How to constrain an object suspended by crane

[FP_11210718]

Hello PTC community,

I'm trying to simulate the stress that results at the mounting points when an object is suspended by a crane using only two points. The stress is caused solely by gravity.
I tried to simulate this by rigidly constraining the mounting points of the object, but I'm concerned that the simulated stresses will be significantly lower than in reality due to the rigid constraint.

The center of gravity (COG) of the object is naturally located directly below the mounting points.

Would it be better to ditch the gravitational load and instead apply a calculated force (F = m·g) directly to the bolt holes? If so, how should I constrain the model? Nothing seems to be working for me.

I have attached a simplified sketch of the problem.

Thanks for your help!

[Chris3]

The gravitational load is the right thing to use. Make sure that the Simulate mass = real life mass.

If you are constraining two cylinders than you are likely over constraining the problem. Most hoisting equipment has point contact loads which allows multiple DOF to be released. I would setup your model with point constraints, ignore the peak contract stress at the points and use a hand calculation for those locations instead. This answer really depends on what hardware you are using and how you are actually hoisting the equipment though.

[FP_11210718]

thanks for the answer. Where exactly would you place these point constraints ? At the bottom of the hole, the edge or somewhere completely different ?

I have attached another picture that clarifies the way it is supposed to be hoisted. 

[Chris3]

If you are hoisting with an eye bolt then there are going to be moments transferred into the part. Ideally you hoist with swivel hoist rings so that the load swings under the swivel and this doesn't happen. Then you just use the rated load (WLL) for the hoisting device and setup your analysis with that threaded interface as a constraint and the constraint  CSYS in the direction of the 1G gravity load and then you apply your 1G load to look at the stresses in your part and rely on the WLL for the lifting device for that part of the analysis.

[FP_11210718]

Sorry for the late reply. 

The problem is that I don't know how the object is going to be hoisted. So Im going with the worst case but still plausable scenario. 

Due to english not being my native language, Im having trouble understanding the placement of the constraints. Are you suggesting using a displacement constraint in the threaded holes and locking them in the vertical axis (direction of 1g gravity load) ?

Maybe a small scetch could be helpful, thanks!

[Chris3 · Accepted solution]

Yes exactly what you said. Constrain the threaded hole surfaces and setup a coordinate system for that constraint that has the vectors in the orientation that you need for your gravity load.

You will likely get singularities around the constraint, but you can ignore those and rely on a bolted joint calculation for that region instead.