower tractor when a housing is removed. 
     FIG. 3 is a side view of the same transmission. 
     FIG. 4 is a diagram of a power transmitting system of the same transmission. 
     FIG. 5 is a diagram of the same system wherein differentials are modified in accordance with a second embodiment of the present invention. 
     FIG. 6 is a perspective view of a linkage interposed between a steering wheel and a control arm connected to a movable swash plate of a hydraulic pump for steering. 
     FIG. 7 is a plan view of the same. 
     FIG. 8 is a graph showing the relationship between a position of a slide link and a steering angle of the steering wheel in the same linkage. 
     FIG. 9 is a schematic view of a linkage interposed between movable swash plates of a hydraulic pump for speed changing and of a hydraulic motor for steering. 
     FIG. 10 is a schematic view of the same in advancing. 
     FIG. 11 is a schematic view of the same in reversing. 
     FIG. 12 is a view of a linkage having a bendable point interposed between the steering wheel and the movable swash plate of the hydraulic pump for steering, wherein the bendable point is moved according to motion of another linkage connected to the speed change pedal. 
     FIG. 13 is a sectional view looking in the direction of the arrows X—X in FIG.  12 . 
     FIG. 14 is a diagram of a power transmitting system of a transmission in accordance with a third embodiment of the present invention. 
     FIG. 15 is a diagram of the same wherein a construction for driving a charge pump is modified. 
     FIG. 16 is a diagram of a power transmitting system of a transmission in accordance with a fourth embodiment of the present invention. 
     FIG. 17 is a diagram of a power transmitting system of a transmission in accordance with a fifth embodiment of the present invention. 
     FIG. 18 is a diagram of a power transmitting system of a transmission in accordance with a sixth embodiment of the present invention. 
     FIG. 19 is a diagram of a power transmitting system of a transmission in accordance with a seventh embodiment of the present invention. 
     FIG. 20 is a diagram of a power transmitting system of a transmission in accordance with an eighth embodiment of the present invention. 
     FIG. 21 is a diagram of a power transmitting system of a transmission in accordance with a ninth embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     At first, a general explanation will be given on a construction of a mower tractor  1  provided with a transmission  2  of the present invention in accordance with FIG.  1 . 
     A front column  13  is provided upright on a front portion of a vehicle chassis  12 . A steering wheel  14  as a steering operating tool projects upwardly from column  13 . A speed change pedal  15  as a speed changing operating tool and brake pedals (not shown) are disposed beside column  13 . One or more caster wheels as follower front wheels  16  are disposed on the front lower portion of chassis  12 . 
     A seat  17  is mounted on a center portion of chassis  12 . A mower  9  is disposed below chassis  12 . Mower  9  comprises a casing  19  with at least one rotary blade provided therein, which is driven by power of an engine  11  through pulleys, belts and the like. Mower  9  is suspended at front and rear portions of its casing  19  by linkages and is vertically moveable. 
     Engine  11  is mounted on a rear portion of chassis  12  and is covered with a bonnet. Engine  11  has a vertically axial crankshaft provided with a vertically downward projecting output shaft  11   a . An output pulley  20  is fixed onto a lower end of output shaft  11   a.    
     Transmission  2  of the present invention is disposed on the lower rear portion of chassis  12 . As shown in FIGS. 2,  3  and  4 , a housing  25  contains therein transmission  2 , which comprises a first HST  21  for speed changing in advancing and reversing, a second HST  22  for steering, a first differential  24  for speed changing in traveling and a second differential  23  for steering. Housing  25  is fixed to chassis  12 . Each of HSTs  21  and  22  is an assembly of a hydraulic pump and a hydraulic motor fluidly connected with each other. 
     An input shaft  26  projects outward from a substantial center of an upper surface of housing  25 , so as to be fixedly provided thereon with an input pulley  27  and a cooling fan  29 . A belt is interposed between input pulley  27  and output pulley  20  of engine  11  and a tension pulley (not shown) is pressed against the belt, thereby transmitting the power of engine  11  from pulley  20  to the pulley  27 . 
     A pulley  31  is also fixed onto input shaft  26 . At a longitudinally intermediate portion of chassis  12  is erected a support boss  32 . A counter shaft  33  passes through boss  32  and is fixedly provided on the upper and lower ends thereof with pulleys  34  and  35 , respectively, for driving mower  9 . A belt  36  is interposed between pulleys  34  and  31 , and a belt  37  is interposed between pulley  35  and a pulley  38  fixed onto an input shaft of mower  9 . A tension pulley as a belt tension clutch may be also disposed between pulleys  35  and  38 , so as to switch on and off mower  9 . 
     Next, referring to FIGS. 2,  3  and  4 , the internal mechanism of housing  25  as transmission  2  comprising HSTs  21  and  22  and differentials  24  and  23  will be described in accordance with a first embodiment of the present invention. 
