In order to improve the durability of the coil springs used in the automotive suspensions against the corrosion fatigue fracture and the delayed fracture, the effects of the alloying and the chemical passivation treatment with HNO3 solution of 63 mass% concentration on that durability were investigated through the following procedures. Firstly, the anordic polarization curves were measured for 4 commercial spring steels and 22 laboratory steels. Secondly, the quasi-pitting potential of V'cq100 was analyzed from these polarization curves to discuss those effects. Then, the commercial spring steels whose pitting resistance was enhanced by the measures noted above were put to the following tests: the slow strain rate tensile test using the test pieces corroded by wet-dry cyclic corrosion in NaCl solution of 5 mass% concentration; and the rotating bending fatigue test under wet condition in dropping NaCl solution of 5 mass% cocentration. The major results are as follows. (1) The effective alloy elements to improve the pitting corrosion resistance are Si, Ni, Cr, Cu, Co, Sb, and B. (2) The chemical passivation treatment for 1 hour is effective against the pitting corrosion for the spring steels with less alloy elements. (3) The guide to the alloying design is proposed from the regression of V'cq100 on the content of alloy elements. (4) The spring steel (HDS13) with the alloy elements effective against pitting corrosion showed superior hydrogen embrittlement resistance and corrosion fatigue strength. (5) It was found that the addition of alloy elements noted in (1) and the chemical passivation treatment were effective to reduce the sensitivity to the corrosion fatigue and the hydrogen embrittlement.
The shot peening process is known to produce a hard layer called white-layer on the surface of coil springs. However, little is known about the fatigue properties of this white-layer. In this study, coil springs with white-layer were investigated. The surface of these springs was examined by micro Vickers hardness tester and FE-SEM. to evaluate fatigue strength of the springs. It was found that a microstructure of the white-layer with an average cell size of about 100 nm was observed, with a Vickers hardness rating of 1,000HV. Sample springs were manufactured in two groups utilizing a double-peening process. These springs had the same residual stress distribution and surface roughness. There is only one difference: one group had a nanocrystalline layer on the surface, while the other did not. The results of the fatigue test shows an increase of the fatigue life for the springs with nanocrystalline surface layer by approximately 10%.
High durability is required for automotive engine valve springs. To ensure the performance of springs, it is necessary to evaluate and measure the residual stress distribution on surface and inner direction of the spring. Theoretically, it is used to be treated that the residual stress of a shot-peened material surface is uniform. However, the shape of a coil spring causes various peening angles corresponding to the surface location, so the uneven shot peening angles may leads to a non-uniform residual stress on the coil spring surface. On the other hand, an analysis method of non-liner 2θ-sin2φdiagram (or φ-splitting analysis) is well known to directionally deformed material such as ground, planed or rolled materials. Recently, an φ-splitting analysis has been reported for the shot-peened materials. In this study, the residual stress distribution of slant shot-peened spring material was measured and, φ-splitting (non-linear 2θ-sin2φdiagram) was studied in the view of chemical compositions and microstructures. As a results, the extent of φ-splitting was mainly affected by the distribution of carbide particles, not the kind of carbide particles itself.
Cu-Al-Mn Shape Memory Alloys (SMAs) have high cold-workability with high cost performance. Their superelasticity (SE) are superior to conventional Cu-based SMAs. In this study, we obtained following results; The SE properties of Cu-Al-Mn SMAs are achieved by microstructural control such as grain size and texture. When the grain size is coarser than wire diameter, the SE stress is nearly constant and the SE strain of 10% is obtained. This is achieved by adding Co to the alloy. When the grain size is finer than wire diameter, the fatigue strength is improved. This is achieved by adding B to the alloy. The addition of Ni to the Cu-Al-Mn ternary alloy shows a drastic effect on the formation of the strong <110> recrystallization texture. Even for a sheet specimens having small grain size, the SE strains of 6% can be obtained in an angle of 45 degree to the rolling direction. Coil springs made by this new SMA show superior properties to conventional SMAs.
