what is a tandem <a href='https://www.ruidapetroleum.com/product/47'>hydraulic</a> <a href='https://www.ruidapetroleum.com/product/49'>pump</a> free sample

www.powermotiontech.com is using a security service for protection against online attacks. An action has triggered the service and blocked your request.

Please try again in a few minutes. If the issue persist, please contact the site owner for further assistance. Reference ID IP Address Date and Time 8bf2006c85a66667641f5dd58dcb3d35 63.210.148.230 03/07/2023 05:34 AM UTC

what is a tandem <a href='https://www.ruidapetroleum.com/product/47'>hydraulic</a> <a href='https://www.ruidapetroleum.com/product/49'>pump</a> free sample

More particularly, a first aspect of the present invention relates to a tandem pump unit with first and second hydraulic pumps, which respectively cooperate with first and second actuators driven through a hydraulic effect.

A hydraulic pump is used in various applications and in particular as the hydraulic pump adapted for operation in association with an actuator driven through the hydraulic effect. The description will hereinafter be made for the tandem pump unit by taking for example the case where it cooperates with first and second hydraulic motors serving as the actuators that respectively drive the right and left drive wheels.

For example, U.S. Pat. No. 4,920,733 discloses a vehicle including first and second hydraulic pumps respectively connected via first and second hydraulic lines to the first and second hydraulic motors for driving the right and left drive wheels. In this vehicle, the first and second hydraulic motors respectively have outputs variable in response to the adjustment of the input/output flow rates of the first and second hydraulic pumps, thereby controlling the rotational speed and rotational direction of the right and left drive wheels.

The vehicle disclosed in the above cited U.S. Pat. No. 4,920,733 has the first hydraulic pump and the second hydraulic pump separately arranged from one another, the former being operated in association with the first hydraulic motor, and the latter being operated in association with the second hydraulic motor. These separate hydraulic pumps pose various problems, such as troublesome mounting operation of the first and second hydraulic pump, troublesome conduit setting work between the pumps and the motors, and requiring separate housings which respectively accommodate the first and second hydraulic pumps.

The first aspect of the present invention has been therefor conceived in consideration of the prior arts. It is an object of the first aspect of the present invention to provide a tandem pump unit with first and second pumps connected to each other in series, while being connected to actuators via first and second hydraulic lines respectively, and is capable of lowering the manufacturing cost through the simplification of mounting operation and the reduction of the number of parts.

The second aspect of the present invention relates to a tandem pump unit with first and second hydraulic pumps, which respectively cooperate with first and second actuators driven through a hydraulic effect.

A hydraulic pump is used in various applications and in particular as the hydraulic pump adapted for operation in association with an actuator driven through the hydraulic effect. The description will hereinafter be made for the tandem pump unit by taking for example the case where it cooperate with first and second hydraulic motors serving as the actuators that respectively drive the right and left drive wheels.

For example, U.S. Pat. No. 4,920,733 discloses a vehicle including first and second hydraulic pumps respectively connected via first and second hydraulic lines to the first and second hydraulic motors for driving the right and left drive wheels. In this vehicle, the first and second hydraulic motors respectively have outputs variable in response to the adjustment of the input/output flow rates of the first and second hydraulic pumps, thereby controlling the rotational speed and rotational direction of the right and left drive wheels.

The vehicle disclosed in the above cited US patent has the first hydraulic pump and the second hydraulic pump separately arranged from one another, the former being operated in association with the first hydraulic motor, and the latter being operated in association with the second hydraulic motor. These separate hydraulic pumps invite a complicated structure of the feeding passage for charging working hydraulic fluid to the pair of the first and second hydraulic lines, and pose various other problems.

A third aspect of the present invention relates to a pump unit with first and second hydraulic pumps that are respectively connected via first and second hydraulic lines to first and second actuators driven through a hydraulic effect.

A hydraulic pump is used in various applications and in particular as the hydraulic pump adapted for operation in association with an actuator driven through the hydraulic effect. The description will hereinafter be made for the pump unit by taking for example the case where it includes the first and second hydraulic motors serving as the actuators that respectively drive the right and left drive wheels.

For example, U.S. Pat. No. 4,920,733 discloses a vehicle including first and second hydraulic pumps respectively connected via first and second hydraulic lines to the first and second hydraulic motors for driving the right and left drive wheels. In this vehicle, the first and second hydraulic motors respectively have outputs variable in response to the adjustment of the input/output flow rates of the first and second hydraulic pumps, thereby controlling the rotational speed and rotational direction of the right and left drive wheels.

The vehicle disclosed in the above cited US patent has the first hydraulic pump and the second hydraulic pump separately arranged from one another, the former being operated in association with the first hydraulic motor, and the latter being operated in association with the second hydraulic motor. Such a separate arrangement of the hydraulic pumps invites a complicated structure of a feeding passage for feeding working hydraulic fluid from a reservoir tank to the first hydraulic line and the second hydraulic line, and poses various other problems.

The third aspect of the present invention has been therefor conceived in consideration of the above prior art. It is an object of the third aspect of the present invention to provide a pump unit with the first and second hydraulic pumps that are respectively connected via the first and second hydraulic lines to the first and second actuators driven through the hydraulic effect, and that is capable of achieving a simplified structure of the feeding passage for feeding working hydraulic fluid to the hydraulic lines extending between the actuators and the hydraulic pumps.

A fourth aspect of the present invention relates to a pump unit with first and second hydraulic pumps that are respectively connected via first and second hydraulic lines to first and second actuators driven through a hydraulic effect.

A hydraulic pump is used in various applications and in particular as the hydraulic pump adapted for operation in association with an actuator driven through the hydraulic effect. The description will hereinafter be made for the pump unit by taking for example the case where it includes the first and second hydraulic motors serving as the actuators that respectively drive the right and left drive wheels.

For example, U.S. Pat. No. 4,920,733 discloses a vehicle including first and second hydraulic pumps respectively connected via first and second hydraulic lines to the first and second hydraulic motors for driving the right and left drive wheels. In this vehicle, the first and second hydraulic motors respectively have outputs variable in response to the adjustment of the input/output flow rates of the first and second hydraulic pumps, thereby controlling the rotational speed and rotational direction of the right and left drive wheels.

The vehicle disclosed in the above cited US patent has the first hydraulic pump and the second hydraulic pump separately arranged from one another, the former being operated in association with the first hydraulic motor, and the latter being operated in association with the second hydraulic motor. Such a separate arrangement of the hydraulic pumps invites a complicated structure of a feeding passage for feeding working hydraulic fluid for an HST (hydrostatic transmission) from a hydraulic fluid tank to the first hydraulic line and the second hydraulic line, and poses various other problems.

