Snap%20fit

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Snap fit
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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

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Snap fit

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

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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

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Snap fit examples

• Toys
• Small Appliances
• Automotive
• Electronic Fields

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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.
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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
• Customers ability to interpret proper use
• Technicians ability to properly disassemble and
  reassemble without damage
• Environment
• Misuse, unexpected loads

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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

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

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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

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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

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Construx
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Knex
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Our application
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