Question about Bolt Preload and Proof Load 3

[Vah1D]

Hi everyone,

Hope it's not a stupid question!

Let's say you have bolted connection and the bolts are preloaded to the recommended value (let's say to 75% of proof load) and the joint is working properly under the external loads applied to joint.

Now let's swap the bolt with much larger one, and preload it exactly to the same amount of case#1; meaning that we turn the nut much less and using only, let's say for sake of this example, 40% of the proof load and that gives us identical amount of clamping load as to case#1. The external loads are exactly the same.

Am I right in thinking that both cases should perform the same? Is there any downside in going with case#2? (well other than the fact that I'm wasting money by using oversized bolts)


Thanks,
Vahid

 

[desertfox]

Hi Vah1D

The easy answer to your question yes you are just wasting money otherwise no different, however in general depending on what the joint function is you are better off with a larger number of smaller bolts than fewer large ones because using more smaller bolts will give a more even spread over a joint.
A good example of the above would a flange with a gasket, a larger number of smaller bolts will spread the sealing load over the gasket much better than a few larger bolts in the same situation, hope this helps.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[r13]

The increased reserve capacity is the reward to your generosity, or waste if additional capacity is unnecessary.

 

[Vah1D]

Thanks Desertfox, what you said about better spread of loads makes sense.

I don't want to make this more complicated that it should be! but I guess one reason I asked this, is that if you use a larger bolt (and keep the clamped members the exactly the same), that means in a sense you increased the relative stiffness of the bolt to clamped members. If I'm not mistaken, that implies that even though the external applied load is still the same, but a larger portion of it is being resisted by the bolt.

Maybe in the example above this is not an issue, but could this be a valid reason to use as small diameter screw as possible (so that they still provide enough clamp to prevent joint separation), so they have less stiffness relative to clamped members? This way if we have unexpected high loads, or fluctuating loads, then by design, the joint is able to refer a higher portion of this external load to the members and not to the bolt.


Vahid

 

[mfgenggear]

yes but if there is same # amount of bolts, there is an increase in preload and tensile strength. and an increase in safety factor.
but it would have to be justified, am I wrong?
edit also if there is enough edge distance and flange material to clear threads and holes.

 

[desertfox]

Hi Vah1D

Generally even with larger bolts the clamped components are usually much stiffer than the bolts but yes if you increase the stiffness of the bolts without increasing the flange stiffness then I would expect the bolts to carry more of the load than previous.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[Vah1D]

mfgenggear,
I think You are absolutely right, if you have a series of larger bolts, joint should be much stronger anyways even though they are resisting larger portion of the external loads.

Desert,
I didn't think about that, that's true, the clamped members are generally much larger in area, etc. so they naturally should have much higher stiffness anyways.

After these past couple of comments, I think a good conclusion would be that, a joint with smaller diameter bolts but with stronger material is more desirable vs. joint with larger diameter bolts with low/medium strength?
This way you can still get high pre-loads, and since the screw is of stronger material you can still have high factor of safety for the external loads, and with better stiffness ratio a larger portion of the loads is resisted by the clamped members and not the bolt.
(btw I'm not trying to make general/rule of thumb! I know each problem should be analyzed per its own unique circumstances.)

Overall, I need to read on this topic a bit more, I'm not sure if I fully understand the importance of the stiffness ratio of the bolt and clamped members.

Vahid

 

[desertfox]

This link might help:-

https://mechanicalc.com/reference/bolted-joint-analysis

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[IRstuff]

One issue that's of keen interest to me is the proportion of friction that is overcome by torque and whether that plays into how much actual preload is being applied and how easily the fastener can be unscrewed under vibration.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

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[RolMec]

Quote < Now let's swap the bolt with much larger one, and preload it exactly to the same amount of case#1 > unquote
The larger bolt, not being preloaded to its proper percentage of proof strength, cannot be considered secure in sense of loosening / getting untightened.
You'll have to take care of that, depending on the required level of safety as per your application.

Roland Heilmann

 

[MintJulep]

You change the bolt:joint stiffness ratio in a way that is generally unfavorable.

 

[rb1957]

how do you swap in a larger bolt and maintain the bolt spacing (and edge distance on the flange) ?

if your original spacing was 6D and you double D, now your spacing is 3D ... which creates it's own issues.

if your edge distance was 2D and you double D, now your eD is 1D ... clearly not good.

if you're doing a "thought experiment" well, that's different. I think you have to ask yourself "why 75% pre-load ?"
If you have one joint with n D sized bolts and a similar joint with n/2 2D bolts, well ...
the 2nd joint is much stronger than the 1st (2D bolts will have an allowable/proof load 4* the 1D bolts)
the 2nd joint flange will probably be heavier than the 1st (more edge distance, more bending between the bolts, being further apart)

another day in paradise, or is paradise one day closer ?

 

[desertfox]

Hi rb1957

I think the question is hypothetical

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[rb1957]

then maybe answer the thought with a question ... if you increase the strength of the bolt (change it's grade) do you increase the preload ?
Your "rule of thumb" would say so, but if the loads on the bolt haven't changed why change the preload ?

A sensible answer could be that the higher (static) strength bolt has a lower fatigue allowable (maybe?) which could say that even the 75% pre-load doesn't provide the same fatigue life ...

another day in paradise, or is paradise one day closer ?

 

[desertfox]

Rb1957

I haven’t mentioned the preload, basically the question I answered was if a stiffer bolt in a joint was used but the clamp load remained identical when the joint is now loaded with the same external force would the larger bolt see more of the load. If the bolt stiffness increases but the stiffness of the clamped parts don’t change then the bolt see more of the load.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[rb1957]

true, but the larger bolt (with the smaller bolt's preload) would have a higher MS.

another day in paradise, or is paradise one day closer ?

 

[desertfox]

Yes the OP mentioned that in his post where is states if I use an oversize bolt am I just wasting money and I agreed he was

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[rb1957]

yeah, but that's where his question falls down ... substitution of a larger bolt is very limited without changing the flange geometry
which is why I changed the question to changing the material (grade/strength) of the bolt (without changing the geometry of the joint).

it may be better to turn the original question completely around and ask "if I put in a smaller bolt, and apply the original preload, will it fail ?"
and the answer relies (in part) on the original MS.

another day in paradise, or is paradise one day closer ?

 

[desertfox]

I think the OP is only interested in the ratio of the joint to bolt stiffness, so if he opens up an existing hole in the joint to fit a larger bolt, then the stiffness of the clamped parts is reduced however the larger bolt is stiffer, now if we apply the same external load to the joint which as the original clamping force as with the smaller bolt will the larger bolt take more of the external load, the answer in my book is yes, maybe the OP will return and comment.

 

[rb1957]

So ...
"Now let's swap the bolt with much larger one, and preload it exactly to the same amount of case#1; meaning that we turn the nut much less and using only, let's say for sake of this example, 40% of the proof load and that gives us identical amount of clamping load as to case#1. The external loads are exactly the same."
is wrong in that the "same" preload application doesn't give the "identical" amount of clamping ... for a host a practical reasons, but also analytically because changing the bolt changes the bolt/joint stiffness.

And ...
"Am I right in thinking that both cases should perform the same?"
is similarly "wrong" (for a host of practical design reasons and several analytical ones too).

And ...
"Is there any downside in going with case#2? (well other than the fact that I'm wasting money by using oversized bolts)"
yes, see above !?

another day in paradise, or is paradise one day closer ?