unrealisticly high stresses in transient structural

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  • Last Post 31 July 2019
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Vanderbezi posted this 30 July 2019

Greetings,

i am running a transient Analysis of a rotating part (crankshaft) from 0° to 2160° with torque loads on the crackpins (Joint) and a rotation joint on one End to be uniformly rotated. The problem is that the Stresses I have been getting are too high to be realistic. (around 9000MPa) and keeps increasing.

Can you help me with any ideas about what might be causing this stresses?

I have tested the part's Mesh through a convergence Analysis in static and it does not seem as that part contains singularities and the maximal Stress calculated in static was around(1MPa).

regards,

Vanderbezi

 

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peteroznewman posted this 30 July 2019

Please attach the .wbpz archive file after you post your reply stating what version of ANSYS you are using.

Vanderbezi posted this 30 July 2019

Hello Peter,

thank you for your response. I am using  the 19.2 version.

I have attached the .wbpz file to this URL: https://filebin.net/lu1g6j0ogdmtzf98

I could not share the exact same Model but I have created a smaller one in which the same stress effect occures.

regards,

Vanderbezi

peteroznewman posted this 31 July 2019

This model has too many constraint elements.

You have one revolute to the two ends. Given the third revolute, this should just be on the left end.

and you have a revolute joint to the center to apply a moment on as a load.

Then you have a revolute joint on the end face, which is the one you decided to apply the rotation displacement load on.

If you are trying to simulate the stress in a crank shaft, you want the forces pushing radially on the correct side of the bearing. Using a Moment on the entire surface does not create forces that just push radially on the surface, it also creates forces that shear on the surface and pull on the surface, which does not represent how forces are actually applied to the crankshaft which is by pure radial compressive force.

Vanderbezi posted this 31 July 2019

Hello Peter,

thank you for you answer. I am using Flexible rotation probes to calculate the rotation in some surfaces of the Body. Does applying the Load as you described have an effect on the values of these rotations?

regards,

Vanderbezi

peteroznewman posted this 31 July 2019

Yes.

Vanderbezi posted this 31 July 2019

would the stresses on the pin be right if I created rigid rings around the pins with a revolute joint and a frictionless contact with a small tolerance and then assigned the torques to the rings? If not what can i do to create such radial compressive forces for the whole cycle?

regards,

Vanderbezi

peteroznewman posted this 31 July 2019

Some people have modeled the connecting rod and piston. The connecting rod has a frictionless contact to the crankshaft pin and a revolute joint to the piston. The piston can be on a translational joint to ground to avoid modeling the cylinder. A tabular force vs time load on the piston is in the model and is tied to the crankshaft rotation.

This is a more realistic model of the loads on the crankshaft because it can include the inertia of the connecting rod, which can put sideways forces on the crankshaft pin that a pure torque on axis cannot.

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