1. Snap fit

2. Snap fit
“A snap-fit is a mechanical joint system where part-to-part attachment is accomplished with locating and locking features (constraint features) that are homogenous with one or the other of the components being joined. Joining requires the (flexible) locking features to move aside for engagement with the mating part, followed by return of the locking feature toward its original position to accomplish the interference required to latch the components together. Locator features, the second type of constraint feature, are inflexible, providing strength and stability in the attachment. Enhancements complete the snap-fit system, adding robustness and user-friendliness to the attachment.”
-The First Snap-Fit Handbook, Bonenberger, 2000

3. Snap fit
…an arrangement of compatible locators, locks and enhancements acting to form a mechanical attachment between parts

4. Snap fit Important Criterion – Flexibility in integral locking feature
“Catch”
Joining method using springy (elastic) properties of metals and plastics
Plastic – Plastic
Plastic – Metal
Metal – Metal
Although most commonly used today with plastic assembled parts, snap-fit has long before existed in metal-metal components in clothing
Results in reduced man-hour, production cost, and or number of parts in assembly

5. Snap fit examples
Toys
Small Appliances
Automotive
Electronic Fields

6. Key Requirements Strength – attachment integrity for life of product
Integrity prevents:
Looseness
Breakage
Squeaks
Rattles
Constraint – prevention or control of relative movement between parts
Constraint Condition
Effect on
Proper
Under
Over
Noise
Allows a close fit btwn pairs
Part misalignment, possible looseness, squeaks and rattles
None
Assembly
Features fir w/out interference
Difficult assembly – interference btwn parts
Cost
Saving (relative to over)
Requires close tolerances
Analysis
Makes feature analysis possible
Interference is statically indeterminate
Reliability
Supports feature strength for reliability
Improper lock loading can lead to lock failure
Possible failure due to residual stain btwn parts
Possible component distortion under extreme temp.

7. Key Requirements
Compatibility – harmony in snap-fit interface between all elements
Often a subtle mistake
Robustness – tolerance of snap-fit to all variables and unknowns that exist in product design, manufacture, assembly, and use
Customer’s ability to interpret proper use
Technician’s ability to properly disassemble and reassemble without damage
Environment
Misuse, unexpected loads

8. Retention – nature of locking pair
Permanent – not intended for release
(not truly permanent, but require more effort or tool for release)
Non-permanent – intended for release
Releasing
Non-releasing

9. Example: Duracon M90-44
Yield strain : 7 - 8%
Thickness: h = 3mm
Height: Y = 2mm
Span: L = 10mm
Strain = 9%

10. Interface between concentric ridges in cylinders
Annular locks
Interface between concentric ridges in cylinders
“catch” is wrapped around cylinder
Ex: cap on ball-point pen
Constrain in 5 degrees of motion
Reasons we are looking at using annular locks:
Compatibility – can be locked in at any rotational position
Constrained in 5 DOM

11. Other important features of snap-fit
Guides
Help with assembly
Protect catch when misaligned
Assists
Finger tab
Tool access
Recessed, or push-in activation
Guards
Protect weak features

12. Troubleshooting Difficult assembly Distorted parts Feature damage
Over-constraint
Assembly motion and constraint feature incompatibility
Basic shape and assembly motion incompatibility
Access and basic shape incompatibility
Access and assembly motion incompatibility
Parts warped
Simultaneous engagement of several features
No guide or clearance enhancement
No operator feedback and/or feedback interface
Mating part is too hard to handle
Distorted parts
Parts warped when made
Distorted in assemble
Feature tolerances and position robustness
Compliant (flexible) parts, often panels are not constrained at enough points
Feature damage
Under-constraint
Incompatibility between features and assembly motion
Long term yield
Damaged during assembly
Poor processing
Abuse in usage
Abuse or damage during service / removal
Loose parts
Weak feature mounting area on mating and base plate
Compliant parts do not provide a stron base for the constrint features

14. Construx

15. K’nex

17. Our application