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1、 3 The illustration above shows a photograph of two snap-fit models taken in polarized light; both have the same displacement (y) and deflective force (P). Top: The cantilever arm of unsatisfactory design has a constant cross section. The non-uniform distribution of lines (fringes) indicates a very
2、uneven strain in the outer fibers. This design uses 17% more material and exhibits 46% higher strain than the opti- mal design. Bottom: The thickness of the optimal snap- fit arm decreases linearly to 30% of the orig- inal cross-sectional area. The strain in the outer fibers is uniform throughout th
3、e length of the cantilever. ASnap Joints/General Common features Types of snap joints Comments on dimensioning BCantilever Snap Joints Hints for design Calculations Permissible undercut Deflection force, mating force Calculation examples CTorsion Snap Joints Deflection Deflection force DAnnular Snap
4、 Joints Permissible undercut Mating force Calculation example EBoth Mating Parts Elastic FSymbols Contents Page 2 of 26Snap-Fit Joints for Plastics - A Design Guide Common features Snap joints are a very simple, economical and rapid way of join-ing two different com- ponents. All types of snap joint
5、s have in common the principle that a protruding part of one component, e.g., a hook, stud or bead is deflected briefly during the joining opera- tion and catches in a depres-sion (undercut) in the mating component. After the joining operation, the snap-fit fea- tures should return to a stress-free
6、condition. The joint may be separable or inseparable depending on the shape of the undercut; the force required to separate the compo-nents varies greatly according to the design. It is particularly im-portant to bear the following factors in mind when designing snap joints: Mechanical load during t
7、he assembly operation. Force required for assembly. 4 Snap Joints General A Page 3 of 26Snap-Fit Joints for Plastics - A Design Guide 5 A Types of snap joints A wide range of design possi-bilities exists for snap joints. In view of their high level of flexibility, plastics are usually very suitable
8、materials for this joining technique. In the following, the many design possibili- ties have been reduced to a few basic shapes. Calculation principles have been derived for these basic designs. The most important are: Cantilever snap joints The load here is mainly flexural. U-shaped snap joints A v
9、ariation of the cantilever type. Torsion snap joints Shear stresses carry the load. Annular snap joints These are rotationally sym-metrical and involve multiaxial stresses. Page 4 of 26Snap-Fit Joints for Plastics - A Design Guide 6 Cantilever snap joints The four cantilevers on the control panel mo
10、d- ule shown in Fig. 1 hold the module firmly in place in the grid with their hooks, and yet it can still be removed when required. An economical and reliable snap joint can also be achieved by rigid lugs on one side in combination with snap-fitting hooks on the other (Fig. 2). This design is partic
11、ularly effective for joining two similar halves of a housing which need to be easily separated. The positive snap joint illus- trated in Fig. 3 can transmit considerable forces. The cover can still be removed easily from the chassis, however, since the snap-fit- ting arms can be re-leased by pressin
12、g on the two tongues in the direction of the arrow. The example shown in Fig. 4 has certain simi- larities with an annular snap joint. The pres- ence of slits, however, means that the load is predominantly flexural; this type of joint is therefore classified as a “cantilever arm“ for dimen-sioning p
13、urposes. Fig 4:Discontinuous annular snap joint Snap Joints/General A Page 5 of 26Snap-Fit Joints for Plastics - A Design Guide Torsion snap joints The tor-sion snap joint of the design shown for an instrument housing in Fig. 5 is still uncom- moninthermoplastics,despitethefactthatit,too, amounts to
14、 a sophisticated and economical join- ing method. The design of a rocker arm whose deflection force is given largely by torsion of its shaft permits easy opening of the cover under a force P; the torsion bar and snap-fitting rocker arm are integrally molded with the lower part of the housing in a si
15、ngle shot. Annular snap joints A typical application for annular snap joints is in lamp housings (Fig. 6). Here, quite small undercuts give joints of considerable strength. Fig 5: Torsion snap joint on a housing made of Makrolon polycarbonate Fig 6: A continuous annular snap joint offers a semi-herm
16、etic seal and is better for single assembly applications A Page 6 of 26Snap-Fit Joints for Plastics - A Design Guide 8 Combination of different snap joint systems The traffic light illus- trated in Fig. 7 is an example of an effective design for a functional unit. All the compo- nents of the housing are joined together by snap joint. Details: Housing and front access door engage at the fulcrum 1a. The lugs 1b (pressure
