A lot of the torque is needed to overcome the friction. That is a massive variable, depending on whether the thread is oiled or not – different materials can also have a huge impact, stainless steel for example can lead to micro welds in the thread (“fressen”, don’t know the correct technical translation). The combined method mentioned above does away with that variable, you just fit it snugly and then lenghten the screw by a certain amount, translated into an angle. The length in turn corresponds to a force:
https://en.wikipedia.org/wiki/Hooke%27s_law
In steel construction usually the bolt is tightened with a machine, then torqued – and then, after a day, torqued again. I thus assume that re-torquing isn’t a problem here.
By the way, since the bolts are comparatively (compared to the pre tensioning elements of bridges) short, a tiny amount of movement is enough to lose a lot of force in the connection, thus in steel construction one needs to take the corrosion protection coating into account, it needs to be stable and not too thick.
(Yes, in Germany we still use the torque only method, as it is cheaper and easier and doesn’t require new skills. But you can use the screw force only up to 85% AFAIR.)
slowflyer wrote:
Interestingly the Savvy article referenced by @Inkognito states that torquing is considered inaccurate
There is also a major difference between torquing in a single operation and re-torquing. Probably because the latter involves a breakout force to overcome stiction.
Interestingly the Savvy article referenced by @Inkognito states that torquing is considered inaccurate
slowflyer wrote:
What is your opinion regarding the „counting flats“ method?
If the manufacturer presents this as a method, then fine. I have found this with some very tightly torqued automotive bolts. I have never encountered it in GA aviation maintenance. The same male thread fastener may have different torques, depending upon how it is intended to carry a load. In rotation, it’s torque will probably be zero (with a locking means). In single shear, or double shear, it’s torque may be low, maybe enough to lock a plain nut, or having a locking nut. In tension, it will be high, and may differ based upon the length of the fastener (how much it stretches). In some cases, the correct torque for a bolt and nut is determined by precisely measuring its torqued length (some connecting rod bolts).
I have learned that the cost of the failure is much greater than the cost of correct torquing.
https://www.aopa.org/news-and-media/all-news/2020/january/pilot/savvy-maintenance-tense-bolts
This article is also available as a pdf download if you google some. For standard bolts, there are tables available:
https://bauforumstahl.de/upload/documents/publikationen/arbeitshilfen/Arbeitshilfe_05-03.pdf
Arbeitshilfe_05_03_pdf
I would advise against defying the manufacturers maintenance handbook – It’s rather an issue for the manufacturers to come to terms with something that even civil engineering grudgingly allows. But basically, you can quite easily calculate the parameters to reach an intended force in the bolt.
Just a word for the wary, this always requires a corresponding nut.
[ google URL replaced with the actual doc ]
Thank you @Pilot_DAR , I do have the correct tools but was just wondering, why the recommended torque values differ in such a way between the publications cited above. What is your opinion regarding the „counting flats“ method?
There are several types of crowsfoot wrenches. If it looks like an open end wrench, which can be slid onto the flats of the hex from the side, it’s the wrong type for a flare nut on plumbing. The correct type of crowsfoot wrench for flare nuts can be found by searching under that term. It will have more than the four internal flats, and have to be slid onto the hex axially along the tube. It should be used with the correct calculations with a torque wrench, to the torque specified in the maintenance manual for the product. AC43.13 torques are for use when no other information is available, and for dry threads.
I am just trying to understand the correct procedure for tightening hydraulic line fittings and found this old thread. Comparing the recommended torque values in the document referenced by @Mark_1 the they are nearly twice the values as recommended in the FAA AC43.13-1b table.
There are several other publications that on the other hand prefer a defined amount of turning the nut either based on counting the flats (FFWR, flats following wrench resistance) or simply turning one quarter turn after hand tightening. Actually the turn method makes more sense to me because you want a defined pressure between the flared tube and the fitting which can be described best by a exact movement on the threads. The torque method in my opinion is dependent on factors like friction which can be influenced by lubricants, re-use of the fitting etc.
Understanding that it is vital not to tighten to much causing cracks in the tube or the fitting but also not risking leaks by undertightening I guess this is important to do it right, What practical approach do others recommend?
Shorrick_Mk2 wrote:
That combo won’t work – i need to tighten a nut that has a pipe going through the middle.
The round end on the torque wrench and the open end on the nut.
AC43.13 chapter 7 discusses how to correctly torque with an extension – http://www.faa.gov/documentLibrary/media/Advisory_Circular/Chapter_07.pdf
For flared pipe fittings, there is an official torque, but it is much less critical than in load bearing structure as the requirement is just a good seal between the pipe flare and the 100° cone surface on the fitting. I would usually apply a little gasket compound to the thread and tighten to a ‘snug’ fit i.e. one finger pressure on the end of the spanner.
If you’re fitting a Hartzell prop to an engine, then you will need a crow’s foot and an accurate torque wrench.
This is a good guide – http://www.vansaircraft.com/pdf/Torque_Spec_Aluminum_Fittings.pdf
