Abstract:
A hydrostatic transmission and a charge pump for supplying operating oil which are housed in a housing, in which a pump casing of the charge pump is mounted to a center section of the hydrostatic transmission without using mounting bolts. A spring is interposed between the pump casing and the inner surface of the housing. For defining pump discharge pressure, the center section receives all of the radial force generated in the pump casing. Between the center section and the charge pump is interposed a plate of material made of less friction resistance so as to reduce the sliding resistance. The charge pump is simple in construction and inexpensive to produce.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a charge pump attached to a center section in a hydrostatic transmission (hereinafter referred to as an &#34;HST&#34;), and more particularly to a charge pump whose support mechanism is simple in construction and inexpensive to produce. 
     2. Related Art 
     It is well-known that a charge pump for supplying operating oil to an HST may be mounted in close contact with one surface of a center section of the HST. The charge pump may be housed in a housing and driven by an input shaft of the HST which perforates through the center section. Such an arrangement disclosed in Japanese Utility Model Publication No. Hei 6-12276 which was filed by the same applicant as the present application. In such an arrangement, the pump casing which contains the charge pump is flexibly supported by a disc spring (rather than by bolts) disposed between the lower surface of the center section and the inner bottom surface of the housing. The charge pump is biased toward the horizontal lower surface of the center section so as to come into close contact therewith. This provides a charge pump with a self-charging relief function. 
     In such the conventional construction, a pin extends from the upper surface of the pump casing and is retained in the horizontal lower surface of the center section. As a result, the pump casing is not rotatable around the input shaft and the discharge port of the charge pump is maintained in proper communication with an operating oil supply port of the center section. Furthermore, the charge pump can move axially of the input shaft during the relief operation. When the charge pump is driven by the input shaft, a radial force or bending stress, generated at the pump casing is placed on the pin which acts as a shearing force on the pin, causing concern that the pin may be severed. Hence, a tough and highly rigid material should be used for manufacture of the pin. Therefore, the number of parts required in this arrangement is greater, leaving room for improvement in this known design. 
     The above-described spring for creating charge relief pressure uses a disc-like spring of corrugated annular plate-type. Such a spring is relatively larger in spring constant, creating a problem when variations exist during assembly in that the relief setting pressure may differ from one device to the next. 
     Further, the horizontal lower surface of the center section, with which the charge pump comes into close contact, needs to be machined with high accuracy in order to reduce the rotational and sliding resistance of the charge pump. This raises the processing cost. Moreover, discharge oil from the charge pump, when the accuracy of the machining processing is poor, leaks from the sliding surface of the charge pump resulting in a reduction in the efficiency of the device. 
     SUMMARY OF THE INVENTION 
     One object of the present invention is to provide a charge pump for supplying operating oil to the HST, which is housed together with the HST in a housing and is driven by an input shaft mounted on one surface of a center section of HST. A spring is interposed between a pump casing, in which the charge pump is disposed, and the inner surface of the housing. The pump casing is biased by the spring so as to be brought into close contact with one surface of the center section and is disposed thereon in a &#34;floating&#34; manner. The biasing force of the spring defines the pump discharge pressure of the charge pump. A coil-type spring is used to obtain less variation in the pressure of the relief setting of the charge relief function. As a result, the radial force of the pump casing is reliably received by a portion of the HST which is of high rigidity and the number of parts is reduced. 
