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Posted: 12/13/2005 3:59:54 PM EDT
How do you get the right torque on your torque wrench when useing an extension like a barrel wrench?
How do I figure out what torque to set the torque wrench on to get 30"Ft Lbs on the barrel nut? 



Just use the extension at a 90 degree angle and you are not adding length so no math to do. what you dial is what you get..



Thanks guys! Great info!



Unless I am misunderstanding your post, when you use an extension at 90 degrees, you ARE adding length. But at a reduced value. What you are doing is forming a triangle, and need to use an a^2 x b^2 x c^2 formula to find the hypoteneuse, which is the actual arm.
I don't know how to explain it any better and don't have a pic to link to. 


Don't worry about it. According to the TM 23, the torque value is measured when both the torque wrench and combination wrench are used together. 


I don't know, to me that means that you have to measure the torque when you use both wrenches together. That's how I understand it. 


DAMN! I spoke to soon! I know this is great info but I'm to stupid to know how to use the formula. I didn't know you had to be a math expert to know how to use a torque wrench. 


You guys are reading too much into it. The 30# torque reference for AR15 barrels is a minimum. You'll most likely need to go more than that anyway to align a notch in the barrel nut for the gas tube, so critical torque values are unnecessary.


"M4Madness, are you asking me if I bark like a dog?"  Pattymcn

Ok then, just forget about the barrel nut torque for now. I just want to figure out how to use a torque wrench with an extension on it. 


If you are talking about a regular socket extension then it doesn't affect torque. The only thing that affects torque is adding length to a torque wrench. Then the formula Squib posted is correct except that if you keep the extra length at a 90 deg angle to the torque wrench then no torque is added.



I'm talking about adding length by the use of the barrel wrench. Thank you for the 90 degree angle tip, but I still am trying to figure out that formula. 


HUH! 


by extention I meanan extension that changes the plane not length. Do a search about using a torque wrench there are a few article's that explain with pictures (worth a thousand words). I don't always explain things well.



http://home.jtan.com/~joe/KIAT/kiat_3.htm
Take a look here and you will se an illustration of what I mean by 90 degress. If I can find something in a manual at work I will scan it in and post. 


He's right! You must realize that torque is the "lever arm" mulitplied by the force component perpendicular to the lever arm. Yes, as 34CAV pointed out, using the Pythagorean theorem (a^+b^2=c^2), your DISTANCE from the point does change, but your perpendicular "lever arm" is not changing. If you do choose to use this longer distance as the lever arm, you notice that now your force is no longer perpendicular to that lever arm. If you work the trigonometry out, and find the perpendicular component of that force, you'll find it is indeed the same! It's crazy stuff, not quite what you'd think! Gundraw 

Bumpfiring made me a better person.


I think I figured out the formula, are there any math experts out there that can tell me if I got it right?
L1: My torque wrench is 16" inches. L2: My torque wrench with extension is 18" inches. M1: The set torque on my wrench is 30"Ft Lbs. M2: The actual torque applied to the barrel nut is 34"Ft Lbs. 30"Ft Lbs. (TIMES) 18"inches (DIVIDED BY) 16"inches = 34"Ft Lbs. Is that the right answer? Did I get it right? Or am I back to this> 


That's what I get! (you'll notice that oddly enough, the length could be in ANY units and come up with the same answer as you end up finding a ratio between the two making the actual length units meaningless). ETA: Just noticed that was pointed out in a picture above... Gundraw 

Bumpfiring made me a better person.

Alright! Now do I subtract the extra unwanted 4"Ft Lbs. from 30"Ft Lbs. to get the number I should set on my torque wrench which would be 26"Ft Lbs., or do I use the formula again using 27"Ft Lbs. as the torque set on the wrench to get 30"Ft Lbs. on the barrel nut? 


Bump.



Remember, now that you are using the extension, your torque wrench is no longer properly calibrated. So your second choice of using 27ftlbs on the wrench is correct for achieving about 30ftlbs at the nut.
Or, as said above, just use the extension at a 90 degree angle. Gundraw 

Bumpfiring made me a better person.

Nah. Just set it on 30# and go with it. There's a pretty good chance that a notch in the barrel nut won't align for the gas tube at whatever setting you use, and you'll be tightening until the notch does line up WITHOUT a torque wrench anyway. You may have close to 80# torque when you are done. The 30# spec is just to make sure that you don't UNDERtorque the barrel nut. You don't just torque a barrel nut to 30# and stop (unless by chance it happens to line up there.) 

This avatar is dedicated to my wife who survived breast cancer, and to Pattymcn, who is currently battling it.

I have some barrel nut shims from Randall Rausch at AR15 barrels.com so I may be able to stop at 30"Ft Lbs. Thanks for all the help guys! 


Remember, 30# is the minimum . Many prefer more than that. 

This avatar is dedicated to my wife who survived breast cancer, and to Pattymcn, who is currently battling it.

I know. 


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