Threads Per Inch vs. Strength

[tonydacrow]

I remember reading, somewhere, something that seems a little counter-intuitive regarding threads per inch. I'd like some elucidation from you more learned than I.

If I remember correctly, the greater the number of threads per inch on any given bolt, the stronger the bolt will hold.

It seems counter intuitive that the smaller threads of a large thread count will hold better than the larger threads of a courser count. On the other hand, there are more of them. Please give me the benefit of your experience and knowledge on this topic. Thanks!

[This message has been edited by tonydacrow (edited 05-12-2005).]

[SGW]

The key, I think, is that with a fine thread you have a larger core diameter of the bolt. If you have more than about 1 1/2 the bolt diamteter worth of thread, the bolt will break before the thread strips out, anyway.

[nt1953]

Generally, bolts fail either by pulling the threads out (shear) or by pulling in two (tensile). They can also fail by being sheared off, but that is not what you are asking.

A rough approximation of thread shear strength can be determined by calculating the surface area to be sheared and assuming a shear strength of roughly half of the tensile strength of the bolt material. If you assume that the nut material is as strong as the bolt material, then the thread shear area is a cylinder half the length of the thread engagement with a diameter equal to the nominal diameter minus the height of one thread (nominal - .866/TPI). Here you can see that as the TPI gets larger, the effective diameter gets larger, and the shear strength goes up.
NOTE: If you can not assume that the bolt and nut material have similar strengths, you need to be much more creative in your assumptions.


Likewise, a rough approximation of the tensile strength of a bolt can be calculated from the nominal diameter of the bolt less the depth of two threads (nominal - 1.73/TPI), times the tensile strength of the bolt material. Again, as the TPI gets large, the effective diameter gets small and the tensile strength goes down.

From this you can also see that bolts that fail by being sheared off are stronger if the effective diameter is larger. Again, that means more TPI for stronger bolts.

FYI, Grade 5 bolts range from about 120,000psi at 1/4"dia to 105,000psi at 1.5"dia. Grade 8 bolts are about 150,000psi up to 1.5"dia. Don't forget to add safety factors!

[ronsmith100]

the finer thread, although having less contact per pitch, has more contacts in an inch.
A 1/4 -20 has 20 engagements in an inch
A 1/4 -28 had 28 engagements in an inch.
The 1/4-20 has only about 70% of the engagements of a 1/4- 28.
Does the increased thread height make up for it?

[ricksplace]

I dunno. I'm trying to find some elucidation for tonydacrow.....

[Mcgyver]

lets say the thread depth is .500. So long as its the same form, fit & dept of thread, the lengh of shear line is same isn't it? if true, #/inch wouldn't effect the thread shear but a high number would give a great core dia as SGW pointed out.

[WLW-19958]

Hi There,

NT1953 did a nice job explaining the relationships between TPI and thread strength. But I think there is one hiccup:

"Likewise, a rough approximation of the tensile strength of a bolt can be calculated from the nominal diameter of the bolt less the depth of two threads (nominal - 1.73/TPI), times the tensile strength of the bolt material. Again, as the TPI gets large, the effective diameter gets small and the tensile strength goes down."

I think you meant to say that as the TPI goes up, the effective diameter gets larger and the tensile strength goes up.

Otherwise very good information!

-Blue Chips-
Webb

[Paul Alciatore]

Good discussion. But much of what has been said is assuming that the threads on both the screw and nut are cut perfectly.

I have done some ROUGH calculations on coarse and fine threads and at first look it appears that they both have the same root area. That's the part that will shear so they are at least close to equal there.

The core diameter is going to be larger with a fine thread so there is a definite advantage there. But it is going to be in the range of 5% to 15%, not double or triple. So not necessairly a big consideration.

But here's the real rub. Most threads are not cut at 100% of the theoretical form. Most internal threads are only 75% to 50% of that number. So the shear stress is not acting at the root of the thread but 25% to 50% up the slope where there is a lot less meat to resist it. I have even seen a 32 TPI nut that only had about 0.005" engagement on each side. And the male thread was in soft plastic. It would fail if you breathed on it. That's breathed lightly, not heavy.

