Abstract:
An axle driving apparatus which has a housing and a hydrostatic transmission therein separate from the housing with fluid conduits in a center section connected to hydrostatic rotatable cylinder blocks operatively connected to axle shafts, the improvement involving check valves associated with the fluid conducting conduits and including check valve balls to affect fluid flow through the conduits and being positioned to move in a vertical path in a direction at right angles to the axes of the axle shafts, and a movable bypass arm mounted in the housing and movable to open or close the fluid conduits.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of U.S. application Ser. No. 10/256,782 filed Sep. 27, 2002; now abandoned
     which is a continuation of U.S. application Ser. No. 09/412,448 filed Oct. 4, 1999; now abandoned   which is a continuation of U.S. application Ser. No. 09/298,676 filed Apr. 22, 1999; now U.S. Pat. No. 5,979,271;   which is a continuation of U.S. application Ser. No. 09/141,550 filed Aug. 28, 1998; now U.S. Pat. No. 5,921,151;   which is a continuation of U.S. application Ser. No. 08/658,618 filed Jun. 5, 1996; now U.S. Pat. No. 5,634,526;   which is a continuation of U.S. application Ser. No. 08/658,747 filed Jun. 5, 1996; now U.S. Pat. No. 5,593,000;   which is a continuation of U.S. application Ser. No. 08/658,535 filed Jun. 5, 1996; now U.S. Pat. No. 5,626,204;   which is a continuation of U.S. application Ser. No. 08/595,276 filed Feb. 1, 1996; now U.S. Pat. No. 5,802,931;   which is a continuation of U.S. application Ser. No. 08/354,525 filed Dec. 13, 1994; now U.S. Pat. No. 5,542,494;   which is a continuation of U.S. application Ser. No. 08/181,983 filed Jan. 14, 1994; now U.S. Pat. No. 5,513,717;   which is a continuation of U.S. application Ser. No. 08/182,769 filed Jan. 14, 1994; now U.S. Pat. No. 5,577,572;   which is a continuation of U.S. application Ser. No. 07/706,279 filed May 28, 1991; now U.S. Pat. No. 5,505,279;   which is a continuation of U.S. application Ser. No. 07/482,656 filed Feb. 21, 1990 (now abandoned),   which is a continuation of U.S. application Ser. No. 07/319,164 filed Mar. 3, 1989, now U.S. Pat. No. 4,903,545.   

   DESCRIPTION 
   1. Technical Field 
   This invention pertains to a center section for a hydrostatic transmission, with the hydrostatic transmission having particular utility as a component of an integrated hydrostatic transaxle. A transaxle of a type used in equipment, such as a lawn tractor, has gear reduction and axle components mounted in a housing providing a sump for lubricating oil. The disclosed center section is directed toward an economical integration of the hydrostatic transmission with the transaxle components in a common housing providing a common sump. 
   2. Background Art 
   Hydraulically driven equipment, such as a lawn tractor, have had transaxle structure mounted in a housing including a drive input connection, a gear reduction drive, and oppositely-extending differentially-connected axles, and a hydrostatic transmission is connected to the exterior of the housing whereby a drive output from the hydrostatic transmission connects to the drive input to the transaxle structure. 
   The known prior art structures have not integrated the hydrostatic transmission with the transaxle components in a common housing to provide a common sump and with the use of a unique center section between the hydraulic components of the hydrostatic transmission as disclosed herein. 
   A hydrostatic transmission has a pair of hydraulic displacement units with fluid connections therebetween. In a typical hydrostatic transmission, the hydraulic displacement units each have a rotatable cylinder block mounting a plurality of reciprocal pistons and with the piston-receiving chambers in the cylinder block communicating with ports for fluid flow to and from the piston-receiving chambers. Many different types of structure are known for achieving fluid communication between the arcuate ports associated with the pair of rotatable cylinder blocks. Such structure can be by means of tubing or by means of a structural section with fluid passages and positioned adjacent both rotatable cylinder blocks. This structural section can be either integral with a housing for the hydrostatic transmission or a separate component mountable between the hydraulic displacement units and separable from the housing. 
