  Cylindrical extension springs have been commonly exploited in mechanical
systems for years and their behavior could be considered as well identified.
Nevertheless, it appears that the influence of the loops on the global
stiffness is not yet taken into account properly. Moreover, it would be of key
interest to analyze how initial tension in extension springs influences the
beginning of the load-length curve. The paper investigates these topics using
analytical, simulation and experimental approaches in order to help engineers
design extension springs with greater accuracy. As a result, the stiffness of
the loops has been analytically defined. It enables to calculate the global
stiffness of extension springs with more accuracy and it is now possible to
determine the effective beginning of the linear load-length relation of
extension springs and thus to enlarge the operating range commonly defined by
standards. Moreover, until now manufacturers had to define by a try and error
process the axial pitch of the body of extension springs in order to obtain the
required initial tension. Our study enables for the first time to calculate
quickly this key parameter saving time on the manufacturing process of
extension springs.
