Torque wrench calculations

[expatriated]

Continuing my rather limited education in mechanics.....Can someone elaborate for me what adjustment (if any) is needed when you use a torque wrench with an extension? I've read various opinions on this and I don't understand it.

For example, if I use a 10 inch extension with a socket on a torque wrench, why should an adjustment need to be made? Isn't the the ratchet mechanism turning the same distance as the socket on the other end of the extension? Or is it gathering more resistance? If it calls for 20 ft lbs, should the wrench be set at 20 ft lbs or some other amount based on the fact that a 10 in extension is factored in?

Does it make a difference if a crowsfoot is used?

Thank you!

 

[Cryptic1911]

I've never read about it myself, but it could be that a 10 inch extension may twist some and throw off the torque? As far as a crows foot, I think that will definately need an adjustment because you are changing the angle of the force

 

[Mike662]

My understanding is that you are correct. If you are using an extension, which does not offset the fastener from the head of the wrench, no adjustment is required.

A crowsfoot, on the other hand, does offset the fastener and would need to be corrected for.

I bought a TQ wrench new a while ago, and it came with the instructions for making these adjustments.

Not sure if I'm explaining this clearly.

 

[Cobra4B]

Subscribed... good question. I've always heard that using extensions/adaptors lowers the resultant torque because some of it is used in twisting the extension/adaptor... i.e. it doesn't reach the item being torqued.

 

[ATTappman]

If your hand is exerting force at a right angle to the torque wrench, as you would normally do, the torque is equal to the force applied multiplied by the distance between your hand and the axis about which the fastener turns. This relationship is taught by saying "torque is equal to force times the lever arm."

A perfectly straight socket extension won't change the torque, because it doesn't change the distance between your hand and the axis of the fastener.

A crowfoot socket effectively makes the torque wrench longer, because it increases the distance between your hand and the fastener axis. If you apply the same force as you did without the crowfoot socket, the torque at the fastener will be higher than it was without the socket.

 

[expatriated]

Subscribed... good question. I've always heard that using extensions/adaptors lowers the resultant torque because some of it is used in twisting the extension/adaptor... i.e. it doesn't reach the item being torqued.

Very good, Cobra--that's the first question I was trying to pose So, thank you for making it clear. I reread my question and it seems a bit confusing.

 

[nissan_crawler]

A crowsfoot, on the other hand, does offset the fastener and would need to be corrected for.

Unless it's 90* to the handle. this is going to turn into a long nasty debate.

Long story short, the change in torque is irrelevant, and no manufacturer really cares. Can it change it? Maybe, like putting a deer whistle on your front bumper changes your mileage.

 

[Cobra4B]

A crowfoot socket effectively makes the torque wrench longer, because it increases the distance between your hand and the fastener axis. If you apply the same force as you did without the crowfoot socket, the torque at the fastener will be higher than it was without the socket.

The way you described it wouldn't it make the resulting torque w/ the crows foot higher? I've never used one, but if it makes the effective lever arm longer and you supply the same amount of force needed to make the wrench click say 100 ft/lbs wouldn't the acutal torque value for the fastener be higher? Because the wrench mechanism doesn't account for the added length?

Very good, Cobra--that's the first question I was trying to pose So, thank you for making it clear. I reread my question and it seems a bit confusing.

 

[Cobra4B]

Can it change it? Maybe, like putting a deer whistle on your front bumper changes your mileage.

You should pass this on to those people that "hyper-mile" their cars... we'll see front ends covered in deer whistles

 

[expatriated]

this is going to turn into a long nasty debate.QUOTE]


Well, a torque wrench calculations debate will be a refreshing change. (We've already gone 9 posts without a brand or country of origin debate)

 

[expatriated]

the torque is equal to the force applied multiplied by the distance between your hand and the axis about which the fastener turns. This relationship is taught by saying "torque is equal to force times the lever arm.".

Does this mean the torque values change slightly if I'm holding the torque wrench at the end (18") from the head(axis) vs. holding it 6" from the head while exerting force?

 

[ATTappman]

The way you described it wouldn't it make the resulting torque w/ the crows foot higher? I've never used one, but if it makes the effective lever arm longer and you supply the same amount of force needed to make the wrench click say 100 ft/lbs wouldn't the acutal torque value for the fastener be higher? Because the wrench mechanism doesn't account for the added length?

