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How to Check the Accuracy of a Torque Wrench

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  • How to Check the Accuracy of a Torque Wrench

    Have seen/heard a few people say "I don't trust my torque wrench" or similar, so thought I would post up this method for checking its accuracy.

    Basics-
    Torque is most commonly measured in N.m (newton meters) or ft.lb (foot pounds)
    I will be using N.m, since it's the metric unit, and hence better.
    All weights are measured in kg's, and distances in meters.
    For reference 1 N.m = 0.74 ft.lb

    I'll avoid getting into all the physics and get straight into you what you need to know.

    What you need-



    -torque wrench
    -vice
    -some weights
    -scales
    -rope or wire (strong enough to hang the weights from)
    -calculator
    -tape measure/ ruler

    Method-

    1) Weigh the torque wrench on the scales in kg's. Ill call this number "mc" for the calculations. It was 1.3kg in my case.
    2) Find the center of mass of the torque wrench.
    To do this, balance the torque wrench on your finger as shown below.



    Measure the distance between your finger and the middle of the square drive. Call this "dc".

    3) Clamp the square drive of the torque wrench in the vice, so that the the wrench is parallel to the ground. You will be hanging the weights from the end of the torque wrench, so pick a spot where the rope will not easily slide about. Measure from this spot to the middle of the square drive. Call this "dt".



    4) Tie a loop in one end of the rope, and the other end to your weight. Using the scales, weigh the rope and weight. Call this "mt".

    When this weight is hung from the torque wrench it will apply a torque of T = (dt x mt x 9.81) + (dc x mc x 9.81) N.m

    If this is greater than the range of your torque wrench, use a smaller weight. Ideally you want to check the wrench around the range you most commonly use. ie dont check it at 2 N.m it you generally do bolts up to 30 N.m

    5) Set the torque wrench to the above value, and gently hang the weight from the previously chosen spot on the wrench. Dropping the weight on to suddenly will give you a false reading.
    If the wrench clicks, increase the torque setting on the wrench. If it doesnt click, reduce the torque setting. Continue to do this until you find the setting where the wrench clicks, but wont if the torque setting is further increased.



    6) You can repeat steps 4 and 5 with different weights as many times as you like. I did it 4 times, using 5, 10, 15 and 20kg weights.

    7) You should be able to see the error between the actual torque applied, and the torque measured by the wrench.
    If there is no error, your done. Grab a beer, sit on the couch, and bask in your glory.
    If there was an error, you have 3 choices:
    a) Re-calibrate your torque wrench and repeat steps 4, 5 and 6 to check. Each torque wrench is different so I can't tell you exactly how to do this. A tool shop may be able to help if you get stuck.
    b) Take the error into account when you do up the bolts.
    c) Ignore the error.

    Notes:
    This method is not 100% accurate for various reasons, but its close enough.
    If your torque wrench is out by 2 or 3 Nm, I wouldnt bother recalibrating.
    If you dont have any suitable weights, use a bucket with water in it.
    Make sure the vice is done up tight. You don't want the weights falling on your feet. For the same reason, make sure you use strong enough rope and tie good knots in it.

    Disclaimer: I accept no responsibility for anything.
    Last edited by Duffman; 10-08-2009, 10:18 PM.
    Respect is earned, not enforced.

  • #2
    nice work. always good to know how to check this by yourself.
    Sponsored by:

    Billetta Imports, Motorcycle Panel & Paint, Pirelli, YAMALUBE

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    • #3
      Nice write-up duffman - I'd +rep ya if I could.

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      • #4
        I wish I had a mill in my garage.....


        Nice writeup.
        EXPERIENCE: noun: Knowledge or skill derived from actual participation or direct contact rather than mere study, interest, or internet.

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        • #5
          Less calibrating, more big bang.

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          • #6
            Originally posted by ApatheticEnd View Post
            I wish I had a mill in my garage.....


            Nice writeup.
            +1

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            • #7
              Bit of topic but how much does a simple torque wrench like yours go for? (no digital).

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              • #8
                Anywhere from $100 upwards.

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                • #9
                  I got my 3/8" for $40 and my 1/2" for $60, new. Mine are budget ones though but does the job.
                  PSB Roller - http://soundcloud.com/talon

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                  • #10
                    Originally posted by Xuaxace View Post
                    Bit of topic but how much does a simple torque wrench like yours go for? (no digital).
                    Like everything, it depends on the quality your after.
                    Mine was given to me so I have no idea of its history, but assume its good quality, and possibly a couple of decades old.

                    It was out by 2Nm across the range I measured, and only took 1 grub screw to reset it.

                    Aslong as you keep checking the accuracy of a cheap one, it can do as good a job as a more expensive one.
                    Respect is earned, not enforced.

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                    • #11
                      Thanks for the advice... I will look further into it when I get some $$$. I definitely need one to replace the clutch and brake rotor.

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                      • #12
                        .. I recently bought a Warren & Brown to replace the one I lost years ago. I was surprised to find that they were cheaper than many of the other brands I looked at and seemingly just as good quality as they used to be.
                        .. and thats Racer # 193 to y'all; my fabulous sponsors (who all do good shit) are: Graeme Fleming IT Consultants, Vision Image, Pacific Safety Wear, Excess Power Equipment, Pro Photo Booth

                        .. and according to Sean'o: 'get the Kwaka (never thought i would say that!)'

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                        • #13
                          Originally posted by smeghead View Post
                          .. I recently bought a Warren & Brown to replace the one I lost years ago. I was surprised to find that they were cheaper than many of the other brands I looked at and seemingly just as good quality as they used to be.
                          Warren & Brown pretty good quality equipment.
                          I've had a 3/8" drive for many years when working in Mining Industry.

                          Use a Caterpillar (Snap-On) 1/2" drive as well.

                          Good write up with calibration check/adjustment.


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                          • #14
                            Awesome writeup mate, ive seriously been doubting the accuracty of my torque (named alex) cause you know how pedantic he can be =P

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                            • #15
                              Good stuff.

                              (that 3rd pic should be 355mm)
                              Increasing my carbon footprint - one 500 @ a time...

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