3.0 ANCHORING OF EXTENDED END 
Constant-force extension springs (NEG'ATOR Springs) provide a strong recoiling force. This force may result in a tendency on the part of the extended material to twist in its effort to regain preset curvature. To inhibit this curling, it is always advisable to engage the end in a manner that will not permit turning axially about the line of force. The attached member must not be free to rotate about that axis. Therefore, the use of a wire or cable as an attachment means for a single exposed band is unsatisfactory.

ENGAGEMENT METHODS               

                

Figure 10 Pin Engagement. The simplest form of engagement is to permit the extreme end of the spring material to seek its preset curvature, and to attach this end to a stud or post at a point on the line of force, in a manner that will not attempt to restrain or deform the material, or introduce a reverse curvature.

          

Figure 11 Straight Engagement. The free end can also be firmly attached and held flat. In thus restraining the material from regaining its preset curvature, the attached member is called upon to exert a resisting moment (M). Care must be taken in the alignment of parts in the assembly so that no reverse-curvature is
imposed on the spring material. Employing paired constant-force springs, mounted back-to-back, cancels the free-end moment otherwise Imparted to the attached member.

4.0 GENERAL DESIGN CONSIDERATIONS 
In order to make most effective use of the NEG'ATOR type of spring the following considerations need to be kept in mind:

FATIGUE 
The anticipated number of cycles of a coiled-band, constant-force extension spring has a direct effect on the size of the unit. A moderate or low number of required cycles permits the use of a heavily prestressed spring with surprisingly high force available from a small package. On the other hand, where space is not restricted, these springs have been designed to provide millions of cycles. To evaluate the compatibility of spring and application, the required number of cycles of the spring should be estimated for the over-all life of the equipment, or in accordance with a practical replacement schedule. A cycle is defined as a complete change of curvature from the