Extension Spring Design Theory
Spring Rate of Extension Springs
Extension-spring coils are wound tightly together, and the wire is twisted as if is wound, creating a preload in the coils that must be overcome to separate them. The figure below shows a typical load-deflection curve for a helical extension spring. The spring rate is linear except for the initial portion. The preload is measured by extrapolating the linear portion of the curve back to the force axis. The spring rate formula for an extension spring is the same as that of a compression spring.
Note that no deflection occurs until the applied force exceeds the preload force (initial tension) that is built into the spring.