Top 5 Facts Regarding Torque Wrench Calibration System

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Torque Wrench Calibration

IS/ISO 6789-1 and 2 : 2017 emphasizes:

  1. Metrological traceability 
  2. Accuracy of torque measuring devices 
  3. Significance 
  4. Effects of avoiding parasitic forces and moments
  5. Application of torque and its geometric effects 
  6. Interface between tool and measuring device
  7. Effect of variation of loading point 
  8. Uniformity in speed of application of torque 
These are the main criteria during calibration of hand torque tools/torque wrench calibration for reliable and consistent results of calibration. Torque wrench calibration The standard IS/ISO 6789-1 and 2:2017 specifies that the calibration system of hand torque tools consists of torque measuring device and torque loading system (as shown). Torque wrench calibration

1. Clause 5.1.5: Maximum permissible deviation of torque tools.


Class

Maximum torque value

10Nm

> 10Nm

A and D

± 6%

B, C and E

± 6%

± 4 %

If a tool is operates in both directions, then the maximum permissible relative deviation shall be met in each direction as specified by the manufacturer. Torque wrench calibration


        Class 

Maximum torque value

10Nm

> 10Nm

A, B and C

± 6%

± 4 %

D, E, F and G

± 6%

If a tool is operates in both directions, then the maximum permissible relative deviation shall be met in each direction as specified by the manufacturer. Torque wrench calibration

  1.  Max permissible deviation of torque tools (Type I)
  2. Maximum permissible relative deviation (Type II)

2. Clause 6.2.4: Setting torque tools (Type II) should be loaded slowly and steadily.

Increasing the torque until attainment of the target torque is signalled by the torque tool. The target torque shall be reached after a minimum time as defined in Table 5.

3) Minimum time for application of torque values

Applied torque value

<10N.m

≥10 N·m
<1000 N.m

≥100 N·m
<1000 N.m

≥1 000 N·m

Minimum time to increase the torque from 80% of target value to target value


0.5 s


1s


1.5 s


2 s


For screwdrivers (Type II, Classes D, E and F) the time to increase the torque from 80% of target value to target value shall be between 0.5 seconds and 1.0 second.

Article no: RSM 2-2022; page 1 of 4          

3. Clause 4.3: Calibration System

The calibration system that is chosen shall be suitable for the measurement of a specified range of the torque tool.
At each target value the relative uncertainty interval of W1md of the measurement of the device shall not exceed ¼ of the expected maximum relative uncertainty interval of the torque tool W1.

4. Clause: Annex C (normative) C.4.1: Reference standard uncertainty.

The measurement uncertainty interval Wref of the reference measurement standard shall not exceed 2/5 of the claimed measurement uncertainty interval W1md of the torque measurement device to be calibrated at each calibration value.

5. Clause 7.2 & 7.3: Torque wrench calibration. The system should facilitate the -

(a) Measurement of error or deviation of the torque. 

(b) Uncertainty due repeatability. 

(c) Uncertainty due to reproducibility, 

(d) Uncertainty due to resolution.                        

(e) Uncertainty due to geometric effects of the output drive of the tool.
(f) Uncertainty due to geometric effects of the interface between the output drive of the torque tool and the calibration system. 

(g) Uncertainty due to the variation of the force loading point. 

(h) Error and uncertainty of torque measuring device for determination of relative expanded measurement uncertainty and relative measurement uncertainty interval as per standard with ease. Torque wrench calibration

A. Torque measuring device

  1. For calibration of hand torque tools/ torque wrench calibration, measurement uncertainty interval (measurement uncertainty plus error) should be considered and not just measurement uncertainty.
  2. The calibration certificate should provide measurement uncertainty interval that is better than 1% (0.25% uncertainty, 0.75% error) at each torque point in the entire range of calibration.
  3. The torque measuring device should be calibrated against reference standard machine having measurement uncertainty that is better than 2/5th (0.1% CMC) or better of the measurement uncertainty of the entire range of the torque measuring device.  
  4. Square drive and hexagonal drive-in torque measuring device as per dimensions in accordance with ISO 1173 and ISO 1174-1 to minimise joint conditions effect. Torque wrench calibration

Article no: RSM 2-2022;   Page 2 of 4       

Solution Note 1. Deadweight torque calibration machines having metrological traceable newton weight lever arm and known gravity and air density values that meet the requirement.

B. Torque loading system

The torque loading system should have provision to meet the following specified requirements during calibration.  

During application of torque:

  • The force applied to the tool shall be perpendicular to the axis of rotation.
  • The handle of the torque tool shall be free to move in all planes of rotation
  • The connection between torque measurement system and torque tool shall permit self-alignment so that parasitic forces and moments are minimised.
  • The force shall be applied slowly and steadily increasing torque from 80% of target value to target value. This shall be applied as per minimum time period  mentioned in 6.2.4.       A timer may be required to control and monitor minimum time required for application of torque value.
  • During torque wrench calibration a spirit level shall be used to maintain the torque tool in a parallel fashion.

Solution Note 2. 6789-2. To avoid parasitic forces during calibration and moments. Balancing of torque wrench using deadweight and pulley is necessary with sprit level to maintain it perpendicular so that the torque wrench handle is floating supported by floating carriage. Without this facility, the loading system is not suitable.  

C. Automatic loading or manual loading

  1. As per standards one must test each torque wrench by loading it 125 to 153 times depending on the type of tool avoiding human fatigue. Is it possible by manual loading?

Solution Note3. Automatic loading system is the only solution to avoid fatigue and to ensure consistent reliable results of calibration.

D. Enclosed is the overview of numbers of loadings

(application of torque) required for different types of torque tools and capacities for ease of understanding.

Torque wrench calibration procedure

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