REALLY Basic Torque Question

[RoyJackson]

Yeah! ... Incorrect stuff. Youre putting people who dont know any better into jeopardy. Get physic.
larry

Not sure if your comment was directed to me, or another post, or posts in general.
However continuous torque being transmited through a driveshaft would transmit all torque, once the "twist" (used for lack of a better term) of the driveshaft has been completed. That's not as clear as I'd like to state it...might come back later to clarify.
However, torquing a fastener is not continuous torque. Once that measuring device (probably a torque wrench) actuates (by a click, indicator, relief valve (used in some pnuematic torque limiting guns), etc), the torquing action or movement (by hand or pneumatics) stops. If the resistance to "twist" in the extention is greater then the torque value of the fastener, you'll get a low torque.
Now, in CUTs, I doubt if I've ever used an extention longer then 8" (as stated in a previous post, wheel torquing requires a 6" extension). That correction factor is probably (if I calculated it) negligible. I add a couple, three ft-lbs from habit. If I was using a 1/4 or 3/8ths extension 12" or longer, and I thought the fastener was critical enough, I'd probably do the research to find those correction calculations (if you want to do the research, aerospace maintenance manuals might be your best bet).

There ya go...I'm sure someone will respond, which is fine...but I reckon we'll be either going around in circles or picking nits. So, use your logic and believe or analyze what you want to and either add a couple foot-lbs to a torque value, or don't...what ever works for you...as long as you're within a few percentage of the torque value, you should be fine.

 

[SPYDERLK]

Continuous torque being transmited through a driveshaft would transmit all torque, once the "twist" (used for lack of a better term) of the driveshaft has been completed. That's not as clear as I'd like to state it...might come back later to clarify.
However, torquing a fastener is not continuous torque. Once that measuring device (probably a torque wrench) actuates (by a click, indicator, relief valve (used in some pnuematic torque limiting guns), etc), the torquing action or movement (by hand or pneumatics) stops. If the resistance to "twist" in the extention is greater then the torque value of the fastener, you'll get a low torque.
Now, in CUTs, I doubt if I've ever used an extention longer then 8" (as stated in a previous post, wheel torquing requires a 6" extension). That correction factor is probably (if I calculated it) negligible. I add a couple, three ft-lbs from habit. If I was using a 1/4 or 3/8ths extension 12" or longer, and I thought the fastener was critical enough, I'd probably do the research to find those correction calculations (if you want to do the research, aerospace maintenance manuals might be your best bet).

There ya go...I'm sure someone will respond, which is fine...but I reckon we'll be either going around in circles or picking nits. So, use your logic and believe or analyze what you want to and either add a couple foot-lbs to a torque value, or don't...what ever works for you...as long as you're within a few percentage of the torque value, you should be fine.

Your description analogy of the driveshaft is clear. Some twist, or torsional give, is present in all drives and can be pretty significant when shafts are long and small diameter. This is the perfect extension scenario -- you seldom have room to run a good sized tubular extension [torsionally rigid] to the fastener. Instead, a relatively small hi strength rod is used to transmit the torque. This extension is a torsion spring. It stores energy. Thats why extensions are such an issue with impact drives. Using the limited power of the gun and the inertia of its rotor it repetitivly delivers great power for discrete instants. If the limited energy present in an instant of great power goes into twisting the extension the bolt wont turn. Then between impulses the extension rebounds feeding the stored energy back against the gun. ... So the non rigid setup performs poorly here. However, where force is applied by hand, thousands of times slower than an impact, the energy is durably stored in the extension and all input torque is available at the fastener. The reason why a correction factor would be applied to this is baffling, and suggests either a misunderstanding by the persons formulating it, or an attempt to assure getting at least adequate fastener tension in the exaggerated stick - slip situation that develops with poor torsional rigidity. This could just as easily result in significant above spec torques however.
larry

 

[RoyJackson]

