# Calculation formula of tension spring

Spring constant: expressed in K, when the spring is stretched, the load (kgf / mm) for each increase of 1 mm distance
Spring constant formula (unit: kgf / mm): k = (g * D4) / (8 * DM3 * NC)

G: Steel modulus of wire rod; D: wire diameter; DM: medium diameter = outer diameter wire diameter; NC: effective number of coils = total number of coils – 2.

Initial tension of tension spring: the initial tension is equal to the force required to pull apart the spring and coil which are close to each other. The initial tension occurs after the spring is rolled and formed. When the tension spring is made, the initial tension of each tension spring is uneven due to the difference of steel wire material, wire diameter, spring index, static electricity, lubricating grease, heat treatment and electroplating.

Therefore, when installing the tension spring of various specifications, it is necessary to pre pull until the distance between the parallel coils is slightly separated. The required force becomes the initial tension.Initial tension = P-K x L = maximum load spring constant x tensile length