Torque spec on rings by Leupold or Nightforce - Dry or with oil?

[taliv]

Sooooo

If that’s true it seems to imply conventional wisdom about retorquing may be in error.

Specifically that once you correctly torque something and make a witness mark and go about your business. At a later date if you need to remove a scope and reinstall with a torque wrench it’s commonly known that you’ll go past your witness mark before the wrench indicates desired torque value.

Conventional wisdom (ie most gunsmiths Have told me) has been to just go to the original mark. But you’re saying the bolt stretched and isn’t elastic and needs to be stretched again so you should continue til the wrench says stop.

 

[RetiredUSNChief]

Sooooo

If that’s true it seems to imply conventional wisdom about retorquing may be in error.

Specifically that once you correctly torque something and make a witness mark and go about your business. At a later date if you need to remove a scope and reinstall with a torque wrench it’s commonly known that you’ll go past your witness mark before the wrench indicates desired torque value.

Conventional wisdom (ie most gunsmiths Have told me) has been to just go to the original mark. But you’re saying the bolt stretched and isn’t elastic and needs to be stretched again so you should continue til the wrench says stop.

Not necessarily.

Let's talk about fracture mechanics a little bit.

Fracture mechanics is, essentially, the study of what it takes for a material to break under applied stress. There are many terms used in this field (I know, say it ain't so for an engineering subject!).

Fracture toughness is the ability of a material to resist brittle fracture by undergoing plastic deformation...by "bending", in other words. A couple terms for this bending are "elastic deformation" and "inelastic deformation".

"Elastic deformation" is the ability of a metal to deform and then return to its original shape after the forces which caused the deformation are removed. If you exceed the maximum allowable force for this, then the metal will not be able to return to it's original shape...it will remain deformed. This would be "inelastic deformation". How much deformation will exist is a function of the material's fracture toughness and the forces applied to it.

If you stay within the design limits of the bolt for a given torque range, then the bolt will only experience "elastic deformation". If the torque range is going to approach the design strength of the bolt, then it's time to consider a differnent bolt material and/or a higher grade bolt.

A "witness mark" (or "match mark" as I'm used to calling it) is useful in and of itself. But it's no substitute for an actual measured torque value in critical jobs where torquing is very important. Many things other than bolt deformation can affect the witness marks upon a subsequent torquing event: lubrication, contact surface areas between the bolt head and the component, temperature variations between the time of the original torque and the newly applied torque, thread damage (even microscopic galling of CRES components), improperly applied or wrong type of lubricant, use of a different torque wrench (even if they were both in calibration), and probably a few other things.

I typically use match marks where it may be important to note whether the bolting hardware has subsequently moved during a period of operation after the torquing event. I did this, for example, on a particular generator repair job on an aircraft carrier after that ship had repeated problems with the hardware mysteriously coming loose, allowing damage to a slip ring assembly. We had done several repair jobs in a short amount of time and it was my firm belief that the problem was NOT hardware related. (Read: personnel related.). This was ONLY happening on ONE particular generator on ONE carrier out of the entire fleet. Strangely enough, once we match marked the hardware such that they were easily visible upon inspections during operation and before/after maintenance evolutions, they stopped having a problem with hardware "mysteriously" allowing components to move.

Things that make you say "hmmm...".


Some engineering instructions I've had to work to direct a re-torque upon completion. So, if it's in the drawing/technical manual, that's what you do. You may or may not observe actual movement during this and there could be many reasons for doing so. Perhaps it's a function of the gasket material being used. Perhaps it's a function of incremental torquing for a given torque sequence which requires a final torquing upon completion to verify the incremental torquing didn't affect the applied torques which came before.

For me, when I'm working on my car, a re-torquing is simply an idiot check I do at the end to make sure I didn't miss anything. Life would suck after an oil change, for example, if I only THOUGHT I properly tightened down the oil pan drain plug or only THOUGHT I got all the lug nuts on all my wheels after a brake job.

 

[RetiredUSNChief]

Damn
I Never knew any of that “

Most of this is "invisible" to the average person who does not have to deal with engineering specifications where these things make a significant difference. This is because most of the stuff which would require a torque value for "things around the house" are a simple matter of "torque to what the booklet says" and be done with it...it's more than enough to ensure things don't fall apart and plenty of conservative error factors built in to ensure you don't break anything during the torquing.

It's kind of like changing the oil in your car...the owners manual says use filter X (or equivalent) and SAE 10W30 oil.

Your car/truck doesn't care if it's a FRAM or Mobil One filter and it doesn't care if it's Quaker State, Kendall, or Mobil One oil...so long as it's the right size filter and the right weight of SAE oil. If you want to put the more expensive Mobil One filter and synthetic oil in, more power to you...but not necessary.

If, however, you're running a high performance engine that will be pushing the limits of performance...perhaps the higher quality filter and synthetic oil will make a difference in the long run.

