Torque Wrench Maintenance: Can A Torque Wrench “Go Bad” and Why?
Torque tools, such as click wrenches, digital torque wrenches, dial wrenches, beam wrenches, torque watches, torque drivers, torque multipliers, hydraulic wrenches, and pneumatic/electric wrenches all have tolerances and accuracies that are stated by the manufacturer of the torque tool. These torque tools experience torque forces that with normal every-day use can cause the accuracy of the tool to “go bad” by being Out of Tolerance (OOT) or even Significantly Out of Tolerance (SOOT).
A tool being OOT or SOOT means that the wrench is no longer providing the manufacturer stated accuracy of applied torque and that using the tool may result in under-tightened fasteners or over-tightened fasteners. Under-tightened fasteners do not apply the necessary clamping load to a joint, which can result in a fastener coming loose and the eventual failure of the joint. Over-tightening fasteners can lead to a fastener shearing off or stripping out which will also result in an eventual joint failure. Joint failures like these can be avoided through the regular calibration of torque tools at intervals determined by individual requirements and tool usage.
Torque wrench maintenance, including routine tool calibrations, are an important part of preserving a tool’s accuracy and identifying tools that are OOT or SOOT. Tools designated as OOT or SOOT will require a calibration adjustment to bring the torque tool back into tolerance.
Aside from regular use, there are also other factors specific to each type of torque tool, that can cause it to go Out of Tolerance. We explore those tools and factors below.
Click Wrenches and Torque Drivers
Click wrenches are manual torque tools that use an internal spring that is compressed and decompressed to load varying amounts of tension against a click block when the micrometer handle of the wrench is adjusted. This spring is designed to provide a specific amount of tension on the click block at specific handle settings. When the wrench is in use, the click block overcomes the tension being applied by the spring and the wrench clicks, indicating it has reached the set torque value. The accuracy of the click wrench is dependent on the spring maintaining its designed elasticity. If the spring remains loaded under compression for extended periods it will start to conform to that shape, causing the spring to lose its elasticity and its accuracy by changing the amount of tension applied to the click block. This makes it very important to release the tension on the spring when not in use. This is done by adjusting the micrometer handle to its lowest setting during storage. If the tool is found to no longer be accurate during its calibration cycle there are usually adjustments that can be made, depending on the brand and model, that can bring the tool back into tolerance if the readings are linear. If the readings are not linear, the spring can be replaced with a new spring from the manufacturer, bringing the tool back into tolerance.
This same principle also applies to torque screwdrivers, which also use an internal spring. In these tools the spring is loaded against steel balls rolling over indented plates which acts as a torque limiting clutch. The more the torque setting on the handle of tool is increased, the more tension is loaded on the balls and plates. This requires the operator to apply more torque to the tool to get the balls to overcome the tension loaded by the spring, at which point the balls “cam-over” the indented plates indicating the torque setting has been reached. Much like with click wrenches, leaving the setting on the handle of the wrench above its lowest setting keeps the springs tension loaded and causes the spring to lose its elasticity and therefore its accuracy. Similarly to click wrenches, if the tool is found to no longer be accurate during its calibration cycle, adjustments can usually be made, depending on the brand and model, that can bring the tool back into tolerance if the readings are linear. If the readings are not linear, the spring can be replaced with a new spring from the manufacturer, bringing the tool back into tolerance.
Digital Torque Wrenches
Digital torque wrenches are manual torque tools that differ from click wrenches in that they generally use internal strain gauges that are wired to an electronic display to digitally show the current or peak torque value applied to the wrench. The strain gauges are adhered to a metal torsion rod within the wrench through which the strain gauges measure the amount of deflection seen by the rod during the application of torque. With torque applied, the strain gauges output a proportional measurement signal to the display which converts the signal into a readable torque value. These types of wrenches, while more accurate than click wrenches, are more susceptible to going Out of Tolerance if they are over-torqued. If the tool is over-torqued past its maximum capacity, it is possible the strain gauges will become deformed and no longer accurately relay the correct torque value to the display. The tool would then have to be sent to a qualified calibration laboratory to be reprogrammed and recalibrated. If the tool was over-torqued too much the torsion rod may have deflected beyond its physical limit and become deformed which would result in the tool needing to be replaced.
