At present, the widely used spring stress and deformation calculation formulas are derived from material mechanics. Without certain practical experience, it is difficult to design and manufacture high-precision springs. As the design stress increases, many previous experiences are no longer applicable.
For example, after the design stress of the spring increases, the helix angle increases, which will transfer the fatigue source of the spring from the inside to the outside of the coil. To this end, sophisticated analytical techniques must be used, and the currently widely used method is the finite element method (FEM).
The characteristics of vehicle suspension springs are that in addition to sufficient fatigue life, their permanent deformation should be small, that is, the anti-relaxation performance should be within the specified range, otherwise the center of gravity of the vehicle body will shift. At the same time, the impact of environmental corrosion on its fatigue life should be considered.
With the increase of vehicle maintenance period, more stringent requirements are put forward for permanent deformation and fatigue life. Therefore, high-precision design methods must be adopted. The finite element method can predict in detail the effect of spring stress on fatigue life and permanent deformation, and can accurately reflect the relationship between material and spring fatigue life and permanent deformation.
In recent years, the finite element method design method of spring has entered the practical stage, and many practical reports have appeared, such as the influence of helix angle on spring stress; the relationship between stress and fatigue life calculated by finite element method.
In addition, an optimized design was introduced in the design of the spring. The structure of the spring is relatively simple, the function is simple, and the parameters that affect the structure and performance are saved. Therefore, the designer has used analytical, graphic or graphic analysis to find the optimal design solution very early, and has achieved certain results. With the development of computing technology, the use of computers to optimize the design of nonlinear programming has achieved results.
Reliability design is a series of analysis and design techniques used in order to ensure the reliability of the designed product. Its task is to make the designed product reach the specified reliability target on the basis of predicting and preventing the possible failure of the product. value. It is a supplement and improvement of traditional design methods. Spring design has made some progress in the use of reliability technology, but further improvement requires data development and accumulation.
With the development of spring application technology, many new problems that need to be paid attention to and solved have also been raised for designers. For example, the influence of material, strong pressure and shot peening treatment on fatigue performance and relaxation performance is difficult to calculate accurately during design; it depends on experimental data to determine; another example is the number of turns calculated according to the current design formula, the stiffness of the made spring is higher than the design The rigidity value is small, and the effective number of turns needs to be reduced to meet the design requirements.
The development of spring materials
The development of spring application technology puts forward higher requirements on materials. Mainly to improve fatigue life and anti-relaxation performance under high stress; secondly, according to different uses, it is required to have corrosion resistance, non-magnetic, electrical conductivity, wear resistance, heat resistance, etc. For this reason, in addition to the development of new varieties of spring materials, beneficial results have also been achieved in strict control of chemical composition, reduction of non-metallic inclusions, and improvement of surface quality and dimensional accuracy.
1. The development of alloy steel Si-Cr steel has been widely used in valve springs and suspension springs. In order to improve fatigue life and relaxation resistance, V and Mo are added to Si-Cr steel. At the same time, the Si-Cr drawn wire was developed, which has better relaxation resistance when working at high temperatures than piano wire. With the high-speed and miniaturization of engines, Ti alloys with good anti-flutter performance, light weight, and small elastic modulus have been widely used, and their strength can reach 2000Mpa.
2. Development of stainless steel wire
1) The strength of stainless steel wire with austenitic structure is better than that with ferrite structure, and its corrosion resistance is also better than that of martensite structure, because the scope of application continues to expand.
2) Low-temperature wire drawing or low-temperature nitriding wire drawing can increase the strength of the steel wire. The structure of martensite is unstable when heated, and wire drawing in low-temperature liquid nitrogen can form cryptic martensite, which can obtain high strength in hot state. This kind of steel wire has been used in many applications in the United States and Japan, but currently it can only handle steel wires under 1mm.
3) Precision springs in electronic equipment require non-magnetic properties. This kind of steel wire cannot generate cryptic martensite during drawing. To this end, elements such as N, Mn, and Ni must be added. To meet this demand, the United States has developed AUS205 (0.15C-17Cr-1Ni-15Mn-0.3N) and YUS (0.17C-21Cr-5Ni-10Mn-0.3N). As the content of Mn increases, cryptic martensite will not be formed during processing. After solution treatment, the strength can reach 2000Mpa, and the fatigue performance is high, which is better than SUS304.
3. Improve material purity For high-strength materials, strictly control inclusions and improve purity to ensure their performance. For example, the oxygen content of valve spring materials has reached 20×10ˉ6.
4. Improve surface quality The surface quality of materials has a great influence on fatigue performance. In order to ensure the surface quality, the surface layer of 0.1mm is used for materials with special requirements. Eddy current flaw detection is used for defects with a depth of 0.5mm. For the unevenness of the surface during the wire drawing process, electrolytic polishing can be used to reduce the surface roughness to Ra=6.5～3.4μm.
5. In special circumstances, the development of electroplated steel wire requires additional properties such as corrosion resistance and electrical conductivity in addition to the spring characteristics, and most of them are solved by electroplating. The corrosion resistance of some stainless steel wire and piano wire is equivalent to that of galvanized steel wire. If another layer of ZnAl (5%) alloy is plated, the corrosion resistance can be increased by about 3 times. Stainless steel wire or piano wire that has requirements for resistance performance. The diameter of the steel wire is less than 0.4mm and can be plated with copper. If the diameter is greater than 0.4mm, it can be made of copper inside and stainless steel outside. Generally, piano wire is plated with 5μm thick Ni to improve its conductivity.
Generally speaking, processes that can harden the surface of the material to form residual stress (such as shot peening and surface nitriding) can improve fatigue strength. Currently, non-electrolytic Ni plating is being studied. By heating (300～500℃), 7% of P can be precipitated as PNi, which can increase the Vickers hardness to HV500. After shot peening, if Ni is heated and plated below 300℃, the hardness can be increased by 10%.
6. Development of shape memory alloys Currently, one-way shape memory alloys with promising applications in springs, with the best performance of 50Ti-50Ni. The spring made of shape memory alloy can be stretched and contracted under the action of temperature. Mainly used in the control system of constant temperature, constant load and constant deformation. Since the actuator is pushed by the expansion and contraction of the spring, the working stress of the spring changes greatly.
7. The application of ceramics. Ceramics have high elastic modulus and low breaking strength, which is suitable for places with little change. Currently, there are ceramics that are heat-resistant, wear-resistant, and have good insulation properties, and superplastic zinc alloy (SPZ) is used, which has high strength at room temperature. In addition, there is high-strength silicon nitride, which can withstand high temperatures up to 1000°C. However, ceramic springs are not suitable for working under impact loads.
8. The application of fiber-reinforced plastic in springs. Glass fiber-reinforced plastic (GFRP) leaf springs have been widely used in Britain, the United States and Japan. In addition to horizontal suspensions, they can also be used for special light vehicles, such as the longitudinal Set the suspension. At present, a carbon fiber reinforced plastic (CFRP) suspension spring has been successfully developed, which is 20% lighter than a metal leaf spring.