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    • FEA How-To: Fixtures, Examples & More

    Owned by Katherine Nguyen
    Last updated: Aug 23, 2023
    7 min read

    Use this to guide you, not as definitive rules! Every part is different, and there are many ways to approach simulating them (some better than others, this is just what we’ve found works!).

    Introduction: Simulations for Beginners

    This page describes how to approach performing a FEA simulation on a part of the dynamics system. We do this to simulate the stress on a part that would occur in real-life situations. Every time a change is made to a part, we have to run a new simulation on it to ensure that it will not break. If you have no experience with FEA, watch this video and then come back to this page! Ok, so now that you know how to run a general study in SolidWorks, what is your job?

    1. Get assigned a part to simulate (Yay! This could be any custom part of the dynamics system. You’ll become super familiar with it.)

    2. Set up your simulation (we’ll guide you through this here!) while also keeping a record of what you do in a report.

    3. Ultimately, your report should not only show the details of how you created your simulation but should show if the part failed or passed under the loads (see below for what is a pass/fail).

    Every material has a yield strength (typically measured in Pa or MPa) which is the limit of stress that the material can take before it breaks (which for your purposes, means it has failed). If at any place on your part it has reached it’s yield strength, the part has failed. If it hasn’t reached it’s yield strength, it passed!

    Message from Stress Smiley

    Hi! I’m Stress Smiley. I’ll be helping with this tutorial. I’m made of 6061-T6 (a material used often in our custom parts!)

    Reading a Stress Plot

    As you can see in the results of this simulation of Stress Smiley, the yield strength of 6061-T6 (highlighted) is 2.750e+08 N/m^2 (aka Pa), and the max stress is indicated at the top of the scale (1.078e+09 N/m^2) which is higher than the yield strength. Therefore, this part failed

    If I take the same part and lessen the load such that the stress on the part is decreased, you can see that the scale changes to a maximum of 2.695e+06 Pa which is well below the yield strength. So something we can do to better represent these results in comparison with the first study is to change the maximum of the scale (Right click on the scale > Chart Options > Unclick “Automatically Define Maximum” > Set maximum to yield strength. Also, select “Show Max Annotation” to see where the maximum stress occurs in your part.

    6 Degrees of Freedom

    Without any fixtures, your part is free to move in all directions! Which is not true in real-life situations, which is where fixtures come in. Depending on your part, you may want to lock all six degrees of freedom or you may want to leave select degrees free for the part to move properly.

    Fixtures

    There are many features that you can now use to make your simulation as accurate as possible. One of which is selecting the correct fixtures for your part. We’ll be running through the most commonly used fixtures (in bold). Jump to Which Fixture When? for a guide on which situations need which fixtures.

    • Basic Fixtures:

      • Fixed Geometry

      • Roller/Slider

      • Fixed Hinge

      • Elastic Support

      • Bearing Fixture

      • Foundation Bolt

    • Advanced Fixtures:

      • Symmetry

      • Cyclic Symmetry

      • Use Reference Geometry

      • On Flat Faces

      • On Cylindrical Faces

      • On Spherical Faces

    • Virtual Wall

    Basic Fixtures

    Fixed Geometry

    Selecting fixed geometry on a feature of your part will inhibit all movement at that point (all 6 degrees of freedom). In many cases, this is not a realistic representation of how an object is fixed in space. Thus, you will rarely use fixed geometry.

    Roller/Slider

    Selecting roller/slider on a feature of your part allows to the part to translate in the horizontal and vertical directions of the face, as if it was sliding on a surface. It also prevents the part from lifting off of the surface.

    Since the roller/slider feature inhibits Z movement of the part, this is generally not very accurate to what happens in real life situations. For example, when an object is resting on a table, it is still able to be lifted off the table. Roller/slider would not allow that to happen. A better option is to use an advanced fixture or a virtual wall, as you will see below.

    Advanced Fixtures

    On Flat Faces

    Adding a flat face fixture on a part allows you to restrict motion along any combination of the three directions on a plane: along, across, and normal to the face. One of the most common use for flat face fixtures are on faces where the head of a bolt makes contact with a surface, and is fixed normally to the surface.

    Flat face fixtures are similar to fixed geometry in that it can constrain 3 DOF, but fixed geometry fully fixes all 3 directions, while flat face fixtures let you select which you prefer.

    On Cylindrical Faces

    Cylindrical face fixtures act very similarly to flat face fixture, but act obviously on cylindrical faces. Instead of restricting motion in the 3 cardinal directions, they restrict radially, axially, and normally. these cylindrical face fixtures will most commonly occur in the slot of a bolted connection, having an axial fixture. Cylindrical fixtures also have to ability to lock their rotation, so if a specified part does not rotate then that option should be checked

     

    On Spherical Faces

    Spherical face fixtures are again similar to flat face and cylindrical face fixtures, but are for (you guessed it!) spherical faces on parts. The motion can be restricted radially, longitudinally about the face or laterally about the face.

    Virtual Wall

    Virtual walls mimics having the part set on a flat surface. The part can slide around on the wall, lift off of the wall, but can’t go through it. This feature is not set under fixtures. Right-click Connections > Local Interactions > Type: “Virtual Wall”. For the plane, you’ll need to set a reference plane coincident to the face you want to have on a wall. For the face, select the face that is touching the wall.

     

    Split Lines

    Split lines in SolidWorks are an easy way to select a certain area of a surface for applying forces or fixtures. Split lines can be created using a sketch on a surface, or even an intersection of a plane and a surface. The split line tool can be found in the features tab under 'Curves.' There are three types: https://help.solidworks.com/2021/english/SolidWorks/sldworks/hidd_dve_pline.htm

     

    Dummy Parts

    Dummy parts are when you create simple geometric parts in a simulation to represent parts in an assembly that interact with the part that you are simulating. Usually, we create dummy parts for bolts, bearings, bearing housings, etc. (i.e. anything that has an important interaction with the part that would be complicated to simulate using typical fixtures). For an example on how dummy parts are used in simulations, see the tutorial of the upright.

    Which Fixture When: Common Cases