   A prior art hydrostatic transmission has a pair of hydraulic displacement units generally in side-by-side relation and with a rotatable cylinder block of each of the hydraulic displacement units being associated with a structural section having arcuate ports for association with both of the hydraulic displacement units. A pair of generally parallel straight passages, formed in the structural section intersect and communicate with the arcuate ports in pairs whereby there is fluid communication between a pair of arcuate ports associated one with each of the hydraulic displacement units. 
   The prior art also includes hydrostatic transmissions wherein the hydraulic displacement units are disposed at a selected fixed angle relative to each other whereby the axes of rotation of the rotatable cylinder blocks thereof are at an angle to each other and a structural section disposed therebetween has had a pair of faces at the selected angle whereby arcuate ports associated therewith may coact with the angularly-related cylinder blocks of the hydraulic displacement units. 
   DISCLOSURE OF THE INVENTION 
   The integrated hydrostatic transaxle disclosed herein has resulted from efforts to reduce the cost, size and weight of a transaxle package which has had a non-integrated relation between the housings for a hydrostatic transmission and the gear reduction, differential and axle components. Elimination of as much machining as possible contributes substantially to cost reduction. 
   A primary feature of the invention is to provide a one-piece, generally L-shaped center section for a hydrostatic transmission which is positionable in a housing and has first and second faces for coaction with rotatable cylinder blocks of a pair of hydraulic displacement units of the hydrostatic transmission and with the center section designed to require a minimal amount of machining to the body thereof with resultant maximum cost savings. 
   The lowest possible machining cost for the center section can be achieved by going to a casting process, such as die casting or the lost foam process. A casting process results in a more porous center section and, with passages therein having fluid at high pressure, it is important to assure that leakage from the center section shall not be a problem. 
   The one-piece generally L-shaped center section being separable from the housing for the hydrostatic transmission and mountable therein permits casting of the center section since leakage from a porous cast center section will leak into a sump defined by the housing for the hydrostatic transmission, rather than through a wall of the housing. 
   An object of the invention is to provide, in combination, a hydrostatic transmission comprising a pair of hydraulic displacement units each having a rotatable cylinder block with reciprocal pistons and a housing for the displacement units providing a fluid sump along with a unique, one-piece, generally L-shaped center section positionable in the housing to facilitate utilization of such a structure with drive components for a hydraulically-driven device all in a common housing having a common sump. 
   Additionally, the center section of the hydrostatic transmission is uniquely designed with passages in addition to first and second generally straight passages interconnecting the hydraulic displacement units to provide for mounting of bypass valves as well as delivery of make-up oil to the hydraulic circuit and provide for bleed of air from the hydraulic circuit during operation of the bypass valves. 
   A further object of the invention is to provide, in combination, a hydrostatic transmission comprising a pair of hydraulic displacement units each having a rotatable cylinder block with reciprocal pistons, and a housing for said displacement units providing a fluid sump, said rotatable cylinder blocks having their axes of rotation generally normal to each other, a one-piece generally L-shaped center section positionable in said housing and having first and second faces generally at right angles to each other, said center section being positioned to have said first face engage an end of one rotatable cylinder block and the second face engage an end of the other rotatable cylinder block, arcuate fluid ports at the face of each of said center section faces for coaction with a rotatable cylinder block, a first straight fluid passage in said center section connecting one of the ports at each face to define a pair of fluid communicating ports and terminating at one of said pair of ports, and a second straight fluid passage in said center section connecting another of the ports on each face to define a second pair of fluid communicating ports and terminating at one of the ports of said second pair. 
   Another feature of the invention is to provide an integrated hydrostatic transaxle having a common housing for a hydrostatic transmission and a pair of oppositely-extending, drivingly-connected axles to provide a common sump, with the hydrostatic transmission having the center section as described in the preceding paragraphs. Cost effectiveness is achieved by use of the common housing, common sump and one-piece center section whereby leakage from the hydrostatic transmission including from a fluid passage in the center section containing fluid pressure may reach the common sump at atmospheric pressure. This makes it possible to cast the center section and minimize costly machining even though the center section may be more porous. 
   An object of the invention is to provide an integrated hydrostatic transaxle having the structure referred to in the preceding paragraph. 
