Reply With QuoteThe strength of bolts loaded in tension can be
easily determined by the ultimate tensile
strength. To determine the amount of force
required to break a bolt, multiply its ultimate
tensile strength by its tensile stress area, As
Determining the strength of the threads is more complicated. Since the male threads pull past the
female threads, or vice-versa, the threads fail in shear and not in tension. Therefore, the stripping
strength of an assembly depends on the shear strength of the nut and bolt materials.
To determine the force required to strip the threads, multiply the shear strength by the cross
sectional area being sheared. The difficulty lies in determining the
cross sectional area in which the shear will occur. Here are three
possible scenarios for this type of failure.
1. The nut material is stronger than the bolt material. In this
example, the nut threads will shear out the bolt threads. The
failure will occur at the root of the bolt threads.
2. The bolt material is stronger than the nut material. In this
scenario, the bolt threads will shear out the nut threads. The
failure will occur at the root of the nut threads.
3. The nut and bolt are the same strength. In this scenario, both
threads will strip simultaneously. This failure will occur at the
pitch line.
The tensile strength of most fasteners is usually specified, whereas
shear strength is not. In order to avoid shearing the threads, ensure
that the length of engagement between the internal and external thread is long enough to provide
adequate cross-sectional thread area.
Failure scenarios #1 and #3 can typically be avoided by ensuring proper thread engagement. With
proper engagement, those scenarios would result in a tensile failure of the bolt rather than thread
stripping.
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Originally Posted by moneytree
