How To Calibrate A Torque Wrench – (Step By Step Guide)

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Nuts and bolts are components that exist in many types of machinery and instrument, and they constantly need to be adjusted so that they can function properly. Obviously, no sane person would try to adjust them with bare hands, because, let's face it, humans do not possess superpowers. So, what magical tool can relieve us common folks from the hassle of adjusting nuts and bolts? A torque wrench of course. A torque wrench is constructed with the sole purpose of fastening nuts and bolts. It may not seem like much, but it is indeed a very effective tool. However, without proper calibration, it may not seem like a step above using our bare hands. There are quite a few easy methods to calibrate a torque wrench. In this article, let us further divide these methods into details to get a better idea of how you can get the work done easily:

How To Calibrate A Torque Wrench

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 A  torque wrench or good impact wrenches are constructed with the sole purpose of fastening nuts and bolts. It may not seem like much, but it is indeed a very effective tool. However, without proper calibration, it may not seem like a step above using our bare hands. There are quite a few easy methods to calibrate a torque wrench. In this article, let us further divide these methods into details to get a better idea of how you can get the work done easily:

Using Bathroom Scale

  • In this method, we use a bathroom scale, any heavy item lying around the house, a measuring tape, and of course, said torque wrench.
  • Firstly, the person in charge of calibrating should get on the scale and measure themselves as a point of reference.
  • In the second step, the person should hold the heavy item and weigh themselves. Now, the weight of the heavy object can be easily derived by subtracting the previous weight taken on the scale of the individual, from the new weight measured on the scale holding the heavy object.
  • In the third step, the measuring tape should be used to measure the length of the torque wrench, to predetermine a position on which the heavy object can be hung from the wrench so that the torque can be calculated. Let’s assume the predetermined length at which the heavy object will be hung from is somewhere around 16 inches, which is equivalent to 1.33 ft., and the weight of the heavy object is 30 Lbs., and multiplying the length and the weight gives us a value of 40 ft.-Lbs.  With these measurements taken, we can move on to calibrating the wrench.
  • For starters, the torque wrench must be calibrated to 40 ft.-Lbs. of torque, since this is the derived measurement that would be used to test the tool against.
  • Next, the individual can take the torque wrench and put it on a lug nut socket, perhaps at the wheel of their vehicle. They should keep in mind to place it in a way so that it is as parallel to the ground as possible.
  •  Now, the individual must make sure the force applied to the torque wrench is perpendicular to where it’s placed.
  • Finally, the individual should hang the heavy object weighing 40 Lbs., at the predetermined position on the torque wrench, using a string.
  • At this point, the torque wrench is meant to make a clicking noise, so we need to tighten the spring on the wrench till we can hear the clicking noise. For this adjustment to be as accurate as possible, an individual needs to apply the weight after the spring adjustment is done so the load may need to be taken off the torque wrench periodically. As soon as the individual hears the torque wrench click, they must make sure they did not over adjust the spring too far.
  • To revert over adjustment of spring, loosen the spring ¼ turn at a time till we no longer hear any clicking sounds.
  •  The procedure needs to be repeated a few more times until the user feels the calibration has been fine-tuned.
  • Different ranges of weight can be used to fine-tune the torque wrench further to increase the accuracy of calibration.

