Quote:
Originally Posted by dradernh
I've seen this type of spec before, and I suspect you're just the person to explain it in plain English. 
Can you do that for us?
TIA. |
The bolts are first tightened to a value that gives a consistent clamping force on the bolt head (the force tensioning the bolt along it’s length), but not enough torque that variability in the thread friction becomes a big factor in what the final clamping force is - this value is the jointing torque and provides about 1/3 to 1/2 of the final clamping force.
The final bolt tightening angle is then applied. As the jointing torque has ensured there is no gap between the bolt head and the components being clamped together by the bolt, this final angle only stretches the bolt. In the case of a 60 degree angle, that would mean a stretch of one-sixth of the bolt thread pitch. Most of this stretch will occur from between where the bolt enters the threaded component and the bolt head.
To make the bolt stretch apply the most consistent clamping force, the amount of stretch for a given bolt size is calculated to stretch the bolt into it’s plastic deformation range as opposed to it’s elastic range. Where the stretch in a material changes from elastic to plastic is the material yield point, hence “stretch to yield”.
Reusing a stretch-to-yield / torque-to-yield fastener is problematic, as it is never the same in properties after it has been deformed in this controlled manner and will certainly have less than the design safety factor if stretched again. In best case for reuse if it absolutely has to be done, the bolt has to be rotated to exactly the same position as when it was undone, as applying the same jointing torque and angle again will just stretch it more, lengthening it beyond what the design envisioned. It is also important to use a bolt with exactly the right grade, head and thread length of the original to ensure designed clamping force and safety margin is achieved.
Sorry this is a lengthy explanation, but hopefully I made it as non-technical as I could. I can throw around some sample calculations for typical bolts to show how the basic maths works