From Materials Selection in Mechanical Design, Third Edition
6.8 Elastic Hinges and Couplings
Nature makes much use of elastic hinges: skin, muscle, cartilage all allow large, recoverable deflections. Man, too, design with flexure and torsion hinges: ligaments that connect or transmit load between components while allowing limited relative movement between them by deflecting elastically (Figure 6.13 and Table 6.13). Which materials make good hinges?
| Function | Elastic hinge |
| Constraints | No failure, meaning ? < ? f throughout the hinge |
| Objective | Maximize elastic flexure |
| Free variables | Choice of material |
Figure 6.13: Elastic or "natural" hinges. The ligaments must bend repeatedly without failing. The cap of a shampoo bottle is an example; elastic hinges are used in high performance applications too, and are found widely in nature.
The Model.
Consider the hinge for the lid of a box. The box, lid and hinge are to be molded in one operation. The hinge is a thin ligament of material that flexes elastically as the box is closed, as in the figure, but it carries no significant axial loads. Then the best material is the one that (for given ligament dimensions) bends to the smallest radius without yielding or failing. When a ligament of thickness t is bent elastically to a radius R, the surface strain is
| (6.26) |  |
and since the hinge is elastic the maximum stress is
| (6.27) |  |
This must not exceed the yield or failure strength ? f. Thus the minimum radius to which the ligament...
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