Patent ID: 7717690

Claim:
A gear pump comprising a first gear and a second gear which are provided rotatably around mutually parallel rotating shafts and which engage with each other, and a casing with housing space to retain said first gear and said second gear in a condition where the tooth tips and both side surfaces slidably contact surfaces within said housing space, said casing being formed with an intake port which draws in oil and a discharge port which discharges oil in conjunction with the rotation of said first gear and said second gear, wherein said casing comprises a main casing which retains said first gear rotatably while restricting movement in the axial direction, and a gear holder which can move in the axial direction of said mutually parallel rotating shafts within said main casing and retains said second gear rotatably, a biasing member which applies a biasing force and biases said gear holder to one side in the axial direction of said mutually parallel rotating shafts, and a piston which is acted on by hydraulic force and presses said gear holder on the other end side in the axial direction of said mutually parallel rotating shafts to counteract said biasing force, are provided in said main casing, and said gear holder is acted on by said hydraulic force from said piston to counteract the bias from said biasing member and moves in the axial direction of said mutually parallel rotating shafts according to said hydraulic force so that the engagement width between said first gear and the second gear retained in said gear holder is changed, wherein said piston is constructed to have a generally disk-shaped flat plate and a protrusion that protrudes in a cylindrical shape from said flat plate and is fitted into said main casing, the center of an outer circumferential surface of said flat plate and the center of an outer circumferential surface of said protrusion are eccentric, and a separate cutout part of the outer circumferential surface of said flat plate is curved to match the tooth tips of said first gear.