Each node in a model meshed for a finite Element Analysis (FEA) study has six degrees of freedom or DOF. Three are translational and three are rotational. In order to stabilize the model, the nodes' DOF must be constrained. Note: this SOLIDWORKS Simulation article is a copanion to ou OLIDWORKS Simulation Tutorial - Fixture Better Practices video.
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| Figure 1 : Meshed cylinder with examples of nodes circled |
6 Degrees of freedom
In any FEA, the model is discretized into segments called elements. This is necessary in order to run stress equations throughout a part. In SOLIDWORKS, these elements are tetrahedrons. The points where tetrahedrons connect are called nodes.
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| Figure 1 : Meshed cylinder with examples of nodes circled |
Each node has six Degrees of Freedom,(6DOF): three translation and three rotational. After a model has been meshed and brocken up into tetrahedrons, its nodes are what need to be constrained.
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| Figure 2 : Degrees of Freedom |
In some more incolved simulation software programs like Abaqus, it's possible to select individual nodes and restrict their 6DOF. In SOLIDWORKS Simulation, we instead apply fixtures to model geometry like faces, edges, and vertices. This essentially selects all nodes on the chosen geometry and restricts their translation and/or rotation.
STANDARD FIXTURES
There are three standard fixtures : Fixed, Roller/Slider, and Fixed Hinge.
- Fixed removes all 6DOF for the selected faces, edges, and vertices.
- Roller/Slider only allows degrees of freedom transalationally in directions sitting on the plane of the selected face. the face's other four degrees of freedom are locked.
- Fixed Hinge removes the three transalational degrees of freedom and allows only one rotational degree of freedom about the selected cylindrical face.
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| Figure 3 : Standard Fixture icons and associated DOF. Top to bottom : Fixed, Roller/Slider, Fixed Hinge |
ADVANCED FIXTURES
The six Advanced Fixtures offer more granular
control : Symmetry, Cyclic symmetry, Reference geometry, on Flat faces, On Cylindrical faces, and On Spherical Faces.
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| Figure 4 : Advanced fixture icons and associated DOF. Clockwise from top left : Symmetry, Reference Geometry, On Cylindrical Faces, On Spherical Faces, On Flat Faces, Cyclic Symmetry |
- On Flat faces is similar to Roller/Slider in that it controls the transalational degrees of freedoù for a selected planar face the translations that can be restricted - or not - are normal to the planar face and the two in-plane directions. to restrict a DOF in any of the Advanced Fixtures, you must select it under the transalations section of the PropertyManager. Leaving the value at 0 prevents the face from moving in this direction.
- On Cylindrical Face allows you to independently control rotational degrees of freedom. These are radial Circumferential, and Axial.
- On Spherical Faces is a combination of radial and longitudinal & latitudinal rotational degrees pf freedom.
- Symmetry and Cyclic Symmetry are special becouse they tell the sover that there is more mass on the other side.
- Reference Geometry is the most powerful and nuanced advanced Fixture. It alone can recreate all other fixtures listed abopve. You can pick vertices, adges, and faces. Depending on the reference geometry you select, you can control either translational or rotational degrees of freedom. If you select a plane or flat face, you can control translational DOF. An axis or cylindrical face will give you options for rotational DOF. Finally, if you seleect an edge, you can control displaceùent along that edge.
APPLICATION
Different combinations of fixtures can produce the same result if they're consraining the same nodes ' DOFs in the same ways. After running your study, if your results don't look right, you may need to go back and add or edit fixtures.For example, if you apply a Fixed fixture to a face that should be able to slide- that is, that should not have restricted translational DOFs - then this can add non-real stiffness to your model.
Too few fixture can result in your model transalating or rotating in a way you don't want. This can throw off your Displacemennt results. Applying a Reference Geometry fixture to one or more vertices is one way to stabilize the model.
I hope you found this article explaining SOLIDWORKS Simulation degrees of freedom and fixutres helpful. Check out more tutorials below. Additionally, join the Noor-Artificiel Community to participate in the conversation, create forum posts, and answer questions from other SOLIDWORKS users.
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