Beam wrenches are manual torque tools that consist of two parallel beams that make up the body of the wrench. One of the beams will have a handle used to apply torque and act as the frame of the wrench. The other beam will deflect to indicate the amount of torque being applied through a graduated scale along the base of the handle of the wrench. The beam wrench design is much simpler than other types of torque wrenches and is typically less expensive and has a higher accuracy than other mechanical torque wrenches. A beam wrench can go out of tolerance if the top beam of the wrench is bent or deformed, causing the reading indicated by the top beam to be inaccurate. These tools are typically replaced when they are found to be Out of Tolerance because they are fairly inexpensive, and it is difficult to bend the beam back into its original factory alignment.
Dial wrenches are manual torque tools that have an analog gauge that indicates the amount of torque being applied using an internal arm attached to the square drive plate of the tool. The other end of the arm is a clamp that holds a small metal rod that has a 90° bend in it. The metal rod engages a clock spring mechanism that transfers the forces seen from the applied torque to the analog gauge which displays the current torque reading. Over time the clock spring mechanism can wear and shift the tool Out of Tolerance. The small metal rod with the 90° bend can be adjusted in or out of the arm to adjust the Dial Wrench back into tolerance.
Hydraulic wrenches are pressure driven torque tools that use hydraulic pressure provided by a hydraulic pump or “power pack” that are used in higher torquing applications. These tools use a Pressure/Torque Conversion Chart that allows the end user to translate the regulated hydraulic pressure being supplied to the tool into a known torque value. These tools operate by using the hydraulic pressure fed into the tool to extend an internal cylinder against a ratchet that uses pawls to turn the output drive, thereby applying torque to a fastener. The cylinder is then retracted allowing the pawls to move freely over the ratchet’s teeth before starting the process over. Hydraulic wrenches rely on seals to prevent leakage outside of the hydraulic system, as well as to prevent fluid from crossing the area of the cylinder head. These seals are critical in maintaining the pressure within the tool and for converting fluid power into the linear motion of the cylinder. Over time these seals can break down and leak, preventing the hydraulic wrench from achieving the manufacturers stated torque values in the Pressure/Torque Conversion Chart. When this occurs, the tool is considered Out of Tolerance and the seals, along with any damaged components, will need to be replaced.
Torque multipliers provide a mechanical advantage by using a gear system to apply greater amounts of output torque from a lower input torque. They typically are used in conjunction with a separate torque tool applying torque to an input drive which is multiplied by the gear system to achieve a higher torque output through the output drive. There are typically two kinds of torque multipliers, direct read multipliers and ratio driven multipliers.
Direct read multipliers use a gauge or electronic display to indicate to the user the amount of output torque being applied. Since the final output torque is shown on the tool itself, the user does not need to know the amount of input torque. Because of this, many direct read multipliers can use a simple handle as an input device as opposed to a separate torque tool. Depending on the specific make and model, the built-in torque indicator can function in a similar manner to dial, digital, and/or hydraulic wrenches. Each type of measurement system will have failure modes similar to the wrench type they represent for the same reasons. If a direct read multiplier is found to be out of tolerance, a calibration lab can reprogram the digital display, adjust the analog gauge, or replace any damaged seals to bring the tool back into tolerance, as is appropriate for each specific tool.
Ratio driven multipliers use a fixed gear ratio, and rely on the user knowing the torque ratio of the gear system to calculate the input torque value required to reach the desired output torque value. For example, a torque multiplier with a torque ratio of 4:1 would require an input torque of 250 lbf∙ft to provide an output torque of 1000 lbf∙ft. In ratio driven multipliers, the internal gearing systems require lubrication in the form of grease to facilitate the movement of the gears. Over time the grease can dry up and cause the torque multiplier to produce less output torque for the same amount of applied input torque. This causes the torque ratio to change and the tool to become Out of Tolerance. To bring the tool back into tolerance requires service by a qualified Calibration Lab that can open the tool, remove the old grease, and apply new grease to the gearing system.
Your Trusted Source for Torque Wrench Calibration Information and Systems
Any torque tools used in critical applications should undergo regular torque wrench maintenance and calibration to ensure the accuracy of their output. If you have questions about the calibration and upkeep of torque tools, A.K.O., Inc. has the Torque Wrench Calibration Systems and technical expertise to help. Give us a call at 1 (860) 298-9765 or email us at firstname.lastname@example.org.