The selection of double torsion spring materials should be determined based on the performance of the double torsion spring under load, stress state, stress magnitude, operating temperature, environmental medium, service life, electromagnetic conduction requirements, process performance, material source, price and other factors.
When determining the shape and size of material cross-sections, priority should be given to the size series specified by national standards and ministerial standards. Small and medium-sized double torsion springs, especially spiral tension springs, should give priority to reinforced steel wires, cold-drawn steel wires, lead bath isothermal oil-hardened and tempered steel wires, which have high strength, good surface quality, fatigue properties better than ordinary tempered hardened steel wires, and are easy to process. Good processing performance and stable quality.
Carbon double torsion spring steel wire and piano steel wire produce large residual stress after cold drawing. The residual stress after processing of the double torsion spring is large, the dimensional change is large after tempering, and the dimensional accuracy is difficult to control. After the oil-quenched and tempered steel wire is drawn to the specified size, its strength should be adjusted. There are essentially no residual stresses. After the spring is formed and tempered at low temperature, the size change is small.
For large and medium-sized double torsion springs, for double torsion springs with higher load accuracy and stress, cold-drawn materials or cold-drawn polished steel should be selected. For low load accuracy and low stress springs, hot rolled steel can be used.
The material sections of the helical double torsion spring are preferably circular. Square and rectangular cross-section materials have strong load-bearing capacity and good impact resistance, which can make double torsion springs miniaturized, but there are few sources of materials. And the price is very high. This material is generally not chosen except for special needs. In recent years, the development of flat round steel wires instead of trapezoidal steel wires has achieved good results.
Double torsion spring materials working at high temperatures need to have good thermal stability, resistance to relaxation or creep, oxidation resistance and corrosion resistance of certain media. As the operating temperature of the double torsion spring increases, the elastic modulus of the spring material decreases, resulting in a decrease in stiffness and load-bearing capacity. Therefore, springs operating at high temperatures must understand the rate of change of elastic modulus (value) and calculate the impact of reduced spring load-bearing capacity on performance. According to gb1239, when the operating temperature of ordinary coil springs exceeds 60°C, the shear modulus should be corrected.
Spring materials used at low temperatures should have good low temperature toughness. Carbon spring steel wire, piano steel wire, 1Cr18Ni9 austenitic stainless steel spring steel wire, copper alloy and nickel alloy have good low temperature toughness and strength. At low temperatures, the brittleness of materials is very sensitive to surface defects. Therefore, strict requirements are placed on the surface quality of materials.
At low temperatures, the degree of corrosion of materials by environmental media is much lower than that in the greenhouse. Cadmium and zinc plating may cause low temperatures and embrittlement. At low temperatures, the material's elastic modulus and expansion coefficient change little and can be ignored in design.
The selection of double torsion steel spring stiffness should be based on the bearing properties and stresses of the spring. However, hardness is closely related to plane strain fracture toughness. As the hardness increases, the plane strain fracture toughness (KIC) value decreases significantly. This means that when determining the spring hardness value, the spring hardness value should be lower on the premise of meeting the spring characteristic requirements.