Fatigue analysis and experiment of leaf-spring pivots for high precision flexural static balancing instruments

Abstract

For distributed-compliance flexure mechanisms, their service life is affected by fluctuating stress which may result in fatigue failure, but only static failure was taken into consideration in most studies. As one of the best-known types of distributed-compliance flexural pivots, the cross-spring pivot applied to the flexural static balancing instruments is subjected to large enough vertical loads and fluctuating stresses. In this paper, the cross-spring pivot is taken to explore the methods of prolonging its fatigue life, and the methods can be extended to the other pivots based on leaf-springs. Meanwhile, another important performance of the pivot-the rotational stiffness remains unchanged. Firstly, in order to develop a stress-life model, the equivalent fatigue stresses of the cross-spring pivots are derived in which the stress concentration factors are obtained by fitting the finite element results. Then, two methods for prolonging the fatigue life of the pivot are proposed, i.e., increasing the ratio of length to thickness of leaf-springs and reducing stress concentration. To verify the theoretical analysis, the fatigue life of five cross-spring pivots with the same rotational stiffness is tested based on a developed fatigue test bench. Experimental results show that the fatigue life of the cross-spring pivot can be prolonged, which verifies the validity of the two methods.