Run-on Torque value added to required torque for self-locking nut.

[Carrollbw74]

Hello all, I am brand new to this forum and I am here for one topic that I need some explanation on. I am an AH-64D Attack Helicopter Technical Inspector and I have been in an on-going discussion/argument over the subject of run-on torque addition to required torque for self locking nuts. Short explanation follows:



Maintenance manual calls for Nut (A) to be installed on Bolt (B) with a required torque of 1000 inlbs.

Nut (A) is a self locking nut with a run-on torque of 75 inlbs.

Run-on torque has been established using a dial indicating torque wrench with bolt (B) fully extended through Nut(A).

Question 1: Will the 75 inlbs of run-on torque be added to the required torque of 1000 inlbs for a total applied torque of 1075 inlbs?

Question 2: What is the Physics explanation behind this answer.

Thanks for any assistance

 

[xxChrisxx]

The important aspect is preload, what you are doing is extending the bolt (adding strain) to clamp up the joint. The bolt needs to be clamped to a certain preload to ensure a secure joint.

Torque is acutally a fairly poor, but easy way of specifying a certain bolt preload.
This is becuase torque depends on the friction between the bolt and the nut.
A high friction means that for a given torque value, you are getting less preload.
Consequently if you used a lubed nut and bolt, for a given torque you'd be getting a higher preload.

What manufacturers sometimes do is 'calibrate' the torque value vs preload for a given clamping scenario.
In this case, it depends if the manufacturer has already taken the prevailing torque (torque required to get the nylock nut moving) into account when specifying the torque to apply.

If it's unknown I'd just add it, it's only a bit extra anyway.

 

[Pkruse]

From an engineer who specifies torque in tech manuals: you already have one good reply, so I'll add to that.

If I call out a torque that means that I'm not terribly concerned about the preload. This is because a torque will result in a huge variation in actually preload. So I'll take that into account and specify a number big enough to make sure I get the minimum required preload anyway. But most the bolts will have a much higher preload.

If that is not OK then I won't call out a torque at all and I will require more elaborate means of preloading the bolts. In one application we stretched the bolts with a hydraulic device, and then simply snugged up the nut before removing the hydraulic cylinder. We double checked the bolt preload by measuring bolt stretch with an ultrasonic device.

All alternatives to controlling preload by torque are expensive, so that is why we use so many torque wrenches.

If I call out a torque with a self locking nut, then I add that to my desired torque because most techs will not ask the questions you ask.

 

[Pkruse]

If I require the tech to measure the run on torque in the field, it will only be to make sure that the self locking feature is still working. They do wear out after a number of repeated uses.