     First HST  21  for speed change in traveling is a well-known combination of a variable capacity type hydraulic pump  52  and a fixed capacity type hydraulic motor  53 . A pump shaft of hydraulic pump  52  constitutes input shaft  26  vertically supported in housing  25 . Hydraulic pump  52  is fitted onto a horizontal portion of a center section  51  fixed to the interior of housing  25 . Hydraulic motor  53  is fitted to a vertical portion of center section  51  behind the horizontal portion thereof. A motor shaft  54  of hydraulic motor  53  is laterally axially disposed. Hydraulic pump  52  and hydraulic motor  53  are fluidly connected with each other through a closed circuit in center section  51 . 
     Hydraulic pump  52  is provided with a movable swash plate  57  as an output speed changing means of first HST  21  or as a capacity changing means of hydraulic pump  52 , thereby changing the discharge direction and volume of pressure oil from hydraulic pump  52 . A control shaft  59  is connected to the swash plate  57 . A neutral biasing spring is wound around control shaft  59  in housing  25 , thereby biasing shaft  59  toward its neutral position. The neutral position thereof is movable and adjustable. As shown in FIG. 1, control shaft  59  is connected to a speed changing operating tool like a lever or a pedal (in this embodiment, a speed change pedal  15 ) through an arm  41 , a link  42  and the like. Pedal  15  is pivoted at its center, thereby looking like a seesaw. 
     Hydraulic motor  53  is rotated in one direction for advancing of a vehicle by treading on a front portion of pedal  15  and is rotated in the opposite direction for reversing of the vehicle by treading on a rear portion thereof. Whichever portion is trod, the rotary speed of hydraulic motor  53  is increased in proportion to the depth pedal  15  is trod. As a result, swash plate  57  is slantingly operated by treading on pedal  15 , so that the pressure oil discharged from hydraulic pump  52  is adjusted in direction and volume and is supplied into hydraulic motor  53  through the closed circuit within center section  51 . 
     Speed change pedal  15  may also be divided into two pedals for advancing and reversing. Anyway, the construction of the speed changing operating tool is not restricted to such a pedal  15 . 
     A traveling drive gear  55  and a steering drive gear  54  are fixedly provided onto one side of motor shaft  54  and a brake  66  is provided onto the other side of motor shaft  54 . Brake  66  is applied by operation of the above said brake pedal or the like, thereby braking axles to a stop. Its position is not limited to the position described above. It can be disposed on any rotary member in the traveling drive system between input shaft  26  of first HST  21  and the axles. 
     Regarding first differential  24 , a shaft  44  is disposed between a pair of first differential output shafts  40  (left and right first differential output shafts  40 L and  40 R). Shafts  40  constitute left and right axles in this embodiment. Shaft  44  is disposed so as to have the same axis with shafts  40 . A center gear  60  is fixed onto shaft  44  so as to engage with drive gear  55 . A pair of sun gears  61  are fixed respectively onto both end portions of shaft  44  with center gear  60  between them. A pair of carriers  62  are fixed respectively onto inner sides of shafts  40 . A plurality of planet gears  63  are pivoted on each carrier  62  so as to engage with the periphery of each sun gear  61 . A pair of large diametric gears  65  (left and right large diametric gears  65 L and  65 R) centering sun gears  61  are disposed at both sides of center gear  60 . Each gear  65  integrally forms a ring gear  64  on its inner periphery. Each ring gear  64  peripherally engages with every planet gear  63  surrounding each sun gear  61 . In this way, there are constructed a pair of planetary gears connected to shafts  40 . 
     With respect to a vehicle employing the conventional lateral arrangement of a pair of HSTs, left and right driving axles are individually driven by the pair of HSTs, whereby the both outputs of the HSTs are necessarily equalized exactly for straight traveling. A vehicle employing the transmission of the present invention, on the other hand, has differential output shafts  40 L and  40 R differentially connected with each other through first differential  24  as left and right axles, thereby requiring no equalization of output between two HSTs. 
     In second HST  22  for steering, an input gear  67  fixed onto an input shaft  70  as a pump shaft of a hydraulic pump  71  engages with the steering drive gear  56 . The shaft  70  is laterally axially disposed in parallel to shafts  40 . Both hydraulic pump  71  and a hydraulic motor  72  of HST  22  are variable capacity types. They are fluidly connected with each other through a closed circuit within a center section  69 . Hydraulic pump  71  is provided with a movable swash plate  76  as an output speed changing member of HST  22  or as a capacity changing means of hydraulic pump  71 . It is connected with steering wheel  14  through a linkage or the like, so as to be rotated in correspondence to rotating operation of steering wheel  14 . 
