Patent ID: 6840747

Claim:
A positive-displacement oil pump received in a central opening formed in a shaft body of a crankshaft, comprising: an insert body fitted into the central opening of the shaft body from a lower end of said opening to a height at which an oil-feeding hole is formed on the shaft body, said insert body thus being rotated along with the shaft body and consisting of: a central hole formed in the insert body along a central axis of said insert body; a cylindrical lip formed around an outlet of said central hole; a fluid discharge diode provided in an inlet of said central hole; and an inclined groove formed around an outer circumferential surface of the insert body such that the inclined groove forms a closed curve; a piston movably fitted over a lower portion of said insert body such that the piston is axially moved relative to the insert body while changing a volume of a displacement space defined between the insert body and the piston, said piston being immersed at a lower portion thereof in oil contained in an oil reservoir, and consisting of: a fluid suction diode provided in a suction hole formed at a bottom wall of said piston; an axial ridge externally formed on a sidewall of the piston; and a projection formed on an inner surface of the sidewall of the piston and movably engaging with the inclined groove of said insert body; and a piston guide fitted over the piston so as to guide an axial movement of the piston, said piston guide being mounted on a support structure, and consisting of: an axial groove formed on an inner surface of a sidewall of the piston guide so as to movably engage with the axial ridge of the piston.