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Dome nuts, what advantages?

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DrSnoosnoo
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PostPosted: 14:55 - 09 Dec 2013    Post subject: Dome nuts, what advantages? Reply with quote
So just a quick question, there's one on each of the exhaust hangers on the ZZR6 and as mentioned in another thread, and on the CG suspension.

Are they just prettier than leaving an edge of a bolt hanging out or do they serve another purpose?
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Raffles
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PostPosted: 15:00 - 09 Dec 2013    Post subject: Reply with quote
They help to reduce thread corrosion and they also look pretty.
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c_dug
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PostPosted: 15:16 - 09 Dec 2013    Post subject: Reply with quote
They do have a practical purpose rather than purely cosmetic, if they're on a long enough thread they tighten up against the inside of the dome to prevent them coming off by providing some torsion to the threads.
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69chris
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PostPosted: 15:20 - 09 Dec 2013    Post subject: Reply with quote
and if the thread is slightly too long it just pushes the dome off Laughing
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c_dug
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PostPosted: 15:29 - 09 Dec 2013    Post subject: Reply with quote
Can't say I've ever had that issue and I've really cranked up some dome nuts before on a bit of thread I needed them never to come off of.
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Walloper
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PostPosted: 18:39 - 09 Dec 2013    Post subject: Reply with quote
Cosmetic as they look 'prettier' than a thread end.
Practical as it can help prevent ingress of moisture and you won't catch your nylons on ragged threads.

The bolt shouldn't really foul the inside of the dome as it can deform the fragile shape.

A locking nut should be used under the dome nut or the bolt cut to the correct length.
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CaNsA
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PostPosted: 18:51 - 09 Dec 2013    Post subject: Reply with quote
c_dug wrote:
They do have a practical purpose rather than purely cosmetic, if they're on a long enough thread they tighten up against the inside of the dome to prevent them coming off by providing some torsion to the threads.


That doesnt sound right to me...

If you think about where the tension on the threads is located, if the bolt is touching the dome then the tension between the nut threads and the bolt threads is reduced as the dome is pushing the bolt away from the nut...
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Fisty
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PostPosted: 18:52 - 09 Dec 2013    Post subject: Reply with quote
c_dug wrote:
They do have a practical purpose rather than purely cosmetic, if they're on a long enough thread they tighten up against the inside of the dome to prevent them coming off by providing some torsion to the threads.


Riiiiiiiiiiiiiiiiight.
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Aff
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PostPosted: 18:53 - 09 Dec 2013    Post subject: Reply with quote
c_dug wrote:
They do have a practical purpose rather than purely cosmetic, if they're on a long enough thread they tighten up against the inside of the dome to prevent them coming off by providing some torsion to the threads.


If you do this, you will have very rattly exhausts. Laughing
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c_dug
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PostPosted: 19:15 - 09 Dec 2013    Post subject: Reply with quote
I promise you all they work that way, never had one deform either, I reckon you'd strip the thread before deforming the dome.

The torsion on the threads thing is the same as tightening down on a piece of metal, only you're tightening on the end the thread instead. Done it plenty of times at work without issue.

I start again tomorrow from my crash, I'll see if I can find an example of something we would use them that way on.
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kestrel
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PostPosted: 19:15 - 09 Dec 2013    Post subject: Reply with quote
c_dug wrote:
They do have a practical purpose rather than purely cosmetic, if they're on a long enough thread they tighten up against the inside of the dome to prevent them coming off by providing some torsion to the threads.


Really?.... If using a dome nut as a fastener then how do you gauge clamping force if the thread has bottomed out inside the dome?

Walloper wrote:
Cosmetic as they look 'prettier' than a thread end.
Practical as it can help prevent ingress of moisture and you won't catch your nylons on ragged threads.

The bolt shouldn't really foul the inside of the dome as it can deform the fragile shape.

A locking nut should be used under the dome nut or the bolt cut to the correct length.

