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There is provided a pump unit including plurality of hydraulic pumps respectively fluidly connected to plurality of hydraulic motors, which are disposed away from the pump unit, so as to form closed circuits, the pump unit including a pump case accommodating the plurality of hydraulic pumps so that

There is provided a pump unit including plurality of hydraulic pumps respectively fluidly connected to plurality of hydraulic motors, which are disposed away from the pump unit, so as to form closed circuits, the pump unit including a pump case accommodating the plurality of hydraulic pumps so that the plurality of hydraulic pumps are operatively driven by a common power source; and a filter mounted at the pump case so as to be inserted in a charge passage for replenishing operation fluid from a fluid sump to the closed circuits.

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What is claimed is: 1. A pump unit including a plurality of hydraulic pumps respectively fluidly connected to a plurality of hydraulic motors, which are disposed away from the pump unit, so as to form closed circuits, the pump unit comprising: (a) a pump case accommodating the plurality of hydrauli

What is claimed is: 1. A pump unit including a plurality of hydraulic pumps respectively fluidly connected to a plurality of hydraulic motors, which are disposed away from the pump unit, so as to form closed circuits, the pump unit comprising: (a) a pump case accommodating the plurality of hydraulic pumps so that the plurality of hydraulic pumps are operatively driven by a common power source; (b) a charge pump having a suction side fluidly connected to the fluid sump and a discharge side fluidly connected to a charge passage, the charge pump operatively driven by the common power source; and (c) a charge pump case accommodating the charge pump; (d) a filter inserted in the charge passage for replenishing operation fluid from a fluid sump to the closed circuits; (e) wherein the filter is mounted at the charge pump case in a threaded manner. 2. A pump unit including a plurality of hydraulic pumps respectively fluidly connected to a plurality of hydraulic motors, which are disposed away from the pump unit, so as to form closed circuits, the pump unit comprising: (a) a pump case accommodating the plurality of hydraulic pumps so that the plurality of hydraulic pumps are operatively driven by a common power source; (b) a charge pump having a suction side fluidly connected to the fluid sump and a discharge side fluidly connected to the charge passage, the charge pump operatively driven by the common power source; (c) a charge pump case accommodating the charge pump; (d) a filter inserted in a charge passage for replenishing operation fluid from a fluid sump to the closed circuits; (e) wherein the plurality of hydraulic pumps includes first and second hydraulic pumps respectively driven by first and second pump shafts that are arranged in parallel to each other; (f) the charge pump is driven by one of the first and second pump shafts, the one pump shaft having an output end that extends outwards from the pump case accommodating the first and second hydraulic pumps; and (g) the filter is mounted at the charge pump case so as to overlap with the hydraulic pump driven by the other pump shaft out of the first and second pump shafts when seen from the front along the axial direction of the first and second pump shafts. 3. A pump unit including a plurality of hydraulic pumps respectively fluidly connected to a plurality of hydraulic motors, which are disposed away from the pump unit, so as to form closed circuits, the pump unit comprising: (a) a pump case accommodating the plurality of hydraulic pumps so that the plurality of hydraulic pumps are operatively driven by a common power source; (b) a charge pump having a suction side fluidly connected to the fluid sump and a discharge side fluidly connected to a charge passage, the charge pump operatively driven by the common power source; and (c) a charge pump case accommodating the charge pump; (d) a filter inserted in the charge passage for replenishing operation fluid from a fluid sump to the closed circuits; (e) wherein the filter includes a filter case having a contact surface contacted to the charge pump case, a filter body accommodated in the filter case, and a filter-side suction passage and a filter-side discharge passage having openings opened to the contact surface, the filter-side suction passage and the filter-side discharge passage being fluidly connected to each other through the filter body; and (f) the filter is mounted at the charge pump case so that the openings of the filter-side suction passage and the filter-side discharge passage face upward on the basis of the mounting state of the pump unit. 4. A pump unit including a plurality of hydraulic pumps respectively fluidly connected to a plurality of hydraulic motors, which are disposed away from the pump unit, so as to form closed circuits, the pump unit comprising: (a) a pump case accommodating the plurality of hydraulic pumps so that the plurality of hydraulic pumps are operatively driven by a common power source; (b) a charge pump having a suction side fluidly connected to the fluid sump and a discharge side fluidly connected to a charge passage, the charge pump operatively driven by the common power source; and (c) a charge pump case accommodating the charge pump; (d) a single filter inserted in a charge passage for replenishing operation fluid from a fluid sump to the closed circuits; (e) wherein the plurality of hydraulic pumps includes first and second hydraulic pumps that are respectively driven by first and second pump shafts arranged in parallel with each other; (f) wherein the charge pump is driven by one of the first and second pump shafts that has an end extending outward from the pump case; and (g) wherein the single filter is mounted at an outer surface of the charge pump case which extends in a direction that is parallel to the longitudinal axis line of one of the first and second pump shafts, and that faces towards the pump shaft that does not drive the charge pump. 5. A pump unit including a plurality of hydraulic pumps respectively fluidly connected to a plurality of hydraulic motors, which are disposed away from the pump unit, so as to form closed circuits, the pump unit comprising: (a) a pump case accommodating the plurality of hydraulic pumps so that the plurality of hydraulic pumps are operatively driven by a common power source; (b) a filter mounted at the pump case so as to be inserted in a charge passage for replenishing operation fluid from a fluid sump to the closed circuits; (c) a charge pump having a suction side fluidly connected to the fluid sump and a discharge side fluidly connected to the charge passage, the charge pump operatively driven by the common power source; (d) a suction fluid passage fluidly connected to a suction side of the charge pump; (e) a discharge fluid passage fluidly connected to a discharge side of the charge pump; (f) a charge passage for replenishing a part of pressurized fluid in the discharge fluid passage into each of the closed circuits; and (g) a working machine operating fluid passage for supplying a part of the pressurized fluid in the discharge fluid passage toward an external hydraulic device. 6. A pump unit according to claim 5 further comprising a working machine switching valve for selectively communicating or shutting off the working machine operating fluid passage. 7. A pump unit according to claim 6 wherein (a) the working machine operating fluid passage includes a proximal end fluidly connected to the discharge fluid passage and distal ends branched so as to have two working machine operating fluid ports; and (b) the working machine switching valve is inserted in one of the two branched passages. 8. A pump unit according to claim 5 wherein hydraulic fluid ports to the plurality of hydraulic motors and a working machine operating fluid port to the external hydraulic device are faced to the same direction with respect to one another. 9. A pump unit according to claim 5 further comprising a relief valve that has a primary side fluidly connected to the discharge fluid passage; wherein (a) the working machine operating fluid passage is fluidly connected to the discharge fluid passage via a sequence valve; and (b) the charge passage is fluidly connected to the discharge fluid passage via a pressure-reducing valve. 10. A pump unit according to claim 9, further comprising: (a) a PTO shaft operatively driven by the common power source; (b) a hydraulic PTO clutch for selectively engaging or disengaging power transmission from the common power source to the PTO shaft; (c) a PTO fluid passage having a proximal end fluidly connected to the discharge fluid passage via an orifice and a distal end fluidly connected to the PTO clutch; (d) a PTO switching valve inserted within the PTO fluid passage; and (e) a PTO relief valve having a primary side fluidly connected to the PTO fluid passage.