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]

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-analysis

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-control.cgi/2008/080371.pdf

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

 

[desertfox]

Here’s the calculation

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

 

[Vah1D]

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!

 

[desertfox]

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

 

[rb1957]

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 ?