tensile stress in bolt 1

[mielke]

I am using Austenitic SS 304 annealed bolts (3/8"-16) and hypothetically say i am torquing it to 290 in-lb. using the following relationships i determine the tensile stress developed in the bolt...

F = T/[KxD] = (290)/(0.2x0.375) = 3875 lb
P = F/A = (3875)/(0.0775) = 50,000 psi

where
F is force in pounds
T is torque in inch pounds
D is nom diameter of bolt
P is stress
A is tensile stress area

I get a stress of 50,000 psi and I am a little confused on how to use this value. the yield strength for this bolt is 30ksi while the tensile strength is 75ksi. For such an application would i use the yield strength or tensile strength?

 

[desertfox]

Hi mielke

I would advise you use the yield stress and not the tensile stress.
Depending on application, bolts are torqued to either 70% or 90% of the proof stress or yield stress.
However all joints should be analyised based on external loads and the strength of materials being clamped together.

Look at this site;-

http://www.roymech.co.uk/Useful_Tables/Screws/index_screws.htm

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

desertfox

 

[SnTMan]

mielke, ask yourself, "Do I want the bolt to YIELD?" The answer will inform your selection.

You should also know that the correlation between torque and bolt stress is very inexact. This has been discussed many times, a site search will no doubt provide you further reading.

Regards,

Mike

 

[seymours2571]

A read of specificaiton VDI 2230 could prove helpful. It is a pretty exhaustive bolt specification pertaining to the design and loading of bolts/fastners.

Stephen Seymour, PE
Seymour Engineering & Consulting Group

http://www.seymourecg.com

 

[Twoballcane]

290 in lb sounds way to high, for this bolt I think that runs to about 156 in lb. So this should give you about 27ksi which is below the Sy. Why the high torque?

Tobalcane
"If you avoid failure, you also avoid success."

 

[rb1957]

if the preload stress (50 ksi) exceeds the yield stress, wouldn't you think you've yielded the bolt ?

normally limit preload to 70% of yield, then compare this preload to the maximum limit in-service load (preload should be higher, to prevent the joint gapping).

then check the ultimate condition, with the ultimate load only (as the joint wil have gapped and all the load is in the bolt)

 

[mielke]

what exactly is the use of the tensile strength then?

 

[SnTMan]

Well, it tells you when the bolt will break, but the yield tells you when you start losing joint load.

In simple terms, all joint load will be lost well before the bolt breaks.

Regards,

Mike

 

[rb1957]

"what exactly is the use of the tensile strength then?"

like SnT says ... exceed yield strength and the bolt will yield, exceed ultimate strength and you have two pieces of bolt.

do you think you want to yield the bolt from preload ?

 

[Twoballcane]

"what exactly is the use of the tensile strength then?"

Now you're scaring me. I'm not trying to be crass, but if you don't know or understand Failure Theories (did not you take this back in college?), I suggest hiring a Mechanical Engineer.



Tobalcane
"If you avoid failure, you also avoid success."

 

[GregTirevold]

The reason minimum ultimate tensile is given in addition to yield or proof is that different specs and codes use different values to determine the allowable stress of a pre-loaded bolt.

Some say 50% or 55% of Minimum Tensile Ultimate while others use 75% of Minimum Tensile Yield and so on.

Hence the ASTM/SAE Grade designation lists Ultimate, Yield and Proof strengths.

Other factors like using a washer or not, the friction of the bolt versus the clamped parts, the strength of the clamped material, etc also play a role. If you are designing to a spec or code most of these decisions have already been assumed for you and the math is just a rote exercise or chart lookup.

 

[rb1957]

arr umm, noo ... the reason ult tensile strength is given is so you know the strength of the bolt, when will it break. The reason yield strength is given is so you know when it'll yield. Why would we have tables of ultimate strength and yield strength of anything, like Steel, Aluminium, ...

 

[GregTirevold]

rb1957, I must not be making myself clear or maybe I misunderstood the OP's area of confusion.

My meaning is that some code books may list the proper amount of pre-load in terms of ultimate strength of the bolt rather than in terms of yield strength. One may say "use 35% of ultimate" and another may say "75% of yield". In effect they are saying the same basic thing since .35*Fu is about the same as .75*Fy with common steel fasteners. Machinery's Handbook explains this better on their section on preloading bolts.

Maybe the OP didn't understand the definition of Ultimate vs Yield but I didn't read it that way. I thought his question was more of "why doesn't everyone use the same point of reference?".

 

[rb1957]

cool ... i thougt you were following on from his rather remarkable post "what exactly is the use of the tensile strength then?"

 

[JTRY]

For a gasketed flanged joint , when the bolts starts yielding the joint integrity is lost and leakage starts(joint failure is a leak).For such an application yield stress is the basis of design.

For a non gasketed joint , leakage is not the criteria. Although exceeding the yield stress will loosen the joint , it is good to know when it will come apart(joint failure), if you compare the bolt stress with ultimate tensile strength.

I would however use only yield stress criterion for a bolted joint design