Correct.
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c_dug
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PostPosted: 19:19 - 09 Dec 2013    Post subject: Reply with quote
I don't mean for clamping down on something, I mean more like for making a rod to go through something where it needs to be tightened from one side. I'll see if I can photo what I'm thinking of tomorrow.
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Aff
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PostPosted: 19:27 - 09 Dec 2013    Post subject: Reply with quote
c_dug wrote:
I don't mean for clamping down on something, I mean more like for making a rod to go through something where it needs to be tightened from one side. I'll see if I can photo what I'm thinking of tomorrow.


You would still need a fixed rod to get it tight, you're better off using 2 full nuts or a welded nut. A domed nut will deform the threads or break the dome.
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CaNsA
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PostPosted: 19:30 - 09 Dec 2013    Post subject: Reply with quote
The only part of the fitting that will provide any kind of "holding force" will be the tip of the bolt and the inside of the dome.... hardly any surface area touching when compared to using a standard bolt.
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Aff
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PostPosted: 19:34 - 09 Dec 2013    Post subject: Reply with quote
CaNsA wrote:
The only part of the fitting that will provide any kind of "holding force" will be the tip of the bolt and the inside of the dome.... hardly any surface area touching when compared to using a standard bolt.


Not quite true, the surface the face of the bolt contacts only really supplies minimal clamping force and acts as a way to hold the part you want fixed, most of it comes from the threads and on non helicoiled threads about 60-70% comes from the first 2 threads.

The contact patch at the head of the bolt is mostly just to stop deformation of the part being clamped by spreading the load.

Seeing as that load is all applied at a small point in the dome, either the dome will give of the thread.

If he can get it tight without blowing the dome out it will work fine apart from the threads will be fucked.
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Walloper
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PostPosted: 20:47 - 09 Dec 2013    Post subject: Reply with quote
Aff wrote:
CaNsA wrote:
The only part of the fitting that will provide any kind of "holding force" will be the tip of the bolt and the inside of the dome.... hardly any surface area touching when compared to using a standard bolt.


Not quite true, the surface the face of the bolt contacts only really supplies minimal clamping force and acts as a way to hold the part you want fixed, most of it comes from the threads and on non helicoiled threads about 60-70% comes from the first 2 threads.

The contact patch at the head of the bolt is mostly just to stop deformation of the part being clamped by spreading the load.

Seeing as that load is all applied at a small point in the dome, either the dome will give of the thread.

If he can get it tight without blowing the dome out it will work fine apart from the threads will be fucked.



I feel a flounce on the way in this thread. Thinking

Bolt/nut clamping force load path is through the 'face' of the bolt head or nut.

Applying the clamping force to the inside of the dome will not transmit force properly.

It can also crush the top thread which can then make removal of the nut impossibru.
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CaNsA
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PostPosted: 21:58 - 09 Dec 2013    Post subject: Reply with quote
Aff wrote:
Not quite true, the surface the face of the bolt contacts only really supplies minimal clamping force and acts as a way to hold the part you want fixed, most of it comes from the threads and on non helicoiled threads about 60-70% comes from the first 2 threads.


If all threads are equally spaced across the nut and bolt then the force will be spread across all touching surfaces within the threads, assuming the force from what ever is being bolted together is applying the force in line with the bolt.
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G
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PostPosted: 22:19 - 09 Dec 2013    Post subject: Reply with quote
c_dug, I believe, is talking about making a threaded rod effectively in to a bolt.

I'd certainly choose welding over that, but can see it could make it easier in some circumstance.

