Abstract: A balanced rotary pump is disclosed of the type including a pumping element (17) having a rotor member (25) defining a plurality of slots (27). Each of the slots receives a radially displaceable roller vane member (29). The pumping chamber is defined by a pair of inlet arc surfaces and a pair of discharge arc surfaces (65) of progressively decreasing radius. Each of the slots (27) includes a driving surface (73), each of which includes a substantial surface portion (77) oriented at a negative angle relative to a radial line passing through the center of the adjacent roller vane member. The engagement of each of the roller vane members (29) and its respective negative surface portion (77) is effective to act to reduce slightly the net radially outward force acting on the roller vane member, to reduce bouncing thereof as the roller vane member passes from the inlet arc surface to the discharge arc surface.