Recently, the following two new items are strongly required for coil springs used for automotive suspensions. One of them is the control of the force action line of the spring. The second item is to design the geometry of the spring so that the spring can be installed within the space allowed. For these requests, the geometry of the spring became complicated. Therefore, it was difficult to produce the spring correctly to the various demands. The developed CAD-CAM system is capable of materializing the designed geometry of the spring without the help of worker's skills and experiences. This system allows us to automatically generate NC data for coiling from the geometry of the spring obtained by the finite element analysis. In the case of creation of NC data, this system makes reference to a database of forming machines and spring materials which affect the dimensions of the product. It also includes the prediction of the geometry change in the process of presetting and tempering.
Today's suspension coil spring design requires not only accounting for one-dimensional force along the coil spring axis, but also exerting multi-dimensional force and torque field between the spring seats. This paper describes the design of a 6-DOF parallel mechanism to simulate the force and torque characteristics of a coil spring. This mechanism can physically generate the 6-DOF force and torque field of a coil spring, allowing designers to experimentally evaluate the quasi-static force effects of a coil spring while still at the design stage. Examples are presented for a physically generated force and torque field of a coil spring used in a McPherson Strut suspension, and its effect is correlated to the side force acting upon the suspension strut. As an extension, this mechanism can be widely used to investigate the relationship between spring characteristics and suspension system characteristics, such as damper friction, self-steering torque and steering pull.
Even under stronger environmental regulation in global society, paints for multi-leaf springs is composed of organic solvent and zinc dust which provides a sacrificial anode effect of corrosion protection presently. Recently Japanese government has set a target which clears a regulatory organic solvent level widely accepted by international society by the year of 2010. In EU, deliberations on zinc have started, even though it has not been declared as harmful metal yet. Under these circumstances, the committee conducted a series of weathering test to compare alternate paints like water solvent zinc rich paint, powder paint, and electro deposit paint with the current zinc rich paints. The test was executed with the help of paint manufacturers. To evaluate candidate paints, equipment cost, necessary floor plan, running cost, and the extent of handling easiness are also generally considered in addition to the essential corrosion protective quality. Since we identified the problems of alternate paints, the development of practically applicable alternate paints is desirable in near future.
Usually, helical compression spring ends require a grinding process. This process sometimes restricts productivity of the production line and needs countermeasures for dust and noise. Fine steel powder generated from the process contains particles of the abrasive wheel. As the steel powder and the abrasive particles are inseparably mixed, they cannot be handled as steel material for scrap metal. They inevitably become industrial waste. As a result, the grinding process needs additional costs for dust control, noise prevention and waste disposal. This committee was launched for the purpose of investigating the new processing method which can replace grinding operation, and the following results were obtained. (1) Study on current practices Springs that have not-ground- ends are widely used for industrial purposes. Wire diameter of the springs range from 0.2mm to 35mm and the spring indexes are higher than 3.5. The production-mix ratio between “ground-ends springs” and “not-ground-ends” springs varies from company to company. It seems that the customers make a decision on necessity of coil-end grinding. When squareness and flatness is essential, the grinding process is employed. (2) Archive research The committee members reviewed relevant literatures written from 1975 to August 2003 and could not find any that conforms to their purposes. (3) Patent research The committee searched through patents applied in the same period and found that 10 of them met with the requirements. (4) Finite Element Analysis (FEA) The committee investigated the applicable range of a spring characteristic formula and a stress formula by applying FEA to the calculation model. Consequently, it seems that the spring characteristic formula is practical. However, when the spring index became less than six, it is necessary to take the influence of contact stress into consideration for the stress calculation formula. (5) Study on existing manufacturing methods Bending, rolling or cutting-off method, are mainly used for the manufacturing of “not-ground-ends springs.” The Rolling method has long been utilized for hot-formed springs while it has not been applied to cold-formed springs.