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





RE: Bolt Fatigue
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RE: Bolt Fatigue
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.
RE: Bolt Fatigue
Why can't you torque these to a greater
load condition?
RE: Bolt Fatigue
RE: Bolt Fatigue
RE: Bolt Fatigue
RE: Bolt Fatigue
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:
σASV = 0.85 (150/d + 45)
where
σASV 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.
RE: Bolt Fatigue
RE: Bolt Fatigue
RE: Bolt Fatigue
Regards,
Cory
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
RE: Bolt Fatigue
RE: Bolt Fatigue
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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