High tensile strength of oil-tempered wire is intensely required for higher fatigue strength of coil spring. For this subject, wire has been developed in terms of increasing carbon level and/or applying high alloy of chemical composition. This study investigates the effect of retained austenite on the toughness of high tensile strength of oil tempered wire and on fatigue strength of coil spring. The experimental results are as follows; (1) Oil-tempered wire developed to increase retained austenite should be tempered inevitably with low value of tempering parameter in order to produce the required strength. As the results, the toughness of the oil-tempered wire is reduced. (2) Nevertheless it is possible that the retained austenite in the oil- tempered wire is transformed into strain induce martensite by coiling spring and/or shot peening, and the surface hardness of the coil spring is induced to increase. And also the toughness of the coil spring is induced to restore by stress relief annealing after coiling. As the results, the fatigue strength of coiled spring with the oil-tempered wire, which contains a lot of retained austenite is higher that of coiled spring with the oil-tempered wire which contains a little of retained austenite.
DK-3 (C19220), which is a precipitation hardening type alloy, is especially appropriate to the applications where high electrical conductivity is necessary, such as lead frames, power relays, busbars and connectors etc.. We have developed a high electrical conductivity type, spring use alloy, DK-3-SH through redevise the work-heat treatment process of DK-3, while remaining the same chemical composition as DK-3. Owing to its new work-heat treatment processes, DK-3-SH alloy has evenly dispersed minute (about 10nm in dia.) precipitates of Fe-P and Fe-Ni-P coherent with Cu matrix, thus, much more improved tensile strength, spring property fatigue strength and softening temperature etc.. The characteristics of DK-3-SH can be summarized: 1) The tensile strength, spring limit longitudinal modulus of elasticity, softening temperature and fatigue strength of DK-3-SH, are increased by 80N/mm2, 25N/mm2, 18N/mm2, 30°C and 50N/mm2 respectively from that of DK-3. 2) DK-3-SH shows fairly good 90°W bend and 90° repetitive bend workability, despite its high tensile strength. 3) DK-3-SH also has very good reliability such as solder ability etc., and excellent corrosion resistance.
There is an urgent need automotive springs with high fatigue strength, which enable fuel efficiency improvement, vehicle weight reduction and higher output. To realize this, spring materials of higher strengths are sought, and formed spring surface is often hardened. Due to this trend, spring materials with much higher strength than previously, are being used. Also, formed springs receive extreme surface hardening, to over 900 HV in some cases. Conventional shot peening media are not effective enough to peen such ultra-hard springs. As a solution to this problem, we have developed cemented-carbide shots. This new shot medium has been applied to spring products, to established the following facts: 1) Cemented-carbide shots, with remarkable hardness and very high density, provide an enormous residual compression-stress, which would be better than any conventional shot medium. The effects of this are more evident in harder workpieces. 2) Compare to conventional shot media, the same residual compression-stress can be obtained at a lower peening speed (pressure). This results in less shot-media and equipment wear as well as lower power demand. 3) The cemented-carbide shots are especially effective for improving the fatigue strength of surface-hardened, for example gas-nitrided, springs.
Despite increasing demand for high dimensional accuracy of helical compression springs for the compact design products in recent years, influencing factors on the dimensional accuracy of helical compression springs have not been clarified thoroughly yet. The characteristics of material (the loop uniformity) and the shape and position of coiling tools (feed rollers and coiling pins) as factors influencing the dimensional accuracy of helical compression springs have been studied by the research and study committee for compression coil forming. However, the effect of back tension, which is the tensile force working in the opposite direction of wire feed, on the dimensional accuracy of helical compression springs has not been clarified yet because quantitative measurement of back tension is difficult to accomplish. In this paper, we conducted an experimental investigation on back tension and estimated its effect on the accuracy of free height of helical compression springs. As the result of the experiments, it is clarified that back tension has direct effects upon the accuracy of free height of helical compression springs.
In this paper, the, experimental method for evaluating a corrosive mode of spring steels to meet with the real usage of car, and the statistic approach for avaluating a corrosive pit characteristic, were studied. The studies were also extended to the influence of alloy elements and shot peering to corrosive fatigue strength. It was found that these approaches may have some possibility to predict corrosive fatigue strength, as described below. 1) The corrosive pit characteristic can be improved by adding Cu, Ni and Cr to spring steels. 2) The proposed statistic approach by using the estimated value of pit dimension can evaluate corrosive pit characteristic of spring steels quantitatively. 3) The shot peering process can improve corrosive fatigue strength for the residual stress can restrain crack propagation at the bottom corrosive pit. 4) It can be realized the prediction of corrosive fatigue strength may become possible by applying the estimated value of pit dimension to Murakami's ewuation.
Effects of tensile residual stress induced by coiling on delayed fracture of coil springs caused by hydrogen embrittlement were investigated The evaluation method using strain gauge installed on coil spring was developed to detect the delayed fracture. This method was made certain of its effect by the experiment applyed to cold formed coil spring having high tensile residual stress dipped in dilute hydrochloric acid. The new method made it clear that higher material strength of coil spring brought the delayed fracture chracteristics drop due to its high strength itself and increase of tensile residual stress.
Formed bellows accumulators have been mass produced far pulse absorbers of the vehicle active suspension systems since 1989, because of the features of no-gas-leakage and high-frequency-response. By these days' needs, pursuing more intensive space efficiency, we began to design it so that some modes of input pressure may be in the range beyond the fatigue limit of the bellows. On the other hand, we must make it clear that how the accumulator gets damaged by using within the range beyond the fatigue limit. In this report, we present a sample analysis and prognosis of the fatigue life of our accumulator. The prognosis is based on precise prediction of the system life and the Miner's law. We can apply this method to design accumulators for automobile parts which we must guarantee the fatigue life for the entire period of the system life.
In spite of a demand for an improvement in the dimensional accuracy of compression coil springs formed with an automatic coiling machine, factors affecting the dimensional accuracy of compression coil springs have not yet been clarified. Under the circumstances the assurance of the dimensional accuracy of compression coil springs has been left to skilled workers. To improve these circumstances, the Research Committee on Compression Coil Forming was established in July of 1990 and implemented to make clear factors affecting the dimensional accuracy of compression coil springs until June of 1997. The purpose of this paper is to show the results of experiments of the committee conducted to demonstrate the influence of loop uniformity, shapes and positions of coiling tools, and initial tension of compression coil on the free height dispersion of compression coil springs which had been taken up from among other factors.
This report deals with the results of survey on the actual situation of springs and the proposal of future research subjects for spring technology at the request of Japan Spring Manufacturers Association. Firstly, this committee investigated the index of literature of small springs from Transaction of Japan Society for Spring Research, and collected the manufacturing data of small springs in Japanese industries. Secondly, the committee analyzed the index of literature and data collected under the title of the above mentioned purposes, and classified them into three categories of design, material and manufacture. In conclusion, this committee is reguested to propose the main research subjects of small springs to be chosen in future for Japan Society for Spring Research.