Torque wrench calculations

[AdamMopar]

I'm going to go ahead an disagree again.

An inline shaft cannot absorb torque. It can twist, but it does not matter.

The simple virtue that a fastener is moving does not affect anything in this problem, so long as there is no acceleration. The coefficients of friction (mu's) will be kinetic, instead of static. This is the reason a fastener is supposed to be rotating prior to the torque wrench clicking/beeping/lighting up etc.

The twist does not necessarily mean that there IS an imbalance of forces, it only means that there WAS. When the extension reaches equilibrium, the imbalance is gone, and the torque applied is the same as the reaction force at the socket. If this were not the case the extension would continue to twist and would end up looking like a pretzel.

The only way I could see an long extension causing any sort of discrepancy is if it was not kept perpendicular to the workpiece. Like this: Credit to THEO on this pic.

All of the major torque wrench manufacturers I have come across suggest that correction factors must be used ONLY for extensions to the length of the wrench itself.


EX:

http://www.cditorque.com/main.html

I could be wrong Elroy, but I don't see where. Lemme know

You are correct.

 

[AdamMopar]

Criminy, I had no idea this topic had already generated so much traffic. I've only been reading this forum for less than a year now.

I did some editing and deleted a couple of my posts. I don't want to be responsible for misleading anybody.

I'm a chemical engineer myself (I've even got a PhD, imagine that - shows how much that means) but I don't remember much from sophomore physics. I talked this "twisting" question over with a couple of the other chemical engineers I work with, and it started a big argument between the two of them (one agreed with me, the other guy was right).

That is why you need to ask the MEs.

 

[porschedude996TT]

Don't confuse this with using an extension that is lenghtwise with the handle. Your discussion is about the twisting of the standard socket wrench extension. I would not consider the torque changes that are taken up in the torsional take-up of the extension. If it really bugs you then maybe a larger drive should be used. Maybe take it to the top of the range for the desired torque value.

 

[ATTappman]

That is why you need to ask the MEs.

Not many of them around here. They can get better jobs.

 

[Elroy]

The only way the torque between the input and output of a torsionally loaded member is if the system is accelerating. Otherwise the input and output torque are constant. It is not dependent upon the stiffness.

A shaft turning at a constant speed is static. There is no difference between the input and output torque unless the system is accelerating.

Acceleration.

As the phrase goes: That's rich. You must be joking.

Actually a shaft that is turning at constant speed would be considered steady state and it sure the hell isn't statistic and can bet your life the torque inputs are NOT balanced.

If what you're saying is true then Mary Poppins must be the one who determines the direction of rotation. I mean really. come on.

Elroy is a little disappointed the real experts haven't weighed in.

With the pot boiling so good here Elroy tells me he's going to go get an after diner drink and read some more entertainment.

Now before everyone gets their ******* in a bunch think back to the springs in series.

Be calm and think. Don't be an emotional slob like that Elroy clown.

 

[krusty the clown]

The only way the torque between the input and output of a torsionally loaded member is if the system is accelerating.

you are forgetting the important part.......we are talking about turning the fastener. lets say the bolt is supposed to be torqued to 100lb-ft. but the bolt stops turning at say 95 lb-ft due to friction of the treads. as the wrench is pulled harder the extension twists making the TW click. IT'S NOT THE SAME TORQUE AT BOTH ENDS OF THE EXTENSION. BUT WITHIN THE ACEPTABLE RANGE.


this is why the TW manufacturer's recomend using the shortest extension possible AND why the fastener's threads should be lubricated.

 

[AdamMopar]

you are forgetting the important part.......we are talking about turning the fastener. lets say the bolt is supposed to be torqued to 100lb-ft. but the bolt stops turning at say 95 lb-ft due to friction of the treads. as the wrench is pulled harder the extension twists making the TW click. IT'S NOT THE SAME TORQUE AT BOTH ENDS OF THE EXTENSION. BUT WITHIN THE ACEPTABLE RANGE.


this is why the TW manufacturer's recomend using the shortest extension possible AND why the fastener's threads should be lubricated.

