Study On Theory And Technology Of Generalized Assembly Principle In Mechanical Product Growth Design

Assembly design is one of important features of computer-aided design and it is a basis and prerequisite to assure a good quality in the product development. In this paper, a generalized assembly principle based on product growth design is studied, and the study is under assurance of a good generalized assembly quality in the whole lifetime of the product. In the phase of concept design, various assembly design i ssues are w ell considered from d ifferent views, i ncluding t he assembly structure, the assembly tolerance and the assembly planning to meet the requirement of assembly constraint in product design, and thus a product growth design with the assurance of the assembly quality as the kernel is realized finally. The paper is organized as:The objective of the paper is determined after a general overview on the available international and internal investigations on the assembly design of product. A detailed analysis is done on the up-to-date technologies applied in the assembly design of the products. The trends in the field of the assembly design are given, and also, some key points are proposed to further study in the field.The theory structure about the product growth design under the constraint of the generalized assembly principle is developed. A definition of the generalized assembly principle and its design protocol are put forward, which consist of a product complexity theory, a functional tolerance theory and a virtual and actual-based structure co-design technology. An expression on the growth model of the product assembly is made, in which a hierarchical product model is built up from the basic unit of the growth design, the assembly relation model of products, the component model and the tolerance model. This study lays down the foundation of the assembly-based decomposition and reconstitution (D&R), the tolerance allocation, and the virtual and actual-based product reconstruction co-design.A method of gene decomposition based on the product complexity theory is put forward. The product growth design can be regarded as a design state transition based on a time sequence, and the product gene decomposition can beregarded as the growth of the products from the function-based design area to the concept-based design area. Apparently, different evolution of the products can be obtained with the different diversity accuracy based on the function requirement. In o rder t o g et a r elatively o ptimal p roduct s tructure, a d erivation method in the design procedure is put forward under the control of the function requirement and under the constraints built up from the complexity theory analysis. It includes a method for the functional decomposition and reconstitution in the process of product gene decomposition, and a method to generate the function based design specifications. Based on the above methods, the product gene decomposition can be well designed as a dynamic decomposition and a static decomposition. In the process of the dynamic decomposition, the transfer chain of products is generated by a derivation from the complexity based kinematics and motion transmission analysis. In the process of the static decomposition, a minimum entropy method to optimize the product gene decomposition is given, through a quantitative treatment on the entropy, which in the paper is used to describe the system complexity, and after a comprehensive consideration on the complexity in the component manufacture and their assembly.Tolerance allocation is one of important points in the assembly design. To guarantee a successful assembly and to make the functionality of the products to reach the objective of the design, a function tolerance design theory in the process of product growth design is put forward, which make the allocation of the dimension tolerance and the geometry tolerance in the whole lifetime of the products realized, with regarding the design cost, manufacturing cost, usage cost, and the depreciation cost of the product, as the control constraint of the tolerance allocation. Tolerance design rules are defined qualitatively as well as quantitatively. In the qualitative definition, 5 rules in the tolerance design are given to describe the theory and the rule in the tolerance design from the views of the tolerance type judgment and the numerical relationship among different types of tolerance. In the quantitative definition, a mathematical model to allocate the function tolerances based on the multifactor-cost function of the q uality loss is built up by developing a correlated sensitivity function between the cost and the tolerance. Then, the dimension tolerance allocation in the whole lifetime ofproducts based on the functionality and the assembly quality is realized through analyzing a designed mathematical model. In the model, the minimum cost is used as the objective function, a reasonable process capability index, the assembly function, and the assembly quality, are taken as the constraints, while the depreciation cost in the objective function is expressed as the discount rate -terminology in the economics.The result of the product gene decomposition is a conceptual functionality gene chain model, which consists of functional components a nd the correlation information among different functional faces. In order to realize the mapping from the functional gene chain model to the concrete product structure, a product reconstruction method based on the virtual and reality co-design is put forward in the paper. A Mapping-reconstruction design method based on a three-layer searching mechanism and a hybrid-reconstruction design method based on the zero thickness entity theory are proposed. Thus structure design to complete new product as well as to reconstruction from design history is realized. While in the process of product growth design, the interference detections on the static, the dynamic and the motion are carried out as well in the virtual reality environment, to give the guideline to the design modification, so that the parallel design of the part structure and assembly capability is realized. In the preprocessing simulation module, a constraint release based program is developed to deal with the global assembly sequence and the local assembly sequence.Based on the developed theories and methods in the paper, the original design system DARFARD is expanded and the developed theories and methods in the paper are verified successfully by some experiments finally.This study laid down the foundation for further investigation in the automation technologies and making these technologies realizable in applications. It provided some helpful theory and experience in the development of assembly design technologies, and would encourage developing a c reative growth design system to meet our country's conditions in the future.