© 2008 AGI-Information Management Consultants
May be used for personal purporses only or by
libraries associated to dandelon.com network.
Paul R. Bonenberger
The First Snap-Fit
Handbook
Creating and Managing Attachments
for Plastic Parts
2nd Edition
HANSER
Hanser Publishers, Munich • Hanser Gardner Publications, Cincinnati
Contents
1 Snap-Fits and the Attachment Level™ Construct
1.1 Introduction
1.2 Reader Expectations
1.3 Snap-Fit Technology
1.4 Feature Level and Attachment Level
1.5 Using this Book
1.5.1 The Importance of Sample Parts
1.5.2 Snap-Fit Novices
1.5.3 Experienced Designers
1.5.4 Design for Assembly Practitioners
1.5.5 Engineering Managers and Executives
1.6 Chapter Synopses
1.7 Extending the ALC to Other Attachments
1.8 Summary
1.8.1 Important Points in Chapter 1
2 Overview of the Attachment Level™ Construct
2.1 Introduction
2.2 The Key Requirements
2.2.1 Strength
2.2.2 Constraint
2.2.2.1 Improper Constraint
2.2.3 Compatibility
2.2.4 Robustness
2.3 Elements of a Snap-Fit
2.3.1 Function
2.3.1.1 Action
2.3.1.2 Attachment type
2.3.1.3 Retention
2.3.1.4 Lock type
2.3.1.5 Function Summary
2.3.2 Basic Shapes
2.3.2.1 Mating Part and Base Part
2.3.2.2 Basic Shape Descriptions
2.3.2.3 Basic Shape Summary
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2.3.3 Engage Direction
2.3.4 Assembly Motion
2.3.5 Constraint Features
2.3.5.1 Locator Features
2.3.5.2 Lock Features
2.3.6 Enhancements
2.3.7 Elements Summary
2.4 Summary
2.4.1 Important Points in Chapter 2
2.4.2 Important Design Rules Introduced in Chapter 2
3 Constraint Features
3.1 Introduction
3.2 Locator Features
3.2.1 Locator Styles
3.2.1.1 Lug
3.2.1.2 Tab
3.2.1.3 Wedge
3.2.1.4 Cone
3.2.1.5 Pin
3.2.1.6 Catch
3.2.1.7 Surface
3.2.1.8 Land
3.2.1.9 Edge
3.2.1.10 Hole
3.2.1.11 Slot
3.2.1.12 Cutout
3.2.1.13 Living Hinge
3.2.2 Design Practices for Locator Pairs
3.2.2.1 Terminology
3.2.2.2 Locator Pairs, Constraint and Strength
3.2.2.3 Locator Pairs and Ease of Assembly
3.2.2.4 Locator Pairs and Dimensional Control
3.2.2.5 Locator Pairs and Mechanical Advantage
3.2.2.6 Locator Pairs and Compliance
3.2.2.7 Locators Summary
3.3 Lock Features
3.3.1 Lock Feature Styles
3.3.2 Cantilever Beam Locks
3.3.2.1 The Deflection Mechanism
3.3.2.2 The Retention Mechanism
3.3.2.3 Cantilever Lock Examples
3.3.2.4 Locators as Cantilever Locks
3.3.2.5 Lock Pairs
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3.3.2.6 Cantilever Lock Assembly Behavior
3.3.2.7 Cantilever Lock Retention and
Disassembly Behavior
