|Budget Amount *help
¥2,000,000 (Direct Cost: ¥2,000,000)
Fiscal Year 2002: ¥1,000,000 (Direct Cost: ¥1,000,000)
Fiscal Year 2001: ¥1,000,000 (Direct Cost: ¥1,000,000)
Constant-force springs are often used in applications that require constant load characteristics, such as lift cord mechanisms for window blinds, sliding shelf mechanisms and the mechanisms for raising/lowering commercial camera stands. There are springs that do not demonstrate this kind of proportional relationship, such as constant-force springs. With these, the load necessary for extending the steel strip spirally wound on the drum is almost constant, regardless of the spring's extension stroke. There are very few theoretical studies about constant-force springs. Considering the strain energy necessary for flattening a coil spring, Votta proposed an analytical theory of such a spring. This theory, however, could not be applicable for all stages of displacement, especially for small displacements, because the uncoiled length of the spring can not be flattened. The load, according to Votta's theory, is unable to accurately represent all the stages of the spring's actual deformation, particularly the transient rapid increase of the load.
In this research, we reconsider the deformation of constant-force springs as a "problem of large elastic deformation" and perform a new theoretical analysis of the constant-force spring based on the nonlinear theory. Then, we clarify not only the accurate load characteristics of the springs, but also the entire state of spring deformation in the process of spring extension, the state of deformation at the spring end, the state of the strip as it leaves the drum, the extension mechanism and the deformed shape of the spring. As a result, it is found that the force rises rapidly and approaches a constant value as the spring is extended infinitely. Furthermore, the experimental verification is carried out using a commercially available constant-force spring. The theoretical values are in good agreement with the experimental ones. Consequently, the new theory proves to be of practical use.