Analysis and Design of Hollow Helical Springs for Shape Memory Actuators

Shape memory alloys (SMAs) can be exploited successfully to reduce the complexity and the weight of actuators, but the main drawbacks that limit the use of SMAs are the low bandwidth, poor energetic efficiency, and unsatisfactory stroke. This article contributes to enhancing the mechanical, thermal, and electric performances of SMA actuators by providing analysis and design equations for helical springs with hollow round section. By emptying the inefficient material from its center, the hollow section features a lower mass, lower cooling time, and lower heating energy than its solid counterpart for given strength, stiffness, and deflection. The advantages of the hollow construction over solid springs are presented and discussed by means of dimensionless functions. A step-by-step procedure leading to the optimal design of hollow springs with minimum energy consumption is finally proposed.