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Torque wrenches + extensions = reading corrections?

Mike Allen

Da' Pipe Fitter.
Joined
Jan 24, 2008
Location
Basement of the Alamo
I have always used beam style torque wrenches when needing to set something to a spec. I started using them years ago when I was managing an aircraft accessory repair shop here in San Antonio. One of the services we offered was the 6 mo. calibration of torque wrenches for FBOs to maintain their FAA requirements for accuracy.

In doing hundreds of calibrations it was clear that the old beam style wrenches maintained more accurate, repeatable readings than any of the 'click' style wrenches. With each one we were required to provide a correction chart for the wrenches range (50ft. indicated, 48ft. actual and so on). Some click style could be adjusted to get the closest to accurate readings in the middle of the design range, some could not. Again and again, the beam style were the most accurate.

Now back to the title.

We always tested with them with no extensions installed as that will give false readings due to the twist of the extension.

so:

1. Has anyone ever seen anywhere any correction charts for the use of different length extensions? Of course, the thickness and composition of the extensions will throw more variables in the equation.

2. When was the last time (if ever) you had your torque wrench calibrated?
 
An extension does not affect the torque readings. Think equilibrium. Same torque at each end of the extension or else something is moving. Adding a "crow's foot" wrench or something like that to the end of a torque wrench does affect the reading because you have lengthened the lever arm.

Along the lines of your reasoning, what an extension does affect is an impact gun's torque delivery to a fastener. The longer the extension, the less torque each pulse delivers to the part because of the loss in twisting the extension. That is a transient response vs. the static state you arrive at when torquing a fastener to a prescribed setting.
 
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I can only report what I have seen and they do, the approved FAA procedure we were required to follow dictated that no extension was to be used during the calibration process for that reason.
 
That's because with an extension you have to hold the torque wrench at the extension end to keep it on axis with the calibration device due to the moment arm the extension adds. There isn't going to be any linear correction for extension length. Call it end user experience if you like. Someone that uses a torque wrench all the time would get the same reading with a foot long extension as they would with a 3" extension. The is no loss if you know what you are doing. The flesh on the palm of my hand doesn't resist much of a load without moving.
 
I can only report what I have seen and they do, the approved FAA procedure we were required to follow dictated that no extension was to be used during the calibration process for that reason.
This used to be something I looked into also, and I never could get it, that an extension did not affect the value, but as said it's not supposed to.
Crow's foot etc, there is a chart or table to calculate that pretty accurately, it does change the setting required.
My Snap-ons I've had checked, both beam and click. Beam as you say is aircraft certified, way back when no other was. My clicker was quite close though through most of its range, which was up to 75 lbs/ft
 
Great subject.
I have heard that extensions should not be used also, but I think that refers to a cheater bar on the handle.
Who calibrates torque wrenches? How often should they be calibrated?
What brands are best?
With click style torque wrenches, is it true that you should not loosen the handle adjustment all the way?
This may be a stupid question but, what is the conversion between inch pounds and foot pounds? Does 12 inch pounds equal 1 foot pound?
 
Yes, 12 inch pounds = 1 foot pound. Torque is defined as the amount of weight added to the end of a lever required to maintain equilibrium. By the same logic, 16 inch ounces = 1 inch pound.
 
Then how do 'Torque Sticks" work to limit torque? They twist and release torque between hits of an impact, can't help but that the same thing would or could happen to some degree using a torque wrench.

http://www.asedeals.com/video_accutorq_sockets.wmv

Snap-On used to recommend that you return their snap or break over wrenches to '0' after use, I suspect they still do.

Anytime one is dropped it should be calibrated, may or may not knock one out of calibration. FAA requires it, your call on how much weight that requirement translates to the real world.

Most aircraft accessory repair and overhaul do them, but I'm sure there are other places, maybe Snap-On provides that service.
 
What 2manyturbos said.

It isn't so much the "flex" or "torsional twist" or "spring" of an extension, but the geometry.

In "statics" we would draw the triangle and our force vector. When you add the extension, some of the force were you pushing or "applying force in the direction" (the exact geometry of human hands matters though, but not the "big idea" here) directly on the handle of the wrench tangent to the arc it would trace a "component" of that force vector is not directly multiplied by the length of the lever arm to generate torque at the fastener, but is at the wrench is sort of the idea (a picture is worth a lot more than words here). A possible way to think of the "twist" not really mattering is just a coil spring. Suppose you put a weight atop it with all the force through the spring aiming straight downward (forget about complex motion or geometry for a minute), the force on the ground is exactly the same if everything is static as if there were no spring. Same thing here. You are twisting slowly and evenly, close to "still." In stark comparison, the impact rapidly accelerates things with a hammer and things are "not static" at all...there is an acceleration component. By hand, everything is smooth and even and there isn't rapid acceleration or dampening...doesn't matter if the extension is glass or gumby.

