Large-gap passive levitation mechanism combined with thrust and magnetic force for pump applications

Blood pumps are used to treat patients with heart failure. In recent years, non-contact support of the impeller by active magnetic bearings or hydrodynamic bearings has improved the durability. In this study, we propose a pump mechanism with a large gap and passive levitation to achieve higher reliability and blood compatibility. According to Earnshaw's theorem, one degree of freedom becomes always unstable when using only static magnetic field. Therefore, in this study, positive stiffness is achieved for unstable degrees of freedom (axial direction) by using thrust. Thrust is the force that acts on the impeller from the fluid as a reaction when the impeller discharges the fluid. In the proposed mechanism, the flow rate inside the impeller changes with the axial displacement of the impeller, and as a result, the thrust force increases as a restoring force against the displacement. Since positive stiffness can be achieved in the axial direction with thrust, an axial motor with an iron core is employed to achieve a smaller size compared to conventional mechanisms. In the experiments, the impeller can move freely only in the axial direction, and is supported by a slider in the radial and tilt directions. Axial levitation experiments were conducted using water as the working fluid, and levitation was confirmed at an impeller speed of 7,000 rpm. From the result, we conclude that the use of an axial motor in a levitation mechanism using thrust has advantageous in terms of miniaturization.