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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So a lot of people are referencing a NASA document from 1990 to say that helical spring washers are useless. I’ve read that document, and frankly, it leaves a lot of questions. It’s not a study on the effectiveness of lock washers, it’s a brief description of different methods of locking fasteners, including a blurb on the helical spring washer. It provides no supporting evidence and cities no study or methodology used to come to such a claim. I’m not trying to suggest that I know better than NASA engineers, but this flies in the face of my own personal experience with helical spring washers, and my understanding of how physics works, which makes me question whether this is being cited out of context, is perhaps a bias of the author, or simply a perspective relevant to aerospace engineering and not broadly applicable.
To answer your question, OP, a spring washer, in theory, uses friction to lock a fastener in place. If you place your palm gently on a table and try to slide it across the surface, it should glide across easily enough. It should be easy enough to rotate your palm against the surface too. Now if you push down on the surface and try to do the same, you should feel more resistance, because you’ve changed something called the coefficient of friction, a measure of the interaction between two surfaces.
When you push down on a spring, you can feel it pushing back as it tries to return to it’s unloaded state. What a spring washer is supposed to do is provide extra force against the fastener to push it against the threads it’s mating with, and increase the coefficient of friction, making it a little bit harder for it to turn itself loose.
This context is why I question the document being cited. A spring does not lose it’s potential energy when it’s bottomed out, anybody can test this with any spring. You can even test it with a helical spring washer – place one on a hard surface, crush it with something that has a flat plane, and you’ll see – it won’t lose spring tension, you’ll need to continue to apply force to keep it bottomed out. If you remove a spring washer that’s been in use for years, it’s typically lost some spring tension and can’t be reused, but it won’t stay completely flat unless it’s been in use for a *very* long time, or was overtorqued. It will return at least partially to form. That should mean that in the case of a threaded fastener, it will continue to modify the coefficient of friction where the threads meet, even if it’s crushed flat. It will lose tension over time, yes, but that’s not the same thing as useless, that’s simply less effective. So I question the claim that they’re useless, and I question the reasoning behind it. Are they useless *specifically for aerospace engineering?* Do they handle vibration poorly, a force that will be present in aircraft, but they’re still useful in more static applications like a loose chair leg? Do they work well, but drop off in effectiveness over time as the spring fatigues, and are therefore a liability that can’t be tolerated in aerospace applications?
Split lock washers have been discussed to death already, so let’s talk about some other lock washer and bolt/nut types.
My favorite type of lock washer is the toothed type. They work by biting in to both the bolt or nut and the surface of whatever the hole is in. Because of this, they don’t work well on hardened surfaces. They also don’t work with standard washers, since they work by preventing rotation between the nut/bolt and the hole.
Nordloc, or wedge type lock washers work similarly, but they’re actually two parts loosely held together. They have stepped wedges on the inside that prevent them from turning counterclockwise against each other without spreading apart. On the outside they have serrations that bite into the surface the hole is in as well as the nut or bolt head. When you turn them counterclockwise the internal wedges try to spread the two halves apart, causing the teeth to dig in harder. This is the most effective type, but they tend to cost a lot more.
Other types of locking fasteners use a deformable section to resist rotation. That means they are hard to turn even when loose. The most common is the nylock, or nylon insert locknut, which has a plastic ring crimped inside the nut. To install the nut you have to force the thread through the nylon. These work great, but will eventually wear out as the threads cut the plastic. They also aren’t good anywhere it gets hot enough to significantly soften the nylon.
You can also get all metal deformable lock nuts. They work like the nylocks but are all metal rather than a metal nut with a plastic ring.
Other types of lock nuts include serrated flange nuts, which have teeth that bite into the surface, nuts with a pre-installed toothed lock washer, castle nuts, which are used with lock wire or cotter pins, and jamb nuts, which is where you have two thinner nuts that you tighten against each other.
Key diagram showing NOT using a lockwasher is better than using one:
boltscience.com/images/preloaddecaycurvehelicalspringwasher.GIF
I used to do R&D for a top automotive fastener supplier. We built custom test equipment to test whether various fastening solutions could withstand vibration. We found lock washers were not sufficient for critical applications like automotive powertrains. Adding adhesives (patches) to joints is very effective, but those bolts are basically a one-time-use product. If you want a serviceable joint that still withstands vibration, you need a mechanical method of keeping the threads under significant tension at all times. We have a product like that, pictured here: [https://www.efc-intl.com/Asset/241756_a.jpg](https://www.efc-intl.com/Asset/241756_a.jpg). The wavy threads take up any slack and keep friction on the threads at all times. (The wavy threads are much stiffer than lock washers.)
Mechanic here. Split washers are important where the surfaces being mated to are prone to expand and contract with changes in temperature. They ensure the threads are under tension even if the fastener or the the bore change length a little bit.
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