There is no such chart. It wouldn't be linear. Experience and knowing how to hold a wrench in the first place don't hurt. Many are dangerous with a wrench in their hands.

If you are using an impact hammer of various types, that's another matter completely. Then the "torsional-twist" or "spring" (like a torsion bar spring (porsche/VW of front of many independent truck suspensions and tons of other applications) action of the extension changes the ending maximum torque value fairly uniformly on the fastener. That's why tire shops often have a (or several, likely) "torque stick" extension(s) for wheel nuts that does just that and they should check them with a wrench for final torque: a quick way to get reasonably even clamping, safety and less user fatigue when done right.

Of course...as with doing it manually (with some additional risks to adding the impact), condition, type and material of stud and fastener affect clamping, safety and longevity...plan accordingly. Were materials and machines perfect, nothing would wear out and clamping force at all fasteners would be assured. Torque sticks have a life span in reality and respond a bit differently to different types of impact, torque wrenches aren't perfect and wear out too and wheel studs get dirty, cross threaded, or otherwise abused by neanderthals. Worse yet, one side the car can "self tighten" more than the other, though that is fairly hair splitting...the fastener torque is usually great enough we hope that friction holds everything "still" or close enough. And who wants to remember "left hand" or "right hand" thread for wheel nuts anyway?

I'll take pics of the 244 when I get home before it gets dark I hope...heading to lunch.
 
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You're confusing the issue, transient vs. static. With the "Torque stick" think damper. It absorbs spikes in energy because it is a spring, nothing more. You could do the same thing with another extension of the same cross sectional area and length. They are calibrating the impact gun in the video. When you are using a torque wrench, you are in a transient state until the fastener reaches the required torque. At that point, you are adding no additional force because you are reading the dial or hearing the click. A state of static equilibrium is reached. The fastener is not moving and neither is the torque wrench. What is in between the two is irrelevant. It could be a chunk of ceramic with a stiffness like diamond, or an extension made of rubber that has three twists in it like a stick of licorice. Either way, the torque at each end is the same.
 
I used to work with a really old timer, if he was still alive he'd be about 93. He used to tell me about his early days of working on cars. One thing he mentioned all the time, was a reference chart that gave values to adjust torque numbers when using extensions. He said that it was more a function of the poor metallurgy of the time, and that the extensions had some tendency to deflect and encourage incorrect readings. Whenever I torque things, I always try to avoid using an extension when possible.
 
Handle extensions, not extensions at the end of the wrench. There is no such relationship that any chart could correct. It's simple physics. You can't break the laws of physics. :)
 
The longer the extension, the more force is diverted from 'twist' to just plain sideways deflection of the wrench. Unless the wrench end of the extension is supported in some manner.
 
The longer the extension, the more force is diverted from 'twist' to just plain sideways deflection of the wrench. Unless the wrench end of the extension is supported in some manner.

Exactly. However, the end result is the torque delivered is actually higher than is indicated on the dial. That's because the effective lever arm length is increased. That's what James was attempting to describe with his triangle referrence. The tilt results in a larger arc. So, back to operator error. Bottom line, no extension whenever possible. If you have to use an extension, keep it as short as possible and do your best to keep the end of the torque wrench centered over the axis of the fastener.
 
The longer the extension, the more force is diverted from 'twist' to just plain sideways deflection of the wrench. Unless the wrench end of the extension is supported in some manner.

the extension in theory can act as a torsion bar or spring, but chances are you'll hit your "number" way before you can get the extension to twist :)

this was brought up in our A&P school as well..... Hey Mike, how much power can a vortex generator make? :-P
 
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Did you ever use digital torque wrenches?

Also, on extensions, use the thickest one you can find to minimize twist and maximize accuracy. I use impact extensions when torquing through an extension.
 
the extension in theory can act as a torsion bar or spring, but chances are you'll hit your "number" way before you can get the extension to twist :)

this was brought up in our A&P school as well..... Hey Mike, how much power can a vortex generator make? :-P

That's not what I was referring to. Twist as in torque applied at the socket end of the extension. The longer the extension is, the more a downward force on the torque wrench's handle is diverted away from torque to a simple downward force on the wrench end of the extension.
 
Did you ever use digital torque wrenches?

Also, on extensions, use the thickest one you can find to minimize twist and maximize accuracy. I use impact extensions when torquing through an extension.

Thicker does not mean it is going to twist less, quality of the metal can make a difference.

Use a QUALITY extension, not the cheap chinese crap, and it should be more accurate. Should does not mean is.
 
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