As a further disadvantage, the working hydraulic fluid between the hydraulic pumps and the actuators may increase in temperature due to the load from the outside. Such an increase in temperature of the working hydraulic fluid may invite various problems such as lowering of the volumetric efficiency, or lowering of the axle revolution speed if the hydraulic motors are used as the actuators for driving the drive wheels of the vehicle, deteriorating of the durability. However, the above-cited US patent does not teach any solutions to limit the temperature of the working hydraulic fluid of the HST.

The fourth aspect of the present invention has been therefor conceived in consideration of the above prior art. It is an object of the fourth aspect of the present invention to provide a pump unit with the first and second hydraulic pumps that are respectively connected via the first and second hydraulic lines to the first and second actuators driven through the hydraulic effect, and that is capable of effectively limiting the increase in temperature of the working hydraulic fluid to be replenished to the hydraulic lines between the actuators and the hydraulic pumps.

According to the first aspect of the present invention, there is provided a tandem pump unit that includes: a first hydraulic pump and a second hydraulic pump respectively having a first pump shaft and a second pump shaft respectively having adjacent ends connected together so that the first and second pump shafts are coaxially aligned and non-rotatably connected in tandem; a common housing for accommodating the first hydraulic pump and the second hydraulic pump; and a first center section and a second center section that respectively support the first and second hydraulic pumps.

The tandem pump unit of the above arrangement can reduce the number of the parts and hence the manufacturing cost, in comparison with a conventional tandem pump unit, which requires a separate housing for each pump unit. As an additional advantage, the tandem pump unit allows both the first and second hydraulic pumps to be mounted in position only by mounting the common housing with the first and second hydraulic pumps therein. Whereby, the efficiency in assembling operation can be enhanced.

Furthermore, the common housing has first and second openings respectively formed at opposed ends of the housing with respect to the pump shaft direction, and a bearing wall located midway between the first and second openings. The first and second openings allow the first and second hydraulic pumps to pass therethrough, and the bearing wall supports the connection portions of the first and second pump shafts. The first and second openings are adapted to be respectively sealed by the first and second center sections respectively supporting the first and second hydraulic pumps.

Therefore, the first and second hydraulic pumps can be respectively placed through the first and second openings into the housing with the first and second hydraulic pumps being respectively supported by the first and second center sections. Thus, the assembling efficiency of the tandem pump unit can be enhanced. In addition, either one or both of the pumps can easily be removed from the housing, while the housing is still mounted on an object such as a vehicle, thereby achieving an improved operation efficiency in maintenance work, or the like.

Furthermore, the tandem pump unit of the first aspect of this invention is designed so that the first and second center sections are disposed at the opposed ends of the housing with respect to the pump shaft direction in the housing excluding the portion between the first and second hydraulic pumps, thereby allowing for great flexibility in designing the adjacent ends of the first and second pump shafts to be connected together.

The tandem pump unit of the first aspect of this invention preferably includes a coupler for non-rotatably receiving the adjacent ends of the first and second pump shafts.

In the case where the first and second hydraulic pumps are axial piston pumps of a variable displacement type respectively having first and second angularly adjustable swash plates of cradle type, the bearing wall is preferably designed to have a side facing the first and second hydraulic pumps that forms guiding surfaces for slidingly guiding spherical convex surfaces formed in rear sides of the swash plates, which rear sides are opposed to surfaces facing the first and second hydraulic pumps.

According to the second aspect of the present invention, there is provided a tandem pump unit that includes the integral arrangement of a first hydraulic pump and a second hydraulic pump. The first hydraulic pump with a first pump shaft is adapted to be connected to a first actuator via a first pair of hydraulic lines. The second hydraulic pump with a second pump shaft is adapted to be connected to a second actuator via a second pair of hydraulic lines. The first pump shaft and the second pump shaft respectively have adjacent ends connected together so that the first and second pump shafts are coaxially aligned and non-rotatably connected in tandem. The tandem pump unit of the second aspect of the present invention further includes a charge line disposed within the tandem pump unit and having a first end communicating with either a reservoir or a hydraulic fluid feeding mechanism and a second end adapted to communicate with the first pair of hydraulic lines and the second pair of hydraulic lines, respectively.

The tandem pump unit of the above arrangement allows hydraulic fluid to be replenished into the first pair of hydraulic lines and the second pair of hydraulic lines, only by connecting a hydraulic fluid feeding mechanism such as a charge pump to the first opening of the charging line. Whereby, the piping structure for charging can be simplified, and the manufacturing cost can be lowered through the reduction of the number of parts and the improved efficiency in assembling operation. Also, the workability in maintenance can be improved.

As an additional advantage, the charge line disposed within the pump unit is unlikely to be damaged by the contact against external parts, thereby effectively preventing the leakage of the hydraulic fluid to the outsides from the charge line. This hydraulic fluid leakage preventive arrangement is advantageous particularly when the pump unit is used in vehicles for traveling on lawns or other grounds such as a riding mower, a walk behind mower, a commercial ride-on mid-mount-deck lawnmower, a tractor, or other device.

In one case, the tandem pump unit of the second aspect has preferably a common center section having oppositely facing sides with respect to the pump shaft direction, respectively supporting the first and second hydraulic pumps via the oppositely facing sides, and a first pump housing and a second pump housing for respectively accommodating the first hydraulic pump and the second hydraulic pump. The common center section forms a first pair of inlet/outlet ports and a second pair of inlet/outlet ports. The first pair of inlet/outlet ports respectively serve as connection ports for connection with the first pair of hydraulic lines and the second pair of inlet/outlet ports respectively serve as connection ports for connection with the second pair of hydraulic lines. The charge line has a first bore portion, a conduit and a second bore portion. The first bore portion is formed in a peripheral wall of either the first pump housing or the second pump housing with a first end opening to the outside of the either the first pump housing or the second pump housing so as to communicate with the either the reservoir or the hydraulic fluid feeding mechanism and a second end opening to the inside of the either the first pump housing or the second pump housing. The conduit is disposed within the either the first pump housing or the second pump housing with a first end connected to the second end of the first bore portion and a second end extending to the common center section. The second bore portion is formed in the common center section with a first end connected to the second end of the conduit and a second end communicating with the first pair of inlet/outlet ports and the second pair of inlet/outlet ports, respectively.