     Referring now to FIGS. 6 and 7, there is shown one embodiment of a linkage between steering wheel  14  and swash plate  76 . This is so constructed as to reduce the sensitivity of replication of swash plate  76  to the steering operation when steering wheel  14  is in vicinity of its straight traveling position, thereby providing a more stable operation of the vehicle when steering wheel  14  is slightly rotated for turning. 
     A cam plate  144  centering a stem  14   a  of steering wheel  14  is provided with a cam groove  144   a . Cam groove  144   a  is curved such that one end thereof is close to stem  14   a  and the other is far from it. A cam follower  145   a  stuck to a slide link  145  is movably inserted into the groove  144   a . The link  145  passes slidably through a slide guide  146 , thereby being slidable in a direction approximately coinciding with a rotational range of a control arm  139 , which is integrally extended from swash plate  76 . A connecting rod  147  is interposed between link  145  and arm  139 , so as to be rotatable according to rotation of arm  139 . 
     In this construction, cam groove  144   a  is so designed in shape that link  145  slides at a small rate to a unit rotational angle of steering wheel  14  when steering wheel  14  (cam plate  144 ) is in vicinity of its straight traveling position. As shown in FIG. 7, the straight traveling position of cam follower  145   a  is designated as 0. The position of cam follower  145   a  moves from 0 to +3 through +1 and +2 in proportion to the leftward rotational angle of steering wheel  14 . Also, it moves from 0 to −3 through −1 and −2 in proportion to the rightward rotational angle of steering wheel  14 . As shown in FIGS. 7 and 8, while cam follower  145   a  is moved between 0 and +1 or between 0 and −1, the increase of the leftward or rightward rotational angle of steering wheel  14  is θ 1 . It is larger than the rotational angle of θ 2  in case of cam follower  145   a  moving between +1 and +2 or between −1 and −2, and that of θ 3  in case of moving between +2 and +3 or between −2 and −3 (θ 1  &gt;θ 2 , θ 3 ). 
     Accordingly, when steering wheel  14  is rotated in its rotational range of left or right turning spaced from the straight traveling position, link  145  slides at a large rate in relation to the rotation thereof, thereby moving the swash plate  76  sensitively. On the other hand, when steering wheel  14  is rotated in vicinity of its straight traveling position, link  145  and swash plate  70  are moved gradually. In other words, swash plate  76  scarcely moves if steering wheel  14  is rotated a little from its straight position, thereby enabling the straight traveling course of the vehicle to remain stable. 
     Now regarding second HST  22  for steering, a motor shaft  73  of hydraulic motor  72  is vertically supported so as to project downward from center section  69 . A bevel gear  74  is fixed onto the lower end of shaft  73 . A pair of side gears  75  (left and right side gears  75 L and  75 R) engage respectively with both sides of gear  74  for the purpose of forming two power transmitting courses. Accordingly, pair of side gears  75  tend to rotate in opposite directions with respect to each other by rotation of shaft  73 . Each gear  75  is fixed onto each of a pair of second differential output shafts  77  (left and right second differential output shafts  77 L and  77 R). A pair of small diametric gears  78  are fixed onto outer ends of shafts  77 . A pair of double gears  86 , each of which consists of a large diametric gear  86   a  and a small diametric gear  86   b , are rotatably provided on shaft  54 . Gear  86   a  engages with small diametric gear  78  and gear  86   b  engages with large diametric gear  65 . 
     In the aforementioned transmission  2  of the first embodiment as shown in FIGS. 2,  3  and  4 , a movable swash plate  79  of motor  72  of second HST  22  is connected with swash plate  57  of hydraulic pump  52  of first HST  21  through a linkage for the purpose of corresponding the turning radius of the vehicle to the traveling speed. This linkage is best shown in FIG.  9 . In this regard, a rhomb control arm  80  is fixed at its center onto control shaft  59 . Control links  81  and  82  are pivoted at their one ends onto side ends of control arm  80 . The other ends of control links  81  and  82  are movably anchored to a fan-shaped control plate  83  for rotation of swash plate  79 . 
     In this regard, control plate  83  is fitted at a projection on an apex thereof to the plate  79  and is fixed at the center thereof onto control shaft  84 . It is provided at the portion near the arcuate edge thereof with a pair of arcuate holes  83   a  and  83   b . The above said other ends of links  81  and  82  are slidably inserted respectively into holes  83   a  and  83   b . A coiling spring  85  is wound around shaft  84  with one end thereof fitted to plate  83 , whereby the swash plate  79  is biased toward its neutral position or the slanting angle thereof tends to a minimum. 
     Explanation will be now given to the operation of transmission  2 , including the linkage shown in FIG. 9, according to a first embodiment of the present invention. 
     While engine  11  is on, input shaft  26  always rotates. When steering wheel  14  is set to its straight traveling position, second HST  22  is neutral. In this state, swash plate  57  of hydraulic pump  52  of first HST  21  rotates in accordance with treading on pedal  15  forward or rearward, thereby driving hydraulic motor  53  of first HST  21 . The rotational power of motor  53  is transmitted into first differential output shafts  40  as axles through traveling drive gear  55 , center gear  60  and first differential  24 , thereby enabling the vehicle to advance or reverse in a straight line. 
