Pin connection with contacts - Ingot tongs

Andrea

I have to simulate contact between pin and tongs components.
Generally I study the components separately with forces obtained from hand calculation. Now I would have a comparison.
For first step is a good practice to set pressure overlosure E/t*10 considering the thickness equals to the pin diameter?

Pin diameter 60mm -> 210/0.06*10 = 35000 GPa*m

Thanks

Victor

Yep, that looks fine except GPa/m. That formula makes the elastic layer between the parts roughly 10 times stiffer than the part whose thickness you used.

Andrea

sorry...GPa/m instead GPa*m

Andrea

First proof shows strange behaviour of the pins: radial deformations. Only for the lower pins. Could it depends from Master/Slave inversion?

Victor

If the deformation scale factor is >>1, they might be rotating, which looks like radial expansion when it's exaggerated. In that case, you might need to attach them to something or constrain them to prevent rigid body rotation.

Andrea

Maybe you're right! But before launching the tongs model I made a test with a simple 3 links hinged where the central pin is restrained only in z direction and seems to be correct deformation shape.
The link and the pins have the same dimension of the studied model to have a first check on K value

Andrea

Well...I think that the problem is related to the pressure overclosure value (too low). In the test model reducing it happens a similar problem: radial deformations of the free pin

Victor

I don't think being too low should have that effect if there's nothing else wrong with the model - it should cause the linkages to spread further apart and intersect the pin.

Any rigid body degrees of freedom that are only constrained by contact, or in this case nothing at all, are at a high risk of flying away with very large displacements or failure to converge. They might not always do that if all forces happen to balance perfectly so it's possible that seemingly unrelated changes could trigger it.

Did you identify if it's rotating or expanding? I can't explain why it would expand without seeing the model, but rotating is what I expect it should do with only an axial constraint.

Andrea

You are right! Now I will try to apply axial constraint to the pins: for the central pins is not a problem because they are on the simmetry plane (only vertical translations) but for the lateral pins, is quite diffucult because they have a translation on a plane.

I think that some trouble are deriving from the solver. In fact for the simple test sometimes the solution is reached with a different numbers of iterations. This is not correct

Andrea

I solved appliyng rigid body to the pin vertical faces and locking the rotations

disla

I would suggest you consider your reference axis aligned (same elevation) with the lateral pins instead of the central , so you can impose they will not move vertically, only laterally.
On this new reference the pins on the axis of symmetry (both) will be free to go up and down but not laterally. This way you avoid more sofisticated BC’s out of plane.

Another aprox. would be to impose the lateral pins to move at a fixed distance from the central pin. (This option affects real tensile/compressive stress on the arms)
I have tested only in very basic models for small rotations and nonlinear, but it worked.
You can constrain initial radial distance to an arbitrary centre as (x-x0) and (y - y0) being x0 and y0 the coordinates of the arbitrary origin.

Victor

The common "dumb" way to solve rigid body motion is to put weak springs on the free part. Use a line2 spring element fully constrained at its other end. Then in the solution, check the reaction forces on the spring's constraints to ensure they're very small which shows it's weak enough not to affect the solution.

Andrea

Of course! Some software (Ansys for example) have an automatic detection of rigid body motions and puts automatically soft springs. In this case, due to the long time ofsolution, I don't know If it is convenient.