In another case, the tandem pump unit of the second aspect also includes a common center section having oppositely facing sides with respect to the pump shaft direction, respectively supporting the first and second hydraulic pumps via the oppositely facing sides, a first pump housing and a second pump housing for respectively accommodating the first hydraulic pump and the second hydraulic pump. The common center section forms a first pair of inlet/outlet ports and a second pair of inlet/outlet ports. The first pair of inlet/outlet ports respectively serve as connection ports for connection with the first pair of hydraulic lines, and the second pair of inlet/outlet ports respectively serve connection ports for connection with the second pair of hydraulic lines. The charge line has a third bore portion and a fourth bore portion. The third bore portion is formed in a peripheral wall of either the first pump housing or the second pump housing with a first end opening to the outside of the either the first pump housing or the second pump housing so as to communicate with the either the reservoir or the hydraulic fluid feeding mechanism and a second end extending to the common center section. The fourth bore portion is formed in the common center section with a first end connected to the second end of the third bore portion and a second end communicating with the first pair of inlet/outlet ports and the second pair of inlet/outlet ports, respectively.

In still another case, the tandem pump unit of the second aspect of the present invention also includes a common pump housing for accommodating the first and second hydraulic pumps, a first center section and a second center section for respectively supporting the first hydraulic pump and the second hydraulic pump. The common pump housing has a first opening and a second opening respectively formed at opposed ends of the pump housing with respect to the pump shaft direction. The first and second openings allow the first and second hydraulic pumps to pass therethrough. The first and second center sections are respectively connected to the common pump housing so as to seal the first and second openings in a liquid tight manner. The first and second center sections respectively form a first pair of inlet/outlet ports and a second pair of inlet/outlet ports serving as connection ports for connection respectively with the first pair of hydraulic lines and the second pair of hydraulic lines. The charge line has a first end opening to the outside of either the first center section or the second center section so as to communicate with the either the reservoir or the hydraulic fluid feeding mechanism, and a second end communicating with the first pair of inlet/outlet ports and the second pair of inlet/outlet ports, respectively.

Preferably, the common pump housing has a bearing wall located midway thereof with respect to the pump shaft direction to support the adjacent ends of the first and second pump shafts. The bearing wall divides the common pump housing into a first hydraulic pump accommodation chamber and a second hydraulic pump accommodation chamber for respectively accommodating the first hydraulic pump and the second hydraulic pump.

Preferably, the common pump housing is designed to allow hydraulic fluid to communicate between the first hydraulic pump accommodation chamber and the second hydraulic pump accommodation chamber.

According to the third aspect of the present invention, there is provided a tandem pump unit for operation in association with actuators. The tandem pump unit includes the integral arrangement of a first hydraulic pump and a second hydraulic pump. The first hydraulic pump with a first pump shaft is adapted to be connected to a first actuator via a first pair of hydraulic lines, and the second hydraulic pump with a second pump shaft is adapted to be connected to a second actuator via a second pair of hydraulic lines. The first and second pump shafts respectively have adjacent ends connected together so that the first and second pump shafts are coaxially aligned and non-rotatably connected in tandem. The tandem pump unit also includes a center section supporting the first hydraulic pump and the second hydraulic pump, a housing accommodating the first hydraulic pump and the second hydraulic pump, and a reservoir tank supportingly connected to the single unit for storing hydraulic fluid to be replenished to the first pair of hydraulic lines and the second pair of hydraulic lines. The first hydraulic pump, the second hydraulic pump, the center section and the housing are integrally connected together to constitute a single unit.

The pump unit of the above arrangement can improve an efficiency in mounting the first and second hydraulic pumps on an object such as a vehicle, and shorten the length of the piping for replenishing the hydraulic fluid from the reservoir tank to the first pair of hydraulic lines and the second pair of hydraulic lines, thereby lowering the manufacturing cost, and improving an efficiency in replenishing the hydraulic fluid through the decrease of the resistance force between the hydraulic fluid and the pipe wall, and producing other desirable effects

Preferably, the single unit of the tandem pump unit of the third aspect of the present invention is designed so that the housing can serve as a hydraulic fluid tank, and the pump unit further includes a hydraulic fluid communication passage for providing a free fluid communication between the reservoir tank and the housing. With this arrangement, the number of the pipes required between the first and second hydraulic pumps, and the first and second actuators can be reduced to substantially four pipes only, specifically the first pair of hydraulic lines and the second pair of hydraulic lines. Thus, as compared with the conventional arrangements, the pump unit of this arrangement can achieve a lower manufacturing cost, an improved assembling efficiency and an excellent workability in maintenance. Since the housing itself also serves as a hydraulic fluid tank, the reservoir tank can compactly be made.

The tandem pump unit of the third aspect of the present invention preferably has the following arrangement. Specifically, the center section forms a first pair of hydraulic passages respectively having first ends communicating with the first hydraulic pump and second ends opening to the outside of the center section to form connection ports for connection with the first pair of hydraulic lines, a second pair of hydraulic passages respectively having first ends communicating with the second hydraulic pump and second ends opening to the outside of the center section to form connection ports for connection with the second pair of hydraulic lines, and a charging passage having a first end opening to the outside of the center section to form an inlet port for charging, serving as an inlet for the hydraulic fluid to be replenished and a second end communicating with the first pair of hydraulic passages and the second pair of hydraulic passages via check valves. The charging passage is connected to a pressure relief line communicating with the housing via a relief vale, and the inlet port for charging is connected to the reservoir tank via a hydraulic fluid replenishing passage.

The tandem pump unit of the third aspect of the present invention also preferably has the following arrangement. Specifically, the tandem pump unit includes a cooling fan provided near the single unit. The cooling fan is adapted to be driven in synchronism with the first and second hydraulic pumps. The reservoir tank is connected to the single unit in such a manner as to form a clearance therebetween, into which a cooling air stream is drawn from the cooling fan. The hydraulic fluid communication passage and the hydraulic fluid replenishing passage are disposed in such a manner to traverse the clearance. The thus arranged pump unit can limit the temperature increase of the hydraulic fluid stored in the reservoir tank and the housing, and also effectively limit the temperature increase of the hydraulic fluid flowing through the hydraulic fluid replenishing passage and the hydraulic fluid communication passage, thereby improving the transmission efficiency between the hydraulic pumps and the actuators.

According to the fourth aspect of the present invention, there is provided a tandem pump unit that includes: a first hydraulic pump and a second hydraulic pump respectively having a first pump shaft and a second pump shaft that are coaxially aligned and non-ratably connected in tandem; a center section supporting the first hydraulic pump and the second hydraulic pump; and a housing accommodating the first hydraulic pump and the second hydraulic pump. The housing is adapted to be used as a hydraulic fluid tank. A hydraulic fluid circulation mechanism is also provided for taking the hydraulic fluid from the hydraulic tank, and again returning the same to the hydraulic tank. The hydraulic fluid circulation mechanism is designed to cool the hydraulic fluid while circulating the same.

