Patent Publication Number: US-7708531-B2

Title: Axial piston device

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an axial piston device such as a pump unit or a motor unit. 
   2. Related Art 
   An axial piston device comprising a cylinder block rotated about an axis and a piston accommodated in a slidable manner in an axial direction with respect to the cylinder block while being rotated about the axis together with the cylinder block has been widely utilized as a pump unit to be used as a hydraulic source with respect to hydraulic equipment such as a hydraulic motor or as a motor unit to be hydraulically driven by a hydraulic source such as a hydraulic pump. 
   Hereinafter, description will be given of a conventional axial piston device by way of a pump unit. 
   A conventional pump unit comprises, for example, a housing which has a housing body opened at a first end thereof and a plate attached to the first end of the housing body, a pump shaft which is supported by the housing and is driven by a drive source, and a pump body which is accommodated inside the housing and is rotatably driven by the pump shaft, wherein each of a discharge port and a suction port of the pump body is hydraulically connected in circulation to a corresponding hydraulic device such as a hydraulic motor. 
   That is to say, a pair of oil passages communicating with the discharge port and the suction port of the pump body, respectively, is formed at the plate. Thus, pressurized oil is supplied from the pump body to the hydraulic device via one of the oil passages, and further, return oil is returned to the pump body from the hydraulic device via the other one of the oil passages. 
   In the pump unit after assembly, air is mixed inside the pair of oil passages; therefore, the pair of oil passages is required to be deaerated. 
   In other words, the pump unit and the hydraulic device are connected via the pair of oil passages, thereby forming a circulation circuit, wherein the circulation circuit is required to be sufficiently deaerated upon filling oil into the circulation circuit. 
   In regard to this point, in the conventional pump unit, a drain oil passage for allowing the pair of oil passages to communicate with an oil sump is formed at the plate, and further, a shutoff valve is disposed inside the drain oil passage in such a manner as to be positionally adjusted in an axial direction (see U.S. Pat. No. 6,332,393). 
   In particular, a valve seat is provided at the drain oil passage. The position of the shutoff valve in the axial direction can be adjusted in such a manner that the shutoff valve can take a shutoff position at which the shutoff valve is in contact with the valve seat so as to have the drain oil passage shut off and a communication position at which the shutoff valve is apart from the valve seat in the axial direction so as to have the drain oil passage communicated. 
   In this conventional pump unit, the pair of oil passages can communicate with or can be cut out of the oil sump by operating the shutoff valve, with an attendant problem of impossibility of speedy switching between the communication and shutoff. 
   Namely, in the conventional pump unit, the position of the shutoff valve in the axial direction can be adjusted with respect to the plate owing to screw connection. Consequently, in order to move the shutoff valve from the shutoff position to the communication position at which a sufficient opening width is secured, the shutoff valve must be rotated on an axis many times. 
   The present invention has been accomplished in view of the above prior art. Therefore, a primary object of the present invention is to provide an axial piston device in which an oil passage can be securely and speedily deaerated. 
   SUMMARY OF THE INVENTION 
   According to one aspect of the present invention, there is provided an axial piston device that includes a housing, a rotary shaft, a cylinder block, and a piston. 
   The housing includes a housing body opened at a first end thereof and a plate disposed at the first end of the housing body. The rotary shaft is rotatably supported about an axis by the housing body and the plate. The cylinder block is rotated together with the rotary shaft and is accommodated inside the housing. The piston is accommodated in the cylinder block in slidable a manner in an axial direction. 
   Furthermore, the plate is provided with a pair of first oil passages having first ends communicating with a discharge port and a suction port of the cylinder block and second ends opened to the surface of the plate, and a drain oil passage for allowing at least one of the first oil passages to communicate with an oil sump. The drain oil passage is provided with a rotary valve which can selectively switch a shutoff position at which the drain oil passage is shut off and a communication position at which the drain oil passage is communicated. 
   With this configuration, since the rotary valve switches the communication/shutoff of the drain oil passage, the pair of first oil passages can be deaerated remarkably speedily with ease. 
   Preferably, the axial piston device further includes a pump body to be driven by the rotary shaft; and a pair of second oil passages communicating with the discharge port and the suction port of the pump body, respectively. And the axial piston device is configured that at least a part of oil, which is supplied to a hydraulic device from the discharge port of said pump body via one of said second oil passages and is returned to the suction port of said pump body via the other one of said second oil passages, is introduced into the pair of first oil passages. 
   With this configuration, the pair of second oil passages can be deaerated by use of the rotary valve. Consequently, the pair of second oil passages can be deaerated remarkably speedily with ease without providing any additional member. 
   In one embodiment of the axial piston device, the housing is configured such that an inside space is used as said oil sump, and the drain oil passage has a first end opened to the inside space of said housing. 
