Mitsutoshi Ishimura

Experimental Evaluation of Screw Thread Loosening in Bolted Joint With Some Parts for Preventing the Loosening Under Transverse Repeated Loadings

In mechanical structures, a lot of bolts and nuts have been used. Sometimes loosening accidents happen in the structures under several types of loadings. Recently, the cause of the screw thread loosening is being elucidated. However, the effect of some parts for preventing the loosening in bolted joints such as spring washer, double nut and so on is not fully elucidated. In this study, the experiments were carried out to measure the reduction of axial bolt force in the bolted joint with some parts for preventing the loosening such as the spring washer, the toothed washer, the double nuts, the eccentric nut, and so on under the transverse repeated loadings. The test machine used was Junker’s type loosening machine. Then, the effects of the washers and some types of nuts are evaluated on the loosening of bolted joints. It is shown that the performance for loosening is getting better as the initial clamping force increases and the effect of the washers is not found on the loosening. However, the effect of eccentric nut is substantial on the loosening. In addition, the loosening tests according to NAS were also carried out. No effect of all the washers mentioned above is found due to NAS testing. Discussion is made on the effect of the parts for preventing the loosening mentioned above.Copyright

Bolt-Nut Loosening in Bolted Flange Connections Under Repeated Bending Moments

Bolted flange connections for wind power generations and chemical plants are subjected to repeated bending moments due to wind and earthquake loads. It is known empirically that the bolts loosening in the bolted connections occur. Thus, it is necessary to examine the bolt loosening in the bolted flange connections under repeated bending moments. The objective of the present paper is to clarify the mechanism of bolt loosening in bolted flange connections subjected to repeated bending moments and to discuss on a solution for preventing the loosening of the bolted flange connections subjected to repeated bending moments, too. The loosening mechanism of the bolted flange connections is examined using the FEM calculations and the experiments. In the loosening experiments using the specified hexagon nuts, it is observed that the loosening of the bolts in the connection occurs due to the permanent-set of the bolts and the movement of the bearing surfaces in the flange due to the external bending moment. In the FEM calculation, it is examined that the bolt loosening in the bolted flange connections qualitatively. In addition, it is found that as the bolt preload increase, the bolt loosening decreases. From the FEM results and the experimental results, it is found that the most effective part for preventing the loosening of the bolted flange connection is the eccentric nut.Copyright

3-D FEM Stress Analysis of Screw Threads in Bolted Joints Under Static Tensile Loadings

The stress concentration factor (SCF) for the roots of screw threads in bolted joints under static loadings is analyzed using 3-D elastic FEM taking account the spiral of screw threads. At first, the stress states at the roots of screw threads in initial clamping state in a bolted joint where two hollow cylinders were clamped with a bolt and a nut were analyzed in initial clamping. The elastic FEM result of SCF for the first root was obtained as SCF=3.2. When the bolt was clamped in initial clamping (preload) at the 60 % of bolt yield stress, the plastic deformations were found at the first and the second roots, and non-engaged screw threads. It was found that as the external tensile loads increased, the development in plastic deformation region increased from the first root to the other roots as well as the non-engaged screw threads. It was found that the rupture occurred from the non-engaged screw threaded part while the plastic deformation increased at each root of screw threads. The numerical result was coincided with the experimental result. In the experiments, it was observed that the rupture occurred from the non-engaged screw thread and not from the first root of screw thread. Also, the bolt fatigue was predicted from FEM and it was shown that a fatigue fracture occurred from the first root.Copyright

Mechanical Behavior of Rotational Screw Thread Loosening in Bolted Joints Under Repeated Temperature Changes

Bolted joints have been used in mechanical structures. However, loosening accidents sometimes happen under several types of loadings. Recently, some studies have been conducted on the screw thread loosening in bolted joints. It is verified that bolted joints are loosened when transverse repeated loadings are applied. A lot of parts for preventing the loosening in the joints have been proposed. Among these parts, eccentric nut is especially expected to be the solution for preventing the loosening. Some studies on no rotational loosening in bolted joints under thermal changes have also conducted. However, few studies on rotational screw thread loosening in bolted joints have conducted. In our previous study, mechanism of rotational screw thread loosening in bolted joint under repeated temperature changes was examined. However, the difference in the loosening mechanism between the FEM results and the experimental results was substantial. In this study, the effect of the incline at the bearing surfaces was taken into consideration on the loosening. In addition, the loosening in the bolted joint with the eccentric nut under repeated temperature changes was examined using finite element method (FEM) calculations. Discussion is made on the mechanism and a solution of rotational screw thread loosening in the bolted joints under the repeated temperature changes. The results show that a reduction in axial bolt force was increased when the incline at the bearing surfaces was taken into consideration and the eccentric nut was shown for preventing the bolt loosening.Copyright