The tandem pump unit of the above arrangement can effectively limit the increase in temperature of the hydraulic fluid stored within the hydraulic tank, thereby effectively preventing deterioration in working efficiency of a hydraulic actuation device.

Preferably, the circulation mechanism of the tandem pump unit of the fourth aspect of the present invention includes a circulation line, at least a portion of which serves as a conduit; the circulation line having a first end communicating with the inside of the hydraulic tank and a second end again communicating with the inside of the hydraulic tank. The conduit has at least a portion provided thereon with cooling fins.

Further, the pump unit of the fourth aspect of the present invention preferably has the following arrangement. The center section forms a first pair of hydraulic passages respectively having first ends communicating with the first hydraulic pump and second ends opening to the outside of the center section to form connection ports for connection with the first pair of hydraulic lines, a second pair of hydraulic passages respectively having first ends communicating with the second hydraulic pump and second ends opening to the outside of the center section to form connection ports for connection with the second pair of hydraulic lines, and a charging passage having a first end communicating with the hydraulic fluid tank to form an inlet port for charging, serving as an inlet for the hydraulic fluid to be replenished and a second end communicating with the first pair of hydraulic passages and the second pair of hydraulic passages via check valves. The tandem pump unit further includes: a charge pump for sucking the hydraulic fluid stored within the hydraulic fluid tank and then discharging the same into the inlet port for charging; and a pressure relief line having a first end connected to the charging passage via a relief valve and a second end forming a drain port through which the hydraulic fluid from the relief valve is drained. The second end of the pressure relief line is connected to the conduit, and the charge pump constitutes a part of the hydraulic fluid circulation mechanism.

Preferably, the pump unit of the fourth aspect of the present invention further includes a reservoir tank, in which the reservoir tank is in free fluid communication with the housing via a hydraulic fluid communication passage, and forms a hydraulic fluid tank in cooperation with the housing, and the inlet port for charging communicates with the reservoir tank via a hydraulic fluid replenishing passage.

Preferably the pump unit of the fourth aspect of the present invention further includes cooling fins provided on the hydraulic fluid replenishing passage and the hydraulic fluid communication passage.

Further, the pump unit of the fourth aspect of the present invention preferably has the following arrangement. Specifically, a cooling fan adapted to be driven in synchronism with the first and second hydraulic pumps is provided near the housing. The reservoir tank is connected to the housing in such a manner as to form a clearance therebetween, into which a cooling air stream from the cooling fan is drawn. The hydraulic fluid communication passage and the hydraulic fluid replenishing passage are disposed to transverse the clearance.

A cooling air duct is preferably provided in the pump unit of the fourth aspect of the present invention, so that a cooling air stream from the cooling fan is drawn into the clearance along the cooling air duct.

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. 2 is a hydraulic circuit diagram of the vehicle to which one embodiment of a tandem pump unit according to the first aspect of the present invention is applied.

FIG. 9 is a longitudinal cross-sectional front view of a portion of the tandem pump unit according to another embodiment of the first aspect of the present invention.

FIG. 12 is a hydraulic circuit diagram of the vehicle to which one embodiment of a tandem pump unit according to the second aspect of the present invention is applied.

FIG. 13 is a longitudinal cross-sectional front view of the tandem pump unit according to the first embodiment of the second aspect of the present invention.

FIG. 19 is a longitudinal cross-sectional front view of the tandem pump unit according to another embodiment of the second aspect of the present invention.

FIG. 22 is a hydraulic circuit diagram of the vehicle to which one embodiment of a pump unit according to the third aspect of the present invention is applied.

FIG. 32 is a hydraulic circuit diagram of the vehicle to which one embodiment of a pump unit according to the fourth aspect of the present invention is applied.

The first embodiment of the pump unit according to the first aspect of the present invention will be hereinafter described with reference to the accompanying drawings.

A pump unit 100 according to the first aspect of the present invention is designed to be operated in association with an actuator that is connected thereto via first and second pairs of hydraulic lines 184 aand 184 band driven through an effect of pressurized hydraulic fluid in the pairs of hydraulic lines. This embodiment will be described by taking for example the case that hydraulic motors 182 aand 182 beach are used as the actuator.

FIG. 1 is an expansion plan view of a vehicle to which the pump unit 100 of this embodiment is applied. The reference codes 185, 197 aand 197 b, 199, 198 aand 198 b, and 192 aand 192 bin FIG. 1 respectively represent a reservoir tank, caster wheels, a driver seat, steering wheels, and a linkage mechanism connecting between the steering wheels and the hydraulic motors.

FIG. 2 is a hydraulic circuit diagram of the vehicle to which the tandem pump unit 100 of this embodiment is applied. FIGS. 3 and 4 are respectively longitudinal cross-sectional front and side views of the tandem pump unit 100. FIG. 5 is a cross section taken along lines V—V in FIG. 4.

As illustrated in FIGS. 2–4, the pump unit 100 is adapted to be used in a vehicle having right and left drive wheels 183 aand 183 bto which first and second hydraulic motors 182 aand 182 brespectively connected. The pump unit 100 is of a tandem type which includes a first hydraulic pump 110 aand a second hydraulic pump 110 brespectively connected to the first and second hydraulic motors via a first pair of hydraulic lines 184 aand a second pair of hydraulic lines 184 b, a pump case 120 that accommodates the first and second hydraulic pumps 110 aand 110 b, and a first center section 130 aand a second center section 130 brespectively supporting the first and second hydraulic pumps 110 aand 110 bwhose first pump shaft 111 aand second pump shaft 111 bare disposed in a tandem arrangement, that is, coaxially disposed as connected together in a non-rotatable manner relative to one another.

The connection form between the right and left drive wheels 183 aand 183 b, and the first and second hydraulic motors 182 aand 182 bmeant in this embodiment includes the direct connection of the drive wheels respectively to those hydraulic motors, and also the operative connection of the drive wheels respectively to those drive wheels via a suitable power transmission mechanism.

In this embodiment, the pump unit 100 is of a vertical type that has the vertically extending first and second hydraulic pump shafts 111 aand 111 b. However, the first aspect of the present invention is not necessarily limited to this arrangement. Rather, it is a matter of course to employ the pump unit of a horizontal type that has the horizontally extending first and second hydraulic pump shafts 111 aand 111 b. The reference codes 180, 181 and 185 in FIG. 2 respectively represent a power source, a cooling fun and a reservoir tank.

