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

• 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

• Important Criterion – Flexibility in integral locking feature

“Catch”

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

None Difficult assembly – interference btwn parts

Cost Saving (relative to over) None Requires close tolerances

Analysis Makes feature analysis possible

None 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%

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

– 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– Over-constraint– Compliant (flexible) parts, often panels are not constrained at enough points

• Feature damage– Over-constraint– 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– Feature damage– Weak feature mounting area on mating and base plate– Difficult assembly– Under-constraint– Compliant parts do not provide a stron base for the constrint features

Construx

K’nex

Our application