Motion Simulation vs. Structural Analysis Results

[MegaStructures]

I'm hoping for a quick sanity check.

I've been called in to help check the results of a motion analysis completed on a CAD package that calls for a seemingly unreasonably high force to lift a small man basket. I have a model in SAP2000 of the basket and mechanical linkages and believe that if I do the following I will be able to find the force required to lift the basket:

1) Replace all members with rigid links
2) Place a pinned boundary condition at the location of force input
3) Run various static analyses, one at each stage of motion
4) Check the reaction at the force input boundary <--- The force applied must be equal to or greater than this value to start motion in the opposite direction

I would do a simple hand calculation to sum the moments about the hinge point, but there is a fair amount of linkages and a couple rotation points and I would prefer to let SAP do the math for me in this case (not my day job just trying to ball park a reasonable figure

“The most successful people in life are the ones who ask questions. They’re always learning. They’re always growing. They’re always pushing.” Robert Kiyosaki

 

[GregLocock]

If you accurately model the mechanism then you'll get accurate results. If...

I'd prefer to take the worst case analysed by the CAD and do it by hand in that configuration. Just from interest, which CAD package running what motion simulation (I haven't used any of the CAD based ones apart from Inventor a million years ago).


Cheers

Greg Locock


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[MegaStructures]

Greg:

The CAD program is CREO I’m not familiar with it so my first plan of action is to do as you said and check the output with a “hand calculation”.

I’m just double checking that my theory is sound that all I need to do is check the reaction at the force input location and thus the motor would need to produce that reaction to balance the load plus an additional force to break equilibrium. I’m basically just proposing to use SAP to do my static equilibrium equations for me.

“The most successful people in life are the ones who ask questions. They’re always learning. They’re always growing. They’re always pushing.” Robert Kiyosaki

 

[GregLocock]

Hand calc=FBD.

Cheers

Greg Locock


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[MegaStructures]

Greg: I do understand what an FBD is... I also understand that a hand calculation and SAP2000 will give me the same reactions at the boundaries, this is why I put hand calculation in quotes. I think I'm on the right track, was just making sure I'm not missing something. Thanks!

“The most successful people in life are the ones who ask questions. They’re always learning. They’re always growing. They’re always pushing.” Robert Kiyosaki

 

[goutam_freelance]

Your theory may work when the man basket is at somewhat uniform speed or at rest. However, during initial start from rest as well as similar situation when the basket stops in between and you need to start again the dynamic forces (acceleration forces or d'Alembert forces) will come into play.
You can reduce the dynamic forces by lifting the basket very slowly, but this may increase the time for completion of work.
Best option is to use a mechanism analysis software, example in the link below:

https://psmotion.com/mechdesigner/feature/kinematics-analysis-and-simulation-software-0

Engineers, think what we have done to the environment !

https://www.linkedin.com/in/goutam-das-59743b30/

 

[3DDave]

I would use an energy balance - F*d (or T *alpha) on input vs mg*h, leaving out kinetic energy and friction loss since it's a man-lift, not a trebuchet. This eliminates the need to look at the details of the intermediate reactions.

 

[CWB1]

I agree with Greg's first post, except I'd pick the best and worst cases from Simulate and evaluate them with a combination FBD & excel for an overall system check. Whoever created the motion sim should also be able to quickly/easily tell you torque and force reactions at every point in the system, so if there's only one input and one output you could probably just do the math in your head while walking through the system live with them. Granted, the fun begins when you get into the multiple inputs & outputs of some clockwork claptraps.

 

[rb1957]

I'd go back to the CAD result (before embarking on a modelling expedition, which can end up in the deepest drkest swamp).

The CAD analysis should show you the forces at each joint of the mechanism. Plot these as a force polygon. At a minimum this'll confirm the CAD result and it may highlight issues in the mechanism and/or the original CAD analysis (possibly some factor fell into the analysis ?). Treating the mechanism as a structure will get you an answer, but one that ignores a whole bunch of mechanical systems issues.

What you could do to check the CAD result is (as greg has suggested) a FBD of the set-up ... sum the external reactions ... what does that tell you about the load ?

another day in paradise, or is paradise one day closer ?

 

[drawoh]

MegaStructures

How complicated can the forces on a man-basket be? You should at least be able to check your CAD data, pick out the maximum force condition, and analyse that by hand.

--
JHG

 

[MegaStructures]

Thanks all. I wouldn't say the forces and structure are that complicated, but the system is indeterminate, which makes hand calculations time intensive. I was able to use SAP to check the motion simulation results and found good agreement.

Appreciate all the input.

“The most successful people in life are the ones who ask questions. They’re always learning. They’re always growing. They’re always pushing.” Robert Kiyosaki

 

[GregLocock]

An indeterminate mechanism sounds a bit... odd.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[MegaStructures]

The mechanism is not indeterminate, but the man basket has two hinge points, so the system ends up being indeterminate.

“The most successful people in life are the ones who ask questions. They’re always learning. They’re always growing. They’re always pushing.” Robert Kiyosaki

 

[rb1957]

how indeterminate ? Two forces/moments sharing a line of action ?

if indeterminate, how did the CAD solve it ?

another day in paradise, or is paradise one day closer ?

 

[CWB1]

How complicated can the forces on a man-basket be?

Not to sidetrack but if most folks' work wasn't proprietary, I'd wager a thread of "overcomplicated mechanism" pictures here might prove humorous. Not to suggest I haven't made similar mistakes on a smaller scale, but I have seen a few projects where someone needed a spoon and ended up with a swiss watch.

 

[3DDave]

It's probably just a parallelogram mechanism to maintain the basket orientation.