     Further, a plate made from a material of less frictional resistance is interposed between the center section and the charge pump so as to reduce the sliding contact resistance when the charge pump is rotated. As a result, a charge pump of lower manufacturing cost can be obtained without raising the processing accuracy for the mounting surface of the charge pump on the center section. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     Other objects and aspects of the present invention will become apparent from the following description of the embodiments with reference to the accompanying drawings in which: 
     FIG. 1 is a cross-sectional view of the HST of the present invention; 
     FIG. 2 is an enlarged partial view of the principal portions of the charge pump shown in FIG. 1; 
     FIG. 3 is an exploded view of the charge pump in relation to the center section of the HST; 
     FIG. 4 is an enlarged partial view of the principal portions of the charge pump according to a second embodiment of the present invention; 
     FIG. 5 is a partial sectional view of the center section of the same; 
     FIG. 6 is an enlarged partial view of the principal portions of the charge pump according to a third embodiment of the present invention; 
     FIG. 7 is an enlarged partial view of the principal portions of the charge pump according to a fourth embodiment of the present invention; 
     FIG. 8 is an enlarged partial view of the principal portions of the charge pump according to a fifth embodiment of the present invention; 
     FIG. 9(a) is an enlarged view showing the check means for the pump casing in relation to the center section of the HST; 
     FIG. 9(b) is an enlarged view showing an alternative check means for the pump casing in relation to the center section of the HST; 
     FIG. 10 is an enlarged sectional view of the principal portions of the charge pump in accordance with a sixth embodiment of the present invention; 
     FIG. 11 is an enlarged view showing an alternative check means for the pump casing in relation to the center section of the HST; 
     FIG. 12 is an enlarged cross-sectional view of the charge pump constructed to permit an oil filter to be removed from the housing without having to remove the charge pump; 
     FIG. 13 is an exploded view of another embodiment of the charge pump in relation to the center section of the HST; 
     FIG. 14 is an enlarged cross-sectional view of the principal portions of a check valve housed in the pump casing; 
     FIG. 15 is a cross-sectional plan view of the center section; 
     FIG. 16 is an enlarged cross-sectional view of the principal portions of the charge pump according to an embodiment of the present invention which disposes a plate between the center section and the charge pump; 
     FIG. 17 is a plan view of the plate of FIG. 16; 
     FIG. 18 is an enlarged cross-sectional view of the principal portions of a charge pump according to a modified embodiment of the present invention in which a plate on which an electric sealing member is mounted; 
     FIG. 19 is a plan view of the plate of FIG. 18; 
     FIG. 20 is an enlarged cross-sectional view of the principal portions of the charge pump of a further modified embodiment of the present invention for holding the pump casing; 
     FIG. 21 is an enlarged cross-sectional view of the principal portions of the charge pump in FIG. 20; 
     FIG. 22 is an enlarged cross-sectional view of the principal portions of the charge pump according to the further modified embodiment in which the oil filter can be easily removed; 
     FIG. 23 is an exploded view of the charge pump in relation to the center section of FIG. 22; 
     FIG. 24 is an enlarged cross-sectional view of the principal portions of the charge pump in a still further modified embodiment for receiving the radial force acting on the pump casing during the relief operation; 
     FIG. 25 is an enlarged cross-sectional view of the principal portions of the charge pump according to yet a further modified embodiment which has a modified detent for the plate; 
     FIG. 26 is an enlarged cross-sectional view of the principal portions of the charge pump according to a further modified embodiment in which the detent for the plate is changed; and 
     FIG. 27 is an exploded view of the charge pump according to a still further modified embodiment for receiving the radial force acting on the pump casing. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The HST shown in FIG. 1 includes a housing which comprises an upper half housing 1 and a lower half housing 2 which are joined to each other along a horizontal surface. The housing contains the HST and forms an oil sump which is filled with operating oil for the HST. A center section 5, which is L-like-shaped when viewed in cross-section is disposed within the housing. Center section 5 is provided with a horizontal portion and a vertical portion and is fixed to the upper half housing 1 by mounting bolts 6. A pump mounting surface 20 is formed on the upper surface of the horizontal portion of center section 5. A cylinder block 16, constituting a hydraulic pump P of the axial piston-type is rotatably disposed on pump mounting surface 20. Pistons 13 are biased by springs 18 into a plurality of cylinder bores formed in cylinder block 16. Pistons 13 are freely moveable into and out of the cylinder bores. The heads of pistons 13 abut against a thrust bearing held to a movable swash plate 8. 