   Preferably, the axial piston device according to the one embodiment further includes a valve plate to be interposed between the plate and the cylinder block. 
   The valve plate is configured to allow the discharge port and the suction port of the cylinder block to communicate with the pair of the first oil passages, respectively, and support the cylinder block in a rotatable manner about the pump shaft. 
   The drain oil passage has a groove formed at a surface located inside the housing inside space of the plate in such a manner as to be opened toward the valve plate. The groove extends outward in a radial direction beyond the valve plate in reference to the rotary shaft. 
   According to the preferred embodiment, the structure of the drain oil passage can be simplified. 
   In the various embodiment of the axial piston device, preferably, the first oil passages are arranged in a substantially linear manner substantially symmetrically with respect to each other in reference to the rotary shaft. And the drain oil passage has a single substantially linear cross oil passage for allowing the pair of first oil passages to communicate with each other. 
   According to this preferred embodiment, the structure of the drain oil passage can be simplified. 
   According to another aspect of the present invention, there is provided an axial piston device that includes a rotary shaft rotating about an axis; a cylinder block fitted around in a non-rotatable manner relative to the rotary shaft; a piston accommodated in the cylinder block in a freely advancing/retreating manner in an axial direction; and a plate having a contact face which is brought into contact with a discharge port and a suction port of the cylinder block. 
   The plate is provided with a pair of first oil passages having first ends opened to the contact face in such a manner as to communicate with the discharge port and the suction port of the cylinder block, respectively, and second ends opened to the surface of the plate, and a drain oil passage for allowing at least one of the first oil passages to communicate with an oil sump. 
   The drain oil passage is provided with a rotary valve that can selectively switch a shutoff position at which the drain oil passage is shut off and a communication position at which the drain oil passage is communicated. 
   Preferably, the axial piston device according to another aspect further includes a housing surrounding the cylinder block. The housing is configured such that an inside space thereof is used as the oil sump. 
   In one embodiment, the housing is configured in such a manner as to surround the plate in addition to the cylinder block, and the second ends of the pair of first oil passages are fluid-connected to a conduit member supported by the housing astride inward and outward of the housing. 
   In another embodiment, the axial piston device further includes a housing body having an opening formed at a first end thereof. The housing body is configured to surround the cylinder block. The plate is configured in such a manner as to be connected to the housing body so as to close the opening formed at the first end of the housing body. The housing body and the plate constitute the housing. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above, and other objects, features and advantages of the present invention will become apparent from the detailed description thereof in conjunction with the accompanying drawings wherein. 
       FIG. 1  is a longitudinal cross-sectional view showing an axial piston unit according to a first embodiment of the present invention. 
       FIG. 2  is a cross-sectional view taken along a line II-II of  FIG. 1 . 
       FIG. 3  is a cross-sectional view taken along a line III-III of  FIG. 1 . 
       FIG. 4  is a view as viewed from an arrow IV of  FIG. 2 . 
       FIG. 5  is a view as viewed from an arrow V of  FIG. 4 . 
       FIG. 6  is a diagram illustrating a hydraulic circuit of the axial piston unit shown in  FIGS. 1-5 . 
       FIG. 7  is a perspective view showing a plate of the axial piston unit shown in  FIGS. 1-6 , as viewed from the inner surface 
       FIG. 8  is a cross-sectional view taken along a line VIII-VIII of  FIG. 1 . 
       FIG. 9  is a longitudinal cross-sectional view showing an axial piston unit according to a second embodiment of the present invention. 
       FIG. 10  is a cross-sectional view taken along a line X-X of  FIG. 9 . 
       FIG. 11  is a cross-sectional view taken along a line XI-XI of  FIG. 9 . 
       FIG. 12  is a view as viewed from an arrow XII of  FIG. 10 . 
       FIG. 13  is a cross-sectional view of a plate of a modified axial piston unit shown in  FIGS. 9-13 . 
       FIG. 14  is a laterally partial plan view showing an axial piston device according to a third embodiment of the present invention. 
       FIG. 15  is a cross-sectional view taken along a line XV-XV of  FIG. 14 . 
       FIG. 16  is a cross-sectional view taken along a line XVI-XVI of  FIG. 14 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Embodiment 1 
   Hereinafter, description will be given of an axial piston device according to a preferred embodiment of the present invention with reference to the attached drawings. 
   An axial piston device according to this embodiment is used as a pump unit, i.e., a hydraulic source with respect to hydraulic equipment such as a hydraulic motor. 
     FIG. 1  is a longitudinal cross-sectional view showing a pump unit  1  according to this embodiment. Furthermore,  FIGS. 2 and 3  are a cross-sectional view taken along a line II-II of  FIG. 1  and a cross-sectional view taken along a line III-III of  FIG. 1 , respectively. Moreover,  FIGS. 4 and 5  are views as viewed from an arrow IV of  FIG. 2  and an arrow V of  FIG. 4 , respectively. 
