Making shape memory springs

With the possible exception of straight wire, the most popular and useful form of Nitinol shape memory alloy for actuators is helical coil springs. Such springs can be used either in extension (tension) or compression, can provide an impressively large stroke, and may be designed to exert significant forces.

The method of making shape memory springs is chosen largely by the quantity required. If only one or a few springs are needed, it is easiest to wind the chosen wire on a cylindrical mandrel having a diameter that will yield the chosen I.D. Both ends of the wire must be fastened to the mandrel, such as by capturing them under a screw head, and one should wind the wire on the mandrel tightly and with the desired pitch (wire spacing). Heat treating on the mandrel (see below) will then set the shape of the spring.

For larger numbers of springs, a standard spring winding machine is used. The shape memory wire should be in the as-drawn condition to respond best to the winding, and because of the very high elasticity of these alloys you will find that you need to set the machine to wind a tighter coil than you would expect to achieve a desired size. The as-wound springs can be heat treated in two different manners. In the first, you can maintain the as-wound dimensions by putting the coils into a holding fixture of the chosen size and then heat treating the coils to set their shape. The second method is to wind the coils tighter than the desired final size, and then heat treat the coils without confinement. Because of the shape memory effect, the coils will grow significantly in diameter (on the order of 25%) during heat treatment. This method requires more trial and error to achieve the desired final size, and gives poorer size control, but is less expensive than the first method.

The heat treatment chosen to set the shape and properties of the spring is critical, and usually needs to be determined experimentally for each desired spring's requirements. In general, temperatures as low as 400 deg.C and times as short as 1-2 minutes can set the shape, but generally one uses a temperature closer to 500 deg.C and times over 5 minutes. Rapid cooling of some form is preferred via a water quench or rapid air cool (if both the springs and the fixture are small). Higher heat treatment will increase the actuation temperature of the spring and often gives a sharper thermal response, but there is usually a concurrent drop in peak force for springs and in the ability to resist permanent deformation.

A final caution that one should observe is that heat treatment fixtures can be surprisingly sluggish in reaching the desired temperature in air or vacuum furnaces. The desired spring properties are imparted largely by the time at the maximum temperature, so be careful that your springs actually reach the desired temperature and time.