 

[taliv]

mmm interesting but i missed a practical answer to the question. assuming same parts and no change to lube, and that 15-18 inch pounds of torque doesn't exceed any of the screw limits and that they therefore remain elastic, why do you always go past the witness mark to get back to the same torque? and should you go back to the same spot, which should stretch the bolt the same, since it's still elastic? or does it not stretch the bolt the same and you should retorque it to whatever the wrench tells you?

 

[Walkalong]

Conventional wisdom (ie most gunsmiths Have told me) has been to just go to the original mark. But you’re saying the bolt stretched and isn’t elastic and needs to be stretched again so you should continue til the wrench says stop.

Let me try in laymans terms.

If over torqued the screw will have stretched to the point it is damaged and not elastic anymore, so it isn't holding well (Trying to spring back) and if brought back to that mark still won't be.

If torqued correctly it will be stretched a hair and pulling/trying to spring back, holding threads tight together, so should be ok if taken out and returned to that spot.

How about it engineers?

 

[RetiredUSNChief]

mmm interesting but i missed a practical answer to the question. assuming same parts and no change to lube, and that 15-18 inch pounds of torque doesn't exceed any of the screw limits and that they therefore remain elastic, why do you always go past the witness mark to get back to the same torque? and should you go back to the same spot, which should stretch the bolt the same, since it's still elastic? or does it not stretch the bolt the same and you should retorque it to whatever the wrench tells you?

I'm not sure on this and would have to do some theory-to-practice tests to see what happens, what conditions is happens under, etc. Questions like "Does it happen all the time, or only part of the time?", "Are there particular materials that are more susceptable to this than others?", etc.

If I were to match mark a torqued screw, disassemble and then reassemble the parts and retorque it repeatedly and re-match mark it each time, would each successive torquing result in going past the most recent match mark?

I don't have the answers to these...and therefore don't have anything to address your question. And I wasn't able to find anything on a google seach, either. If I were an actual mechanical engineer, I might be more qualified to give something definitive.

Regardless, if a component is supposed to be torqued, one should never rely on match marks from a previous torquing to acheive the required torque value. Match marks are really for position indication.

This DOES, however, give me an idea for something to occupy my time should I have a slow night at work sometime and a supply of fasteners and a torque wrench to play with!

 

[RetiredUSNChief]

Let me try in laymans terms.

If over torqued the screw will have stretched to the point it is damaged and not elastic anymore, so it isn't holding well (Trying to spring back) and if brought back to that mark still won't be.

If torqued correctly it will be stretched a hair and pulling/trying to spring back, holding threads tight together, so should be ok if taken out and returned to that spot.

How about it engineers?

If over torqued, a screw MAY have some inelastic damage. Hard to say for any given circumstance, because it involves the amount of over torquing and the bolt material. Damage to a Grade 2 bolt for a given over torque may not cause any damage to a Grade 5 or Grade 8 bolt.

And yes...in general, a properly torqued bolt is typically reusable UNLESS there's an engineering requirement which says to replace any used bolts for some reason.

I imagine those who are actually deep into the mechanical engineering field on this would LOVE to discuss the minutia of this issue!

There's a hysteresis associated with elastic deformation, for example, (which might explain @taliv's question), effects of cyclic stress on fracture troughness due to changes in the crystaline structure of the metal and so forth.

 

[JFrank]

Cycle stress on fracture toughness due to crystalline structure?
I may have to go back to school

 

[Slamfire]

Thank you for all I learned about bolt torquing. It will be useful not only in my shooting sports, but anything time I screw a bolt into a hole, or turn a nut on a bolt.

 

[RetiredUSNChief]

Cycle stress on fracture toughness due to crystalline structure?
I may have to go back to school

Heh...here's a demonstration you've probably run into before, but which illustrates the effect of cyclic stress on crystaline structure and thus fracture toughness.

Ever look around for that pair of diagonal cutters that was nowhere to be found when you needed to cut up a coat hanger for a piece of stiff wire for something?

Well, if you're like me you probably said "heck with it" and started bending the wire back and forth at some spot so you could break it, right?

The wire bends at the chosen spot pretty easily...a few times. Then the bend rapidly heats up while you're doing this and suddenly you notice that the wire isn't bending exactly where you started any more. In fact, it's getting pretty hard to KEEP bending it at that spot, especially since you don't have pliers or gloves and you're trying to do this bare handed and it's gotten HOT!

What's happening in this instance is the physical act of bending the wire coat hanger is affecting the crystaline structure of the metal right at the bend. Bend it back and forth a few times and the overall effect of this is to actually harden the metal at that location...which is why it wants to quit bending at that spot and the wire starts bending next to it as you continue to flex it back and forth.

The bending is introducing tiny point defects in the crystaline structure of the metal. These defects work to counter crack propagation and "hardens" the metal.

 

[Walkalong]

The bending is

To me, the same as over torquing at this point. There is damage to the fastener before it snaps off. First we stretch it, then over stretch it, then it fails completely.