3.3.3 Planar Locks
3.3.4 Trap Locks
3.3.4.1 Trap Assembly Behavior
3.3.4.2 Trap Retention and Disassembly
3.3.4.3 Traps and Lock Efficiency
3.3.5 Torsional Lock
3.3.6 Annular Lock
3.3.7 Lock Pairs and Lock Function
3.4 Selecting a Locking Feature
3.5 Summary
3.5.1 Important Points in Chapter 3
3.5.2 Design Rules Introduced in Chapter 3
4 Enhancements
4.1 Introduction
4.2 Enhancements for Assembly
4.2.1 Guidance Enhancements
4.2.1.1 Guides
4.2.1.2 Clearance
4.2.1.3 Pilots
4.2.2 Product Example 1
4.2.3 Product Example 2
4.2.4 Product Example 3
4.2.5 Operator Feedback
4.2.6 Product Example #3 Revisited
4.2.7 Assembly Enhancements Summary
4.3 Enhancements for Activating and Using Snap-Fits
4.3.1 Visuals
4.3.2 Assists
4.3.3 User Feel
4.4 Enhancements for Snap-Fit Performance
4.4.1 Guards
4.4.2 Retainers
4.4.3 Compliance
4.4.3.1 Compliance through Local Yield
4.4.3.2 Compliance through Elasticity
4.4.3.3 Isolators
4.4.4 Back-Up Locks
4.5 Enhancements for Snap-Fit Manufacturing
4.5.1 Process-Friendly
4.5.2 Fine-Tuning
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4.6
Summary
4.6.1 Important Points in Chapter 4
4.6.2 Design Rules Introduced in Chapter 4
5 Fundamental Snap-Fit Concepts
5.1
5.2
5.3
The Importance of Constraint
5.1.1 Constraint Review
5.1.2 Constraint Principles
5.1.2.1 Perfect Constraint
5.1.2.2 Proper Constraint
5.1.2.3 Proper Constraint in Less than 12 DOM
5.1.2.4 Under-Constraint
5.1.2.5 Over-Constraint
5.1.2.6 General Constraint Rules
5.1.3 The Constraint Worksheet
5.1.4 Additional Comments on Constraint
Lock Decoupling
5.2.1 The Lock Feature Paradox
5.2.2 Decoupling Examples
5.2.3 Levels of Decoupling
5.2.3.1 No Decoupling (Level 0)
5.2.3.2 Level 1 Decoupling
5.2.3.3 Level 2 Decoupling
5.2.3.4 Level 3 Decoupling
5.2.3.5 Level 4 Decoupling
5.2.4 Decoupling Summary
Summary
5.3.1 Important Points in Chapter 5
5.3.2 Design Rules Introduced in Chapter 5
6 Feature Design and Analysis
6.1 Pre-Conditions for Feature Analysis
6.2 Material Property Data Needed for Analysis
6.2.1 Sources of Materials Data
6.2.2 Assumptions for Analysis
6.2.3 The Stress-Strain Curve
6.2.4 Establishing a Design Point
6.2.4.1 For Applications Where Strain is Fixed
6.2.4.2 For Applications Where Strain is Variable
6.2.4.3 Materials With a Definite Yield Point
6.2.4.4 Materials Without a Definite Yield Point
6.2.4.5 The Secant Modulus
6.2.4.6 Maximum Permissible Strain Data
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6.2.5 Coefficient of Friction (/j.)
