Bolt Fatigue 4

[robsb123]

Does anyone have any specs on the fatigue strength of grade 5 and grade 8 hardware?

I have an alternating tensile load, alternating from "zero" (just the tension from the nut) to a larger positive value.

 

[NickE]

There has to be somewhere...

http://www.google.com/search?hl=en&...Fatigue+Strength+of+Grade+8+bolts&btnG=Search

http://www.google.com/search?q=Fatigue+of+Bolted+Joints&client=netscape-pp&rls=com.netscape:en-US

 

[TVP]

robsb123,

What exactly does "just the tension from the nut" mean? The fatigue strength is affected by the mean stress, the alternating stress/stress amplitude, the thread geometry (M vs. MJ thread), the surface quality (rolled vs. cut threads, decarburization, etc.), and the manufacturing process (cold heading vs. hot heading vs. machining). Unless you are controlling all of these variables it is difficult to determine a fatigue strength for SAE grade 5 or 8 bolts.

 

[dimjim]

You have the worst case load situation.
Why can't you torque these to a greater
load condition?

 

[swall]

What is the joint configuration and how many bolts are there? Threads rolled after heat treat or before?

 

[robsb123]

The joint configuration is a large flange (94" bolt circle diameter). There are 24 bolts. The flanges are on large rotating cylinders. As the cylinder rotates it puts alternating stresses on the bolts. When the bolt is at the bottom of the rotation it experiences maximum tension. When it is at the top of the rotation the bolt only experiences the stress from the pretension caused by tightening the nut (an some shear stress).

 

[arto]

ASME B&PV Code Sec. VIII Div 2, App5 par 5-120 has some info on bolting fatigue

 

[CoryPad]

Joint loading can be complicated, and this is a case where a picture is worth more than a thousand words.

Are the holes clearance holes? That means that the joint preload and friction at the mating surfaces is what holds the parts together. The bolts should not have any shear stress from external loading.

There are some basic numbers for fatigue strength (tension-tension fatigue) of fasteners. According to VDI 2230, the following equation is good for metric fasteners heat treated after thread rolling:

[σ]_ASV = 0.85 (150/d + 45)

where

[σ]_ASV is the fatigue strength in MPa
d is the bolt basic diameter in mm

Regards,

Cory

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

 

[PriamEngineering]

BS7608 gives the fatigue strength of steel bolts as 6% of the UTS. Note that irrespective of the actual UTS it should not be taken to be greater than 785MPa.

 

[btrueblood]

Uh, 6%? Did you not mean 60%?

 

[CoryPad]

No, it is approximately five or six percent. This is for a threaded fastener (full of fatigue initiating notches, also known as threads). If you machine a smooth specimen from the fastener, the fatigue strength would be much higher.

Regards,

Cory

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

 

[TVP]

Keep in mind that this 5-6% of UTS number is the alternating stress that is applied to a bolt with a mean stress of usually 50-60% of the UTS. It is extremely important to be clear about the loading conditions when discussing fatigue (mean stress, alternating stress, stress amplitude, stress ratio).

 

[Metalguy]

>"As the cylinder rotates it puts alternating stresses on the bolts. When the bolt is at the bottom of the rotation it experiences maximum tension. When it is at the top of the rotation the bolt only experiences the stress from the pretension caused by tightening the nut (an some shear stress)."<

If you can tighten the bolts high enough, the difference in applied loads between top and bottom will be a few % at most.

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