SEBASTIANOFA,
You need a precise knowledge of your thread's major diameter and its pitch to do any meaningful calculations. You need to be way more precise than 1-1.5cm! The metric unit of length on engineering drawings happens to be millimetres.
By the sound of it, you are being asked to torque your bolts to 100N.m, then, turn the bolt another 1/4[ ]turn. This makes sense to me. You preload the bolt to some known value, and then apply a fixed rotation to it. I usually think in terms of finger-tight plus nominal rotation, but just what is finger tight? Any liquid will give you better control over friction.
I wonder about your bolt sizes. I am just quickly evaluating some metric grade[ ]10.9 bolts here at 75% of yield stress and a friction factor of 0.2. The 0.2 friction factor is generally used on the assumption of no-lubricant, which means you have poor control over friction.
M10x1.5, [—] 72N.m
M12x1.75 [—] 126N.m
M14x2 [—] 201N.m
M16x2 [—] 312N.m
Your initial torques have to be comfortably below your initial yield stress ones. In other words, you can apply 40N.m[ ]torque plus 1/4[ ]turn to an M10x1.5[ ]bolt. You cannot apply 100N.m.
My best guess is that your original manufacture made assumptions and they did calculations. Their instructions are based on these, so you had better follow them. Your next choice would be to do your own analysis and figure out what assumptions they made. Friction factor? Percentage of yield?
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JHG