6.2.6 Other Effects
6.3 Cantilever Hook Design Rules of Thumb
6.3.1 Beam Thickness at the Base
6.3.2 Beam Length
6.3.3 Insertion Face Angle
6.3.4 Retention Face Depth
6.3.5 Retention Face Angle
6.3.6 The Threshold Angle
6.3.7 Beam Thickness at the Retention Feature
6.3.8 Beam Width
6.3.9 Other Features
6.4 Initial Strain Evaluation
6.5 Adjustments to Calculations
6.5.1 Adjustment for Stress Concentration
6.5.2 Adjustment for Wall Deflection
6.5.3 Adjustment for Mating Feature Deflection
6.5.4 Adjustments for Effective Angle
6.5.4.1 Effective Angle for the Insertion Face
6.5.4.2 Effective Angle for the Retention Face
6.5.5 Adjustments Summary
6.6 Assumptions for Analysis
6.7 Using Finite Element Analysis
6.8 Determine the Conditions for Analysis
6.9 Cantilever Hook Analysis for a Constant Rectangular Section Beam . . .
6.9.1 Section Properties and the Relation between Stress and Strain . . .
6.9.2 Evaluating Maximum Strain
6.9.2.1 Adjusting Maximum Allowable Strain for
Stress Concentrations
6.9.2.2 Calculating the Maximum Applied Strain in a Constant
Section Beam
6.9.2.3 Adjusting the Calculated Strain for Deflection
Magnification
6.9.3 Calculating Deflection Force
6.9.4 Adjusting for Mating Part/Feature Deflection
6.9.5 Determine Maximum Assembly Force
6.9.5.1 Determine the Effective Insertion Face Angle
6.9.6 Determine Release Behavior
6.9.6.1 Calculate the Effective Retention Face Angles
6.9.6.2 Calculate the Separation Forces
6.9.6.3 Other Retention Considerations
6.10 Cantilever Hook Tapered in Thickness
6.11 Cantilever Hook Tapered in Width
6.12 Cantilever Hook Tapered in Thickness and Width
6.13 Modifications to the Insertion Face Profile
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6.14 Modifications to the Retention Face Profile
6.15 Other Feature Calculations
6.16 Summary
6.16.1 Important Points in Chapter 6
7 The Snap-Fit Development Process
7.1
Introduction
7.1.1 Concept Development vs. Detailed Design
7.1.2 A General Development Process
7.2 The Snap-Fit Development Process
7.2.1 Is the Application Appropriate for a Snap-Fit? (Step 0)
7.2.2 Define the Application (Step 1)
7.2.3 Benchmark (Step 2)
7.2.3.1 Rules for Benchmarking
7.2.4 Generate Multiple Attachment Concepts (Step 3)
7.2.4.1 Select Allowable Engage Directions (Step 3.1)
7.2.4.2 Identify All Possible Assembly Motions (Step 3.2)
7.2.4.3 Engage Directions, Assembly Motions and
Worker Ergonomics
7.2.4.4 Select and Arrange Constraint Pairs (Step 3.3)
7.2.4.5 Add Some Enhancement Features (Step 3.4)
7.2.4.6 Select the Best Concept for Feature Analysis and
Detailed Design (Step 3.5)
7.2.5 Feature Analysis and Design (Step 4)
7.2.5.1 Lock Alternatives
7.2.6 Confirm the Design with Parts (Step 5)
7.2.7 Fine-Tune the Design (Step 6)
7.2.8 Snap-Fit Application Completed (Step 7)
7.3 Summary
7.3.1 Important Points in Chapter 7
8 Diagnosing Snap-Fit Problems
8.1
Introduction
8.1.1 Rules for Diagnosing Snap-Fit Problems
8.1.2 Mistakes in the Development Process
8.2 Attachment Level Diagnosis
8.2.1 Most Likely Causes of Difficult Assembly
8.2.2 Most Likely Causes of Distorted Parts
8.2.3 Most Likely Causes of Feature Damage
8.2.4 Most Likely Causes of Loose Parts
8.3 Feature Level Diagnosis
8.4 Summary
8.4.1 Important Points in Chapter 8
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9 Creating a Snap-Fit Capable Organization—Beyond
Individual Expertise
9.1 Introduction
9.2 Terminology
9.3 Harmful Beliefs
9.4 Suggested Initiatives
9.4.1 Initiatives for Getting Started
9.4.2 Initiatives for Developing Organizational Capability
9.5 The Snap-Fit Capability Plan
9.5.1 Vision, Mission, and Values
9.5.2 Objectives
9.5.2.1 Essential Objectives for Individual Capability
9.5.2.2 Recommended Objectives for a Snap-Fit
Capable Organization
9.5.3 Strategies
9.5.3.1 Near-Term Strategies
9.5.3.2 Long-Term Strategies
9.6 Details of the Initiatives
9.6.1 Provide Education and Training
9.6.2 Provide Technical Resources
9.6.3 Identify Low-Risk Applications as a Starting Point
9.6.4 Use Physical Models
9.6.5 Provide Benchmarking Opportunities
9.6.6 Include Snap-Fit Technical Requirements in the Bidding
and Purchasing Processes
9.6.7 Identify Intermediate Applications
9.6.8 Identify and Empower a Snap-Fit "Champion"
9.6.9 Identify and Empower a Snap-Fit "Technology Leader"
9.6.10 Make Snap-Fits Visible within the Organization
9.6.11 Link the Snap-Fit Effort to Other Business Strategies
9.6.12 Create and Maintain a Library of Preferred Concepts
9.6.13 Have a Model of the Snap-Fit Technical Domain
9.6.14 Reward Teamwork and Make Snap-Fits Interesting
9.6.15 Identify Supportive Customers and Suppliers
9.6.16 Initiatives Summary
9.7 Summary
9.7.1 Important Points in Chapter 9
9.7.2 Cautions
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Appendix A—Resources
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Index
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