Analysis and design of hollow helical springs for shape memory actuators

The 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, the poor energetic efficiency and the unsatisfactory stroke. This paper contributes to enhancing the mechanical, the thermal and the electric performances of SMA actuators by providing analysis and design equations for helical springs with hollow round section. Emptied of the inefficient material from its centre, the hollow section features a lower mass, a lower cooling time and a 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.