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

How do I calculate the force on a threaded rod?

How do I calculate the force on a threaded rod?

(OP)
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!

RE: How do I calculate the force on a threaded rod?

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)

RE: How do I calculate the force on a threaded rod?

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

RE: How do I calculate the force on a threaded rod?

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??

RE: How do I calculate the force on a threaded rod?

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

RE: How do I calculate the force on a threaded rod?

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.

RE: How do I calculate the force on a threaded rod?

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.

RE: How do I calculate the force on a threaded rod?

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

RE: How do I calculate the force on a threaded rod?

(OP)
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!smile

RE: How do I calculate the force on a threaded rod?

(OP)
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,

RE: How do I calculate the force on a threaded rod?

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.

RE: How do I calculate the force on a threaded rod?

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

RE: How do I calculate the force on a threaded rod?

@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" ?

RE: How do I calculate the force on a threaded rod?

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

RE: How do I calculate the force on a threaded rod?

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.

RE: How do I calculate the force on a threaded rod?

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

RE: How do I calculate the force on a threaded rod?

play nicely guys ...

RE: How do I calculate the force on a threaded rod?

"'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,"


It's mind reading 101...

RE: How do I calculate the force on a threaded rod?

I lauded Cockroach for his effort and presentation, on an OP which was poorly defined and presented. The poor, incomplete question reflects badly on the OP’er. and his engineering experience and abilities, no one else. Cockroach made a few assumptions, consistent with what little info. was given, out of dozens of possible interpretations, and offered one possible tack on the problem. He also showed a set of hand calcs. for these types of problems, which would be a good example for many of these people to be able to emulate. Since, many of them couldn’t engineer their way out of a paper bag which was open at both ends, without $20,000 worth of computer stuff at their disposal. And, then, they still wouldn’t know if they had a real answer to their problem.

I agree with Zekeman on the issue that we see way too many poorly posed questions here a E-Tips. I can’t help but wonder if some of these are from high school science or tech students, certainly not college students or graduate engineers, one hopes. And, it scares the hell out of me that some of these might be real tech people designing products or structures which will be foisted on the public and maybe hurt someone, for lack of any real engineering input into their design. And, I don’t know who to blame/damn for this state-of-affairs; the company which expects non-engineers to engineer their products, on the cheap; or the engineer (young or otherwise) who has oversold his abilities to the point that he can’t even define his design problem, and is trying to get us to help him dig out of the hole he made for himself.

I just tend to ignore questions which don’t have enough info. for an intelligent discussion. If the guy couldn’t waste the time to post an intelligent question with sufficient info., why should I waste my time trying to decipher what he wants or needs? He probably wouldn’t understand what was said anyway, or worse yet might misuse it in a dangerous way. By the way, I am all for helping other engineers with a problem they may not have seen before, but I would like them to show a little native ability in the engineering area they claim to be practicing in. And like Zekeman, I wonder, if they can’t even pose an intelligent engineering question, should I help them pretend to be engineers.

RE: How do I calculate the force on a threaded rod?

Dhengr,

Spot on. I couldn't agree with you more.

What scares me that in addressing these poorly defined problems and giving answers sometimes with caveats, the socalled "engineer" may then take the info and "design" something completely out of whack that may be unsafe, putting somebody at risk of life and limb.

Perhaps a policy change at the administrator level should give us, the responders, better guidelines in giving answers to these oblique inquiries.

RE: How do I calculate the force on a threaded rod?

(OP)
I realize my question was very poorly described and not enough information was given to solve the problem.I was just hoping to get some heads up. ... Next time I;ll make sure I provide every piece of information when I ask a question.

Yes I am a fresh graduatesmarty and have no real experience.
I went though two textbooks on statics from university and none of them mentioned thread engagement.sad

Again, my apologies for being ignorant.neutral

and thank you again cockroach for your help.

regards,

RE: How do I calculate the force on a threaded rod?

GogiBOB:
Nobody else said ignorant, so don’t you either; lazy maybe, inexperienced probably... But, you should start thinking like a real working engineer, where every aspect of a problem is not given to you on a golden platter. You actually have to dig for some of the needed design info. And, you have to step through the problem in a logical fashion, just as Cockroach did for you.

The take-away here should be to provide a sketch; read through your word-picture (description), word-for-word, and it better be detailed enough so any of us can only draw one picture as we read it. If there can possibly be two or three interpretations, then you’ve got us guessing (wasting our time) and there’s a good chance that you might get the wrong answer. We can’t see, from here, what you are imagining or CADing, you gotta show us. Even if you are a new grad. engr., and we all were once upon a time, think through your problem, what info. is needed to come to a solution, these guys will need that info. to give you good advice. There are a bunch of really sharp people here willing and able to help. At the same time, they are giving you their time and experience, so don’t waste it just because you are lazy in your OP or further posts to the thread. Then follow your thread, and don’t leave people who have helped hanging, finally when you have your answer close out the thread, with a Thank You, as you have done.

If you aspire to do this kind of design and to be a good Mech.E., you should have a few good Strength of Materials, Engineering Mechanics and Machine Design text books as your best friends. Maybe a good Mech. Engr’s. Handbook, Marks’ comes to mind, for general info. Also, get a copy of Machinery’s Handbook, pub. by Industrial Press, Inc. for a wealth of mechanical info. Cockroach has undoubtedly done that set of calcs. more than a few times in his life, and I’ll bet a dollar to a doughnut that he used a few ref. books while he did the above calcs. I certainly don’t remember all those values and formulas without looking them up, and I don’t have the time to develop them each time, our advantage is we already have those ref. pages marked, and know where to look.

RE: How do I calculate the force on a threaded rod?

(OP)
Lessons I learned from this single thread is more then I learned from my engineering ethics class. Thank you for your words!

RE: How do I calculate the force on a threaded rod?

for better, or for worse ?

RE: How do I calculate the force on a threaded rod?

(OP)
rb1957/ for better of course :)

I recently had some time to work on this project and tried to prove the model by adapting the options Cockroach suggested. While I understand adding a washer/nut would help the rod to resist bending stress I am not too sure which equations to mathematically prove. Can anyone give me a little more explanation?

Also, I don't have much mechanical background and having hard time looking for references.
Is there any good mechanical engineering reference books other then Machinery's handbook?(I just ordered one!)

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