Spring constant: Represented by k, it is the load (kgf/mm) required for each additional 1mm of distance when the spring is stretched.
Spring constant formula (unit: kgf/mm): k = (G*d⁴) / (8*Dm³*Nc)
G: Wire stiffness modulus; d: Wire diameter; Dm: Mean diameter = Outer diameter - Wire diameter; Nc: Effective number of coils = Total number of coils - 2.
Initial tension of a tension spring: The initial tension is equal to the force required to separate tightly wound spring coils. The initial tension occurs after the spring is coiled. During manufacturing, differences in wire material, wire diameter, spring index, static electricity, lubricating grease, heat treatment, electroplating, etc., result in uneven initial tension for each tension spring. Therefore, when installing tension springs of various specifications, they should be pre-stretched to create a slight gap between the coils. The force required at this point is called the initial tension.
Initial tension = P - k × l = Maximum load - Spring constant × Stretch length