The reason why a correction factor would be applied to this is baffling, and suggests either a misunderstanding by the persons formulating it, or an attempt to assure getting at least adequate fastener tension in the exaggerated stick - slip situation that develops with poor torsional rigidity. This could just as easily result in significant above spec torques however.
larry

As I wrote, I got my info from working with aircraft and that is what was done on military birds. If you think the aeronautical engineers were mistaken, so be it.
If we were installing the bolts that attach the wings to a fuselage, we could go into this with great detail about torsional rigidity, moments of inertia and all that cool stuff. However, we're discussing torquing wheel bolts on a tractor where the values might be measured in grunts or grunts and a half and be perfectly safe.
So, reckon we have to agree to disagree and call it a day...you're welcome to the last word if it makes ya happy.
But I'll still add my 2-3 extra ft-lbs and go happily tractoring.

 

[Raspy]

It seems clear that ALL torque is transmitted through an extension in most cases. However, when torquing a nut, using a click type torque wrench, the nut can be actively turning when the click occurs. In this case if the torque is held steady, but the rotation continues, the nut will have a chance to actually come to a stop at the rated torque. In other words, it continues to rotate after the click and come to it's final bolt stretching value. But only if you allow it to. Also, when using an extension, there can be a lot of side loading on the nut that causes additional thread friction and takes some torque to overcome. So the extension should be supported out near the wrench to get the full nut torque value by not causing extra thread friction with a side, binding, load.

I've noticed at some tire shops they use extensions designed to "wind up" a bit when used with impact wrenches on lug nuts. These are rated in torque values. So as the impact hammers on one end, the other applies a springing torque. It dampens the hammering blows to limit the torque at the nut. A different story than claiming an extension absorbs or uses up some torque with a torque wrench, but with hammering input blows it can be designed to limit the torque at the nut.

Torquing nuts is not the best way to get the proper stretching load on the bolt, but it is much easier than measuring stretch in most cases. So it will do where there is a wide rage of acceptable torque, like lug nuts. Nut face friction, thread friction and lubrication on these are huge variables. I always lube the lug nut tapered face and the threads. Then I'm nervous about applying full rated torque because of the far greater stretch. So I back off on the torque to compensate.

 

[SPYDERLK]

If we were installing the bolts that attach the wings to a fuselage, we could go into this with great detail about torsional rigidity, moments of inertia and all that cool stuff. However, we're discussing torquing wheel bolts on a tractor where the values might be measured in grunts or grunts and a half and be perfectly safe.

Greetings Learned Forum:

I need to torque my tractor's lug nuts. The rear rims are deep enough that I need to use an extension on my torque wrench. Way in the back of my mind, I have a memory from high school shop class (God bless you, Mr. Sveck) that adding an extension changes the torque actually being applied, as compared to the setting on the wrench. Is this true, or just the aftermath of too much partying after school? And if true, is there an easy way to account for the difference?

I greatly appreciate your thoughts and insights. Thank you.

Nick

Regardless that that is correct, we are actually talking about a "REALLY Basic Torque Question". The answer is no, unless the extension is being driven by impulse ... or yes to an uncertain + or - amount if stick slip is prevalent in the rotation of the fastener.
larry

 

[merrickvilleguy]

2 things ...

1. adding an extension to the handle (is not advised by my torque wrench as it will damage the wrench) will make the force you have to apply (to your extension/cheater) less, because you swing through a longer arc to apply the force.

torque is radius (length of cheater) cross force (i.e. 10' cheater with 10lbs on the end applies twice the torque to a 5' cheater and the same weight)

2. adding an extension between the wrench and socket makes no difference whatsoever as the force being applied is perpendicular to the length of the extension

 

[SPYDERLK]

2 things ...

1. adding an extension to the handle (is not advised by my torque wrench as it will damage the wrench) will make the force you have to apply (to your extension/cheater) less, because you swing through a longer arc to apply the force.

torque is radius (length of cheater) cross force (i.e. 10' cheater with 10lbs on the end applies twice the torque to a 5' cheater and the same weight)

2. adding an extension between the wrench and socket makes no difference whatsoever as the force being applied is perpendicular to the length of the extension

1] Most clicker types sense torque at the head even tho the adjustment is out at the handhold. Youre dead right that you wont have to put as much force at the end of the cheater. But, as long as its a good cheater -- straight and overlapping the handle snuggly and well, it will not change the click point. Itll just make it so your wife or kid can get a click at the 400ft-lb setting.

