Combined torsion and axial load 2

[jhamil1]

All,

I have a heavy hex nut approximately 3 inches in size which was threaded onto a 1.75" diameter coarse thread stud; Grade B steel. It is about 5" long. The nut failed under a pre-stress of 40,000 psi. Due to dirt or some other means the nut also experienced some torque applied by the stud up to a maximum of 2500 ft-lbs. The nut failed due to shear/fracture. My question is;

Is there a way to analytically determine the combined effects of these loads on the shear stress?

I have found combined load examples for torsion-bending but not torsion-axial.

 

[Bribyk]

As far as Mohr's Circle is concerned, bending is the same as axial. In bending you have one surface in axial tension (and the other in axial compression). It's the same, you have a shear stress and a normal stress.

 

[desertfox]

Hi jhamil1

What is the nut material? normally the grade of the stud is made of a material that is not as strong as the nut.
There are a number of failure theories you can apply ie:-
maximum shear stress theory for ductile materials, maximum normal stress theory etc.
Did the nut fail under static load or dynamic load? did the thread merely strip on the nut or did the nut fall apart in pieces.
If you can expand a bit more on the failure maybe we can advise an appropriate failure critera.
A picture of the fractured parts would be very helpful.

regards

desertfox

 

[dinjin]

I would also question the flatness of the interface
surfaces.

 

[HeviiGuy]

jhamil, Some more details please:

[ul]
[li]What you mean the nut failed? The bolt was okay?![/li]

[li]"B" what ?[/li]

[li]How do you know the nut (or bolt, perhaps?) failed at 40,000 psi?[/li]

[li]Just how did the "...stud apply 2500 ft-lbs to the nut"??[/li]
[/ul]





Ciao,

HevïGuy

http://www.heviitech.com

 

[jhamil1]

Sorry guys,

I have been on the road. More details. The heavy hex nut is a "coupling". One end is threaded onto an existing stud and then a new stud is threaded into the opposing end. A torque value of 2500 ft-lbs was recommended to impose a 40,000 psi pre-stress on the coupling. The nut sheared at about where the two studs are threaded into the nut. I don't have a camera with me but you can see on the fracture face radial lines, which tells me that the coupling had some amount of torque applied. For a coupling you do not want this situation. You want pure tensile force. This lead me to believe that the nut failed due to a combination load. Torsion and the tensile load.

-----Proposed Solution-----
Using Distortion-Energy Theory for ductile materials says that ductile materials will fail due to shear and the maximum shear that a ductile material will handle is given by the following equation.

S (maximum stress due to pure torsion) = 0.577*Sy (yield strength)

So then I calculated the amount of shear stress; assumed that all 2500 ft-lbs was translated to the coupling.

shear stress (tau) = Torque*radius/J (polar moment of inertia)

Does anyone have the formula for J; for a hexagon with a hole in it?

Then I assumed that the 40,000 psi pre-stress was applied.

sigma = 40,000 psi

Then just use the principal stress equations to calculate the combined effects or use Mohr's circle.

I ran the calculation for a cylindrical shape and depending on the strength of the steel it is very close to the failure ellipse do to combined effects.

Does this method sound correct to solve this problem?
--------------------------------------------------------

---- Answers to questions ----

+ This is a static loading condition.

+ The only way that the stud could apply a torque to the coupling is due to dirty threads. If anyone else has some ideas they would be appreciated. But, this is the only way that I can think of.

+ I don't know if the nut failed at 40,000 psi, it might of been more, might of been less. I have to assume this value because it was recommended.

+ The stud was okay, the bolt just broke in half. No yielding, just pure shear.

+ All I know about the steel, is that the recommended grade is B87. I do not know what this means. I am still tracking this down. I know that this is standardized. Does anyone know what the standard is for a heavy hex coupling?

I hope this helps all, and I will try to check it periodically. One more question to throw out, wouldn't you also have stress concentrations at the points of the hex nut? If so, how would you factor this into the equations.