Mechanism of Rotational Screw Thread Loosening in Bolted Joints Under Repeated Temperature Changes

Bolted joints have been used in mechanical structures. However, loosening accidents sometimes happen under several types of loadings. Recently, some studies have been conducted on the screw thread loosening in bolted joints. It is verified that bolted joints are loosened when transverse repeated loadings are applied. Some studies on no rotational loosening in bolted joints under thermal changes have also conducted. However, few studies on rotational screw thread loosening in bolted joints have conducted. In this study, rotational screw thread loosening in bolted joint under repeated temperature changes is examined. Effects of the linear thermal expansion coefficient of clamped parts and number of repeated temperature changes are examined by the measurements and finite element analyses. In the experiments, a bolted joint, which consist of two rectangular plates fastened at room temperature by two bolts and nuts, are put in a furnace and heated by an electric heater. Then, the bolted joint is cooled at room temperature. These thermal cycles are repeated. Bolt temperature and bolt load were measured. Loosening is evaluated from the reductions in the bolt load. Finite element analyses were carried out for clarifying the loosening mechanism. Discussion is made on the mechanism of rotational screw thread loosening in bolted joints under repeated temperature changes. The results show that loosening occurs under repeated temperature changes due to difference in the linear thermal expansion coefficients of the different clamped parts.Copyright

Mechanical Characteristics and Design Method for Bolted Circular Flange Joints Subjected to External Tensile Loads

The characteristics of bolted circular flange joints tightened with bolt number N are analyzed using FEM such as the interface stress distributions in initial clamping state, those in operation and the load factor under tensile loadings. Then the effects of the bolt pitch circle diameter D and the bolt number N on the load factor and the interface stress distributions are examined. The FEM results of the load factor and a load when the interfaces starts to separate are fairly coincided with the experimental results. The value of the load factor is the smaller and it is less than 0.1. In addition, it decreases as the value of D increases. Another objective is to demonstrate a new design method and two typical cases are described. One is a problem how to determine the bolt nominal diameter when the bolt strength grade, bolt number N, the interface stress for keeping the joint function and external load are provided and the other is how to determine the bolt strength when the bolt nominal diameter, interface stress and external load are provided. The measured tightening coefficient Q is introduced in the design and the bolt strength is evaluated using the newly introduced equivalent stress taking account the shear stress due to torque in bolt initial tightening. Using the obtained nominal diameter, the bolt maximum stress, the bolt stress amplitude and the critical stress at the bearing surfaces are checked. The availability of the method is shown with the flowchart for design.Copyright

3-D FEM Stress Analysis and Mechanical Characteristics in Bolted Joints Under External Tensile Loadings

In practical design for bolted joints, the stress concentration factor (SCF) of the first root in screw threads is important. In the present paper, the SCF in screw threads taking account of spiral in the screw threads is analyzed using three-dimensional Finite Element Method (FEM). In addition, the stress states of screw threads under repeated and static loadings are analyzed in elasto-plastic deformation range. The effect of bolt nominal diameter (M12, M24 and M33) on SCF is also examined. In addition, the experiment to measure the strains at the roots of screw threads was carried out newly. It is found that the FEM result is fairly coincided with the measured results. It is also found that the SCF increases as the bolt nominal diameter increases and the maximum stress is found to occur at the half pith from the engaged screw threads. The value of SCF is smaller in the coarse screw threads of M12 than that in the fine screw threads of M 12. Finally, the bolt ruptures for several nominal diameters are shown to occur from screw threads in the non-engagement in bolted joints under static loadings and under the repeated loadings the bolts are ruptured from the first root of screw threads due to fatigue even if the bolt preload is the higher.Copyright

FEM Stress Analysis and Design for Bolted Circular Flange Joints Under Tensile Loading