But yes - mostly to stop it seizing, especially if there's a little grease in there (yes, I know it's generally not advised to grease threads) - there should be a lot higher change of removing it when it's been in an adverse environment.
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poleposition
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PostPosted: 04:52 - 10 Dec 2013    Post subject: Reply with quote
The only time i have ever seen a capped nut being tightened down onto a threaded bar is on a cheap childs go-kart...But it is NOT the way to tighten these type of nuts..either a lock nut underneath or the correct length threaded bar should be used.
If you ever see a capped nut with the top broken off it has been fitted incorrectly.
the word "bodge" springs to mind.
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c_dug
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PostPosted: 08:50 - 10 Dec 2013    Post subject: Reply with quote
Ok, having spoken to the guys at work (being engineers that I trust rather than random people off the Internet) I'll concede that they're not really designed to be used that way. My confusion came from us having used them that way on a couple of jobs, but apparently it was more for lack of better options than for it being the proper thing to do, that said it does work for the reasons Aff mentioned. Also they agreed that with a decent stainless dome nut you're more likely to strip the thread than deform the dome.

For the record this is far from a flounce, it's just admitting I was mistaken. Do I really seem like a flouncer?
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Aff
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PostPosted: 09:04 - 10 Dec 2013    Post subject: Reply with quote
CaNsA wrote:

If all threads are equally spaced across the nut and bolt then the force will be spread across all touching surfaces within the threads,


As you tighten a nut or bolt the thread pitch will change.

Because the bolt is normally loaded in tension and the housing in compression, the bolts thread pitch will increase while the housings decreases. This makes the threads near the loaded face take much more force than any of the others. (like I said earlier usually 60-70% of total load in the first 2 threads.)

You can solve this with helicoil as they have designed them to effectively have a varying pitch, which once tightened will mate with the bolt much more evenly. I have a picture somewhere, I'll try and find it.

:edit: here

https://i.imgur.com/QApXSas.jpg
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CaNsA
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PostPosted: 09:12 - 10 Dec 2013    Post subject: Reply with quote
Huzzah!
And i'm nowhere near being an engineer yet Very Happy

@aff, I see your point Thumbs Up
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poleposition
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PostPosted: 15:02 - 10 Dec 2013    Post subject: Reply with quote
Aff wrote:
CaNsA wrote:

If all threads are equally spaced across the nut and bolt then the force will be spread across all touching surfaces within the threads,


As you tighten a nut or bolt the thread pitch will change.

Because the bolt is normally loaded in tension and the housing in compression, the bolts thread pitch will increase while the housings decreases. This makes the threads near the loaded face take much more force than any of the others. (like I said earlier usually 60-70% of total load in the first 2 threads.)

You can solve this with helicoil as they have designed them to effectively have a varying pitch, which once tightened will mate with the bolt much more evenly. I have a picture somewhere, I'll try and find it.

:edit: here

https://i.imgur.com/QApXSas.jpg


Great diagram....however I very much doubt that diagram was drawn up to illustrate a varying pitch on a thread, there is no such thing as a varying pitch, only the result of overtightening a specific area of a thread.
The load area of a thread, IE the points of contact where the nut meets the bolt, remain constant..therefore any overloading of a thread will affect the whole points of contact (in other words the width of the nut) and any stretching on the threads themselves will apply to this whole section since equal force is being applied across this area.
It will not change the pitch (the distance between 2 adjacent threads) across this area....it will only change the pitch between the lower thread directly underneath the nut.
Smile
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Aff
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PostPosted: 15:30 - 10 Dec 2013    Post subject: Reply with quote
poleposition wrote:

Great diagram....however I very much doubt that diagram was drawn up to illustrate a varying pitch on a thread, there is no such thing as a varying pitch, only the result of overtightening a specific area of a thread.


How would you overtighten a specific area of a thread if the pitch was consistent?
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lihp
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PostPosted: 15:38 - 10 Dec 2013    Post subject: Reply with quote
poleposition wrote:

It will not change the pitch (the distance between 2 adjacent threads) across this area....it will only change the pitch between the lower thread directly underneath the nut.
Smile


The pitch changes as the tension starts to deform the material, thus holding the bolt tight.

Therefore, the pitch does actually vary as the bolt is fastened.
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