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Measuring prevailing (running) torque for nylon lock nuts

Draker

Well Known Member
AC43.13 talks about nylon locknut "prevailing torque" which is the torque from the locking mechanism that must be overcome when attaching them to bolts. This amount must be added to the specified torque value for the bolt. I've found empirically, that simply tightening a nylon locknut to the bolt's specified torque is insufficient (the torque wrench "clicks" before the bolt even snugs down). How can I measure this extra torque that will be needed, both to get the bolt snug, then to tighten it properly? Is there a special tool needed to do this, or is there a rule of thumb given the fastener type + size? Is this value going to be different for each individual lock nut??
 
The most accurate way is to purchase a dial style torque wrench but the following is close enough....

Incrementally adjust your torque wrench to find the torque value where your torque wrench just "trips" when turning the nut on the end of a bolt, but not yet in contact with the washer.

This is the prevailing torque value that you should add to the spec'ed torque value.

For the spec'ed value, use the center of the range to allow for slight variation from one nut to the other.
 
Prevailing Torque is a Measure of the Effectiveness of the Lock

It?s really a check to make sure the ?lock? is within specification. Typically, the fasteners are test to a specification for ?first on? i.e., is too much torque required to run the nut down. Next test is ?first off?- is the specified amount of lock or prevailing torque still present. Last element is ?fifth off?, where prevailing torque is measured after a fifth re-use to see if the lock is still within a specified range. The same type of test are done for both nylon as well as all metal locknuts. These are SAE or IFI standards. The application torque is spec?d for The joint and/or the fastener system. You need not add prevailing torque to it.
Terry, CFI
RV9A N323TP
 
I've found empirically, that simply tightening a nylon locknut to the bolt's specified torque is insufficient (the torque wrench "clicks" before the bolt even snugs down). ?

Follow Scott's post. But, if you a locknut that will not even go on with specified final torque, that doesn't sound right. e.g., if you have an AN3 bolt and nut and 25 inch pounds won't move the nut, before it's tight, something's wrong. Make sure you haven't run out of threads.
 
The application torque is spec?d for The joint and/or the fastener system. You need not add prevailing torque to it.
Terry, CFI
RV9A N323TP

You do if it is influencing the value measured by a torque wrench.

The torque value in most charts referenced by RV builders is for a plain nut. If a self locking nut (or nutplate) is being used instead, the prevailing torque is measured by the torque wrench and will cause the actual torque of the nut to be below the desired value if prevailing torque is not added.
 
The most accurate way is to purchase a dial style torque wrench

Agreed, however:

but the following is close enough....

Incrementally adjust your torque wrench to find the torque value where your torque wrench just "trips" when turning the nut on the end of a bolt, but not yet in contact with the washer.

This is the prevailing torque value that you should add to the spec'ed torque value.

Uhh NO! This way you'd be measuring the stiction of the entire assembly, not the extra drag while moving. Stiction is significantly higher than the moving drag, so you would be systematically overtorqueing everything if you added the value so measured to the nominal torque on the nut. The difference is not too large with nylocks, but becomes a worry with the thin metal nuts used in the later kits.

Too, most torque wrenches have increasing errors toward the edges of their range, so your measurement is likely to be doomed from the outset absent mucho$$$$ gear.

Just use the values recommended in the Van's manual: clearly those ten thousand aren't falling out of the sky from undertorquing.
 
Agreed, however:



Uhh NO! This way you'd be measuring the stiction of the entire assembly, not the extra drag while moving. Stiction is significantly higher than the moving drag, so you would be systematically overtorqueing everything if you added the value so measured to the nominal torque on the nut. The difference is not too large with nylocks, but becomes a worry with the thin metal nuts used in the later kits.

Too, most torque wrenches have increasing errors toward the edges of their range, so your measurement is likely to be doomed from the outset absent mucho$$$$ gear.

Just use the values recommended in the Van's manual: clearly those ten thousand aren't falling out of the sky from undertorquing.


I agree with Scott, with the fine tuning that you would like to know the torque required while turning, not to just get it to start. That much I kind of agree with what Aluminum said.

HOWEVER--

The AN fastener torque spec is already on the low side of normal engineering practice. I've written about this before.

The slight overtorque from following Scott's advise exactly (which may result in measuring the 'stiction' torque rather than the running torque) is far better than substantially undertorquing by not adding in any correction at all.

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.
 
Incrementally adjust your torque wrench to find the torque value where your torque wrench just "trips" when turning the nut on the end of a bolt, but not yet in contact with the washer.

Thanks, I'll give that a try.

But, if you a locknut that will not even go on with specified final torque, that doesn't sound right. e.g., if you have an AN3 bolt and nut and 25 inch pounds won't move the nut, before it's tight, something's wrong. Make sure you haven't run out of threads.

I'm sure I've encountered this quite a bit. 23 inch pounds is middle of the AN3 range. It does turn the nut, but not all the way down the bolt so it snugly rests against the work piece. The wrench will sometimes trip with the nut about 3/4 of the way down the bolt--way too loose. For now, I'm just making them snug, and plan to revisit them all later on when my technique is better. I wonder if there exists a definitive check list of "all the bolts on an RV" so I can make sure I hit them all.
 
Just use the values recommended in the Van's manual:

You mean the manual I help write??????????? :rolleyes:

I agree that the prevailing torque of an AN365 10-32 nut is not very high and wont have a very large influence, but the OP's question was general in nature and we should not ignore it for all fasteners. Additionally, the newer RV kits use other style of self locking fasteners like all steel MS nuts, etc. which have a much higher prevailing torque than a nylock nut.
 
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You mean the manual I help write??????????? :rolleyes:

LoL! Time for an update, then. :D


I agree that the prevailing torque of an AN365 10-32 nut is not very high and wont have a very large influence, but the OP's question was general in nature and we should not ignore it for all fasteners. Additionally, the newer RV kits use other style of self locking fasteners like all steel MS nuts, etc. which have a much higher prevailing torque than a nylock nut.

Yes, my main worry are the newer MS nuts: dry steel on steel stiction is about 100% higher than friction (can't find measurements for cadmium, perhaps someone can link if available), so if the OP is starting from ~25 in*lbs of uncertainty he might actually end up torquing some of the AN3s close to the fastener's limit with just traces of skin grease--way beyond optimum for the aluminum alloys being joined. Over-torquing across aluminum will reduce fatigue life of that joint; this has nothing to do with tensile strength of the fasteners (2-3 times greater!), most of which operate below fatigue threshold for steel as used on these airframes. Think two pieces of acrylic bolted together: more torque cracks the acrylic, not the fastener.

Anyhow, there are by now hundreds of RVs flying with MS nuts, and those don't seem to be falling out of the sky from under-torquing either, so the ancient wisdom of 28 in*lbs for AN3s appears to work just fine. :cool: Don't fix what's not broken, and all that.

Bonus: these torque adapters are great for the bigger bolts (too coarse for the 10-32s) and more versatile than a wrench.
 
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!

il92sw.jpg
 
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.
 
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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Not rocket science

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.
 
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)
 
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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