By Chul Lee, BASF
PART II – IMPROVING SNAP-FIT DESIGN: CLASSICAL BEAM THEORY AND SNAP-FIT DESIGN
In every snap-fit application, the design engineer’s main challenge is to find the optimal balance between integrity of the assembly and strength of the cantilever beam, i.e., how it fits and stays together, and how it supports a structure joined to a wall. Before arriving at the desired balance of snap-fit properties, it is not unusual for the designer to go through several iterations – changing length, thickness, deflection dimensions and other factors.
The typical snap-fit assembly consists of a cantilever beam with an overhang at its end (Figure 1). The depth of overhang defines the amount of deflection during assembly. Modifying the angles on the entrance and retraction sides of the overhang can optimize, respectively, assembly and disassembly forces (Figure 2 Mating Force).
Assembly Integrity and Beam Strength
The stiffness of the beam and the amount of deflection required for assembly or disassembly determine the integrity of the structure. A designer can increase the beam’s rigidity by using either a higher modulus material or a thicker cross section. The product of these two parameters determines total rigidity of a given beam length.
Increasing the overhang depth can also improve the integrity of assembly. However, as beam deflection increases in reaction to greater overhang depth, beam stress also rises. Failure results if the working stress exceeds the strength limit of the beam material.
Optimizing beam section geometry is one way to ensure that the deflection required for assembly integrity can be reached without exceeding the strength or strain limit of the beam material.
Cantilever Beam: Deflection-Strain Formulas
Assembly and disassembly forces increase with both stiffness (k) and maximum deflection of the beam (Y). The force (P) required to deflect the beam is proportional to the product of the two factors:
P = kY
The Deflection-Strain Formulas in Figure 3 illustrate the effect of different beam section geometries on stiffness, as well as the effect of deflection on beam stress or strain.
Designers should be careful when selecting the flexural modulus of elasticity (E) for hygroscopic (moisture absorbing) materials, e.g., polyamide. In the dry-as-molded (DAM) state, datasheet values are valid to calculate stiffness, deflection or retention force of a snap-fit design. However, physical properties decrease under normal 50% relative humidity conditions. Therefore, stiffness and retention force can decline while deflection increases. Both scenarios require verification and the lower values should be used in evaluating the structural performance of the assembly. This will ensure a built-in safety factor.
Improved Formulas in Part III
Classical cantilever beam formulas work well with the most rigid materials, such as stone and metal. When applied to thermoplastic snap-fit designs, however, the same formulas overestimate the amount of strain at the beam/wall interface because they do not consider the deformation in the wall itself. The next article in this series Part III – “Improved Cantilever Design” will present new formulas that incorporate the effects of wall deformation. The new formulas provide engineers the information they need to arrive at the optimal balance of snap-fit design variables, which produces the most successful snap-fit assemblies.
Part III of the five-part series “Improving Snap-Fit Design” will combine the subject “Improved Cantilever Design” with “Guidelines to Avoid Common Difficulties.” 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
About the Author
|Chul Lee, Applications Technology Leader
|Chul Lee, an Applications Technology Leader at BASF, has been involved with plastics applications and developmental activities for the past three decades. Working out of BASF Corporation’s Wyandotte, Michigan facility, Chul has a wealth of experience in various areas of plastics research, including service life prediction research for the plastics pipeline industry, and application of CAE technology for plastics product development. Chul’s current endeavors in the field are focused on the development of plastics joining technology, such as vibration welding, laserwelding and mechanical fastening.During the last 10 years, Chul has been actively involved in applications development work pertaining to the automotive and industrial market segments. Concentrating on application developments for the automotive powertrain market, including plastic air intake manifold developments and oil supply systems, Chul has coordinated BASF Global Research on Joining Technologies, such as vibration welding, laserwelding, snap fits, mechanical fastening and adhesive joining. Results of Mr. Lee’s research is often disseminated both internally and in the public domain by virtue of participation in seminars and publication distribution at major technical conferences.Chul, a Ph.D. graduate from the University of Michigan’s Mechanical Engineering department, has published many papers on various technical development topics. His academic specialization in the field of Fracture Behavior of Plastics Materials enables his astute research and subsequent publication of a multiplicity of topics, ranging from Snap Fit Design, Ribbing Design Optimization, Vibration welding optimization, and NVH (Noise Vibration and Harshness) experiments and simulations of air intake manifolds, and Laserwelding. Chul is also responsible for several patents, such as a patent on NVH, multiple patents on vibration welding, and currently has a patent pending on stone impact resistant ribbing design.|
The views, opinions and technical analyses presented here are those of the author, and are not necessarily those of UL, ULProspector.com or Knowledge.ULProspector.com. While the editors of this site make every effort to verify the accuracy of its content, we assume no responsibility for errors made by the author, editorial staff or any other contributor. All content is subject to copyright and may not be reproduced without prior authorization from Prospector.