The fatigue property of shot-peened specimens with a semispherical artificial pit of varied diameter was investigated. Shot peening (SP) and stress shot peening (SSP) were administered to the specimens, followed by etching hole diameter of 0.2, 0.4, and 0.8 mm. After that, bending fatigue tests were carried out on the specimens. The fatigue strength of the SP specimen was improved substantially because of the compressive residual stress generated on the surface, furthermore, the SSP specimen shows a higher fatigue strength due to the formation of greater compressive residual stress than SP.Å@In the case of shot-peened specimens and stress shot peened specimens, the specimen having an artificial pit with d=0.2mm fractured from outside the artificial pit. Therefore, d=0.2mm diameter artificial pit was acceptable in the case of SP or SSP specimen. Multiple non-propagating cracks were observed in a peened specimen after fatigue testing. It is considered that the reason why the artificial pit is acceptable is the stress intensity factor range, which is reduced by compressive residual stress by SP or SSP, becomes smaller than the threshold stress intensity factor range compared to the fatigue limit stress of un-shot peened material.
Accidental temperature rise will cause a significant decrease in the fatigue limit of the steel which has been subjected to the multiple surface treatment of vacuum carbonitriding and double shot peening. The phenomenon was investigated by the volume fraction of retained austenite and residual stress level altered by the tempering temperature. Based on the knowledge of this study, the new methodology to overcome the degradation was proposed. The residual stress and high hardness introduced by vacuum carbonitriding and double shot peening were decreased by tempering with the temperature of 573 K and above due to the decomposition of the retained austenite. The phenomenon makes fatigue limit to the 56 % reduced level. It turned out that the residual stress did not decrease by heating at 573 K, if the residual stress is introduced by double shot peening after tempering with high temperature where the retained austenite was completely decomposed.
This paper describes a new coil spring design method for automated, optimized and designer-independent design. In conventional coil spring design, a spring profile at free state is usually designed first and it is then compressed in FEA (Finite Element Analysis) to check if the spring characteristics satisfy the required specifications. However, the spring requirements such as rate, load, FLP (Force Line Position), deformed profile, etc. are usually given at the compressed state of the spring. Therefore, if the result does not meet the requirements, a designer must modify the free state profile while predicting how it reflects to the spring characteristics at deformed state. In the new method, the ideal deformed spring profile is designed first and its corresponded unique free profile is then reversely determined by automated FEA iterations in order to satisfy the requirements. By scanning the total number of turns in a certain range for the deformed spring profile designed at the first stage, the optimal spring design can be selected in terms of weight, stress and manufacturability. This paper validates the new method with actual design example. The method and resultant spring shape are currently international patent pending.
In many cases, the compact type hard disk drive (HDD) is employed in mobile devices, and a shockproof design is essential for this kind of application. A swing arm type positioning actuator is vulnerable to the vibration of arm bending modes excited by natural frequency when the shock impulse is imposed. In this paper, we studied different kind of swing arm type positioning actuators. Their mass and geometry are close each other, however, their vibration characteristics are quite different when the shock impulse is imposed. It turned out that the big difference in vibration response comes from the frequency interval of two arm bending modes in the actuator. The vibration system of three arm actuator consists of the main vibration system with top & bottom arm and the sub vibration system with the middle arm. We established the design method to minimize the frequency interval of two arm bending modes in order to improve the shockproof capability by utilizing the dynamic damping effect.
In this research, a new type valve system using a conical coil spring and an electromagnetic actuator was developed. This system uses a conical coil spring whose spring constant is low, and therefore, the valve system has a low torque in a low-speed rotation mode. As a result, the energy loss in a low-speed rotation is decreased. However, the jump and bounce phenomena would happen in the high-speed rotation mode. The phenomena are suppressed by using an electromagnetic actuator consisting of two permanent magnets and electromagnet. The electromagnetic actuator strengthens the restoration force of the spring by the repulsive force, and controls the jump and bounce phenomena. It was confirmed that the use of a conical coil spring lessened the load torque in this experiment and that the jump and bounce phenomena were suppressed by using the present electromagnetic actuator.
A high speed actuator consisting of coil springs with iron particle layers was developed. In this system, the move of actuator head is controlled by electromagnets. Using this newly developed actuator, the two dimensional position control mechanism that controls the position of the instruction point on the screen by changing the angle of the laser pointer is introduced. The mechanism consists of two spring actuators, sliders, and a laser pointer. One actuator is parallel to X- axis and the other to Y-axis, and the laser pointer is rotated by flexing the springs. The head displacement in X-direction is independent of Y-direction. Therefore, the instruction point can be scanned all over the range by operating the one actuator with high frequency and the other with low frequency. Moreover, the instruction point moves to the target point by using usual PID control method.
The method for measuring a decarburized zone depth is conventionally listed as follows; (1) Micro structure observation by optical microscope, (2) Measurement of hardness distribution, (3) Measurement of carbon concentration by EPMA analysis. The second method is considered to be convenient for easy and practical handling. In some cases, this hardness measuring method is specified in the inspection agreement between a supplier and a customer. Because of the restriction of applicable loads and measuring pitch in Vickers hardness tests, this method is deemed to be difficult for the measurement of small decarburized depth which is often seen in oil quenched and tempered wires for springs, though it can be used for measurement of large decarburized depth in general carbon steels. Round robin tests were carried out in the committee for the oil tempered and quenched wires, and the following conclusions were obtained. It was clarified that the present measurement accuracy of Vickers hardness was suitable for judging large differences such as 100HV or more, but not suitable for judging small difference such as 50HV or less. In order to insure accurate decarburized depth measurements in the latter case, an error inspection by using the reference blocks should be carried out, and statistical processing should be used on a larger number of measurement.
It is known that the acceptable defect size of steel is smaller for materials having a higher strength. If the acceptable defect size can be increased by shot peening, the reliability of spring could be raised, and it can be anticipated the decrease of production cost of the spring. Then, at this research committee the effects of shot peening on the fatigue strength of spring steel specimens (SUP9A) containing an artificial small hole were investigated. Shot peening (SP) or stress shot peening (SSP) were carried out with the specimens containing an artificial drilled hole 0.2-0.8 mm diameter or a semi-circular slit with 0.3 and 0.6 mm surface length. Then, bending or torsional fatigue tests were carried out with these specimens. SP and SSP improve substantially the fatigue strengths of specimens containing an artificial small hole. Furthermore, the sizes of surface defects, which can be acceptable, by SP or SSP were discussed from the viewpoint of fatigue limit. In order to assess the fatigue limit of spring steel, which possesses surface defects, the fatigue limit assessment diagram (FLAD) was proposed based on the Dugdale model, and the cyclic crack tip opening displacement criterion. The validity of the FLAD was discussed comparing with the past literature data.