topic Re: Help with simple simulation question - constraints in Fusion Design, Validate & Document Forum

Help with simple simulation question - constraints

walker.payne — Mon, 28 Sep 2020 18:48:26 GMT

Hello, I am trying to learn Fusion 360 simulation and have a quick question about constraints.

I have a simple 2" thick aluminum plate that I will be hoisting with eye bolts threaded through 4 holes in the corners. The plate will hold an object that weighs 2.308e4 (ouncemass, according to Fusion) or 1442 lbs.

To see if the aluminum plate will handle that amount of weight, I take it to the simulation environment and remove the eyebolts (they are rated for 5000 lbs a piece on McMaster Carr). I apply 11601 lbf of vertical force to each of the cylindrical holes in the plate ( [1442 lbs * 32.17 ft/s^2] / 4).

I guess I technically have two questions:

Are the cylindrical holes the proper place to apply the force?
Which face of the plate (top or bottom) should be constrained? I think it should be the top face (which is how I have it constrained in the above picture) , as that is the one that is having pressure applied to it from the object that's resting on it, but I am unsure. I am unclear on how exactly the constraints work.

Any advice would be greatly appreciated. Thanks a lot!

Re: Help with simple simulation question - constraints

TheCADWhisperer — Tue, 29 Sep 2020 10:28:46 GMT

Can you File>Export your *.f3d file to your local drive and then Attach it here to a Reply?

Re: Help with simple simulation question - constraints

walker.payne — Tue, 29 Sep 2020 15:09:14 GMT

@TheCADWhisperer Yes I can do that, here it is. Please let me know if I've messed anything up! I really appreciate your help.

Re: Help with simple simulation question - constraints

TheCADWhisperer — Wed, 30 Sep 2020 20:39:14 GMT

Bumping this to the top - I forgot about it.

Hopefully I will get a chance (or someone else) to take a look at it soon.

Examine the Attached file.

I split the face.

I was unsure of what you intended for the load.

If you want 4 times, you need separate load for each fastener location - when you apply one load on 4 faces - it is divided by the number of faces.

Re: Help with simple simulation question - constraints

John_Holtz — Thu, 01 Oct 2020 12:43:37 GMT

Hello @walker.payne

The part shown in your image only weighs 1440 lbf. Why are you multiplying that by 32.17? (The 11,600 lbf load that you are applying exceeds the eye-bolt allowable by a factor of 2, so that is not a good design. 😁)

The next question is how is the object attached to the lifting plate.

If it is somehow bonded to the plate glued, rigidly bolted, etc), then applying the constraint to the middle of the plate is okay. That says that the object makes the plate infinitely stiff in that area (fair to say).
If the object is just resting on the lifting plate, then the constraint on the lifting plate is wrong. In the real situation situation, the plate is able to bend (like a banana), but you are preventing that behavior with the constraint. You should not stand underneath this device when you use it! The correct way of doing this in Fusion 360 is difficult. You would need to add rigid body connectors to each hole and put the anchor point at the center. (To define the anchor point at the center of the hole requires another "fake" body that you can use to select the point.) Each of the 4 anchor points need to be constrained in the vertical direction (UY) but free in the other directions. The weight of the object would then be added to the lifting plate as a force.

The other thing to consider is how the lifting harness is attached to the eye bolts. If the cables are vertical, then the above description is reasonable. If the 4 cables all meet at a common point, then the force in the cables create a moment on the plate which increases the bending and stress! You would need to take this into account also.

Re: Help with simple simulation question - constraints

walker.payne — Thu, 01 Oct 2020 13:35:18 GMT

@John_Holtz Thank you very much for your response. You've given me a lot to think about.

Thank you for clarifying the lbf issue - I was confused by lbf vs lb, but it looks like if I convert the weight of the object to pounds, that will be the equivalent total "lbf" force that the simulation should experience.

As far as how they're attached, the object will be resting on it, constrained with brackets to keep it from sliding around on the plate. Which sounds closer to your second bullet point than the first. I will try what you suggested with the anchor points and report back.

Lastly, yes you are correct as well that the lifting cables will probably not be vertical but instead meet at a common point (or two common points) which further adds to the complexity. Hmmm. Suffice it to say I underestimated the "simplicity" of this simulation. Thanks a lot for your input - I hope it's ok if I maybe come back with some additional questions. I really appreciate it.