Patent ID: 8047120

Claim:
A hydraulic piston pump with a balance valve, the hydraulic piston pump comprising: a valve plate having an absorption port and a discharge port which communicate with an absorption path and a discharge path of a pump case, respectively; a cylinder block which slides on the valve plate to rotate; a plurality of cylinder bores formed in the cylinder block; and pistons which slide in the respective cylinder bores to do reciprocating motion in response to a rotation angle of the respective cylinder bores, wherein the hydraulic piston pump includes: a through hole formed between the absorption port in the valve plate and a timing hole to introduce a chamber pressure in the cylinder bores, the timing hole is formed in such a way that the chamber pressure in the cylinder bore does not enter the discharge port abruptly; a first oil path for introducing pressure oil of the chamber pressure from the through hole via the valve plate; a second oil path for introducing pressure oil of a system pressure from the discharge port; and a balance piston, constructed as a free piston having one end surface that receives the pressure oil from the first oil path and an other end surface that receives the pressure oil from the second oil path, the balance piston being activated in response to a pressure difference between the chamber pressure and the system pressure; and when cylinder ports that are formed at the bottom of a cylinder bore communicate with the absorption port and the through hole, a pressure of a first pressure chamber of the balance valve which slides the balance piston decreases to a pressure of the absorption port and consequently the balance piston returns to an initial position at which the first pressure chamber is compressed.