How to calibrate a torque wrench in your own shop.
Posted: Fri Jul 11, 2003 1:38 pm
I figured this out when someone on another forum<br>asked where he could get a torque wrench calibrated.<br>I hate to pay for anything I can do myself...<br>-------------------------------------------------------<br>Put the square drive of the wrench in a vise, making sure<br>that the body of the wrench isn't touching (only the square<br>drive touches the vise)<br><br>Ratchet the wrench to a horizontal position.<br><br>Okay, first, how to do the "micrometer" or "Click" type of<br>torque wrench (the beam needle type is below that, but bothe<br>start with the wrench held horizontal by clamping the square<br>drive in a vice in such a way that ONLY the square drive is<br>in contact with the vice).<br><br>Measure out from the center of the square drive (this coincides <br>with the center of the fastener, the wrench may or may not ratchet<br>about this same point).<br><br>You can determine the center of the square drive by the point <br>at which two diagonal lines from the corners of the square intersect.<br>Mark this distance on the handle. Choose somewhere convenient,<br>like at an even number of inches near where your hand would be.<br><br>Write down this distance in inches or feet, depending if your<br>wrench is marked in inch pounds (in. lb) or foot pounds (ft lb).<br><br>Get a weight, somewhere near the amount of force you might<br>typically extert on the handle (say, 20 lb, or 40 lb).<br><br>Multiply the distance from the center of the square drive by<br>the weight you will use.<br><br>If you wrench is a clicker type, set it to the number you<br>got when you multiplied the distance by the weight (inch lb <br>or ft lb). If it isn't the clicker type, read this anyway,<br>and then read the section on beam/pointer type wrenches.<br><br>Using bailing wire, hang the weight at the mark on the handle.<br><br>If the wrench clicks, lift the weight, move it closer to the handle,<br>let the weight hang at the new location. You might start by moving it<br>an inch or two, and see if it still clicks. <br><br>If it still clicks, keep moving it closer until it doesn't, then gradually<br>move farter out (away from the vice) until you find the transition<br>point between where it clicks, and where it doesn't.<br><br>Measure this distance. This is a new distance. You can now<br>use the ratio of the new distance, and the first (expected)<br>distance to determine a calibration factor.<br><br>If the wrench didn't click when you first tried hanging the weight,<br>find how much farther out you have to hang the weight to make it<br>click. try to find the transition point. Measure this distance.<br>You will similarly use this distance to determine a calibration ratio.<br><br>Let:<br><br>Ts = the torque setting on the wrench.<br>D1 = distance measured first (to the mark you made)<br>D2 = distance to point at which wrench actually clicked.<br>Ta = actual torque the wrench is applying.<br><br>So, if you set your wrench to a given torque value (Ts)<br>the torque it is actually applying is:<br><br>Ta = Ts x (D2/D1)<br><br>And, if you want a certain actual torque (Ta) applied to a fastener,<br>you would set your wrench to a value given by this equation:<br><br>Ts = Ta x (D1/D2)<br><br>Okay, put the wrench in the vise as described.<br><br>Mark it at 24" from the center of the square drive.<br><br>Get a 20 Lb weight.<br><br>Set the wrench to 480 inch lbs or 40 ft lbs.<br><br>Hang the weight on the line.<br>If it clicks, move it towards the vice, if it diesn't, move it <br>towards the free end (away from the square drive).<br><br>Find where the threshold between where it wil & won't<br>click is. Lets say that's at 26 inches.<br><br>Okay, the torque it applied when you first hung the weight was<br>480 inch lb.<br><br>The torque it took to make it click at that setting was 26 in. x 20 lb<br>= 520 in lb (divide by 12 to get ft lb)<br><br>But from now on, with this info, you can do the following...<br><br>If you want to apply a certain amount, say 50 ft. lb.,<br>Just multiply 50 by 24/26 (or 12/13 if you're watching)<br><br>so 50 x 12/13 = 46.15 <br><br>So, to torque a bolt to 50 lb, set your wrench to 46 "and a hair" ft lb.<br><br>Easy enough, eh? <br><br>Actually, once you know the ratio (like 12/13 = .923)<br>all you have to do is multiply the torque you want on the<br>bolt by that number (.923) to tell you what setting to<br>put the wrench at.<br><br>I'll have to give instructions for the beam / needle (pointer) <br>type wrench later.<br><br>[img]<!--EZCODE AUTOLINK START--><a href="http://www.imagestation.com/picture/sra ... 627d91.jpg[/IMG">www.imagestation.com/pict...1.jpg[/IMG</a><!--EZCODE AUTOLINK END-->]<br><br>Okay, for the beam / needle pointer type torque wrench:<br><br>Mark the torque wrench handle at a known distance from<br>the center of the square drive (you don't really even have<br>to mark it, just choose a convenient number like 10", 12",<br>24"...). Record this distance (D1).<br><br>Multiply the distence above by the weight. <br><br>Now move the hang point of weight along the handle until <br>the torque wrench needle is pointing to the value of torque<br>calculated above.<br><br>Measure the distance from the center of the square drive <br>to the point that the weight is now hanging, and that is the<br>the other distance (D2). These two distances will be used to <br>calculate a calibration ratio.<br><br>Now the equation is similar:<br><br>Let:<br>Ts = the torque that you will read on the pointer<br>D1 = the initial distance to apply the actual torque<br>D2 = the distance to make the needle point to the value calculated<br>Ta = the calculated torque from the cosen distance X known weight.<br><br>Okay here is an example.<br><br>I'll choose 24" (D1) and 20 lb again. We know that if we hang <br>a 20 lb weight 24" fron the center of the square drive, we are <br>applying 480 inch Lb (40 ft lb).<br><br>So, now we will move the weight until the needle points to<br>480 inch lb, or 40 ft lb. (if the needle was pointing to less<br>than the known torque, move the weight away from the square<br>drive (which is in the vice).<br><br>Now measure that distance (D2). Let's say it's 26"...<br><br>So, to torque a fastener to a desired value (say, 50 ft lb)<br><br>Using Ts = Ta x (D1/D2) = 50 x 24/26 = 50 x 12/13= 46.15<br><br>So if you pull on the torque wrench until the needle points to<br>46.15 ft lb you will be applying the desired 50 ft lb.<br><br>And there you have it.<br>Essentially, your torque wrench was reading lower than the<br>torque it was actually applying, so if you pulled on it until<br>it read the torque you wanted, you would be overtightening<br>the fastener.<br><br>And, once again, Actually, after you know the ratio <br>(like 12/13 = .923) all you have to do is multiply the torque you <br>want on the bolt by that number (.923) to tell you what setting to<br>read on the wrench to get that actual torque. <p></p><i></i>
