KEVOS® MAKEOS · Materials
Snap-Fit Joint Design (Deflection, Assembly and Retention Forces)
How far can this snap-fit finger deflect, how hard is it to push together, and how much force will hold it closed?
Design of a cantilever snap-fit joint in plastic: the largest undercut the finger can take at the permissible strain, the assembly strain for the undercut chosen, deflection force and mating force over the lead-in angle (as the snap-fit tool), the separation force over the retaining angle (or self-locking when the retaining face cannot slide), and the bending stress at the root.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Undercut (deflection) | mm | Height of the catch the finger must ride over. |
| Finger thickness at the root | mm | Finger thickness at the root. |
| Finger length | mm | Length of the finger from root to catch. |
| Finger width | mm | Width of the finger. |
| Geometry factor (1 = constant section; higher for a tapered finger) | Geometry factor for the finger’s taper (1 for constant thickness; tapering to half thickness makes it near 1.6). | |
| Secant modulus at the strain | MPa | Secant modulus of the plastic at the strain (from the material maker). |
| Permissible strain | % | Permissible short-term strain for a single or repeated assembly (from the material maker). |
| Friction coefficient | Friction between the parts (plastic on plastic about 0.2–0.6). | |
| Lead-in angle | ° | Angle of the lead-in face. |
| Retaining (return) angle | ° | Angle of the retaining face (90° makes a joint that cannot be pulled apart). |
| Retention force wanted | N | Pull-apart force the joint must resist. |
Method and formulas
- Strain ε = 3 Y h ÷ (2 L² K); largest undercut Y_max = 2 ε_perm L² K ÷ (3h).
- Deflection force P = b h² E ε ÷ (6L).
- Mating force W = P (μ + tan α₁) ÷ (1 − μ tan α₁); separation force uses the retaining angle α₂ and locks when μ tan α₂ ≥ 1.
- Root stress σ = E ε.
- Example: the finger takes up to 2.3437 mm of undercut; it pushes on with 6.3852 N and holds 11.724 N.
Limitations
- Elastic beam with a rigid root; a flexible base lowers forces and strain.
- Strain limits must come from the material maker for the temperature and number of assemblies.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Design a clip for a plastic housing
- Decide whether a snap is separable or permanent
- Trade finger length against assembly force
Who it is for
- Product designers
- Plastics engineers
- Design engineers
What you get
- Largest undercut and the strain at the undercut chosen
- Deflection and assembly forces
- Separation force and root stress
Interpreting the result
- A longer finger cuts strain with the square of the length, so it usually beats a thinner one.
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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Open the Snap-Fit Joint Design (Deflection, Assembly and Retention Forces) calculator
