In this study, the mechanics of snap-fit for automobile during assembly and separation process was studied. Two types of test machines were developed; micro-tensile tester and a gantry-type tester. The micro-tensile tester controlled displacement and measured force precisely, which was suitable for formal shaped small specimen. The gantry-type tester consisted of a gantry on a vibration isolation table. A hook was installed on a frame of the gantry, and matching mate was laid on the table, which was suitable for large sized arbitrary shaped mate component. In this test method, assembling motion was controlled by a manual handle, and force was measured by a load cell. The performance of the two testers was verified with similar sized snap-fit specimens. Two types of snap-fit were evaluated; trap type snap-fit and back side supported type snap-fit.
The trap type snap-fit showed an elastic behavior during the assembly process. However, it showed plastic behavior during the separation process due to an excessive compressive stress at the contacting surface of mate. The back side supported type snap-fit showed plastic behavior during both assembly and separation process.
An intuitive analytic formula to calculate the assembly force of trap type snap-fit was proposed. The force was derived from a force balance equation of an object placed on an inclined plane, accompanying with the stiffness of a hook obtained from a semi-numerical analysis.
Finite element analysis(FEA) was performed to analyze the assembly and separation force. Implicit dynamic analysis scheme was applied. Frictional contact conditions were considered. Fixed boundary condition was applied to mate part, and displacement condition was applied to hook part. It was found out that all assembly forces obtained from experimental measurement, finite element analysis, and analytic solution well agreed with each other.