Screws are notched components. Why are they considered as such? For their threading. And where the screws are subjected to repeated and irregular stress? In some cases the break occurs in the first load-bearing part of the thread, at the entry into the internal (female) thread. (in 90%)
What is it necessary for the design?
In these cases the design must allows for the fatigue strength ±σᴬ of the screws. This amounts to a fraction of the tensile strength, independent of the static loading.
The fatigue strength of fine threads decreases:
- with increasing rigidity
- with fineness of thread.
The fatigue strenght of fine threaded fasteners with a property class of 12.9, it can be up to 30% lower than for coarse threads.
How can further increase the resistance?
Basically, all measures which can reduce the effective peak stresses or prevent combined loading (loading along more than one axis), are suitable for increasing the fatigue strength of the screwed connections. What’s else?
- Long rather than short screws
- Screws with waisted shanks rather than screws with normal shanks
- Pins or fitted shoulder screws to absorb lateral forces, adequate and above all controlled prestressing of the screws.
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