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Proper Torque Wrench Technique?

dustin_96

Member
Section 5.20 spells out a straightforward process of setting the torque for various sized nuts and bolts. While I understand the theory behind adding the friction drag torque to the standard torque, I get a little tripped up in implementation.

Currently I am going through the process of torquing the AN3 bolts that attach the wing ribs to the main wing spar on an RV-14A. These utilize self locking nuts so the chart prescribes a torque setting of 28 in-lbs.

My struggle has to do with determining the friction drag torque figure.

First question: is friction torque to be determined for each bolt? There seems to be a significant difference in drag from one to the next.

Second question: my torque wrench is incapable of measuring below 10 in-lbs. Most of the bolts seem to has a friction drag torque of something less than 10. Is there just a "plug-in" figure that I should use if not able to measure? For example, if less than 10 should I just assume 5, therefore the torque wrench should be set at 28+5=33?

Thanks! I may be overthinking this but just want to make sure I'm doing it right.
 
This is one place where either a beam-type toque wrench, or modern, a torque load cell (check Harbor Freight) comes in handy - either one allows you to measure the drag of the nut, which you then add to the desired torque value.

Paul
 
1. Yes.

2. What Paul said - those things are so handy for this. But in many cases you'll need to get used to reading the scale with a mirror because it'll be on the "bottom" side as you're holding the wrench.

Dave
 
Thanks!

This information really helped! Harbor Freight didn’t have a 1/4 drive beam-type but I did find one at Oreilly auto! Works great. I measured16 different bolts with drag torque measuring 8-15 in-lbs. significant difference! I feel much more confident now.

Paul….maybe a topic for a metal magic episode???
 
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This information really helped! Harbor Freight didn’t have a 1/4 drive beam-type but I did find one at Oreilly auto! Works great. I. Ensure 16 different bolts with drag torque measuring 8-15 ft-lbs. significant difference! I feel much more confident now.

Paul….maybe a topic for a metal magic episode???

You mean inch lbs. right???
 
I use a torque screwdriver to measure the drag. It measures pretty low.

And right now, there is a deal on Amazon....
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Testing for "Rule of Thumb" Use

I made this testing device to "run" enough drag torque tests, of all normally used AN bolts (and some anchor nuts), to generate a "Rule of Thumb" that could easily be applied to allowable torque ranges, per AC 43.13-2B, to yield a simpler application procedure when applying final torque. All testing done at 68 - 72*F.

The testing device uses a 12" "torque" arm resting on a digital scale, coupled to a lever arm for applying the torque necessary to initiate "slipping" (rotate enough to develop "steady state" condition). Convert the scale reading to pounds, from pounds/ounces indicated, multiply that result by 12 (in.), and you have the running drag in in-lbs.

After running a couple dozen test nuts (AN365 -) of each size used in our aircraft, I came up with the following "Rule of Thumb" I use when otherwise convenient running drag values are difficult to come by. I wouldn't take this to the "bank" without independent corroboration, but it's what I found and what I use.

For sizes -3 & -4 bolts - If you torque to the upper end of the range + a couple of in-lbs, you will be "inside" the range allowed by AC 43.

For sizes -5 & -6 bolts - If you torque to the upper end of the range = 15-18 in-lbs, you will be "inside" the range allowed by AC 43.

YMMV - so don't take my word for it without independent support.

HFSRunning Torque Tester .jpg
 
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