Optimizing rubber boot design with FEA

22
Feb

Rubber boots are used in many industries to protect flexible connections between two bodies. To perform well, boots need to walk a fine line between being too stiff and not stiff enough – they should have enough stiffness to retain their shape, but not be so stiff that they interfere with the flexible connection.

In the automotive industry, “constant-velocity” joints on drive shafts are usually sealed with rubber boots in order to keep dirt and moisture out. These rubber boots are designed to accommodate the maximum possible swing angles at the joint, and to compensate for changes in the shaft length. One of the design challenges is ensuring that during bending and axial movements, the individual bellows of the boot do not come into contact with each other. Contact would result in abrasions and wear when the shaft rotates, which would ultimately lead to the premature failure of the joint. Bellows shape optimization, fatigue life, maintainability and replaceability, and cost are also important design goals.

FEA of rubber boots offers many interesting features: (1) large displacements; (2) large strains; (3) incompressible material behavior; (4) susceptibility to local buckling; and (5) varying boundary conditions caused by the 3-D contact between various parts of the boot.

panel aPanel bPanel cPanel d

This example (figures a-d) shows the analysis of the axial compression and bending of a rubber boot. The boot is clamped on one side to a rigid surface, and on the other side to a translating and rotating shaft. Axial compression is first applied (figure b), followed by bending (figures c-d). The Cauchy stress contours on the deformed shapes are shown for the axial compression and rotation of the shaft. Once in place, the shaft rotates and the boot must rotate about the axis of the shaft in the tilted position.

A leading U. S. rubber boot manufacturer used similar 3-D contact analysis techniques to evaluate and optimize new boot designs (one design has a longitudinal seam to facilitate installation). Improved fatigue life was the design goal, and nonlinear FEA was successfully used to minimize time and cost—and come up with a boot design that achieved an acceptable product life cycle. The analysis was correlated with test results, and showed that a modified design with a seam attained a similar fatigue life as the original design (without a seam). The new design with a seam substantially reduced the installation costs. “Do-it-yourself” kits using this split boot design are now available to replace worn-out boots.

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