   As shown in  FIGS. 1 to 3 , the pump unit  1  according to this embodiment includes a housing  10 , a pump shaft  40  to be operatively driven by a drive source (not shown), and a first pump body  50  to be driven by the pump shaft  40 . 
   The housing  10  is configured in such a manner as to accommodate the first pump body  50  therein while rotatably supporting the pump shaft about an axis. 
   In this embodiment, the housing  10  has a hollow housing body  20  opened at a first end thereof, and a plate  30  disposed at the first end of the housing body  20 . 
   Here, in this embodiment, the housing body  20  is bottomed by closing a second end thereof. 
   Specifically, the housing body  20  is provided with a side wall  21  having a positioning boss for installing a pump body, and a circumferential wall  22  extending from the peripheral edge portion of the side wall  21  toward a direction of the pump shaft. 
   The plate  30  is preferably configured in such a manner as to liquid-tightly close an opening  20   a  at the first end of the housing body  20 , and therefore, an inside space  11  of the housing  10  can be used as an oil sump. 
   The pump shaft  40  is rotatably supported on an axis by the housing body  20  and the plate  30  in a state in which an input end extends outward in such a manner as to be operatively connected to the drive source. 
   In the pump shaft  40  in this embodiment, a first end  41  located upstream in a transmission direction (i.e., a right end in  FIGS. 1 and 2 ) extends outward of the side wall  21  of the housing body  20 , and further, a second end  42  located downstream in the transmission direction (i.e., a left end in  FIGS. 1 and 2 ) also extends outward of the plate  30 . 
   Incidentally, a second pump body  80 , described later, is supported at the second end  42  located downstream in the transmission direction of the pump shaft  40 . 
   The first pump body  50  is accommodated inside the housing  10  in such a state as to be freely driven by the pump shaft  40 . 
   The first pump body  50  in this embodiment is configured in a variable displacement type in which a suction/discharge oil rate can be varied according to a slanting position of an output adjusting member  53 . 
   In particular, the first pump body  50  includes a cylinder block  51  supported by the pump shaft  40  in a relatively non-rotatable manner, a piston  52  slidable in the pump shaft direction with respect to the cylinder block  51  while rotating on the pump shaft  40  together with the cylinder block  51 , and the output adjusting member  53 . 
   The output adjusting member  53  is provided with a movable swash plate  54  defining a sliding range in the pump shaft direction of the piston  52  according to a position of the piston unit  52  around the pump shaft  40 , a connecting arm  55  having a first end connected to the movable swash plate  54 , and a control shaft  56  supported by the housing  10  in a rotatable manner on the axis so as to have a first end connected to a second end of the connecting arm  55  and have a second end located outward of the housing  10 . 
   An operating arm  61  is connected to the second end of the control shaft  56 , and thus, the control shaft  56  is rotated on the axis by oscillating the operating arm  61  on the axis of the control shaft  56 . 
   As shown in  FIGS. 2 to 4 , the first pump body  50  in this embodiment includes a neutral position returning mechanism  60  for returning the movable swash plate  54  to a neutral position. 
   The neutral position returning mechanism  60  is provided with the operating arm  61 , a locking pin  62  disposed at a first end  61   a  of the operating arm  61 , a fixed pin  63  fixedly disposed at the housing  10 , and a coil spring  64  wound around the outer portion of the control shaft  56 . 
   A second end  61   b  of the operating arm  61  functions as an operating portion. That is to say, the control shaft  56  is rotated about its axis by oscillating the second end  61   b  of the operating arm  61  about the control shaft  56 , so that the movable swash plate  54  is slanted. 
   The coil spring  64  includes a central portion  64   a  wound around the outer portion of the control shaft  56 , and a first end  64   b  and a second end  64   c  extending from the central portion  64   a . The fixed pin  63  and the locking pin  62  are held between the first end  64   b  and the second end  64   c  of the coil spring  64 . 
   With this configuration, the fixed pin  63  is adapted to position the movable swash plate  54  at the neutral position in a state in which no operating force is applied to the operating arm  61  from the outside. In other words, the fixed pin  63  functions as a neutral position setting member defining the neutral position of the movable swash plate  54 . 
   Particularly, when the operating arm  61  is oscillated toward one side about the control shaft  56 , the movable swash plate  54  is oscillated in a corresponding direction according to the rotation of the control shaft  56  about the axis, and further; the locking pin  62  is also oscillated toward one side about the control shaft. 
   When the locking pin  62  is oscillated in the above manner, the coil spring  64  is oscillated at only the first end  64   b  toward one side about the control shaft  56  in a state in which the second end  64   c  is held by the fixed pin  63 , whereby the coil spring  64  retains its resiliency. 
