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Displacement is way too high

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Displacement is way too high

Let me start by saying I am essentially self-taught when it comes to simulate. I have no training aside from a few examples on SolidWorks back in college (six years ago). I understand the mathematical concepts of FEA, but my practical knowledge has proven to be frustratingly lacking. So please forgive me if I am unable to frame my question properly in technical terms.

I am doing an analysis on a lifting device designed to retrieve and replace large tools from shelves. It is basically a "hook" to reach into the shelf to grab the tools using a hoist.

So far, I have only attempted static analysis. Is that sufficient? Do I need to do a more complex analysis? If so, that's a whole other can of worms for a later discussion.

My immediate problem is a huge discrepancy between stress and displacement in my static analysis. You can see my method and results via the attached screen captures.

Stress looks good - well under yield strength. The displacement is another story. Where am I going wrong? Are my constraints/loads incorrect? Is it a units problem?

For reference, the distance between the two holes is 44-3/8". The part was plasma cut from 1/2" SA36 plate. And FYI, this tool has already been in use for a while. This company is run by veteran welders and machinists. Engineering is somewhat of an afterthought.

Any guidance would be appreciated.

Thanks!

Luke

9 REPLIES 9

Re: Displacement is way too high

I would start by checking the Young's Modulus of the material.  Unfortunately I can't comment on inch-lbf units, but if it's steel I'd expect YM of around 206 GPa (= N/mm^2); if alumin(i)um, around 71 GPa.

Depending on your basic modelling units set, it can be easy to get confused with more complex derived units such as stress.  I try to set my models to N, mm, s when I'm doing analyses because it ensures that forces and stresses are in units I'm familiar with - if you're using IPS then I'd suggest similarly setting your units to inch, lbf, sec.  It does mean that any masses (and densities) will have to be in... is it slugs?  For me it's tonnes, but that's not too hard to work with.

Re: Displacement is way too high

Hello

What version of Creo Simulate use you.

Kind regards.

Denis

Re: Displacement is way too high

Thank you for the reply. I think the answer to your question is yes. That is, I am opening a design study with the part exactly as shown in my attached pictures and running the analysis.

Here is the .prt if you want to play with it.

Re: Displacement is way too high

Could you tell me what you use?

The conversion of young's modulus is not good.

Re: Displacement is way too high

Thank you Steven! That was it. The lbm/(in sec^2) unit was pulled in from out material file. I'll have to get those corrected.

Now I'm curious if you think this is an adequate analysis of this part to give it a rating, or is a more involved analysis necessary?

Re: Displacement is way too high

Steven,

Thanks again for your help. What I'm concerned about at this point is not the result of the analysis, but the method I've used to get there.

The top hole is hung on a hook hanging on a chain from a ceiling hoist. Is the displacement constraint I've placed on it a good representation of that scenario? I tried to remove the constraints in the X and Z direction, because nothing will be holding the hook in place in those directions, but the analysis fails every time I try it that way. It shows parts of the mesh in red and tells me my constraints are wrong. The same thing happens with a "pin" constraint on the top hole. I ended up with the constraint on the top hole the way it is just to make the analysis run without failing.

Also, is the bearing load correct? How exactly does creo define a "bearing load" and in what directions is it free to move/rotate? Is the gravity load set up correctly? As I said before, I just don't know what i'm doing with any level of confidence on this. I really need to take a class or something, but I don't see that happening.

Re: Displacement is way too high

Steven ,

I thought the gravity load was to include the weight of the part itself. I knew it would not have a significant effect on the result in this case, but included it just to be thorough. Is that not the purpose of the gravity load tool?

I have requested the guys in the shop snap some pictures the next time they use this tool. I don't know when that will be, but I will update this thread as soon as I get them.

Until then , thank you very much for your help.

Re: Displacement is way too high

The slotted holes are hand-holds for manipulating the position of the part hanging from the hook while it is suspended - that would be primarily rotating it and fine adjustments to set it down in the proper place on the floor/shelf. I don't think they would ever see more than 10 blf in any direction. Do you think it should be included in the analysis?

Re: Displacement is way too high

What's the overall factor of safety?  My view would be that unless this is being designed right down to the limit (is it going into space? ) then minor additional loadcases such as that one can be absorbed into the safety factor, unless they're likely to trigger buckling or somesuch, which shouldn't be an issue as this is in tension.

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