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Bolted joints are elements used to create resistant assemblies in the mechanical system, whose overall performance is greatly affected by joints’ contact stiffness. Most of the researches on contact stiffness are based on certainty theory whereas in real applications the uncertainty characterizes the parameters such as fractal dimension D and fractal roughness parameter G . This article presents an interval estimation theory to obtain the stiffness of bolted joints affected by uncertain parameters. Topography of the contact surface is fractal featured and determined by fractal parameters. Joint stiffness model is built based on the fractal geometry theory and contact mechanics. Topography of the contact surface of bolted joints is measured to obtain the interval of uncertain fractal parameters. Equations with interval parameters are solved to acquire the interval of contact stiffness using the Chebyshev interval method. The relationship between the interval of contact stiffness and the uncertain parameters, that is, fractal dimension D , fractal roughness parameter G , and normal pressure, can be obtained. The presented model can be used to estimate the interval of stiffness for bolted joints in the mechanical systems. The results can provide theoretical reference for the reliability design of bolted joints.

Analyzing of nonlinear carrying performance of hydrostatic ram is among key research questions to improve heavy machine tools because the machining precision will be directly influenced by it. A dynamic model of nonlinear supporting characters for hydrostatic ram is developed in this work. Modified Reynolds equation is resolved by finite difference method numerically to determine the carrying capability. Dynamic equilibrium equation of hydrostatic ram under cutting force impact is solved by Runge–Kutta method to obtain the variation of tool tip position and evaluate the machining accuracy. An optimal oil supply rate allocation and oil pad size is proposed to improve the static and dynamic performance simultaneously based on method of bisection.

Bolted joints are widely used for the mechanical assembly of engineering structures and friction coefficient of the contact surface of a bolted connection is the main factor that influences its structural performance. This article presents a mathematical model based on the Florida contact model to obtain the friction coefficient for the contact surface of a bolted joint. The pressure distribution function of the bolted joint is introduced into the mathematical model and elastic, elastic–plastic, and full plastic deformation of asperities at the microscale is also considered. By varying the peak height of asperities, an improved exponential distribution function was constructed. In addition, the proposed model was verified by comparing simulation results to experimental values obtained using a CETR UMT-5 high-precision ball-on-disk friction and wear tester. Then the theoretical friction coefficient was used in the finite element model of the bolted joint, and through numerical simulation, the state of the contact surface of the tightened bolted connection was analyzed. Increasing the friction coefficient was shown to improve the structural performance of the bolted joint. The results provide a theoretical basis for designing uniform preloading of bolted joints and reducing micro-slip at the joint interface.

The quality of heavy-duty machine tool foundations can drastically affect the operating life and working precision of the tool, and the high cost of manufacture has drawn a lot of attention. This article summarized the research status of the relevant literature on the characteristics, vibration isolation, foundation optimization, and quality inspection of heavy-duty machine tool-foundation system, induced the influencing laws of the influencing factors of the system, reviewed the highlights and achievements in the research of heavy machine tool-foundation system at present, and put forward some problems and development directions existing in the research of heavy machine tool-foundation system. It lays a foundation for realizing the judgment of the concrete foundation quality and improving the processing precision and the maintenance of the heavy machine tool.

Joint flexibility has a major impact on the motion accuracy of a robotic end effector, particularly at high speeds. This work proposes a technique of precisely modeling the torsional stiffness of the rotational joints for the industrial robots. This technique considers the contacts that exist in the joint system, which can have a significant effect on the overall joint stiffness. The torsional stiffness of the connections that commonly exist in the rotational joints, such as the belt connection, the connections using key, bolts, and pins, were modeled by combining the force analysis and the fractal theory. Through modeling the equivalent stiffness for the springs in serial and in parallel, the torsional stiffness of all joints for the ER3A-C60 robot were calculated and analyzed. The results show that the estimated stiffness based on the proposed technique is closer to the actual values than that based on the previous model without considering the contacts. The analysis is useful for controlling the dynamic characteristic of the industrial robots with the rotational joints while planning the trajectory for the end effector.

Hydrostatic bearing is a key part that provides precision and long life to machine tools. It is one of the embodiments of tribology, mechanics, optimization method, and structural design in engineering practice. Articles about hydrostatic bearings since 1990 are collected in this review. Researching status is evaluated in two aspects: basic theory and typical application. This article presents a review of research articles related to introducing developments in hydrostatic bearings. Basic theory contains equations and analysis methods which include analytic, numerical, and experimental methods. Typical applications are based on rectangular oil pad, circular oil pad, and journal bearings. Moreover, this article focuses on the analysis of the relevant model, solution, and optimization and summarizes the hotspots and development directions.

The vibration acceleration signal mechanisms in normal and defect gears are studied. An improved bending-torsion vibration model is established, in which the effect of time-varying meshing stiffness and damping, torsional stiffness for transmission shaft, elastic bearing support, the driving motor, and external load are taken into consideration. Then, vibration signals are simulated based on the model under diverse sampling frequencies. The influences of input shaft's rotating frequency, the teeth number, and module of gears are investigated by the analysis of the simulation signals. Finally, formulas are proposed to calculate the acceleration signal bandwidth and the critical and recommended sampling frequencies of the gear system. The compatibility of the formulas is discussed when there is a crack in the tooth root. The calculation results agree well with the experiments.

The chatter vibration in high-speed machining mostly originates from the flexible connection of spindle and toolholder. Accurate identification of spindle-toolholder joint is crucial to predict machining stability of spindle system. This paper presents an enhanced stiffness identification method for the spindle-toolholder joint, in which the rotational degree of freedom (RDOF) is included. RDOF frequency response functions (FRFs) are formulated based on finite difference technique to construct a completed spatial FRF for the joint, where the measured data can be obtained from the piezoelectric acceleration sensors. In order to depress the influence of “modal truncation” and measurement noises, residual compensation theory is introduced to regenerate the RDOF FRF. Experiments are conducted to demonstrate the efficiency of the proposed model in stiffness identification of spindle-toolholder joint, and the accuracy is significantly improved compared to the traditional model.