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Small-Scale Catastrophic Failure

I had a kitchen cabinet door abruptly fall off recently, because the hinges broke. The hinge part that was fastened to the cabinet stayed fastened to the cabinet; the part that was fastened to the door stayed fastened to the door. No people, animals, or bottles of soy sauce were harmed.

In retrospect, I should have seen this coming. First, the attachment of the hinges was not the best:

Second, and more importantly, the hinges were being subjected to an unusual force every time the cabinet door was opened. As you can see in the top picture, the door had to address two sections of cabinet at right angles to one another. (For people who don’t live in New York City: many of us have very small kitchens, and cabinets are jammed in wherever they may fit.) The two sections of door (the section mounted to the cabinet, on the left, and the outer leaf, on the right) have a hinge connecting them. I’m not entirely sure why, other than maybe that was easier than building a rigid door consisting of two planes at right angles to one another. In any case, the outer leaf created an extra large moment that had to be resolved at the hinges. 

All doors create a moment at the hinges. No matter where the door is in its swing, the top hinge is in tension parallel to the plane of the door and the bottom hinge is in compression. If there are three hinges, this is still true; if there are four, both top hinges are in tension, but the top one carries significantly more tension than the second from the top. The moment is the product of the weight of the door times half the door width, and the tension in the top hinge is that moment divided by the distance between the hinges. In this case, the moment varied depending on what position the outer leaf was in relative to the cabinet-mounted section. The overall weight was more than a normal door and the direction of the moment could leave the plane of the door depending on that relative position of the two parts. In short, the top hinge was subject to greater tension (sometimes) and an extra force pulling it sideways (sometimes). And the cardboard shim seen in the second picture meant that the hinges probably didn’t stay perfectly aligned when loaded to the maximum.

The new hinges are heavier and better mounted than the old.