New designers of plastic parts wishing to use snap-fit had nowhere to turn unless they were fortunate enough to have access to an experienced snap-fit designer. This book organizes and presents all design aspects of snap-fits with an emphasis on the systems level thinking required to create world-class attachments. Beginning, as well as experienced, product designers will find the information they need to develop snap-fits more efficiently and avoid many common snap-fit problems.
The third edition has been thoroughly revised to include new case histories and applications. The text has been extensively rewritten for clarity and user-friendliness and there are many new figures with expert explanations.
Paul R. Bonenberger is President of FasteningSmart, Inc., a position he has held since 2006. He is an widely recognized expert on mechanical attachments, especially those involving threaded fasteners or snap-fits. He consults on design solutions, problem diagnosis, patents, and technology management issues, and undertakes training courses in this area. Previously, he spent 37 years at General Motors as Staff Project Engineer specializing in mechanical attachments. He holds degrees from Oakland University, University of Detroit-Michigan, and Kettering University (previously General Motors Institute).
Paul R. Bonenberger is President of FasteningSmart, Inc., a position he has held since 2006. He is an widely recognized expert on mechanical attachments, especially those involving threaded fasteners or snap-fits. He consults on design solutions, problem diagnosis, patents, and technology management issues, and undertakes training courses in this area.Previously, he spent 37 years at General Motors as Staff Project Engineer specializing in mechanical attachments. He holds degrees from Oakland University, University of Detroit-Michigan, and Kettering University (previously General Motors Institute).
Foreword to Third Edition 7
Preface to Third Edition 9
Foreword to Previous Editions 11
Prefaces to Previous Editions 13
Preface to First Edition 13
Preface to Second Edition 16
Contents 17
1 Introduction 25
1.1 Reader Expectations 26
1.2 Harmful Beliefs 27
1.3 Snap-Fit Technology 28
1.4 Snap-Fits and Loose Fasteners 30
1.5 Snap-Fits as Interface Systems 30
1.5.1 Feature Level 31
1.5.2 Attachment Level 31
1.6 The Attachment Level Construct© (ALC) 33
1.6.1 Attachment Level Terminology 33
1.6.2 Applying the ALC to other Attachment Methods 34
1.6.3 Required Capabilities for Snap-Fit Development 34
1.6.4 Justifying the ALC 35
1.7 Using This Book 36
1.71 Sample Parts 38
1.7.2 Snap-Fit Novices 39
1.7.3 Experienced Product Developers 40
1.7.4 Design for Assembly/Manufacturing Practitioners 40
1.7.5 Executives and Engineering Managers 41
1.8 Summary 41
2 Key Requirements 43
2.1 Constraint 43
2.2 Compatibility 45
2.3 Robustness 48
2.4 Strength 48
2.5 Summary 50
3 Introduction to the Snap-Fit Development Process 53
3.1 Concept vs. Detailed Design 54
3.2 The Value of Multiple Concepts 55
3.3 Step 0: Is a Snap-Fit Appropriate? 56
3.4 The Demand-Complexity Matrix© 60
3.5 Summary 62
4 Descriptive Elements 65
4.1 Function 65
4.1.1 Action 66
4.1.2 Purpose 67
4.1.3 Retention 67
4.1.4 Release 68
4.2 Basic Shapes 69
4.2.1 Mating-Part and Base-Part 69
4.2.2 Basic Shape Descriptions 70
4.2.3 Basic Shape Combinations 71
4.3 Engage Direction 74
4.4 Assembly Motion 76
4.5 Summary 78
5 Physical Elements: Locators 79
5.1 Protrusion-Based Locators 80
5.1.1 Pins 80
5.1.2 Prongs 81
5.1.3 Tabs 82
5.1.4 Lugs 82
5.1.5 Tracks 82
5.1.6 Cones 83
5.1.7 Wedges 83
5.1.8 Catches 84
5.2 Surface-Based Locators 84
5.2.1 Surfaces 84
5.2.2 Edges 85
5.2.3 Lands 85
5.3 Void-Based Locators 85
5.3.1 Holes 86
5.3.2 Slots 86
5.3.3 Cutouts 86
5.4 Living Hinges 87
5.5 Using Locators 87
5.5.1 Locator Pairs 87
5.5.2 Providing Constraint 89
