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Thread: Torque Wrench "Crows Foot"

  1. #1
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    Arrow Torque Wrench "Crows Foot"

    I recently read this in a UL report:

    Q:Reportedly, one manufacture of Rain tight EMT fittings includes a Torque requirement for the installed fitting. How can the torque be verified and enforced in the field?

    A:EMT Fittings are Listed under the product category Electrical Metallic Tubing Fittings (FKAV), located on page 33 in the White Book and in UL's Online Certifications Directory at www.ul.com/database. Each of the manufacturers of EMT fittings Listed for rain tight or wet locations achieves their rain tightness by a different method. Therefore it is important to follow the assembly/installation instructions located on the shipping container of the fitting. A “crows foot” attachment is one method available as a torque wrench attachment for verifying compliance with torque specifications in the field.
    Does anyone know what a "crows foot" is and how it works?
    Bryan P. Holland, MCP

  2. #2
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    Quote Originally Posted by bphgravity
    I recently read this in a UL report:



    Does anyone know what a "crows foot" is and how it works?


    Assuming this is the correct thing, it solves the "How to get a wrench around an EMT fitting" problem so you can attach a torque wrench to the fitting.
    Julie in Austin

    Born to brew, forced to work ...

  3. #3
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    Yes but remember that because you have just moved the center of rotation you will now need to correct or adjust the values you use when tightening because of the longer arm applying the torque, or am I wrong about this?
    Last edited by DaveTap; 11-09-06 at 12:56 PM.
    (yes it's REALLY my name) HVAC/ Electrical Contractor licensed Chicago,IL ... I said its OFF! Don't you trust me?

  4. #4
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    If you install the crow's foot on the wrench so it increases the effective length, then you must increase the measured torque by the ratio of the new length to the old length. You won't be far off if you add 10% to the measured torque.

    If you install the crow's foot so the toes point in the direction you are pulling, then there is no change in length and you don't need to make any correction.

  5. #5
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    Quote Originally Posted by Bob NH
    If you install the crow's foot on the wrench so it increases the effective length, then you must increase the measured torque by the ratio of the new length to the old length. You won't be far off if you add 10% to the measured torque.

    If you install the crow's foot so the toes point in the direction you are pulling, then there is no change in length and you don't need to make any correction.
    The length of the moment arm is different, because you aren't turning the torque wrench output shaft about the center of the fitting. However, your new center of rotation is also moved out, so it's back to r X F. Since you are no longer coaxial to the fitting, the net result is about unity, meaning "don't use a fudge factor". You're gaining torque because of the slightly longer moment arm, but you're losing torque between the square socket and the center of the jaws because of the different centers of rotation. The crow's foot rotates about the fitting, and the torque wrench rotates about the square hole in the crow's foot. Studying the two motions would be an interesting study in Coriolis acceleration, while they're moving :-)

    Dan
    Last edited by dsteves; 11-09-06 at 05:07 PM.

  6. #6
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    Do they make a 2.5" or 3" crow's foot for large EMT? I doubt it. The idea of using a torque wrench on an EMT fitting is ridiculous to begin with. A more appropriate method would be for the manufacturer to design the fitting so that it can be field installed wrench tight with a pair of pump pliers. This is how they are installed on a daily basis.
    Rob

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    All responses based on the 2011 NEC unless otherwise noted

  7. #7
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    Quote Originally Posted by infinity
    Do they make a 2.5" or 3" crow's foot for large EMT? I doubt it. The idea of using a torque wrench on an EMT fitting is ridiculous to begin with. A more appropriate method would be for the manufacturer to design the fitting so that it can be field installed wrench tight with a pair of pump pliers. This is how they are installed on a daily basis.
    Dunno. It seems somewhat likely that we may have to resort to the "spot torque" methodology.

    Are you familiar with that spec? The way I remember it, it has something to do with the spots you see while you are grunting....

    One of my electricians is known by his peers as "Torque". He has the distinction of being able to connect any pair of threaded connectors irreversibly unless you accept destruction in the act of disassembly.