The point is that a given allowance will have a much larger effect on a fine thread than on a coarse one. A 1/2-13 UNC thread has a design thread height of 0.050" while the 1/2-20 UNC has only 0.0325". So a 0.010" allowance will reduct the effective base area by about 31% on the UNF but only by 20% on the UNC. That's a 16.5% difference in strength. With threads that have an even looser fit, this can be a much larger effect than the increase in core strength which is about 15% for this particular size.

I admit that this is not anywhere near to a rigorous analysis. But it does show that the accuracy of the thread is at least as significant a factor in determining it's strength as is the choice of coarse or fine thread. In the end, the grade of the screw/nut if a far better determining factor. This is doubly true as higher grade fasteners are far more likely to be made to higher tolerances.

Paul A.

[Your Old Dog]

I'm hoping that somebody out there is waiting until this subject has been aired out completely to tell us how it really works. I have to believe the coarser thread will have superior strength even though it's shank is severly reduced by the thread depth due to shear strength of the thread base. Most of what I've read so far reads more like speculation to me rather than engineering fact. Keep it going till we reach conclusive proof. Where the hell's an Engineer when you need one? !!

[This message has been edited by Your Old Dog (edited 05-12-2005).]

[ronsmith100]

<font face="Verdana, Arial" size="2">Originally posted by Your Old Dog:
I'm hoping that somebody out there is waiting until this subject has been aired out completely to tell us how it really works. I have to believe the coarser thread will have superior strength even though it's shank is severly reduced by the thread depth due to shear strength of the thread base. Most of what I've read so far reads more like speculation to me rather than engineering fact. Keep it going till we reach conclusive proof. Where the hell's an Engineer when you need one? !!

[This message has been edited by Your Old Dog (edited 05-12-2005).]</font>

Old dog I am hoping to do that.
Sart by thinking of stripped threads.
Go back and read my post about 1/4-20 vs 1/4-28
Which sounds to you like it would strip first? (let us never forget that disaster in St Louis)

Ron

[zl1byz]

Well I don't have the answer!

Actually unless you define the parimeters that you are working with, I'm not sure there is a conclusive answer.

I guess you could ask, if I went and bought 2 bolts of the same size and grade only one was corse and the other fine and using the standard nuts supplied. Put them in a testing machine. Which one will the thread fail first under the same test conditions?

Beyond that you could argue till hell freezes over, because there are so many different applications, materials etc that make one type of thread superior for that use than the other. Guess that is why we still have both corse and fine threads.

So who has the scientific report done in a lab?

Then we can throw it in the bin and make our selection by considering all the other vairables that need to be considered for that application.

Good discussion guys, but I don't think we have the answer yet.

John.

[garyphansen]

There in nothing like real world testing to find you if the Engeering you were taught in school is really, really always true. I remember finding out with the use of a tensile strength testing machine that some of the math equations did not always prove correct. Normally things in the real world are stronger than the math says they should be. (May be this is a built in safty factor.)

Anyway does anyone have access to a tensile strenght tester and would try a 1/4-20 and 1/4-28 thread and report back?

Gary P. Hansen

[JCHannum]

Same discussion a while back on merits of the different systems; http://bbs.homeshopmachinist.net//Fo...ML/007855.html

[nt1953]

Blue Chips did indeed find an error in my text. What I should have said is:

Likewise, a rough approximation of the tensile strength of a bolt can be calculated from the nominal diameter of the bolt less the depth of two threads (nominal - 1.73/TPI), times the tensile strength of the bolt material. Again, as the TPI gets large, the effective diameter gets LARGER and the tensile strength goes UP.

That's what I get for not proofing my text.

I have the right degree, experience, and 25 years of structural test time, but because I am not a PE, I can't claim anything else in a public forum.

[J Harp]

I'm not an engineer by a long shot, but I keep my eyes open around machinery of any kind. If you will check aircraft bolts, the threads on the tie rod ends of your car, or the U bolts on truck axles, I believe you will find them all to be fine threads, that should tell us something about the strength of fine threads. thats my two cents for what it's worth.

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Jim