2] Yes! However, this is the point of contention. I believe the correction spoken of by Roy is applied as a safety factor to assure at least spec torque when the extension is winding up and the fastener is doing a stick slip routine due to difference in static and dynamic friction in the thread helix and under the head. ... Suppose your tightening to 100ft# using a long 1/2 extension. As you approach you note that the handle moves several degrees before the bolt moves. Then when the bolt moves it may do it suddenly because the friction characteristic changes, becoming less, and all the energy in that twisted extension snaps it tighter while it feels as if your ratchet has skipped a tooth. This is not the way its supposed to be - the bolt is supposed to smoothly approach torque without the hesitations that change its friction characteristic. A rigid torsion setup assures this. A flexible one allows the other, and may snap/jump to 97 or 102 while your applying 99 ... and then not move again until you apply 105.
larry

 

[c23380]

We are talkin about lug nut for a tractor but this sparked the interest of so many, so if we want, we can discuss the standards set by our military for nuclear and non nuclear subs.

VOLUME V

As for me, I am with Roy on this. If it says 230ft/lbs to+ or - 5, then if I have a window for error of 10 lbs. Set it on 233 lbs and rock on..........ON a side note, there is a ford 860 running around the farm with 2 of the nuts broken off and have been that way for 20 years. Grandpa installed the tires with the grunt method because he never owned a torque wrench.

 

[Raspy]

I know a guy that kept snapping lug bolts off on his car. Finally, I figured out he was tightening them as tight as he could. It just so happened he was strong enough to sometimes break them.

I once had a guy tell me about some torque spec on a piece of farm equipment I was working on. "The rule is", he stated so authoritatively, "go as tight as you can get it, and then a quarter turn more".

 

[SPYDERLK]

We are talkin about lug nut for a tractor but this sparked the interest of so many, so if we want, we can discuss the standards set by our military for nuclear and non nuclear subs.

VOLUME V

As for me, I am with Roy on this. If it says 230ft/lbs to+ or - 5, then if I have a window for error of 10 lbs. Set it on 233 lbs and rock on..........ON a side note, there is a ford 860 running around the farm with 2 of the nuts broken off and have been that way for 20 years. Grandpa installed the tires with the grunt method because he never owned a torque wrench.

Read your ref. It is talking about adapters that extend laterally from the head of the wrench to apply torque in the same plane as the wrench. Its a no brainer that this causes the torque at the delivery point to be some proportion of the wrench setting other than 1.

The extensions we are talking about extend perpendicular to the wrench head and deliver torque in a plane somewhat removed, but // to the plane the wrench acts in. These deliver torque equal to that of the wrench click point regardless of how long they are. The "correction factor" is 1. So ... I have to wonder how it is youre with Roy? A case of "if a littles good then a lots better, and too much is just right?
larry

 

[c23380]

I guess I stated it wrong. IF he feels it changes with a 6in or 12in or just a socket on the torque wrench, then ad a couple. If not ok. We started out torquing a tractor lug nut. To me we are arguing over something so small it really does not matter.

 

[patrick_g]

Although this might not stir up the heated controversy of a magic conveyor belt and an airplane....

Here is the challenge:

Anyone who believes adding (standard not torque limiting) extensions to a torque wrench changes the results, please supply any credible reference to support your contention and I will enter your name into a drawing where you may win a swell prize.

Gut feelings, third party hearsay, and such as I thought I remember something about... and similar are not qualifying! A reasoned logical argument based on accepted principles of physics may be substituted for credible references.

SPYDERLK You qualify for 10 bonus points for detailing the basic truth as regards the torque issue and why extensions DO NOT make a change in the results. I couldn't have wished for a more cogent explanation nor have supplied it myself.

