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?
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 ]
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.
Interestingly the Savvy article referenced by @Inkognito states that torquing is considered inaccurate
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.
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.)
