Determinant factors of efficiency of a motor-driven ventricular assist device.

Limiting factors of the system efficiency of an electric-motor-driven ventricular assist device (VAD) consisting of a brushless dc motor and a ball screw in the actuator were analyzed from the balance of force and torque in the pump and actuator. The results suggested that several mechanical components in the actuator should be improved to increase the efficiency. According to the suggestion, rotor material was changed from stainless steel to a lighter engineering plastics; smaller ball bearings were used to reduce rotor inertia; the ball screw was changed from rolled- to ground-execution type; the friction loss in the anti-rotation column inside the ball screw shaft was reduced by changing sliding mechanism to rolling one. These improvements elevated the total system efficiency from 11% to 15%. The new system cleared durability testing of longer than 900 h. More elaborate assembly of the mechanical components of the actuator would increase the efficiency.