A bolt being tightened all the way and then some is what constitutes preload. A bolt connects two parts via preload: the tension in the bolt. You must overcome that much force to separate those parts.
The bit of tension in some dainty washer before the bolt is tightened means absolutely nothing.
If there were some application that for some strange reason requires a very light torque, the washer could be useful; but it wouldn't be locking anything. How it could be useful is by creating a softer range of torque: a range where the torque increases slowly relative to the turn rate.
A bolt normally has no torque until it's tightened all the way, and then rapidly rising torque from there. The washer could create a more rounded knee there; and the only possible reason that would ever be useful is if the application stayed in the operating area of that knee!
There are devices in which a nut compresses a spring to a variable amount, like preload in suspensions, or resistance in exercise equipment and such. That is not a fastener application; it has nothing to do with locking.
Only if you restrict yourself to the "real engineering" that the internet loves.
>and the only possible reason that would ever be useful is if the application stayed in the operating area of that knee!
Which is frequently what happens when you try and build shit out of slip fit "what's the thinnest wall tubing we can get away with" held together with bolts acting as pins and the assembler is expected to just kinda eyeball it and not crush the tube lock washers are beneficial. We ship with nylocks now though because the extra part count is more expensive than the difference between nylock and split washer at our volume.
>There are devices in which a nut compresses a spring to a variable amount, like preload in suspensions, or resistance in exercise equipment and such. That is not a fastener application; it has nothing to do with locking.
This is just an argument of semantics/taxonomy. See also: belville washer or spring washer.
There is your problem. However, even if you're consciously building for low quality, lock washers are still not useful or necessary. You will still have low quality if you remove them.
I think if you want to use a bolt and nut as just a pin that holds some slip tubing together, without unduly compressing the tubing, you want two nuts that are tightened against each other. The tubing likely has a bit of elasticity (likely, all the elasticity you would want from a washer and then some)*, so one nut can be lightly tightened against the tubing, so that the assembly doesn't rattle loose under ordinary movement of the product. There can be washers so you don't scratch the tubing, if that matters. Then you put on the other nut and tighten it hard against the previous nut, while preventing that one from turning any farther. This generates tension in the bolt section between the two nuts, binding them firmly together and preventing them from working loose.
This will cost you more in assembly time/steps and parts; that's what you get from building something that is less likely to fall apart in use.
--
* It's entirely likely that the thin wall tubing has a much lower spring constant than a split lock washer. So that is to say, if you tighten the nut to the point that you're deforming the tubing, the washer may still be almost entirely uncompressed. Adding a relatively stiff spring into a "sandwich" that already includes soft springs generally doesn't do anything except past the point where the soft springs are fully compressed.
It sounds as though you are using lock washers as a compliant element to avoid crushing fragile components rather than to prevent unscrewing - so outside the scope of this discussion.
I omit them whenever I can, now.
Tooth lock washers are pretty bad, too-- though sometimes they are slightly beneficial. I don't think they're generally worth the surface damage they cause for minimal benefit.
Mostly, you can ignore any locking device. And if you need one, a nylock nut, or a threadlocker crayon, or a wedgelocking nut, etc, is infinitely superior. (And of course, there's castellated nuts and safety wire for really critical stuff).
[1] https://www.boltscience.com/pages/helicalspringwashers.htm
It is beyond a condemnation that a professional such has yourself has not developed a sense for "authority X recommends Y but everybody is doing Z, there's probably a reason for that".
Split lock washers persist because they are a cheap way to get a decent amount of preload into a bolted joint without torquing the bolt very much relative to typical torque values for fasteners of that size. For highly price sensitive widgets where the nature of the joint means it flex/vibrate too much for "nothing" to not be a acceptable solutions split lock washer is often the cheapest acceptable solution. I understand why NASA says "no locking value". That's a fine rule of thumb for what they do. But they're not making value priced widgets. Having a fastener maintain preload has "locking" value in many practical real world applications.