     During such advancing or reversing, power is transmitted from motor shaft  54  into input shaft  70  of second HST  22  through gears  56  and  67 , whereby hydraulic pump  71  of second HST  22  is driven. The volume per minute of pressure oil discharged from hydraulic pump  71  is increased in proportion to the rotational speed of shaft  54 , that is, the traveling speed of the vehicle. Accordingly, if the advancing or reversing vehicle turns by operation of steering wheel  14 , the replication of turning thereof is more sensitive in proportion to the traveling speed, whereby an operator can obtain a good feeling of steering. 
     If steering wheel  14  is operated for turning while pedal  15  is trod for advancing, swash plate  76  of hydraulic pump  71  is rotated so as to drive hydraulic motor  72 . When steering wheel  14  is in right turning operation, or is rotated rightward, motor shaft  73  of hydraulic motor  72  is regularly rotated, thereby rotating left side gear  75 L regularly and right side gear  75 R reversely through bevel gear  74 . The regular rotation of gear  75 L accelerates left large diametric gear  65 L of the left planetary gear, which rotates together with left differential output shaft  40 L in advancing rotation through planet gears  63 . The reverse rotation of gear  75 R decelerates the right planetary gear regularly rotating together with right differential output shaft  40 R. Thus, the vehicle turns right. 
     When steering wheel  14  is in left turning operation, swash plate  76  is rotated to the opposite side, thereby rotating shaft  73  reversely. Thus, gear  75 L is reversely rotated and gear  75 R is regularly rotated, so that the rotation of shaft  40 L is decelerated and that of  40 R is accelerated, whereby the vehicle turns left. 
     When steering wheel  14  is in right turning operation while pedal  15  is trod for reversing, swash plate  57  is in its reversing range, whereby shaft  54  is rotated in reverse direction so as to rotate the shafts  40 L and  40 R reversely. Also, input shaft  70  is reversely rotated, whereby gear  75 L is reversely rotated and gear  75 R is regularly rotated. Thus, the reverse rotation of the left planetary gear are accelerated together with shaft  40 L and the regular rotation of the right planetary gear are decelerated together with shaft  40 R, whereby the vehicle turns right. 
     On the other hand, when steering wheel  14  is in left turning operation while pedal  15  is trod for reversing, gear  75 R is reversely rotated, so that the right planetary gear together with shaft  40 R is accelerated, whereby the vehicle turns left. 
     Thus, the reversing vehicle can also turn to a side in coincidence with a rotational direction of steering wheel  14 , so that it can be steered as easily as a passenger car. 
     When pedal  15  is neutral, hydraulic motor  53  for speed changing is not driven, so that hydraulic pump  71  for steering is not driven. Accordingly, even if the steering wheel  14  is rotated for turning, hydraulic motor  72  is still off, whereby first differential output shafts  40  are not accelerated or decelerated. The vehicle is thus prevented from moving, even if an operator on the stopped vehicle touches steering wheel  14 . 
     By means of the above mentioned linkage shown in FIG. 9, motor  72  for steering is decelerated in inverse proportion to the depth of tread on pedal  15 . Thus, the vehicle turns along a large turning radius when traveling fast, and it turns along a small one when traveling slowly, for easy and safe steering. 
     Explanation will be now given to the operation of this linkage. As shown in FIG. 10, when pedal  15  is trod forward for advancing, swash plate  57  is rotated so as to drive motor  53  in accordance with the rotation of control shaft  59 . Simultaneously, control arm  80  rotates so as to pull control link  81 . When trod pedal  15  reaches the predetermined depth, the end of link  81 , which is movably inserted into arcuate hole  83   a , comes to abut against an end of hole  83   a  of control plate  83 . If pedal  15  is trod more deeply, link  81  pulls the arcuate edge of plate  83  so as to rotate it, whereby swash plate  79  is rotated so as to increase its slanting angle in proportion to the depth of trod pedal  15 . 
     The strokes of pistons of hydraulic motor  72  are longer in proportion to the slanting angle of its swash plate  79 . When pedal  15  is trod slightly, swash plate  79  is slanted slightly, thereby shortening the strokes of the pistons. If the depth of trod pedal  15  is less than the predetermined degree, the end of control link  81  is freely slidable in arcuate hole  83   a , so that plate  83  stays, swash plate  79  is slant at the smallest angle and the strokes of the pistons of motor  72  are kept to a minimum. 