   Still another object of the invention is to have, in combination, a hydrostatic transmission comprising a pair of hydraulic displacement units each having a rotatable cylinder block with reciprocal pistons, and a housing for said displacement units providing a fluid sump, said rotatable cylinder blocks having their axes of rotation normal to each other, a one-piece L-shaped center section separate from said housing and having first and second faces at right angles to each other, said center section being positioned to have said first face engage an end of one rotatable cylinder block and the second face engage an end of the other rotatable cylinder block, each of said center section faces having arcuate fluid ports for coaction with a rotatable cylinder block, a first straight fluid passage in said center section connecting one of the ports on each face and terminating at one of said ports, a second straight fluid passage in said center section connecting another of the ports on each face and terminating at one of said ports, said center section being of material which may be sufficiently porous to permit leakage of high pressure fluid from whichever one of said straight fluid passages contains high pressure fluid with said leakage flowing to said fluid sump, and said center section having third and fourth straight fluid passages intersecting said first and second fluid passages, respectively, and opening to a surface of said center section opposite to one of the faces thereof for mounting of check valves. 
   Still another object of the invention is to have the combination as set forth in the preceding paragraph wherein said first and second fluid passages are generally parallel, said center section has a through bore extending perpendicular to and positioned between said first and second fluid passages, a fifth fluid passage extending generally parallel to and positioned between said first and second fluid passages and opening to said bore for delivery of make-up fluid to said bore, and a sixth fluid passage extending between the fifth fluid passage and a recess set back from the surface to which the third and fourth fluid passages open for communication with a source of filtered make-up fluid. 

   
     DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a side elevation view of the integrated hydrostatic transaxle, taken looking toward the left in  FIG. 2 ; 
       FIG. 2  is a plan view of the integrated hydrostatic transaxle, with parts broken away; 
       FIG. 3  is a vertical section, taken generally along the line  3 - 3  in  FIG. 2 , and on an enlarged scale; 
       FIG. 4  is a fragmentary section of the bottom part of the housing and structure related thereto, as shown generally along section  4 - 4  in  FIG. 3 ; 
       FIG. 5  is a fragmentary plan view of structure shown in  FIG. 2 ; 
       FIG. 6  is a fragmentary section, taken generally along the line  6 - 6  in  FIG. 5 ; 
       FIG. 7  is a vertical section of the center section, taken generally along the line  7 - 7  in  FIG. 8  and with check valve and bypass structure shown in association therewith; 
       FIG. 8  is a top view of the center section for the hydrostatic transmission; 
       FIG. 9  is a bottom view of the center section of the hydrostatic transmission; 
       FIG. 10  is a side elevation of the center section, looking toward the right side thereof, as shown in  FIG. 8 ; and 
       FIG. 11  is a vertical section of the center section, taken generally along the line  11 - 11  in  FIG. 8  and with the structure associated with the center section being omitted. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The integrated hydrostatic transaxle is shown generally in  FIGS. 1 to 3 . 
   The integrated hydrostatic transaxle has a common housing  10  for the components thereof. The common housing  10  is of two parts, with a top part  12  and a bottom part  14  which are joined together along a split line  16  which is disposed generally horizontal when the integrated hydrostatic transaxle is installed in operative position. The housing parts  12  and  14  are held in assembled relation by a series of bolts  18  extending through peripheral flanges of the top and bottom housing parts which abut at the split line  16 . 
   The shape of the housing parts in plan is shown in  FIG. 2  wherein a portion of the top housing part  12  is seen in the lower left part of the Figure and with the remainder thereof broken away to show the bottom housing part  14 . 
   The common housing  10  encloses a hydrostatic transmission having a pair of hydraulic displacement units, indicated generally at  20  and  22 , respectively, and also houses transaxle components, seen particularly in  FIG. 2 . The transaxle components include a pair of oppositely-extending axles  23  and  24  having ends extended beyond the bottom housing part for mounting of drive wheels (not shown) and their centerlines are coincident with the housing split line  16 . The bottom housing part  14  has bearings  25  and  26  at the outboard ends and thrust bearings  27  and  27   a  at the inboard ends of the axles for rotatable support thereof and with the axles being geared together through a differential, indicated generally at  28 . This differential includes bevel gears  29  and  30  at the inner end of the respective axles  23  and  24  with drive input gears thereto including a gear  31  which meshes with an output gear  32  of a gear reduction drive. The gear reduction drive has a drive input connection from the hydraulic displacement unit  22 , with the output shaft  35  ( FIG. 3 ) of the latter having a gear  36  which meshes with a gear  37 . The latter gear is rotatably fixed to a gear  38  which meshes with the previously-mentioned gear  32 . 
