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How To Calibrate A Torque Wrench

Torque wrenches used on aircraft need to be calibrated at least once a year. For personal torque wrenches that are used on non-aircraft applications, depending on the amount of use of the torque wrench, a calibration could wait a little while.

The torque wrench is a key instrument in precision work, so even the smallest obstacle could offset a calibration enough to break a stud, loosen a nut or leave a stripped thread. A torque wrench that is prepped before use will guarantee the most accurate result.

Clean and undamaged threads specifically define torque values on this type of wrench. This is critical when knowing the settings for each torque wrench job for your aircraft. 

What is calibration?

A calibration is an adjustment of an object such as a tool with a standard set of readings through terms of comparison, checking for accuracy of a measuring instrument. Calibration could also be used for checking the alignment of the standard measurements. The piece of equipment or device with the known accuracy is called the standard.

Calibration is an effective and essential element that ensures valid performances in order to report the best results. Specific application of the instrument, the amount of use and industry requirements determine the settings for calibration of measuring tools.


Why Calibration is Necessary?

Calibration is essential in providing accurate results when a calibrated tool is in use. It is imperative to maintain the servicing of your instrument to be sure that the results of the measurement tool are exact.

If a tool is not calibrated regularly, you could increase your chances of offsetting measurements and quantifying uncertainty in results. In high-risk tasks that mandate exact results, conclusions that aren’t close to accurate can be dangerous.

You want your torque wrench to be as accurate as possible. Again, this will guarantee precise results when tightening. An accurate torque wrench decreases the chances of damaging bolts and loosening nuts.

How To Calibrate A Torque Wrench

While professional torque wrench calibrations are required for aviation applications, you can always learn a do-it-yourself calibration technique that can be used when professional calibrations are not applicable.

In order to understand how a calibration is done, it is critical to know the mathematics behind it. The formula used in this adjustment is for a straight, non-spinning object such as a lever: torque (T) = force (weight) x distance (T = F x D). You can also use the formula Ta = Ts x (D1/D2), in which applied torque (Ta) = torque setting (Ts) x (distance one/distance two).

The load on the bolt will depend upon such a factor as the friction of the thread. If applying a torque load to an oiled bolt, the amount of an applied load will increase, excluding a regular, lightly oiled cylinder head bolt. Any other oiled bolt will increase the load as well as increase the risk of overloading and breaking the bolt and crushing a gasket.

Here is how you can conduct a DIY calibration:

  • Bench vise
  • 20-pound weight
  • Thin rope or string
  • Measuring tape

Step 1:

Measure the length from the square drive to the point of the torque wrench on which your hand rests when using the tool.

Step 2:

Place the square drive of the wrench in the vise and secure it. No other part of the wrench should be locked in the vise.

Step 3:                                       

Align the torque wrench to the value of the settings you got in step one (the distance from the square drive to your hand) and multiply it by 20 pounds.

Step 4:

From the position your hand rests, tie the 20-pound weight with the thin rope or string to the wrench.

Step 5:

If you hear a click from the wrench, lift the weight and move it towards the head of the wrench until the clicking stops. If you don’t hear a click at first, move the weight farther from the head of the wrench and try again.  You can make a definitive mark after two or three stops in the same location of the wrench. IMPORTANT: Make sure the wrench is horizontal when conducting this step.

Step 6:

Measure the distance from the square drive to the location in which the rope is located after step five. Multiply your finding distance by 20 pounds.

Step 7:

With the formula Ta = Ts x (D1/D2), you can calculate the applied torque with your findings. Applied torque (Ta) = torque setting (Ts) x (distance in step one/distance in step five).

What Factors Affect Calibration?

Just like with anything that is set, a torque wrench calibration will lose its accuracy over time. With that being said, it is important to understand what factors have an effect on calibration and how you can make a calibration last longer.

The average temperature at which the tool is used can tie in to how accurately the tool is calibrated. Whether you’re working with the tool in cool or warm conditions, it is important to set the measurements of the tool to a comfortable setting in the climate in which the instrument will be used. This will reduce the chance of errors in solution and provide more accurate results.

In calibration, mathematics is applied to determine the set measurements at which the tool will be used. If a factor in the equation is incorrect, this can offset the calibration and cause major errors in results and performance. It is important to properly measure the distance between the square drive and the hand position in both measuring times to ensure a precise solution to the equation.

Calibrators used for regulating precision in instruments (such as torque wrenches) must meet specific requirements from a dependable manufacturer before being used in the process of calibrating your work instrument. A defective calibrator can and will cause mistakes in calibration results, thus leading to the misuse of your tool. It is important to inquire about a calibrator’s certainty before it is used in the process. This way, before any calculations are found, you can rule out the defectiveness of the calibrator.

How To Use A Torque Wrench Properly

Mechanics know that a specific amount of torque should be applied to certain fasteners in order to secure them correctly. If you apply too much torque, you run the risk of damaging the nut or bolt or the component on which you are working.

A major factor in making a calibration last longer is proper use of the torque wrench. Torque wrenches are must-have instruments in a maintenance facility. This tool is used for tightening a variety of vital fasteners.

Before you put the tool to use, one of the most important steps is finding the recommended torque setting for the job that you are attempting to complete. You can find these settings in the airframe, engine or accessory maintenance manual. The torque setting will typically be measured in foot-pounds (ft-lb) or inch-pounds (in-lb).

Once you have located the setting, adjust the dial on the bottom of the wrench to the correct setting. Then begin tightening the nut or bolt just as you would with a regular wrench, but stop when you hear a clicking noise (also known as a “break”). This lets you know that the recommended torque has been applied to the nut or bolt. Some applications require backing off the fastener (not using the torque wrench) and re-torquing. Other times, a second torque sequence needs to be accomplished. Always refer to the maintenance manual for proper torquing procedures.

Preventative maintenance should be scheduled when taking care of and servicing your torque tool. Now that you can calibrate your own personal torque wrench and use it properly, you are equipped with more safety and precision, even when not working on aircraft. If you don’t feel comfortable with self-calibration, you can always refer to professional calibrations performed by mechanic experts.

Back it Down

Once you are done using a torque wrench, be sure to back it down to zero. Torque wrenches need to be stored at this state to prevent errors in calibration from being introduced. Some torque wrenches don’t require this procedure because of how they are designed, but it is always a good practice to back down a torque wrench before storage.

Aubrey Ivy is partner/operations manager for Pro Torque Tools (PTT). In his five-year tenure with PTT, Ivy has worked one-on-one with customers to provide solutions to fit their torque tool needs. Ivy holds a master’s degree in business administration from the University of Alabama at Birmingham  and an undergraduate degree from Auburn University. PTT is an online specialty torque tool retailer of Snap On/CDI torque tool products, featuring micro adjustable click and dial torque wrenches, multipliers, screwdrivers and calibration testing equipment.