Force characteristics of nickel-titanium tension coil springs.

Nickel-titanium closed coil springs are commonly used for space closure. The springs possess a high resistance to permanent deformation and the potential for relatively constant force delivery. A study was designed to determine whether relatively constant forces can be delivered and whether the force magnitudes approach the manufacturer's targeted force values. Heavy, medium, and light springs were activated 15 mm at temperatures that ranged from 15 degrees C to 60 degrees C. The forces were measured during deactivation with a specially constructed force transducer temperature chamber. Relatively constant forces can be achieved with an over-activation procedure that allows relaxation to the desired activation. The light springs delivered forces that were near the targeted force; no difference was found between the heavy and medium springs in the constant force range. The force magnitudes varied markedly depending on mouth temperature.