About Torque Calibration

Torque Wrench Calibration

Torque wrenches are tools used to apply a specific torque to fasteners such as nuts and bolts. They are used in many cases where the tightness of a screw or a bolt is crucial. Common applications of torque wrenches include the automotive industry (i.e. how tight the bolts of a vehicle wheels must be) and piping applications, such as high pressure natural gas stations (the bolts used for flanged connections must be tightened according to the manufacturer’s specifications; otherwise there is the possibility to damage critical equipment or even cause a serious accident or gas leakage).

There are several types of torque wrenches. Most common types are the following:

Beam Type Torque Wrench

They are the most frequently used tools in the industry since they are the most affordable. A Beam Torque Wrench consists of a long lever arm between the handle and the wrench head. It has a scale attached to it, so the user can see the torque applied to the fastener. When the desired torque is reached, the user stops applying force. This type of wrench is simple and used in several applications where special accuracy is not required.

Click Type Torque Wrench

In this type of torque wrenches the desired torque level is pre-set (with a calibrated clutch mechanism) so, when the desired torque is reached, an audible sound is generated. The advantage of Click Torque Wrenches is that over-tightening of the fastener is prevented, so, in the case where the torque wrench is properly calibrated, possible damages due to over-tightening are avoided. Click Torque Wrenches need a minimum annual calibration and they must also be set to zero, after each use.

Electronic Torque Wrenches

This type of instruments measures torque by means of a strain gauge attached to the torsion rod. The generated signal is converted by a transducer to the desired unit (N×m or lb×ft) and is shown on the digital display.

Torque wrenches are divided into two categories according to ISO 6789:2003 “Assembly tools for screws and nuts — Hand torque tools — Requirements and test methods for design conformance testing, quality conformance testing and recalibration procedure”. They are also classified according to their operation:

Type I: Indicating Torque Tools

Tools belonging to this category indicate by means of a mechanical scale, dial or electronic display, showing the amount of torque exerted by the tool at the output drive. Type I Torque tools are classified as follows:

• Class A: Wrench, torsion or flexion bar (coil spring or deflecting beam torque wrench)
• Class B: Wrench, rigid housing, with scale or dial display
• Class C: Wrench, rigid housing and electronic measurement
• Class D: Screwdriver, with scale or dial display
• Class E: Screwdriver, with electronic measurement

Type II: Setting Torque Tools

A wrench belonging to this category is pre-set to a certain torque value, so that, when the desired torque value is exerted by the tool at the output drive, an audible or visible signal is released. Type II Torque tools are classified as follows:

• Class A: Wrench, adjustable, graduated or with display
• Class B: Wrench, fixed adjustment
• Class C: Wrench, adjustable, non-graduated
• Class D: Screwdriver, adjustable, graduated or with display
• Class E: Screwdriver, fixed adjustment
• Class F: Screwdriver, adjustable, non-graduated
• Class G: Wrench, flexion bar, adjustable, graduated (adjustable beam type torque wrench with graduated scale)

Depending on their category and class, torque wrenches have a specified accuracy, as defined in ISO 6789:2003:

Type I (Indicating Torque Tools)

Type II (Setting Torque Tools)

Torque Wrenches are tools which need a regular calibration since their usage causes them to loose easily their accuracy and setting points. Their calibration integrity depends upon the use and environment to which the tool is subjected. A torque wrench should remain within its tolerances, as they are defined in the above tables, for at least 5000 cycles. Nevertheless, this period is often reduced due to many factors, such as ingress of dust which causes wear, damage caused by using the tool to loosen fasteners, or by often dropping the tool. In order to determine the proper re-calibration period for a specific torque wrench it is required to initially set a calibration interval (such as six months) and after evaluating the results of several calibrations, increase or reduce the calibration interval. In this way we can always be sure that our torque wrench is inside specifications. Of course, regardless of the re-calibration interval, in case the torque wrench has been subjected to an overload, or has been repaired, or has been improperly handled, as to affect the measurement results, the torque wrench must be recalibrated.

Calibration Equipment

In order to calibrate torque wrenches, calibration laboratories use accurately calibrated Torque Testers. These instruments are quality control devices used to calibrate torque control instruments. Modern Torque Testers can measure in clockwise and counter-clockwise directions, they can convert the reading to many torque units and they have three modes of operation (peak, first peak, and track). Torque testers comprise of the following components:

Transducer

The torque transducer is an electronic device which converts torque into an electrical signal. This type of transducer usually consists of four strain gauges in a Wheatstone bridge configuration. The electrical output of the transducer is typically a few millivolts and usually requires amplification by an instrumentation amplifier before it is used.

Digital display

The digital display is used to measure the output signal from the transducer and calculates the reading into a torque value which is displayed on the screen. In most of the cases, the display and the transducer are contained inside the same housing.

Joint Simulator

The joint simulator is used during the calibration process to help test torque tools. The joint simulator is placed on top of the transducers. By using different types of springs, the joint simulator can replicate soft, medium or hard joints.

Calibration Method

The calibration of torque wrenches must be performed under correct environmental conditions. These are defined by ISO 6789:2003. The temperature shall be in the range of 18 ^oC up to 28 ^oC and the maximum relative humidity shall be 90%. The temperature should not change by more than 1 ^oC during the calibration. The environmental conditions during calibration must be controlled, monitored and documented.

Before starting the calibration, the torque wrench under test must be left for sufficient time to stabilize in the environmental conditions of the laboratory.

For indicating torque tools (type I), prior to testing, one pre-loading up to the maximum value must be carried out in the operating direction to be tested and after releasing the load, the pointer or electronic indication must be set to zero. For testing in any other operating direction, this procedure shall be repeated.

For setting torque tools (type II), prior to testing, five releases without measurement must be carried out at the maximum value (nominal capacity of the torque tool) in the operating direction to be tested. For testing in any other operating direction, this procedure shall be repeated.

Torque wrenches are calibrated at the values of 20%, 60% and 100% of their maximum value (or of the nominal value in case if type II, class B or E wrenches).

The number of readings in each direction must be as follows:

• Type I (all classes): 5 readings for each measurement point.
• Type II (classes A, D and G): 5 readings for each measurement point.
• Type II (classes B and E): 5 readings at nominal value.
• Type II (classes C and F): 10 readings for each measurement point.

Example: For the calibration of a click type torque wrench with a range of 0 – 100 N×m, the following measurements must be performed:

Before starting the calibration the torque wrench must be set five times to 100 N×m.

During the calibration the torque wrench must be tested at the following measurement points:

20%: 20 N×m (5 readings)

60%: 60 N×m (5 readings)

100%: 100 N×m (5 readings)

The tolerance of this click type torque wrench in the specific test points (> 10 N×m) is ± 4 %. So, the readings at 20 N×m must be between 19.2 N×m and 20.8 N×m, at 60 N×m must be between 57.6 N×m and 62.4 N×m and at 100 N×m must be between 96 N×m and 100 N×m.

Torque wrenches are generally well used mechanical devices that require regular calibration. Imagine an aircraft that has been manufactured with a torque wrench that does not tighten bolts to the correct specification!

Written by Sofia