Free body diagrams are old-school, and when I was in school I didn’t think I’d use them much.
Truth is, I actually use them alot.
When I’m doing a feasibility study for a mechanism concept or calculating rough safety factors and don’t have time for an FEA simulation, free body diagrams are the best tool for the job.
Some things I’ve had to calculate recently:
- The reaction force at the pivot of a lever to determine the safety factor of the pin the lever pivots on.
- Torque = I*alpha. How fast is this lid going to open if I’m using torsion springs?
- The amount of torque I need at a pinion for the rack to overcome static friction
Sure those could all be done using FEA, but what’s the best case scenario? Everything goes smoothly and it takes me 45 minutes to set up and run the analysis? That’s too long, especially when each of these calculations should take no more than 20 minutes to do by hand.
Still, when I’ve gone a few months without doing a free body diagram and am all of a sudden required to do so, I’d be lying if I said I had no trepidation. That’s when I turn to a the textbooks (this one rocks) to do a gut-check and make sure I still got it.
The bottom line for me is that my employer/client hired me as mechanical engineers, and when necessary, I need to be able to rock out on the fundamentals.
What do you think? Do you use free body diagrams? Love em? Hate em? Haven’t thought about them since school?
Leave it in the comments.
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Article by chris loughnane
Chris is Design Engineer + Sustainability Advocate at a Boston area design consultancy and has written 127 sweet posts for the Product Development Notebook.
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