Torque Wrench formulas and concepts expert needed. Apply within.

[Ratsan]

Ok, we here at work could really use a Torque wrench expert to explain some things to us and so far our in house “experts” are clueless. Maybe someone here can shed some light on this for us.

Here is our problem. We have to teach a course on torque tools. We are all fairly good with how to use our various torque tools but not so good on all the theory. Especially when it comes to using extensions with a torque wrench.

First problem. We have a statement that says. “When using an extension (Crows foot/dog bone), the length of the extension may not exceed the length of the wrench plus the length of the Handle Extension if used.” Sounds great but we don’t know why. If someone asks us why we need to be able to answer intelligently. I also don’t know why they mention the handle extension. As far as I know that affects nothing mathematically.

Second problem. We have another statement that says. “When an extension is used (Crows foot/Dog bone) torque to the middle 50% of the stated torque range.” Again, we understand what they are saying but we don’t know why they would say it. We assume this is some kind of method of reducing possible error but we just don’t know.


Third Problem. When calculating the correction for an angled extension, exactly what is the method. Do you measure a direct line from the center of rotation and center of connection to the wrench or is it more complicated then that?


Anyway, just shooting in the dark.

[Whtstang]

Originally Posted by Ratsan

First problem. We have a statement that says. “When using an extension (Crows foot/dog bone), the length of the extension may not exceed the length of the wrench plus the length of the Handle Extension if used.” Sounds great but we don’t know why. If someone asks us why we need to be able to answer intelligently. I also don’t know why they mention the handle extension. As far as I know that affects nothing mathematically.

Any extensions on the wrench will cause the torque settings on the wrench to be off. Since these wrenches are calibrated according to the length of the wrench as measured from the center of the grip along the axis of the handle to the centerline of the drive, large enough changes in this length could overstress the wrench & cause permanent damage. At least at some point it would cause the corrected setting to be out of the wrenches range.

Originally Posted by Ratsan

Second problem. We have another statement that says. “When an extension is used (Crows foot/Dog bone) torque to the middle 50% of the stated torque range.” Again, we understand what they are saying but we don’t know why they would say it. We assume this is some kind of method of reducing possible error but we just don’t know.

I would agree with your assumption. In the case of a wrong calculation being used when adjusting the torque setting for the extension, (or in the case of a wrench that is out of calibration) this would give you some breathing room.

Originally Posted by Ratsan

Third Problem. When calculating the correction for an angled extension, exactly what is the method. Do you measure a direct line from the center of rotation and center of connection to the wrench or is it more complicated then that?

Nope, thats it. The angle of the extension doesn't matter. Only the axial length that it adds to the wrench.

[Ratsan]

Good stuff. Thanks for the info. That helps a lot.

[Chris-90LX]

Originally Posted by Whtstang

[font=Times New Roman][size=3]


Nope, thats it. The angle of the extension doesn't matter. Only the axial length that it adds to the wrench.

Correct!

T= F(R)

Torque = FORCE x the Length of the moment arm, hence foot/lbs or inch/ounces.

With torque wrenches there can't be any excessive play when using extensions onto the wrench. The wrench could recording 100ft/lbs, but less than 100ft/lbs would be getting to the fastener. It's like over coming friction.

[KS 82 GT]

Originally Posted by Chris-90LX

Correct!

T= F(R)

Torque = FORCE x the Length of the moment arm, hence foot/lbs or inch/ounces.

With torque wrenches there can't be any excessive play when using extensions onto the wrench. The wrench could recording 100ft/lbs, but less than 100ft/lbs would be getting to the fastener. It's like over coming friction.

I don't know if I agree. Say you have an 18" long handle and an angle that effectively adds 2" to your radius. You need to apply 100 ft/lbs of torque, so you set your torque wrench to 100 ft/lbs. 100/1.5=66.67 pounds of force. But if you calculate the torque applied at the bolt you're actually over tightening 66.67*(20/12)=111.11 ft/lbs. The reading at the wrench is what is applied at the end of the wrench. Maybe I'm misunderstanding the question, but it looks to me that you have to trig out the added length and recalculate the torque.

