This was a very good read, Una. Thanks.
This alone was worth the price of admission:
As the bolt loosened even slightly, would not the springiness off the washer re-engage the lock?
Just askin’.
rwj
Spiff, thank you for the positive feedback!
Intuitively, that is what I thought as well. I suppose there is a further debate as to whether or not that constitutes a “lock” function. It wasn’t totally clear to me whether or not going into that might expand the article too much. I had to discuss the properties of stacked Belleville washers with people on my Board for a bit before I understood what was happening too, and even then decided finally to omit the details to keep the article a bit more accessible.
Wonderful article, Una. I always enjoy learning more about the tiny things we are prone to take for granted. I’ve always wondered how split ring lock washers work – your piece turned the light on for me.
I bow before your beautiful mind.
Great report, and nicely done.
Of course, being an engineering geek, I must nitpick very slightly, and that is in regards to the split ring lockwasher. The purpose of the split ring lock is not to provide spring tension, (at which point it cannot be torqued to spec)but to lock the bolt from unthreading. The ends of the spring are sharp and dig into the metal if the nut starts to back off. they are invariably put on the nut end of a fastening because it is felt the friction of the assembly itself will keep the bolt from turning. For this reason, though it is common practice, a flat washer should never be placed under a lockwasher, as it diminishes the effectiveness. Additionally, left threaded bolts require left hand lockwashers- not common, but not unheard of. For this reason, it is important that one chooses a lockwasher that is harder than the nut or base material. Lockwashers are seldom used on dies, for that reason.
All in al, though, again, great report! I love the pictures and descriptions.
You know what Billy? That’s a very good point which you make, and one which both I, and my references, all seemed to miss entirely with respect to split ring lockwashers. Thank you for noticing that and pointing it out; I learned something too as a result.
And thanks for the praise as well, AugustWest!
to have in any way educated Una. Still, you knew this info instinctively, I know you did, you just didn’t run across it in your research.
Damn, you got to it before me. 
I do want to make note, though, that split-ring washers are considered highly questionable as a means for locking the nut; a Grade 5 bolt and mating nut will generally be too hard for the washer “dig in”. As Bill Rubin notes, the washer will be flattened long before the appropriate torque is applied for any size of bolt used for structural applications (1/4" or larger in diameter) and thus contributes nothing to the locking capability of the nut. Star washers are somewhat more effective, particularly on small fasteners in highly vibratory applications but you don’t see them on larger nuts, and like cotter pins, they should never be reused after the fastener is loosened (but often are).
In overhead (cranes, manlifts, telescopic material handlers) and aerospace applications with which I’m familiar, lock washers of all kinds are eschewed in any “mission critical” applications in favor of lock nuts (nylock, thread deformed), lock-wired nuts, and castlated nuts/cotter pins. Torque specs are also tightly controlled (at least, on the drawings…it’s hard to tell what the tech assembling the machine is actually doing) and should prevent fatigue and vibratory/shock failure, providing the engineer has made all the correct assumptions. ( :dubious: ) Although effective, thread adhesive (Loctite) and jam nuts are almost always avoided, owing to to the lack of control on the former, and the propensity for getting the order wrong (jam nut should be inboard of the primary nut) of the latter. Where I’ve seen lock nuts used, they’re nearly always (as as noted in the SR, incorrectly) used with a flatwasher, often to prevent the lockwasher from damaging paint or surface finish, but ultimately defeating whatever purpose it may have. I don’t think I’ve ever seen belleville springs used as a locking device except on cheap retail products, though I’ve seen them used in spring-retainer and vibration isolation apps. They’re valuable because they work over spans too narrow for coil springs and because they can be made strictly linear or constant force by a simple control of geometry.
There is, I’ve found, a great deal of ignorance regarding the appropriate use of fasteners, even with large aerospace concerns; for instance, I was involved in “heritage” review of an ongoing program last year in which nearly every torque spec stated on drawings for this program was undervalued by 20%-40% according to mil-spec and the primary contractor’s specifications. I’d like to see an expanded report that goes into some detail (in layman’s terms of course) into exactly how a bolted joint works, i.e. as a compound spring in which the bolt shaft and the joint act as members giving a total k[sub]eq[/sub] for the joint. (I say this in pure selfishness, 'cause I’d like a simple reference to point to when explaining to someone why their preload assumptions are wrong. Shigley and Mischke are a little obscure on the issue, and the fastener handbook I have at work goes into too much detail to serve as a primer on the topic.)
Stranger
A few comments:
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Why should jam nuts be used inboard of the primary nut? (By “inboard” I assume you mean “underneath;” i.e., “threaded on first”). I’ve never specced a jam nut, but I’ve never seen them put on underneath the primary nut. That makes intuitive sense to me: the primary nut is tightened first to establish the correct tension, and then the jam nut is used to lock the threads. Why would you put them on the other way around?
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Just to clarify, when you say, “Where I’ve seen lock nuts used, they’re nearly always (as as noted in the SR, incorrectly) used with a flatwasher…” I’m reasonably certain you mean lockwasher, as I can’t think of any reason not to use a lock nut with a flat washer.
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You might perhaps try “Practical Stress Analysis in Engineering Design” by Alexander Blake. One of my favorite books. It includes fifteen pages or so on the design of bolted joints. The language is a bit technical, so it might not be exactly what you’re looking for, but it is concise and accurate, and it has a whole lot of other cool stuff in it.
I just want to mention that one other purpose for flat washers on materials softer than the nut like wood or fiberglass is to avoid the nut cutting into the material as the nut is being tightened.
Why use a locking washer when you have superglue? As to using a flat washer to distribute force, I made the mistake of taking my S 2000 to Walmart to get an oil change. They left out the washer on the drain plug and split my oil pan.
So it only needs to be harder than the nut or the base, not both? And what am I supposed to use if I’m using stainless bolts on a steel surface?