As illustrated in FIGS. 3 and 4, the common pump case 120 of a box shape has a first opening 120 aand a second opening 120 bwhich are respectively formed in the opposed ends along the longitudinal direction thereof, and a bearing wall 120 cwhich is disposed midway between the first and second openings 120 aand 120 b. The first and second openings 120 aand 120 ballow the first and second hydraulic pumps to pass therethrough into the common case 120.

The first and second center sections 30 aand 30 bare respectively connected to the common pump case 120 in such a manner as to respectively have the first and second openings 120 aand 120 bsealed. Specifically, the pump case 120 is designed to have a first pump unit accommodation chamber defined between the bearing wall 120 cand the first center section 130 a, and a second pump unit accommodation chamber defined between the bearing wall 120 cand the second center section 130 b, and also to serves as the reservoir tank 185.

FIG. 5 is a cross section taken along lines V—V in FIG. 4. As illustrated in FIGS. 3–5, in this embodiment, the first and second hydraulic pumps 110 aand 110 bare axial piston pumps of a variable displacement type. The pumps 110 aand 110 brespectively include the first hydraulic pump shaft 111 aand the second hydraulic pump shaft 111 b, both of which are coaxially disposed and non-rotatably connected to one another at the adjacent ends thereof, a first piston unit 112 aand a second piston unit 112 bthat are reciprocatingly movable according to the rotation of the pump shafts, a first cylinder block 113 aand a second cylinder block 113 bthat respectively and reciprocatingly support the piston units, a first angulary adjustable swash plate 114 aand a second angulary adjustable swash plate 114 bthat regulate the stroke length of the piston units by varying their tilting angle to vary their input/output flow rates, and a first control shaft 115 aand a second control shaft 115 bthat control their tilting angles of these swash plates.

In the above pump units, by operating the swash plates to respectively vary the input/output flow rate of the first and second pump units 110 aand 110 b, there occurs a pressure difference of hydraulic fluid between the first pair of hydraulic lines 184 a, and/or between the second pair of hydraulic lines 184 b. The pressure difference causes a motor shaft of the first hydraulic motor 182 aand/or a motor shaft of the second hydraulic motor 182 bto rotate at a speed proportional to the amount of the pressure difference, thereby driving the drive wheels 183 aand 183 boperatively connected to the motor shafts.

As described above, the first and second hydraulic pumps 110 aand 110 baccording to this embodiment are of the variable displacement type, and the first and second hydraulic motors in association with the first and second hydraulic pumps 110 aand 110 bare of the fixed displacement type. However, the first aspect of the present invention is not necessarily limited to this arrangement. That is, it is possible to employ the hydraulic pumps of the fixed displacement type, and the hydraulic motors of the variable displacement type driven by the hydraulic pumps, or the hydraulic pumps and the hydraulic motors, both of which are of the variable displacement type.

In this embodiment, the first and second hydraulic pumps 110 aand 110 bare of the axial piston type. Alternatively, the pump unit may employ the hydraulic pumps of a radial piston type.

The first and second control shafts 115 aand 115 brespectively have inner ends that extend into the first and second pump accommodation chambers to be connected to the first and second swash plates 114 aand 114 bat the center points of the tilting areas thereof, and outer ends that protrude outwards from the common pump case 120 in the vehicle width direction orthogonal to the pump shafts 111 aand 111 bto allow these shafts 115 aand 115 bto extend away from one another. This arrangement is advantageous when the pump unit 100 is installed on the vehicle having push-pull control levers 198 aand 198 bas illustrated in FIG. 1, since the first and second control shafts 115 aand 115 bcan have the rotational axes parallel to the pivotal axes of the control levers, thereby achieving the simplification of a link mechanism between these control shafts and the control levers.

FIG. 6 is a cross section taken along lines VI—VI in FIG. 5. As illustrated in FIGS. 3, 4 and 6, the first center section 130 ahas a first surface facing the common pump case 120 (or the upper surface in this embodiment) connected to the common pump case 120 so as to seal the first opening 120 aof the common pump case 120 in a liquid tight manner, with the first hydraulic pump 110 asupported on the first surface. The first pump shaft 111 aof the first hydraulic pump 110 ahas the upstream end in the power transmitting direction (or the lower end in this embodiment) extending downwardly through the first center section 130 ato form an extension. The extension is designed to receive the power for the pump unit 100 and the cooling fun 181 from the power source 180 through a suitable power transmission mechanism such as a belt type power transmission device.

On the other hand, as illustrated in FIGS. 3, 4 and 6, the second center section 130 bhas a first surface facing the common pump case 120 (or the lower surface in this embodiment) connected to the common pump case 120 so as to seal the second opening 120 bof the common pump case 120 in a liquid tight manner, with the second hydraulic pump 110 bsupported on the first surface. The second pump shaft 111 bof the second hydraulic pump 110 bhas the downstream end in the power transmitting direction (or the upper end in this embodiment) extending upwardly through the second center section 130 bto form an extension through which a charge pump 150 described below is driven.

The first and second pump shafts 111 aand 111 bare non-rotatably connected relative to one another, by the connection between the downstream end portion of the first pump shaft in the power transmitting direction (or the upper end) and the upstream end portion of the second pump shaft in the power transmitting direction (or the lower end). The connection portions of the shafts 111 aand 111 bare supported in the bearing wall 120 cof the common pump case 120. In this embodiment, the pump unit 100 is provided with a coupler 116 for receiving the connection portions of the first pump shaft 111 aand the second pump shaft 111 b, and coupling the same together. The coupler 116 is rotatably supported via a bearing member 117 in a bearing hole 120 dformed in the bearing wall 120 c. Whereby, the upper end portion of the first pump shaft and the lower end portion of the second pump shaft are non-rotatably connected relative to one another, and rotatably supported by the bearing wall 120 c.

In this embodiment, as illustrated in FIGS. 3 and 4, two ball bearings are disposed parallel to one another to be used as the bearing member 117. Rather, it is a matter of course to employ just one ball bearing for supporting the coupler 116.

FIGS. 7 and 8 are respectively cross sections taken along lines VII—VII and VIII—VIII in FIG. 4. As illustrated in FIGS. 2 and 8, the first center section 130 aforms a first pair of hydraulic passages 131 afor the first hydraulic pump 110 ahaving first ends that respectively open to the inside of the common pump case 120 through one side of the first center section 130 ain the pump shaft direction so as to communicate with inlet/outlet ports of the first hydraulic pump 110 a, and second ends that respectively open to the outside of the common pump case 120 through the other side of the center section 130 aso as to form a first pair of inlet/outlet ports 132 awhich serve as connection ports with the first pair of hydraulic lines 184 abetween the first hydraulic pump 110 aand the first hydraulic motor 182 a.

