## 4 Results and Discussion  
The case studies present the development, application, and evaluation of two DAs, following the introduced systematic approach. The knowledge from the designers' experience is formalized in an application (i.e., the DA tool and its interface) that supports the solution of the design problems in a transparent and traceable process. The developed interface was tested by the company designers and finally judged as easy to use, based on well-known tools (e.g., CAD software, spreadsheet, etc.). Thanks to the DA graphical interface, the designer is able to interactively modify the requirements and again examine their effect on the machine model. Therefore, on the one hand, the DA user has clear control over the design choices, since the DA layout splits the requirement domains into different solution concepts, improving the design process awareness. In particular, the DA avoids impossible fields of application and leaves the possibility to move with operational certainty and without constraints within established ranges. On the other hand, the DA does not hinder the designer's creativity, since it is open to modifications, if new research leads to an innovation. The implementation of changes and the creation of functional variants is guaranteed by the structure of the DA. On the highest level (first layer), the DA can be updated with new technical solution thanks to its functional deployment, substituting working principle solutions, and therefore generating new variants. On the lowest level (second layer), the DA allows dimensional and geometric changes thanks to the use of parametric modeling and tools, since most CAD software support the configuration of parts and subsystems through changing the parameters in linked spreadsheets, as well as by means of equations or specific programming interfaces. Concerning creativity in design, fixation is a risk. Design fixation can occur due to the involvement of expert designers, who can show blind adherence to a set of already known ideas [55]; the designer consideration of the original functional structure of the product (e.g., functional decomposition, and morphology) [56; 57]; the use of CAD tools; and related CAD facilitated design [58]. If these factors represent a general limit to design creativity, they only partially affect the DA for two main reasons. The first one is due to the selected target of systems and products for the DA development. In fact, the DA approach originates from the need for knowledge management of mature products, with stable technologies, which the company has a complete view over their whole lifecycle. We cannot exclude  
Figure 12: Design process (inputs, outputs) for the generation of the computer-aided design (CAD) model. innovation in these fields of application, but this is not be the main goal of design efforts. Designers, as well as companies, aim to finely tune and optimize these mature products instead of completely upsetting their structures [59]. The second reason is due to the two layers structure of the DA, Starting from the first layer, the designer can question the entire structure of the DA, modifying the systems or subsystems, as well as generating simpler morphological variants in the second layer. Finally, some remarks about the two case studies. A DA is developed and applied for designing clutches in tractor transmissions, as a first case study. The clutch DA is assessed and proves to be a suitable KBE application, while the knowledge retrieved was found to be sufficient for the DA definition. However, the high number of clutch parameters limits the definition of a few concept models with continuously variable parameters. Therefore, the design problem is simplified by fixing a number of main parameter levels. Accordingly, 54 possible solution concepts are produced, with further possible adjustments in the detail design phase.