Yes, that's right.

Precision Instruments explains it here, but past the first sentence, the explanation is not very good. Here's the first sentence (boldface is mine):

"If an adaptor or extension is attached to the square drive of a click-type torque wrench and this adds to its length, then the applied torque will be greater than the pre-set torque."

 

[Daedalus]

Can it change it? Maybe, like putting a deer whistle on your front bumper changes your mileage.

Well put. For an extension (perpendicular to the wrench) it depends on final torque required, torsional stiffness of the extension, and how fast the torque is applied. But practially speaking you don't need to make any adjustments. Go slow when reaching final torque and claim victory.

A crow's foot is a different story. Anything the places the fastener closer to or farther away from the line of action of the square drive (or from the driver extension) should be accounted for, and by a ratio of old distance over the new distance.

E.g. if handle to square drive of the wrench is 10", and the crow's foot grabs the fastener 1" further out (11" total), then you should use 10/11 = 91% of the original torque spec called out.

Similarly, if the crow's foot is toward the handle, you should use 10/9 = 111% of original torque spec called out.

10% probably won't make a difference, but it depends on the size of the wrench and the amount of offset. For these calculations using a driver extension doesn't change the numbers.

 

[Mike662]

Does this mean the torque values change slightly if I'm holding the torque wrench at the end (18") from the head(axis) vs. holding it 6" from the head while exerting force?

Assuming you are using a click type torque whench, it will click once the preset torque is reached at the fastener. Placing your hand 6" from the head will make it 3 times as hard to apply the set amount of torque as placing your hand at the end of the handle, 18" from the head.

Think of it this way...let's say the TQ wrench is set for 120 Ft Lbs, which is the specified value for the fastener. Putting your hand 6" (1/2 a foot) from the head means that you'll have to apply 240 lbs of force to the wrench (240 lbs x 0.5 ft = 120 ft lbs). If instead you put your hand 18" from the head, you would only have to apply 80 lbs to the wrench. Same torque at the fastener, just a lot easier for you!

 

[ATTappman]

Unless it's 90* to the handle.

Yes, excellent point.

Torque is a vector quantity. The magnitude of the torque is the force times the lever arm times the sine of the angle between them. A crowfoot socket attached at a right angle to the handle will lengthen the lever arm, but decrease the angle between the force and the lever arm. These two effects cancel exactly, and the torque is the same.

At least, that's what I get on the back of an envelope.

 

[forceyoda]

It depends on the country of origin of the torque wrench.

 

[Mike83]

Does this mean the torque values change slightly if I'm holding the torque wrench at the end (18") from the head(axis) vs. holding it 6" from the head while exerting force?

Not slightly, by 3 times.

 

[Mike83]

Assuming you are using a click type torque whench, it will click once the preset torque is reached at the fastener. Placing your hand 6" from the head will make it 3 times as hard to apply the set amount of torque as placing your hand at the end of the handle, 18" from the head.

Think of it this way...let's say the TQ wrench is set for 120 Ft Lbs, which is the specified value for the fastener. Putting your hand 6" (1/2 a foot) from the head means that you'll have to apply 240 lbs of force to the wrench (240 lbs x 0.5 ft = 120 ft lbs). If instead you put your hand 18" from the head, you would only have to apply 80 lbs to the wrench. Same torque at the fastener, just a lot easier for you!

The force needs to be applied at the center of the handle for accurate torque output. Most torque wrenches even have a note indicating this on the tool itself. The only exception I know of is dial type which I *think* use strain gauges to measure torque output.

 

[CraigFL]

Actually, the torque is not dependant on the stiffness of the extension. The torque 'travels through" the twisiting extension no matter how weak or strong it is. Only something that is an external force or friction in the opposite direction of the applied force would weaken the applied torque. Even if you had to twist the torque wrench 360 degrees due to the flexibility of the extension(winding it up) to get 100 ft-lb, there would still be 100 ft-lb at the nut. Of course if the extension was long and flimsy, you may have to support it with your other hand to twist this far which then may subtract some of the torque.

 

[expatriated]

Ok, let's get practical.

Example: Spark plug installation. Manual calls for 20 ft lbs torque. I am using a 10" extension. What should I set the torque wrench to to ensure that the plug gets 20 ft lbs?

