The testing evaluation of several digital torque wrenches by using a torque wrench tester


ACTA IMEKO | www.imeko.org December 2020 | Volume 9 | Number 5 | 189 

ACTA IMEKO 
ISSN: 2221-870X 
December 2020, Volume 9, Number 5, 189 - 193 

 
 

THE TESTING EVALUATION OF SEVERAL DIGITAL TORQUE 

WRENCHES BY USING A TORQUE WRENCH TESTER 
 

K. Ogushi1 

 
1 National Metrology Institute of Japan, AIST, Tsukuba, Japan, kji.ogushi@aist.go.jp  

 

Abstract: 

Hand torque wrenches (HTWs) with electrical 

indicating devices (Type I, Class B defined in ISO 

6789-1:2017; called “digital torque wrenches”) 

manufactured by several manufacturers were tested 

and evaluated. The digital torque wrench is the most 

accurate in all types of HTWs in general. The 

reference standard was a torque wrench tester (TWT) 

with a rated capacity of 50 N∙m. This TWT has been 

calibrated by using a traceable reference torque 

wrench (RTW). Testing results are described with 

some characteristics. 

Keywords: Torque; Standard; Torque wrench; 

Torque wrench tester; Testing evaluation 

1. INTRODUCTION 

Although hand torque tools (HTTs) are just tools 

which fasten elements such as bolts, nuts, and 

screws, they also have a function of measuring 

devices which indicate the fastening torque or give 

out a kind of signal when achieving a pre-set torque. 

HTTs include hand torque wrenches (HTWs) 

and hand torque screwdrivers (HTSs). ISO 6789 

[1][2], which is an international standard of the 

requirement for HTTs, has been revised in 2017. It 

has been separated into two documentations. Part 1 

describes the testing method and the conformity 

assessment of HTTs, whereas Part 2 describes the 

calibration method and evaluation of uncertainties. 

The author supposes that the former is for both 

manufacturers and customers, and the latter is 

mainly for manufacturers and calibration 

laboratories. 

There are some excellent investigation results 

concerned with HTWs. Röske proposed the 

evaluation method of measurement uncertainty for 

the HTWs calibrated according to ISO 6789:2003 

[3]. Brüge investigated the measurement 

uncertainty of setting type HTWs when they were 

used as transfer devices for the comparisons or 

proficiency tests [4]. Bangi et al. showed a simple 

uncertainty evaluation method for the calibration 

results of HTWs in action [5]. Khaled et al. 

proposed a new calibration procedure for setting 

type HTWs to reduce the measurement uncertainty 

differentiating from ISO 6789-1:2017 [6]. Whereas, 

there might not be detailed investigations for 

electrical indicating HTWs manufactured by 

different manufacturers so far. 

ISO 6789-1:2017 classifies HTTs as follows: 

a) Indicating torque tools (Type I): 
1) Class A: HTW, torsion or flexion bar; 
2) Class B: HTW, rigid housing, with scale or 

dial or display; 

3) Class C: HTW, rigid housing and electronic 
measurement; 

4) Class D: HTS, with scale or dial or display; 
5) Class E: HTS, with electronic measurement.  

b) Setting torque tools (Type II): 
1) Class A: HTW, adjustable, graduated or 

with display; 

2) Class B: HTW, fixed adjustment; 
3) Class C: HTW, adjustable, non-graduated; 
4) Class D: HTS, adjustable, graduated or with 

display; 

5) Class E: HTS, fixed adjustment; 
6) Class F: HTS, adjustable, non-graduated; 
7) Class G: HTW, flexion bar, adjustable, 

graduated. 

In Japan, some customers criticize that 

conformity assessments are not correct because 

more significant deviations occur than accuracies 

declared by manufacturers themselves, particularly 

in lower capacities of HTWs. Then, the National 

Metrology Institute of Japan (NMIJ), in the National 

Institute of Advanced Industrial Science and 

Technology (AIST) examined testing of HTWs 

with electrical indicating devices (Type I, Class C 

defined in ISO 6789-1:2017; called “digital torque 

wrenches: “DTW”), manufactured by several 

manufacturers. The DTW is the most accurate in all 

types of HTWs in general (around 1 %, as a relative 

accuracy declared by manufacturers). The reference 

standard was a torque wrench tester (TWT) with a 

rated capacity of 50 N∙m. This TWT has been 

calibrated by using a reference torque wrench 

(RTW). The RTW has also been calibrated by using 

a torque standard machine (TSM) at NMIJ. Some 

characteristic results are described. 

