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

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

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

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.

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

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

This link might help:- https://mechanicalc.com/reference/bolted-joint-ana...

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

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)
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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

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

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 ?

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

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 ?

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

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 ?

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

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 ?

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.

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 ?

I did a quick calculation on a joint up using an M16 and an M20 bolt and assumed a external load of 30kN was placed on both joints, initial preload for both bolts was 40KN although that didn't really come into the equation for what I was trying to show.
Based on Shigleys Frustum on this link https://mechanicalc.com/reference/bolted-joint-ana... I worked out joint and bolt stiffness.
Based on page 27 of this link I got the formula for the ratio of load taken by bolt and that by the clamped parts:-https://prod-ng.sandia.gov/techlib-noauth/access-c...

The final conclusion was that using the larger bolt transferred more of the external load to the bolt than with the smaller bolt which I believe the OP wanted to know. There was a change in joint stiffness due to the increased bolt size which was expected and the actual external load transferred to the bolt was marginally but this would probably increase had I used a bigger step between the bolts.
I will post the calculation shortly.

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

Here’s the calculation

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

Thanks Desertfox,

Yes, that's what I wanted to know, thanks for the link and taking the time doing the calculations + the fact that in Juvinall machine component design book, under Increasing Bolted-Joint Fatigue Strength, Chapter10, there are some general recommendation, one of them is "Modify stiffnesses to decrease the portion of the external load that increases bolt tension." and then proceeded to say you can do this by increasing the stiffness of clamped members and/or by decreasing the stiffness of bolts, which makes sense and is consistent with your calc. and what has been said in this thread.

RolMec mentioned that bolts not preloaded to proper tensions are not secure in terms of self-loosening. In Shigley, under fatigue load section, there's a similar recommendation, quoting from the textbook "A rule of thumb is that preloads of 60 percent of proof load rarely loosen.", but there was no detailed explanation provided for this (maybe it was beyond the scope of the book), so I don't know how that number was calculated, so I guess I just take Shigley advice that regardless of bolt size, it's best to preload the bolts to recommended values not just for clamp load but also to avoid them getting loose.

Rb1957,
I agree with your comment that the original question wasn't well put, haha. My goal was to know if achieving certain clamping load that satisfies the required factors of safety for fatigue and static loading, regardless of how you reach to that clamping load (X% preload of using a larger bolt vs. X+Y% preload using smaller size bolt) is all that maters for a joint or are there other consequences. Which from your reply and other comments, it seems they are other things to thin about, I got the stiffness comment from Desertfox, comments self-loosening from RolMec & IRStuff etc.

I don't know about rest, but this was great learning experience for me. Thanks all!

Hi Vah1D

You’re very welcome and thanks for returning and clarifying things.

There are a couple of other points, one being torquing or preloading screws and bolts to 60 or 90% of there proof strength, this is okay provided the material that’s being clamped under the head of the bolt/screw doesn’t exceed its yield stress when tightened.
Another point is that with the larger bolt the joint separation force increases.

Fs = external force/ (1-C)

So in that example I gave C for the M20 bolt = 0.3266 and C for the M16 = o.3070
Staying with the 30KN external force then the separation force for each situation is:-
Fs= 30KN/(1-0.3266) = 44.55KN and Fs= 30KN/(1-0.3070) = 43.29KN

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

there's a tonne of report out there on bolt preload, and fastener loads, ... VDI2230 is one.

it should be obvious that changing any one parameter in a complex problem affects the solution.

75% preload is "only" a guide (for tension bolts) and not a "requirement"

Changing a bolt size, the impact on fastener stress is only part of the problem and there are many practical things to be addressed as well.

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