5.5.3 Assembly Motion and Strength 90
5.5.4 Fine-Tuning 92
5.5.5 Dimensional Robustness 93
5.5.5.1 Positioning 93
5.5.5.2 Compliance 95
5.5.5.3 Datum Points 96
5.5.6 Constraint Efficiency 96
5.5.7 Mechanical Advantage and Stability 97
5.5.8 Ease of Assembly 98
5.6 Summary 98
6 Physical Elements: Locks 101
6.1 Lock Deflection and Separation Behavior 102
6.2 Lock Styles 104
6.3 Cantilever Beam Locks 105
6.3.1 Hooks 108
6.3.1.1 Hook Assembly Behavior 110
6.3.1.2 Hook Separation Behavior 112
6.3.1.3 Hooks and Retainers 115
6.3.1.4 Hooks and Prongs 116
6.3.2 Loops 117
6.3.2.1 Loop Assembly Behavior 118
6.3.2.2 Loop Separation Behavior 119
6.3.2.3 Loops and Knit Lines 120
6.3.3 Traps 122
6.3.3.1 Trap Assembly Behavior 125
6.3.3.2 Trap Separation Behavior 125
6.3.4 Low Deflection Lugs 127
6.3.5 Other Cantilever Beam Locks 128
6.4 Planar Locks 129
6.5 Torsional Locks 131
6.6 Annular Locks 131
6.7 Using Locks 132
6.7.1 Lock Pairs 132
6.7.2 Short Grip-Length and Low-Clearance Applications 133
6.7.3 High Demand Applications 134
6.7.4 Tamper Resistant Applications 135
6.7.5 The Case against Cantilever Hooks 135
6.8 Summary 137
7 Lock Strength and Decoupling 141
7.1 Level 0 No Decoupling 143
7.2 Level 1 Decoupling 144
7.3 Level 2 Decoupling 145
7.4 Level 3 Decoupling 148
7.5 Level 4 Decoupling 149
7.6 Summary 154
8 Constraint in Snap-Fit Applications 157
8.1 Perfect Constraint 158
8.2 Proper Constraint 160
8.3 Under-Constraint 161
8.4 Over and Improper Constraint 163
8.4.1 Redundant Constraint Features 164
8.4.2 Opposing Constraint Features 165
8.5 The Constraint Worksheet 169
8.6 Using the Constraint Worksheet 175
8.7 Constraint Rules 180
8.8 Summary 181
9 Physical Elements: Enhancements 183
9.1 Assembly Enhancements 184
9.1.1 Guides 185
9.1.2 Clearance 187
9.1.3 Pilots 188
9.1.4 Example – Switch Application 189
9.1.5 Example: Reflector Application 192
9.1.6 Feedback 196
9.2 Activation Enhancements 200
9.2.1 Visuals 200
9.2.2 Assists 203
9.2.3 User-Feel 204
9.3 Performance Enhancements 206
9.3.1 Guards 206
9.3.2 Retainers 207
9.3.3 Compliance 208
9.3.3.1 Local Yield 209
9.3.3.2 Elasticity 211
9.3.3.3 Isolators 211
9.3.4 Back-Up Features 211
9.4 Manufacturing Enhancements 213
9.4.1 Process-Friendly Design 214
9.4.2 Fine-Tuning Enablers 217
9.5 Summary 221
10 Applying the Snap-Fit Development Process 227
10.1 Step 1: Define the Application 228
10.2 Step 2: Benchmark 230
10.3 Step 3: Generate Multiple Concepts 234
10.3.1 Engage Direction 235
10.3.2 Assembly Motions 236
10.3.3 Identify Constraint Pairs 239
10.3.4 Add Some Enhancements 244
10.3.5 Select a Concept for Analysis 245
10.4 Step 4: Design and Analyze Features 246
10.4.1 Lock Alternatives 247
10.4.1.1 Threaded Fasteners 247
10.4.1.2 Plastic Push-In Fasteners 249
10.4.1.3 Spring-Steel Clips 250
10.5 Step 5: Confirm Design with Parts 251
10.6 Step 6: Fine-Tune the Design 254
10.7 Step 7: Snap-Fit Application Completed 255
10.8 Summary 255
11 Feature Development: Material Properties 257
11.1 Sources of Material Property Data 257
11.2 Material Property Assumptions 258
11.3 The Stress-Strain Curve 259
11.4 Determining a Design Point 263
11.4.1 Applications with Fixed Strain 263
11.4.2 Applications with Variable Strain 264
11.4.3 The Secant Modulus 266
11.4.4 Maximum Permissible Strain Data 266
11.5 Coefficient of Friction 268
11.6 Other Effects on Material Properties 270
11.7 Summary 273
12 Lock Feature Development: Rules-of-Thumb 275
12.1 Beam-Based Locks 275
12.1.1 Beam Thickness at the Base 277
12.1.2 Beam Length 279
12.1.3 Beam Thickness at the Retention Feature 280
12.1.4 Beam Width 281
12.2 Retaining Member: Catch 283
12.2.1 The Insertion Face 283
12.2.2 The Retention Face 284
12.3 Loops 286
12.4 Traps 287
12.5 Other Lock Styles 289
12.5.1 Torsional Locks 289
12.5.2 Planar Locks 289
12.5.3 More Lock Styles 290
