FEA Fundamentals for Housing Integrity

1. The FEA Workflow: From CAD to Confidence

Every FEA study, regardless of the software, follows a logical sequence. It begins with a CAD model and ends with an engineering judgment based on the results. The mantra "garbage in, garbage out" is paramount here; the quality of your results is entirely dependent on the quality of your setup.

Onshape's philosophy is to make FEA a fast and intuitive tool for designers, not just for specialist analysts. It streamlines many of the tedious steps, like meshing and defining contacts, by leveraging the data you've already built into your assembly (like mates).

2. Setting Up the Simulation for Our Housing

Let's apply this process to our project. We will set up a linear static analysis on the housing to simulate its behavior under operational load. For this exercise, it's often best to work in a simplified context rather than running the analysis on the full, complex machine.

Step 1: Prepare the Analysis Assembly

1. Create a new assembly in your document named "Housing FEA".
2. Insert the support plate and both halves of the housing.
3. Mate the housing to the support plate just as you did in the main assembly.
4. This isolates the key structural components we need to analyze.

Step 2: Define Fixtures and Connections
An FEA model must be constrained to prevent it from flying off into space when loaded.

1. In the instance list, right-click the support plate and select Fix. This is our "ground" or immovable reference.
2. On the right side of the screen, open the Simulation panel.
3. Under the Connections section, you can see the Method. The default, Mates and touching faces, is a good start. It tells Onshape to treat mated parts as connected according to their mate type (e.g., Fastened parts are bonded) and to bond any other parts that are touching.

Step 3: Define Loads
This is the most critical step for getting meaningful results. We must apply forces that realistically represent the operational loads. The primary load on the housing comes from the belt tension acting on the pulleys, which is transferred through the bearings.

1. Let's assume the total force from belt tension on the output shaft is 100 N, acting horizontally. This force is supported by two bearings, so each bearing applies a reaction force of 50 N to the housing.
2. From the Assembly toolbar, select the Bearing load tool.
3. In the dialog:
   • Instance: Select one half of the housing.
   • Faces to load: Select the cylindrical face of the bearing bore in that housing half.
   • Load direction: Select a mate connector or edge that defines the direction of the force (e.g., perpendicular to the shaft axis).
   • Magnitude: Enter 50 N in the appropriate direction.
4. Click the green checkmark.
5. Repeat the process, applying another 50 N bearing load to the bore in the other housing half.

Step 4: Assign Material and Run

1. Ensure your housing part has the correct material assigned (e.g., ABS or Aluminum 6061). You can verify this in the BOM table. The material's Yield Strength is crucial for calculating the factor of safety.
2. In the Simulation panel, click the Show results checkbox. Onshape will begin computing the analysis on its cloud servers. You will see results update in real-time as the solution converges.

For a detailed reference on these steps, you can consult the official Onshape documentation. The section "Basic workflow: Create and simulate a load" in resource LINK, "Simulation," provides a comprehensive guide.

3. Interpreting the Results

Once the simulation status shows a green checkmark, the analysis is complete. Now we must interpret the results to make an engineering decision.

Let's do the same for our housing. At the top of the graphics area, you'll see the results toolbar.

1. Sanity Check: Displacement

• From the dropdown, select Displacement.
• Click the play button to animate the deformation. It will be highly exaggerated.
• Ask yourself: Does the housing deform in the way I expect? The areas under the bearing loads should deflect, and the part should bend away from the force. If it's twisting unexpectedly or deforming in a strange way, it might indicate a problem with your fixtures or loads.

2. Stress Analysis: von Mises Stress

• Switch the result type to von Mises stress. This scalar value represents the overall stress state at any point in the material.
• The color plot shows you where stress is highest (typically red/yellow) and lowest (blue).
• Look for "hotspots." Are they in areas you expect, like sharp corners or around the mounting holes? High stress in the middle of a flat face might be unexpected.
• Note on Singularities: FEA can sometimes produce artificially infinite stress at sharp corners or point contacts. Onshape helps by setting the legend's maximum value to the 95th percentile of stress, effectively ignoring these non-physical peaks. If you see a tiny red dot with an astronomically high stress value, it's likely a singularity and can often be disregarded for a basic analysis.

3. The Verdict: Factor of Safety (FOS)

• This is the most direct result for design validation. Select Safety factor from the dropdown.
• Factor of Safety is calculated as:
  $FOS=\frac{\text{Material Yield Strength}}{\text{von Mises Stress}}$
• Any region with an FOS less than 1.0 has theoretically yielded and permanently deformed. Our goal is to ensure the minimum FOS is well above 1.
• For a project like this, a minimum FOS of 2.0 to 3.0 is a good target. This provides a margin of safety for unexpected loads, material imperfections, and simplification in our analysis.
• Look at the color plot. If you see large areas of red (FOS near 1.0) or blue (very high FOS), it tells you where your design might be too weak or potentially over-engineered (too heavy/costly).

Conclusion

Fantastic work today. You have successfully taken your design from a geometric model to a validated engineering component. By performing this basic FEA, you've engaged in a core practice of modern product development, gaining confidence in your design's structural integrity before ever manufacturing a physical part.

Key Takeaways:

• The FEA workflow involves defining geometry, materials, fixtures (constraints), and loads to create a realistic digital test.
• Onshape's simulation tool is designed to be an intuitive part of the design loop, handling complex setup steps automatically.
• Interpreting results is a three-step process: sanity check with Displacement, identify hotspots with von Mises Stress, and make a go/no-go decision with the Factor of Safety.
• The goal of FEA is to inform design decisions, helping you identify weak points to strengthen and overbuilt areas to optimize.

Preview of the Next Lesson:
Structural integrity is only one aspect of a successful design. We also need to ensure the components will last for their intended service life. In our next lesson, we will shift our focus to another critical type of analysis: Perform bearing life calculations for the selected shaft bearings. This will involve using standard engineering equations to predict the lifespan of our bearings under the defined operational loads.