   A brake for the drive is mounted externally of the common housing  10  and associated with an end of the drive output shaft  35 , with this brake structure, including a brake  40 , a brake drum  41  and a brake cover  42 . 
   Each of the hydraulic displacement units  20  and  22  is shown in detail in  FIG. 3  and is of generally the same construction. The hydraulic displacement unit  20  has a rotatable cylinder block  45  connected by a splined connection  46  to a drive input shaft  47  having an internal end rotatable in a journal  47   a  positioned in a center section, indicated generally at  48 , of the hydrostatic transmission. The outboard end of the drive input shaft  47  is rotatably supported by the top housing part  12  by means of a bearing  49 . A lip seal  50  seals the shaft opening in the top housing part  12 . 
   The rotatable cylinder block  45  has a series of piston-receiving chambers, each of which movably mount a piston  51  of a relatively large diameter and with each of the pistons  51  being urged by an associated spring  52  into following engagement with a swashplate structure. The hydraulic displacement unit  20  has overcenter variable displacement, with this operation being achieved by angular adjustment of a swashplate  54  which, as well known in the art, can have its angle varied from the clockwise position shown in  FIG. 3  to an opposite extreme position in a known manner and by manually operable structure, not shown. The swashplate can pivot about a pivot axis in a counterclockwise direction and past a horizontal center position, as viewed in  FIG. 3 . The swashplate  54 , as known in the art, mounts a thrust plate  55  against which the pistons abut and a bearing and bearing guide structure rotatably support the thrust plate  55  relative to the body of the swashplate. 
   Each of the piston-receiving chambers has a passage  57  opening to a face of the rotatable cylinder block  45  for coaction with arcuate ports of the center section  48  which will be described subsequently. 
   The hydraulic displacement unit  22  is a fixed displacement unit and has a rotatable cylinder block  58  with a plurality of piston-receiving chambers each movably mounting a piston  59  which is spring-urged by a spring  60  toward a swashplate  61 . The swashplate  61  has a thrust plate  62  against which an end of the pistons engages and a ball thrust bearing  63  interposed between the thrust plate and the swashplate to rotatably mount the thrust plate. 
   The rotatable cylinder block  58  drives the drive output shaft  35  through a splined connection  64  therebetween. 
   An inner end of the drive output shaft  35  rotates within an opening  65  in the center section  48  which may optionally receive a journal  66  and, if the journal is not used, the opening  65  is cylindrical as shown in  FIG. 11 . The outboard end of the drive output shaft  35  is sealed by a lip seal  67  and with bearing structure disposed interiorly thereof including a ball bearing  68 . 
   Each of the piston-receiving chambers of the rotatable cylinder block  58  has a passage  69  opening to a face thereof which coact with arcuate ports associated with a face of the center section  48  to be subsequently described. 
   Since the hydraulic displacement unit  22  is of a fixed displacement, the swashplate  61  need not be adjustably mounted and, therefore, can be supported by the common housing  10  against hydraulic forces exerted through the pistons  59 . As seen in  FIG. 3 , the centerline of the drive output shaft  35  is located on the split line  16  of the housing parts  12  and  14  and extends through a central opening  69  in the swashplate  61 . The swashplate  61  spans the split line and support thereof against fluid forces is provided by the common housing at both sides of the split line. 
   The foregoing description generally describes the integrated hydrostatic transaxle wherein the bottom housing part  14  provides a common sump for the transaxle components as is evident in  FIGS. 1 and 2  and also for the hydrostatic transmission as is evident from  FIGS. 1 to 3 . 
