[robz71lm7]

So a certain student's machine design professor has pushed the idea into people's heads that we should use SAE Grade 5.2 or greater for all thing structural or important. Good luck finding those bolts, I knew they existed, but have never seen any. His logic is that the yield strength of a grade 5 bolt changes when you get over a certain diameter (1" IIRC) whereas the 5.2 grade's yield strength is constant. We also had a machine design project where we had to specify a size and preload for a grade 7 bolt. (IMG:http://www.frrax.com/rrforum/style_emoticons/default/rolleyes.gif) Also according to him, nobody makes bolts with cut threads any more. Aren't most cheap chinese bolts manufactured with cut threads?

[35th_Anniversary...]

For our machines we always use rolled threads (not cut). Of course our threads are holding up to 270,000 lbf of load.

We also hydraulically tension the main bolts. That is we put on a collar, apply hydraulic pressure to stretch the bolts to get the proper pre-stress then hand-tighten the nut with a metal stick (nut is round with several holes drilled in it.