Torque Wrench Calibration Verification and Usage
A definitive, highly technical SOP for validating torque wrench accuracy (dial, beam, clicker, and electronic) per ISO 6789 and implementing correct fastening techniques to prevent joint failure, bolt preload deviation, and equipment damage.
To establish a standardized procedure for the periodic verification of torque wrench calibration accuracy and to define the correct methods of usage. This ensures specified bolt preload is achieved, preventing joint loosening, fatigue failure, stripped threads, or catastrophic equipment damage caused by over- or under-torquing critical fasteners.
Applies to all dial, beam, clicker (micrometer), and preset torque wrenches used during critical fastening operations in mechanical maintenance, assembly, and overhaul tasks across the facility. This SOP specifically excludes hydraulic torque wrenches, torque multipliers, and pneumatic/electric impact tools, which are governed by separate maintenance procedures.
- Ensure the equipment being worked on is properly locked and tagged out if the fastening operation involves stored energy (hydraulic/pneumatic pressure), rotating parts, or electrical isolation.
- Wear approved safety glasses at all times. Use cut-resistant gloves when handling fasteners with sharp edges. Use hearing protection when working near pneumatic tools or high-torque operations.
- Keep fingers and loose clothing clear of the square drive, reaction bar, and fastener head during torque application. Sudden joint slip or tool release can cause severe pinch injuries or lacerations.
- Never use a torque wrench to break loose fasteners. Always use a properly rated breaker bar or a designated impact wrench to loosen seized or torqued fasteners. Misuse can cause sudden calibration drift or internal mechanism failure.
- Torque wrench under test (dial, beam, clicker, or electronic type).
- Torque tester / calibration rig (e.g., Norbar TST, Crane Electronics, or CDI Torque Ranger) with a current calibration cert traceable to NIST.
- Properly fitting chrome-moly socket (avoid impact sockets and universal joints/adapters if possible, as they introduce error).
- Clean, lightly lubricated fastener or grade-matched bolt simulator.
- Calibration log sheet, digital form, or CMMS (ServiceGrid) PM record interface.
- Manufacturer’s operation manual for the specific torque wrench model under test.