    Dan

  8. #8
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    Quote Originally Posted by dsteves
    The length of the moment arm is different, because you aren't turning the torque wrench output shaft about the center of the fitting. However, your new center of rotation is also moved out, so it's back to r X F. Since you are no longer coaxial to the fitting, the net result is about unity, meaning "don't use a fudge factor". You're gaining torque because of the slightly longer moment arm, but you're losing torque between the square socket and the center of the jaws because of the different centers of rotation. The crow's foot rotates about the fitting, and the torque wrench rotates about the square hole in the crow's foot. Studying the two motions would be an interesting study in Coriolis acceleration, while they're moving :-)
    Dan
    No Dan. The torque wrench applies the measured torque at the square drive of the wrench where it connects to the crow's foot, and it applies a force to the drive point of the crow's foot equal to the force that you pull on the wrench handle. The torque corresponding to the product of that force, times the distance from the crowfoot drive to the center of EMT, is added to the torque measured by the torque wrench.

    It will be clear if you draw the force and moment diagram for the point where the torque wrench applies the torque, and then use those forces to calculate the torque applied to the nut.

    Here is my invention for you guys who need a tool for measuring torque when torquing up big EMT nuts:
    Get a pipe wrench and cut off the handle. Weld something with a 1/2" female square drive to it. The applied torque is the measured torque with the wrench times the ratio of the new total length to the old length from drive point to pivot in the torque wrench handle.

    Torque = (measured torque) x (extension + original length) / original length

  9. #9
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    They day we start torquing EMT nuts is the day I'll retire. I'm all for following instructions, but a fella's gotta draw the line someplace.

  10. #10
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    Quote Originally Posted by Bob NH
    No Dan. The torque wrench applies the measured torque at the square drive of the wrench where it connects to the crow's foot, and it applies a force to the drive point of the crow's foot equal to the force that you pull on the wrench handle. The torque corresponding to the product of that force, times the distance from the crowfoot drive to the center of EMT, is added to the torque measured by the torque wrench.

    It will be clear if you draw the force and moment diagram for the point where the torque wrench applies the torque, and then use those forces to calculate the torque applied to the nut.
    All right, I'm trying to follow you on this one, Bob. Let's say you've got your crow's foot wrench at the nine o'clock position, as viewed from above, on a nut with a right hand thread. So, your square hole for the torque wrench is at 180 degrees on the cartesian coordinate system I have arbitrarily defined in this system as positive X right, positive Y up. There are several situations I can see on this one.

    Assume that r1 is by definition the mean distance from the square drive of the crow's foot to the center of the crow's foot jaw (i.e. the conduit).

    Assume that r2 is the length of the torque wrench handle from the center of the square drive to the effective position of the electrician's hand.

    Assume the electrician always provides force normal to the line defined by r2.

    Situation one: Wrench handle at zero degrees in my defined plane - torque is diminished by r2-r1.
    Situation two: Wrench handle at 180 degrees - torque is increased by r2+r1
    Any time the variation in torque will be defined by (r1-r2) * the cosine of the angle subtended between r1 and r2.

    When the wrench is orthogonal to either of these two conditions (cos theda = 0), the resultant torque on the object nut is exactly equal to the applied torque. The applied torque is r times F times sin [angle between r and F], always. r is the distance from the center of rotation of the fastener you're twisting to the effective point of where you're making the twisting motion. F is the force you're applying to make something happen. sin theda is the sine of the angle between the radial line passing through the center of "desired" rotation and the vector in line with the force you are applying.

    Conclusion: I'm right on only two unique positions out of an infinite number of possibilities. Of the rest of the possibilities, half of the time I'm high, and half of the time I'm low. To correct my original statement, make sure you have the handle of the torque wrench at a 90 degree angle to the line from the middle of the square socket on the crow's foot to the center of the conduit, and you'll not have to compensate for any crow's foot geometry ;-)

    An exaggeration to prove the point goes like this. Let's say the crow's foot is of infinite length from the center of the jaws to the center of the square drive. Question: If you apply force on a 3 foot wrench at a 90 degree angle to the line from the center of the jaw to the center of the square drive on that, how much torque are you applying to the nut?

    Answer: None. You'll pull the crow's foot straight off the nut.

    If you put your wrench handle in line with that infinitely long axis, any force you apply orthogonally will twist the nut right off or on. I think the math still works with realistic dimensions. Thanks for pointing out the error; I only considered the orthogonal axis situation.

    It will be clear if you draw the force and moment diagram for the point where the torque wrench applies the torque, and then use those forces to calculate the torque applied to the nut
    Yes, you're right. Sorta ||r X F|| is by definition r * F * sin theta, right?

    Marc, only "spot torque" the biggens.

    Edited 09 Nov 2006 - 2239 - Of course, the infinitely long linkage would have to be massless or you wouldn't be able to budge it...
    Last edited by dsteves; 11-09-06 at 11:41 PM.

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