A Short Discourse on Safety Factor
A friend of mine is doing design and calculation for a mechanical component using safety factor of 5 in design. He read one of my article here, Design Considerations in Heavy Construction, and asked a very good question: “Why do you need many safety factors in design?” I asked him in return: “Where did you get the figure 5 for your safety factor from?”
The concept of safety factor has always been a deep thought for me since I was introduced to design. It is partly science and partly art of design. My friend said the safety factor of 5 is from his boss. When he asked his boss where he got that mysterious figure 5 from, the boss said: “It’s intuition, experience.”
Is that an educated answer? It reminds me of Mbah Maridjan, the famous caretaker of Mount Merapi in Java, who had intuition that the volcano would not erupt. That’s why I break down and simplify the safety factor into several factors, each covers major element affecting the structure: stress analysis (Finite Element Analysis), fabrication, material, dynamic body load, and safety factor. Of course different design case will require different combination of factors or even a new factor. Design is a living process. It is flexible. For example, a gas turbine designer will add thermal effect on material as critical element in design. This is almost the same with aircraft design in which all possible elements affecting the aircraft is studied thoroughly and informations are collected as many as possible, leaving only very small room for uncertainty.
Designing heavy equipment is much simpler than aircraft. A lot of simplifications and practical figures drawn from experience are used to speed up design process. But still we cannot shoot in the air by instantly applying safety factor of 5 or 6 and say it is from intuition. Design is a reasonable process. There must be a reason behind every step. Mbah Maridjan’s intuition doesn’t provide any slightest clue about what is happening inside the volcano. But Engineer must understand what happens to his design. We don’t need to understand thoroughly because it will be a wasting of time, cost and resources. But we can identify factors affecting design and roughly assign certain value to each factor to cover their uncertainties. This is much better than a single shot of mysterious figure derived from intuition.
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Entry filed under: Design & Analysis.