Well if you want to get into what is actually going on with the fastener then torque doesn't make much difference.

However, in your example the torque in the will still be the same at BOTH ends of the extension. As stated before the extensions rotational deflection has no bearing on the torque. Continuing to turn the torque wrench until it clicks, even after the fastener stops, still develops an equal torque at either end.

 

[krusty the clown]

Well if you want to get into what is actually going on with the fastener then torque doesn't make much difference.

However, in your example the torque in the will still be the same at BOTH ends of the extension. As stated before the extensions rotational deflection has no bearing on the torque. Continuing to turn the torque wrench until it clicks, even after the fastener stops, still develops an equal torque at either end.

if you not considering the fastener what the hell are we talking about?


a torque wrench is used to torque FASTENERS and the torque of the FASTENER is our goal. common sense is not included in the text book


i proved my theory in another long heated debate with some college trained engineers........

 

[AdamMopar]

Acceleration.

As the phrase goes: That's rich. You must be joking.

Actually a shaft that is turning at constant speed would be considered steady state and it sure the hell isn't statistic and can bet your life the torque inputs are NOT balanced.

If what you're saying is true then Mary Poppins must be the one who determines the direction of rotation. I mean really. come on.

Elroy is a little disappointed the real experts haven't weighed in.

With the pot boiling so good here Elroy tells me he's going to go get an after diner drink and read some more entertainment.

Now before everyone gets their ******* in a bunch think back to the springs in series.

Be calm and think. Don't be an emotional slob like that Elroy clown.

Sorry Elroy, the system is static at a constant speed and the torque input and output are balanced. What determines the direction of rotation is considered in the initial dynamics of the system and any the dynamics of any disturbances after.

Something is considered static if it is acted on by balanced forces, meaning either at rest or uniform motion. A moving block on a surface at a constant speed is static. (Reference Statics by Leroy D. Sturges for more information or any statics text.)

Your springs in series have nothing to do with the torque balance in the extension.

 

[AdamMopar]

if you not considering the fastener what the hell are we talking about?


a torque wrench is used to torque FASTENERS and the torque of the FASTENER is our goal. common sense is not included in the text book


i proved my theory in another long heated debate with some college trained engineers........

The tension in the fastener is the concern, not any residual torsional load.

A torque strategy is just one way to get the approximate desired tension in the fastener to maintain the desired clamp load.

 

[krusty the clown]

The tension in the fastener is the concern, not any residual torsional load.

A torque strategy is just one way to get the approximate desired tension in the fastener to maintain the desired clamp load.

all of that is fine and dandy.........

but torque is the only way we have to measure the clamping load of the fastener.


seem's were picking the fly **** out of the pepper. for all practical purposes it's just a "good enough" measurement. that's why there is an acceptable range for a torque spec....

i'll stand by my assertions, and have tested the theory

 

[AdamMopar]

It is one of the more pratical ways to do it without employing other methods.

 

[Elroy]

Your springs in series have nothing to do with the torque balance in the extension.

But it has a lot to do with transmitting a solid non-mushy signal to the spring in the torque wrench.

Sorry Elroy, the system is static at a constant speed and the torque input and output are balanced. What determines the direction of rotation is considered in the initial dynamics of the system and any the dynamics of any disturbances after.

Please consider the small diameter, long extension that's attached firmly to the fastener and your back on the other end driving it with a big ole torque wrench and in the process your twisting that skinny extension up like a pretzel and you're going to tell me with a straight face the torques are balanced.

That's a joke surely

One of us is missing something here. It must be Leroy D. Sturges.

 

[franzdom]

But it has a lot to do with transmitting a solid non-mushy signal to the spring in the torque wrench.



Please consider the small diameter, long extension that's attached firmly to the fastener and your back on the other end driving it with a big ole torque wrench and in the process your twisting that skinny extension up like a pretzel and you're going to tell me with a straight face the torques are balanced.

