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I've got what is basically a hammock frame design, made from tubing, that I'm trying to make as light as possible, while meeting a necessary strength safety factor.

I've hired a few different mechanical engineers to tell me if the design is feasible and, if so, what the best combination of aluminum/steel and diameter/wall thickness would be optimal.

Before hiring anyone, I went through a few iterations of a physical prototype and have been able to see some of the problems when a load is applied.

The problem is that some of those problems aren't showing up on the FEAs, and, additionally, there seems to be a lack of familiarity with the different aluminums and steels (comparative strengths, weldability, etc.).

Is this more of a job for a structural engineer, or am I just not hiring good enough mechanical engineers?

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  • $\begingroup$ Bike frame builder would be a good experience. $\endgroup$ – paparazzo Mar 18 '18 at 11:15
  • $\begingroup$ is this actually a hammock (for a person to lie in)? Something you intend to sell? The actual application matters to answer this question. $\endgroup$ – agentp Mar 18 '18 at 15:35
  • $\begingroup$ Yes, it's commercial product intended for a person to lie in. $\endgroup$ – Scotty Mar 18 '18 at 15:39
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    $\begingroup$ I'd look for someone familiar with the FEM package you are using to see if they can Id what you are doing wrong. You might even get help if you post details here. $\endgroup$ – agentp Mar 18 '18 at 16:02
  • $\begingroup$ re weldabilty, if the guy doing the welding cant help Id find a new welder. $\endgroup$ – agentp Mar 18 '18 at 16:04
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It sounds like what you really need is some one with a good mix of theoretical knowledge and practical fabrication experience.

In practice relatively few things are designed from scratch using purely theoretical analysis. even with FEA you need a bit of practical knowledge to set the right parameters and judge what is a genuine issue and what is an artefact of the simulation for example FEA tends to over estimate the significance of stress raisers like holes unless you have a well tuned high resolution model.

Actually your approach of optimising based on physical testing and reinforcing the bits which break is not terrible, especially if you have an experience fabricator on hand who has a good intuitive sense of what is likely to work and the ability to accurately diagnose the root cause of failures. For example if a welded joint fails is it inherent bad design or a bad weld ?

Equally it is sometimes better to start with a conservative design and then lighten it rather than the other way around.

Personally I woudl look at small companies rather than individual consultants custom or small volume motorcycle or sports/racing car builders may be a good place to start as they tend to have a lot of expertise in designing and building lightweight space frames.

Also you probably need to accept that there may not be a single optimal solution and you will help any technical collaborators a lot of you can provide them with a concise list of priorities going from essential through desirable to nice to have if possible.

Eg

  • Minimum weight capacity
  • Target weight
  • target price
  • any additional features, easy of transport and disassembly etc.
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