Learning Forum

[rezvanrakhshan]

I’m performing a static structural analysis in ANSYS Mechanical and have defined a cylindrical local coordinate system for a hole in my model. I want to extract the reaction forces from this hole, but I’ve noticed that when I switch between the global and local coordinate systems, the total reaction force value appears to change. Given that this is a static structural analysis, I would expect the total value of the reaction force to remain constant, regardless of the coordinate system. Could anyone provide insight into why this discrepancy is occurring or suggest a method to ensure consistent results across coordinate systems?

[peteroznewman]

Please reply with a screen image of the two reaction force results.

[rezvanrakhshan]

 

Sure, here is the image of the reaction force results in global and local coordinate systems

 

[rezvanrakhshan]

Sure, here is the image of the reaction force results in global and local coordinate systems

 

[Ashish Khemka]

 

 

Hi Rezvan,

I performed a simple analysis on a cantilever fixed at a hole at one end and force (100N acting in X, Y, and Z directions) acting at the other end of the cantilever. The reaction force in global coordinate is as expected (100N, 100N, 100N) but in cylindrical I get a force reaction of 100N for Z component and higher magnitude for other directions.

Regards,

Ashish Khemka

 

 

[rezvanrakhshan]

Hi Ashish,
thank you for the analysis. However, my concern is not about the reaction forces changing in the x, y, and z directions across different coordinate systems. The issue is that the magnitude of the total force (sqrt(fx^2 + fy^2+fz^2​​) in the global coordinate system is different from that in the local coordinate system.

Specifically:

• In the global coordinate system, the total force magnitude is 4312.1 N.
• In the local coordinate system, the total force magnitude is 1651.5 N.

As you know, the total force magnitude in space should remain constant, regardless of the coordinate system, because different coordinate systems only affect how the force is represented (its components), not its actual magnitude or direction. This discrepancy suggests that there might be an error in the calculations or transformations between coordinate systems.

Could you please investigate this further? Let me know if you need additional details from my side.

[peteroznewman]

Hi Rezvan,

I like the simple analysis Ashish did on a cantilever fixed at a hole at one end and force (100N acting in X, Y, and Z directions) acting at the other end of the cantilever. The reaction force in global coordinate is as expected (100N, 100N, 100N).

I don't expect reaction force components to stay the same value in a different coordinate system. Extending the simple example, create a rectangular coordinate system that is rotated 45 degrees about the Z axis from global.  The value expected along one of these new X or Y axes would be the RSS of the two components in global or 141.42 N.  Your expectation that the values should not change is wrong.

[Ashish Khemka]

Hi Rezvan,

Thanks for your comments. Yes, the total reaction force should not change but I too did notice that they are not the same. Let me check with the team and I will get back to you.

Regards,

Ashish Khemka

[dlooman]

If Coordinate System_Bolt1 is a cylindrical system, X is the sum of the radial forces and Y is the sum of the tangential forces.  Only the cylindrical Z force is comparable to the Global Z and they do match.   So the tangential forces will tend to be large as they are resisting the moment in addition to the reaction force.

[rezvanrakhshan]

Thank you for the explanation! Could you elaborate more on the moment part? Specifically, are there any resources or references that could help illustrate how these additions to the tangential forces occur due to the moment?

Also, does this mean that if I want to determine the reaction force, using the global coordinate system will provide the correct answer?

 

[dlooman]

I was visualizing a rectangular plate in the xy plane with a hole near the left end and a vertical force on the right end.  The cylindrical surface of the hole is fixed.  The moment, equal to the force times the distance to the hole, is carried by tangential forces on the surface of the hole summing to the moment divided by the radius of the hole.  You could use another coordinate system besides Global Cartesian, but to get meaningful force resultants it should be a cartesian coordinate system, not cylindrical.

[rezvanrakhshan]

Understood. Thanks for your help.