     An input shaft 3 engages with cylinder block 16 at the axis of rotation thereof. The rotational axis of input shaft 3 extends vertically. Input shaft 3 is supported at the upper end thereof by the upper half housing 1 and at the lower end by pump mounting surface 20 of center section 5. The upper end of input shaft 3 projects upwardly and outwardly from the upper surface of upper half housing 1. At the upper end of input shaft 3 is fixed a pulley (not shown), by which power is inputted to input shaft 3 from an engine or the like through a belt (also not shown). 
     A motor mounting surface 21 is formed on the outside surface of the vertical portion of center section 5. A cylinder block 17, constituting an axial piston-type hydraulic motor M, is rotatably disposed on motor mounting surface 21. Pistons 14 are inserted into a plurality of cylinder bores formed in cylinder block 17 and are biased by springs 19 such that they are movable into and out of the cylinder bores. The heads of pistons 14 abut against a thrust bearing held to a fixed swash plate 9, which is fixedly sandwiched between upper half housing 1 and lower half housing 2. 
     An output shaft 4 engages with cylinder block 17 at the axis of rotation thereof and is horizontally disposed. A bearing for supporting one end of output shaft 4 is provided at the joint surface of upper half housing 1 and lower half housing 2. Output shaft 4 is supported at the other end to motor mounting surface 21 of center section 5. 
     A pair of left and right axles (not shown) are provided in the housing. A gear 25 is provided on output shaft 4 and is interlocked through a power transmission mechanism, such as gears, with the axles. Output shaft 4 projects at the utmost end thereof outwardly from the housing. A brake disc 15 for braking the axles is provided at the utmost end of output shaft 4. 
     As shown in FIG. 15, a pair of oil passages 5a and 5b are provided in parallel to each other in center section 5 and communicate with a pair of kidney ports open at pump mounting surface 20 and a pair of kidney ports open at motor mounting surface 21 to form a closed fluid circuit for circulating operating oil between cylinder block 16 of hydraulic pump P and cylinder block 17 of hydraulic motor M. Check valves C for supplying operating oil are disposed at the open ends of oil passages 5a and 5b. 
     An operating oil supply port 5c is open at the horizontal lower surface of center section 5 and communicates with the discharge side of a charge pump 11, to be discussed below. Operating oil supply port 5c also communicates with an oil passage 5d in center section 5. Oil passage 5d extends from oil supply port 5c and communicates with an oil passage 5e extending between check valves C. Oil passage 5d has an open end closed by a sleeve 6a which has an insertion bore for mounting bolt 6. Sleeve 6a functions at the outer periphery as a plug for closing the open end of oil passage 5d. 
     As seen in FIG. 1, movable swash plate 8 is interlocked with a speed changing member, such as a lever or a pedal provided on the vehicle to which the HST is mounted. As shown in FIG. 1, movable swash plate 8 may be tilted from a horizontal neutral position to vary the amount and direction of oil discharged from hydraulic pump P to hydraulic motor M through the closed fluid circuit in center section 5. This results in an HST creating a stepless output rotation of motor shaft 4 of hydraulic motor M. 
     Check valves C, as shown in FIG. 15, are provided with push pins 33 for allowing the input and output sides of check valves C to open into the oil sump. The utmost ends of push pins 33 are biased by springs to center section 5. Push pins 33 project outwardly in parallel to each other and are opposite to a single biasing member 22. The center of biasing member 22 has an elongated bore 22a. A cam pin 34 is inserted into elongated bore 22a and is fixed to the lower end of an operating shaft 35 (shown in FIG. 1). The upper end of operating shaft 35 projects outwardly from the housing so as to fixedly support a by-pass arm 23. When by-pass arm 23 is rotated for hauling the vehicle, cam pin 34 moves biasing member 22 forwardly, and push pins 33 are retreated so as to open the check valves C, thereby opening the closed fluid circuit into the housing to obtain free rotation of hydraulic motor M. 
