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. Also according to him, nobody makes bolts with cut threads any more. Aren't most cheap chinese bolts manufactured with cut threads?

I don't think I know wht the markings look like, so i have no idea if I've ever seen one. Grade 5 usually has 3 "tick" marks on top and grade 8 has six. The metric ones usually say the grade number on the head. What does a 5.2 look like? A grade 7?

http://www.americanfastener.com/technical/...kings_steel.asp Three tick marks in a special pattern. My guess is in the real world 5.2 bolts will probably be more expensive than readily available grade 8 bolts.

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.

Yuck.

I'm a guy who never made it past the "weed out" classes, but do you guys have to do budgeting or anything for your design projects? If so, make sure you check out the next grade up that is readily available to see the price difference. I say this because I've seen some amazing things in reference to purchasing vs. engineering. Purchasing doesn't know the difference between Grade 5, 5.2, or Grade 12345. They order what they're told, even if something equivalent would get the job done for 1/10th the money.

If not, remember it for when you're designing things and the one ordering them isn't an engineer

Good luck with the class, sounds like a pain.

I thought most hardware had cut threads, but you can always get stronger rolled threads if you order them. I always thought the stuff you find in the hardware store is cut threads, because cut threads are a hair cheaper.

Rob I can tell you that alot of cut treads come out of China on bicycle's They are a pain in my side!

But I also get rolled tread's too, tread's that are rolled in a perfect circle

I've seen one of those. A bolt with infinate threads per inch.

i'v seen the 3 tick bolts. i had to replace the tranny on the POS honda and the tranny bolts were those. they were cut threads.

from what i learned at the lumber yard when i was younger is: all bolts and screws are made round, with the heads still on, and no threads. then they are shaped and sized down, then the threads are cut last.

about the rolled threads...i have no clue. never even heard of them

Instead of using a die to cut the threads they use small rollers that deform the materal into shape to form threads. Rolled thread are stronger then cut threads, but cut threads are cheaper and sometimes quicker to make.