## General Mechanical

#### Question about the acting point of reaction force of displacement

• Haiquan
Subscriber

Hello

I was performing a simple compression simulation for a metal spring. I found confused result in case 1 as shown below.

I applied displacement in case 1, due to insufficient constraint, the moment should be unbalanced and the spring should rotate, this should cause convergence issue unless I put a block to prevent such rotation. But simulation shown no rotation in spring and it converged! I checked the moment equilibrium carefully and found that the acting point of reaction force of displacement was not in the location where displacement was applied, it was near the middle of the spring as shown in the red arrow in case 1.

In case 2, I used a tool to apply displacement instead of applying it directly on spring, this was more realistic, the spring rotated. That is why I put a block to prevent such rotation. Deformation shape in case 2 matches the reality!

My question is why the acting point of reaction force of displacement is not in the location where displacement is applied?

• Akshay Maniyar
Ansys Employee

In first case your model is not fully constraint, so solver would have created weak springs and tried to converge it. That is why you are not able to see any rotation in first case. You can try switching off weak springs and try running again. (analysis settings regards amaniyar
• 1shan
Ansys Employee
"I checked the moment equilibrium carefully...... the middle of the spring as shown in the spring" - How did you arrive at this conclusion? You could simply insert a force/moment reaction probe and check if they balance out. When you apply a fixed displacement in Z, all nodes at the scoped geometric entity translate in Z together. This implies that the scoped faces cant rotate, because if they do then each node would have a different z displacement. In case 2 rotation is possible because there is no constraint which restricts this movement.
Regards Ishan.
• Haiquan
Subscriber
Thank you very much for your comment. I checked the week spring, it is turned off by default
Thank you very much for your comment. For the force and moment balance, they are balanced as shown below. Why I said the acting point of reaction force of displacement moved to center of the spring is based on this calculation: 169.89Nmm/31.798mm=5.3mm as shown below. The displacement reaction force is not at the location where displacement is applied(16mm), how Ansys still gets this balance result? This is what I feel confused.
Yes, I agree the scoped faces cannot rotate as this is displacement load. So If I donÔÇÖt introduce the tool, what should I do if I want to have a Z direction displacement, should I apply in an edge, or use remote displacement instead?

• peteroznewman
Subscriber
Hello You should use a Remote Displacement on that face, leave 5 DOF Free and only apply the Z displacement.
• Haiquan
Subscriber
Thank you very much for your comment. Remote displacement works well.