That's a joke surely

One of us is missing something here. It must be Leroy D. Sturges.

The torques are balanced. Straight face. ME, new product design, Eaton Corp.

 

[AdamMopar]

But it has a lot to do with transmitting a solid non-mushy signal to the spring in the torque wrench.



Please consider the small diameter, long extension that's attached firmly to the fastener and your back on the other end driving it with a big ole torque wrench and in the process your twisting that skinny extension up like a pretzel and you're going to tell me with a straight face the torques are balanced.

That's a joke surely

One of us is missing something here. It must be Leroy D. Sturges.

The only concern is if a sufficient torisional load is developed such that the stress in the material exceeds the yield point and plastic deformation occurs.

I could twist the extension or shaft multiple revolutions, relative to the opposite end, not exceeding the yield stress, and still transmit a constant load from one end to the other.

This can occur in extremely long shafts.

I'm not missing anything.

 

[mjozefow]

you are forgetting the important part.......we are talking about turning the fastener. lets say the bolt is supposed to be torqued to 100lb-ft. but the bolt stops turning at say 95 lb-ft due to friction of the treads. as the wrench is pulled harder the extension twists making the TW click. IT'S NOT THE SAME TORQUE AT BOTH ENDS OF THE EXTENSION. BUT WITHIN THE ACEPTABLE RANGE.


this is why the TW manufacturer's recomend using the shortest extension possible AND why the fastener's threads should be lubricated.

By this logic Krusty the extension has nothing to do with it. This situation can and does arise regardless of whether or not an extension is used.

This is why the bolt is supposed to be rotated up to the spec torque, not stopped. Easier said than done at times.

 

[krusty the clown]

This situation can and does arise regardless of whether or not an extension is used.

but it is more likely when using an extension............

 

[mjozefow]

but it is more likely when using an extension............

How?

 

[krusty the clown]

How?

come by the shop if you can't understand........i'll show you. i really don't feel like retyping my last several posts

 

[mjozefow]

you are forgetting the important part.......we are talking about turning the fastener. lets say the bolt is supposed to be torqued to 100lb-ft. but the bolt stops turning at say 95 lb-ft due to friction of the treads. as the wrench is pulled harder the extension twists making the TW click. IT'S NOT THE SAME TORQUE AT BOTH ENDS OF THE EXTENSION. BUT WITHIN THE ACEPTABLE RANGE.

I believe what you are describing is the difference between static and kinetic friction.

When the bolt stops, it takes more torque to get it to turn again than it would have to keep it going. This is the same as sliding a box on the floor: Hard to get it moving, easier to keep it moving.

Therefore, when you try re-apply torque, the extension may minutely twist, but the fact of the matter is, that when you twist something, there must be an equal and opposite reaction torque at the other end. Other wise the extension would spin, not twist.

It is not possible to apply a torque on something and not have it react equally and oppositely. Just the same as you cannot push on something any harder than it pushes on you. Your **** only exerts ___Lbs on your chair, and your chair exerts the same on your ****. Otherwise you would fall or fly off.

So when you have your fastener stop prematurely and then you re-apply the only reason the wrench clicks is because the "break-out" torque needed to get the fastener moving again is greater than that of the torque setting on the wrench.

 

[krusty the clown]

I believe what you are describing is the difference between static and kinetic friction.

When the bolt stops, it takes more torque to get it to turn again than it would have to keep it going. This is the same as sliding a box on the floor: Hard to get it moving, easier to keep it moving.

Therefore, when you try re-apply torque, the extension may minutely twist, but the fact of the matter is, that when you twist something, there must be an equal and opposite reaction torque at the other end. Other wise the extension would spin, not twist.

It is not possible to apply a torque on something and not have it react equally and oppositely. Just the same as you cannot push on something any harder than it pushes on you. Your **** only exerts ___Lbs on your chair, and your chair exerts the same on your ****. Otherwise you would fall or fly off.

