Effects of the angle between the spring arm on spring characteristics on the single coiled torsion spring using Ti-Ni shape memory alloy wire.

Abstract

We have been investigated one-coil torsion springs using shape memory alloy (SMA) wire for application to welfare equipment (for example, lower limb orthoses), because one-coil torsion springs using SMA wire have small size and large displacement. It is known that the spring properties of a torsion spring depend on the shape of the spring, and design calculation method for springs made of normal metals have already been established. However, SMA has different characteristics from normal metals, and it is difficult to apply conventional design calculation methods. Therefore, we fabricated several SMA torsion springs with different the angle between the spring arm and investigated the effects of shape of the spring on the spring characteristics of SMA torsion springs by spring characteristic tests and fatigue tests. The experimental results show that increasing the angle between the spring arm improves the fatigue life of the SMA torsion springs but the spring generating force decreased. On the other hand, the functional degradation characteristics are independent of the angle between the spring arms.