Measuring prevailing (running) torque for nylon lock nuts
 

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

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.

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.

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.

Thanks, I'll give that a try.

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.

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.

LoL! Time for an update, then. :D


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.