     Thus, when steering wheel  14  is rotated for turning to some degree so as to make hydraulic pump  71  discharge pressure oil into motor  72 , the rotary speed of motor shaft  73  is reduced in inverse proportion to the depth of trod pedal  15 . If motor shaft  73  is rotated slowly, the rotations of gears of second differential  23  for accelerating and decelerating the shafts  40  are also slow, so that the turning radius of the vehicle becomes large. As a result, when traveling fast, the vehicle can turn along a large turning radius, thereby improving safety by preventing sudden turning. On the other hand, when traveling slowly, the same steering angle turns the vehicle along a small turning radius, thereby enabling the vehicle to turn around swiftly and easily. 
     As shown in FIG. 11, when pedal  15  is trod rearward for reversing, arm  80  together with shaft  59  rotate in the direction opposite to that described above. In this case, arm  80  pulls control link  82 . An end of link  82  abuts against an end of arcuate hole  83   b  when trod pedal  15  reaches the predetermined depth. If pedal  15  is trod more deeply, link  82  pulls plate  83  and rotates it, so that the slanting angle of swash plate  79  is increased in proportion to the depth of trod pedal  15 , whereby the rotary speed of motor shaft  53  is reduced in inverse proportion to the same. Thus, similar to the above described operation when advancing, when utilizing the same steering angle, the vehicle turns along a large turning radius when traveling fast and turns along a small turning radius when traveling slowly. 
     A second embodiment for obtaining the similar relation between the traveling speed and the steering angle is shown in FIGS. 12 and 13. In this embodiment, hydraulic motor  72  for steering is a fixed capacity type, and two links are interposed between steering wheel  14  and swash plate  76  of hydraulic pump  71 , wherein a ratio of one link to the other is changeable in length corresponding to the depth of trod pedal  15 . 
     The base of stem  14   a  integral with steering wheel  14  interlocks with a Pitman  130 . An utmost end of Pitman  130  is pivotally connected with a rotary link  132  through a connecting rod  131 . Link  132  is pivoted at its intermediate portion around a pivotal shaft  133 . The portion of link  132  opposite to the utmost end of Pitman  130  with respect to shaft  133  is bored by a longitudinally long hole  132   a . A pivotal pin  134  slidably passes through hole  132   a.    
     Pin  134  also slidably passes through a long hole  135   a  longitudinally bored in a portion of a control link  135  close to its one end. The other end of link  135  is fixed onto a control shaft  137  rotatably supported by housing  25 . Control arm  139  projects from shaft  137  and engages with one end of swash plate  76 . 
     Thus, a first linkage consisting of links  132  and  135  has shaft  133  as a first fixed pivotal point, pin  134  as an intermediate movable pivotal point and shaft  137  as a second fixed pivotal point. The first linkage is bendable at the movable pivotal point thereof in accordance with the steering operation of steering wheel  14 . 
     Pin  134  still also slidably passes through a long hole  136   a  bored in a ratio changing link  136 . Link  136  is fixed at its one end onto a slide rod  140  axially slidably supported by a guide  141 , so that link  136  does not rotate around pin  134  but slides according to motion of pin  134  within hole  136   a . An utmost end of rod  140  is pivotally connected to one end of a bell crank  143  through a connecting rod  142 . Bell crank  143  is connected at its to pedal  15  through a link or the like. Thus, a second linkage comprising link  136  is extended from pedal  15  to the movable pivotal point as pin  134 . 
     Accordingly, when steering wheel  14  is rotated for turning, Pitman  130  is rotated so as to rotate link  132  around shaft  133  through rod  131 , whereby link  135  is rotated, through pin  134 . Accordingly, arm  139  is rotated around shaft  137 , so that swash plate  76  is slanted, thereby adjusting the discharge volume of hydraulic pump  71  similar to the above described first embodiment. 
     In this state, when pedal  15  is trod, hydraulic motor  53  of first HST  21  is driven so as to drive shafts  40 . Simultaneously, pedal  15  pushes or pulls one end of bell crank  143  so as to rotate it. The rotation of bell crank  143 , which is connected at the other end thereof with rod  142  as described above, moves link  139  toward shaft  133  through rod  140 . Accordingly, a ratio of a distance between shaft  133  and pin  134  to a distance between pin  134  and shaft  135  (in other words, a ratio of a distance between the first fixed pivotal point and the movable pivotal point to a distance between the movable pivotal point and the second fixed pivotal point) is reduced in inverse proportion to the depth to which pedal  15  is trod. 
     When link  132  (steering wheel  14 ) is rotated to some degree, the rotational angle of arm  139  and the slanting angle of swash plate  76  are reduced in inverse proportion to the depth of trod pedal  15 . Thus, at the same steering angle, the discharge volume from hydraulic pump  71  and the rotary speed of hydraulic motor  72  are small when traveling fast and are large when traveling slowly. As a result, the vehicle, which employs transmission  2  comprising hydraulic motor  72  of fixed capacity type together with the linkages shown in FIGS. 12 and 13, has a large turning radius when traveling fast and has a small one when traveling slowly, when steering wheel  14  is at the same steering angle. 
