Maximum Torque to Break a Thread 1

[Mechotron]

Hi,

I have a threaded connection which has a direct tensile load to break the thread is 12,000 lbf. I am trying to find the torque that can supply this load to the thread to break it. The thread is 0.4375-20UNF.

The load on thread and the torque load is perpendicular as you are aware of. can i apply the "direct tensile load to break the thread" into the torque formula on the machinarys handbook to calculate the maximum torque to break the thread.

If you have any questions, please let me know.

Mechatron

 

[CoryPad]

Is it a threaded fastener with a head? If so, the torque/tension relationship is here: faq725-536

If it is only a threaded connection, then you need to remove the bearing friction component and keep the thread friction and thread pitch components.



Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Mechotron]

The link above is for calculating the torque within its elastic limit. But i happen to go beyond elastic limit and break it.

Is there a formula for find the maximum torque (ie before the thread breaks) for threaded connections.

 

[dgowans]

If you need to calculate the torque needed to break the screw, and the tensile load needed to break the screw is 12000#, would you not use 12000# as the preload in the equation CoryPad provided?

 

[drawoh]

Mechotron,

What is it you are looking for, an absolute do-not-exceed torque, or a guaranteed breaking torque? Do you need to be accurate?

Is there any chance you can do this by counting turns of the nut? This may be more predictable and nut friction and the force required to cause plastic deformation.

JHG

 

[Mechotron]

Hi

I am theoritically trying to the find the absolute, do not exceed torque. I want the thread material to go beyond its elastic limit and reach the ultimate clamping force which is in the plastic region. If you torque beyond this point the thread will elongate but the tension will drop and eventually fracture.

can you help

 

[rb1957]

sounds like you want the plastic ultimate torsional modulus of rupture of the minimum thread diameter. I thought this was somewhere in Bruhn but couldn't find it (quickly), so i'd suggest ... dT = Fsu*rdA (allow the entire section to work at the ultimate shear stress). and dA = pi*2*r*dr

then dT = Fsu*2*pi*r^2dr and Tult = Fsu*3*pi*r^3

 

[CoryPad]

I didn't notice that you are requesting a fracture calculation. Is the torsion applied at the same time as tension (i.e., by loading with an internal thread)? Or, is the torsion the only stress? If it is the former, then it is a complicated calculation. VDI 2230 Systematic Calculation of Bolted Joints has an equation that is similar to what rb1957 has, although it only assumes reaching the yield stress for the entire cross-section. I think it could be modified to estimate fracture. If you only have torsion, then you can use:

T = 2/3 [·] [π] [·] r³ [·] [τ]

where

T = torque
[π] = 3.141 592 654
r = shaft radius
[τ] = shaft material shear strength



Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[drawoh]

Mechotron,

You need a stress strain curve for your bolt material so that you can find the maximum stress point. Your material looks like a grade five bolt to me. A similar steel material in my machine design book elongates 19% at fracture. For a reasonably long bolt, this should be quite a few turns of your nut, allowing for accurate control of force.

The primary cause of wrenching torque is friction. This is hard to predict accurately.

JHG

 

[rb1957]

cory's right ... intergal of r^2 is r^3/3 (sigh, shouldn't do this stuff in my head)

also a good point ... is the torque applying preload (like in the real world) or is this a theoretical application (maybe torquing against a jam nut, so there wouldn't be any preload) ?

 

[Tmoose]

I'd start with a basic value from a table, then spend a few hours testing the actual bolts I'll be buying.

 

[Cockroach]

Very good, CoryPad, star for you. Maximum Distortion Theory uses 0.577 rather than your 2/3 or 0.667, but close enough since this is known to be conservative.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[Tmoose]

"I am theoritically trying to the find the absolute, do not exceed torque. I want the thread material to go beyond its elastic limit and reach the ultimate clamping force which is in the plastic region. If you torque beyond this point the thread will elongate but the tension will drop and eventually fracture. "

Do you need 12,000 lb preload, or are you specifically looking to achieve the "maximum"?

It looks like about 1200 lb-in would exert the 12,000 lbs you're after. If you could use a SHCS the bolt would take it easily.

http://www.holo-krome.com/pdf/technical catalog.pdf

HoloKrome catalog page 32 (pdf page 39)

I don't think intentionally using the thread region as the limit is commonly done. Too many variables. Torque-to-yield bolts and angle tightened bolts (same thing) plastically stretch the shank, for several thousandths of an inch of initial elongation and greater tolerance for joint preload load loss due to embedment,etc.

Designing with relieved shank bolts longer than 5 or so diameters assigns the stretching function to the entire bolt shank, which can be finished nicely to avoid stress concentration.

http://www.mondello.com/Pages/Articles/1/Cycle_bolt_story_pics.022.jpg

 

[CoryPad]

Cockroach,

The equivalent stress with many names (distortion energy/octahedral shear stress/Hencky/von Mises ) is a yield criterion based on shear. For fracture in shear, it is common to use the value 0.6 [·] tensile fracture stress, which is obviously close the 0.577 value. However, the non-uniform stresses due to torsion require a different analysis. The sand hill analogy has been used to find the limit torque of polygonal shafts. This, and other techniques, result in the 2/3 value. A really good article on the subject is Strain 41 (2005) 31-32.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Cockroach]

Thanks for the update, CoryPad. I am studying a polygon shaft right now, in this case, decagon, for a client. I used a more complicated spline analogy and doctored it up to suit my situation. Came very close to an FEA model showing stress due to torsion.

I will check out the Sand Hill Analogy. Thanks for the tip.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[unclesyd]

You might want to checkout this paper by Dose. It looks at thread stripping and breakage of a fastener.

http://www.hexagon.de/dose/dose-1e.pdf