KEVOS® MAKEOS · Materials
Cantilever Snap-Fit Design (Plastics)
Will this snap-fit finger survive assembly, and what forces does it need?
Assembly strain of a straight or tapered cantilever snap-fit finger by the handbook formula, the deflection force and the assembly (insertion) force from the lead-in angle and friction.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Deflection during assembly (undercut depth) | mm | How far the finger tip must deflect to pass the undercut. |
| Finger thickness at the root | mm | Thickness of the finger where it joins the part. |
| Finger length | mm | Length from the root to the hook. |
| Finger width | mm | Width of the finger. |
| Taper factor K (1 = straight) | Factor from the handbook chart for fingers that taper to half thickness (about 1.6–1.7); 1 for a straight finger. | |
| Secant modulus at the strain | MPa | Secant modulus of the plastic at the expected strain (lower than the initial modulus). |
| Permissible dynamic strain | % | Permissible short-term strain for the material from the resin supplier (e.g. about 2–4% for many unfilled thermoplastics, less for filled grades). |
| Friction coefficient | Friction between the mating materials. | |
| Lead-in angle | ° | Angle of the lead-in ramp on the hook. |
Method and formulas
- Strain ε = 3Yh ÷ (2L²K) (K = 1 for a straight finger).
- Deflection force P = b h² E ε ÷ (6L).
- Assembly force W = P (μ + tan α) ÷ (1 − μ tan α).
- Example: the default finger strains 1.28% and needs about 6.3852 N to assemble.
Limitations
- Elastic beam theory with a rigid root; real roots flex, which lowers strain but also holding force.
- Sharp corners at the root multiply the strain; always add a root radius.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Design snap-fit latches in moulded plastic parts
- Check a finger against the material’s strain limit
- Estimate insertion and deflection forces
Who it is for
- Product designers
- Plastics engineers
- Design engineers
What you get
- Assembly strain
- Deflection force
- Assembly force
Interpreting the result
- If the strain is too high, lengthen or thin the finger, or taper it.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 593–605. Topic reference; no source tables or recommended settings are reproduced.
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