     At the horizontal lower surface of center section 5, opposite to pump mounting surface 20, is disposed a pump casing 10 which contains charge pump 11. Pump casing 10 is molded from sintered metal material. A recess is formed on the upper surface in which trochoid-type charge pump 11 is disposed. Charge pump 11 supplies oil into the closed fluid circuit of the HST. 
     Although a trochoid-type charge pump is shown in the Figures, the invention is not limited to this. A gear-type charge pump or a vane-type charge pump may also be used, for example. Charge pump 11 comprises an inner rotor 11a and an outer rotor 11b. Outer rotor 11b is rotatably guided at the outer periphery thereof by the inner periphery of pump casing 10. Inner rotor 11a is engageable with internal teeth of outer rotor 11b and is disposed in outer rotor 11b. The depth of the recess in pump casing 10, as shown in FIG. 2, is smaller than the thicknesses of inner rotor 11a and outer rotor 11b, so that one end surface of inner rotor 11a and outer rotor 11b is brought into slidable contact with the horizontal lower surface of center section 5. 
     Inner rotor 11a is provided with grooves 11k for axially slidably and not-relative-rotatably engaging with a pin 36 mounted to the lower end of input shaft 3 which projects from the horizontal lower surface of center section 5. The axis of rotation of charge pump 11 extends vertically. Input shaft 3 drives the hydraulic pump P and charge pump 11. 
     A suction port is formed at one side of the engaging portion of inner rotor 11a and outer rotor 11b. A discharge port is formed at the other side thereof. The discharge port is opposite to operating oil supply port 5c and communicates therewith. 
     A coil spring 26 is interposed between the lower surface of pump casing 10 and the inner bottom surface of lower half housing 2 so that pump casing 10 is biased by spring 26 to bring one end surface of inner rotor 11a and outer rotor 11b into close contact with the horizontal lower surface of center section 5. Thus, pump casing 10 is mounted in a floating manner whereby the biasing force of coil spring 26 defines the discharge pressure of charge pump 11. When pressure in the discharge port of charge pump 11 becomes greater than a predetermined biasing force, pump casing 10 together with inner rotor 11a and outer rotor 11b move axially down input shaft 3 so as to create a gap between one end surface of inner rotor 11a and outer rotor 11b and the horizontal lower surface of center section 5, whereby the discharge port of charge pump 11 is open to the oil sump in the housing to discharge oil having a pressure greater than the predetermined value. Thus, pump casing 10 and coil spring 26 function as a charge relief valve. Since coil spring 26 has a smaller spring constant than a conventional corrugated spring, variations in the individual relief pressure setting can be minimized even when the mounting length of the spring is not exact due to an error in the assembly thereof. 
     An annular oil filter 12, having an inner diameter which is larger than an outer diameter of pump casing 10, is disposed between the horizontal lower surface of center section 5 and the inner bottom surface of lower half housing 2 and surrounding the peripheral surface of pump casing 10. Oil filter 12 is molded of porous material. Both annular ends of oil filter 12 are covered by sealing members 12a and 12b, respectively. One sealing member 12a is brought into close contact with the horizontal lower surface of center section 5 and the other sealing member 12b is brought into close contact with the inner bottom surface of lower half housing 2 so as to separate the exterior and the interior of oil filter 12. A suction port 10a which communicates with the suction port of charge pump 11 is open at the lower surface of pump casing 10 so as to communicate with the interior of oil filter 12. 
     In summary, the present invention is designed so that charge pump 11 is mounted to the horizontal lower surface of center section 5 in a floating manner, rather than being fixed by bolts for example, as is conventional. 
     Next explanation will be given on a first embodiment of the present invention in accordance with FIGS. 2 and 3. 
