The method of predicting the maintenance time of mechanical products based on Virtual Prototype

Aim: To predict the mechanical product maintenance time is difficult in the situation of lack of physical prototype or similar products’ statistics in stage of design Method: According to the theory of time accumulative estimation method, a product maintenance time prediction method framework based on virtual prototype was constructed, which described the prediction process. The virtual maintenance environment which contains virtual prototype, virtual human and maintenance tools was developed. The virtual human’s position and posture information during the maintenance process was obtained by implementing VBScript language. Result: Basic maintenance motions that constitute the whole maintenance process were classified into 4 categories: body movement, upper limb movement, grasp/replace and operation. Based on MODAPTS (Modular arrangement of predetermined time standard) method and virtual maintenance simulation, corresponding time prediction methods for each categories were proposed. Discussion: Take a maintenance dissassembly and assembly task of engine as an example, through the comparison between the measured actual maintenance time and predicted time of several methods, feasibility and effectiveness of proposed method are verified

Heat Transfer Enhancement of Finned Copper Foam/Paraffin Composite Phase Change Material

Background: The employment of Phase Change Materials (PCMs) provides a potential selection for heat dissipation and energy storage. The main reason that hinders the wide application is the low thermal conductivity of PCMs. Combining the proper metal fin and copper foam, the fin/composite phase change material (Fin-CPCM) structure with good performance could be obtained. However, the flow resistance of liquid paraffin among the porous structure has seldom been reported, which will significantly affect the thermal performance inside the metal foam. Furthermore, the presence of porous metal foam is primarily helpful for enhancing the heat transfer process from the bottom heat source. The heat transfer rate is slow due to the one-dimensional heat transfer from the bottom. It should be beneficial for improving the heat transfer performance by adding external fins. Therefore, in the present study, a modified structure by combining the metal fin and copper foam is proposed to further accelerate the melting process and improve the temperature uniformity of the composite. Objective: The purpose of this study is to research the differences in the heat transfer performance among pure paraffin, Composite Phase Change Materials (CPCM) and fin/Composite Phase Change Material (Fin-CPCM) under different heating conditions, and the flow resistance of melting paraffin in copper foam. Methods: To experimentally research the differences in the heat transfer performance among pure paraffin, CPCM and Fin-CPCM under different heating conditions, a visual experimental platform was set up, and the flow resistance of melting paraffin in copper foam was also analyzed. In order to probe into the limits of the heat transfer capability of composite phase change materials, the temperature distribution of PCMs under constant heat fluxes and constant temperature conditions was studied. In addition, the evolution of the temperature distributions was visualized by using the infrared thermal imager at specific points during the melting process. Results: The experimental results showed that the maximum temperature of Fin-CPCM decreased by 21°C under the heat flux of 1500W/m2 compared with pure paraffin. At constant temperature heating conditions, the melting time of Fin-CPCM at a temperature of 75°C is about 2600s, which is 65% less than that of pure paraffin. Due to the presence of the external fins, which brings the advantage of improving the heat transfer rate, the experimental result exhibited the most uniform temperature distribution. Conclusion: The addition of copper foam can accelerate the melting process. The addition of external fins brings the advantage of improving the heat transfer rate, and can make the temperature distribution more uniform.

Numerical Simulation and Experimental Research of Oil-Free Scroll Air Compressor Based on CFD

Background: Compared with other types of compressors, although the scroll compressor has a simple structure and low noise, due to the limitation of the structure, it has defects such as excessively high discharge temperature, and has extremely high requirements on the machining accuracy of parts. The oil-free scroll compressor is a new type of scroll machine, which does not contain oil during the working process; it can be applied to cases with low displacement requirements and a high-pressure ratio. Objective: Taking a scroll compressor with a rated displacement of 0.6 as the research object, the analysis and research of the working performance parameters of the scroll compressor in actual work provide a certain theoretical basis for the improvement and optimization of the test prototype. Methods: The thermodynamic model of the scroll compressor in the actual working process is established by the variable-mass system thermodynamics and the control volume method. Based on the CFD method, a three-dimensional unsteady-state numerical simulation of the flow characteristics of the working fluid in the scroll compressor's working chamber is carried out in order to verify the thermodynamic model. Considering the accuracy of numerical simulation, a test platform with air as the working fluid is set up. Results: Through the thermodynamic model and numerical simulation, the changes of temperature, pressure and velocity in the working chamber of the scroll compressor with the orbiting angle of the main shaft as well as the gas force and torque acting on the orbiting scroll tooth are obtained. Through experiments, the law of volume flow and shaft power of the scroll compressor with the speed of change, and the law of the change of discharge temperature with pressure at different speeds are obtained. Conclusion: At the same time, the thermodynamic model established by considering heat transfer and leakage is more in line with the actual working process of the compressor; the mass exchange between adjacent working chambers of the scroll compressor has a greater impact on the temperature and pressure in the working chamber. Due to internal leakage and irreversible loss, such as heat transmission, there is a deviation between the theoretical volume flow and the actual volume flow.

Optimized Rod Size Design of Denim Fabric Grinding Robot Based on Improved Cuckoo Search Algorithm

Background: Dexterity is an important index for evaluating the motion performance of a robot. The size of the robot connecting rods directly affects the performance of flexibility. Objective: The purpose of this study is to provide an overview of optimal design methods from many pieces of literature and patents, and propose a new optimal design method for ensuring the robot completes its tasks flexibly and efficiently under workspace constraints. Methods: The kinematics and working space of the robot are analyzed to determine the range of motion of each joint. Then, a dexterity index is established based on the mean value of the global spatial condition number. Finally, an improved cuckoo algorithm is proposed, which changes the step size control factor with the number of iterations. Taking the dexterity index as the objective optimization function and the working radius as the constraint condition, the improved cuckoo search algorithm is used to optimize the size of the robot rod. Results: The improved cuckoo algorithm and proposed rod size optimized method are fully evaluated by experiments and comparative studies. The optimization design process shows that the proposed method has better solution accuracy and faster convergence speed. The optimized design results show that the robot's dexterity index has increased by 26.1%. Conclusion: The proposed method has better solution accuracy and faster convergence speed. The method was suitable for optimizing the rod parameters of the robot, and it was very meaningful to improve the motion performance of the robot.