BIZOMAT 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. |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Undercut (deflection)
- What it is
- How far the tip of the finger must bend sideways to ride over the catch: the height of the undercut.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 50 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1.2 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Strain, forces and root stress rise in direct proportion.
- Get it right
- Use the full interference of the hook over the catch, including tolerances at their worst.
What this means: Finger thickness at the root
- What it is
- The thickness of the finger at its root, measured in the direction it bends.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 50 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1.6 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Strain rises in proportion to thickness, and deflection force with its cube.
- Get it right
- Measure in the bending direction, not the width across the finger.
What this means: Finger length
- What it is
- The length of the finger from its root, where it joins the part, to the catch.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 500 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 15 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Strain falls with the square of length and deflection force with its cube.
- Get it right
- Measure from the root to the point where the catch contacts, not to the very tip.
What this means: Finger width
- What it is
- The width of the finger, measured across it at right angles to the direction of bending.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 500 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 6 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Forces are directly proportional to width; strain does not change.
- Get it right
- Do not swap width and thickness: thickness is in the bending direction.
What this means: Geometry factor (1 = constant section; higher f…
- What it is
- A factor for the finger’s shape along its length: a constant section is the reference, and a finger that tapers in thickness spreads the strain.
- Units and format
- Enter a plain number with no unit. The form accepts 0.5 to 2. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Strain is inversely proportional to it; the largest undercut is directly proportional.
- Get it right
- Use 1 for a constant section; take the value for a tapered finger from your snap-fit design guide.
What this means: Secant modulus at the strain
- What it is
- The modulus of the plastic taken as stress divided by strain at the strain reached, lower than the initial modulus for most plastics.
- Units and format
- Enter a number in MPa: megapascals (1 MPa = 1 N/mm²). The form accepts 0.000001 MPa to 100000 MPa. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2300 MPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Forces and root stress are directly proportional; strain does not change.
- Get it right
- Use the secant modulus at this strain and temperature from the material maker, not the tensile modulus on the first page of the data sheet.
What this means: Permissible strain
- What it is
- The largest short-term strain the plastic can take at the finger root without yielding or cracking during assembly.
- Units and format
- Enter a number in %: percent: the number is read as a percentage, so type 8 for 8 %. The form accepts 0.000001% to 50%. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2.5% as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Sets the largest undercut and the strain check.
- Get it right
- Type the percentage itself; the limit for repeated assembly is lower than for a single snap.
What this means: Friction coefficient
- What it is
- The friction coefficient between the catch and the mating part as they slide during assembly and separation.
- Units and format
- Enter a plain number with no unit. The form accepts 0 to 1.5. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.4 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Higher friction raises both mating and separation forces.
- Get it right
- Take it for the actual material pair; it varies with surface finish and lubrication.
What this means: Lead-in angle
- What it is
- The angle of the sloping face of the catch that the mating part first slides over during assembly.
- Units and format
- Enter a number in °: degrees of angle. The form accepts 5° to 80°. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 30° as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A steeper lead-in raises the mating force.
- Get it right
- Measure from the direction of the finger deflection as drawn in the tool’s method; too steep a face with high friction locks instead.
What this means: Retaining (return) angle
- What it is
- The angle of the face that holds the parts together and has to be overcome to pull them apart.
- Units and format
- Enter a number in °: degrees of angle. The form accepts 5° to 90°. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 45° as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A steeper retaining angle raises the separation force until it self-locks.
- Get it right
- A right-angled face, or one where friction stops sliding, makes the joint permanent.
What this means: Retention force wanted
- What it is
- The pull-apart force the joint must resist in service without opening.
- Units and format
- Enter a number in N: newtons. The form accepts 0 N to 100000 N. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 10 N as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- It does not change the forces, only the retention check.
- Get it right
- Include loads from handling, drops and vibration, not just the static weight.
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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