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  • Torque Wrench Check/Calibration

    I got a torque wrench from E-Bay this week. From the box, the brand name is MAC and it reads 0-50 ft-lbs. It is well used, and needed a good cleaning. It is a dial type, as you can see in the photo below, and the dial was sluggish, especially in one direction. So I took the cover off and did a good cleaning with WD-40 and alcohol. That was followed with a careful application of a light oil. It now appears to work almost like new, even if it still shows it's years of wear on the outside.



    That brings me to checking and perhaps adjusting the calibration. As you can (almost) see circled in the photo there are a pair of notches cast into the handle portion of the bottom cover. So I measured the distance from the center of the 3/8" drive stud to these notches and got 12 5/16 inches. I would have expected a round number, like 12 inches. Then I could hang a weight and read directly in Ft-Lbs. Since it also has metric units on the dial, I suspected possibly a metric distance but 12 5/16 inches translates to 312.74 mm so that is not very likely.

    This sketch shows these notches a bit better.



    My question is this, is there some reason for this odd distance? As I said, I would have expected a nice, round 12 inches. But even if I take the measurement from the side of the drive stud nearest to the handle, it is still 12 1/8 inches. Still odd. Is there some reason why this particular type of wrench would be calibrated at an odd distance? Perhaps the effective center of rotation is shifted a bit due to the internal bar being bent over a distance instead of at a point?

    So, do I use the 12 inch dimension or do I use 12 5/16 inch dimension when checking the readings?
    Paul A.

    Make it fit.
    You can't win and there is a penalty for trying!

  • #2
    I suspect the notch is simply there to help retain a soft moulded handle that has been lost.

    If you want to use it for calibration, it doesn't matter what mechanism is inside - the actual distance is all that matters.

    Cheers

    .

    Comment


    • #3
      Could it be that another handle slipped over that piece? The way it is now looks uncomfortable to handle

      Comment


      • #4
        This one looks the same -

        http://www.ebay.ca/itm/CDI-502DF-dia...item56538e290a

        Nev.
        Nev.

        Comment


        • #5
          The distance to the handle point is irrelevant if you make a calibrating lever. Make whatever distance you want on it, and then you have choices. Make it to fit the stud on the wrenches you have.

          if the wrench is held steady, and the weights put on the lever, you get the same result as a stationary stud-holder, and the wrench used as the lever.

          For a more interesting project, a short shaft in ball bearings, with a replaceable socket allowing any size stud to be used, and provisions for a lever to hold the weights in the manner of an old-fashioned balance scale.

          When the lever is just lifted off the stop, and is horizontal, the torque is known. For the person who must calibrate everything correctly......
          1601

          Keep eye on ball.
          Hashim Khan

          Comment


          • #6
            So, do I use the 12 inch dimension or do I use 12 5/16 inch dimension when checking the readings?
            Does it really matter?
            At a full scale reading of 50 ft. lbs. the difference between the two distances is only 1.3 ft. lbs. I would venture to say that the placement of your hand on the grip could easily change the actual applied force more than that.
            Anything less than half scale one should change to an in. lb. wrench anyway for greater resolution and accuracy.
            Home, down in the valley behind the Red Angus
            Bad Decisions Make Good Stories

            Comment


            • #7
              Let's say you have a 25 # weight, hang it at the notch , multiply by 1.078, 12.9375 decided by 12. It is important though,that the weight hangs at 90 degrees to the handle. Bob.

              Comment


              • #8
                How accurate does a torque wrench really need to be? So many others factors involved; oiled threads, dry threads, hand position, etc... I figure that 'reasonable' is fairly accurate. I have a 3/8 x 3/8 square drive bit that I attach to two torque wrenches, one of which is a click type [chucked up in a vice]. I set a torque value on the breakover and then pull on the dial wrench. When the break occurs I compare the two. I check all my wrenches this way. They have always been VERY close, though I guess one could argue that both were goofed up.

                Comment


                • #9
                  I'd say its missing a hand grip of some sort



                  Brian

                  Comment


                  • #10
                    Think assembly lube between the rubber and the handle.
                    Gene

                    Comment


                    • #11
                      Since it also has metric units on the dial, I suspected possibly a metric distance but 12 5/16 inches translates to 312.74 mm so that is not very likely.

                      This sketch shows these notches a bit better.
                      One thing to think about is that the angular force applied to the measuring spring is not located at the center of the drive stud, but some distance away from that center. IF the notches are placed to be the calibration point, I would suspect the actual distance to be either 312mm or 313mm. It might be that neither of those numbers is the actual, but that's my guess and I'm sticking to it.

                      Pops

                      Comment


                      • #12
                        CDI Torque Products made that one for MAC. They were absorbed by Snap On in 1995 I believe.

                        Paul for what it's worth I've included a current link to CDI's dial type torque wrench, Repair,Maintenance, and Troubleshooting Manual in pdf format.
                        Lots of good info on both of these links.

                        http://www.cditorque.com/RepairManua...enchRepair.pdf
                        Home, down in the valley behind the Red Angus
                        Bad Decisions Make Good Stories

                        Comment


                        • #13
                          It seems likely to me that mark is a standard pull point. Since those wrenches are fairly rigid the pull point has only a small effect unless extensions are used. (Not common extensions in line with the spindle, special purpose extensions in line with the handle.) When those are used there is a correction factor applied but the pull point has a large effect on the reading.

                          If you loosen or take the cover off, be aware that the calibration changes quite a bit because of increased flex of the body. Torque wrenches bend just like lathes and mills do!
                          Don Young

                          Comment


                          • #14
                            Yes, it looks totally identical, including the box. I can see the same notches on it. Well, the one you reference is in better condition. But neither of them has a hand grip.


                            Originally posted by NiftyNev View Post
                            Paul A.

                            Make it fit.
                            You can't win and there is a penalty for trying!

                            Comment


                            • #15
                              Yes, that is one way to do it, but since I saw the notches at a point that is very close to one foot, I just assumed that they MIGHT be for calibration.

                              I did a quick check by grabbing the 3/8" tool stud in my bench vise and I hung 20 pounds at the notch. The reading I got was about 20.7 foot-pounds. So I ask my question, hoping that someone here has had some relevant experience.

                              Originally posted by J Tiers View Post
                              The distance to the handle point is irrelevant if you make a calibrating lever. Make whatever distance you want on it, and then you have choices. Make it to fit the stud on the wrenches you have.

                              if the wrench is held steady, and the weights put on the lever, you get the same result as a stationary stud-holder, and the wrench used as the lever.

                              For a more interesting project, a short shaft in ball bearings, with a replaceable socket allowing any size stud to be used, and provisions for a lever to hold the weights in the manner of an old-fashioned balance scale.

                              When the lever is just lifted off the stop, and is horizontal, the torque is known. For the person who must calibrate everything correctly......
                              Paul A.

                              Make it fit.
                              You can't win and there is a penalty for trying!

                              Comment

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