General Mechanical

Joint – Displacement vs. Displcement

• jonsys
Subscriber

What is the main difference between Joint - Displacement and Displacement (or Remote Displacement)?

Are there some rules of thumb on when to use which?

What are some advantages and disadvantages of using one over the other?

• Sandeep Medikonda
Ansys Employee

Jon,

There is a very good article on this in the PADT Inc blog. Search for:

Donny Don’t – Remote Objects

If you haven't already, Can you take a look at it and come back with any specific questions you might have?

Regards,

Sandeep

• peteroznewman
Subscriber

Jon,

That was a great blog about Remote objects that Sandeep mentioned in his post.

You ask about the pros and cons of Displacement vs. Remote Displacement.

Displacement doesn't add any extra equations to the model, it actually removes some equations from the model, so that might have some small effect on solution time compared with Remote Displacement. Maybe Displacement allows the model to converge more reliably than a Remote Displacement. If you are doing just displacement, then a Displacement Support is generally going to be better than a Remote Displacement.

Joint - Displacement requires a joint, while Remote Displacement doesn't. A joint can define a relationship between two bodies (face or edge) or between a body and ground, while a Remote Displacement is only to ground.

Regards,

Peter

• jonsys
Subscriber

Hello Peter and Sandeep,

• By the definition, "Remote Displacement enables you to apply both displacements and rotations at an arbitrary remote location in space". What I don't understand properly is what example can there be, in which we apply displacement at a remote location?

• So, when the relationship is between a body and ground, using Remote Displacement will give same results as using Joint-Displacement?

Regards,

• peteroznewman
Subscriber

Hello Jon,

An example of when I would use a remote displacement is where I have an 800 mm long cantilevered structure (bracket) that carries a load at the tip and has a base bolted to a wall.  I define plasticity for the metal of the structure and I want to plot the force-displacement curve to determine the ultimate load capacity of the bracket for a load at the tip. It is generally best to apply displacements to generate a force-displacement plot when you want the force to reach a maximum and then continue plotting lower forces as the bracket fails.

The bracket design is such that no high stress occurs along the most of the length of the 800 mm, all the high stress is less than 100 mm away from the wall.  I could mesh the whole structure and apply a 300 mm displacement to the tip and that would give me a result. Maybe the mesh has 80,000 nodes and takes 80 minutes to pull the tip down 300 mm.

Or, I could go into geometry and cut 600 mm off the structure and just leave 200 mm from the wall on out. Now I still want to pull the tip down 300 mm, but the tip is gone. That is where a remote displacement comes in. I can put the coordinates of the remote point 800 mm off the wall, and scope it to the cut face that is 200 mm off the wall. The mesh on this model may only have 20,000 nodes and take 20 minutes to pull the remote point down 300 mm.  Since all the plastic deformation is happening in the first 100 mm from the wall, the ultimate load calculated from the maximum value of the force-deflection plot is the same as above, but I saved 60 minutes compared with the full model solution.

A remote displacement and a joint to ground may create the same code, under-the-hood. They seem to be equivalent to me.

Regards,
Peter

• jonsys
Subscriber

Peter,

this is the best explanation I ever read regarding remote displacement. thank you very much

Regards,

• peteroznewman
Subscriber

Jonsys made a careful study of remote displacements on a cantilever beam and showed that the stress is not equal on the full model and the remote displacement model.

In my reply on that thread, I show that the stress is equal for remote force, not remote displacement.