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02-16-2013, 08:34 AM
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Join Date: Jul 2009
Location: Mojave
Posts: 4,652
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I was also surprised at the relative low tension of smaller fasteners when I went through initial technical training in the Air Force. Since that "calibration" many years ago, I now reserve the torque wrench only for critical fasteners.
In fact, we used to challenge each other to see how close we could get with the "calibrated elbow" and most of us could get within a few percent.
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WARNING! Incorrect design and/or fabrication of aircraft and/or components may result in injury or death. Information presented in this post is based on my own experience - Reader has sole responsibility for determining accuracy or suitability for use.
Michael Robinson
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Harmon Rocket II -SDS EFI
RV-8 - SDS CPI
1940 Taylorcraft BL-65
1984 L39C
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02-16-2013, 09:14 AM
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Join Date: Dec 2009
Location: Boulder, CO
Posts: 4,435
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A fastener only has so much strength. If you think about it, it's really only got so much strain energy that it can withstand before it fails. If you load too much on to it by overtorquing, then the amount of load it can carry is reduced.
Torque does two significant things to a bolt. It clamps the parts together, helping to carry shear, and it applies a tension load that the bolt can carry before it gaps. That helps the fatigue life of the bolt.
Bottom line -- torque using a torque wrench, to the specification.
Dave
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02-16-2013, 09:44 AM
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Join Date: Jul 2009
Location: Lee's Summit, MO
Posts: 747
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Torque
Just for fun, put a AN3 in your vice and tighten to failure or to the point it begins to yield. It takes a lot. I like the little snap-on 1/4 drive stubby rachets for these little bolts. Using the correct wrench, it seems unlikely that the bolts would be over torqued to any significant degree.
In the steel business we calibrate our bolt installation procedure on a daily basis. Torque is an indirect method of measuring tension. It doesn't take long to figure out that torque is variable based upon thread cut, lubrication, manufacturing tolerance and many other factors. I have always suspected that the torque/tension relationship for small fasteners varies more than we often realize.
Here is a device which measures tension in large fasteners. While the info on the site is not directly applicable to our fasteners, it may be interesting to some. I have always been surprised that someone doesn't offer tension control bolts or washers for other industries.
http://www.skidmore-wilhelm.com
I do agree that all critical fasteners should be installed with a TW as this is the best tool we have in most cases.
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Robert Williams
Lee's Summit, MO
RV-8 - Empennage & Wings Done
Working on Fuse
O-360-A1A
1946 Cessna 120
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02-16-2013, 09:55 AM
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Join Date: Oct 2010
Location: La Feria Texas
Posts: 3,822
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Quote:
Originally Posted by rgmwa
Don, don't you ever sleep? 
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Are you comparing my schedule to those on the "wrong" side of the world? 
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02-16-2013, 10:07 AM
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Join Date: Feb 2005
Location: LSGY
Posts: 3,198
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Torque Wrenches
Now that we all agree that we should use a torque wrench probably more than we do - any good suggestions for good torque wrenches? Some of the new digital units look amazing, but probably a bit of overkill for our application.
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02-16-2013, 10:30 AM
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Join Date: May 2011
Location: Perryville, MD
Posts: 93
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Torque wrenches
I've had my two torque wrenches, 1/4 and 1/2 inch click type for about 40 years. I have always stored them at their lowest torque settings but they have not been calibrated since I've had them. Should they be ok or should they be calibrated periodically. I know in the Air Force our torque wrenches were calibrated quite often.
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Joe Mikus
MSgt USAF Ret Avionics Tech
Perryville, MD
Student Sport Pilot
Future RV-12 Builder?
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02-16-2013, 11:20 AM
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Join Date: Feb 2010
Location: Gardnerville Nv.
Posts: 2,828
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I wonder if any one has lost a control surface in flight, due to improper Tq, that would be my biggest concern with these tiny fasteners.
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7A Slider, EFII Angle 360, CS, SJ.
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02-16-2013, 04:33 PM
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Join Date: Dec 2005
Location: Oliver, B.C. Canada (Okanagan valley)
Posts: 786
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Quote:
Originally Posted by rv8ch
Now that we all agree that we should use a torque wrench probably more than we do - any good suggestions for good torque wrenches? Some of the new digital units look amazing, but probably a bit of overkill for our application.
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I use the click type of inch pound torque wrench. I wish I would have bought a dial type instead. It gives you some indication of how close you are to you torque goal.
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Lorne
RV 7a tip-up
Pre-cover MD-RA Inspected.
Canopy completed. Bonded with Sika-Flex.
Up on her mains, Firewall Fwd and wiring on going.
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02-16-2013, 04:37 PM
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Join Date: Sep 2012
Location: Omaha, NE (KMLE)
Posts: 2,247
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I bought a Parktool TW-1 inch-pound wrench. 1/4" drive with a 3/8" adapter. It's apparently used mostly for bicycle work, but I think the in/lbs are the same on an airplane.  Anyway, it was FAR FAR less expensive than most of the alternatives, dial or click. I paid under $40 shipped for it, brand new. There's a bigger TW-2 as well.
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Dale
Omaha, NE
RV-12 # 222 N980KM "Screamin' Canary" (bought flying)
Fisher Celebrity (under construction)
Previous RV-7 project (sold)
Last edited by DaleB : 02-16-2013 at 05:06 PM.
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02-16-2013, 05:04 PM
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Join Date: Jul 2007
Location: Delaware, OH
Posts: 435
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Torque vs fastener size
Years ago I read a paper on bolted connections. It indicated that when torquing by "feel", small fasteners tend to be over torqued and large fasteners tend to be under torqued. The rationale given is that humans tend to scale things up linearly (i.e. a 1/2 inch bolt should be torqued twice as much as a 1/4 inch bolt), but the bolts themselves follow an exponential curve.
I always use a torque wrench.
FYI, an over torqued bolt will have less shear capability than a correctly torqued bolt.
__________________
Mike Cencula
RV-7A (fuselage)
www.our7a.com
Betcha my pile-o-scrap is bigger than your pile-o-scrap.
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