In designing a bolted joint, it is necessary to know a ratio of increment Ft in axial bolt force to an external tensile loading W, that is, the load factor φ=Ft/W. In our previous paper, the new formulation for obtaining the value of the load factor φ for a bolted joint in which two hollow cylinders and two T-shape flanges were clamped was proposed by introducing the tensile spring constant Kpt for clamped parts. Then, the values of the load factor of bolted joint obtained from our formulation are shown to be in a fairly good agreement with the experimental values. In the present paper, the mechanical characteristics of bolted circular flange joints are analyzed by Finite Element Method (FEM) and axi-symmetrical theory of elasticity such as the load factor, the contact stress distributions at the interfaces and a load when a separation occurs at the interfaces. For verification of the analyses, experiments were carried out to measure the load factor, the maximums stress and a load when a separation occurs. In the analyses, the effect of the bolt pitch circle diameter D on the value of the load factor is examined. The numerical results of the load factor are fairly coincided with the experimental results. As the results, it is found that the load factor decreases as the bolt pitch circle diameter D increases and that the value of the load factor is less than 0.1. It is also found that the load when the interfaces start to separate decreases as the value of D increases. Based on the obtained results, a design method for the bolted circular flange joints under tensile loadings is demonstrated, that is, the method how to determine the bolt nominal diameter, bolt strength grade, the bolt preload for the external tensile loading are described.Copyright

Mechanism of Screw Thread Loosening in Bolted Joints With Dissimilar Clamped Parts Under Repeated Temperature Changes

It is well known that bolted joints are sometimes loosened under transverse repeated loadings. Recently, some studies have been carried out on the mechanism of the bolt loosening under transverse repeated loadings. In the present research, the mechanism of bolt loosening was examined for a bolted joint using Junker’s type loosening test machine under transverse repeated loadings. Furthermore, a rotational deformation of bolt axis and nut was analyzed using FEM. As a result, it is found that the rotational load is applied and the bolt loosening occurs due to the deformation between the bolt axis and the nut. The measured change in axial bolt force was fairly good agreement with the numerical results. In addition, the bolt loosening in bolted joints with dissimilar clamped parts was examined under temperature changes. Due to the temperature changes, it is assumed that a relative displacement among the bolt bearing surface and the bearing surfaces of the clamped parts and then it is found that a bolt loosening occurs. In FEM calculations, the behavior of the bolt loosening was examined and found to occur a slight loosening. In the experiments, the bolt loosening was measured under temperature changes. The material of dissimilar clamped parts is assumed steel and aluminum. The thickness of the clamped parts is 10mm. The tendency of the bolt loosening is fairly coincided between the numerical and the measured results. It was observed that the bolt loosening occurred easily as the clamped length (gap length) decreased. In the FEM calculations, the slope θ of the bearing surfaces between the bolt head and the clamped part is assumed to be 0.5 degree to 2 degree. The effects of the slope of the bearing surfaces and on the bolt loosening are examined in the FEM calculations. The calculated results are in a fairly good agreement with the measured results. As the conclusions, 1) the bolt loosening of bolted joints under transverse repeated loadings was elucidated using 3-D FEM calculations, 2) Under repeated temperature changes, it was found that the bolt loosening occurred due to the slips between the bearing surfaces and the interfaces of clamped parts. Using FEM calculations, the behavior of the bolt loosening was elucidated.Copyright

Mechanical Characteristics and Design of Bolted T-Shape Flange Joints Subjected to Tensile Loadings

Bolted joints have been widely used in mechanical structures. However, a design of bolted joints has been carried out empirically. In designing a bolted joint, it is necessary to know a ratio of increment Ft in axial bolt force to an external tensile loading W, that is, the load factor φ=Ft/W. In our previous paper, the new formulation for obtaining the value of the load factor φ for a bolted joint in which two hollow cylinders were clamped was proposed by introducing the tensile spring constant Kpt for clamped parts. Then, it is shown that the values of the load factor of bolted joint obtained from our formulation are in a fairly good agreement with the experimental values. In addition, the effect of the position where an external load is applied to the joint is significant on the value of the load factor. In the present paper, a method for obtaining the value of the load factor for bolted T-shape flange joints in which two T-shape flanges are clamped by two bolts and nuts under tensile loadings is demonstrated using two-dimensional theory of elasticity. In the analysis of the load factor φ, the compressive spring constant Kc for a hollow cylinder around the bolt hole is calculated. The value of the correction factor Kc′/Kpt is also analyzed using the two-dimensional theory of elasticity. When the external tensile loads are applied to the joints, the bolts are inclined and as the result, the bending moment occurs in the bolts. A method for analyzing the bending stress in the bolts is also demonstrated. In the numerical calculations, the distance C between the bolt position and the center of T-shape flange is varied and the effect of the distance C on the load factor is examined. For the validation of the analyses, calculations are also carried out. Experiments to measure the load factor and the maximum stress due to the bending moment occurred in the bolts were carried out. The numerical results of the load factor and the maximum stress in the bolts are fairly coincided with the experimental results. Furthermore, a design method for the joints is discussed, that is, how to determine the bolt position C, the bolt preload for the external tensile loading, how to choose the bolt strength.Copyright