Abstract:

In machine design courses it is not enough that students are taught design models and problem solving processes but also a practical sense of proportion, design for purpose and an understanding from which models can be developed. Although computer aided design techniques and sophisticated simulation packages assist engineers there is a sense that students are becoming out-of-touch with physical understanding and ultimately at risk of arriving at design solutions which do not address the fundamental design problem in an appropriate manner: this is not good engineering practice and an attempt to avoid such results should be taught as early as possible. In particular, before embarking on machine design the problem needs to be understood and serving purpose(s) clearly identified. This paper looks at some common problems arising with current engineering students and the education system that may give rise to them. It then proposes some possible solutions and identifies some key aspects that could be taught to the cohorts to avoid such problems. To that end it revisits the modulus concept in mechanical engineering design.
Starting from a convenient choice of the modulus based on the design’s function, reaching the design goal becomes somewhat more straightforward. These ideas are not new, however their origins appear to have lost appreciation in recent times. This approach was once at the core of engineering design as can be seen in old texts [1-3] in which the proportions indicate a design for purpose from a physical understanding view point. Many of the engineers of the past were more reliant on these methods due to a lack of resources (calculation, manufacturing, material etc) and a lack of energy to power design solutions. The idea of identifying the main purpose for a machine and then its modulus before embarking on the design is clearly depicted in the work of Vitruvius from the 1st century BC. In the 10th book of Vitruvius some explicit, practical rules to design machines, such as a crossbow and a catapult starting from the length of the arrow and from the weight of the stone to be thrown, are given. It is from appropriate identification of purpose that the modulus can be determined and then the whole project organised somewhat more systematically.
The fact that theoretical models cannot account for every aspect of what will happen in the “real world” means that valuable lessons are to be learned from experience. Vitruvius notes that this technique helps to reduce design time; essential in wartime.
To illustrate the above concepts a number of simple examples are considered as: to design a bicycle the distance between the seat and the main movement is used, to design a bolted joint the diameter of the shank, to design a press-fitted coupling the diameter of the shaft, and others. The full paper provides a brief historical review of the modulus concept in mechanical design and points out the fluctuation in its appreciation over time. It also draws attention to its appropriateness for today’s engineer and how it can also assist them in organising the drawing hierarchy and developing a model for assessing the solution(s).

References:
[1] Vitruvius, P. M., De Architectura, Venetiis Ioannis de Tridino alias Tacuino, 1511
[2] Francesco di Giorgio Martini, Trattati di Architettura ingegneria e arte militare, a cura di Corrado Maltese, Edizioni il Polifilo, Milano, 1967
[3] Reuleaux, F., Le Constructeur, Librairie F. Savy, Paris, 1881

Keywords:

machine design, design for purpose, design modulus, bologna process.