12.6 Summary 292
13 Lock Feature Development: Calculations 293
13.1 Assumptions and Allowances 294
13.2 The Deflecting Member: Cantilever Beam 296
13.2.1 General Equations for Rectangular Sections 297
13.2.2 Constant Section Beam Bending 298
13.2.3 Adjusting the Design Strain for Stress Concentration 301
13.2.4 Calculating the Initial Beam Strain 303
13.2.5 Adjusting for Deflection at the Beam’s Base 303
13.2.6 Calculating the Initial Beam Deflection Force 307
13.2.7 Adjusting for Mating Feature/Part Deflection 307
13.2.8 Example Beam Strain and Deflection Calculations 309
13.2.9 Deflection Graphs for a Straight Beam 316
13.3 Deflecting Member: Tapered Beams 320
13.3.1 Taper Error Example 321
13.3.2 Beams Tapered in Thickness 323
13.3.3 Beams Tapered in Width 328
13.4 Beam Calculation Summary 331
13.5 Other Deflecting Member Styles 332
13.5.1 Other Beam-Based Styles: Loops and Traps 332
13.5.2 Other Styles: Torsional, Annular, and Planar Deflection 334
13.6 The Retaining Member: Catch 335
13.6.1 Lock Assembly Force 336
13.6.1.1 Adjusting for the Insertion Face Effective Angle 336
13.6.1.2 Example Assembly Force Calculations 338
13.6.1.3 Modifying the Insertion Face Profile 339
13.6.2 Catch Separation Force 343
13.6.2.1 Adjusting for the Retention Face Effective Angle 343
13.6.2.2 Example Assembly Force Calculations 345
13.6.2.3 Modifying the Retention Face Profile 347
13.7 Stationary Catches and Traps as Retaining Members 349
13.7.1.1 Other Separation Considerations 352
13.8 Using Finite Element Analysis 353
13.9 Calculation Spreadsheets 354
13.10 Summary 357
14 Diagnosing Snap-Fit Problems 361
14.1 Common Snap-Fit Mistakes 363
14.2 Attachment Level Diagnosis 364
14.3 Feature Level Diagnosis 365
14.4 Summary 371
15 Gaining a Competitive Advantage in Snap-Fit Technology 373
15.1 Terminology 375
15.2 Managing Expectations 376
15.3 Harmful Beliefs 377
15.4 The Demand-Complexity Matrix 379
15.5 The Snap-Fit Capability Plan 384
15.5.1 Vision, Mission, and Values 385
15.5.2 Objectives 385
15.5.3 Strategies 385
15.6 Initiatives for Getting Started 387
15.6.1 Provide Education and Training 388
15.6.2 Provide Technical Resources 388
15.6.3 Identify Low-Impact Applications as a Starting Point 388
15.6.4 Use Physical Models 389
15.6.5 Provide Benchmarking Opportunities 389
15.6.6 Include Snap-Fit Technical Requirements in the Bidding and Purchasing Processes 390
15.6.7 Identify Intermediate Applications 392
15.7 Initiatives for Organizational Capability 393
15.7.1 Identify and Empower a Snap-Fit Champion 393
15.7.2 Identify and Empower a Snap-Fit Technical Leader 393
15.7.3 Make Snap-Fit Technology Visible in the Organization 394
15.7.4 Link Snap-Fits to Other Business Strategies 394
15.7.5 Create and Maintain a Library of Preferred Concepts 394
15.7.5.1 Example of a Preferred Concepts Initiative 396
15.7.6 Have a Model of the Snap-Fit Technical Domain 399
15.7.7 Reward Teamwork and Make Snap-Fits Interesting 399
15.7.8 Identify Supportive Customers and Suppliers 399
15.8 Summary 400
Appendix – Resources 403
About the Author 407
Index 409
| Prefaces to Previous Editions |
Preface to First Edition
This book is a reference and design handbook for the attachment technology called snap-fits or sometimes, integral attachments. Its purpose is to help the reader apply snap-fit technology effectively to plastic applications. To do this, it arranges and explains snap-fit technology according to an Attachment LevelTM knowledge construct. The book is intended to be a user-friendly guide and practical reference for anyone involved with plastic part development and snap-fits. It is called The First Snap-Fit Handbook for two reasons: I believe it is the first book written that is devoted exclusively to snap-fits. I also hope it leads to increased interest and more books on the subject.