   The hydraulic displacement units  20  and  22  have their respective rotatable cylinder blocks arranged with their axes of rotation generally at right angles to each other. It is the primary function of the center section  48  to provide communication between selected piston-receiving chambers of the respective cylinder blocks  45  and  58 . In achieving this primary function, center section  48  has been uniquely designed to minimize costly machining operations and enable formation of the body of the center section by casting. Examples of such casting, without limitation, are lost foam casting and die casting. The resulting material of the cast body of the center section has a relatively high degree of porosity as compared to a conventional machined center section for a hydrostatic transmission and in order to assure any leakage problem of high pressure fluid contained within a passage in the center section, because of porosity, is confined within the common housing, the center section  48  has been constructed as a separate one-piece center section which is positionable within the bottom housing part  14  as seen in  FIG. 3 . The one-piece center section  48  is generally L-shaped to have a pair of faces generally at right angles to each other with one planar face  72  coacting with a face of the rotatable cylinder block  45  of the variable displacement unit  20  and a second planar face  73  coacting with a face of the rotatable cylinder block  58  of the hydraulic displacement unit  22 . The center section body has two integral parts  74  and  75  oriented to have the two parts define the legs of the L shape of the center section, with the part  74  having the planar face  72  and the part  75  having the planar face  73 . 
   The planar face  72  has a pair of arcuate ports  76  and  77  and the planar face  73  has a pair of arcuate ports  78  and  79 , as seen in  FIGS. 8 and 10 , respectively. 
   First and second straight, generally parallel passages  80  and  81  are cast into the center section body and function to intersect the arcuate ports and place the arcuate ports in paired relation for fluid communication. The first passage  80  intersects with arcuate port  76  and arcuate port  78  to provide a first pair of ports in fluid communication. The second passage  81  intersects arcuate ports  77  and  79  and places them in paired fluid communication. 
   In operation of the integrated hydrostatic transaxle, one or the other of the first and second fluid passages functions to deliver fluid under pressure from the variable displacement unit  20  functioning as a pump to the fixed displacement unit  22 , functioning as a motor, and with the other fluid passage providing for return of fluid from the motor to the pump. The first and second fluid passages  80  and  81  terminate at one end at their intersection with the arcuate ports  78  and  79  and are closed at their other end as formed in the casting process. 
   The center section  48  has a third passage  84  intersecting said first passage  80  and a fourth passage  85  intersecting the second passage  81 , with the passages  84  and  85  opening to a surface  86  of the center section opposite to the planar face  72 . 
   A through bore  87  extends perpendicular to and is positioned between the first and second fluid passages  80  and  81  and a fifth fluid passage  88 , sealed intermediate its ends by journal  47   a,  extends generally parallel to the through bore  87  and is positioned between the first and second fluid passages  80  and  81 . A sixth fluid passage  90  extends between and normal to the fifth fluid passage  88  and a recess  91  in the center section set back from the surface  86  of the center section. 
   The utility of the through bore and third through sixth passages will be readily understood by reference to  FIGS. 3 to 7  and the following description. 
   The third and fourth fluid passages  84  and  85  mount a pair of check valves which each having a tubular seat member  93  and  94 , respectively, fitted therein and which form seats for a pair of check valve balls  95  and  96  spring-urged downwardly against the seats. The check valves function, when closed, to block fluid flow from either of the first and second passages  80  and  81  to a recess or well  100  ( FIG. 3 ) formed by a cavity in the bottom housing port  14 . This recess is generally oval and is defined by a continuous upstanding wall on the bottom housing part with wall sections shown at  101  and  102 . The lower ends of the third and fourth passages  84  and  85  open into this generally oval recess. The oval recess  100  is sealed off, at its top, by a generally oval-shaped wall  103  on the underside of the center section  48  and which has a sealing O-ring  104  therebetween. This is a sealed recess or well so that filtered fluid in the recess may be a source of make-up fluid to the hydrostatic transmission. Structure associated with the check valves also provides for a bypass function wherein, even though the pump is set at a displacement and is operable, there is no drive of the motor since the first and second passages  80  and  81  are cross-connected through opening of the check valves and the generally oval recess  100 . 
   The make-up fluid is delivered to the generally oval recess  100  from the common sump within the bottom housing part  14  by flow through an open space beneath the center section  48  ( FIG. 3 ) and through a cylindrical filter  110  having O-ring seals at its top and bottom. The interior of the filter  110  communicates with the sixth fluid passage  90  in the center section. As previously described, the sixth fluid passage  90  communicates with the fifth fluid passage  88  and the fifth fluid passage  88  communicates with the through bore  87  so that fluid reaches the recess  100 . 