 

[taliv]

If torqued correctly it will be stretched a hair and pulling/trying to spring back, holding threads tight together, so should be ok if taken out and returned to that spot.

but i can't imagine 15 inch pounds would stretch a screw beyond it's elastic limit

If I were to match mark a torqued screw, disassemble and then reassemble the parts and retorque it repeatedly and re-match mark it each time, would each successive torquing result in going past the most recent match mark?

yes. based on my experience. try it.

another example is the conventional wisdom about torquing barrel nuts onto AR15 receivers. you're supposed to put grease on them and tighten then loosen, tighten then loosen, then tighten to the appropriate torque and find the nearest hole. as you tighten the second and third times, the nut goes farther than it did the first and second times. so what is happening? are you saying the aluminum threads on the receiver are stretching past their point of elasticity? there hasn't been any change to the lube or parts

 

[murf]

but i can't imagine 15 inch pounds would stretch a screw beyond it's elastic limit


yes. based on my experience. try it.

another example is the conventional wisdom about torquing barrel nuts onto AR15 receivers. you're supposed to put grease on them and tighten then loosen, tighten then loosen, then tighten to the appropriate torque and find the nearest hole. as you tighten the second and third times, the nut goes farther than it did the first and second times. so what is happening? are you saying the aluminum threads on the receiver are stretching past their point of elasticity? there hasn't been any change to the lube or parts

first and second tightening is probably sucking up the slack in all the parts. third try is with all parts lined up. kinda like setting the preload on wheel bearings.

murf

 

[JFrank]

I’ll probably get torched for this;
The Reason IMHO Vortex says15-18 inch pounds is because their scope tube is so thin.
A more robust scope will take 20 inch pounds all day long without a problem.

 

[Walkalong]

but i can't imagine 15 inch pounds would stretch a screw beyond it's elastic limit

Me either, I imagine it is the aluminum ring it screws into that is the limiting factor.

All steel? Yea, would be hard to hurt, and 15 inch pounds is not very much at all, they can easily take more.

 

[Garandimal]

Wet or dry this happens at a certain point, and why they give torque specs.

But not the Same point.

Dry threads also create high Friction forces - greased threads do not.

So which is it - Friction torque that rattles loose?

...or deflection torque that holds tight.




GR

 

[Garandimal]

Bear in mind that the lubrication of threads can be done wrong. I wouldn't recommend flying in an airplane or riding an amusement park ride that had Never-Seez applied to all the bolts when it was assembled.

Hyperbole. (or FUD)

Do you know anything with more vibration than a Diesel pickup truck engine?

For Thirty Years, have greased all the bolts/nuts when servicing (belts/pulleys/brackets/accessories/covers/lugs/etc..., some up to 5/8" dia., and never used a torque wrench.

100's of Thousands of miles later, never had a bolt seize, never had a bolt come loose.


Threads hold - because they are flexed.




GR

 

[Walkalong]

For Thirty Years, have greased all the bolts/nuts when servicing (belts/pulleys/brackets/accessories/covers/lugs/etc..., some up to 5/8" dia., and never used a torque wrench.

100's of Thousands of miles later, never had a bolt seize, never had a bolt come loose.

From post #15...

If you have been turning wrenches all of your life you get a good feel for tightening fasteners down tight enough but without thread damage/galling/screw stretching, but it is real hard to feel the difference between 10 in lbs and 12

 

[RetiredUSNChief]

yes. based on my experience. try it.

I'll give it a go on a quite night! Just the kind of thing that interests me!

 

[RetiredUSNChief]

I’ll probably get torched for this;
The Reason IMHO Vortex says15-18 inch pounds is because their scope tube is so thin.
A more robust scope will take 20 inch pounds all day long without a problem.

No reason to "get torched" over anything.

This is as valid a reason for a torque limit as any other. Most people think torque limits are to ensure parts are "tight enough". There are plenty of torque limits which are meant not to cause damage to components other than the fasteners by over torquing. For example, the new processor cards in Type II Instrumentation and Control equipment has a Flash Memory card which is held onto the card by screws. The technical manual says to tighten the screws snug "not to exceed 5 in-lbs". The reason why has nothing to do with damaging the screws and everything to do with not stripping out the screw holes on the processor card.

The technical manual doesn't SAY that's the reason...but it turns out an inadvertant theory-to-practice with over-torquing these screws did exactly that.

 

[murf]

a caution to know the difference between foot pounds and newton meters. a mechanic ripped a couple head bolts out of an engine block because he used foot pounds instead of newton meters. quite the mess.

murf

 

[redneck2]

There are some engines that emphasize that head bolts are to never be reused. Dunno if that’s due to stretch from torque to specs or the fact that they’re subjected to stress while running

 

[murf]

the head bolts in my example were new.

murf

 

[RetiredUSNChief]

a caution to know the difference between foot pounds and newton meters. a mechanic ripped a couple head bolts out of an engine block because he used foot pounds instead of newton meters. quite the mess.

murf

Heh!

I'd also like to add a caution to not only know which way to set the dial on a bidirectional torque wrench, but also to understand if that in-lb gage needle doesn't move as soon as you start applying torque, double check the dial setting BEFORE you twist the stud completely off a 900 pound rod control transformer... the night before a scheduled underway!

 

[JFrank]

Thanks Chief,
I just had to google Newton Meters”
J