The reader should consider this book to be a “good start” in the ongoing process of understanding and organizing snap-fit technology. There is much more to be done, but one must begin somewhere. Although the original “attachment level” construct (created in 1990 and 1991) has proven to be fairly robust and complete, many details have evolved over the years as I learned more about the topic. The construct will continue to evolve and I encourage and welcome reader’s comments on the subject; they will certainly help in the process.
My interest in the subject of snap-fits grew out of a very real need at General Motors. As a long-time fastening expert, I had typically been involved with threaded fasteners and traditional mechanical attachments. In the late 1980s and early 90s, as GM embraced design for manufacturing and assembly, the philosophies of Dr. Geoffrey Boothroyd and Dr. Peter Dewhurst [Product Design for Manufacture and Assembly, 1988, G. Boothroyd and P. Dewhurst, Department of Industrial and Manufacturing Engineering, University of Rhode Island, Kingston, RI] were formally adopted as the corporate direction, and were rolled out in a series of intensive training/workshop sessions. As a result, product designers and engineers began looking for alternatives to traditional loose fasteners, including threaded fasteners. Snap-fit attachments immediately became popular but we soon discovered that there was little design information available in the subject. Calculations for cantilever hook performance could be found in many supplier design guides or as software, but beyond that, no general snap-fit attachment expertise was captured in design or reference books. GM needed to bootstrap itself to a level of snap-fit expertise that was not written down anywhere. An intensive study of snap-fit applications resulted and eventually patterns of good design practices began to emerge. A “systems level” understanding of snap-fit attachments began to grow.
I called this systems level organization of snap-fits attachment level to emphasize its focus on the interface as a whole and to distinguish it from the traditional feature level approach. I have been teaching about snap-fits according to this attachment level model since 1991. The reaction after each class has been that attendees had indeed reached a new or better understanding of snap-fits. I trust and hope this book will have the same results for the reader.
The Attachment Level Construct (ALC) was only a personal vision in 1990. I believed it had potential and that it represented a unique approach to understanding snap-fit applications but I needed much more than that to make it reality. I needed verification that I was not just reinventing or paraphrasing some existing but obscure snap-fit design practices; an extensive literature search verified that systems-level snap-fit practices were not documented anywhere. I also needed impartial validation that the model was indeed useful and worth pursuing. A colleague, Mr. Dennis Wiese, who was Manager of the Advanced Product Engineering Body Components Group at that time, provided that initial validation. He also gave moral support and generously provided resources including his own engineers and significant amounts of his own time for debate and discussion of the fledgling snap-fit design methodology. Those discussions, sometimes lively and always useful, drove the insights that helped shape the original attachment level model. Dennis was certainly the midwife of the attachment level approach and I cannot thank him enough for his help. Other GM people involved with the infant methodology included Florian Dutke, Tom Froling, Daphne Joachim, Colette Kuhl, Chris Nelander, Tom Nistor, Tim Rossiter, and Teresa Shirley.
Finally, Mike Carter, of GM University, deserves special thanks because in the early 1990s he asked me, what are you fastening guys going to do about too many loose fasteners in our products? That question was the beginning of my involvement with design for assembly. Mike, this book is your answer.
