This paper relates to achieving even distribution of internal load on ball screws in heavy duty drive system, aimed at extending the ball screw life in such applications. Based on the observation that the uneven distribution of load on the load carrying balls of a ball screw reduces the durability of the ball screw, the authors have analyzed primary factors causing uneven load distribution in ball screws of tube recirculation type. Findings by case studies indicate that variation components of load distribution in a narrow range have relation with arrangement of ball recirculation tubes and that those in a broad range are connected with the application points of the load upon the nut as well as with nut diameter. The authors developed an improved ball screw structure that minimizes the variation components of load distribution. The ball screw having the improved structure exhibits 2.4 times longer life and, practically, 3.5 times higher durability under heavy load conditions.