A Short Discourse on Safety Factor

August 29, 2006 at 4:22 am 3 comments

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.

** End of Article **


Entry filed under: Design & Analysis.

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3 Comments Add your own

  • 1. Goio  |  August 29, 2006 at 7:12 am

    probably it was just your friend who were to lazy to find out what exactly it is.. I mean, probably his boss has already caculate all of them (the safety factors)and then give your friend the result.. and your friend just eat it alive *halah!* .. hihihihi… the point is, IMHO, there is another factor that was not stated (in your article) in regards to safety factors, which is economic factor… I’ll tell you all about it later lahhh .. 🙂

  • 2. isadikin  |  August 29, 2006 at 2:35 pm

    You are encouraged to post your economic factor here, mate. I think it would be classified into Project Management category since it concerns cost. About the safety factor, most people I know seldom try to explain the reason behind a safety factor selection. Understanding and comprehension mean we can give satisfactory and simple explanation on related matter. Jean Henri Fabre, the famous entomologist, said: “To simplify is to have knowledge; to complicate is to be ignorant.” If we really understand, then why can’t we spend our little time to give simple and brief explanation? So that when something goes wrong, people do not just dig nose and scratch ass in confusion and say: “I don’t know why, it’s mysterious.” I still have many things which I experience and cannot explain completely, such as excessive deflection of structure I design. Still digging nose and scratching ass in confusion. 😀

  • 3. Mari  |  August 29, 2006 at 8:48 pm

    Engineering products usually involve lives. I think it is the right procedure to have as little uncertainty as possible, and in this case the right safety factor 😀
    Mr. Engineers, stop scratching your asses and get cracking! hehehe….


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This blog is intended to accommodate sharing of thoughts, ideas, and experience in heavy equipment design and construction. You are free to copy, print, and distribute material in this blog provided that you refer back to its source and you do not use it for commercial purpose. Feel free to drop comment. Have a nice day, mate. //




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