Why Calibration Services are Essential for Torque Wrench Precision
April 13, 2022
The Importance of Torque Wrench Precision
All torque-type equipment requires calibration due to the demanding nature of how they are utilized. Each type of torque wrench, whether it is click, dial, hydraulic or powered, has unique load application features that must be routinely addressed.
A reputable calibration services facility, such as Constellation PowerLabs, will conduct a set of laboratory procedures based on the type of unit under test (UUT) and its application to ensure the highest level of torque wrench precision. These protocols are fundamental to achieving consistently accurate fastener loads during the processes of manufacturing, assembling, and repairing hardware.
When testing click-type wrenches, Constellation PowerLabs calibration services team employs a standard (STD) transducer to achieve accuracy and provide ample leeway over range. The transducer converts the LBF-FT (unit of torque) into a digital readout with accurate values. For example, if a wrench is rated for 100 LBF-FT (the unit of torque) a transducer with a range of 10 – 150 LBF-FT is used.
First, the wrench is examined for damages and defects such as a cracked socket holder or a binding adjustment. It is then preloaded a minimum of three times prior to calibration, when changing the direction of the applied force clockwise or counterclockwise and after any repairs are made. Next, the equipment is mounted to the loader at a level position while the handle is aligned with the reaction arm at the center point. Calibration services are then performed at 20, 40, 60, 80 and 100% of maximum indicated range. Finally, the click-type wrench is verified in sequence from the lowest to the highest test points, in the same direction of the preload. Any units that do not align with manufacturer specifications are marked for repair.
A similar procedure is used for click-type torque drivers, with the exception that the unit is positioned perpendicular to the STD transducer. At this point, calibration services verify that all click-type torque wrenches are functioning in all four quadrants of the 360-degree rotation. However, if a particular quadrant is significantly different, the wrench is flagged for repair.
During testing, the middle reading, or an average of the three are recorded on the requisite format data sheet. Upon completion, the wrench is returned to the lowest indicated scale setting. The calibration adjustment access area is then sealed shut using an epoxy, torque seal or lacquer. The same sealants are then applied to the handle adjustment screw.
Small Differences for Dial-type Wrenches
For dial-type wrenches, the calibration services team applies a similar procedure to that of click-type wrenches, with a few exceptions. Specifically, the wrench is returned to zero after each test point and the back of the cover plate is sealed with epoxy, torque seal or lacquer to prevent user adjustments.
Hydraulic Torque Wrench Calibrations
Prior to test and calibration, the equipment and any adaptors or drives are inspected for damage and all hydraulic connections, hoses and fittings are verified to be secure and leak-free. Then, the appropriate pressure standard torque transducer is selected. Next, fluid level of the pump is checked, two hydraulic hoses are attached to the piston and one is attached to the pressure STD. The hex or square drive link is mounted to the torque transducer adapter. The reaction arm is then secured and the wrench is exercised at 50% of full scale once to cycle the piston.
The pump is then unloaded, all standards are zeroed, and the pressure STD is used to pressurize the piston to each test point. Three sets of readings are taken, and the torque generated is recorded on the data sheet. The “as found” test points must be the same as the “as left” test points while also matching the values on the manufacturer’s torque chart.
Energized Torque Wrenches
Constellation PowerLabs calibration services also utilize transducers when calibrating energized torque wrenches, as it is not uncommon for them to overshoot their marks during a test. Once the range is identified, the appropriate stem-type transducer, test stand and rundown are selected. Next, the transducer is mounted on the test stand, the rundown is inserted into the transducer, the hex attachment and reaction foot to the unit are attached and powered up.
The unit is preloaded three times prior to taking any found data in order to exercise the mechanics and electronics. The equipment is then taken to the first formatted test point, three to five readings are taken, and the average is recorded. The same procedure is used for the remaining formatted test points and all data is recorded. If an inoperable condition is identified, it will be sent for repair.
An uncalibrated torque wrench can yield vastly inaccurate loads upon fasteners, creating safety hazards in the workplace and other end-user environments that can have far-reaching consequences. The best way to avoid such circumstances is to establish a routine schedule for regular testing of all torque-based tools.