As pressure of other work grew, the development team dwindled back to one (me). In 1992, Tony Luscher, the project manager of a planned snap-fit program at Rensselaer Polytechnic Institute (RPI), and I learned of each other’s work and made contact (once again, thanks to Mike Carter). The RPI program was originally designed around feature level research but Tony enthusiastically embraced the concept of attachment level thinking. Tony, with the concurrence of Dr. Gary Gabrielle, the project leader, modified the RPI program to include some aspects of the attachment level method. Tony’s technical insights, contributed during many hours of personal discussion and through exchange of correspondence, helped drive more refinements to the method. Under his guidance, some work to apply and extend the methodology occurred under the RPI program. Tony is now a professor at the Ohio State University and he has carried his interest and enthusiasm for the subject to his new position. Tony and I shared a longterm vision for snap-fit technology: that attachment level thinking will lead to evolution of the snap-fit design and development process from an art to an engineering science.
The original motivation for the attachment level work was to provide support for Design for Manufacturing and Design for Assembly initiatives at General Motors. Joe Joseph, then the Director of the GM DFM Knowledge Center, supported my early efforts by providing a site for snap-fit training classes. This also gave the kind of validation needed to justify continued efforts to develop the methodology. Joe is now Dean of the Engineering College of the GM University and he continues to provide valued moral support. The patience and support of Jim Rutledge, Dave Bubolz, and Roger Heimbuch is also greatly appreciated. They provided an environment in which ongoing development work could flourish and gave me much encouragement. Tony Wojcik also deserves thanks because he first sent a publisher my way. That marked the beginning of the snap-fit book project.
I must also acknowledge the creative people who designed and developed the numerous snap-fit applications that I have studied. In products from around the world, the level of cleverness and creativity evident in many snap-fits is truly impressive. My admiration for and fascination with these designs helped to drive the original ideas behind the Attachment Level Construct in the following manner:
-
Observation: There are many clever, well-designed, and complex snap-fit applications in existence; there are also many poor snap-fits.
-
Hypothesis: Many snap-fit designers must possess tacit knowledge that allows them to develop good snap-fits; others do not.
-
Problem: Snap-fit application design information could not be found as documented knowledge. Principles of good snap-fit application design were not written down anywhere.
-
Solution: Discover the information and define it. Study successful snap-fit applications and look for patterns of good design practices. Capture and organize the concepts behind good snap-fit design.
-
Result: A deep understanding of snap-fit concepts and principles organized in a knowledge construct.
I cannot claim credit for the clever snap-fit applications I describe here. Most were found on existing products or inspired by products. I simply interpreted them, inferred a logical process by which they could have been developed, and organized everything I found into a knowledge structure. The only new “invention” here is the construct itself. Hopefully, it will inspire readers to create their own product inventions.
My wife and son have provided endless encouragement and understanding through the long process of writing this book, putting up with my long hours at the computer, and tolerating (barely) my monopolization of same.
With thanks and appreciation to all.
Rochester, Michigan 1999
Paul Bonenberger
Preface to Second Edition
The first edition of this book introduced a systematic way of thinking about snap-fit attachments. By intent, it did not spend a lot of time or space on calculations of feature behavior because this information was available elsewhere. That information is still available in various resources, including online sources; therefore, no new calculations have been added. However, equations for locking feature analysis are available online. The reader should check Appendix A for resources providing snap-fit feature calculations.
This second edition is an opportunity to add clarification and more detail in some areas. Most significantly, a new chapter, “Creating a Snap-Fit Capable Organization — Beyond Individual Expertise” has been added. This chapter is targeted primarily toward engineering executives and managers. It explains how engineering...
| Erscheint lt. Verlag | 16.1.2017 |
|---|---|
| Verlagsort | München |
| Sprache | englisch |
| Themenwelt | Naturwissenschaften ► Chemie |
| Technik | |
| Schlagworte | Konstruieren • Kunststoffe • Kunststofftechnik • SPE • Verbindungstechnik |
| ISBN-10 | 1-56990-679-3 / 1569906793 |
| ISBN-13 | 978-1-56990-679-8 / 9781569906798 |
| Haben Sie eine Frage zum Produkt? |
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