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.

 

[PJC01]

.

Hello there,

I can provide you with a response to this topic. First of all, let's define what torque is. Torque is the measure of rotational force or the force that causes an object to rotate around an axis. In this case, the torque is being applied to the nut (A) in order to secure it onto the bolt (B).

Now, to answer your first question, yes, the run-on torque of 75 inlbs should be added to the required torque of 1000 inlbs for a total applied torque of 1075 inlbs. This is because the run-on torque is an additional force that is required to properly secure the nut onto the bolt. Without it, the nut may not be fully tightened and could potentially loosen over time.

As for the physics explanation behind this, it is important to understand the concept of static friction. Static friction is the force that prevents two stationary surfaces from sliding against each other. In this case, the nut and bolt are the two surfaces in contact, and the static friction between them is what keeps the nut from loosening.

When you apply the required torque of 1000 inlbs, you are essentially increasing the force of static friction between the nut and bolt. However, as the bolt continues to rotate, the static friction decreases due to the changing contact points between the nut and bolt. This is where the run-on torque comes into play. The additional force of 75 inlbs helps to maintain the static friction and prevent the nut from loosening.

In conclusion, the run-on torque is necessary to ensure that the nut is properly secured onto the bolt and should be added to the required torque for a total applied torque. I hope this explanation helps to clarify any confusion and assists in your ongoing discussion/argument. Thank you.