Recently the downsizing and the weight saving of mechanical structures asked for springs lighter, and the light metal alloys are paid attention to as spring materials. Magnesium is the lightest in the practical metals, and it is excellent in the damping capacity. But the tensile strength and the proof stress of the pure magnesium is much lower than the ones for other spring materials. Hence, it is necessary to improve its strength by alloying, hardning, heat treatment, and by other processes. The workabilities of magnesium alloy springs had not been studied, too. Therefore the authors studied the mechanical properties of magnesium alloy AZ31 and made coil springs from wires of a magnesium alloy AZ31 by a lathe type coiling machines. The workabilities of coils were studied. The effects of annealing on the dimensions of the coils and on the spring property were also studied. The main results are as follows; (1) Magnesium alloy wire AZ31 is much inferior to other spring wires, but not much inferior in specific strength because of small specific gravity. (2) It was possible to form coil of about 5.0 in spring index from wires of a magnesium alloy AZ31 by a lathe type coiling machines, so wires of a magnesium alloy AZ31 fill the workabilities as spring wires. (3) In magnesium alloy wire AZ31 the spring back of coil diameter is large compared with other spring steel wires. (4) By annealing the outside diameter increases but the free length hardly changes. (5) When the annealing temperature is over 180(°C), the elastic limit of coiled springs deteriorates.
There are many kinds of irregular shaped coil spring like an elliptically coiled, a rectangularly coiled and a squarely coiled spring. Especially an elliptically coiled spring is used in limited space like a gun magazine or precision instruments. Recently, elliptic coil springs are used in the coin shooter of vending machines thanks to their flexibility. Though the irregular shaped coil spring has numerous uses, its design formulae on the spring constant and the maximum stress of the wire have not been derived yet. In this study, general formulae for the spring constant and the principal stress for the springs with relatively small pitch angle are derived theoretically. Moreover, experimental study and FEA (Finite Element Analysis) are conducted. The theoretical results are compared with experiment and FEA. Then the stress correction factor for an elliptically coiled spring is derived by comparing theoretical value with FEA. The theoretical results multiplied by the correction factor are compared with FEA.
The coiled wave spring is considered as a multiple layered waved washer, which shows powerful spring force in a limited space. In recent years, some automobile makers adopted them for clutch springs. The application of the spring to other fields has been expanding more and more. But the industrial standard has not been established, yet. In this research, the spring characteristics and the stress of coiled wave springs are analyzed. And the formulae on the spring constant and the stress are derived. Furthermore the formula on the nonlinear spring characteristics is derived. Moreover, some experiments and the Finite Element Analysis are conducted to confirm the validity of the analytical results. As a result, it is confirmed that the analytical results on the spring characteristic agree with the experimental ones. The formula on the spring constant in linear analysis gave the almost average slope of the spring characteristic curve, which means that the formula is useful practically. But the ones on the stress only agree qualitatively. Therefore the theoretical results on the stress must be multiplied by a correction factor of 0.8.
Delayed fracture sensitivity is one of the most important problems for strengthening the spring steels. Recently, some papers are reported to improve the delayed fracture life by adding the alloy elements such as Vanadium and Boron or by refining the grain size. But, in usual case, delayed fracture properties are only evaluated by the total life for fracture and there are few studies on initiation of crack and its propagation. AE (Acoustic Emission) method is already well known to detect the initiation of (initial) crack of structural materials. Hence, we developed a new system for testing the delayed fracture properties of coil springs by AE method. In this study, we clarified the superiority of this new method by the fact that the location of initial crack and its propagation of coil springs are detected in detail by analyzing AE signals. Using this method, we researched the effects of hardness distribution on the behavior of delayed fracture. The incubation time of initial crack occurrence and its frequency of events vary depending on the hardness distribution of coil spring, especially on surface hardness. Many events of AE signals in lower hardness of coil springs were considered to be related with the depth of quasi-cleavage fracture region detected by scanning electron microscope.
In application of flexible materials for mechanical springs, it is very important to check mechanical properties like Young's modulus. This report describes the development of a new measuring method to investigate Young's modulus of flexible materials by considering large deformation behaviors. Two kinds of flexible materials (a high-polymer material and a steel material) were tested. As a result, it is made clear that the new method is suitable for flexible thin plates (sheets) and thin rods. Incidentally, a measured modulus by using this method is secant modulus. The new method developed in this report can be applied suitably to high flexible materials (composites, glass fiber, carbon fiber, optical fiber, etc.).
In the first half of 1980s, The Japan Society for Spring Research held the joint committee which researches shot-peening condition for small springs. This committee proposed optimum shot-peening conditions for steel grades and wire diameters. Resent years, strength of spring material has been targeted to be higher and higher levels. Shot-peening technology has been improving, and new shot-media and multi-shot-peening technology have become to be employed. Therefore, the joint research was needed for high strength spring. Under these circumstances, a research committee for optimizing a shot-peening process was established in September 1996. In this committee, the experimental shot-peening conditions were widely chosen in the possible range of manufacturing parameter. Single shot-peening, double shot-peening, stress shot-peening and hardness of spring were studied. The effect of each shot-peening process on fatigue strength were considered in relation to surface compression stress and surface roughness. For control of shot-peening condition, effectiveness of a harder Almen strip, relation between X-ray radiation field size and wire diameter, and the correction method of residual stress were studied. The results are summarized as follows. (1) For high strength spring, a harder Almen Strip with hardness of 58HRC, thickness 1.3mm, is proposed. (2) Research on suitable shot-peening conditions for high strength spring: (a) The difference of torsional and rotary fatigul test is negligible. (b) In the case of single shot-peening, higher surface residual stress and smaller surface roughness induced high strength of fatigue (c) In the case of double shot-peening, harder and finer shot indicated higher strength of fatigue. (d) Stress shot-peening could not be better than double shot-peenig with higher and finer shot. (e) Double shot-peening with higher and finer shot indicated the best fatigue strength. (3) Analysis of fatigue phenomenon for a shot-peened spring (a) Compression residual stress on surface is the most effective for improving fatigue strength. (b) Stage I type crack first initiates and propagates and then stage I crack transfers into stage II type crack. Magnitude of stage I crack is almost equal to depth of compression residual stress. (4) X-ray stress measurement: (a) In the case of coil spring, X-ray radiation field size should be smaller than 1/2∼1/3 of wire diameter. (b) By using FEM method, Heyn's correction equation is practically useful for correction of residual stress.