why does ARP use the stud/nut instead of the stock style of bolts?
it seems you can get a more uniform accurate torque setting with the stock bolts than the ARP because there are less moving parts...one verses two.
do i make sense?
it says to hand tight studs, then tighten nuts...won't the studs back out over time if they are only hand tightened and then the nuts will also back off?
where as stock ones you just thread and tighten to proper torque settings...


[Modified by koy, 8:15 PM 2/8/2002]

 

This is what I understand:

When you tighten bolts, you are applying both a torque and tension to the shaft. The torque portion is a sort of necessary evil with bolts, as it doesn't actually do anything for it's function, and it stresses the shaft more, on top of making the tightening process less accurate. You just need the tension to clamp parts together.

Because studs have two separate parts, the shaft receives no torque during tigtening, only tension. The nut receives the torque, and it twists because of it. The shaft doesn't twist.
You tighten the stud by hand, which would let it come loose because the friction between the threads on the shaft and the block isn't high enough to prevent it from coming loose. However, when you properly tighten the nut, it pulls on the stud, increasing this friction to the point that it won't come loose.

Bolts are cheaper than studs, hence why they're used by manufacturers.
Studs are an engineer's decison. Bolts are an accountant's decision (or something like that, according to Corky Bell).

Feel free to correct or simplify any of my explanation....

 

Interesting. I've never thought about this before.

Re torquing bolts, you said, "...it doesn't actually do anything for it's function, and it stresses the shaft more, on top of making the tightening process less accurate"

I thought the purpose of torquing was to establish the stress (tension) on the shafts accurately... or at least consistently.

 

You're right, that is why you torque bolts. It's the most accurate way of establishing tension in the shaft. My point is that it also establishes a shear stress in the shaft. If there was a way do establish the tension without also causing shear, we'd all be better off. That's what a stud lets us do.

 

The most accurate way of determining tension in a bolt is by measuring strain, not torque.

No matter what type of fastener you are using, be it a cap screw, bolt, or stud, there will always be both tensile and torsion stress. Heres my analogy: The threads on a stud are like an inclined plane and the nut is like a heavy box that you are trying to push up the inclined plane. Not only is the box pushing directly down on the plane (tensile stress), but the force you apply to push the box up the plane attempts to cause the plane to slide horizontal to the ground (torsion stress). Hopefully you can visualize what I'm talking about.

Studs are chosen over cap screws and bolts because they eliminate the stress concentrations found at the interface between the bolt shank and head. The rapid change in geometry from the shank to the head creates a stress riser that will cause the head to shear from the bolt long before the shank is critically stressed. Obviously, studs don't really have this problem because the head (nut) and shank (threads) are two separate pieces.

 

Interesting. Does that apply to a nut on a bolt also? I mean, if you torque the nut instead of the bolt, then the bolt (shaft) won't get the shear forces. Is that right?

Day-um. The things you learn here!