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The machine tools are consisted of many parts and most of them are connected by the bolts. Accurate modeling of contact stiffness and damping for bolted joint is crucial in predicting the dynamic performance of machine tools. This paper presents a modified three-dimensional fractal contact model to obtain the stiffness and damping of bolted joint. Topography of the contact surface of bolted joint is fractal featured and determined by fractal parameters. Asperities in microscale are considered as elastic, elastic-plastic, and full plastic deformation. The expand coefficient is introduced to the size-distribution function of asperities. The real contact area, contact stiffness, and damping of the contact surface can be calculated by integrating the microasperities. The relationship of contact stiffness, damping, fractal dimension D, and fractal roughness parameter G can be obtained. Experiments are conducted to verify the efficiency of the proposed model. The results show that the theoretical mode shapes are in good agreement with the experimental mode shapes. The relative errors between the theoretical and experimental natural frequencies are less than 3.33%, which is less than those of the W-K model and L-L model. The presented model can be used to accurately predict the dynamic characteristic of bolted assembly in the machine tools.

Background: Many previous studies lack sufficient quantitative evidences about changes in biomechanical properties of the knee in response to proximal fibular osteotomy (PFO). Therefore, the aim of this study was to compare the preoperative and postoperative effects of PFO on mechanical stresses in the knee joint and provide with a biomechanical basis for PFO in the treatment of mild knee osteoarthritis (KOA) with varus deformity. Methods: A total of 10 patients suffering mild KOA with varus deformity were enrolled in this study. Their image data from computerized tomography (CT) and magnetic resonance imaging (MRI) were used for finite element models, and PFO models were established. Static structural analysis was carried out using ABAQUS to compare the von Mises stress distribution and values of the maximal von Mises stress of femoral cartilage, meniscuses, tibial cartilages, and tibial plateau before and after surgery. Results: The stress distribution in the cortical bone of the tibial plateau showed that stresses were transferred from the anterior medial area to the posterior medial area after PFO. Values of the maximal von Mises stress in femoral cartilage, medial meniscus, medial tibial cartilage, and tibial plateau after surgery were significantly lower than the preoperative values, with statistically significant differences (P < 0.05). Postoperative values of the maximal von Mises stress of lateral meniscus and lateral tibial cartilage were significantly higher than the preoperative ones, with statistically significant differences (P < 0.05). Conclusion: PFO could reduce the stresses in the medial compartment of the knee joint with stress pathways transferring from the anterior medial area to the posterior medial area of the tibial plateau. Therefore, PFO is recommended for the treatment of mild KOA with varus deformity featuring favorably pain-relieving effects.

In most existing bolt assembling operations, the human-machine collaboration (HMC) is used to guarantee both the operational efficiency and the connection quality. However, the huge labor intensity and costs are still needed when there exist bolts with huge quantity and multiple models in some special working environments, for example locomotive maintenance and automobile assembly. Actually, the robotic application can better solve the above problem. The main reason why this application is limited in the bolt assembly is due to the technological barriers related to the intelligent control. This paper firstly introduces the HMC-based and the robot-guided bolt assembly processes in detail, then summarizes the latest researches related to the significant technologies needed in the robot-guided assembly to show their present status and future trends. Finally based on the technology limitations this paper further presents some suggestions for future studies of each technology, which attempts to help readers broaden their research thoughts and speed up the development of the robot-guided bolt assembly.INDEX TERMS Automatic bolt assembly, robotic application, technological barrier, intelligent control.

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Due to the influence of centrifugal force, accurate contact stiffness model of spindle-toolholder joint at high speeds is crucial in predicting the dynamic behavior and chatter vibration of spindle-toolholder system. In this paper, a macromicro scale hybrid model is presented to obtain the contact stiffness of spindle-toolholder joint in high speeds. The hybrid model refers to the finite element model in macro-scale and three-dimensional fractal model in micro-scale. The taper contact surface of spindle-toolholder joint is assumed flat in macro-scale and the finite element method is used to obtain the pressure distribution at different speeds. In micro-scale, the topography of contact surfaces is fractal featured and determined by fractal parameters. Asperities in micro-scale are considered as elastic and plastic deformation. Then, the contact ratio, radial and torsional contact stiffness of spindle-toolholder joint can be calculated by integrating the micro asperities. Experiments with BT40 type toolholder-spindle assembly are conducted to verify the proposed model in the case of no speed. The reasonable intervals of spindle speed and drawbar force can be obtained based on the presented hybrid model, which will provide theoretical basis for the application and optimization of the spindle-toolholder system.

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