Now to await the reasoned response from the other side of the issue...

Pat

 

[Redneck in training]

I do not know which side of the argument is right but my guts feeling is that extension makes some though negligible difference due to torque applied dynamically. Here is an experiment I would propose. Take two clickers and connect them someway together by a short connection. Then adjust the setting such a way that both click when force is applied. Then put a long extension in between and apply a force. I have only one clicker otherwise I would try it.

 

[mwb]

I do not know which side of the argument is right but my guts feeling is that extension makes some though negligible difference due to torque applied dynamically. Here is an experiment I would propose. Take two clickers and connect them someway together by a short connection. Then adjust the setting such a way that both click when force is applied. Then put a long extension in between and apply a force. I have only one clicker otherwise I would try it.

Here you go; I am the technical manager of a calibration lab. We do thousands of torque wrenches a year, all brands and types (beam, click, dial, electronic).

Comparing any of the above wrench types to a Norbar torque transducer (0.1% IV) with no extension (wrench directly engaged to the transducer) and with a 12" extension produces NO difference in recorded value from the Norbar analyser (peak function). There is greater uncertainty from the repeatability of the wrench itself. This is a static test (i.e calibration of the wrench), there is no nut or threads involved and only the deflection of the strain gage transducer.

IMO, the designer (engineer) for the engine or whatever would have specified the correct torque to be used with consideration for all of the concerns listed (lubrication, bolt stretch, fastener type...) and to try to re-engineer this at the workbench is probably not worth the time and trouble - and could also cause problems (miscalculation).

If you are worrying about adding / subtracting 2% for whatever reason then you might want to purchase something better than a 2 or 3% wrench. Also, unless you are loading the wrench using a load fixture (torque calibrator with worm gear ) most competent users will still slightly exceed the click point when pulling by hand. When pulling by hand the torque calibrator will on average record higher values than when using a mechanical device (it's hard to release the wrench exactly on the click).

 

[SPYDERLK]

When i was a boy my father, who stopped just short of a PhD in Physics, told me offhandedly while doing mechanical work that a long screwdriver could tighten a screw better because it gave more leverage. I was intuitively skeptical of this but didnt say anything for awhile. When I did, it took him no time to come to the realization. ... It seems that any af us will sometimes adopt and pass along a plausible "truism" without really thinking about it. Its easy to see how it could go on in this case if no skepticism were applied. -- Long means bigger handle and blade, thus better purchase on each end for turning. Also less sensitivity to lateral offset of the driven axis. Performance shows a seeming validation, however skirting the Physics.
larry

 

[merrickvilleguy]

The problem with that is that has nothing to do with torque ... the torque remains the same regardless of the length of the handle so long as the handle is perpendicular to the plain the force is being applied in.

Heck the handle could be 20' long and so long as it was the same diameter as a short one, you would apply the same torque with the same force (assuming the handle didn't just twist)

 

[merrickvilleguy]

Although this might not stir up the heated controversy of a magic conveyor belt and an airplane....

That one always ticked me off ... most people only have experience with cars (wheels apply force to the ground) so the idea that the wheels on a plane don't propel it at all (less friction in the bearings but i digress) led to many an argument prior to that mythbusters episode

 

[BeeferMan]

Hey, this thread seems to be quieting down a little... let's see if I can stir it up some.

If the length of an extension makes a difference in the torque applied at the far end, then the diameter of the extension must also make a difference...

Have at it...

 

[c23380]

I would be more interested in the magic carpet airplane thing. I fly, work on em and build-em from scratch. Someone tell me about the myth we are joking about, please. Thanks, Mike

 

[SPYDERLK]

Hey, this thread seems to be quieting down a little... let's see if I can stir it up some.

If the length of an extension makes a difference in the torque applied at the far end, then the diameter of the extension must also make a difference...

Have at it...

diameter would make a difference only when the smaller of the two could not maintain a straight drive axis. This condition would be pretty obvious as it became a pretzel.
larry