关键紧固件的扭矩与张紧：结构和机械连接最佳实践

Torque wrenches should be calibrated at least annually, and more frequently for heavy use. For workshop use (daily), quarterly calibration is recommended. For field use, a calibration certificate should accompany the wrench and be verified annually. Any torque wrench that has been dropped or subjected to misuse should be re-calibrated immediately before further use. The calibration certificate must show the wrench was tested at multiple points across its range, not just at one setting.

Common causes of flange leakage despite proper torque: gasket damage — the gasket was damaged or previously over-compressed; incorrect gasket — wrong material or grade for the service conditions; flange surface damage — scratches, pitting, or corrosion on the flange face creates leakage paths; inadequate lubrication — if threads or bearing surfaces were not lubricated as assumed in the torque calculation, the actual preload was lower than calculated; thermal cycling — gasket relaxation under thermal cycling can reduce preload; pipe strain — external loads on the flange from pipe thermal expansion, gravity, or vibration can unload the joint.

Snug tight is the condition where the bolt is tightened to a relatively low preload just to draw the joint members into firm contact — typically achieved by a few impacts from an impact wrench or 2-3 hand turns of a wrench. The purpose is to hold the joint together while all bolts are installed before final torque. Snug tight is not a measured value; it is a feel-based condition. Full torque (or 'turn-of-nut' completion) is the final specified torque applied in a controlled pattern. The 'turn-of-nut' method uses snug tight + a calculated additional rotation to achieve preload, which is often more consistent than direct torque for large bolts.