Torque wrenches are tools that are used to measure the torque applied to various screws, bolts and other interconnecting fasteners. They can either be used as a measurement tool, or they can be used to set the appropriate torque to be applied to the fastener so it can meet the specifications designed by the manufacturer. Under-tightening a fastener can result in loose parts, while over-tightening may result in broken parts.
Torque Wrench Calibration Accuracy
Torque wrench accuracy is typically specified by the manufacturer however, international standard ISO 6789 outlines the maximum permissible relative deviation for various types and classes of torque wrenches. Typically, torque wrenches are calibrated to a maximum error of ± 4% of reading in the clockwise direction and ± 6% of reading in the counter-clockwise direction. The calibrated range on most torque wrenches is 20% to 100% of full scale and calibration is performed at 20%, 60%, and 100% of full-scale capacity.
Torque Wrench Calibration Interval
There are a number of factors and circumstances that will affect the accuracy of torque wrenches, so regular calibration is necessary to ensure that they can still meet their manufacturers specifications.
ISO 6789 states that a torque wrench should be re-calibrated at a minimum of every 5000 operations or every 12 months, whichever comes first. Specific organization accuracy requirements, process control procedures, or industry standards may dictate a more frequent re-calibration interval. The standard also states that if a torque wrench is subjected to an overload of 25% or more above the maximum range of the torque wrench, it should be recalibrated and checked for damage.
Between calibration intervals, many organizations with a large number of torque wrenches or higher accuracy requirements may also utilize a calibrated Torque Tester to quickly check the torque wrench accuracy on the production floor. Torque wrenches can be checked daily or weekly to ensure the wrenches maintain the required accuracy between formal calibrations.
All torque wrenches or testing equipment used to calibrate the torque wrenches should be calibrated by an accredited calibration laboratory to maintain the measurement traceability that is required in many industries such as in the aviation and medical fields.
Factors Affecting Torque Wrench Calibration
Proper calibration is important for torque wrenches to ensure that they are accurate, however, the technique in how they are calibrated has a significant effect on the accuracy obtained. The factors listed below need to be implemented to ensure the measurement errors are minimized.
Adjustable torque wrenches should be allowed to settle at their lowest marked setting prior to calibration. If a torque wrench is stored with a higher torque setting, the increased load on the internal tension spring can stress the spring and cause measurement errors. If a torque wrench is received for calibration in this condition, the wrench should be set to the lowest marked setting and allowed to stabilize at that setting for 24 hours before calibration.
The force on the torque wrench handle should be applied slowly and smoothly. If the force is applied too rapidly or with an irregular motion, extra torque could be applied after the indicated reading is reached and produce a measurement error.
The force must be applied to the center of the handle, not just anywhere on the torque wrench body. Torque wrench accuracy depends on the length from the torque pivot point to the handle, so any changes in the length will give inaccurate readings and could overload the wrench and damage it. Many torque wrenches have a marked load point on the handle to show the proper place that the load should be centered when making a measurement.
The beam should be at a 90° angle to the axis of the calibrator or torque cell drive. Even a few degrees of angle can cause an error in the readings obtained. A torque wrench loader is available for most calibrators that can assist in the alignment of the torque wrench during calibration to eliminate human errors and ensure repeatable readings.
Many torque wrenches are only manufactured to indicate properly in the clockwise direction. It is necessary to always check the wrench’s manufacturer specifications to determine if it requires calibration in the counter-clockwise direction.