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How to prepare this model for FEA?

Question asked by Babar Firasat on Aug 17, 2020
Latest reply on Aug 17, 2020 by Shawn Mahaney

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Hi All,

I'm reading this FEA study and it states the following used to prepare the model:

The components are constrained as depicted in image 1.
The component is attached to an actuator at the arm and is
braced by supports with bearings at the pivot that give a hinge
like mechanism.
The mast is attached via the plate using bolted connectors,
which are not modelled.
The actuator and pivot are assumed within the model as a
rigidly pinned support (all three directions constrained) with
rotations allowed.
Kinematic (RBE2) couplings are used to transfer the boundary
conditions to the surfaces.
The moment load is acting about the z-direction and is
distributed on the plate with a diameter of 100mm, again using
a kinematic coupling.
The moment is calculated at 425.46Nm.

So I'm wondering how I should prepare the model by applying forces where and what fixture to use and any boundary conditions. I've attached a file, if someone could prepare it for me to show me how it looks from running simulation will be very helpful.

I've also attached image 2 to show simulation results from this preparation.

Edit:

The Assembly file contains the mast 14kg, camera kg and head with lights 17kg. It's shown how its connected.

Thanks,

Babar.

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Jim Riddell Aug 17, 2020 5:35 AM

Is this for your homework project? (edit: n/m I see that you asked this question 3 months ago.)

1. The 'constraints' shown are unrealistic and invalid in the real world. (Bearings do not have a suction effect.)

2. How can you have something 'rigidly fixed' yet allow rotation?

3. Why would you have a kinematic coupling on the plate? Are the bolts acting as stand-offs? Again, totally unrealistic.

4. Why do you have your stress scale going to 417 MPa while it appears as if your max stress is 215 MPa? You seem to be truncating information which would be useful.

5. How could anyone run your simulation for you if you don't supply the loading forces, i.e. the cylinder that is rotated, mass, CG, etc.?

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Babar Firasat @ Jim Riddell on Aug 17, 2020 5:51 AM

No, this isn't a homework project. This is an actual part that bent at a company I work in. I don't have much experience with FEA especially in assembly and I'm the only engineer there. I'm trying to learn as I go along. We got a company to do this analysis for us. These are the results they got back. Now, I'm trying to figure out how this was done.

I have edited the question with the rest of the information. Thanks.

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Jim Riddell @ Babar Firasat on Aug 17, 2020 5:54 AM

Babar Firasat wrote:

No, this isn't a homework project. This is an actual part that bent at a company I work in. I don't have much experience with FEA especially in assembly and I'm the only engineer there. I'm trying to learn as I go along. We got a company to do this analysis for us. These are the results they got back. Now, I'm trying to figure out how this was done.

I have edited the question with the rest of the information. Thanks.

They did it wrong. You should get your money back. Those results are nonsense since the FEA is set up unrealistically from the get-go.

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Jim Riddell Aug 17, 2020 6:05 AM

Go back to my answer on the previously time you asked about this part. Follow those steps. Run at least 3 load cases, maybe 4, i.e. mast at 0°, 45°, & 90°. You will need to figure out your 'restraint' direction based on the kinematics of the linkage in the various positions then you can apply your mass loads to the appropriate surfaces. You might want to do a little more with the attachment condition at 0° but that could be a hand calculation, nothing more needed (eg. toe at bottom lip and two bolt restraint ignoring the lower two bolts would be sufficient.) Easy-Peasy.

Do not over-constrain - the solid blue on the pivot shaft was a dead give way that your FEA supplier has no clue what they are doing.

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Shawn Mahaney Aug 17, 2020 8:04 AM

This is at least the third time the forum has been asked to look at that problem.

One could set it up exactly as in the schoolwork problem statement shared here (the "fixed hinge" constraint is what you want).

The most general and realistic solution is to model all the solids and use gravity and contact for all the loads and constraints. This I presented in How to conduct FEA on this part? .

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5 Replies

Jim Riddell Aug 17, 2020 5:35 AM
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Correct Answer
Is this for your homework project? (edit: n/m I see that you asked this question 3 months ago.)
1. The 'constraints' shown are unrealistic and invalid in the real world. (Bearings do not have a suction effect.)
2. How can you have something 'rigidly fixed' yet allow rotation?
3. Why would you have a kinematic coupling on the plate? Are the bolts acting as stand-offs? Again, totally unrealistic.
4. Why do you have your stress scale going to 417 MPa while it appears as if your max stress is 215 MPa? You seem to be truncating information which would be useful.
5. How could anyone run your simulation for you if you don't supply the loading forces, i.e. the cylinder that is rotated, mass, CG, etc.?
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- Babar Firasat @ Jim Riddell on Aug 17, 2020 5:51 AM
  Mark Correct
  Correct Answer
  No, this isn't a homework project. This is an actual part that bent at a company I work in. I don't have much experience with FEA especially in assembly and I'm the only engineer there. I'm trying to learn as I go along. We got a company to do this analysis for us. These are the results they got back. Now, I'm trying to figure out how this was done.
  
  I have edited the question with the rest of the information. Thanks.
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  - Jim Riddell @ Babar Firasat on Aug 17, 2020 5:54 AM
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    Babar Firasat wrote:
    
    No, this isn't a homework project. This is an actual part that bent at a company I work in. I don't have much experience with FEA especially in assembly and I'm the only engineer there. I'm trying to learn as I go along. We got a company to do this analysis for us. These are the results they got back. Now, I'm trying to figure out how this was done.
    
    I have edited the question with the rest of the information. Thanks.
    They did it wrong. You should get your money back. Those results are nonsense since the FEA is set up unrealistically from the get-go.
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Jim Riddell Aug 17, 2020 6:05 AM
Mark Correct
Correct Answer
Go back to my answer on the previously time you asked about this part. Follow those steps. Run at least 3 load cases, maybe 4, i.e. mast at 0°, 45°, & 90°. You will need to figure out your 'restraint' direction based on the kinematics of the linkage in the various positions then you can apply your mass loads to the appropriate surfaces. You might want to do a little more with the attachment condition at 0° but that could be a hand calculation, nothing more needed (eg. toe at bottom lip and two bolt restraint ignoring the lower two bolts would be sufficient.) Easy-Peasy.

Do not over-constrain - the solid blue on the pivot shaft was a dead give way that your FEA supplier has no clue what they are doing.
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Shawn Mahaney Aug 17, 2020 8:04 AM
Mark Correct
Correct Answer
This is at least the third time the forum has been asked to look at that problem.
One could set it up exactly as in the schoolwork problem statement shared here (the "fixed hinge" constraint is what you want).

The most general and realistic solution is to model all the solids and use gravity and contact for all the loads and constraints. This I presented in How to conduct FEA on this part? .
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How to prepare this model for FEA?

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