Tried explaining to my 4 year old the purpose of the lockwasher and she asked how it worked? I came to the realization I didn’t know. Help my educate my child by educating me please!
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NASA says: “When a lock washer is tight, it’s flat so it’s the same as a flat washer.” Really? Not if it’s a true spring.
This NASA document is a high-level fastener review, but the author makes the mistake of disregarding the hold force of the spring without providing either a reference or a test. If it’s a real lock washer (and not just a split flat washer) this spring force doesn’t *go away* when the washer is flat; the force is always there. Just like an old-style leaf spring for a vehicle suspension doesn’t suddenly cease being a spring because it’s been clamped flat — don’t make that mistake!
To answer OP’s question:
A screw holds tight in a threaded nut because of the friction between the screw’s thread and the thread of the nut. When the screw is still loose it has enough play to slide down the inclined ramp (which is what the thread is) as you tighten it. But when the head contacts the final surface, the screw’s thread is pulled up against the nut’s thread causing a force which a) holds the screw in the hole so it acts as a fastener and b) creates friction on the “ramp” so that the screw can’t slide slightly back out of the nut, freeing it to turn. But once a screw gets slightly loose, continued vibration or a repeated pull on the screw can gradually work the screw up out of the nut.
A regular washer is used to distribute the load of the tightened screw head across a larger area so that the final surface isn’t compressed, which would allow the screw to loosen as noted above. This is generally done if the material the screw head will contact is softer than the screw material, e.g. a bolt passing through wood.
A split ring washer is made of spring steel that is biased to return to it’s split position with a gap between the bottom face and the top face. It’s *not* just a cut flat washer that’s been twisted a bit, because that will not retain the “spring” force when compressed (and it’s why cutting one out of a regular washer is useless). A spring washer needs to be made of hardened spring material. What the split ring washer does is to keep a degree of force between the screw head and the nut (or final surface) even if the two elements are vibrated or subjected to a repeated strain.
In my years of experience with machine assemblies and with fastening structural timbers, the correct use and layering of flat and split-ring washers *absolutely* affects how long and under what conditions a threaded assembly remains tightly fastened.
Now maybe NASA evaluates this using different criteria and use conditions — I don’t know zero gravity might result in different behavior. And maybe under continuous vibration of just the right frequency, the extra “hold” from the spring could be overcome. But in my personal real-world experience, omitting required washers results in loose fastenings down the road.
(edit: I have read the NASA article and it’s a fastener overview, no special conditions but obviously biased for aerospace use cases. In general it’s a good guide.)
Now, if you never intend to remove a fastener again, there are locking adhesives you can put into the nut which make loosening or extracting a bolt virtually impossible. And also makes repairs impossible, so there’s that.
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