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  #11  
Old 10-14-2018, 10:12 PM
fl-mike's Avatar
fl-mike fl-mike is offline
 
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Responding to the original question, here is what I measured with an assortment of torque wrenches checked with old-school torque calibrators (dig the Cleco name!)

I found the running and "stiction" torque to be nearly the same, except at the AN5 size.

New nut, elastic portion fully engaged, (slow) running torque:
AN3 - 5-7 in-lb
AN4: 16-17 in-lb
AN5: 25-28 in-lb

#8 metal nutplate: 7 in-lb
AN3 metal nutplate: 7.5 in-lb

I also notice some variation if the bolt threads were not so precise, especially in the AN5 size. That increased the numbers quite a bit. You could feel it in the metal threads before you reach the elastic. Keep that in mind.

This is just one or two samples. I make no guarantees as to the accuracy of these numbers! I encourage you to measure for yourself!

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  #12  
Old 10-15-2018, 12:15 AM
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Draker Draker is offline
 
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Thanks everyone for the discussion. I didn't intend to start a fight over Van's manual!

It looks like the tool I'm looking for to measure the nut's prevailing torque is something like this.
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  #13  
Old 10-15-2018, 01:50 AM
scsmith scsmith is offline
 
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Quote:
Originally Posted by Draker View Post
Thanks everyone for the discussion. I didn't intend to start a fight over Van's manual!

It looks like the tool I'm looking for to measure the nut's prevailing torque is something like this.
Yes, that should do nicely. Make sure the pointer is on zero when unloaded, and make sure you apply pressure exactly at the intended spot. Most flex-beam torque wrenches have a handle that is free to pivot about a single point, and you apply load being sure that the handle is floating in between stops, so the force is going in at the pivot.

With low-capacity torque wrenches like the one you linked, often they just have a spherical ball on the end. Be sure to press straight at the center of the ball, with no extra twisting moment.
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Last edited by scsmith : 10-15-2018 at 01:52 AM.
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  #14  
Old 10-15-2018, 02:58 PM
BobTurner BobTurner is offline
 
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Location: Livermore, CA
Posts: 5,355
Default Not rocket science

Biggest issue is many torque wrenches arenít accurate at very low settings, but if yours is:
Thread the nut on the bolt but stop short of the nylon or metal locking area. Put your torque wrench on the nut and smoothly turn until the locking area of the nut is fully engaged. If the wrench clicked, repeat with a higher setting. If it didnít, repeat with a lower setting. When it is borderline click/no click youíre at the running torque drag setting.

If you donít trust your wrench at low settings:
Put the bolt head in a vice, bolt shank horizontal. Thread the nut on until the locking mechanism is fully engaged, and the wrench is (1) headed down and (2) about 30 deg above the horizontal. Tie a string to the end of the wrench, X inches from the bolt centerline. Put a weight on the end of the string. To break the static friction push down on the wrench, letting go as it becomes horizontal. If the wrench immediately slows and stops, increase the weight until it wants to keep turning as it goes thru horizontal. The running torque is:
X times the weight, plus LW/2, where L is the length of the wrench and W is the weight of the wrench.
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  #15  
Old 10-15-2018, 10:19 PM
scsmith scsmith is offline
 
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Location: Ashland, OR
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Quote:
Originally Posted by BobTurner View Post
Biggest issue is many torque wrenches aren’t accurate at very low settings, but if yours is:
Thread the nut on the bolt but stop short of the nylon or metal locking area. Put your torque wrench on the nut and smoothly turn until the locking area of the nut is fully engaged. If the wrench clicked, repeat with a higher setting. If it didn’t, repeat with a lower setting. When it is borderline click/no click you’re at the running torque drag setting.

If you don’t trust your wrench at low settings:
Put the bolt head in a vice, bolt shank horizontal. Thread the nut on until the locking mechanism is fully engaged, and the wrench is (1) headed down and (2) about 30 deg above the horizontal. Tie a string to the end of the wrench, X inches from the bolt centerline. Put a weight on the end of the string. To break the static friction push down on the wrench, letting go as it becomes horizontal. If the wrench immediately slows and stops, increase the weight until it wants to keep turning as it goes thru horizontal. The running torque is:
X times the weight, plus LW/2, where L is the length of the wrench and W is the weight of the wrench.
Just to clarify in case that was confusing for some, the torque is the hanging weight times the length of the wrench to the point where the string is tied plus the weight of the wrench itself times half its length. (assumes that the center of mass of the wrench is in the middle of the wrench, which is probably close enough for us)
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Steve Smith
Aeronautical Engineer
RV-8 N825RV
IO-360 A1A
WW 200RV
"The Magic Carpet"
Hobbs 515 in 9 years (would have flown more this year if not for fire smoke)
also LS-6-15/18 sailplane
VAF donation Dec 2017
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  #16  
Old 10-15-2018, 10:35 PM
n982sx n982sx is offline
 
Join Date: Apr 2010
Location: Chicago, IL
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Quote:
Originally Posted by scsmith View Post
An in-between approach, what I do, is to use a flex-beam type torque wrench. Not as accurate as a dial type, but they are pretty good, and with that, you can see the needle on the scale while the nut is turning, giving you a running torque measurement.
This is the method I use. A good beam torque wrench that reads up to 75 in pounds is perfect for this and doesn't cost that much. You can check all your common nuts and just make a small table of values to add to the standard torque values to account for the run on torque.
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