FEA for a chair? 2

[vicbee]

I'm designing a pretty basic metal frame armchair for small production. I need to make sure the alloy used and design pose no risk of breaking under a human load... I am debating whether to use aluminum or stainless steel and which types. Also, depending on cost and feasibility, I don't yet know if the frame will be forged or diecasted.
This may sound a bit much, but should I get a FEA report for this and how much can that cost me?... I just don't want to be sued by an angry 300 lbs football player who would have broken the chair (and hurt himself) after having sat on the chair 12 hours in a row in front of the tv, watching game reruns (and jumping up and down, and shaking the chair, and...whatever!) Thanks for your feedback.

 

[MadMango]

You either pay for the FEA or lawsuits. I think the FEA would cost less in the long run, plus provide you peace of mind. I'm not sure what this service might cost though. "The attempt and not the deed confounds us."

 

[tld23]

Vicbee,

Do not think FEA first. Basic calculations should be carried out first. You may not need to go any further.

I suspect the chair design will be dictated by stiffness charecteristics rather than strength as such.

Keep the calculations with the design log. Should the worst come to the worst you will have to show that you have taken all reasonable steps to make a safe design.

TERRY

 

[pjhype]

Generally, the time to consider using FEA is when you must reduce your margins of safety to meet performance or weight requirements. For example, if you must design the lightest possible chair for the heaviest possible person, your calculations might determine that you must operate close to the yield strength of the material. In this case, you would perform the FEA to ensure that you have enough design margin to avoid failure and to determine what areas you could eek out the last bit of weight.

In real life, though, for everyday items such as chairs, you would grossly overdesign it to account for uncertainties in how the chair will be used. This might involve designing the chair for the weight of a 99th percentile human, and multiplying the weight by a large factor of safety (like 2 or 3).

In litigation, analysis results (or FEA) will not usually amount to a hill of beans, as a jury does not typically have enough technical background to understand how these results are obtained. In cases like these, there is no substitute for a test. You should therefore test your chair under all potential (normal) operating scenarios just to cover yourself.

pj

 

[KSshopper]

I don't think an Finite element analysis will buy you any more than hand calculations for a chair. Mistakes can be made in Finite element models just as easily as with hand calculations. Therefore a test is the best, and probably hold up better in court.

One last note: I've seen litagation filed against a porch swing manufacturer, not because it broke, but because it flipped over.

 

[corus]

It would be cheaper to build the chair and sit someone in it to see if it broke. To avoid being punched by the tester after, try sitting in it yourself first. Make notes after.