The First Snap-Fit Handbook

Foreword to Third Edition

Preface to Third Edition

Foreword to Previous Editions

Prefaces to Previous Editions

Preface to First Edition

Preface to Second Edition

Contents

1 Introduction

1.1 Reader Expectations

1.2 Harmful Beliefs

1.3 Snap-Fit Technology

1.4 Snap-Fits and Loose Fasteners

1.5 Snap-Fits as Interface Systems

1.5.1 Feature Level

1.5.2 Attachment Level

1.6 The Attachment Level Construct© (ALC)

1.6.1 Attachment Level Terminology

1.6.2 Applying the ALC to other Attachment Methods

1.6.3 Required Capabilities for Snap-Fit Development

1.6.4 Justifying the ALC

1.7 Using This Book

1.71 Sample Parts

1.7.2 Snap-Fit Novices

1.7.3 Experienced Product Developers

1.7.4 Design for Assembly/Manufacturing Practitioners

1.7.5 Executives and Engineering Managers

1.8 Summary

2 Key Requirements

2.1 Constraint

2.2 Compatibility

2.3 Robustness

2.4 Strength

2.5 Summary

3 Introduction to the Snap-Fit Development Process

3.1 Concept vs. Detailed Design

3.2 The Value of Multiple Concepts

3.3 Step 0: Is a Snap-Fit Appropriate?

3.4 The Demand-Complexity Matrix©

3.5 Summary

4 Descriptive Elements

4.1 Function

4.1.1 Action

4.1.2 Purpose

4.1.3 Retention

4.1.4 Release

4.2 Basic Shapes

4.2.1 Mating-Part and Base-Part

4.2.2 Basic Shape Descriptions

4.2.3 Basic Shape Combinations

4.3 Engage Direction

4.4 Assembly Motion

4.5 Summary

5 Physical Elements: Locators

5.1 Protrusion-Based Locators

5.1.1 Pins

5.1.2 Prongs

5.1.3 Tabs

5.1.4 Lugs

5.1.5 Tracks

5.1.6 Cones

5.1.7 Wedges

5.1.8 Catches

5.2 Surface-Based Locators

5.2.1 Surfaces

5.2.2 Edges

5.2.3 Lands

5.3 Void-Based Locators

5.3.1 Holes

5.3.2 Slots

5.3.3 Cutouts

5.4 Living Hinges

5.5 Using Locators

5.5.1 Locator Pairs

5.5.2 Providing Constraint

5.5.3 Assembly Motion and Strength

5.5.4 Fine-Tuning

5.5.5 Dimensional Robustness

5.5.5.1 Positioning

5.5.5.2 Compliance

5.5.5.3 Datum Points

5.5.6 Constraint Efficiency

5.5.7 Mechanical Advantage and Stability

5.5.8 Ease of Assembly

5.6 Summary

6 Physical Elements: Locks

6.1 Lock Deflection and Separation Behavior

6.2 Lock Styles

6.3 Cantilever Beam Locks

6.3.1 Hooks

6.3.1.1 Hook Assembly Behavior

6.3.1.2 Hook Separation Behavior

6.3.1.3 Hooks and Retainers

6.3.1.4 Hooks and Prongs

6.3.2 Loops

6.3.2.1 Loop Assembly Behavior

6.3.2.2 Loop Separation Behavior

6.3.2.3 Loops and Knit Lines

6.3.3 Traps

6.3.3.1 Trap Assembly Behavior

6.3.3.2 Trap Separation Behavior

6.3.4 Low Deflection Lugs

6.3.5 Other Cantilever Beam Locks

6.4 Planar Locks

6.5 Torsional Locks

6.6 Annular Locks

6.7 Using Locks

6.7.1 Lock Pairs

6.7.2 Short Grip-Length and Low-Clearance Applications

6.7.3 High Demand Applications

6.7.4 Tamper Resistant Applications

6.7.5 The Case against Cantilever Hooks

6.8 Summary

7 Lock Strength and Decoupling

7.1 Level 0 No Decoupling

7.2 Level 1 Decoupling

7.3 Level 2 Decoupling

7.4 Level 3 Decoupling

7.5 Level 4 Decoupling

7.6 Summary

8 Constraint in Snap-Fit Applications

8.1 Perfect Constraint

8.2 Proper Constraint

8.3 Under-Constraint

8.4 Over and Improper Constraint

8.4.1 Redundant Constraint Features

8.4.2 Opposing Constraint Features

8.5 The Constraint Worksheet

8.6 Using the Constraint Worksheet

8.7 Constraint Rules

8.8 Summary

9 Physical Elements: Enhancements

9.1 Assembly Enhancements

9.1.1 Guides

9.1.2 Clearance

9.1.3 Pilots

9.1.4 Example – Switch Application

9.1.5 Example: Reflector Application

9.1.6 Feedback

9.2 Activation Enhancements

9.2.1 Visuals

9.2.2 Assists

9.2.3 User-Feel

9.3 Performance Enhancements

9.3.1 Guards

9.3.2 Retainers

9.3.3 Compliance

9.3.3.1 Local Yield

9.3.3.2 Elasticity

9.3.3.3 Isolators

9.3.4 Back-Up Features

9.4 Manufacturing Enhancements

9.4.1 Process-Friendly Design

9.4.2 Fine-Tuning Enablers

9.5 Summary

10 Applying the Snap-Fit Development Process

10.1 Step 1: Define the Application

10.2 Step 2: Benchmark

10.3 Step 3: Generate Multiple Concepts

10.3.1 Engage Direction

10.3.2 Assembly Motions

