# How to apply a torque in PrePoMax

For anyone who is new to Calculix and has used the FreeCAD FEM interface, you will know that applying a torque is not straight forward. So here is how I learned how to do it.
watch this video:

Its a little repetitive, but basically
1)create a part with a hole or some surface where the torque will be applied.
2)open in PrePoMax and create a mesh
3)Create a node set and select the surface you want (it can be any surface really)
4)create a “surface” using the previously created node set
5)create a “reference point”. this is where the torque rotates from. In the video, they use the node set centroid, which is probably a good place if you are applying torque around a bolt or from a wrench’s hex center.

6)Go to constraints and create a rigid body using the surface and point previously created
7)create a moment load by selecting the rigid body previously created.

it seems like a lot of steps where one is used to just clicking “moment” and then the surface. but I would file that in the who-cares bin. its great and makes the application of the moment much more general. You can pick any combination of XYZ moment components. It would be interesting if you could also pick a line/axis to be the axis of rotation. In FreeCAD you can create a line and use it as the direction for the applied force for example.

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This is also how you would apply a moment/torque or rotation displacement step by step:

1)create the part in any CAD system and export to STEP:

2)import the file into PrePoMax:

3)right click create mesh:

4)create the node set:

5)create a surface for that node set:

6)Create the reference point that will be the pivot of rotation:

7)Apply displacement (alternative 1):

1. Apply Moment (alternative 2):

9)Check out the results!:

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In most FEA programs (apart from CAD-embedded simulation modules) you can’t apply torque to the surface of the solid model because solid elements don’t have rotational DOFs at their nodes. This is also the case e.g. in Abaqus. Usually we use rigid links like this rigid body constraint discussed here.

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Here’s a recent tutorial video showing how to simulate torsion in PrePoMax: