How to constrain a suspended load?

[dbower-2]

Guys,

I have modelled a frame structure that is designed to be lifted by a single crane hook attaced to it by a two-leg slingset. I want to analyse the structure in a raised position.

I have set up a zero dof constraint at the crane hook location, and attached the constraint to the two lifting points on the structure by simulating the slingset using two point-to-surface rigid links. The rigid links are sonstrained at the lifting point surfaces with freedom in the vertical and horizontal (towards each other) directions.

I have set up a gravitational load in the -y vertical direction attached at the COG of the structure (which happens to be in free space).

The COG is not directly below the crane hook constraint, but its not too far away.

For me, this ought to be sufficiently constrained, as gravity is acting downwards away from the hook constraint.

However, CREO fails for being under constrained.

I have to add a constraint at the base limiting the freedom to the vertical direction to get the analysis to work. This isn't quite realistic though, as it doesn't allow the load to swing so that the COG is directly under the hook constraint.

How do you guys simulate a hanging load?

[danderson]

How about trying 3 very light (almost no force) Spring to Ground Constraints to give the assembly inital stability so Simulate has a starting point to work from?

Don

[danderson]

Steven,

Since the frame CG is not centered under the hook, would the ridged links hide true stress at the tie in points on the frame?

He may be trying to gain insight into how much load one tie in point will be taking veruses the other tie in point. Just a thought.

Don

[dbower-2]

Absoultely correct Don, I need to analyse the stresses with the COG directly below the hook.

[SweetPeasHub]

David,

I think there is a simple solution. I do not see any elements at the hook end (point) of your rigid link. A rigid link is not an element so you need something to resolve that. My preference would be to put a low stiffness spring from that point to somewhere on the structure. It is just there to give an element to attach to and so as long as the stiffness is very low it is OK. I run into this same scenario when attaching a concentrated mass out in space.

James

[dbower-2]

James,

I tried attaching a v low stiffness spring from the hook to the top surface of the structure but it failed to sufficiently constrain the model.

Thanks

[dbower-2]

Thanks for the welocme and the helpful suggestions guys.

I initially tried to simulate the slingset using springs, but found rigid links prefereable as I could attach them to the padeye hole surfaces. Springs require a point connection and you get high localised stresses when you make the springs really stiff (a wire rope sling has a stiffness about 50kN/mm). Rigid links work fine, I have constrained them to allow movement in y and x (towards each other so that I get compressive stress in the top beam) and rotation about the hook CSYS. The hook constraint is zero DOF as a point in space so the analysis hangs off that, but I have now made it able to rotate about the x axis to allow some swing towards the COG.

So as not to impart unrealistic loads into the padeyes at the connection with the rigid links I have made the link constraints able to rotate about the hook CSYS.

It's constraining the frame to the ground thats the problem. I have had some success constraining it on the bottom surface with movement in the y and z (direction of swing) direction. I don't know why CREO needs this though, g x mass in the y vector is surely all the constraint it should need? CREO ought to allow the load to swign to align the COG directly below the hook constraint. Maybe I am asking too much of it?

I'm finding that different types of lower constraint produce a result, but the result can vary significantly depending on how that constraint is configured. This is what concerns me. The load has to swing to aling the COG or else it puts bending moments in the padeyes that are not there in real life.

I guess another approach would be to reorient the hook constraint y vector to allign with the CSYS vector at the COG, and apply g in that direction so it imposes a concentric load. It's more work though that I hoped CREO would do for me.

This is my apporach, will le tyou know how it goes.

[gkoch]

Hi David,

have you been successful with your approach?

[dbower-2]

Hi Gunter,

I'm afraid my attempt at aligning the gravity vector with the hook constraints and the COG didn't really change anything. I still get really high stresses at the padeyes. You can't easily model the rotation of a slingset around a shackle so the padeyes have artificially high bending stresses because of the rigid links.

I have had to analyse the padeyes separately with a concentric force in the direction of the slings and an additional calculated side force to simulate the angle as the structure tilts to align the COG.

[gkoch]

So there was no simple way to do it.

But thank you for sharing the information about how you finally evaluated the padeyes in an extra analysis.

This may help users in similar situations a lot (sometimes you are so focused on doing things in one step, that you miss the obvious resolution of taking two steps for it)