     The linkage shown in FIGS. 6 and 7 can be additionally adapted to transmission  2  employing the linkage shown in FIGS. 12 and 13. In this case, cam plate  144 , link  145 , guide  146  and rod  147  are interposed between stem  14   a  and the utmost end of link  132  instead of link  130  and rod  131 . In other words, the above first and second linkages comprising links  132 ,  135  and  136  shown in FIGS. 12 and 13 are interposed between rod  147  and arm  139  shown in FIG.  6 . 
     Referring to FIG. 5, there is shown transmission  2  in accordance with a second embodiment including a first differential  24 ′ and a second differential  23 ′ both comprising differential gear units. In this regard, motor shaft  54  of first HST  21  is directly axially connected with an input shaft of second HST  22 . Power of the traveling driver gear  55  fixedly provided on shaft  54  is transmitted into a differential ring gear  92  of differential  24 ′ through gears  90  and  91 . A plain gear  93  is fixed onto motor shaft  73  of second HST  22 . Power from gear  93  is transmitted into a differential ring gear  95  of differential  23 ′ through a double gear  94 . Double gear  94  is relatively rotatably provided on the right first differential output shaft or, as in this embodiment, on second differential  23 ′ for a more compact transmission  2 . It may be, however, provided on left shaft  40 L. Second differential  23 ′ is provided with a pair of second differential output shafts  96  (left and right second differential output shafts  96 L and  96 R). Gears  97 L and  97 R are fixed respectively onto shafts  96 L and  96 R, and gears  99 L and  99 R are fixed respectively onto shafts  40 L and  40 R. Gears  97 L and  99 L engage with each other through a reversing gear  98 , so that they tend in the same rotational direction. Gears  97 R and  99 R engage with each other directly, so that they rotated in opposite directions. Gear  98  may be also interposed between gears  97 R and  99 R. In this case, gears  97 L and  99 L engage with each other directly. 
     In this construction, when motor shaft  73  is rotated in one direction by turning operation of steering wheel  14  in advancing or reversing, one of shafts  40 L and  40 R is accelerated and the other is decelerated because of the differential rotations of shafts  96  and the interposition of reversing gear  98 , whereby the vehicle turns. 
     The linkage as shown in FIGS. 6 and 7 can be adapted to transmission  2  of this second embodiment. Transmission  2  can obtain replication of turning in correspondence to the steering angle by adaptation thereof. Also, the linkage shown in FIG. 9 can be adapted to transmission  2  of the same embodiment. Transmission  2  can obtain a changeable turning radius corresponding to the traveling speed by adaptation thereof. Instead of hydraulic motor  72  of variable capacity type and the linkage shown in FIG. 9, transmission  2  of the second embodiment may also employ hydraulic motor  72  of a fixed capacity type and the linkage shown in FIGS. 12 and 13. 
     FIGS. 14 through 21 show various embodiments of transmissions  2  comprising a pair of HSTs and a pair of differentials. Explanation will be now given to transmissions  2  of the various embodiments. 
     A third embodiment shown in FIG. 14 is a modified first embodiment. In this regard, each of first HST  21  and second HST  22  is so constructed that its hydraulic pump  52  or  71  and a hydraulic motor  53  or  72  are juxtaposed on its center section  51  or  69 . Power from motor shaft  54  of hydraulic motor  53  of first HST  21  is transmitted into shafts  40  through first differential  24  for driving driving wheels  43  (rear wheels) similar to the first embodiment. A transmitting gear  103  and a bevel gear  100  are fixedly provided on a transmitting shaft  102 . Steering drive gear  56  fixed on an end of shaft  54  engages with gear  103 . A bevel gear  101  is fixed onto input shaft  70  of hydraulic pump  71  of second HST  22  instead of input gear  67  so as to engage with gear  100 . Thus, power from shaft  54  is transmitted into shaft  70  through gears  56  and  103  and gears  100  and  101 . The transmitting system between second HST  22  and second differential  23  is similar with that of the first embodiment. 
     In FIG. 14, it will be also noted that first and second differentials  24  and  23  are contained in housing  25  and first and second HSTs  21  and  22  are disposed on an exterior of housing  25 . 
     A fourth embodiment shown in FIG. 15 is also modification of the first embodiment. In this regard, power from motor shaft  54  is transmitted into first differential  24  for driving wheels  43  through gears  55  and  60  similar to the first embodiment and is also transmitted into a transmitting gear  104  relatively rotatably provided on one of shafts  77  of second differential  23  through the same gears  55  and  60 . 
