How do I calculate the force on a threaded rod? 3

[GogiBOB]

Hi,
I am trying to calculate how much force can my threaded rod withstand.
My 5/16-18" 304 S/S threaded rod is inserted into the metal plate of 0.25" thickness so there are 4.5 threads inserted in the plate.

There's upward force from the center of the plate which is 0.95" away from the rod.

Can anyone give me a heads up?
Thanks!

 

[KENAT]

Maybe you can use one of the FAQ's here.

http://www.eng-tips.com/faq.cfm?pid=725

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[rb1957]

so you'd consider the threaded rod as a 5/16" SS bolt, but how strong is the tapped hole ? (material) maybe consider the plate as a nut (reduced thickness)

 

[CoryPad]

The plate likely is the weak link, the thread below shows an equation for thread engagement:

thread725-35222

 

[Cockroach]

You have to use a free body diagram and apply statics. Clearly there is a force with vertical component and a bending moment.

The shear in threading is the result of that vertical force. Since you state four and one- half threads of engagement, shear can be computed on the basis of length of engagement based upon the thread pitch. You will find that the Box is always of greater shear strength based on thread geometry, so the Pin is the weakest component of the thread. You then calculate stress based upon the vertical force and Pin shear area.

As mentioned, we have a bending moment. You simplify the bolt extending from the plate as a simple cantilever and find the maximum allowable bending. These are recipe equations generated from first principles, but closed form solutions are found in most first year static textbooks. So get the bending stress.

Since the material is defined, you know the yield strength. Find the factor of safety based on the ratio of yield stress to those two values of computed stress. This provides you with a value to base your judgement as reasonable or not. Typical FOS in threads are 3.0 or grater.

This is an easy static problem. Find an example in the first year statics course on a cantilever beam and work it through. The only difference here is you would need to find thread shear.

I note you can also get the maximum allowable load permissible by the thread and find the maximum moment based upon material and thread geometry. Comparing that to your application also gives FOS. That would be a second approach of equal merit.

Regards,
Cockroach

 

[zekeman]

Another example of incomplete information and we guess to give meaningless answers.

Why can't we get the OPs to post diagrams of the thing and then answer ?
Otherwise ignore the question.

For one thing he doesn't say how the other end of the rod is mounted, nor does he show any other rod(s).

Is he saying that this rod is holding a plate and there is an offset force; let's guess.
Then what about the couple to the rod??

 

[Cockroach]

I'm use to dealing with salesmen who don't understand what they are talking about. So I get it.

The rod will fail in bending after a force of 41 lbf is applied.

Regards,
Cockroach

 

[GogiBOB]

Thank you cockroach, its more than I had expected.
And sorry for incomplete information I provided.
I wasn't sure where to start

 

[rb1957]

just be aware that cockroach made a bunch of assumptions in his analysis. a key one being the material of the plate, i think he's assumed same as the bolt.

FWIW, i'd've thought that a 5/16" SS bolt would be able to carry more than 41 in.lbs moment and 41 lbs tension. i'd've thought that a 5/16" bolt should be able to carry 500 in.lbs moment or 5,000 lbs tension ... i'd've thought the critical load would be closer to 410 lbs.

 

[dhengr]

You calculate the force on the rod very carefully, once you understand your problem well enough so you can actually define the problem. What you might actually want are the stresses in the rod and plate, once you know the forces. You might want to hire someone who can draw a few sketches of what you are talking about. Then get yourself some basic (first year) Engineering Mechanics and Strength of Materials text books and learn a little actual engineering, so you can start to understand your problem. This forum or the internet in general is not the best place to come for your basic engineering education. You usually gets about what ya pays for it.

 

[Cockroach]

You can see that the maximum tensile load of the threaded rod, RB1957, is 7049 lbf. This is acting through the centre line of the rod.

