In 2017, the International Organization for Standards (ISO) updated its guidelines for the accuracy and calibration of hand torque tools, previously modified in 2003. It was a comprehensive update that split the standard into two parts. The first part, ISO 6789-1:2017, lays out requirements for the design and testing of torque tools to ensure they meet the minimum requirements. The second part, ISO 6789-2:2017, lays out the requirements for calibration and measurement to ensure compliance over years of use. It isn’t surprising that these updates are so comprehensive and, for many, these changes were long overdue.
The machinery that keeps modern society running smoothly depends on threaded fasteners tightened with exactly the right amount of torque. The range of torque-dependent machinery includes everything from electronics assembled with clean room tools to the heavy boring machine parts that dig our sewers and subways. Understanding torque wrench accuracy ranges and calibrating torque screwdrivers precisely are vital to ensuring that our civilization continues to thrive.
Understanding ISO Torque Tools and Torque Wrench Accuracy Ranges
Torque is a measure of the force used to turn or twist an object, and it is found by multiplying the amount of force applied by the distance from the point of application. When turning a bolt, for example, the torque is the force applied by your hand multiplied by the length of the wrench. However, with a regular box wrench, this is an inexact science; your hand’s position on the wrench can change the amount of force applied, and the way the wrench sits upon the bolt can affect the exact distance. Accurate torque tools, though, measure the amount of force that is actually applied so the user knows when they have applied too much or too little. ISO divides torque wrenches and screwdrivers into classes based on whether they only indicate torque, whether they can be set for torque, and by their different form factors. A table of these ISO torque wrench classes is given below
|ISO TORQUE TOOL CLASSES|
|Torque Wrench Types||Torque Tool Tolerances|
|Type||ISO Class||Description||? 10 Nm||>10 Nm|
|Indicating Torque Tools||Class A||Wrench with torsion or flexing bar||±6%|
|Class B||Wrench with rigid body and indicator||±6%||±4%|
|Class C||Wrench with rigid body and electronic measurement||±6%||±4%|
|Class D||Screwdriver with indicator||±6%|
|Class E||Screwdriver with electronic measurement||±6%||±4%|
|Setting Torque Tools||Class A||Adjustable wrench with indicator||±6%||±4%|
|Class B||Fixed torque wrench||±6%||±4%|
|Class C||Adjustable wrench without indicator||±6%||±4%|
|Class D||Adjustable screwdriver with indicator||±6%|
|Class E||Fixed screwdriver||±6%|
|Class F||Adjustable screwdriver without indicator||±6%|
|Class G||Adjustable wrench with flexing bar and indicator||±6%|
ISO uses the newton meter (Nm), which is equal to the torque resulting from a force of one newton applied perpendicularly to a lever that is one meter long. The equivalent American Customary Measure is the foot-pound (lbft)—the equivalent of applying one pound of force on a lever one foot in length—and 10 Nm is equivalent to approximately 7.38 lbf.ft.
The value deviations specified by ISO for each class of tools account for discrepancies in the torque indicated by the tool or calibration machinery. For instance, when an adjustable Class A torque wrench with settings that range from 10 to 100 Nm is set to 10 Nm of force, then it is in compliance if it returns an indicated value of 10 Nm and calibration machinery shows that the actual torque was between the ranges of 9.8 and 11.2 meters (plus or minus 6%).
The deviations allowed between torque tool indicators and calibration equipment is largely the same between the present ISO 6789 standards and those issued in 2003. However, the previous ISO 6789 standards specified a measuring range from 20% to 100% of the maximum torque value of the tool in question, so a 10-100 Nm wrench could be calibrated at 20 Nm and up. The new ISO standards given in both parts of ISO 6789:2017 are much more precise, dictating that torque tool calibration starts at the lowest end of its given range (which would be 10 Nm for the wrench example used above). It’s a much more precise standard of calibration.
Choosing Torque Tools that Meet or Exceed ISO Requirements for Torque Wrench Accuracy Ranges
Complying with both parts of ISO 6789 requires establishing a standard calibration regimen for your company’s torque tools. Laboratories that are certified by the National Institute of Standards and Technology (NIST) are qualified to ensure that your tools are tested and in compliance with ISO standards. However, companies that use large numbers of torque tools for their assembly lines may wish to set up their own torque calibration labs. They will, of course, need the appropriate torque analyzers, sensors, testers, and calibration equipment to measure their torque tools and determine the actual force being applied in order to ensure that it falls within the standards laid out in ISO 6789. Purchasing precision torque wrenches, torque screwdrivers, and power assembly tools should mean that you are set for the 5,000 cycles or 12 months— whichever comes first—that ISO 6789 requires.
Quality torque tools ensure precision fastening so that the electronics, heavy equipment, or airframes that your company assembles are to spec, reliable, and have reduced maintenance times. When you’re choosing torque tools, you should select those that make it easy to comply to ISO standards in order to ensure quality.