The first center section 130 ahas mounting bosses integrally formed therewith at four corners, for mounting the first center section on a chassis of the vehicle (see FIG. 8).

On the other hand, as illustrated in FIGS. 2 and 7, the second center section 130 bforms a second pair of hydraulic passages 131 bfor the second hydraulic pump 110 bhaving first ends that respectively open to the inside of the pump case 120 through one side of the second center section 130 bin the pump shaft direction so as to communicate with inlet/outlet ports of the second hydraulic pump 110 b, and second ends that respectively open to the outside of the common pump case 120 through the other side of the center section 130 bso as to form a second pair of inlet/outlet ports 132 bwhich serve as connection ports with the second pair of hydraulic lines 184 bbetween the second hydraulic pump 110 band the second hydraulic motor 182 b.

The second center section 130 balso forms a first bore 133 ahaving a first end that opens to the outsides of the second center section 130 bthrough the upper surface thereof to form an inlet port for charging 134, and bifurcated second ends so as to communicate with the second pair of hydraulic passages 131 b, while opening to the second hydraulic pump accommodation chamber through the lower surface of the second center section 130 b. The inlet port for charging 134 communicates with an outlet port 151 of the charge pump 150 so as to receive pressurized hydraulic fluid directly from the charge pump 150.

As illustrated in FIG. 6, the one end of the bifurcated second ends of the first bore 133 a, which opens to the second hydraulic pump accommodation chamber, is connected with a first end of a conduit portion 133 b. The conduit 133 bis disposed within the pump unit 100, as extending through the second hydraulic pump accommodation chamber, the bearing wall 122 cand the first hydraulic pump accommodation chamber, and having the second end reaching the first center section 130 a.

In this embodiment, the bearing wall 120 cforms a hydraulic fluid communication hole 120 ffor communication between the first hydraulic pump accommodation chamber and the second hydraulic pump accommodation chamber. The hydraulic fluid communication hole 120 falso allows the conduit 133 bto pass therethrough. With this arrangement, the conduit 133 bcan extend through the bearing wall 120 c(see FIGS. 5 and 6).

The first center section also forms a second bore 133 chaving one end that communicates with the second end of the conduit 133 b, and bifurcated second ends that respectively communicate with the first pair of hydraulic passages 131 a.

The thus arranged first bore 133 a, conduit 133 band second bore 133 ctogether form a common charge passage 133 for feeding pressurized hydraulic fluid from a suitable hydraulic fluid feeding mechanism such as the charge pump to the first pair of hydraulic lines 184 aand the second pair of hydraulic lines 184 bvia the first pair of hydraulic passages 131 aand the second pair of hydraulic passages 131 b(see FIG. 2).

Instead of the conduit 133 b, it is possible to employ a bore formed in a side wall of the common pump case 120, and the first and second center sections 130 aand 130 b.

The charge line 133 of this embodiment thus disposed within the pump unit 100 is advantageous in the fact that the pressurized hydraulic fluid can be replenished to the first and second pairs of hydraulic lines 184 aand 184 bvia the first and second pair of inlet/outlet ports 132 aand 132 bonly by feeding pressurized hydraulic fluid from a suitable hydraulic fluid feeding mechanism to the inlet port for charging 134. Whereby, the piping structure for charging can be simplified, and the manufacturing cost can be lowered through the reduction of the number of parts and the improved efficiency in assembling operation can be obtained.

As an additional advantage, the above arrangement where the charge line 133 is disposed within the pump unit 100 can make the charge line 133 unlikely to be damaged by the contact against external parts. Whereby, the leakage of the hydraulic fluid to the outsides from the charge line 133 can be effectively prevented. The hydraulic fluid leakage preventive arrangement is advantageous particularly, when the pump unit 100 is used in vehicles such as a lawnmower.

The outlet port 151 of the charge pump 150 is communicated with the inlet port for charging 134, and also a first end of a pressure relief line 152 that is provided with a relief valve 152 for regulating the hydraulic pressure of the charge line 133 (see FIGS. 2 and 6). The pressure relief line 153 has a downstream end or a second end that communicates with the inside of the common pump case 120, also serving as a reservoir tank 150, via a drain port 135 formed in the second center section 130 b(see FIGS. 2 and 6). The reference codes 150 and 156 in FIG. 3 respectively represent an inlet port of the charge pump, and an inlet port which acts as a connection port with the reservoir tank 185 and communicates with the inlet port of the charge pump 185.

The second end of the first bore 133 aconstituting a part of the charge line 133 is, as illustrated in FIGS. 2, 6 and 7, respectively connected with each of the first pair of hydraulic passages 131 bvia check valves 161 cand 161 d.

Similarly, the bifurcated second ends of the second bore 133 bconstituting a part of the charge line 133 are, as illustrated in FIGS. 2, 6 and 8, respectively connected with each of the first pair of hydraulic passages 131 avia check valves 161 aand 161 b.

These check valves 161 a, 161 b, 161 cand 161 dare designed to allow the flow of the pressurized hydraulic fluid from the charge passage 133 to the lower pressured line of the first pair of hydraulic lines 184 aand the lower pressured line of the second pair of hydraulic lines 184 b, while preventing the reverse flow.

Bypass lines 162 aand 162 beach having a throttle valve are preferably formed between the charge passage 133 and at least one of the first pair of hydraulic passages 131 a, and between the charge passage 133 and at least one of the second pair of hydraulic passages 131 b(see FIGS. 2, 7 and 8)

The bypass lines 162 aand 162 bare designed to assure the neutralization of the hydraulic pumps 110 aand 110 b. Specifically, even if the swash plates 114 aand 114 bof the hydraulic pumps 110 aand 110 btilt from the neutral positions by a small angle, there occurs the pressure difference between the first pair of hydraulic lines 184 a, and/or between the second pair of hydraulic lines 184 b. This pressure difference causes the rotation of the hydraulic motors 182 aand 182 b. That is, even a slight amount of the displacement between the actual neutral positions and the predetermined design positions of the swash plates 114 aand 114 bdue to assembling error or the like causes an unintentional rotation of the hydraulic motors 182 aand 182 b. On the contrary, the bypass lines 162 aand 162 b, as described above, allow the pressurized hydraulic fluid of the small amount to leak therethrough from the first pair of hydraulic lines 184 aor the second pair of hydraulic lines 184 b. Thus, the swash plates can have the neutral positions of a broadened effective area by effectively limiting the pressure difference between the pair of first hydraulic lines 184 a, and/or between the second pair of hydraulic lines 184 b, thereby effectively avoiding the unintentional rotation of the hydraulic motors 182 aand 182 b, even for the swash plates 114 aand 114 bhaving the actual neutral position displaced from the design neutral position due to the assembling errors or the like.