     In this embodiment, two guide pins 27 project from the horizontal lower surface of center section 5 and are fitted into recesses 10e formed on the outer periphery of pump casing 10 which act as detent means for receiving guide pins 27, whereby the mounting position of pump casing 10 with respect to center section 5 is restrained from rotatably shifting around the axis of rotation of input shaft 3, while still allowing pump casing 10 to move axially, away from and towards, center section 5. Hence, the discharge port of charge pump 11 is always properly maintained in communication with operating oil supply port 5c. 
     In order to receive the radial force generated in pump casing 10 when charge pump 11 is driven, a pivotal support bore 10c having an inner diameter which is about equal to the outer diameter of input shaft 3 is disposed coaxially therewith at the bottom surface of the recess in pump casing 10. The lower end 3a of input shaft 3 extends into pivotal support bore 10c and is relatively, rotatably and axially fitted therein, yet it allows pump casing 10 to move axially. The radial force is received by center section 5 through input shaft 3. As a result, guide pins 27, for preventing pump casing 10 from rotating together with charge pump 11, are not subjected to the radial force of pump casing 10, thereby enabling pins 27 to be made of relatively low rigidity material, which lowers the manufacturing cost of the pins. 
     Next, explanation will be given on a second embodiment of the invention shown in FIGS. 4 and 5. In this embodiment, in order for center section 5 to directly receive the radial force generated in pump casing 10, a recess 5k is provided at the horizontal lower surface of center section 5. Recess 5k is substantially similar in external shape to pump casing 10. The upper portion of pump casing 10 is axially slidably fitted into recess 5k of pump casing 10. The utmost ends of guide pins 27, disposed on the upper surface of pump casing 10, are axially slidably retained in a boss provided on the horizontal lower surface of center section 5. Pump casing 10 returns the relief oil discharged through the discharge port to the oil sump through recess 5k. In particular, as best seen in FIG. 5, oil grooves 5s and 10s are provided on the inner periphery of recess 5k of center section 5 and on the outer periphery of pump casing 10, respectively. 
     In a third embodiment shown in FIG. 6, a projection 2k is integrally formed in a coaxial position with input shaft 3 at the inner bottom surface of lower half housing 2 and is axially slidably fitted into a pivotal support bore 10c formed on the lower surface of pump casing 10. The radial force generated on pump casing 10 is received by the housing while allowing pump casing 10 to slide axially. Pump casing 10 is detained by guide pins 27 in the same manner as described above with respect to the first embodiment. 
     As shown in a fourth embodiment in FIG. 7, a projection 10k is provided in a coaxial position with respect to input shaft 3 at the lower surface of pump casing 10. Projection 10k is axially slidably fitted into a recess 2n provided on the inner bottom surface of lower half housing 2. As in the other embodiments, pump casing 10 is slidable axially and the radial force of the charge pump is received by the housing. The remainder of this embodiment is similar to the embodiment shown in FIG. 6. 
     A fifth embodiment of the present invention is shown in FIGS. 8 and 9(a). This embodiment is the same as the first embodiment, in that the radial force of pump casing 10 is received by center section 5 through input shaft 3. This embodiment is different from the above embodiment in that the pump casing is detained, not by guide pins as in the first embodiment, but by a projection 5m which is integrally provided along the horizontal lower surface of center section 5. Projection 5m is axially slidably retained into a recess 10e provided in the outer periphery of pump casing 10. 
     In the modified embodiment shown in FIG. 9(b), a pair of projections are integrally provided along the horizontal lower surface of center section 5 and a projection 10n is integrally provided on the outer periphery of pump casing 10. Pump casing 10 is detained by the retention of projection 10n between both projections 5m. 
     In a sixth embodiment of the present invention shown in FIGS. 10 and 11, a cylindrical projection 10p is integrally provided on the upper surface of pump casing 10. Cylindrical projection 10p is axially slidably fitted into an engaging bore 5p provided along the horizontal lower surface of center section 5, which detains the rotation of pump casing 10. 