 

[desertfox]

hi jhamil1

Did the coupling fail during tightening? or later during normal operation whatever that might be.
How are you applying the pre-load by means of a torque wrench? if that is your method, thats not very accurate because of friction ie dry or lubricated threads, thread form,surface finish etc, you may have over loaded the threads during tightening hence the failure.
The only time the coupling would see a combined load is during tightening, once tightened the threads only see tensile load, unless there was some relative twisting of one stud to the other in service.
You cannot assume that all the torque is absorbed as shear stress on the coupling because of friction losses.
You state it is a static loading case and whilst there will be stress concentration factors with threads, they will have little influnence on ductile materials under this type of loading.
We need some more information about the material, I don't understand why you think the nut would fail under the 40000 psi pre-load stress if that was the pre-load stress recommended.
What is the application of the coupling after assembly? what method is used to tighten the joint? what loads or temperature does the joint see in service?
Finally you are assuming a failure of ductile materials under combined loading in which case the failure would be along the principle planes which run at approx 45 degrees to the normal plane, I would expect the fracture faces to be angled roughly at 45 degrees. However a ductile failure under pure torsion should have fracture faces more or less vertical to the coupling central axis.

regards

desertfox

 

[dinjin]

I think the B87 is the hardness of the steel and I assume it is the min hardness.

 

[jhamil1]

Background

The foundation under a large reciprocal compressor was cracked due to years of fatigue. This foundation was replaced by jack hammering the old foundation out. During this process the old stud was cut which mounted the engine to the old foundation and a new coupling-stud was used after to secure the engine to the new foundation.

Desertfox,

1) The coupling did not fail during loading, a vibration was noticed and they found the nut securing the engine to the foundation was loose. They tried to re-torque the nut and it would not hold the torque. They picked up on the nut/washer assembly and found that the coupling had broke. This was probably a week after the foundation/coupling was replaced.

2)The threads were lubricated and torqued using a hytorq torque wrench. This was one question which I had, and we are currently checking into the last calibration.

3) As far as assumptions, I am assuming the worst case scenario. Which is that some way all torque was transferred to the coupling and the pre-load was at the recommended value. This was the original question. If this situation did exist does it apply enough stress to the coupling to fail.

4) For stress concentration I was referring to the six corners on the outside of the heavy hex coupling.

5) The material, still trying to track that down, but I was assuming a value for a low carbon steel with a yield of 105,000 psi.

6) The assembly may see, max 100 degF.

7) The fracture face is perpendicular to the central axis of the coupling. Which I think you were saying is a failure due to torsion.

Dinjin,

8) That's what I was thinking the B87 was, just the Rockwell hardness.

 

[MikeHalloran]

By "pre-stress", do you mean that the new stud was run into the coupling until its face touched the end of the old stud, and then the new stud was torqued? Given any stress raiser, e.g. the internal threads, that would tend to crack the coupling apart at its midplane, even with no external load applied. I don't understand why one would want to do such a thing.

Normal practice for studs in bulk parts, e.g. engine blocks, is to _not_ allow the stud to bottom out in the hole. I don't know why a coupling nut would be treated differently.





Mike Halloran
Pembroke Pines, FL, USA

 

[desertfox]

hi jhamil1

Now were seeing a different picture, if this coupling didn't fail during tightening then it failed under cyclic load from engine vibration ie fatigue.
Given this situation the nut on the stud is torqued up and the shear stress due to tightening is removed (ie wrench removed) were does the coupling now get its torsional shear stress from? Given Mikes assumption above to be true if the new stud touched the old stud and the new stud itself was torqued that may have caused the initial crack in the coupling and the vibration finished it off, however this would not answer the question I just posed about the shear stress.
I think you need to look at the loads generated by the compressor into the foundation bolts as a starting point.
If those threads on the stud were lubricated then check that the torque figure was for lubricated threads and not dry threads.I think B87 is the hardness also, however who selected the grade of coupling and stud because they should have been chosen on knowing the load conditions on the studs from the compressor.

regards

desertfox

 

[Bribyk]

I think Mike and desertfox have hit it on the head. If they torqued the coupling down to the recommended/required torque for the old stud you now have the recommended/required preload (tension) on the coupling. Now, if you put the compressor frame on and torque the nut down to the recommended/required torque you have 2X the desired preload on the coupling and "the vibration finished it off". The stress concentration from the internal threads is going to have much greater effect that the SC from the points on the external hex.

Can you distinguish the tell-tale fatigue beach-marks on the failed surface of the coupling?

 

[desertfox]

Hi Bribyk And jhamil1

I think if you look at the radial marks you mentioned in your earlier post Jhamil1 they could be the classical fatigue marks.

