Posted by fdt860 on December 04, 2021 at 00:36:34 from (90.93.199.3):
In Reply to: Why posted by SVcummins on December 03, 2021 at 22:35:13:
Because the bolt stretch is much more accurate this way.
When you are tightening a bolt, the clamping force is directly proportionnal to the bolt stretch.
When you do torque only, the variables such as amount of oil,under head surface condition, threads conditions and threads fits, give to much uncertainity on the friction factor between the bolt and the parts, and you may end up with different clamping force.
The torque and turn solution also has variables during the first torque sequence, but the torque being smaller, you have less error.
The torque and turn method also allow to have the final tightening right so the bolt is stretched up to the striction point (where it plastically elongates and will not return to initial length). That is the biggest advantage, but also why you never reuse torque and turn bolts.
This allow for very consistent clamping force, because the force does not vary much between beginning of plastic stretching and breakage. That point is totally unobtainable with torque only, because you may break half of the bolts in the head due to uncertainity of the friction factor.
If you reuse the torque and turn bolts, you may stretch them too much this time and go in the breakage zone.
All of the above also explains why the one time use made to stretch bolts have a smaller shank diameter.
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