   Therefore, when the operating force exerted on the operating arm  61  is released, the locking pin  62  and the operating arm  61  are returned to the neutral position by the resiliency retained by the coil spring  64 , and accordingly, the movable swash plate  54  is returned to the neutral position. 
   Preferably, the neutral position returning mechanism  60  may be configured such that the position of the fixed pin  63  can be adjusted relative to the axis position of the control shaft  56 . 
   In particular, the fixed pin  63  can have an eccentric structure. Namely, the fixed pin  63  can be configured to include a first portion  63   a , at which the position relative to the axial position of the control shaft  56  is made invariable, and a second portion  63   b , which is eccentric from the first portion  63   a  and is held between the first end  64   b  and the second end  64   c  of the coil spring  64 . 
   With this configuration, the position of the second portion  63   b  relative to the axial position of the control shaft  56  can be readily varied by rotating the first portion  63   a  about the axis. 
   Consequently, the position of the second portion  63   b  relative to the axial position of the control shaft  56  can be easily adjusted to a proper position corresponding to the neutral position of the movable swash plate  54 . 
   Although the first pump body  50  is of a variable displacement type in this embodiment, it may be of a fixed displacement type. If the first pump body  50  is of a fixed displacement type, a fixed swash plate is replaced with the output adjusting member  53 . 
   Next, description will be given of a hydraulic circuit in the pump unit  1  according to this embodiment. 
     FIG. 6  is a diagram illustrating a hydraulic circuit of the pump unit  1  according to this embodiment. 
   As illustrated in  FIGS. 3 and 6 , the plate  30  is provided with a pair of first oil passages  101   a ,  101   b  which have first ends forming kidney ports so as to communicate with a discharge port  50   a  and a suction port  50   b  of the first pump body  50 , respectively, and a drain oil passage  110  for allowing the pair of first oil passages  101   a ,  101   b  to communicate with the oil sump. 
   Each of the pair of first oil passages  101   a ,  101   b  has a second end opened to the outer surface of the plate  30 . The opening ends constitute pressurized oil supplying/discharging ports  102   a ,  102   b  for communicating with a hydraulic device such as a hydraulic motor in cooperation with the pump unit  1 . 
   The first oil passages  101   a ,  101   b  are arranged in a substantially linear manner substantially symmetrically with each other in reference to the pump shaft  40  in this embodiment, as shown in  FIG. 3 . 
   The drain oil passage  110  has a first end communicating with at least one of the first oil passages  101   a ,  101   b , and a second end communicating with the oil sump. 
   In this embodiment, the drain oil passage  110  includes a single substantially linear cross oil passage  111  for allowing the pair of first oil passages  101   a ,  101   b  to communicate with each other, and a connecting oil passage  112  having a first end communicating with the cross oil passage  111  and a second end opened to the surface of the plate  30 , as shown in  FIGS. 1 and 3 . 
   As described above, in this embodiment, the inside space  11  of the housing  10  commonly serves as the oil sump. Consequently, the second end of the connecting oil passage  112  is opened to an inner surface facing to the housing inside space  11  of the plate  30 . 
   Here, to the inner surface of the plate  30  is opened also the pair of first oil passages  101   a ,  101   b  in addition to the drain oil passage  110 . 
   The pump unit  1  according to this embodiment adopts a configuration below in order to prevent any interference of the pair of first oil passages  101   a ,  101   b  and the drain oil passage  110  and to allow these oil passages to communicate with the housing inside space  11 . 
     FIG. 7  is a perspective view showing the plate  30 , as viewed from the inner surface. 
   As shown in  FIGS. 1 ,  2  and  7 , the pump unit  1  according to this embodiment includes a valve plate  70  interposed between the plate  30  and the first pump body  50 . 
   The valve plate  70  is configured such that it can rotatably support the cylinder block  51 , and further, that it allows the discharge port  50   a  and the suction port  50   b  of the first pump body  50  to communicate with the first ends of the first oil passages  101   a ,  101   b , respectively. 
   At the inner surface of the plate  30 , a groove  113  is formed in such a manner as to be opened toward the valve plate  70 . The groove  113  extends outward in a radial direction beyond the valve plate  70  in reference to the pump shaft  40 . 
   With this configuration, the second end of the connecting oil passage  112  is opened to the groove  113 . 
   Namely, in this embodiment, the drain oil passage  110  also includes the groove  113  in addition to the cross oil passage  111  and the connecting oil passage  112 . 
   Most part of the groove  113  except for an outer end in a radial direction is designed to be closed by the back surface of the valve body  70  (i.e., a surface in contact with the plate  30 ) when the valve plate  70  is disposed at the inner surface of the plate  30 . As a consequence, a simple structure can allow the drain oil passage  110  to communicate with the oil sump, i.e., the housing inside space  11  without exerting any adverse influence on the oil supplying/discharging function of the cylinder block  51  while preventing the interference with the pair of oil passages  101   a ,  101   b  and the drain oil passage  110 . 
