Sufficient preload in a bolted joint is key to ensuring the reliability of mechanical products; however, under vibration, preload decrease often occurs. The mechanism of preload decrease has not yet been fully clarified. In this study, finite element models of bolted joints with and without helix angles were constructed to study the mechanism of preload decrease under transversal vibration. Based on the finite element analysis results, a new cause of preload decrease, denoted as stress release and redistribution, was discovered and explained in detail. The mechanism of preload decrease caused by stress release and redistribution, cyclic plasticity deformation and rotation loosening is studied systematically, and the typical mode of preload decrease is proposed. Based on the preload decrease curve, more comprehensive evaluation criteria are established, quantified using three parameters to represent the locking behavior of bolted joints. Finally, experiments were conducted to verify the reliability of the preload decrease results.

Surface topography of sealing interface is a key factor affecting sealing performance. In industry, it has always been desirable to optimize the performance of static seals by optimizing the surface topography. The evolution of leak channels and the quantitative effects of surface topography on leak rates are expected to be clarified. This paper proposes a novel approach to calculate leak channels and leak rates between sealing surfaces for specific surface topographies, based on three-dimensional (3D) finite-element contact analysis. First, a macromechanical analysis of the entire sealing structure is conducted to calculate the interfacial pressure. Second, the surface topography data are measured and processed. Third, the interfacial pressure is used as the boundary condition applied on the microscale 3D finite-element contact model, which is built based on the specific surface topography. Fourth, the geometrical models of leak channels are extracted from the finite-element contact model, and the leak rates are calculated using the computational fluid dynamics (CFD) method. The proposed approach was applied to a hollow bolt-sealing structure. Finally, experimental results verified the accuracy and effectiveness of the proposed approach, which can provide valuable information for optimizing surface processing techniques, surface topography, and static seal performance.

An understanding of conditions that trigger the loosening of bolted joints is essential to ensure joint reliability. In this study, a three-dimensional finite element model of a typical bolted joint is developed, and a new simulation method is proposed to quantitatively identify the critical transverse force for initiating loosening. This force is used to evaluate the anti-loosening capacity of bolted joints. Using the proposed simulation method, the effects of factors affecting critical loosening are systematically studied. It is found that the preload, frictional coefficients at the thread and the bearing surfaces, clamped length, and fit tolerance mainly affected loosening. When the preload and friction coefficients are increased, and the clamped length and fit tolerance are reduced, loosening is inhibited. Experiments are performed to demonstrate the reliability of the results. Finally, a suggestion is proposed to improve the design guideline VDI 2230 for bolted joints, which considers the requirement of avoiding loosening under vibrational loading.

Recently, the wedge self-locking nut, a special anti-loosening product, is receiving more attention because of its excellent reliability in preventing loosening failure under vibration conditions. The key characteristic of a wedge self-locking nut is the special wedge ramp at the root of the thread. In this work, the effect of ramp angle on the anti-loosening ability of wedge self-locking nuts was studied systematically based on numerical simulations and experiments. Wedge self-locking nuts with nine ramp angles (10 deg, 15 deg, 20 deg, 25 deg, 30 deg, 35 deg, 40 deg, 45 deg, and 50 deg) were modeled using a finite element (FE) method, and manufactured using commercial production technology. Their anti-loosening abilities under transversal vibration conditions were analyzed based on numerical and experimental results. It was found that there is a threshold value of the initial preload below which the wedge self-locking nuts would lose their anti-loosening ability. This threshold value of initial preload was then proposed for use as a criterion to evaluate the anti-loosening ability of wedge self-locking nuts quantitatively and to determine the optimal ramp angle. Based on this criterion, it was demonstrated, numerically and experimentally, that a 30 deg wedge ramp resulted in the best anti-loosening ability among nine ramp angles studied. The significance of this study is that it provides an effective method to evaluate the anti-loosening ability of wedge self-locking nuts quantitatively, and determined the optimal ramp angle in terms of anti-loosening ability. The proposed method can also be used to optimize other parameters, such as the material properties and other dimensions, to guarantee the best anti-loosening ability of wedge self-locking nuts.

The evaluation of sealing performance is important in the design stage and assembly process of sealing structures. To date, analyses of sealing structures have rarely quantitatively evaluated sealing performance: They tend to neglect microsurface topographic information or simplify rough surfaces. Herein, we propose a new and efficient simulation approach to analyse sealing performance based on a multiscale contact model. In this simulation approach, a new parameter, critical preload, is used as the index of sealing performance. The validity of the proposed index was verified using the case of a cone seal. The evolution of the contact characteristics during the assembly process, such as the real contact area, the distribution of contact pressure, and contact status, were studied in detail.

Torque control is usually the only method for tightening bolts in some precision assembly applications. However, the scatter of the torque–tension relationship may significantly decrease the accuracy of the preload, which conflicts with the high requirement for mechanical accuracy in such precision assemblies. An important, but often ignored, factor affecting the accuracy of the torque–tension relationship is the effective bearing contact radius. In this article, a three-dimensional finite element model of a typical bolted joint was developed to obtain the actual bearing pressure distribution, based on which the effective bearing contact radius can be further calculated. Then, a parametrical study was conducted to systematically investigate the effects of various geometrical, material, and frictional factors on the effective bearing contact radius. Based on the numerical results, a comprehensive and quantitative evaluation of the relative accuracy of each traditional method of calculating the effective bearing contact radius was made. In particular, it was found that the effective bearing contact radius, calculated based on the assumption of uniform bearing pressure distribution, was always relatively accurate regardless of the geometrical, material, and frictional conditions considered. This study will be helpful in increasing the accuracy of preload, thus ensuring mechanical accuracy and quality for precision assemblies.