     In the embodiment of the present invention shown in FIGS. 12 and 13, the same construction as in the sixth embodiment has been adopted to detain the rotation of pump casing 10 and to receive the radial force of pump casing 11. However, this embodiment is different in that the maintenance efficiency for the oil filter 12 has been improved. In particular, an opening 2a is formed in the lower surface of lower half housing 2. Opening 2a is larger than the external diameter of oil filter 12. A disc-like shaped sealing member 12b, which covers the lower surface of oil filter 12, and is larger than opening 2a, is detachably mounted to the outer bottom surface of lower half housing 2 by mounting bolts, thereby closing opening 2a. Hence, for maintenance and inspection of oil filter 12, sealing member 12b is removed from the housing to enable oil filter 12 to be simply removed therefrom. In addition, since the base of coil spring 26 abuts against the bottom surface of sealing member 12b, when oil filter 12 is removed from the housing, coil spring 26 is also removed therefrom which relieves the biasing force against pump casing 10. A snap ring 50 is mounted onto an end portion of the extension of input shaft 3. A gap allowing for the axially slidable movement of pump casing 10 is formed between snap ring 50 and the lower surface of pump casing 10. Therefore, even when pump casing 10 is no longer biased against center section 5, it drops only to the extent of the aforesaid gap and is received by snap ring 50. Thus, projection 10p remains fitted in engaging bore 5p and there is no concern that charge pump 11 will become accidentally dismantled from input shaft 3. 
     In each of the above-described embodiments of the present invention, pump casing 10, in which charge pump 11 is housed, never rotates around input shaft 3 even though it is not fixed by mounting bolts to center section 5. Pump casing 10 is simply mounted to one surface of center section 5 in a floating manner by being biased in close contact therewith through spring 26 while exclusively receiving the radial force created by input shaft 3. In the case where the pressure of the discharge oil from charge pump 11 becomes greater than the corresponding biasing force of spring 26, pump casing 10 together with charge pump 1 automatically move away from one surface of center section 5 toward spring 26. As a result, pressurized oil discharged in part from charge pump 11 may leak from between charge pump 11 and center section 5, whereby pump casing 10 and coil spring 26 function as a charge relief valve. Hence, there is no need for specially providing a charge relief valve within center section 5. This enables the HST to be more inexpensively produced. 
     An alternative embodiment of the present invention is shown in FIGS. 16 and 17. In this embodiment, a relatively thin plate 7 of metal material or sintered alloy is interposed between the horizontal lower surface of center section 5 and one end surface of charge pump 11. Plate 7 reduces the manufacturing cost and the rotating frictional resistance on center section 5 without increasing the processing accuracy of center section 5. As shown in FIG. 17, open kidney-shaped oil bores 7a and 7b are formed in plate 7 in positions corresponding to the position of the suction port and discharge port of charge pump 11, respectively. Oil bore 7a communicates with operating oil supply port 5c provided in center section 5. An opening for inserting input shaft 3 therein is provided in the center of plate 7. At the periphery of plate 7 are provided cutouts 7c in which guide pins 27 for detaining pump casing 10 are received. Plate 7a is kept in a fixed position between center section 5 and charge pump 11 so as to maintain communication of the discharge port of charge pump 11 with operating oil supply port 5c. A spring 26, the same as that described above with respect to the previous embodiments, is interposed between the lower surface of pump casing 10 and the inner bottom surface of lower half housing 2. One end surface of charge pump 11 presses against plate 7 to maintain an oil tight seal between oil bore 7a of plate 7, operating oil supply port 5c and the discharge port of charge pump 11. The remainder of the embodiment is similar to the first embodiment discussed above. 