   As shown in  FIGS. 1 and 3 , a disposing hole  120 , which has a first end opened to the outer surface of the plate  30  and a second end communicating with the drain oil passage  110 , is formed at the plate  30  in addition to the above-described various oil passages. 
   Furthermore, a rotary valve  130  is inserted into the disposing hole  120  in a rotatable manner about its axis in the state in which the outer end extends outward of the plate  30 . 
   The rotary valve  130  shuts off the drain oil passage  110  when it is located at a predetermined shutoff position about the axis with respect to the disposing hole  120  (see  FIG. 3 ); in contrast, it allows the communication of the drain oil passage  110  when it is located at a communication position at which it is rotated about the axis by a predetermined angle from the shutoff position. 
   In other words, the rotary valve  130  is switchably operated between the shutoff position and the communication position according to the position about the axis with respect to the disposing hole  120 . 
   Incidentally, in this embodiment, the shutoff position and the communication position can be selectively switched by rotating the rotary valve  130  at 90° about the axis. 
   Moreover, in this embodiment, the rotary valve  130  includes a detent mechanism  130   a  which holds the rotary valve  130  at the shutoff position and the communication position. 
   That is to say, a seal cap  131  coaxial with the disposing hole  120  is screwed at the disposing hole  120  opened to one side end face of the plate  30 , and an operating shaft  132  of the rotary valve  130  projects outward of the seal cap  131  and is provided with a handle  133 . 
   Additionally, at the outer edge of the handle  133  are formed two projections  133   a ,  133   b  having the same shape as each other at an interval of 90° in a circumferential direction, as shown in  FIG. 5 . 
   Furthermore, a positioning plate  134  having a substantial L-shape as viewed in cross section is disposed at the one side end face of the plate  30 . The positioning plate  134  includes a lateral plate portion in contact with the one side end face of the plate  30  and a vertical plate portion extending from the lateral plate portion along the axial direction of the rotary valve  130 . At the vertical plate portion is formed a recess  134   a  into which the projection  133   a  or  133   b  can be fitted. 
   The detent mechanism  130   a  is configured in the above-described manner. Therefore, the projection  133   a  is fitted into the recess  134   a  when the rotary valve  130  is located at the shutoff position, so that the handle  133  is held at that position; in contrast, the projection  133   b  is fitted into the recess  134   a  when the rotary valve  130  is located at the communication position, so that the handle  133  is held at that position. 
   In the pump unit  1  having this configuration, the pair of first oil passages  101   a ,  101   b  can be remarkably speedily and readily deaerated in comparison with the conventional pump unit. 
   In the prior art in which the shutoff and communication of the drain oil passage are switched by moving the shutoff valve screwed into the plate in the axial direction, a communication opening width of the drain oil passage cannot be sufficiently secured unless the shutoff valve is rotated about the axis several times. 
   Furthermore, with this conventional configuration, the valve seat is required to be disposed at a deep portion of the oil passage into which the shutoff valve is screwed. 
   In contrast, in the pump unit  1  according to this embodiment, the shutoff and communication of the drain oil passage  110  can be switched without rotating the rotary valve  130  once about the axis (only by rotation at 90° in this embodiment), and thus, the pair of first oil passages  101   a ,  101   b  can be remarkably speedily deaerated. 
   Furthermore, in this embodiment, no valve seat is required to be disposed, unlike the prior art, and therefore, the drain oil passage  110  can be readily bored. 
   Moreover, in the pump unit  1  according to this embodiment, a charge oil passage  140  for supplying charge oil to the pair of first oil passages  101   a ,  101   b  is formed at the plate  30 , as illustrated in  FIGS. 3 and 6 . 
   The charge oil passage  140  includes a first bypass oil passage  141  for allowing the pair of first oil passages  101   a ,  101   b  to communicate with each other, and a suction oil passage  142  which has a first end connected to the first bypass oil passage  141  and a second end communicating with the housing inside space  11 . 
   Check valves  150   a ,  150   b  for allowing an oil flow from the suction oil passage  142  to the pair of first oil passages  101   a ,  101   b  and preventing a reverse oil flow are interposed between a connecting point of the first bypass oil passage  141  to the suction oil passage  142  and the pair of first oil passages  101   a ,  101   b , respectively. 
   In this embodiment, a throttle  155  is disposed in the check valve  150   b  interposed between the first oil passage  101   b  of the first oil passages  101   a ,  101   b  and the charge oil passage  140 , thereby increasing a neutral width of the first pump body  50 . 
   Additionally, a self-sucking throttle  145  in the case where either one of the first oil passages  101   a ,  101   b  becomes low in pressure due to oil leakage is provided on the charge oil passage  140 . The inside of each of the first oil passages  101   a ,  101   b  can be kept in a state full of oil all the time by providing the throttle  145 . As a consequence, in the case where the pump unit  1  according to the present invention is used as, for example, a drive source for a vehicle traveling hydraulic motor, there is no danger that a vehicle cannot be rolled down toward a ravine even if the vehicle is parked on a slope without applying parking brake. 
