Torque Wrench Selection, Use and Calibration — ISO 6789
Selection + calibration guide compiled by cBallast. 3-page PDF; the full content is on this page.
Compiled by
cBallast
Subject
Torque wrench, torque multiplier, torque analyser
Document type
Selection + calibration guide
Reference
ISO 6789 : 2017
Issued
2026-07-15
Pages
3
Format
Web page and PDF
Torque wrench selection, correct use technique, and calibration interval per ISO 6789. Covers wrench types (click, dial-indicator, digital, hydraulic), Class I (indicating) vs Class II (setting) tolerance, torque unit conversion, and calibration frequency guidance for industrial and maintenance workshops.
Wrench types
Type
Accuracy (Class)
Typical range
Best for
Beam / deflection
Class I (indicating) ±4%
0 – 350 Nm
Simple, robust, low cost; occasional use
Dial-indicator
Class I ±4%
0 – 3000 Nm
Bench work, laboratory, calibration reference
Click (mechanical)
Class II (setting) ±4%
1 – 2000 Nm
Production line, general workshop
Digital electronic
Class I / II ±2%
0.3 – 2000 Nm
High-accuracy critical joints, data logging
Torque multiplier (planetary)
Accuracy from input wrench × ratio
200 – 100,000 Nm
Heavy bolts, flanged joints, wind turbines
Hydraulic torque wrench
±3%
500 – 200,000 Nm
Large-flange bolts, offshore, mining
Class I vs Class II (ISO 6789)
ISO 6789 distinguishes two wrench classes. Class I ("Indicating") — the wrench continuously displays the applied torque via a scale, dial or digital readout. The operator reads the value and stops applying force at the target. Class II ("Setting") — the wrench is set to a pre-selected torque value; a mechanical click, break or slip signals the operator to stop. Class I gives visibility into the actual applied torque; Class II gives repeatability across many joints at the cost of no readback if the mechanism drifts. Both have the same ±4% tolerance when new; both require the same calibration interval.
Correct use technique
Set to zero after use — a click-type wrench left set at high torque puts the spring under continuous load; the setpoint drifts. Wind it back to the lowest scale mark after each use.
Grip at the marked pivot point — the wrench is calibrated for the moment arm from tool tip to a specific handle grip position. Gripping elsewhere changes the effective moment arm and the applied torque.
Pull, don't push — pushing risks hand injury when the joint yields suddenly and gives less controlled force application.
Steady, slow application — jerking the wrench gives a peak-detection error on digital, and can over-click a click-type wrench past its setpoint.
Never use as a breakout tool — undoing a stuck bolt overstresses the torque cell / click mechanism; use a plain breaker bar for undoing.