Tension spring (tension spring) is a spiral spring that bears axial tension and is generally made of circular cross-section material. When there is no load, the coil of the tension spring is usually tight and there is no gap. It is widely used in machines and instruments to control the movement of parts, store energy and measure force. Its hook forms include side hook tension spring, long hook tension spring, English hook tension spring, German hook tension spring, semi-circular hook tension spring, duckbill hook tension spring, etc., and the materials are stainless steel, piano steel, high carbon steel, phosphor copper, oil tempered alloy spring steel, etc.