How to Set Up Gravity Loads in FreeCAD FEM

Setting up gravity loads—also referred to as self-weight—in FreeCAD's Finite Element Method (FEM) workbench allows you to simulate the internal stresses and deformation caused by an object's own mass. This process requires creating an FEM analysis container, generating a solid mesh, assigning a material with a defined mass density, applying fixed boundary conditions, and configuring the self-weight constraint before solving the simulation.

Step 1: Prepare the Model and FEM Container

  1. Open your 3D model in FreeCAD and switch to the FEM Workbench using the workbench selector.
  2. Click Analysis container (the yellow ball icon) in the toolbar to create a new analysis.
  3. Select your solid part and generate a mesh using either Netgen or Gmsh. Ensure the generated mesh is placed inside the active Analysis container in the tree view.

Step 2: Assign a Material with Density

Gravity calculations depend entirely on the mass of the object, which is derived from volume and density.

  1. In the FEM toolbar, click Material for solid (the red mechanical part icon).
  2. Choose a preset material from the dropdown list (such as Steel-Generic or Aluminum-Generic) or create a custom one.
  3. Verify that the Density field contains a valid value (e.g., \(7850\text{ kg/m}^3\) for steel). If density is missing or zero, the gravity load will not generate any forces.
  4. Add your solid object to the material's geometry selection list and click OK.

Step 3: Apply Boundary Conditions

A model subjected only to gravity must have at least one anchor point to prevent unconstrained rigid body motion:

  1. Click Constraint fixed (the padlock icon).
  2. Select the face, edge, or vertex where the part is supported.
  3. Click Add, select the geometry, and click OK.

Step 4: Add the Self-Weight (Gravity) Constraint

  1. In the toolbar, click the Constraint self weight icon.
  2. In the task panel, configure the gravity direction vector and acceleration value:
    • Direction: Gravity acts downward relative to your coordinate system. If the vertical axis is Z, set the vector to x: 0, y: 0, and z: -1.
    • Acceleration: The default acceleration is standard Earth gravity (\(9.81\text{ m/s}^2\) or \(9810\text{ mm/s}^2\), depending on your unit settings). You can increase this value to simulate higher g-forces or load factors.
  3. Click OK to add the constraint to the analysis container.

Step 5: Run the Solver and Inspect Results

  1. Double-click the SolverCcxTools object inside the Analysis container.
  2. In the task panel, select Static analysis.
  3. Click Write .inp file to generate the input deck, then click Run CalculiX.
  4. Once the solver finishes without errors, close the task panel.
  5. Double-click the resulting CCX_Results object to inspect the deformation, displacement vectors, and Von Mises stress generated by the gravity load.