How To Use The Bolt Analysis Tool
The bolt analysis tool is straightforward to use once you're familiar with the variables. Follow these steps to perform an analysis:
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Fill out the form: Begin by entering all the required inputs in the analysis form above. These inputs include material properties, bolt dimensions, forces, moments, and other relevant parameters. Ensure all values are accurate, as they directly influence the analysis results. A detailed explanation of each input is provided below.
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Select the materials: Choose the appropriate materials for the plates, bolt, and nut from the dropdown menus. The material properties, such as yield strength and ultimate strength, are preloaded based on standard data.
- If the bolt is threaded into the second plate, select the same material for the nut as for plate 2. Additionally, if you want to be conservative in your analysis, make sure that the "threads in bearing" option is selected.
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Input forces and moments: When using an FEA element like an RBE-2 in NASTRAN, the analysis tool will provide you with the results, including axial force, shear forces (in both X and Y directions), and any moments or torque that the bolt is subjected to. Input these values accurately to reflect the true loading conditions.
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Set geometry parameters:
This is the most detailed part of the process, so I've included graphics for clarity. Geometry parameters like minimum edge distances, thread engagement length, and other constraints are critical for calculating stress concentrations and potential failure modes.
- Let's consider an example: Assume you have one plate bolted down securely (the gray plate), with a smaller black plate bolted on top. Something will be mounted to the black plate (not shown). The software simulates the loading on the plate caused by whatever it mounted to the plate when it is being used, but we'll focus on explaining the geometry for the inputs required by the tool.
- "Minimum edge distance 1" is the distance from the center of the bolt to the closest edge of either plate in the X direction.
- "Minimum edge distance 2" is the distance from the center of the bolt to the closest edge of either plate in the Y direction.
- In the example below, the green lines indicate the minimum edge distances. Note that an edge distance in one direction could be limited by another bolt, but in this case it isn't.
- This metric is crucial for determining the potential for bolt tearout through the edge of the plate.
- "Torsion Constraint Distance" measures the distance from the bolt's center to the point where rotational force along the bolt's axis is constrained. In simpler terms, this is the distance to the element preventing the bolt from rotating around its center.
- Typically, this would be another bolt, but it could also be a surface or feature that stops the plate from turning. In the example below, the other bolt serves as this constraint.
- "Moment X Constraint Distance" is the distance in the Y axis that resists the plate's rotation around the X axis.
- "Moment Y Constraint Distance" is the distance in the X axis that resists the plate's rotation around the Y axis.
- In the example below, Mx indicates the "Moment X Constraint Distance" and My indicates the "Moment Y Constraint Distance".
- This constraint is usually provided by another bolt or the edge of the plate. In this case, it's the edge of the plate since there are no closer bolts.
- "Engaged Thread Length" refers to the thickness of the nut, if present, minus one pitch. For example, for a 1/2-20" screw, it would be the nut's thickness minus 1/20". If there is no nut, use the thickness of the second plate that is being threaded into.
- In the cross-section below, the engaged thread length is the nut's thickness minus one pitch. If the nut is 1/4" thick and has a pitch of 1/28", the engaged thread length would be 1/4" - 1/28" = 0.2143".
- "Threads in Bearing?" indicates whether the bolt's threads are in contact with the inside of the holes in the plates. Some bolts have a shaft that prevents the threads from bearing contact.
- In the cross-section below, you can see that the threads are in bearing, meaning they contact the sides of the holes.
- The second image shows a screw with a shaft, which prevents the threads from being in bearing, potentially increasing the screw's strength.
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Specify safety factors: Enter the factor of safety for both yield and ultimate conditions. The factor of safety is the multiple of the bolt's strength required for it to be considered safe. Common values are 3x or 5x, depending on the criticality of the application.
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Submit the form: After all fields are filled out, click the "Calculate" button. The tool will analyze the inputs and provide detailed results, including stress values, safety margins, and any potential failure risks.
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Review the results: The results will be displayed along with the equations used in the analysis. Carefully review these to understand how each result was derived and to verify the analysis's accuracy.
This tool is a valuable asset for any mechanical engineer, allowing for quick and accurate bolt analysis directly from FEA models. Use it to validate your designs and ensure that every component meets the necessary safety and performance standards.