Tolerance Stack Up Analysis:
How To Handle Clearances
In this guide, we will explore practical examples of handling clearances during tolerance stack-up analysis.
First, we will cover a simple, generalized approach to managing clearances. Then, we will dive into a more specific scenario involving pin-joints.
See "Episode 2" of the "How to set up a tolerance stack"-series
Basic Example of Clearances and Best Practice
• Set up your tolerance stack as usual when working with clearances.
• For every clearance, add a help/leader-line on each side.
• This helps to clarify interface boundaries.
• Imagine that in a worst case scenario - all the boxes are pushed to the left. Setting up a tolerance stack in neutral position and in a worst case position yields the same results.
Handling Stack-Up with Pin-Joints
• When performing tolerance stack-up analysis with pin-joints, best practice is to place the pin-joint in a neutral position
• Alternatively, pin joints can be pushed to their worst case position to make illustrations simpler.
Advanced Tolerance Stack Example: Part 1
In this example, the purpose is to evaluate if there will be at least 1 mm overlap between the yellow arm and the blue arm (always).
• The image shows is a cross section - a good fit for an illustration.
• In this case the assembly is setup with clearances (it's neutral position).
Advanced Tolerance Stack Example: Part 2
• Set up help/leader-lines and mark the "Point of Interest"
• Place a help/leader-lines on each side of every clearance
• Now setup parameter arrows, they should follow the alternating pattern shown in the image.
Advanced Tolerance Stack Example: Part 3
• Add parameter values as diameters, not radii.
• Diameters are converted to radiis when defining the transfer function
Advanced Tolerance Stack Example: Part 4
• When defining the transfer function, manually convert diameter values that should be radii by multiplying them by 0.5 (or by dividing by 2)
• Add your requirements (Targets for USL, Mean, USL) and run the simulation.
Advanced Tolerance Stack Example: Final Result
• Choose evaluation method: Worst Case, RSS, Statistical or Monte Carlo.
• The final result now shows as passed, which means that our design passes the given requirements.
Best Practice: Use Local Compliant Features to Remove Clearances
• Use local compliant features (with an overlap) to eliminate play.
• You achieve best performance of the fit by only making 'one side' flexible - so you always know where the part is positioned.
• Different solution for each case - if you work in plastic injection molded parts, sheet metal or CNC.
Best Practice: Use Crush Ribs to Eliminate Play by Plastic Deformation
• Use crush ribs to eliminate play. Make small features that plastically deform during assembly to take up any variation in a given fit.
• Pay respect to material, process and use case.
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