HELP: self-locking nut run-on torque check

Main Question or Discussion Point

Hi, i would like to the correct method for self locking nut run-on torque check procedure. are we going to measure the torque to overcome friction for nut to move when 1)Fully engage which mean certain thread of bolt already exposed at the bottom of nut OR 2)Hand tighten until cannt move anymore then we measure it?

Related Mechanical Engineering News on Phys.org
wolram
Gold Member
Hi, i would like to the correct method for self locking nut run-on torque check procedure. are we going to measure the torque to overcome friction for nut to move when 1)Fully engage which mean certain thread of bolt already exposed at the bottom of nut OR 2)Hand tighten until cannt move anymore then we measure it?
I have never allowed any extra torque for self locking nuts, but then i hardly ever use them.

Q_Goest
Homework Helper
Gold Member
Hi Stan, The torque required to overcome the self locking feature of a fastener is called "prevailing torque". That torque simply resists rotation of the nut on the bolt and does nothing to increase the clamping load. If self locking nuts or bolts are used, the correct procedure is to measure this torque using a beam type or dial type torque wrench while the nut is rotating. Add that amount of torque to the torque needed to properly tighten the fastener. Bolt torque should always be measured while the nut/bolt is rotating, not when stationary.

"If self locking nuts or bolts are used, the correct procedure is to measure this torque using a beam type or dial type torque wrench while the nut is rotating"
ok, but are we going to measure that torque only after full engagement(which mean bolt end thread already protruded from bottom of nut) of nut to bolt? or......

Q_Goest
Homework Helper
Gold Member
ok, but are we going to measure that torque only after full engagement(which mean bolt end thread already protruded from bottom of nut) of nut to bolt?

Forces which resist torque on a fastener are generally broken up into 3 categories:
1. Frictional resistance at the thread. (frictional load x moment arm = torque)
2. Frictional resistance at the nut or bolt face. (frictional load x moment arm = torque)
3. Load created within the bolt. (modeled as an inclined plane)

Modeling the bolt is generally done as if the bolt is an inclined plane, but there are differences. When you torque a bolt, you have to overcome friction at the threads as well as friction at the nut face. The remaining torque goes into 'pushing' this load up the inclined plane (so to speak). The two frictional forces are resisting that pushing force, and as it turns out, most of the torque put into a fastener goes into overcoming friction at the thread and nut/bolt face. Less than half goes into applying a clamping force. Take a look at a typical http://www.boltscience.com/pages/quality.htm" [Broken]and note the amount of clamping load (they call "Thread Extension Torque") in comparison to the other two.

The point of measuring prevailing torque is to determine a fourth load that must be overcome. That load is the frictional resistance of the threads that exists without any other load on the fastener. It is strictly a function of the locking feature of the nut/bolt which resists tightening or loosening.

To answer your question then, the prevailing torque is measured with the nut and bolt fully engaged, but without any of the other three above forces resisting the torque. So you wouldn't want there to be any load on the bolt. You could even measure the prevailing torque with just the nut and bolt together with nothing in between, but that's not necessary and adds time to assembly. Note you could also measure a group, take the statistical mean, and simply add that to the total torque required. For something as simple as a fastener, you'd think the science behind them is equally simple, but this just scratches the surface. There are entire books written on the subject that cover more information about fasteners than you'd ever want to know.

Last edited by a moderator:
thank you very much

Keep in mind that self locking nuts loose some grip every time they are removed. Every fastener on an old airplane ends up with a lot more tension than they were intended to have. This is because only one torque spec is published and mechanics aren't smart enough to compensate for used hardware.

Keep in mind that self locking nuts loose some grip every time they are removed. Every fastener on an old airplane ends up with a lot more tension than they were intended to have. This is because only one torque spec is published and mechanics aren't smart enough to compensate for used hardware.
Just a bunch of "Grease Monkeys"?? Not "Smart Enough"...... Wow. you shouldn't be reusing self locking in the first place on aircraft. Also, most of the mechanics that I know use NEW hardware.

FredGarvin