How do washers (for screws) work, and why?
In: Engineering
Both of the two main purposes of the washer according to the common description are – providing a larger surface area to press on, and allowing pressure to be applied better on uneven surfaces – but these can be be built into the screw itself. Just make the screw head larger in diameter so that it’s its own washer.
In real life it doesn’t work well. These types of screws do exist and they tend to suck just as bad as a screw with no washer.
The important part is that the washer isn’t a part of the surface or a part of the screw. If a screw can’t go into a surface anymore, it means that it can still be backed out fairly easily (even just from vibration). Also if you keep reefing on it to prevent that you will just break the screw. The washer mediates the tension and lets the screw press down harder than it could if it were all one solid piece. Since there isn’t a clear stopping point it isn’t easy for it to be backed out, which is what is desired.
Fittings with NPT threads work in a similar way without a washer, because the threads are tapered (hard to see but they are, very slightly). As you thread one fitting into another the resistance is graduated. There is no clear stopping point, so it can’t be backed out easily nor will it break easily unless you go way too far. The “washer” in this case is that the threads themselves are very gradually distorted as you apply more pressure, so there is no clear stop point.
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