Main stud preload is important as torque levels rise, to keep from unloading the cap-to-block joint.
If the joint stays together, then the fastener only experiences the steady preload.....no cyclic fatigue loads. So the fastener will live a long happy life if it's preloaded above operating loads. Cap fretting is a warning sign that the joint is unloading.
The overall strength of the threaded fastener looks at (3) factors:
a) tensile strength of fastener which is the thread stress area times the ultimate tensile strength of the material
b) thread stripping strength of the fastener which is governed by the fastener material, thread shear area and the length of thread engagement
c) thread stripping strength of the tapped hole which is governed by the same 3 factors. But the shear area of the thread in the block is larger than the shear area of the fastener since the effective diameter is bigger.
So the weaker block material can hold a stronger fastener material if the engagement length is sized correctly and the differences in thread shear area are understood.
Eric
__________________
"Everybody has a plan until they get punched in the mouth" noted philosopher Mike Tyson
Life begins at the end of your comfort zone.
“The mind, once stretched by a new idea, never returns to its original dimensions.”
|