Seems like from above that I should just set it at 20?

 

[CraigFL]

Click type torque wrenches use torque limiting devices so they aren't as susceptible to hand position. The beam type torque indicators need you to hold the handle properly so it balances on the pivot for accurate readings.

 

[CraigFL]

Ok, let's get practical.

Example: Spark plug installation. Manual calls for 20 ft lbs torque. I am using a 10" extension. What should I set the torque wrench to to ensure that the plug gets 20 ft lbs?

Seems like from above that I should just set it at 20?

10" extension, 5" or 20" -- set it to 20 ft-lb

 

[flashpuppy]

What's it say on those dogbone torque extensions?

1

2

3

 

[ATTappman]

Actually, the torque is not dependant on the stiffness of the extension. The torque 'travels through" the twisiting extension no matter how weak or strong it is.

Yes, you're right.

 

[Mike662]

Good point...isn't that the principle on which "torque sticks" work?

 

[franzdom]

It's sum of forces, and Craig is correct. As for having to hold the end of the torque wrench a bit to keep it from moving, I don't think that affects the values either, as long as you don't grab the extension.

 

[franzdom]

Good point...isn't that the principle on which "torque sticks" work?

Those only work for impact wrenches, not for static torque applications.

 

[raflomo]

Ok, let's get practical.

Example: Spark plug installation. Manual calls for 20 ft lbs torque. I am using a 10" extension. What should I set the torque wrench to to ensure that the plug gets 20 ft lbs?

Seems like from above that I should just set it at 20?

WHAT? You mean we had 2 pages for this....Why didn't you just ask if you needed to change the setting while putting in your plugs...j/k with you bro!!!

This topic crossed my mind today as I was torqueing a bolt to 120 in/lbs and was using a 3" extension.Of course it was only my 2nd time using a TW so it is all new to me...

 

[mjozefow]

Here is what seems to be a novel idea with this topic:

Go to a well known manufacturers website, and see what they recommend. They will tell you how to calculate correction factors and everything.

This has blown up twice before in my knowledge, lets not make it three.

PS- They will say an inline extension does not mess with the torque reading. A simple moment diagram will show that.

 

[expatriated]

WHAT? You mean we had 2 pages for this....Why didn't you just ask if you needed to change the setting while putting in your plugs...j/k with you bro!!!

This topic crossed my mind today as I was torqueing a bolt to 120 in/lbs and was using a 3" extension.Of course it was only my 2nd time using a TW so it is all new to me...

Ha! I did the plugs awhile ago. It was just an attempt to organize my question in a more lucid way.

I appreciate everyone's posts. Informative. Am I right in gathering from reading through these responses that it is not something you guys worry about too much? Or, more specifically, that I, as a home mechanic need to worry about?

 

[AdamMopar]

I don't see how this can be. Imagine that the extension is made out of stiff rubber, but just flexible enough so that the force applied to the wrench handle rotates the top of the extension, but the bottom of the extension doesn't move at all. The extension twists like a pretzel. Not realistic, I know, but the principle is the same. Torque is only transmitted perfectly though an extension if the extension is perfectly rigid. Absorbing rotational force is the function of torsion bars on independent front suspensions.

Cheap impact extensions (or maybe expensive ones too, I don't know) will significantly reduce the torque applied to the fastener.

Craig is correct.

 

[AdamMopar]

There is no correction needed period. This comes up every couple of months it seems like, some incorrect information is posted, and then eventually it comes to the correct conclusion that there is no correction needed.

 

[glenmore]

Ok, let's get practical.

Example: Spark plug installation. Manual calls for 20 ft lbs torque. I am using a 10" extension. What should I set the torque wrench to to ensure that the plug gets 20 ft lbs?

Seems like from above that I should just set it at 20?

As long as your extension and plug are at 90 degrees to your torque wrench, you still set it at 20. Same with any length extension, as long as the line of the nut and extension is perpendicular to the wrench.

 

[Elroy]

Craig is correct.

Actually Mr. Craig is only "partially" correct.

There is only one little problem here. To tighten a bolt or screw or spark plug you have to turn it and it's no longer static.

Elroy has really just about pissed down this road enough and he's going to give one last input on the subject.