So when you have your fastener stop prematurely and then you re-apply the only reason the wrench clicks is because the "break-out" torque needed to get the fastener moving again is greater than that of the torque setting on the wrench.

nope...........

 

[Shadowdog500]

Im trying to grasp what you are saying. If I torque a fastener through an extension the torque at the the fastener would be less than the torque at the torque wrench?

Chris

 

[nissan_crawler]

I knew this would turn into a **** sandwich.

 

[CraigFL]

I believe what you are describing is the difference between static and kinetic friction.

When the bolt stops, it takes more torque to get it to turn again than it would have to keep it going. This is the same as sliding a box on the floor: Hard to get it moving, easier to keep it moving.

Therefore, when you try re-apply torque, the extension may minutely twist, but the fact of the matter is, that when you twist something, there must be an equal and opposite reaction torque at the other end. Other wise the extension would spin, not twist.

It is not possible to apply a torque on something and not have it react equally and oppositely. Just the same as you cannot push on something any harder than it pushes on you. Your **** only exerts ___Lbs on your chair, and your chair exerts the same on your ****. Otherwise you would fall or fly off.

So when you have your fastener stop prematurely and then you re-apply the only reason the wrench clicks is because the "break-out" torque needed to get the fastener moving again is greater than that of the torque setting on the wrench.

Yup...


What we are really trying to achieve is a preload in the bolt of approximately 90% of the yield strength. This gives us maximum clamping force, within a safety factor without going beyond the yield strength of the material which would actually lower the clamping force as the bolt stretches more. The torque required depends on a lot of variables including the strength of the material, the angle of the ramp on the threads(coarse & fine are different), the friction between the threads as well as under the head/nut. The most accurate method is the turn-of-the-nut method because actual stretch is proportional to the yield of the material but it is also very difficult to do because you need to know the effective length of the bolt/stud too.

Because friction enters into the equation, stick/slip friction can make a difference as described above. In high strength structural bolted connections like those used in highway bridges, washers are required under the bolt heads to assure friction consistancy. If the connection needs to be checked or retorqued, nuts usually are loosened 1/4 turn and tightened from there, moving up the final torque without stopping until the required torque is reached.

 

[mjozefow]

I knew this would turn into a **** sandwich.

This one is civil so far though... I have hope that this one stays that way.

 

[AdamMopar]

Yup...


What we are really trying to achieve is a preload in the bolt of approximately 90% of the yield strength. This gives us maximum clamping force, within a safety factor without going beyond the yield strength of the material which would actually lower the clamping force as the bolt stretches more. The torque required depends on a lot of variables including the strength of the material, the angle of the ramp on the threads(coarse & fine are different), the friction between the threads as well as under the head/nut. The most accurate method is the turn-of-the-nut method because actual stretch is proportional to the yield of the material but it is also very difficult to do because you need to know the effective length of the bolt/stud too.

Because friction enters into the equation, stick/slip friction can make a difference as described above. In high strength structural bolted connections like those used in highway bridges, washers are required under the bolt heads to assure friction consistancy. If the connection needs to be checked or retorqued, nuts usually are loosened 1/4 turn and tightened from there, moving up the final torque without stopping until the required torque is reached.

Yep exactly right.

Krusty I don't know what you are seeing in your shop, but something strange is going on for you there.

 

[dgarage]

This one is civil so far though... I have hope that this one stays that way.

It won't matter. The Admin will simply lock the thread instead of addressing the particular individual(s) responsible. I assume due to their post count. It's the reason I haven't posted since the last torque wrench debacle. I suspect others have responded similarly.

Now back to your regularly scheduled programming...

 

[Shadowdog500]

How the heck does the extension know which end to make less? If I had someone hold the torque wrench with an extension real steady and turned my engine around the head bolt until the torque wrench clicked, do you now think that the bolt would be too tight since some of the torque that I applied to the socket through the bolt was lost going through the extension to the torque wrench?

Do you believe that the torsion spring on your garage door has less torque at one end than the other because it has a half dozen turns in the spring? If so, How does the spring know which end to make less?

Chris