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