Safety Margins and Design Margins: A Differentiation between Interconnected Concepts

The life cycle of complex products is subject to multiple uncertainties. Designers include margins into the product to cater for these uncertainties: safety margins, which is typically included in the requirements against the uncertainties of use and design margins against the uncertainties of design, such as changing requirements or engineering change. In practise the terms design margins and safety margins are sometimes used interchangeably. At the end of the design process, a product might include considerable overdesign, which can increase the initial cost as well as the running cost of many complex systems. Late discovery of both overdesign and lack of safety margins are also expensive to deal with in late phases. This paper explore the role of safety margins and design margins in the design process based on two case studies in truck design and jet engine component design. The paper shows that margins are added both to the requirements and the design parameters, so that companies run the risk of duplicating margins throughout the process without different teams being aware of it. This paper argues that a clearer and common description of design margins can reduce undesired iteration in development processes arising from misconceptions and aggregation effects in the development of complex systems.