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Torquing locking fasteners

mfleming

mfleming

Well Known Member
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So I received my new issue of KitPlane magazine and was reading a tip about modifying a tool for accessing tight areas (I think it was written by our own Larry Larson)

In the article he admonishes us to do the math for the additional length of the wrench.

I used to know this stuff in a previous life but I have forgotten the math and haven't had much luck with my searches.

Soooo... The question is:

How does one do the math for an extended torque wrench (crows foot for instance)?

And even more mystifying.

How to we compensate for the locking fasteners?

ie. I set the torque wrench to the proper value and it clicks before the nut is fully bottomed out.

When I was wrenching on certified iron, I don't remember using many locking nuts for critical items...everything was safetied.
 
AC43.13 has you covered (as well as a lot of the aircraft mechanic manuals such as Standard Aircraft Handbook, etc.).
They also show you how to add an extension and not need not compute an alternate torque value.
 
Lock nuts: you need to add the ?running torque? of the nut to the desired torque, to get the wrench setting. e.g., if the nut has 3 inch pounds of drag, and you want 25 in lbs, set the wrench to 28 in lbs. The running torque of the locknut should be measured with all the threads, including the locking mechanism, engaged, but with the nut not snugged up yet. Measure it in motion, not static. Best to have a beam type wrench calibrated for low (like 2-5) in lbs for the AN3 size.
Torque with adaptors: Torque is always r x F, where r is the straight line distance from the center of the nut to where you apply the force; F is the force; and x means multiply r times F times the sine of the angle between the direction of r and the direction of F. In normal useage the angle is 90 deg, the sine of 90 = 1. Knowing the desired torque, calculate F. Then set your wrench to F times the length of the torque wrench.
 
rvbuilder2002 said:
AC43.13 has you covered (as well as a lot of the aircraft mechanic manuals such as Standard Aircraft Handbook, etc.).
They also show you how to add an extension and not need not compute an alternate torque value.
Click to expand...

Thanks Scott.
Not sure how I missed this information in AC43.13...I have a hard copy as well as an digital version.
Thanks for redirecting me to the bible :rolleyes:

BobTurner said:
Lock nuts: you need to add the ‘running torque’ of the nut to the desired torque, to get the wrench setting. e.g., if the nut has 3 inch pounds of drag, and you want 25 in lbs, set the wrench to 28 in lbs. The running torque of the locknut should be measured with all the threads, including the locking mechanism, engaged, but with the nut not snugged up yet. Measure it in motion, not static. Best to have a beam type wrench calibrated for low (like 2-5) in lbs for the AN3 size.
Torque with adaptors: Torque is always r x F, where r is the straight line distance from the center of the nut to where you apply the force; F is the force; and x means multiply r times F times the sine of the angle between the direction of r and the direction of F. In normal useage the angle is 90 deg, the sine of 90 = 1. Knowing the desired torque, calculate F. Then set your wrench to F times the length of the torque wrench.
Click to expand...

Bob. Thanks for your very precise answers which correlate perfectly with AC43.13.

I just ordered a 0-80 in/lb. beam type torque wrench to measure nut drag!

"43.13-1B
7-40.
d. Add the friction drag torque to the desired torque. This is referred to as “final torque,” which should register on the indicator or setting for a snap-over type torque wrench."
 

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Tips

mfleming said:
So I received my new issue of KitPlane magazine and was reading a tip about modifying a tool for accessing tight areas (I think it was written by our own Larry Larson)

In the article he admonishes us to do the math for the additional length of the wrench.

I used to know this stuff in a previous life but I have forgotten the math and haven't had much luck with my searches.

Soooo... The question is:

How does one do the math for an extended torque wrench (crows foot for instance)?

And even more mystifying.

How to we compensate for the locking fasteners?

ie. I set the torque wrench to the proper value and it clicks before the nut is fully bottomed out.

When I was wrenching on certified iron, I don't remember using many locking nuts for critical items...everything was safetied.
Click to expand...

Wow. Thanks. Nice to know someone reads them. And thanks to everyone for explaining what I should have in the tip. It's simple math for those times we can't use a torque wrench. You can always use an accurate fishing scale. Just compute the length of the arm X the force applied. If you're curious about the tension of the fastener, clamp the bolt in a vice. Use a wrench and scale to measure the amount of force needed to turn it on the bolt. Usually a couple inch/pounds.
By the way, I post all my Kitplanes tips on my blog for anyone interested.
 
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Drag

Not sure I'm a 100% in to adding in drag on torque because as the bolt starts to load up in tension surely the drag changes as the tightens.
Bob
 
6 Gun said:
Not sure I'm a 100% in to adding in drag on torque because as the bolt starts to load up in tension surely the drag changes as the tightens.
Bob
Click to expand...

That occurs for all threaded fasteners whether they have a self locking feature or not, so the specified torque value takes it into account.

Because different locking features have different prevailing (a more official name for it used within the industry) torques, that must be taken into account separately.
 
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