Helical coil springs are the most popular and usable form of Nitinol shape memory alloy for actuators, with the exception of straight wire. These springs can be utilized in either extension (tension) or compression, have a long stroke, and are capable of exerting enormous forces.
The number necessary determines the process of creating nitinol shape memory springs. It is easiest to wound the chosen wire on a cylindrical mandrel with a diameter that will provide the desired I.D. if only one or a few springs are required. Both ends of the wire should be secured to the mandrel, such as by trapping them under a screw head, and the wire should be wound tightly and with the desired pitch on the mandrel (wire spacing). The shape of the spring will next be established by heat treating it on the mandrel (see below).
A typical spring winding machine is utilized for greater quantities of springs. To respond best to the winding, the shape memory wire should be in the as-drawn condition, and due to the high elasticity of these alloys, you will need to arrange the machine to wind a tighter coil than you would expect to obtain the necessary size. Heat treatment of as-wound springs can be done in two ways. In the first case, you can keep the coils as-wound by placing them in a holding fixture of the desired size and then heat treating them to determine their shape. The second method involves winding the coils tighter than the final size and then heat treating them without restriction. The coils will increase significantly in diameter (on the order of 25%) after heat treatment due to the shape memory effect. This method needs more trial and error to get the desired end size and has less size control than the first, but it is less expensive.
The heat treatment used to set the spring’s shape and qualities are crucial, and it must usually be determined experimentally for each specific spring’s requirements. Temperatures as low as 400 degrees Celsius and times as short as 1-2 minutes can create the form, although most people use temperatures closer to 500 degrees Celsius and times longer than 5 minutes. Rapid cooling in any manner, such as a water quench or a rapid air cool, is preferable (if both the springs and the fixture are small). Higher heat treatment raises the spring’s actuation temperature and often results in a sharper thermal response, but it usually comes at the cost of a reduction in peak force and the capacity to resist permanent deformation.
Finally, keep in mind that in air or vacuum furnaces, heat treatment fixtures might be unexpectedly slow in attaining the necessary temperature. The time spent at the maximum temperature imparts a lot of the required spring qualities, so make sure your springs reach that temperature and time.
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