   The center section has a series of through mounting holes at  115 ,  116 , and  117  whereby, as seen in  FIG. 3 , in assembly, the center section  48  can be secured to the upper housing part  12 , as by self-tapping screws  118  and the final assembly achieved by bringing the bottom housing part  14  into association with the top housing part  12  along the split line  16 . 
   All of the first through sixth fluid passages of the center section as well as the through bore  87 , recess  65 , recess  91  and through mounting holes  115 - 117  can be formed in the center section in a casting process. There is only a limited amount of machining required to finish the center section. As previously stated, a cast center section has a higher porosity than a conventional machined center section, which could create the possibility of leakage from whichever of the first and second passages  80  and  81  may have pressure fluid therein; however, the one-piece, integral center section which is independent of the housings avoids any problem from such leakage since such leakage would merely be into the common sump of the integrated hydrostatic transaxle and which is open to atmosphere through a bleed tube  140 . 
   The bypass operation previously referred to is effected by opening the check valves by raising the check valve balls  95  and  96  off their seats. The structure for this includes a bypass actuator structure including a bypass actuator plate  120  and a bypass rod  121 . The bypass actuator plate  120 , as seen in  FIGS. 4 and 7 , is positioned in the generally oval recess  100  in the bottom housing part  14  and, at its middle, is connected to the lower end of the bypass rod  121  and has a pair of upturned ends ( FIG. 7 ) positioned beneath the check valve balls  95  and  96 . Lifting of the bypass rod  121  causes the bypass actuator plate to lift the check valve balls and place the center section first and second passages  80  and  81  in fluid communication. Lifting of the bypass rod  121  is achieved by rotation of a handle  125  positioned above top housing part  12  and, as seen particularly in  FIGS. 2 ,  3  and  5 . The bypass rod  121  is longitudinally movable in an opening  126  in the top housing part  12  as well as having its lower part extending downwardly through the through bore  87  of the center section and is normally urged downwardly by a spring  127 . As seen in  FIG. 6 , the handle  125  has cam shapes  130  formed thereon which coact with ends of a through pin  131  fitted into an end of the bypass rod  121 . Rotation of the handle  125  from the position shown in the drawings to bring the cams  130  under the through pin  131  raises the through pin and the bypass rod  121  to establish the bypass operation. 
   The bypass rod  121  and center section  48  are uniquely associated with the housing structure whereby a bypass operation also results in bleeding air from the system fluid. When the bypass rod  121  is in its lower position and the check valves are closed, the upper end of the through bore  87  of the center section  48  is closed by a seal washer  135  backed up by peripheral flange on the bypass rod, so that there is no fluid communication between the through bore  87  and the interior of the common housing  10 . When the bypass rod  121  is raised to effect a bypass operation, the seal washer  135  is moved upwardly from its seat whereby the upper end of the through bore  87  is open to the interior of the common housing and air can bleed off to the housing interior. Air that accumulates in the common sump can bleed off to atmosphere through the bleed tube  140  ( FIG. 1 ). 
   It is believed that the operation of the integrated hydrostatic transaxle is clearly apparent from the foregoing description. However, it may be briefly summarized as follows. An engine drives the drive input shaft  47  for the variable displacement unit  20  (functioning as a pump) to cause operation of the displacement unit  22  (functioning as a motor) and the drive output shaft  35  drives the transaxle components shown in  FIG. 2  for rotation of the wheel axles  23  and  24 . The direction of rotation of the wheel axles can be shifted from forward to reverse by shifting the swashplate  54  of the variable displacement unit  20  to a position opposite side of center from that shown in  FIG. 3  and with resulting reversal of pressure fluid flow through the center section  48  from the pump to the motor. In the event there is to be no rotation of the wheel axles  23  and  24  while the pump is still operating and set for displacement, a bypass operation is achieved by rotation of the handle  125  to raise the bypass rod  121  and open the check valve balls  95  and  96 . As previously mentioned, any air in the passages in the center section can bleed to the sump of the common housing. Either one of the check valves can automatically open to provide make-up fluid to the transmission circuit from the generally oval recess  100  when the pressure existing in one or the other of the first and second straight passages  80  and  81  in the center section is sufficiently less than that of the fluid in the oval recess to overcome the spring closing force on a check valve ball.