In view of transmission efficiency between the hydraulic pumps 110 a, 110 band the hydraulic motors 182 a, 182 b, the leakage of the pressurized hydraulic fluid from the first and second pairs of hydraulic lines 184 a, 184 bthrough the bypass lines 162 a, 162 bis not preferable. Therefore, the bypass lines 162 a, 162 bare preferably provided in portions from the first charge passage 133 to one of the first pair of hydraulic passages 133 a, and to one of the second pair of hydraulic passages 133 b, and more preferably to one of the first pair of hydraulic passages 133 awhich has a higher pressure during rearward movement of the vehicle. This is because the forward movement of the vehicle frequently occurs as compared with the rearward movement.

The check valves 161 a, 161 b, 161 cand 161 dare more preferably provided with release means 163 to forcibly bring the first pair of hydraulic passages 131 ainto communication with one another, and the second pair of hydraulic passages 131 binto communication with one another, if an emergency arises, as illustrated in FIGS. 7 and 8. The release means 163 are designed to easily move the vehicle, when the vehicle must forcibly be moved or the vehicle wheels must forcibly be rotated by man power or the like due to the disorder of the power source 180, the hydraulic pumps 110 a, 110 bor the like. Specifically, when the vehicle wheels connected to the hydraulic motors 182 aand 182 bare forcibly rotated with the first pair of hydraulic lines 184 aand/or the second pair of hydraulic lines 184 blying in the closing state, there occurs the pressure difference between the first pair of hydraulic lines 184 a, and/or between the second pair of hydraulic lines 184 b. As a result, the vehicle is hardly moved, or the vehicle wheels are hardly rotated. On the contrary, the release means can easily achieve the communications between the first pair of hydraulic passages 131 a, and between the second pair of hydraulic passages 131 bby mechanically releasing all the check valves 161 ato 161 d. Whereby, the vehicle can easily be moved by man power or the like.

As illustrated in FIG. 5, all the release means 163 are preferably disposed in the same side of the center section 130, so that the link mechanism linking these release means 163 for operation of the same can have a simplified structure.

The pump unit 100 of this embodiment includes the charge pump 150, as a hydraulic fluid feeding mechanism for the first and second pair of hydraulic lines 184 aand 184 b, to forcibly feed the pressurized hydraulic fluid into the inlet port for charging 134. As an alternative to the arrangement using the charge pump, the pump unit 100 may have an arrangement where the inlet port 134 is connected to the hydraulic fluid tank, thereby allowing the hydraulic fluid to spontaneously flow into the inlet port 134 when the pressure in a lower pressure line of the first pair of hydraulic lines 184 aand/or the pressure in a lower pressure line of the second pair of hydraulic lines 184 bdrops from a predetermined value.

As described above, the pump unit 100 of this embodiment is designed to have the first and second hydraulic pump 110 aand 110 baccommodated within the common pump case 120, thereby achieving a lower manufacturing cost through a relatively small number of parts in comparison with a prior pump unit which needs pump cases respectively used for the first hydraulic pump and the second hydraulic pump.

As an additional advantage, in the pump unit 100, both first and second hydraulic pumps 110 aand 110 bcan be mounted on the vehicle only by mounting the unitary common pump case 120 with the first and second pumps 110 aand 110 btherein on the vehicle. Whereby the work efficiency in assembling the vehicle can be also enhanced.

As described above, in the pump unit 120, the common pump case 120 has the first and second openings 120 aand 120 bat the opposed ends thereof along the pump shaft direction, which openings 120 aand 120 brespectively allowing the first and second hydraulic pump 110 aand 110 bto pass therethrough into the pump case, and the bearing wall 120 cwhich is disposed between the first and second openings to support the connection portion of the first and second pump shafts 111 aand 111 b. The pump case 120 also has the first and second openings 120 aand 120 brespectively sealed by the first and second center section 130 aand 130 b, with the first and second hydraulic pumps 110 aand 110 brespectively supported by the center sections 130 aand 130 b. Hence, the pump unit 100 allows the first and second hydraulic pumps 110 aand 110 bto be respectively placed into the common pump case 120 through the first and second openings 120 aand 120 b, after mounting the first and second hydraulic pumps 110 aand 110 brespectively on the first and second center sections 130 aand 130 b. Whereby, the assembling efficiency of the pump unit can be enhanced. Furthermore, this arrangement allows either one or both of the hydraulic pumps 110 aand 110 bto be detached from the common pump case 120 with the housing being mounted of the vehicle, thereby achieving the improved operation efficiency in a maintenance, or the like.

Since the pump unit 100 is designed so that the first and second center section 130 aand 130 bare respectively disposed at the opposed ends of the pump case in the longitudinal direction thereof not to be located between the first and second hydraulic pumps 110 aand 110 b. This allows for great flexibility in designing connection portions of the first and second pump shaft 111 aand 111 b. Specifically, if the center section is disposed between the first and second hydraulic pumps, the first and second pump shafts must be connected together without interfering passages formed in the center section, resulting in a lowered flexibility in designing the connection portions and, in some cases, a large-sized pump unit. On the contrary, the pump unit 100 of this embodiment can effectively avoid these problems.

In this embodiment, the swash plates of the first and second hydraulic pumps 110 aand 110 bare of a trunnion type. However, it is of course to employ the swash plates of a cradle type. FIGS. 9 and 10 are respectively a longitudinal cross-sectional front view and a longitudinal cross-sectional side view of a modified pump unit according to the present invention, which includes hydraulic pumps having swash plates of a cradle type.

When the hydraulic pumps have angularly adjustable swash plates 414 aand 414 bof a cradle type, as illustrated in FIG. 34, the bearing wall 120 c′ may preferably forms, on its side facing the hydraulic pumps 110 a′ and 110 b′, spherical concave surfaces 120 erespectively adapted to spherical convex surfaces 118 formed in the rear sides of the swash plates which rear sides being opposite to the surfaces facing the piston units 112 aand 112 b. With this arrangement, the spherical concave surfaces 120 ecan slidingly guide the spherical convex surfaces 118 of the swash plates 114 aand 114 b, thereby securely resting the swash plates 114 aand 114 bthereon.

