bigs

OK Stupid question time. You wrenches can educate me quickly.

I've read a bunch of threads emphasizing the importance of tightening nuts/bolts to their proper torque. Who at the factory determines what the proper torque is for each item, and how?

macreel

Ah, (Dr.) Bigs,... a question easy to ask but it has a long answer.
The short version is: physics, engineers and experience determine
torque specs depending upon use, environment and materials.

Testing labs, engineering design handbooks and regs have a say.
(That's where we get: SAE, DIN, ANSI, ASTM, IEEE, NIST, etc.)

G'luck.

Garth S

As macreel infers, the truth about torque is shrouded within 'engineering speak' ** - and has a lot to do with the symmetric distribution of clamping forces for the assembly in question. How much clamping force is required depends upon the service and whether or not the surfaces are gasketed: oil pans and valve covers torque to a certain gasket compression, main bearing caps torque to the modulus range (elasticity) of the grade of bolt/stud selected, and head gaskets are a bit of both. So it all depends ....
For the average dufus with a wrench in hand ( like yours truly ) a torque spec. is a reasonable guideline in the tightening of a fastener so as to apply adequate clamping force that
a) the fastener doesn't fall out [5%]
b) the gasket doesn't squirt out the sides [5%]
c) the bolt/stud is sufficiently stretched to stay within its elastic limits [90%]

The 90% is a guesstimate of the average DIY urge to " lets giv'er one more turn" - which results in the elastic limit of the fastener in question being exceeded: Once exceeded, the fastener is in the 'plastic state'. In essence, the bolt fills the hole but applys no clamping force or either the bolt/stud/nut/threads fail.
That sickening feeling of having things suddenly go slack in your hands after that extra snug, or the sharp metallic 'clack' heard when the torque wrench is still in its case are two of the indications of failure known to most who have turned a wrench.... "Oh, mother ... "

** probably a lot like 'gyno talk', but ... well, you get the idea.

bigs

So...

If I'm designing a new car from scratch (as in the 928 way back when) do I define my torque specs based on engineering theory alone? Or is that just the starting point, then the fasteners get stress tested for "x" amount of time before the car is actually mass produced?

macreel

So... Engineering 101.

Mother Nature (raw); Father Physics (Theory); Practical engineer (make it work).

Blank sheet project transforms ideas by application of what has worked before and
throw in a few new, but well-tested wrinkles. Then out to the test track.

That's what kept me gainfully occupied for 35 yrs.

bigs

I'm envious.

928ntslow

Don't forget about actual materials and molecular structure. Aluminum, steel, iron, plastic. Material length, blah, blah blah...geez, there are so many factors that one would HAVE to go to engineering school JUST to understand how to properly use that torque wrench.

That is why designers design and engineers engineer...then the final product ends up somewhere in between.

I just torque 'em to spec and ask no questions.