Spring constant: expressed in k, when the spring is stretched, the load (kgf/mm) for every 1mm increase
Spring constant formula (Unit: kgf/mm): k=(GXd4)/(8XDm5XNc)
G: wire modulus of wire; d: wire diameter; Dm: medium diameter = outer diameter - wire diameter; Nc: effective turns = total number of turns -2.
The initial tension of the tension spring: The initial tension is equal to the force required to pull apart the springs that are in close contact with each other, and the initial tension occurs after the spring rolls are formed. When the tension spring is manufactured, the initial tension of each tension spring is not uniform due to the difference in the material, wire diameter, spring index, static electricity, grease, heat treatment, and plating of the wire. Therefore, when installing various specifications of the tension spring, it should be pre-pulled to a small distance between each circle. The force required at this time becomes the initial tension.
Initial tension = P-k X l = maximum load - spring constant X extension length 
Many different terminal devices or "hooks" are used to ensure the pull force of the tension spring. The working principle of tension spring and compression spring is opposite. The compression spring acts in the opposite direction when pressed, and the tension spring acts in the opposite direction when it is stretched or pulled apart. When the tension spring is pulled apart at both ends, the spring will try to pull them back together. Like compression springs, tension springs absorb and store energy. But unlike compression springs, most tension springs are usually under a certain degree of tension, even without any load. This initial tension determines how tightly the tension spring coils without any load.
Stretching springs, also known as spiral tension springs, are generally pitches with many circular cross-sections. They can be used in many applications such as production assembly, experimentation, research and development, and maintenance. The tension spring plays an important role in the global market and is widely used in defense, marine, computer, electronics, automotive, mold, medicine, biochemistry, aerospace, railway, nuclear power, wind power, thermal power, construction machinery, mining machinery, construction machinery, Elevators and other fields.