     Each of gears  78  fixed on each of shafts  77  of the fourth embodiment engages directly, whereas that of the first embodiment engages through double gear  86 , with each of the gears  65  of first differential  24  involving the pair of planetary gears. Furthermore, wheels  43  of the fourth embodiment are attached respectively to a pair of axles  107 , each of which engages with each of shafts  40  through speed reduction gears  105  and  106 , whereas each of wheels  43  of the first embodiment is directly attached to its respective shaft  40 . 
     As a result, the axle driving rotary speed is reduced after output from first differential  24  in the fourth embodiment, whereas it is reduced between shafts  77  and gears  65  of first differential  24  in the first embodiment. 
     In this embodiment, first and second HSTs  21  and  22  are supplied with pressure oil in common by a charge pump  124  which is driven by engine  11 . Reference numeral  123  designates a relief valve for setting a charging pressure of pump  124 . Pump  124  may be also driven by an additional electric motor  125 , as shown in FIG.  16 . 
     FIGS. 17 through 21 show modifications to the second embodiment, described above with respect to FIG.  5 . 
     A fifth embodiment shown in FIG. 17 details the disposal of first and second HSTs  21  and  22  of the second embodiment, wherein the oil circuit and the gear train are similar to those of the second embodiment. In this regard, HSTs  21  and  22  are laterally separately disposed with respect to traveling drive gear  55 . Center sections  51  and  69  are disposed at utmost outsides of transmission  2 , so that hydraulic motor  53  of first HST  21  and hydraulic pump  71  of second HST  22  face each other. Motor shaft  54  is disposed in common or along the same axis with the input shaft of hydraulic pump  71  of second HST. This arrangement allows a more compact transmission  2 . 
     In a sixth embodiment shown in FIG. 18, center sections  51  and  69  face each other. Motor shaft  54 , which is in common with the input shaft of hydraulic pump  71  of second HST  22 , passes through center section  51 . Traveling drive gear  55  fixed on shaft  54  directly engages with differential ring gear  92  of first differential  24 ′. Small diametric speed reduction gear  105  fixed on the outside portion of each first differential output shaft  40  engages with large diametric speed reduction gear  106  fixed on each axle  107 . 
     Motor shaft  73  of second HST  22  passes through center section  69  so as to project into the space between center sections  51  and  69 . Plain gear  93  fixed on shaft  73  directly engages with differential ring gear  95  of second differential  23 ′. One of gears  97  fixed on second differential output shafts  96 , directly engages with gear  106  on one of axles  107  and the other gear  97  engages with gear  106  on the other axle  107  through a reversing gear  98 . 
     Transmission  2  of this embodiment is compact as a whole because the speed reduction mechanism is simplified, gears  55  and  93  are disposed between the center sections  51  and  69  and first and second HSTs  21  and  22  are disposed so as to be surrounded by the pairs of shafts  40  and  96  and the pairs of gears  105  and  106 . 
     In accordance with a seventh embodiment shown in FIG. 19, transmission  2  is compact and mechanically simplified. The hydraulic pumps and hydraulic motors of both of first and second HSTs  21  and  22  are fitted onto a common center section  109 . In this embodiment, the hydraulic pump and hydraulic motor of the same HST are disposed respectively on both surfaces of the center section  109  opposite to each other. In particular, on one surface of the center section  109  are fitted hydraulic pump  52  of first HST  21  and hydraulic motor  72  of second HST  22 , and on the opposite surface of center section  109  are fitted hydraulic motor  53  of first HST  21  and hydraulic pump  71  of second HST  22 . Traveling drive gear  55  and Steering drive gear  56  are fixed on motor shaft  54  of hydraulic motor  53 . Traveling drive gear  55  directly engages with differential ring gear  92  of first differential  24 ′ and steering drive gear  56  engages with input gear  67  fixed on input gear  70  of hydraulic pump  71  on the same side with hydraulic motor  53 . Plain gear  93  fixed on motor shaft  73  directly engages differential ring gear  95  of second differential  23 ′. The construction for power transmitting to the axles  107  and the construction for steering are similar with those of the sixth embodiment. The HSTs  21  and  22  are surrounded by differential output shafts  40  and  96  and speed reduction gears  105  and  106 , thereby providing a compact transmission  2 . 
     Transmission  2  in accordance with an eighth embodiment, as shown in FIG. 20, is similar to that of the seventh embodiment in that first and second HSTs  21  and  22  are fitted to common center section  109  and are surrounded by shafts  40  and  96  and gears  105  and  106 . However, in this embodiment, hydraulic pump  52  and hydraulic motor  53  of first HST  21  are fitted on one surface of center section  109  and hydraulic pump  71  and hydraulic motor  72  of second HST  22  are fitted on the opposite surface of center section  109 . Motor shaft  54  is in common with the pump (input) shaft of second HST  22 , thereby omitting gears  56  and  67 . First differential  24 ′ between shaft  54  and shafts  40 , second differential  23 ′ between shaft  73  and shafts  96  and the speed reduction gears between shafts  96  and shafts  40  are similar to those of the seventh embodiment. 
