In a general definition, engineering design consists of performing calculation, drawing and execution procedures for machines, equipment or installations in order to meet certain requirements.
The mechanical design process generally includes the following stages:
- Recognition of a need and general instructions for the proposed theme which defines the problem.
- Analysis of different structural and functional schemes that define the problem and choosing one that will be investigating in detail.
- Preliminary design of the machine, structure, system or process.
- Component design, drawing and detailed specifications required to solve the project.
In the early stages of the design process, the designer also performs a creative role. In this case, ingenuity and imaginative power should be at maximum capacity.
The detailed drawings and specifications for a complete design are the result of multiple decisions of greater or lesser importance. In the final stages of the design process, the designer is a decision maker. The whole volume of work is based on scientific principles supplemented by a series of empirical data. However, it must be understood that science can only establish the boundaries between a decision must be made or give a statistical picture of the effects of a particular decision. The decision itself is taken by the designer. Therefore, judgement in decision-making is one of the basic features of a good designer.
Product life cycle
In general, designing a product is done for a well-established service life, which is referred to as the product’s life cycle. Once the life cycle of the product has been completed, a new product must be designed and developed. In Figure 1 the essential life cycle phases, including the production and use sequences, are presented.
Figure 1. The product life cycle
The life cycle of technical products is closely related to the material cycle (Figure 1). It starts with a product idea, emerging from a market need or a customer requirement, which is materialised in the first phase of the product life and is defined as the product planning. The result is the setting of a task which provides the basis for the second phase of product life: development and construction. At this stage, the implementation of the idea and/or task-setting into a viable product takes place in individual steps. The life cycle then continues onward with the manufacturing process with the manufacturing of the parts, assembly and quality testing. The process ends with the product manufacturer when the product is passed on to the distribution department. This product life phase is the interface to the application of the product, something which can be described as the product’s usage or consumption. Intermediate maintenance steps can serve to extend the useful life. Product recycling follows the primary use of the product, something which can lead to a further use for the same invention or changed product functions (reuse/further use) or to secondary usage with the same or changed characteristics to the secondary materials (recycling or further utilisation). Non-recyclable components will then end up in a landfill site or burnt to produce thermal energy.
Except for recycling or landfill, the life cycle applies both to the physical products of machinery, equipment and devices as well as to software products. It is usual for companies to use such structuring techniques for product tracking.
The second phase of product life is development and construction. This is also often referred to as product development. To further structure this phase of product life, it is usual to break stages down into individual steps. This procedural approach in handling constructive tasks is based on general solution methods and/or working method approaches as well as the general relationships in building technical products. It is not a rigidly prescribed approach, but instead, it is an essential tool for the engineer in product development. The individual working steps are the basis for other activities, e.g. the preparation of schedules or the planning of product development costs. They also help the engineer in finding where he is in the development process. A possible structure can be seen in Fig. 2.
Figure. 2. General approach to design products
The approach begins with clarifying and specifying the task, something which is especially important for new design tasks. The basis for this is the setting of tasks with individual needs which are developed from product planning tasks. From the wealth of specified requirements, the designer engineer must identify the essential problems to be solved and formulate these in the language of his field of design. The result is a requirements list, which is also known as a specification sheet. It is not only the technical but also, the legal basis for all other activities. In the next work step, the solution-neutral definition of the task takes place, i. e. adopting the assumption that solutions are not prefixed. This has been proven in the form of functions, whose links lead to functional structures. As a result of this work step, such functional structures already demonstrate an abstract form of a solution concept and as such, must be subsequently implemented gradually. The search then follows for solution principles for the key sub-functions. For mechanical products, these are based on physical effects and their fundamental realisation with the help of geometric and material characteristics. The individual solution principles are represented with the help of a morphological box. At the same time, for each key sub-function, a maximum of three to four solution principles should be worked out. With the help of the morphological box, the linking of individual solution principles leads to an effective structure. Even here, it is not usual to work out more than three effective structures. The effective structures which are generated in this way (or principle solutions) are compared with each other through appropriate evaluation mechanisms. Thus, it is possible, as a result of this work, to give a green light to a principle solution (also known as a concept) for further treatment. In further treatment, the principle solution is divided into realistic modules which lead to a structure and allows functional design or design priorities to take shape before the labour-intensive concretisation stage. Furthermore, it is necessary to consider the possibility of realising a structure for further work steps which is production-like, easy to assemble, easy to maintain and recycle and/or, which is building block-like. The result is the modular structure. The designing of relevant structural modules takes place in the next work step, e.g. in mechanical systems, the assemblies, components and the necessary connections are specified. Essentially, this work step includes to the following activities: Procedural calculations, stress and deformation analyses, arrangement and design considerations, as well as manufacturing and assembly examinations. As a rule, these procedures do not yet serve production-related and material engineering-related detail specifications, but first of all, the specification of key characteristics of the design structure in order to be able to optimise these according to technical and economic considerations. The results are draft concepts. The next work step includes the design of other, usually dependent functional elements, i. e. micro-designing all sub-assemblies and components, and their combination in the overall design. For this a variety of calculation and selection methods, material catalogues, machine parts, norm parts and purchased parts are used as well as costing procedures for finding out costs. The result is the overall design. The last work step serves to prepare tasks related to execution and usage, i. e. the development of full diagrammatic documents, including parts lists for manufacturing and assembly as well as the development of operating manuals and maintenance specifications. The result is the complete product documentation. In practice, several work steps in the development and construction phases are often combined, e.g. for organisational or work-related reasons. Thus, in mechanical engineering, the first three sections are seen as a concept phase, the next three sections as a draft stage and the final section is seen as a preparation phase.