Fatigueproof bolt failure

[SSWWRITE]

I am manufacturing a hex shaft( fatigueproof material) with a 1/2-13 o.d. thread 1'' long on the end. Customer is having parts break at the threads. They are telling me that the torque on this should go up to 165ft. lbs. - Is this correct? I could not find a formula for the fatigueproof. They are telling that these are breaking at 80ft. lbs.

any help is appreciated.

 

[desertfox]

Hi sswwrite

We need lots more information like:-

hexagon shaft size.

material grade and any heat treatment etc

length of shaft and application, loads are they constant,
alternating etc.

For a fatigue failure you need varying stress cycle, at present all we have is the torque at failure and a theoretical figure of what the shaft should withstand.
What do the failed parts look like:- is it a helical fracture or are the faces of the failed shaft roughly parallel to each other.

For a hexagon shaft the torsional stress is given by:-

stress(shear)= 1.09*T/(b^3)

t=torque in in-lb

b= length of one hexagon flat side


regards

desertfox

 

[SSWWRITE]

shaft size is 3/4 hex about 22in. long. the thread is turned and machined on both ends.

material is fatigueproof from niagara lasalle. no heat treat. not sure of load. They tell me that they put a nut on the thread and torque to 80ft. lbs and it snaps. the fracture looks like Styrofoam in a way. It is breaking at the root of thread where i would think it would fail. but the break is not clean.
thanks

 

[desertfox]

Hi SSWWRITE

Okay I found the info from niagara lasalle and the material
tensile strength is given as 140000psi.
Now a quick calculation shows the shear stress under 80lb ft
torque to be about 67,426 psi.(at root of thread)
Is it failing at the undercut at the back of the threads which is where depending on your machined profile you will have a large stress concentration locally and possibly
why its failing.
When they put the nut on the shaft are they using any lubricant or not?
Reason I ask is that when you tighten a nut on a thread the
bolt or shaft will see a combined stress of tension and shear and I can work this out, however if they are using a lubricant on the threads the tension in the shaft can be double what it would be, if they weren't using lubricant.
From what your saying in your post the failure occurs while the torque is still being applied.
Not sure what you mean when you say it looks like Styrofoam
but if the failure is jagged and angled it suggests a brittle failure but can't be 100% on that.
I will run another calculation and come back soon.

regards

desertfox

 

[hydtools]

I think the thread is breaking because the tension due to tightening the nut is exceeding the capacity of the 1/2-inch thread.

You might have to consider a larger thread for the torque the customer wants to apply to the nut. Or much higher strength material than Stressproof.

See this reference

http://www.zerofast.com/torque.htm

for 'standard' torques for common bolts and grades.

Ted

 

[hydtools]

Here is Stressproof page on the Lasalle site.

http://www.niagaralasalle.com/products/stressproof.html#specs

It show tensile of 110,000 psi(min) and yield of 100,000 psi(min).

Ted

 

[MikeHalloran]

I think my old Ford FE engine had 1/2-13 head bolts and main bearing cap bolts, made of damn strong stuff with rolled threads, and they're only supposed to be torqued to 110 ft-lb. 165 ft-lbs is a _lot_ of torque for that thread size... as is 80, on a cut thread.

Perhaps your customer has been confused by the zoomy names that LaSalle uses for their high strength machinable steels.

(I was. Big Stressproof(r) bars come with almost 1/4" of extensively cracked 'bark' on their surface, useless material that you have to pay for and then remove. To get a clean surface you have to buy the G/P stuff or buy way oversize. I tossed a couple tons of beautifully machined parts and bought new ones made from something else without the cracks.)

I'm intrigued by your description of the fracture surface. I wonder if it has to do with the microstructure that makes the stuff machinable.



Mike Halloran
Pembroke Pines, FL, USA

 

[TVP]

Mike,

I love the "bark" descriptor for the decarb/scaled surface on the Lasalle bars. The answer to your question about microstructure is "yes". The large amount of inclusions/stringers create a fracture appearance that is somewhat different from that of conventional or clean steels.

 

[hydtools]

I apologize for my error referring to Stressproof and not the subject Fatiqueproof.

I still believe the material is not strong enough for the 1/2-inch thread tightened to 80 ft-lbs.


Ted

 

[desertfox]

Hi SSWWRITE

Any chance you can post a picture of a fractured shaft it may help.
I can't get it to fail with a combination of torque and stress but maybe using a dry friction fiddle factor isn't
realistic.

regards

desertfox

 

[hydtools]

The thread is failing in shear due to the torque. If we can assume that the maximum shear theory applies, then the maximum torque capacity of the thread root is 68.2 lb-ft.

From shafting equations, shear stress = 16*T/(pi*d^3). Solve for T(torque,lb-in) using maximum shear stress = 0.5*yield stress.
Fatiqueproof yield strength is 125,000 psi.
Maximum shear stress = 62,500 psi.
Thread root diameter = 0.4056 inches.

Ted

 

[TVP]

SSWWRITE,

Do you know what type of nut is being torqued onto this shaft, i.e. are they using a standard machine nut or are they using a nut with a prevailing torque feature? If they are using an all-metal prevailing torque nut with poor lubrication, then the failure mode is quite understandable. If it is a standard machine nut with very good lubrication, then the torque is too high. Otherwise, it is likely a combination of torque application, surface defects, variable friction, etc.