How to find "Tightening Torque" of an existing Nut-Bolt assembly?

[Rebel_without_Clause]

Hello,
How to find “Tightening Torque” (not the loosening torque) of an existing Nut-Bolt assembly using mini/micro torque wrench?
Online search didn’t help. My guess is the following:
(1) Increase torque setting in steps till the bolt just moves;
(2) Count number of turns (including fractional ones)

Thank you for your inputs!
Best Regards,
RK

[toma]

What makes you think it’s torqued correctly in the first place?

I’d look up the correct torque values in the manual or refer to a torque
chart. Bolt Torque Chart - Portland Bolt

[Jarrett]

What are you trying to torque down?

That’s a good point about not assuming that it was done properly in the first place.

SMA connectors often have published torque specs. Many other machine components have general guidelines that are like “more than this, but not enough to break stuff”

[Gear105]

Just how critical is this nut-bolt assembly.

[Rebel_without_Clause]

Thank you for your suggestions and the reference link!
In order to replicate the model, torque on three nut-bolts have to precise (did not measure while assembling); it is one of the important parts of the design
It is badly designed mechanical assembly hack + design (by me) as the three torques have to be “tuned” and it can be a painful two hours for that.

I am thinking of a more repeatable design rather than tune it by feel (moving the mechanism); that’s what is on the to-do list.

Thanks,
RK

[Rob]

It’s not clear enough what you’re actually wanting to measure. Screw threads are complex. Like for example why does an impact wrench remove a nut with less damage than a breaker bar, even though higher torque is involved? I think the condition of the nut/screw interface probably dominates if they’ve been together a long time.

[JDMc]

I’ve read this a couple times and now I understand what you are asking:

Given a (custom) assembled and torqued nut-bolt joint with an unknown tightening torque, how do you estimate the tightening torque to recreate / reassemble it?

The methods you have proposed sound pretty reasonable.

You could also try the “guitar tuning approach”:

1. Mark the position of the nut and bolt relative to the assembly,
2. Loosen off the nut slighlty
3. Re-tighten the nut to match the same position using the torque wrench. Easier to read the torque if it has a dial, or just start small and increment up if if the torque wrench has incremental settings.
   Note that differences in lubrication may affect the torque.

Sounds like you are on the right track for the next version of the design. Always desirable from manufacturability / maintenance to use standard tightening torque to produce the clamping force and other design elements to control spacing / tolerance / clearance. Let us know if you need some ideas on how to accomplish that.

[Rebel_without_Clause]

Thank you for suggesting the tuning approach and to use a dial torque wrench to note down the torque.
Good rephrasing of the question, that makes the do to task much clearer.
Best,
RK