     FIGS. 18 and 19 show a modified embodiment of that shown in FIGS. 16 and 17, in which elastic sealing members 28, such as O-rings, are fitted onto the peripheral edges of oil bores 7a and 7b in plate 7. The elastic sealing members improve the close degree of contact between plate 7 and the horizontal lower surface of center section 5 to effectively guide the discharge oil from charge pump 11 into the closed fluid circuit. 
     The embodiment shown in FIGS. 20 and 21 is similar to that shown in FIGS. 18 and 19 however a projection 2b is provided on the inner bottom surface of lower half housing 2 and spring 26 of the previous embodiment is not used. Instead, the top of projection 2b abuts directly against the lower surface of pump casing 10 to support pump casing 10 in a floating manner. As is apparent in FIG. 21, elastic sealing members 28 are thicker than the depth of the groove into which elastic sealing members 28 are fitted. As a result, elastic sealing members 28 extend from the surface of plate 7 toward the horizontal lower surface of center section 5. Hence, when plate 7 is interposed between center section 5 and pump casing 10, a gap e is formed between the horizontal lower surface of center section 5 and plate 7. Discharge port 7b of plate 7 communicates with the discharge port of charge pump 11 and operating oil supply port 5c through elastic sealing members 28. 
     When the discharge oil from charge pump 11 is supplied to supply port 5c through discharge port 7a, pressure of the discharge oil acts on the inner periphery of elastic sealing members 28, so that when the pressure of the oil is greater than the corresponding elastic force of elastic sealing members 28, elastic sealing members 28 are deformed and oil leaks into the oil sump in the housing through the gap e. 
     In summary, a spring as used in the above-described embodiments is not used for biasing pump casing 10 in close contact with plate 7, rather, pump casing 10 is fixed to center section 5 and elastic sealing members 28 mounted onto plate 7 are utilized to provide an elastic force on pump casing 10. As a result, parts such as spring 26 can be omitted and pump casing 10 can function as a charge relief valve. Thus, a charge pump with a self-charging relief function can be produced at low cost. 
     The embodiment of the present invention shown in FIGS. 22 and 23 is similar to that described above with respect to FIGS. 20 and 21, however in this embodiment, an opening 2a is provided in the bottom wall of lower half housing 2 and is closed by a lid 2c, as shown in FIG. 12. When lid 2c is removed from lower half housing 2, oil filter 12 can be simply removed from the bottom of the same, the remainder construction of this is like that described above. 
     The embodiment of the present invention shown in FIG. 24 is similar to that shown in FIGS. 22 and 23 however, on the inner surface of lid 2c, which is mounted to the bottom wall of lower half housing 2, is integrally formed a guide pin 2e to be fitted into a pivotal support bore 10c provided along the lower surface of pump casing 10. Guide pin 2e is similar to projection 2k shown in FIG. 6, for example. 
     The embodiment of the present invention shown in FIG. 25 is similar to that shown in FIG. 8, however, projections 5m provided on the horizontal lower surface of center section 5 engage with recesses 7c in plate 7 and recesses 10e in pump casing 10 to detain rotation of plate 7 and pump casing 10. 
     In the embodiment of the present invention shown in FIG. 26, spring 26 (as described above in previous embodiments) is interposed between a disc-like shaped sealing member 12b for oil filter 12 and the inner surface of lid 2c. Sealing member 12a is retained by a flange formed along the outer periphery of the pump casing 10. Pump casing 10 is biased against plate 7 by the biasing force of spring 26 through oil filter 12. As in the embodiment shown in FIG. 12, cylindrical projections 10p are formed on the upper surface of pump casing 10 and are inserted into fitting bores 5p provided on the horizontal lower surface of center section 5, thereby restraining rotation of pump casing 10. Projections 10p are retained in recesses 7c in plate 7 so as to restrain rotation of plate 7. 
