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This is a fair question but the answer can get complex. Honestly the design/manufacturing of this aircraft joint is way above my expertise and pay grade. I would hope that Boeing has some extremely specialized and talented people working on this. In short, the question of how a gap might affect this assembly is far outside my expertise. However if you want a simple example: Consider a geometrically perfect cylinder resting on a perfect plane. The contact is a line, with zero width. Therefore a contact area of zero. Pressure is force divided by area. So the nonzero weight of the pin, divided by area (zero) is... infinite? You run into the same problem with a pin in a slightly larger hole. How does this seemingly infinite pressure not lead to failures in wheels (think of train wheels on tracks), ball bearings (spherical balls in torroidal raceways with slight clearance), roller bearings, etc? We are surrounded by geometries that have seemingly zero area points of contact, but they support tremendous loads. Hertz (yeah, the same guy for whom the 1/s unit is named) figured out the math behind these contact stresses. Basically, for round (and round-ish) things in 2d and 3d, the contact stress has a lot to do with the deformation of the materials. To answer the riddle above (of the cylinder on plane infinite contact stress), you have to consider the deformation of the cylinder and the plane. The stiffness of the materials comes into play, as well as the geometry. You can read up on Herz (or Hertzian) contact stresses if you would like to know more. The math is not terribly difficult, especially for 2d geometries. For a 2d case of a pinned joint, you can often find that a change of a couple thousandths of an inch can mean the difference between a comfortable factor of safety and failure. I have given a hand-waving example of the importance of tight tolerances on clearances for a small class of problems. I hope it is close enough to the subject matter at hand to be of some use. My comment is from memory, so please forgive (and correct!) any mistakes I've made. edit: I am rereading my comment, and realize that I didn't make explicit the importance of tight fit for Hertzian contact stress. The smaller the gap between a pin and hole, the greater the contact area (with the same amount of deformation). Think of it this way--for a fixed amount of deformation (say strain at failure), you can carry way more load if the contact area is greater. How do you increase this contact area? By a smaller difference in diameters (smaller gap) of pin and hole. So all things equal (material properties, load), a smaller difference between pin and hole diameters will increase load the joint can carry. Another point: calculating these contact stresses is doable for most metals, but is far more complex for anisotropic materials (mechanical properties vary in different directions) materials like the carbon fiber composites. |
I think others might be forgetting (or not know) that the factors of safety* for the parts in airplanes (around 2, or less?) are very different than factors of safety for the structural parts of bridges (around 5?). Compared on those terms, planes are light and fragile, on purpose, so you can't f around with cheating tolerances.
* https://en.wikipedia.org/wiki/Factor_of_safety