The invention concerns an element comprising an annular body (9) provided with two series of pins (10, 12) projecting from at least one side of the body (9), the pins (10) of a first series being engaged in grooves (7) of the fixed volute (3) and the pins (12) of a second series being engaged in grooves (8) of the mobile volute (4), to provide an orbital movement of the mobile volute (4) relative to the fixed volute (3), the contact surfaces of the pins (10, 12) of a common series with the walls of the grooves of the corresponding volute being parallel and the contact surfaces of the pins (10, 12) belonging to two series of pins being perpendicular. The annular body (9) is made of a first metal with a specific density and the pins are made of a second metal having a lower density than that of the first metal.