I want to address the specific issue of torque transmission and loss through a standard socket extension. One where the "moment" is axial.

In these discussions the fact of springs in series has been totally missed.

The first spring in the series is the torque wrench. The second spring is the 10 inch long extension. The third spring is the fastener and the last spring in the system is the joint. The "joint" in the case of the OP is the plug seat or gasket.

Springs in series are a sum of their inverse spring constant.

Now lets look at that 10" long extension. To make the discussion easy to comprehend lets assume it's a 1/4" drive extension and you're pushing it with a 1/2" torque wrench.

The angular deflection of a circular shaft is TL/JG

T= torque
L= Length
J= Polar Moment of inertia (A property of the cross section)
G= Shear Modulus of rigidity (A property of the material)

SO you can see that the longer the extension, the higher the torque, or the smaller the diameter, or the crappier the material the more the extension is going to twist.

So what does all the **** mean ? I'll get to that in just a second.

Lets talk about the twist of the extension in a free body diagram first. First off if the the extension is turning as a result of the input torque the resisting torque (at the fastener) is less. That's because there is an imbalance of forces! If the extension turns the input torque is higher than the resisting torque. Very simple

Hope you all got that.

Elroy is going to go eat now, diner is on the table. I'll, come back here and finish up the springs in series deal. Make sure your torque wrenches are calibrated and your minds are working before posting.

Elroy thanks you kindly

later

 

[Charles (in GA)]

Oh God! not this again..........

I assume you are talking extensions and NOT offsets.

If you have concerns about calculating the correct torque when using OFFSETS, use this as a general reference.

Torque Wrench Offset Adapter Calculations

And Here Is a Whole Thread Devoted to Offset Torquing

If you are worried about a straight extension eating up some of the torque and you not getting the correct torque to the bolt, try.......

This Thread

Or This Thread

But MOST CERTAINLY This Thread

Charles

 

[ATTappman]

Oh God! not this again..........

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).

 

[krusty the clown]

yep, elroy and myself have battled this topic several times. i will reasert my opinion that an extension WILL affect torque although no correction is necessary (as any difference will fall into the aceptable range).

 

[mjozefow]

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

 

[expatriated]

i will reasert my opinion that an extension WILL affect torque although no correction is necessary (as any difference will fall into the aceptable range).

This is all I needed to know.

I didn't realize this was such a debated topic; I kinda regret bringing it up.

(I consider myself a fairly handy guy with numbers but I have to admit that much of this is zooming right past me. I'm more comfortable with ROI, TVM and diminishing marginal valuation than I am with "moment"--I actually thought that was a typo for "movement" when I first saw it.)

Thanks to everyone who responded.

 

[AdamMopar]

Actually Mr. Craig is only "partially" correct.

There is only one little problem here. To tighten a bolt or screw or spark plug you have to turn it and it's no longer static.

Elroy has really just about pissed down this road enough and he's going to give one last input on the subject.

I want to address the specific issue of torque transmission and loss through a standard socket extension. One where the "moment" is axial.

In these discussions the fact of springs in series has been totally missed.

The first spring in the series is the torque wrench. The second spring is the 10 inch long extension. The third spring is the fastener and the last spring in the system is the joint. The "joint" in the case of the OP is the plug seat or gasket.

Springs in series are a sum of their inverse spring constant.

Now lets look at that 10" long extension. To make the discussion easy to comprehend lets assume it's a 1/4" drive extension and you're pushing it with a 1/2" torque wrench.

The angular deflection of a circular shaft is TL/JG

T= torque
L= Length
J= Polar Moment of inertia (A property of the cross section)
G= Shear Modulus of rigidity (A property of the material)

SO you can see that the longer the extension, the higher the torque, or the smaller the diameter, or the crappier the material the more the extension is going to twist.

So what does all the **** mean ? I'll get to that in just a second.

Lets talk about the twist of the extension in a free body diagram first. First off if the the extension is turning as a result of the input torque the resisting torque (at the fastener) is less. That's because there is an imbalance of forces! If the extension turns the input torque is higher than the resisting torque. Very simple

Hope you all got that.

Elroy is going to go eat now, diner is on the table. I'll, come back here and finish up the springs in series deal. Make sure your torque wrenches are calibrated and your minds are working before posting.

Elroy thanks you kindly

later

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.