As described above, in this embodiment, the bearing members 117 are interposed between the outer circumferential surface of the coupler 116 with the connection portions of the first and second pump shafts 111 aand 111 bnon-rotatably inserted thereinto, and the inner circumferential surface of the bearing hole 120 dformed in the bearing wall 120 c, so that the adjacent ends of the first and second pump shafts 111 aand 111 bare supported on the bearing wall 120 cand are connected non-rotatably relative to one another. Alternative to this arrangement, as illustrated in FIG. 11, it is possible that the bearing members 117 are interposed between the outer circumferential surface of the pump shaft 111 aand the inner circumferential surface of the bearing hole 120 d, and between the outer circumferential surface of the pump shaft 111 band the inner circumferential surface of the bearing hole 120 d.

In this embodiment, the bearing wall 120 cis formed integrally with the pump case 120. Instead of this arrangement, it is also possible to employ a bearing wall separately arranged from the pump case to be mounted thereto.

The preferred embodiment of the pump unit according to the second aspect of the present invention will be hereinafter described with reference to the accompanying drawings.

A pump unit 200 according to the second aspect of the present invention is designed to be operated in association with an actuator that is connected thereto via first and second pairs of hydraulic lines 284 aand 284 band driven through an effect of pressurized hydraulic fluid in the pairs of hydraulic lines. This embodiment will be described by taking for example the case that hydraulic motors 282 aand 282 beach are used as the actuator.

FIG. 1 is an expansion plan view of a vehicle to which the pump unit 200 of this embodiment is applied. The reference codes 285, 297 aand 297 b, 299, 298 aand 298 b, and 292 aand 292 bin FIG. 1 respectively represent a reservoir tank, caster wheels, a driver seat, steering wheels, and a linkage mechanism connecting between the steering wheels and the hydraulic motors.

FIG. 12 is a hydraulic circuit diagram of the vehicle to which the tandem pump unit 200 of this embodiment is applied. FIG. 13 is a longitudinal cross-sectional side view of the tandem pump unit 100. FIGS. 14 to 16 are respectively cross sections taken along lines XIV—XIV, XV—XV, and XVI—XVI in FIG. 13.

As illustrated in FIGS. 12 and 13, the pump unit 200 is adapted to be used in a vehicle having right and left drive wheels 283 aand 283 bto which first and second hydraulic motors 282 aand 282 bare respectively connected. The pump unit 200 is of a tandem type which includes a first hydraulic pump 210 aand a second hydraulic pump 210 brespectively connected to the first and second hydraulic motors 282 aand 282 bvia the first pair of hydraulic lines 284 aand the second pair of hydraulic lines 284 b, which first hydraulic pump 210 aand second hydraulic pump 210 brespectively have a first pump shaft 211 aand a second pump shaft 211 bdisposed in a tandem arrangement, that is, coaxially aligned as connected together in a non-rotatable manner relative to one another.

The connection form between the right and left drive wheels 283 aand 283 b, and the first and second hydraulic motors 282 aand 282 bmeant in this embodiment includes the direct connection of the drive wheels respectively to those hydraulic motors, and also the operative connection of the drive wheels respectively to those drive wheels via a suitable power transmission mechanism.

In this embodiment, the pump unit 200 is of a vertical type that has the vertically extending first and second hydraulic pump shafts 211 aand 211 b. However, the second aspect of the present invention is not necessarily limited to this arrangement. Rather, it is a matter of course to employ the pump unit of a horizontal type that has the horizontally extending first and second hydraulic pump shafts 211 aand 211 b. The reference codes 280, 281 and 285 in FIG. 12 respectively represent a power source, a cooling fun and a reservoir tank.

As illustrated in FIG. 13, the pump unit 200 also includes a common center section 230 which has a first surface facing one side along the pump shaft (or the lower surface in this embodiment) to support the first hydraulic pump 210 aand a second surface facing the other side of the pump shaft (or the upper surface in this embodiment) to support the second hydraulic pump 210 b, and a first pump case 221 and a second pump case 222 which respectively accommodate the first and second hydraulic pumps 210 aand 210 b.

As illustrated in FIG. 13, in this embodiment, the first and second hydraulic pumps 210 aand 210 bare axial piston pumps of a variable displacement type. The pumps 210 aand 210 brespectively include the first hydraulic pump shaft 211 aand the second hydraulic pump shaft 211 b, both of which are coaxially disposed and non-rotatably connected to one another at the adjacent ends thereof, a first piston unit 212 aand a second piston unit 212 bthat are reciprocatingly movable according to the rotation of the pump shafts, a first cylinder block 213 aand a second cylinder block 213 bthat respectively and reciprocatingly support the piston units, a first angularly adjustable swash plate 214 aand a second angularly adjustable swash plate 214 bthat regulate the stroke length of the piston units by varying their tilting angle to vary their input/output flow rates, and a first control shaft 215 aand a second control shaft 215 bthat control their tilting angles of these swash plates.

In the above pump units, by operating the swash plates to respectively vary the input/output flow rate of the first and second pump units 210 aand 210 b, there occurs a pressure difference of hydraulic fluid between the first pair of hydraulic lines 284 a, and/or between the second pair of hydraulic lines 284 b. The pressure difference causes a motor shaft of the first hydraulic motor 282 aand/or a motor shaft of the second hydraulic motor 282 bto rotate at a speed proportional to the amount of the pressure difference, thereby driving the drive wheels 283 aand 283 boperatively connected to the motor shafts.

As described above, the first and second hydraulic pumps 210 aand 210 baccording to this embodiment are of the variable displacement type, and the first and second hydraulic motors in association with the first and second hydraulic pumps 210 aand 210 bare of the fixed displacement type. However, the second aspect of the present invention is not necessarily limited to this arrangement. That is, it is possible to employ the hydraulic pumps of the fixed displacement type, and the hydraulic motors of the variable displacement type driven by the hydraulic pumps, or the hydraulic pumps and the hydraulic motors, both of which are of the variable displacement type.

In this embodiment, the first and second hydraulic pumps 210 aand 210 bare of the axial piston type. Alternatively, the pump unit may employ the hydraulic pumps of a radial piston type.

The first and second control shafts 115 aand 115 brespectively have inner ends that extend into the first and second pump cases 221 and 222 to be connected to the first and second swash plates 214 aand 214 b, and outer ends that protrude outwards from the first and second pump cases 221 and 222 in the vehicle width direction orthogonal to the pump shafts 211 aand 211 bto allow these shafts 215 aand 215 bto extend away from one another. This arrangement is advantageous when the pump unit 200 is installed on the vehicle having push-pull control levers 198 aand 198 bas illustrated in FIG. 1, since the first and second control shafts 215 aand 215 bcan have the rotational axes parallel to the pivotal axes of the control levers, thereby achieving the simplific