1. SAE, Manual on Design and Application of Helical and Spiral Springs, 1997 ed., HS-795, pp.48-51, (1997).
2. Spring Manufacturers Institute, Handbook of Spring Design, pp.30-34.
3. Y. H. Zhang and H. H. Liu, Spring Handbook (in Chinese), Chinese Mechanical Engineering Press, pp.407-418, (1997).
4. M. Shimoseki, T. Hamano, and T. Imaizumi, FEM for springs, Springer, Berlin, New York, (2003).
5. http: /en. wikipedia. org/wiki/Castigliano's_method.