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This is the third in a series of five articles on Snap-Fit design presented by BASF Engineering Plastics. Our intent is to help design engineers achieve better results through the use of improved principles and procedures for snap-fit design, application, processing and fabrication.
PART III. IMPROVING SNAP-FIT DESIGN:
IMPROVED CANTILEVER DESIGN
and GUIDELINES TO AVOID COMMON DIFFICULTIES
IMPROVED CANTILEVER DESIGN
The classical cantilever beam strain formulas discussed in Part II of “Improving Snap-Fit Design” work well when a flexible cantilever beam is anchored to a rigid wall, such as wood to stone. In such cases, deformation of the cantilever under a given load is the primary cause of movement at the tip of the cantilever. However, the typical plastic snap-fit design involves a snap-fit finger attached to a flexible wall – usually a plastics plate with thickness in the 3 mm range. When applied to these conventional plastic snap-fit designs, however, the classical formulas fail to account for the amount of deflection from the beam/wall interface. They simply neglect the effect of deformation in the wall itself – a factor that becomes more significant with somewhat more flexible wall materials, such as wood, composites and thermoplastics. The classical formula does predict deflection fairly well when a beam length-to-thickness ratio is greater than 10:1, but the calculated deflection deviates further from actual values as the ratio gets smaller or when the beam, in other words, gets “stubbier.”
To obtain a more accurate prediction of the total allowable deflection for short beams in snap-fits, the design engineer must apply a magnification factor to compensate for the classical formulas’ shortcomings. Doing so allows the design engineer to take full advantage of a material’s strain-carrying capability and, therefore, to enjoy greater design flexibility.
Magnification Factors
BASF Plastics has developed a method for determining these magnification factors for a variety of snap-fit beam/wall configurations, including beams with either uniform or tapered cross sections (graphically depicted in Figures IV-1 and IV-2).
BASF has verified the results of this method both by finite element analysis and actual part testing.
 Improved Formulas
To determine maximum strain at a given snap-fit’s base, design engineers can now use the improved formula which incorporates a corrective magnification factor (See Figure IV-3). BASF also provides formulas for determining the Push-on and Perpendicular Mating Forces (Figure IV-3).
To review examples that employ these formulas to determine accurate solutions for two different snap-fit designs, see Figures IV-4 and IV-5. The first example incorporates allowable strain and coefficient of friction values for a specific grade of BASF Ultradur® PBT; the second example, for a grade of BASF Ultraform® POM. Refer to BASF’s Snap-Fit Design Manual Page IV-4 for tables providing allowable strain and coefficient of friction values for eight different materials appropriate for snap-fit designs.
U-shaped and L-shaped Designs
Design engineers interested in the application of these formulas to U-shaped and L-shaped snap-fit designs – and a more detailed discussion – can refer to BASF’s Snap-Fit Design Manual, Pages V-1, -2, -3 and -4.
Conclusion
The formulas developed by BASF represent a significant improvement over the classical cantilever beam deflection and strain formulas. Working with BASF’s improved formulas, design engineers can now more accurately calculate and predict the forces encountered – and allowable deflection limits – for different configurations of cantilever-type snap-fit assemblies. With the greater degree of certainty these formulas provide, design engineers can get the most of their materials and their designs.
GUIDELINES TO AVOID COMMON DIFFICULTIES
Before finalizing any snap-fit design, the design engineer should review three basic considerations:
• Stress Concentration
• Creep/Relaxation
• Fatigue.
The single most common cause of failure in snap-fits is the concentration of stress caused by a sharp corner between the snap-fit beam and the wall to which it is attached – the normal point of maximum stress. One solution is to incorporate a generous fillet radius at the juncture between the beam and the wall, either on both sides of the beam or particularly on the beam’s tensile stress side (See Figure VI-1).
Another cause of failure is that, over time, creep and stress relaxation can gradually reduce the retention force between the two components connected by a snap-fit. In some cases, lower retention force can lead to leakage from a formerly tight seal; in others, excessive play that creates noise and vibration (or “BSR” - Buzz, Squeak and Rumble). Ways to minimize the effects of creep and stress relaxation include designing a low-stress snap beam, incorporating a 90° return angle to avoid bending, or increasing the land length in the area of a large return angle (See Figures VI-2 and VI-3).
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The third major cause of snap-fit failure is fatigue, or repetitive loading, primarily in applications involving hundreds or thousands of cycles. The first, most obvious, way to avoid fatigue failure is to choose a material known to perform well in fatigue situations. Compare different materials’ S-N curves, which show expected number of cycles to failure at various stress levels and at different temperatures. A second way based on S-N curves is to select a sufficient design stress level at the application’s correct exposure temperature. In the real application, the frequency of cycles is usually a lower number than in testing, thereby providing a margin of safety for the design.
Conclusion
There are a number of ways to overcome stress concentration, stress relaxation or creep, and fatigue. A well thought-out design, plus the right choice of material, will allow your application to benefit from all the advantages of snap-fit design.
“Materials Selection”
Part IV, “Materials Selection”, of the five-part series “Improving Snap-Fit Design” will provide an overview of materials appropriate for snap-fit design and guidelines for selecting the material best suited to your applications. The complete series of articles includes
Part I. Introduction and Overview of
General Applications and Types
Part II. Principles of Classical Beam Theory and Design
Part III. Improved Cantilever Design
and Guidelines to Avoid Common Difficulties
Part IV. Materials Selection Coming soon!
Part V. Processing Considerations Coming soon!
Additional information is available through the following links:
BASF Plastics Snap-Fit Design Manual
BASF Plastics Snap-Fit Design Calculator
BASF Plastics Seminar: Part Design - Assembly of Components (recorded course)
This seminar is for engineers and designers involved in the design of injection molded components. The course provides an overview of various assembly methods of plastic-to-plastic components and plastic-to-metal components. Techniques covered are snap-fits, self-threading fasteners, welding, and press-fitting. Advantages and limitations of each will be discussed in detail, as well as several examples with actual design calculations. This seminar will help educate the engineer/designer about which assembly method to use for their particular application, allowing them to design a cost-effective and efficient joint the first time.
For additional information or to ask questions about Snapfit Design, contact:
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