A rotary-oscillating sub-assembly for positive displacement pumping of a fluid, said sub-assembly comprising:
- a hollow body defining a cylindrical cavity having a longitudinal axis and a wall with at least two ducts passing therethrough and opening out radially into said cavity;
- a piston housed in said cavity so as to define a working chamber with said cavity, the piston having an outside cylindrical surface with a longitudinal channel or recess that opens out longitudinally into said working chamber, said piston having a sealing gasket that is made of a material having a modulus of elasticity that is less than moduli of elasticity of said piston and of said body, the gasket positioned beside said channel so as to form a seal between said piston and said cavity, the piston movable angularly relative to the body and thereby configured to put said working chamber into sequential fluid-flow communication with at least one duct of the two ducts, then no duct, then at least the other of said two ducts when said piston is moved angularly relative to the body, said piston also movable in longitudinal translation relative to the body so as to reciprocate relative to the body and thereby configured to cause a volume of said working chamber to vary and successively suck in and then discharge said fluid via one of said two ducts, then the other of said two ducts when said piston is moved in longitudinal translation relative to the body, wherein the piston includes a first axial end remote from a second axial end, said second axial end being in contact with the working chamber;
- said sealing gasket having a plurality of portions comprising:
- a first sealing portion in the shape of a ring that extends around the cylindrical surface of the piston beside a first axial end of the sealing gasket;
- a second sealing portion in the shape of a half-ring that extends around the cylindrical surface of the piston beside a second axial end of the sealing gasket, the half-ring having two ends that are spaced apart from each other over a cylindrical periphery of the piston;
- and a third sealing portion that is formed by two sealing strips that extend axially over the outside surface of the piston respectively between a first end of the half-ring and the ring, and between a second end of the half-ring and the ring;
- wherein the two strips are angularly spaced from each other and each defines a first sealing line and a second sealing line:
- wherein the first sealing line of one of the two strips angularly borders said channel, wherein said first sealing lines are spaced apart from each other by an angle containing said channel, said angle greater than each angle between the opposite edges of either one of the at least two ducts, and said angle less than each angle between the adjacent edges of either one of the at least two ducts and the other of the at least two ducts;
- wherein each second sealing line is spaced apart from one of said first sealing lines by an angle that does not contain said channel, said angle less than each angle between the edge of one duct of the at least two ducts and the adjacent edge of the corresponding duct, and said angle greater than each angle between the opposite edges of either one of the at least two ducts;
- and
- wherein an angle between each first sealing line from at least one of the second sealing lines, and containing said channel, is greater than an angle between axially-opposite edges of the at least two ducts.