   Here, in this embodiment, the second end of the suction oil passage  142  is opened to the groove  113 . As described above, most part of the groove  113  except for the outer end in the radial direction is closed by the valve plate  70 . As a consequence, the simple structure can allow the suction oil passage  142  to communicate with the housing inside space  11  without exerting any adverse influence on the oil supplying/discharging function of the cylinder block  51  while preventing the interference with the pair of first oil passages  101   a ,  101   b  and the drain oil passage  110 . 
   In addition to the above configurations, the pump unit  1  according to this embodiment includes the second pump body  80  to be driven by the pump shaft  40 , and a pair of second oil passages  201   a ,  201   b  communicating with a discharge port  80   a  and a suction port  80   b  of the second pump body  80 , respectively. 
   The second pump body  80  is adapted to supply pressurized oil to the hydraulic device in cooperation with the first pump body  50  or another hydraulic device other than the hydraulic device. 
   In this embodiment, the second pump body  80  is supported at the second end  42  downstream in the transmission direction of the pump shaft  40  (i.e., the left end in  FIGS. 1 and 2 ). 
     FIG. 8  is a cross-sectional view taken along a line VIII-VIII of  FIG. 1 . 
   As shown in  FIGS. 1 to 8 , the pair of second oil passages  201   a ,  201   b  is bored in a pump case  90  surrounding the second pump body  80 . 
   That is to say, the pump unit  1  according to this embodiment includes the pump case  90  connected to an outer surface on a side opposite to the inner surface of the plate  30  in such a manner as to surround the second pump body  80 . The pair of second oil passages  201   a ,  201   b  is formed in the pump case  90 . 
   In particular, the second oil passages  201   a ,  201   b  have first ends communicated with the discharge port  80   a  and the suction port  80   b  of the second pump body  80 , respectively, second ends opened to the surface of the pump case  90 , thereby forming a discharge port  202   a  and a suction port  202   b , respectively. 
   As shown in  FIGS. 6 and 8 , a relief valve  210  for setting an operating oil pressure for the hydraulic device in cooperation with the second pump body  80  is inserted into the positive pressure oil passage  201   a  communicating with the discharge port  80   a  of the second pump body  80  out of the pair of second oil passages  201   a ,  201   b.    
   In this embodiment, a bypass oil passage  220  for allowing the second oil passages  201   a ,  201   b  to communicate with each other is formed in the pump case  90 , and thus, the relief valve  210  is inserted into the bypass oil passage  220 . 
   In contrast, the negative pressure oil passage  201   b  communicating with the suction port  80   b  in the second pump body  80  out of the pair of second oil passages  201   a ,  201   b  is connected to the pair of first oil passages  101   a ,  101   b.    
   Namely, at least a part of the oil, which is supplied from the discharge port  80   a  of the second pump body  80  to the hydraulic device via one of the second oil passages (i.e., the positive pressure oil passage  201   a ) and is returned to the suction port  80   b  of the second pump body  80  via the other one of the second oil passages (i.e., the negative pressure oil passage  201   b ), is designed to be introduced to the pair of first oil passages  101   a ,  101   b , thereby speedily deaerating the pair of second oil passages  201   a ,  201   b  by use of the rotary valve  130 . 
   In this embodiment, the plate  30  includes a first connecting oil passage  231  which has a first end communicating with the charge oil passage  140  and a second end opened to the surface in contact with the pump case  90 , as shown in  FIG. 1 . 
   Furthermore, the pump case  90  is provided with a second connecting oil passage  232  which has a first end communicating with the negative pressure oil passage  201   b  and a second end opened to the surface in contact with the plate  30 , so as to communicate with the first connecting oil passage  231 . 
   In other words, the negative pressure oil passage  201   b  is designed to communicate with the pair of first oil passages  101   a ,  101   b  via the second connecting oil passage  232 , the first connecting oil passage  231  and the charge oil passage  140 . 
   Moreover, a charge relief valve  240  for setting an oil pressure of the pressurized oil flowing to the charge oil passage  140  from the negative pressure oil passage  201   b  is inserted into the negative pressure oil passage  201   b.    
   Additionally, in the pump case  90  is formed a suction oil passage  250  which has a first end opened to the surface so as to form a suction port  250   a  and a second end communicating with the negative pressure oil passage  201   b.    
   Incidentally, reference numeral  260  in  FIG. 1  designates a drain port for draining the oil reserved inside the housing inside space  11 . 
   In addition, reference numeral  270  in  FIG. 6  designates a leak oil passage from the first pump body  50  to the oil sump (i.e., the housing inside space  11  in this embodiment). 
