{"id":1228,"date":"2007-06-13T14:43:38","date_gmt":"2007-06-13T20:43:38","guid":{"rendered":"https:\/\/www.ulprospector.com\/knowledge\/?p=1228"},"modified":"2018-02-28T14:35:29","modified_gmt":"2018-02-28T20:35:29","slug":"pe-snapfit-2","status":"publish","type":"post","link":"https:\/\/ulprospector.ul.com\/1228\/pe-snapfit-2\/","title":{"rendered":"Improving Snapfit Design (Part 2)"},"content":{"rendered":"

Paid content by BASF<\/h4>\n

By Chul Lee<\/a>, BASF<\/strong><\/p>\n

Read Improving Snapfit Design (Part 1)<\/a><\/strong><\/p>\n

PART II – IMPROVING SNAP-FIT DESIGN: CLASSICAL BEAM THEORY AND SNAP-FIT DESIGN<\/strong><\/p>\n

\"snapfitpt2-1\"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.\u00a0 Before arriving at the desired balance of snap-fit properties, it is not unusual for the designer to go through several iterations \u2013 changing length, thickness, deflection dimensions and other factors.<\/p>\n

The typical snap-fit assembly consists of a cantilever beam with an overhang at its end (Figure 1).\u00a0 The depth of overhang defines the amount of deflection during assembly.\u00a0 Modifying the angles on the entrance and retraction sides of the overhang can optimize, respectively, assembly and disassembly forces (Figure 2 Mating Force).<\/p>\n

\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 Assembly Integrity and Beam Strength<\/strong><\/p>\n

\"snapfitpt2-2\"The stiffness of the beam and the amount of deflection required for assembly or disassembly determine the integrity of the structure.\u00a0 A designer can increase the beam’s rigidity by using either a higher modulus material or a thicker cross section.\u00a0 The product of these two parameters determines total rigidity of a given beam length.<\/p>\n

Increasing the overhang depth can also improve the integrity of assembly.\u00a0 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.<\/p>\n

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.<\/p>\n

Cantilever Beam: Deflection-Strain Formulas
\n<\/strong>Assembly and disassembly forces increase with both stiffness (k) and maximum deflection of the beam (Y).\u00a0 The force (P) required to deflect the beam is proportional to the product of the two factors:<\/p>\n

\u00a0\u00a0\u00a0\u00a0 P = kY<\/p>\n

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.<\/p>\n

Designers should be careful when selecting the flexural modulus of elasticity (E) for hygroscopic (moisture absorbing) materials, e.g., polyamide.\u00a0 In the dry-as-molded (DAM) state, datasheet values are valid to calculate stiffness, deflection or retention force of a snap-fit design.\u00a0 However, physical properties decrease under normal 50% relative humidity conditions.\u00a0 Therefore, stiffness and retention force can decline while deflection increases.\u00a0 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.<\/p>\n

\"snapfitpt2-4\"Improved Formulas in Part III
\n<\/strong>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.\u00a0 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.<\/p>\n

Part III of the five-part series “Improving Snap-Fit Design” will combine the subject “Improved Cantilever Design” with “Guidelines to Avoid Common Difficulties.”\u00a0 The complete series of articles includes:<\/p>\n

Part I\u00a0\u00a0 \u00a0Introduction and Overview of General Applications and Types<\/a>
\nPart II\u00a0\u00a0 \u00a0Principles of Classical Beam Theory and Design
\nPart III\u00a0\u00a0 \u00a0Improved Cantilever Design and Guidelines to Avoid Common Difficulties<\/a>
\nPart IV\u00a0\u00a0 \u00a0Materials Selection<\/a><\/p>\n

 <\/p>\n

 <\/p>\n

<\/a><\/p>\n

About the Author<\/strong><\/h4>\n\n\n\n
Chul Lee, Applications Technology Leader
\nBASF
\nEmail: chul.lee@basf.com<\/a>
\nWebsite: http:\/\/www.basf.com\/group\/corporate\/en\/<\/a><\/td>\n		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\u2019s 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\u2019s 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\u2019s 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\u2019s 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.<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n","protected":false},"excerpt":{"rendered":"

Paid content by BASF By Chul Lee, BASF Read Improving Snapfit Design (Part 1) 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 … Continued<\/a><\/p>\n","protected":false},"author":30,"featured_media":1229,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_monsterinsights_skip_tracking":false,"_monsterinsights_sitenote_active":false,"_monsterinsights_sitenote_note":"","_monsterinsights_sitenote_category":0,"episode_type":"","audio_file":"","podmotor_file_id":"","podmotor_episode_id":"","cover_image":"","cover_image_id":"","duration":"","filesize":"","filesize_raw":"","date_recorded":"","explicit":"","block":"","itunes_episode_number":"","itunes_title":"","itunes_season_number":"","itunes_episode_type":"","footnotes":""},"categories":[27,21],"tags":[39,274],"ppma_author":[1250],"class_list":{"0":"post-1228","1":"post","2":"type-post","3":"status-publish","4":"format-standard","5":"has-post-thumbnail","7":"category-design","8":"category-plastics-2","9":"tag-basf","10":"tag-design","11":"entry"},"yoast_head":"\nImproving Snapfit Design (Part 2) - 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