The Fyzix of Fasteners (Part drei of many thrilling installments)

Here, we'll be looking at "over center devices," for lack of a better term. Traditionally, this term is applied to the weird little hinged things in the arms of dentist chairs, where a pivot goes over the center of the mechanism, so applying more force doesn't affect the position, instead one must physically move the pivot.

Instead, we'll be referring to over center devices as anything which requires a large force to get over a "hump," before remaining locked in place (think about calculator covers and how they slide on, for instance). These work by using some sort of flexible material that, with enough force, can be deformed to fit over something else. By pushing up on a calculator cover, the angled "humps" on either side transfer that into a normal force against the calculator, pushing out the sides of the cover and slipping it over, before locking it into place.

Naturally, a mechanism like this is susceptible to fatigue. Typically, this takes the form of permanent deformation of the plastic or outright breakage. The first is caused by repeated use; squish the plastic around enough and eventually it begins to wear. Breakage is more rare, and typically only results from excessive force, or underclassmen.