It appears that the biggest rule here is to avoid using extentions with your torque wrench if at all possible.

[Ratsan]

Originally Posted by KS 82 GT

I don't know if I agree. Say you have an 18" long handle and an angle that effectively adds 2" to your radius. You need to apply 100 ft/lbs of torque, so you set your torque wrench to 100 ft/lbs. 100/1.5=66.67 pounds of force. But if you calculate the torque applied at the bolt you're actually over tightening 66.67*(20/12)=111.11 ft/lbs. The reading at the wrench is what is applied at the end of the wrench. Maybe I'm misunderstanding the question, but it looks to me that you have to trig out the added length and recalculate the torque.

It appears that the biggest rule here is to avoid using extentions with your torque wrench if at all possible.

Actual = Indicated(Wrench Length + ext. Length)/Wrench Length


Thus

100 = Indicated(1.5ft + .166ft)/1.5ft

Solve for indicated

100 = indicated (1.111)

Indicated = 90.009

So with a 2” extension on a 18” wrench you are looking for a reading of just over 90 to achieve 100 actual.

[bryant99]

One thing that some people do is when a torqe wrench is not in use they will leave it set on a torqe setting which will throw the calibration off on the torqe wrench.I would get in to arguements over that.When I bought mine new it came with a paper that said when not in use set it to zero to where there is no stress on it.I used a friends one time and broke a bolt and then got my uncles and doubled checked and my friends was way off by 30 foot # of torqe.Mine came with an extention abot 3 inches long and my papers that came with it said the same thing that you are talking about.

[Whtstang]

Originally Posted by bryant99

One thing that some people do is when a torqe wrench is not in use they will leave it set on a torqe setting which will throw the calibration off on the torqe wrench.I would get in to arguements over that.When I bought mine new it came with a paper that said when not in use set it to zero to where there is no stress on it.I used a friends one time and broke a bolt and then got my uncles and doubled checked and my friends was way off by 30 foot # of torqe.Mine came with an extention abot 3 inches long and my papers that came with it said the same thing that you are talking about.

This is true. My wrenches are reset to the lower 25% of it's range when in storage(as per the instructions that came with them).

[Whtstang]

Originally Posted by Ratsan

Actual = Indicated(Wrench Length + ext. Length)/Wrench Length


Thus

100 = Indicated(1.5ft + .166ft)/1.5ft

Solve for indicated

100 = indicated (1.111)

Indicated = 90.009

So with a 2” extension on a 18” wrench you are looking for a reading of just over 90 to achieve 100 actual.

Here's the way I learned to do it:

TW = Torque setting on wrench
TE = Torque applied with extension = 100 lb/ft
L = length of wrench = 18"
E = Length of extension = 2"

TW = TE X (L / L + E) = 100 x (18/20) = 90lb/ft

It doesn't matter what units are used on the lengths as long as they are both the same. This will work with ft/lb's, in/lbs or N-M's.

Keep in mind that the length of the wrench is measured from the center of the grip. So if you hold the wrench grip with a fist facing fingers down, this center would be between the middle two knuckles of your hand.
Another tip: On high settings where two hands are needed, hold the second hand directly over the first.

[Ratsan]

That's actually a serious problem we have to address in the training. People don't set them back when there done with them.

[Shakerhood]

When we have to use a Dogbone we keep it at 90 Degrees so that the torque setting does not have to be changed.

[Whtstang]

Originally Posted by Shakerhood

When we have to use a Dogbone we keep it at 90 Degrees so that the torque setting does not have to be changed.

Good point. This would make the effective length of the extension zero. A 2" extension at 45* to the axis of the wrench would only add 1.414 to the length of the wrench.