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The clamping force of a bolted joint greatly influences its performance. But the clamping force is seldom checked after tightening. In this study, a new method to detect the clamping force after tightening has been proposed. In the proposed method, the clamping force is detected by pulling the bolt threads which protrude through the nut. Clamping forces of pre-tightened bolted joints were detected experimentally by the proposed method. Results show that the proposed method can detect the clamping force within to ±10% accuracy. Furthermore, a prototype wrench to detect clamping force has been developed using a commercial torque wrench. Results of the verification show that this wrench can also detect the clamping force within to ±10%.

Our previous study described loosening-fatigue tests under small transverse vibrations that had been performed to elucidate loosening-fatigue mechanism in the long life region. In this study, the influences of various factors of a bolted joint on loosening-fatigue characteristics under transverse vibration have been investigated. Results show that bolt property class has little influence on the transverse loosening-fatigue limits. Results also show that the transverse loosening-fatigue limit is reduced if a bolted joint which has been tightened to within the plastic region is subjected to transverse vibration. And if the grip length and the engaging length are long, the apparent transverse loosening-fatigue limits also decrease.

Fatigue failure of bolted joints is a very serious problem for vehicles that are subjected to vibration loading. Fatigue characteristics of bolted joints under transverse vibration have been investigated in our previous study. According to the results, apparent fatigue limits (the highest amplitude of transverse vibration force which can be applied to the bolted joint without generating fatigue) differ significantly according to tightening conditions although real fatigue limits of bolts are the same if the property classes are the same. The difference in apparent fatigue limits is due to changes in distribution of bending moment applying to the bolt due to tightening conditions such as grip length and engaged thread length. In this study, the relationship between the apparent fatigue limit and the real fatigue limit has been experimentally revealed and a method to predict the apparent fatigue limit using the real fatigue limit has been developed.

The scatter of bolt clamping force tightened by the conventional torque control method very often causes loosening and fatigue failure of bolted joints. Different values of frictional coefficients between mating screw surfaces and between mating bearing surfaces cause the scatter of clamping force in each tightening. In this study, a new tightening method to reduce the scatter of clamping force of bolted joints has been developed. In the new method, the frictional coefficients at screw and at bearing surface of bolt head can be separately detected during a tightening process by simultaneously application of tightening torque and compressive force to a bolt. The tightening experiments have been successfully conducted to demonstrate the efficiency and usefulness of the proposed method. Furthermore the optimum configuration of bolt head to improve the accuracy of the proposed method has been investigated.

Bolt loosening fatigue tests under small transverse vibrations have been performed to understand the behavior in the long life region. The current study is focused on the loosening-fatigue mechanism and on the estimation of nominal stress at the root of the first thread caused by transverse vibration. Results show that if a bolt has loosened within 103∼104 vibration cycles, damage such as crack nucleation at the root of the first thread is not observed, and loosening is due to bolt rotation. However, if loosening does not occur until approximately 105∼106 cycles, then a crack is observed at the root of the first thread in the all experiments. Loosening occurs due to loss of bolt stiffness caused by crack nucleation and propagation which then leads to bolt rotation. Results also show that the initial clamping force above a threshold level does not significantly influence the loosening-fatigue life of a bolted joint.

Multi-material structures are demanded to reduce weight of vehicles. We have to reduce the weight of not only structural material but also joining elements to achieve multi-material structures. Some aluminum alloy bolts have begun to be used in the automotive fields recently. In our previous study, we investigated the tightening characteristics of Aluminum alloy A5056 bolt. The results showed that friction coefficients of thread surfaces and bearing surfaces are obviously different in comparison with those of steel bolt. The tightening strength, especially the proof clamp force, is very important to determine the target clamp force of bolted joint. However the proof clamp force of bolt is different from the proof tensile strength of bolt because the proof clamp force significantly depends on the friction coefficient of thread surfaces. Therefore we can easily know the proof clamp force for each bolt material if we can estimate the friction coefficients from the tensile strength.

If a bolted joint is subjected to axial vibration, the internal force the bolt receives is determined based on the load factor that is a function expressed by bolt stiffness and clamped part stiffness. The stiffness of the bolt and clamped part are generally calculated based on VDI2230, but if the structure of the clamped parts is complicated or made of multiple materials, we have to calculate the clamped part stiffness using FE analysis. FE analysis is very useful for calculations of clamped part stiffness. However it is not easy to conduct FE analysis for all parts and structures. A new method to detect clamped part stiffness of bolted joints was developed in this study. The proposed method is based on a clamp force detection method that we had already proposed. In the method, a protruding bolt thread portion is first pulled while holding down the nut’s upper surface and the displacement at the pulling point is measured.