New method for improving properties of SMA coil springs

This work is related to a SMA coil spring which shows perfect recovery and greater recovery force. When tension spring of common metals is fabricated, `initial tension' can be provided by minus pitch winding. The initial tension is the force for tension springs to contract further in closed state. If a SMA spring has the initial tension, perfect recovery is expected with a greater recovery force. However, the initial tension cannot be given to SMA coils in the conventional way, since heat treatment for shape setting eliminates the tension generated by the minus pitch winding. We have developed a new method of supplying the initial tension to SMA springs by changing coil orientation after the shape setting treatment. In this paper, some examples are introduced that a perfect recovery with a greater force, though permanent deformation is involved in the deformation, can be attained by the initial tension.