Using a Luggage scale

  • In this method, a digital luggage scale is used to calibrate a torque wrench. The additional components required for this method include a socket, a tape measure and a tape. A vise may also be required to place the different components in a rigid way. 
  • First of all, we should place the socket on the vise, and then put the torque wrench inside the socket. 
  • Next, we need to adjust the torque wrench. Typically, a value mid-range in the torque wrench is used, suppose, for example, a value of 35 ft.-Lbs. is used. Now, take a piece of tape and place it on the handle of the torque wrench.
  • In the next tape, we use the measuring tape to take a measure of a length from the center of the head of the torque wrench to a convenient position on the handle and then use a marker to mark it. Suppose we mark it off at 1 foot. Even if the torque wrench is shorter than a foot than it should not be a problem either.
  • Avoid pulling the torque wrench by grabbing on to the handle, because they were designed only to withstand a perpendicular force on the handle.
  •  Now we use a digital luggage scale and place it where we marked the torque wrench. We have to pull the digital luggage scale at a perpendicular angle from the torque wrench until we here a clicking noise.
  • Now, we have to repeat this procedure a few times until we hear the clicking noise, to get a few readings on the luggage scale. 
  • We can use the different readings obtained to work out an average of the measure. Suppose, the average that was obtained through repeating this procedure is 32 ft.-Lbs., however, our torque wrench is adjusted at 35 ft.-Lbs. which indicated that we are about 3 ft.-lbs. off.
  • In order to calibrate the torque wrench we have to unlock it and bring it down to its lowest setting, and then, we loosen the coupler nut which holds the threaded spring tensioner and handle together. 
  • Our aim here would be to tighten the spring tensioner of the instrument to just the right amount because the torque wrench is reading low at this point. 
  • After tightening the spring tensioner a few turns, we have to put back the coupler nut and lock it in place. 
  • Next, we have to unlock the spring tensioner and readjust the wrench to 35 ft.-lbs. and relock it. 
  • After adjusting the torque wrench, we repeat the procedure with the luggage scale once again. If we get the clicking sound when the reading on the luggage scale shows 35 ft.-lbs., it means we have been able to adjust the torque wrench accurately. If for some reason, the correct reading is not observed in the first try, one must repeat the steps of calibration all over again, till the reading on luggage scale matches the reading on torque wrench after adjustment. 
  • If the handle of the torque wrench is more than 1 foot away from the head of the wrench, i.e. if it is longer, it would have to be set at a higher value of ft.-lbs. The procedure for calibrating longer torque wrenches is a bit different from the procedure for calibrating shorter torque wrenches. 
  • Suppose, a long torque wrench is set at the middle of its range at 80ft.-lbs., and a mark is made 23 inches from the center of the head. We may get a weight reading of somewhere about 42ft.-lbs. if the torque wrench is new and factory calibrated. 
  • The first step for calibrating would be to keep in mind that the measurement observed from the luggage scale is 42ft.-lbs. and the distance from the head of the wrench to the mark is 23 inches. 
  • Now multiplying these values and dividing them by 12 inches should give us the conversion to pound-feet of torque. This should give us a value of 80.5 ft.-lbs. of torque, which is pretty close to the value at which the torque wrench was configured earlier. 
  • One way to ease the process of calibration would be to make a reference guide in Excel by repeating the calibration procedure with different types of wrench set at different values of pound-feet of torque. 
  • There is also a way to introduce significant error to the calculation. As mentioned earlier, when pulling the scale, it has to be at a 90-degree angle from the torque wrench. If the angle is off, it is likely to throw off the amount of force that the torque wrench is experiencing from the pole. Taking a cosine of the angle above or below 90 degrees intended position of the scale can let us account for this error.

Other Tools frequently used for calibration

Torque Analyzer

This device is equipped with a display which aids the user to obtain information regarding the calibration of a torque wrench. This device can be connected to a torque wrench by means of external torque sensors, and also lets the user perform tests.

How To Calibrate a Torque Wrench

How it works

  • In the first step, you must boot up the device. 5 options will be displayed on the menu screen, they are Quick Test, Units, Mode, Setup, and Filter.
  • In the input of the device, apply torque using a torque wrench.
  • Next, decrease the torque reading on the screen from maximum to zero
  • The device will then conduct a comparison between the readings on the scale of the wrench with the reading on the tester.
  • Readings indicating whether calibration is required or not will be displayed by the Torque Analyzer.
  • Follow the test results of the device to determine whether the torque wrench needs to be calibrated or not.
  • If you want, you can also transfer and store these readings to a computer as well.

Torque Loading Bench

One other alternative for conducting torque wrench calibration, apart from the usual Do It Yourself (DIY) method is using a device known as the Torque Loading Bench. Through this device, the torque wrench can be calibrated swiftly and painlessly. The wheel of this device allows for easy manual loading. It can also be operated in clockwise and counterclockwise directions.

How it works:

  • The first step would be to find a stable flat surface to place the loader on for safe usage.
  • Connected to the loading bench, there should be a transducer. The next step would be to put the torque wrench on the transducer.
  • Adjustments may occur accidentally during the entire process. Therefore, to avoid accidental adjustments, use the presetting position to ensure and maintain high accuracy.
  • Loading can be done easily by rotating the wheel of the device either clockwise or counterclockwise.
  • Next, along the reaction bar of the Torque loading bench, the loading device assembly should be moved. You must also check if the reaction bar has provided adequate clearance for the torque wrench to be installed.
  • After this, we test the torque wrench by means of the lowest capacity torque transducer.
  • Also, make sure that the torque wrench is aligned properly along the length of the reaction bar.
  • In the female square drive of the torque transducer, we mount the torque wrench in the following step.
  • After the torque wrench has been mounted move the loading assembly along the reaction bar in such a way that the support spindle reacts against the normal hand position of the wrench. 
  • In case a wrench is being tested which consists of square drive ratchet, you must ensure the support spindle allows maximum movement of the wrench handle.
  • The spindle should also be positioned in such a way so that the wrench can be kept at a horizontal position during the entire testing period.
  • Always secure the clamp plate before an equipment is loaded.
  • We can slowly rotate the wheel in order to place load to the torque wrench.
  • Finally, a display instrument can be attached to a torque transducer to measure applied torque. The display will also show us whether the torque wrench is properly calibrated or not.

Let us see a youtube video from Car Fanatic channel to get a clear idea about torque wrench calibration.

Final Verdict

There are many ways a torque wrench can be calibrated. Some of the methods do not require spending on any additional device and follows more of a DIY approach to calibration. These methods can be carried out at a low cost, while some methods require more sophisticated instruments. The effectiveness of calibration may also vary depending on the different methods applied. It is therefore upon you to choose which method best suit your needs.

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