   Embodiment 2 
   Hereinafter, description will be given of an axial piston device according to another preferred embodiment of the present invention with reference to the attached drawings. 
   An axial piston device  1 B according to this embodiment is also configured to be used as a pump unit in the same manner as in the first embodiment. 
     FIG. 9  is a longitudinal cross-sectional view showing the pump unit  1 B according to this embodiment. Furthermore,  FIGS. 10 and 11  are a cross-sectional view taken along a line X-X of  FIG. 9  and a cross-sectional view taken along a line XI-XI of  FIG. 9 , respectively. Moreover,  FIG. 12  is a view as viewed from an arrow XII of  FIG. 10 . 
   Here, in  FIGS. 9 to 12 , the same or corresponding components as or to those in the first embodiment are designated by the same reference numerals; therefore, the detailed description for those components will not be given herein. 
   The pump unit  1 B according to this embodiment is configured in substantially the same manner as that in the first embodiment except that the movable swash plate  54  in the pump unit  1  in the first embodiment is replaced with a trunnion-type movable swash plate  54 B and that the seat faces of the check valves  150   a ,  150   b  are constituted of components independent of the plate  30 . 
   In particular, the pump unit  1 B includes the trunnion-type movable swash plate  54 B in place of the movable swash plate  54 , as shown in  FIGS. 9 and 10 . 
   The above-described movable swash plate  54 B of a trunnion type has small sliding resistance, so that the movable swash plate  54 B can be speedily returned to a neutral position of the movable swash plate  54 B by means of the neutral position returning mechanism  60 . 
   Furthermore, the pump unit  1 B includes a pair of seat members  151  to be inserted into the first bypass oil passage  141 . 
   More particularly, the first bypass oil passage  141  includes a small-diameter portion  143  communicating with the suction oil passage  142  and a pair of large-diameter portions  144  whose diameter is enlarged with steps continuous from the small-diameter portion  143  and which communicates with the pair of first oil passages  101   a ,  101   b , respectively, as shown in  FIG. 11 . 
   The pair of seat members  151  are disposed inside the large-diameter portions  144 , respectively, so that each seat face  155  is oriented toward the corresponding first oil passages  101   a ,  101   b.    
   Incidentally, the seat member  151  is fixed to the large-diameter portion  144  by, for example, a stopper ring (see  FIG. 11 ) or press-fitting. 
   In this manner, a repairing work in the case of degradation of the seat face  155  can be readily performed at low cost by forming the seat face  155  of a member independent of the plate  30  (the seat member  151  in this embodiment). 
   In a situation in which the first pump body  50  is operated for a long period of time in the state of, for example, application of a high load, the check valves  150   a ,  150   b  are frequently opened and closed, whereby the seat face  155  is abraded, thereby inducing a possibility of leakage of operating oil from the pair of first oil passages  101   a ,  101   b.    
   Especially in the case where the plate  30  is made of aluminum, the possibility of leakage is tended to become stronger. 
   In such a case, the seat face  155  can be repaired by only replacing the seat member  151 , if the seat face  155  is formed of a member independent of the plate  30 , like in this embodiment. 
     FIG. 13  is a cross-sectional view showing the plate  30  provided with cartridge-type check valves  152   a ,  152   b.    
   As described above, the seat member  151  provided with the seat face  155  is used in this embodiment. Alternatively, there may be provided the cartridge-type check valves  152   a ,  152   b  each including a valve case having a seat face  155 , as shown in  FIG. 13 . 
   Embodiment 3 
   Hereinafter, description will be given of an axial piston device according to still another preferred embodiment of the present invention with reference to the attached drawings. 
     FIG. 14  is a laterally partial plan view showing an axial piston device  1 C according to this embodiment. Furthermore,  FIGS. 15 and 16  are a cross-sectional view taken along a line XV-XV of  FIG. 14  and a cross-sectional view taken along a line XVI-XVI of  FIG. 14 , respectively. 
   The axial piston device  1 C according to this embodiment is configured to be used as a motor unit, unlike the first and second embodiments. 
   In other words, each of the axial piston devices  1 ,  1 B according to the first and second embodiments includes the pump shaft  40  as the rotary shaft and the pump body  50  serving as the rotor rotatable together with the rotary shaft; in contrast, the axial piston device  1 C according to this embodiment includes a motor shaft  340  as the rotary shaft and a motor body  350  serving as the rotor. 
   Specifically, the axial piston device  1 C comprises the motor shaft  340 , the motor body  350  including a cylinder block  351  fitted around in a non-rotatable manner relative to the motor shaft  340  and a plate  330  which is brought into contact with a discharge port and a suction port in the motor body  350 . The motor block  351  is configured in such a manner as to be rotated with the application of an oil pressure from an oil source such as a hydraulic pump unit which is liquid-connected via the plate  330 , thereby outputting rotational drive force from the motor shaft  340 . 