But this IS NOT the loading case for the rod is displaced 0.95 inches from the centre of the plate, which carries the load. Therefore the load is actually bending the rod at the base of the threads. Hence the bending moment computation. It turns out that this is the killer to the rod, assuming the rod is threaded directly into the plate, worst case scenario. So I offered two options to remedy that situation pointing out that stainless steel is very poor in bending as well as shear.

Of course you make assumptions, those are inherent to the mathematics in question. I outline these in the paradigm in order to point out limitations to the model. Clearly more analysis would be needed, but pointless in the discussion for as pointed out, we have missing information.

I don't think GogiBOB is going to take the calculation and run with it. That wouldn't be smart. I have provided the model that would provide a starting point for further analysis, clearly the equations pertaining to the Box and the material used would be accurately described as I have done in the detail. Also, stainless is notorious for galling, no mention of treatment of the threads here or if they are doped to mitigate the metal-to-metal transfer along the flanks of threading. I should also point out that the threads cut to the rod are full form, most likely done with a die. This is yet another assumption, if they are cut on a lathe then they would probably have thread reliefs which would form yet a thinner part of the rod and lower the tensile load somewhat. No mention of machining.

But the methodology behind the model is sound. Bending moment would unnecessarily shear the threads at the root, obvious for us who know line pipe threads and stainless steel applications. In the geometry, I painfully point out that the direct load through the rod would not be possible given the rod is offset and has a moment arm relative to the centre of the plate.

I am just hoping this isn't a homework problem or assignment question. Seems to be fairly generic, something I would expect in a first year exam.

Regards,
Cockroach

 

[GogiBOB]

Hi Cockroach,
This is for a project I am currently working on.
I realize this is a very simple problem but I wasn't familiar with the equation for thread engagement so I wasn't sure where I could begin.
I think I know what I am going to do from here.

Thanks!

 

[GogiBOB]

Just one more questions, is there any website I can find properties of different materials?
Every website gives me different data and I don't know which one is the most liable one.
Like this one

http://www.azom.com/article.aspx?ArticleID=965

says yield strength of 304 SS is 205MPa (30kpsi).

Regards,

 

[dhengr]

Cockroach:
Very nice work and presentation, as always. That’s well above and beyond the call-of-duty, with some details and ideas that a young engineer would never even think of. What worries me is that companies are foisting products on the public, designed by people who have trouble even knowing what info. is required to define their problem, let alone knowing how to approach the problem solution. They don’t/can’t provide sketches as you have; they don’t/can’t step through the problem as you have, thinking how is the force resisted by my structure, where can the max. stresses be, and on which part, what kind of stress, etc. etc. Just a clean, simple, logical engineering approach to breaking the problem down into its basic parts. Nicely done.

 

[mcgyvr]

http://www.matweb.com

is the best source of material properties I've found...

 

[rb1957]

@cockroach,

i saw the 7049 lbs in your calc. i was responding to "The rod will fail in bending after a force of 41 lbf is applied.".

surely the moment in the bolt is P*0.95" ?

 

[Cockroach]

Thanks DHengr, very kind words. Hope the mathematics and problem solving strategy can be of value to others who parouse this forum on a regular basis. I see some cute problems from time-to-time, some very good contributors to this forum also. Nice to see a varied approach and some lively discussions thereof.....LOL.....

Regards,
Cockroach

 

[zekeman]

Cockroach

Didn't know you could read minds.

I have a lot of trouble with engineers who cannot give me sufficient info to help them.

Making assumptions about the problem is not my way.

They never gave me a course in mind reading.

 

[Cockroach]

It's not mind reading, Zekeman, it's taking a look at the problem and using common sense. As pointed out by a few in this discussion for example, there is not enough information to properly answer the problem. Working through the problem in a step-by-step fashion, I made an assumption based on the missing information. This uncertainty led me to speculate that it was possible to at least get a theoretical answer.

This computation is only a preliminary study. Obviously a more detailed analysis is required.

Regards,
Cockroach

 

[rb1957]

play nicely guys ...