calculation for tightening torque

[rainaaniket]

Can someone suggest what percentage of proof load for bolt should I consider for calculating bolt clamping load if the bolt is of class 8.8 and the threaded hole is made from aluminium. (Most charts available calculate the clamping load at 75% of proof strength) One of my seniors suggested to go for 60%.

 

[MintJulep]

You'll be limited by the pull out strength of the aluminum.

See my post #32 in this thread for his to calculate it.

Tapped axial holes on tube - Mechanical engineering general discussion

I have to design a fixture that is a roughly a 23" ID tube w/ a 1" WT that will have a tapped bolt pattern around the flat base portion of the tube. I've been trying to find how I can calculate what kind of stress would cause the part to fail. My biggest concern is that I'm aligning with 1/2"...

www.eng-tips.com

 

[CWB1]

Why reduce it?

With BJA you're primarily juggling the 1. number of fasteners, 2. diameter of fasteners, and 3. diameter of washers to ensure you're A. stretching fasteners to 75-90% for durability, B. without yielding material underneath your bolt head, and C. providing the necessary clamp load.

If you're stripping threads then your fasteners are too large. If you're yielding the material underneath the bolt head then you need a larger bearing area (washer). If you need more clamp load then you need more and/or larger fasteners.

If your shop has a standard set of torque values that they use when prints dont callout specific torques, you should be using that to communize print callouts AFTER running a BJA, but otherwise engineers should never need a torque chart. Usually that indicates somebody's guessing rather than engineering, which inevitably causes mistakes.