   The axial piston device  1 C according to this embodiment further comprises a housing  320  surrounding the motor body  350  and the plate  330 , wherein its inside space serves as an oil sump. 
   As shown in  FIGS. 14 to 16 , an axle case for supporting a pair of drive axle shafts  400  for driving a pair of drive wheels is commonly used as the housing  320  in this embodiment. 
   That is to say, the axle case  320  includes first and second case bodies  321 ,  322  which are detachably connected to each other, so that a liquid-tight inside space can be defined by connecting the first and second case bodies  321 ,  322 . 
   More particularly, the inside space of the axle case  320  is divided into a motor unit accommodating space  320   a  for accommodating therein the motor body  350  and the plate  330 , a deceleration gear train accommodating space  320   b  for accommodating therein a deceleration gear train  410  operatively connected to the motor shaft  340 , a differential gear unit accommodating space  320   c  for accommodating therein a differential gear unit  420  operatively connected to the deceleration gear train  410 , and a drive axle shaft accommodating space  320   d  for accommodating therein a pair of drive axle shafts  400  operatively connected to the differential gear unit  420 . 
   Incidentally, reference numeral  430  in  FIG. 14  designates a brake mechanism capable of applying brake force to the motor shaft  340 . 
   The motor shaft  340  has a base end supported by the plate  330  and a tip end supported on a partition wall of the axle case  320  in such a manner as to be exposed to the deceleration gear train accommodating space  320   b.    
   The motor body  350  includes the cylinder block  351  fitted around in a non-rotatable manner relative to the motor shaft  340 , a piston  352  accommodated inside the cylinder block  351  in a freely advancing/retreating manner in an axial direction, and a swash plate  354  defining an advancing/retreating range in the axial direction of the piston  352 . 
   Here, the axial piston device  1 C according to this embodiment is of a variable displacement type. 
   Consequently, the motor body  350  includes a movable swash plate serving as the swash plate  354 . Furthermore, the motor body  350  includes a connecting arm  355  having a first end connected to the movable swash plate  354 , and a control shaft  356  supported by the housing  320  in a rotatable manner about an axis so as to have a first end connected to a second end of the connecting arm  355  and a second end positioned outward of the housing  320 . 
   As shown in  FIG. 15 , at the plate  330  are disposed a pair of oil passages  301   a ,  301   b  having first ends communicating with a discharge port and a suction port of the motor body  350 , respectively, and a drain oil passage  310  for allowing the pair of oil passages  301   a ,  301   b  to communicate with the oil sump. 
   More particularly, as shown in  FIG. 14 , each first end of the pair of oil passages  301   a ,  301   b  is opened to a contact face  331  in contact with the motor body in outer surface of the plate  330 . 
   Furthermore, each second end of the pair of oil passages  301   a ,  301   b  is opened to a back face  332  on a side opposite to the contact face  331 . 
   As described above, the plate  330  is also surrounded by the housing  320  in this embodiment. 
   As a consequence, each second end of the pair of oil passages  301   a ,  301   b  is fluid-connected to a hydraulic source such as a hydraulic pump via a conduit member  305  supported by the housing  320  astride inward and outward of the housing  320  (see  FIGS. 14 and 16 ). 
   The drain oil passage  310  has a first end communicating with at least one of the oil passages  301   a ,  301   b , and a second end communicating with the oil sump (i.e., the inside space of the housing  320  in this embodiment). 
   According to this embodiment, the drain oil passage  310  includes a single cross oil passage  311  of a substantially linear shape for allowing the pair of oil passages  301   a ,  301   b  to communicate with each other, and a connecting oil passage  312  having a first end communicating with the cross oil passage  311  and a second end opened to the back face  332  of the plate  330 , as shown in  FIGS. 14 to 16 . 
   Moreover, a disposing hole  120  is bored at the plate  330 , like in the first and second embodiments and, further, a rotary valve  130  is inserted into the disposing hole  120  in a rotatable manner about an axis. 
   Incidentally, according to this embodiment, the outer end of the rotary valve  130  extends outward of the housing  320  (i.e., the axle case) such that the rotary valve  130  can be operated outward of the housing  320 . 
   Additionally, a handle  133  is attached to an outward extending portion  132  at the rotary valve  130 , like in the first and second embodiments. 
   As shown in  FIGS. 15 and 16 , a projection  133   a  is formed at the handle  133 . 
   An engaging recess  134   a  formed is integrally with the housing  320 . The projection  133   a  and the engaging recess  134   a  constitute a detent mechanism  130   a  for holding the rotary valve  130  at cutoff/communication positions. 
   This specification is by no means intended to restrict the present invention to the preferred embodiments set forth therein. Various modifications to the axial piston device may be made by those skilled in the art without departing from the spirit and scope of the present invention as defined in the appended claims.