Reduced friction? Torque absorbsion? Strong-arm washerwrights’ union? Don’t laugh - I bet a lot of us don’t know.
(In what promises to be a series of threads “questions my daughter asks that I can’t answer”)
Regular washers don’t keep nuts and bolts tight. Lock washers however do and they have small raised ridges on one or both sides to help adhere to the material being bolted.
I’m not sure that washers themselves keep nuts tight on the bolts. I think a lock washer exerts an additional force on the threads, creating more friction.
But I’m not a mechanic.
Well, there are washers and then there are washers. For example, Maytag makes some mighty fine washers… oh? that’s not what you meant? ok… ahem
There are washers and there are washers. a flat washer is meant mainly to spread the load, not to prevent the nut from coming loose. A locking washer designed to prevent the nut from coming loose works by increasing friction. This is generally accomplished by teeth or grooves that cut into the metal, by use of plastic or rubber or by other means.
For big solid things like engine blocks and wheel hubs a bolt can be tightened enough to stretch it slightly but not permanently. This keeps the threads under pressure so they are less likely to slip and loosen. When a bolt can’t be stretched much a split or star lock washer can accomplish the same purpose. The split and star segments also dig into the mating surface of the nut slightly to keep it from moving.
There are also things like castellated nuts, which have a jagged edge on the back. You put a castellated nut on a special bolt which has a tiny hole drilled laterally through its shaft (ouch!). Then a lock wire is inserted through the hole and engages the nut. There’s no way that’s coming off unless the wire breaks.
You can also use stuff like “Loc-Tite” or some other liquid thread locking compound.
But washers to keep nuts and bolts tight? Naaah. That’s sissy stuff.
Seconding what Padeye said…
Bolts 'n screws stay tight because of the slight elasticity of the metal, or of the material that they have been screwed into. The shaft and threads stretch slightly which greatly increases the frictional force retaining the threads in place. The washer itself provides a uniform surface to distribute the load across whatever you have screwed into, but doesn’t affect the tightness or retention of the bolt directly.
Aside from lock washers, there is another class of washers designed specifically to maintain tightness (preload) in bolted connections called Spring Washers. They’re used because bolted joints can loosen up over time due to relaxation of the bolts, gasket creep, vibration, and thermal transients.
Sorry if this one was mentioned already, but this thread is driving ME NuTS TOO!!
There are washers that are split in one spot from center to outer edge (making a break in the circle). One edge (or end) is bent one way, and the other edge is bent the other way.
As it is smashed between said Nut n Bolt, it acts as a spring & exerts outward pressure creating increased friction, I believe. IT’S NUTS!!!
Split washer locknuts are not springs; they’re made out of about the same grade of steel as the nut and bolt, not spring steel. Split washers are usually put between a nut and a flat washer and work by digging into the faces of the nut and flat washer which keeps them from turning.
The Book, more than you wanted to know…
Sure they are. Pressing a lockwasher flat doesn’t exceed its elastic limit in most cases so it’s providing extra pressure to the edges digging in and springs back to shape when the nut is removed.
In some high load applications where there is a large force to counter or contain, rather than just holding bits of metal together, the bolts are pre-stressed.
These are first measured accurately for length and are then tightened to achieve a certain stretch length.
Thes types of fastening are sometimes called fitted or machine bolts, you see them in places such as large conrods, or high compression cylinder heads on very large diesels(no not truck engines I mean LARGE engines)
Uh, (how should I put this), after checking the very book that I offered as a reference, I’ll have to admit that I was wrong on this one. Sorry, people…