Snap fit

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

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

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

Snap fit examples Toys Small Appliances Automotive Electronic Fields

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.

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

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

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

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

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

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

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