Regards

Desertfox

 

[jhamil1]

All,

Been on the road again, sorry for the delay. I think you guys a lot for your contributions. After looking at the failure several times, and Mark's comments on the studs touching at interface makes more sense. At the break interface, the threads are broken in the valley, which is the greatest stress concentration. I don't think it can be pinned down to one thing, I think it is a combination of events.

1.) They ran the studs in the coupling until they touched and torqued the nut on the stud.

2.) I think some torque had been transferred to the coupling via dirty threads.

3.) Thermal growth or vibration as desertfox suggested.

I think the root cause was the studs touching and the coupling cracked in the valley of thread, once this happened the vibration, thermal growth, and torque finished the coupling off.

Recommendations

1.) Make sure threads are clean and lubricated with the proper lubricant.

2.) Run new stud into coupling until they touch and then unscrew new stud until an inch of clearance is left between new and old studs.

3.) Change torque value from 2500 ft-lbs to 1875 ft-lbs, which is still within specification to reduce the operating condition versus theoretical stress combination if the torque is totally transferred to the coupling under a 40,000 psi pre-stress.

 

[HeviiGuy]

Jhamil,

Please forgive my absolute ignorance but, I simply can't grasp how you're going to preload the top-half of this foundation-bolt configuration to 40,000 psi without the other end of the stud being afixed.

If you run the new stud into the coupling and leave 1" or so between the old and new studs as you plan to do, nothing is going to prevent the "new" stud from turning while the nut is being tightened. It'll simply continue to turn down into the coupling until it meets an obstruction, namely the "old" stud. At that point, and only at that point is when preload will be generated (and of course, you'll also apply torsional load to the coupling). It's quite likely that you'll end up with nary enough clamp force to prevent the cyclic loading to once again finsh off the coupling.

Ciao,

HevïGuy

http://www.heviitech.com

 

[desertfox]

Hi jhamil1

thanks for the feedback, but you cannot assume all the torque is transfered to the stud, the combined load only exists during tightening. Have a look at this site on pre-loading bolts:-

http://www.roymech.co.uk/Useful_Tables/Screws/Preloading.html

Also you cannot just reduce the torque on the stud without considering the external loads exerted on it.
If the stud pre-load is to small then a fatigue failure will
follow.

Regards

desertfox

 

[jhamil1]

Sorry for the long over due post but I wanted to finalize the information so that it could help someone else with this problem. I contacted the company that used the coupling and stud combination. I finally got a mill spec. on the coupling material. The yield strength for the coupling was 155,000 psi. Just if anyone was wondering. I also discussed the studs touching and sent them the failed coupling. They also agreed with the root cause of the failure. Heviiguy, to answer your question of how we will keep the studs from touching. Primary discussions have lead to two possibilities.

1. On the new stud measure an inch up from the bottom of the thread and punch the threads so that the stud can no longer be threaded into the coupling.

2. Place a washer (smaller than the i.d. of the coupling bore) between the studs. This will eliminate the wedging effect that the studs cause when they come in contact.

Again, thank you all for your help.

 

[HeviiGuy]

jhamil1,

With all due respect, this "solution" is nothing of the kind. Punching the treads will likely not stop the stud turning within the nut, bottoming out on the top of the lower stud and then spreading and twisting the coupling into oblivion. At the very least, the threads will be so chewed-up that you'll have scant hope of retaining much load. The configuration is likely to fail yet again.

Here's an alternative:

Eliminate the torsional loading altogether. Rather than turning, stretch the exposed stud until you've achieved the 40,000 psi bolt stress and then wind-down the nut "finger-tight" until it meets the washer at the base. This will retain the load when you release the pressure used to "stretch" the stud. This is the fundamental process of bolt tensioning. You can do this by using a bolt tensioner as a tool to develop axial load or by using a hydraulic nut which replaces the hex nut altogether and stays in place.

Here's a link explaining the tensioing process:

http://www.heviitech.com/Hevii_TensioningBasics.html

Here's one describing hydraulic nuts:

http://www.heviitech.com/Hevii_HydraulicNut_Intro.html

Ciao,

HevïGuy

http://www.heviitech.com

 

[HeviiGuy]

One more thing. It's so obvious that I didn't even bother to note it: "lock" the both the lower and upper studs onto the coupling with a nut at either end. You'll have a convoluted mess which should actually be replaced by a new stud with the proper length but, at least by tensioning this mess you'll be doing much better than before.

Ciao,

HevïGuy

http://www.heviitech.com