     As seen in FIG. 27, a guide recess 5k large enough to receive the upper end portion of pump casing 10, and for receiving the radial force acting thereon, is formed on the horizontal lower surface of center section 5. Projections 10p are formed on the upper surface of pump casing 10. Engaging bore 5p, for receiving projections 10p, are also formed in recess 5k. Oil grooves 10s, similar to those shown in the embodiment of FIGS. 4 and 5, are provided along the outer periphery of pump casing 10. When oil pressure greater than the biasing force of spring 26 acts on pump casing 10, and when pump casing 10 moves axially in guide recess 5k, an oil passage is formed for discharging the pressurized oil from charge pump 11 between the peripheral surface of guide recess 5k and oil groove 10s. 
     In operation, when a moving vehicle provided with the HST having the charge pump of the present invention stops on a slope and the engine is turned off, the charge pump stops and does not supply operating oil into the closed fluid circuit. The hydraulic motor is provided with a rotational force for moving the vehicle down the slope due to the load corresponding to the weight of vehicle, whereby the hydraulic motor operates as a pump to circulate the operating oil in the closed fluid circuit. However, when the closed fluid circuit is filled with the operating oil, the hydraulic pump in the neutral state prevents the circulation of oil, thereby preventing rotation of the hydraulic motor. 
     However, the operating oil in the closed fluid circuit is subjected to pressure generated by the hydraulic motor, so that over time, operating oil leaks through a sliding portion of each piston contained in the cylinder block or the rotational sliding surface between the cylinder block and the center section. When negative pressure is generated in the closed fluid circuit, minute amounts of air in the oil expand and the hydraulic motor may freely rotate to cause the vehicle to runaway. In order to prevent this freewheeling of the vehicle, an auxiliary oil passage 10y, as shown in FIG. 14, for communicating between the discharge port of charge pump 11 and the oil sump in oil filter 12 is provided. In particular, a check valve 31 opens only when the closed fluid circuit is under negative pressure, is provided in auxiliary oil passage 10y. Check valve 31 enables the oil to be supplied into the closed fluid circuit through auxiliary oil passage 10y. 
     In particular, auxiliary oil passage 10y, which communicates with the discharge port of charge pump 11 is open at the lower surface of pump casing 10. A cylindrical member 45 containing therein a ball 31a is inserted from the bottom surface of recess 10x in charge pump casing 10 so as to constitute check valve 31. Cylindrical member 45 is formed by press processing. At the center of the bottom of cylindrical member 45 is provided an opening 45a through which the interior of oil filter 12 communicates with auxiliary oil passage 10y. The inside peripheral edge of opening 45 is used as a seat surface for ball 31a. At the peripheral edge of the upper end of opening 45a are radially provided a plurality of projections, one of which is bent inwardly of cylindrical member 45 so as to prevent ball 31a from escaping therefrom. Another one of the projections is bent outwardly and is retained to pump casing 10 to thereby prevent cylindrical member 45, which receives discharge pressure from charge pump 11, from separating from auxiliary oil passage 20y. A further one of the projections is also outwardly bent and is retained to the inner periphery of auxiliary passage 10y, thereby preventing cylindrical member 45 from moving toward the inner rotor 11a and outer rotor 11b of charge pump 11. 
     In the case where the engine stops and the vehicle is parked on a slope, when negative pressure is generated in the closed fluid circuit, even if charge pump 11 is not driven, ball 31a in check valve 31 is subjected to negative pressure so as to move ball 31a away from the seating surface of opening 45a in cylindrical member 45, so that clean operating oil in oil filter 12 opens check valves C at the low pressure side of the HST by way of the discharge port of charge pump 11, operating oil supply port 5c in center section 5 and oil passages 5d and 5e, so that some of the oil which is leaked is supplied back into the closed fluid circuit. Hence, the closed fluid circuit is always filled with the operating oil so as not to cause rotation of hydraulic motor M, whereby freewheeling of the vehicle is prevented. 
     Although the invention has been described with reference to several different embodiments, these embodiments are merely exemplary and are not limiting of the invention, which is defined solely by the appended claims.