10.3.3 Identify Constraint Pairs

10.3.4 Add Some Enhancements

10.3.5 Select a Concept for Analysis

10.4 Step 4: Design and Analyze Features

10.4.1 Lock Alternatives

10.4.1.1 Threaded Fasteners

10.4.1.2 Plastic Push-In Fasteners

10.4.1.3 Spring-Steel Clips

10.5 Step 5: Confirm Design with Parts

10.6 Step 6: Fine-Tune the Design

10.7 Step 7: Snap-Fit Application Completed

10.8 Summary

11 Feature Development: Material Properties

11.1 Sources of Material Property Data

11.2 Material Property Assumptions

11.3 The Stress-Strain Curve

11.4 Determining a Design Point

11.4.1 Applications with Fixed Strain

11.4.2 Applications with Variable Strain

11.4.3 The Secant Modulus

11.4.4 Maximum Permissible Strain Data

11.5 Coefficient of Friction

11.6 Other Effects on Material Properties

11.7 Summary

12 Lock Feature Development: Rules-of-Thumb

12.1 Beam-Based Locks

12.1.1 Beam Thickness at the Base

12.1.2 Beam Length

12.1.3 Beam Thickness at the Retention Feature

12.1.4 Beam Width

12.2 Retaining Member: Catch

12.2.1 The Insertion Face

12.2.2 The Retention Face

12.3 Loops

12.4 Traps

12.5 Other Lock Styles

12.5.1 Torsional Locks

12.5.2 Planar Locks

12.5.3 More Lock Styles

12.6 Summary

13 Lock Feature Development: Calculations

13.1 Assumptions and Allowances

13.2 The Deflecting Member: Cantilever Beam

13.2.1 General Equations for Rectangular Sections

13.2.2 Constant Section Beam Bending

13.2.3 Adjusting the Design Strain for Stress Concentration

13.2.4 Calculating the Initial Beam Strain

13.2.5 Adjusting for Deflection at the Beam’s Base

13.2.6 Calculating the Initial Beam Deflection Force

13.2.7 Adjusting for Mating Feature/Part Deflection

13.2.8 Example Beam Strain and Deflection Calculations

13.2.9 Deflection Graphs for a Straight Beam

13.3 Deflecting Member: Tapered Beams

13.3.1 Taper Error Example

13.3.2 Beams Tapered in Thickness

13.3.3 Beams Tapered in Width

13.4 Beam Calculation Summary

13.5 Other Deflecting Member Styles

13.5.1 Other Beam-Based Styles: Loops and Traps

13.5.2 Other Styles: Torsional, Annular, and Planar Deflection

13.6 The Retaining Member: Catch

13.6.1 Lock Assembly Force

13.6.1.1 Adjusting for the Insertion Face Effective Angle

13.6.1.2 Example Assembly Force Calculations

13.6.1.3 Modifying the Insertion Face Profile

13.6.2 Catch Separation Force

13.6.2.1 Adjusting for the Retention Face Effective Angle

13.6.2.2 Example Assembly Force Calculations

13.6.2.3 Modifying the Retention Face Profile

13.7 Stationary Catches and Traps as Retaining Members

13.7.1.1 Other Separation Considerations

13.8 Using Finite Element Analysis

13.9 Calculation Spreadsheets

13.10 Summary

14 Diagnosing Snap-Fit Problems

14.1 Common Snap-Fit Mistakes

14.2 Attachment Level Diagnosis

14.3 Feature Level Diagnosis

14.4 Summary

15 Gaining a Competitive Advantage in Snap-Fit Technology

15.1 Terminology

15.2 Managing Expectations

15.3 Harmful Beliefs

15.4 The Demand-Complexity Matrix

15.5 The Snap-Fit Capability Plan

15.5.1 Vision, Mission, and Values

15.5.2 Objectives

15.5.3 Strategies

15.6 Initiatives for Getting Started

15.6.1 Provide Education and Training

15.6.2 Provide Technical Resources

15.6.3 Identify Low-Impact Applications as a Starting Point

15.6.4 Use Physical Models

15.6.5 Provide Benchmarking Opportunities

15.6.6 Include Snap-Fit Technical Requirements in the Bidding and Purchasing Processes

15.6.7 Identify Intermediate Applications

15.7 Initiatives for Organizational Capability

15.7.1 Identify and Empower a Snap-Fit Champion

15.7.2 Identify and Empower a Snap-Fit Technical Leader

15.7.3 Make Snap-Fit Technology Visible in the Organization

15.7.4 Link Snap-Fits to Other Business Strategies

15.7.5 Create and Maintain a Library of Preferred Concepts

15.7.5.1 Example of a Preferred Concepts Initiative

15.7.6 Have a Model of the Snap-Fit Technical Domain

15.7.7 Reward Teamwork and Make Snap-Fits Interesting

15.7.8 Identify Supportive Customers and Suppliers

15.8 Summary

Appendix – Resources

About the Author

Index