     Referring to transmission  2  in accordance with a ninth embodiment shown in FIG. 21, first HST  21  and first differential  24 ′ for speed changing are contained in housing  25 . Second HST  22  and second differential  23 ′ for steering are disposed outside housing  25  or are attached to an exterior thereof. Sprockets and chains instead of gears are used for the power transmitting mechanism from HSTs  21  and  22  to differentials  24 ′ and  23 ′. 
     In housing  25 , traveling drive gear  55  fixed on motor shaft  54  of first HST  21  engages with differential ring gear  92  of differential  24 ′ through gears  90  and  91 . At the outside of housing  25 , a chain  111  is interposed between a sprocket  110  fixed on shaft  54  projecting from housing  25  and a sprocket  112  fixed on input shaft  71  projecting therefrom, whereby power is transmitted from first HST  21  to second HST  22 . 
     Outside housing  25 , motor shaft  73  of second HST  22  is drivingly connected with a decelerator  113 . A sprocket  115  is fixed onto an output shaft  114  of decelerator  113 . A sprocket  117  is fixed onto a differential casing of differential  23 ′. A chain  116  is interposed between sprockets  115  and  117 , thereby transmitting power from second HST  22  to differential  23 ′. 
     A pair of sprockets  119 L and  119 R are fixed onto outside portions of second differential output shafts  96 L and  96 R of differential  23 ′, and a pair of sprockets  120 L and  12 OR are fixed onto outside portions of first differential output shafts  40 L and  40 R. A chain  12 IR is interposed between sprockets  119 R and  120 R, so as to rotate both of them in the same direction. A sprocket  122  is rotatably disposed outside either sprocket  119 L or sprocket  120 L. If sprocket  122  is disposed outside sprocket  119 L, for example, sprocket  119 L is positioned between sprockets  120 L and  122 . A chain  121 L is interposed between sprocket  122  and the outside sprocket  119 L or  120 L. The other intermediate sprocket  120 L or  119 L engages with an intermediate portion of chain  121 L. Thus, sprockets  119 L and  120 L are rotated in opposite directions, whereby shaft  40 L is rotated in the opposite direction of shaft  96 L. Sprocket  122  may engage with the chain  121 R instead of the chain  121 L. In this case, left shafts  96 L and  40 L are rotated in the same direction and right shafts  96 R and  40 R are rotated in opposite directions. 
     As described above, transmission  2  of the a embodiment using the sprockets and chains has the same effect as using gear trains. 
     Referring to FIGS. 2,  11  and  18 , there are respectively shown three types arrangements of first and second HSTs  21  and  22  and first and second differentials  24  and  23  ( 24 ′ and  23 ′) in relation to housing  25 . In FIG. 2, HSTs  21  and  22  and differentials  24  and  23  ( 24 ′ and  23 ′) are contained in housing  25 . In FIG. 11, HSTs  21  and  22  are disposed outside housing  25  and differentials  24  and  23  ( 24 ′ and  23 ′) are contained in housing  25 . In FIG. 18, HST  21  and differential  24 ′ ( 24 ) for speed changing are contained in housing  25  and HST  22  and differential  23 ′ ( 23 ) for steering are disposed outside housing  25 . Each of transmissions  2  described above in the first through the ninth embodiments may employ any of the three arrangements described. 
     Additionally, contrary to the arrangement shown in FIG. 11, transmission  2  may be constructed such that HSTs  21  and  22  are contained in housing  25  and differentials  24  and  23  ( 24 ′ and  23 ′) are disposed outside. Also, contrary to the arrangement shown in FIG. 18, HST  21  and differential  24 ′ ( 24 ) for speed changing may be disposed outside housing  25  and HST  22  and differential  23 ′ ( 23 ) for steering may be contained in housing  25 . Otherwise, only one of HSTs  21  and  22  may be disposed outside the housing  25 , wherein the other of HSTs  21  or  22  and differentials  24  and  23  ( 24 ′ and  23 ′) are contained therein. 
     One skilled in the art would recognize that each of transmissions  2  between the third and the ninth embodiments shown in FIGS. 14 through 21 can employ the linkage shown in FIGS. 6 and 7, so as to make the sensitivity of replication of turning correspond to the steering angle. It will be also recognized that each of them can employ the linkage shown in FIG. 9 or that shown in FIGS. 12 and 13, so as to make the turning radius correspond to the traveling speed of the vehicle. Motors  72  of variable capacity type shown in FIGS. 15,  16  and  21  may be replaced for those of fixed capacity type, thereby employing the linkage shown in FIGS. 12 and 13. 
     The transmission of the present invention can be adapted to a working vehicle like a small mower tractor which needs to be compact, requires a small turning radius and needs to be operated for speed changing and steering as easily as a passenger car.