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2. EXPERIMENTAL METHOD 

2.1. Experimental apparatus 
The specifications of DTWs which were tested 

in this research are shown in Table 1. Those 

specifications are declared by manufacturers on 

their catalogues. The DTWs were categorized into 

two groups, which were Group I (rated capacities of 

20 N∙m to 34 N∙m) and Group II (rated capacities of 

50 N∙m to 135 N∙m). A photograph of these DTWs 

is shown in Figure 1. Effective lever lengths and 

sizes of the square drive are different depending on 

manufacturers. All torque wrenches show their 

“accuracies” on the catalogues. The definitions are 

generally unclear, although a few manufacturers 

define the accuracies as “the relative deviations 

from the reference values plus one digit.” It would 

be better to recognize the accuracy as the 

“Maximum permissible relative deviation,” which 

is defined in ISO 6789-1:2017. 

 

 

Figure 1: Digital torque wrenches (DTWs) used for the testing and evaluation 

 

Table 1: Specifications of digital torque wrenches 

 
 

A TWT of DOTE50N4, manufactured by 

TOHNICHI Mfg. CO., LTD., was used as a 

reference standard for these tests. A photograph of 

the TWT is shown in Fig. 2. The rated capacity and 

resolution of the TWT are 50 N∙m and 0.005 N∙m. 

A maximum effective length is 410 mm. The 

accuracy declared on the catalogue is ±1 % + 1 digit. 

Nevertheless, the uncertainty of calibration of the 

TWT was evaluated by using a reference torque 

wrench (RTW). The RTW is the combination of 

TTS/100Nm (as a torque transducer in the shape of 

torque wrench) and MGCplus with ML38 (as an 

indicator/amplifier), as shown in Fig. 3. An 

appearance of the calibration of TWT by using 

RTW is also shown in Fig. 4. The RTW was 

calibrated by using a torque standard machine (TSM) 

at NMIJ as well. Therefore, the traceability of 

measurement was utterly established in the series of 

experiments. 

Relative expanded uncertainties of the RTW 

calibration by using the TSM were as follows for 

clockwise (CW) and counterclockwise (CCW) 

torques: 

CW:  0.045 % (5 N∙m ~ 100 N∙m) 

CCW:  0.056 % (-5 N∙m ~ -100 N∙m) 

Relative expanded uncertainties of the TWT 

Calibration by using the RTW were as follows: 

CW:  0.38 % (5 N∙m ~ 50 N∙m) 

CCW:  0.51 % (-5 N∙m ~ -50 N∙m) 

ATECH1FS240N

GEK-030-R2

T2DT30H

CEM20N3X10D

EXACTA 2-25

ATECH2F100BN

GEK-060

-R3

T3DT60H

CEM50N3X12D

Group I

Min Max

1 ATECH1FS240N Snap-on Inc. --- 30

2 GEK-030-R2 Kyoto Tool Co., Ltd. 6 30

3 T2DT30H TONE Co., Ltd. 6 30

4 EXACTA 2-25 Sturtevant Richmont --- 34

5 CEM20N3X10D Tohnichi Mfg. Co., Ltd. 4 20

Square Reso-

drive lution

/ mm / N・m CW CCW
1 ATECH1FS240N Snap-on Inc. 6.3 0.01 2 3

2 GEK-030-R2 Kyoto Tool Co., Ltd. 6.3 0.02 4 4

3 T2DT30H TONE Co., Ltd. 6.3 0.01 3 3

4 EXACTA 2-25 Sturtevant Richmont 10 0.01*
3) 1 1

5 CEM20N3X10D Tohnichi Mfg. Co., Ltd. 10 0.02 1
2)

1
2)

Group II

Min Max

6 ATECH2F100BN Snap-on Inc. --- 135

7 GEK-060-R3 Kyoto Tool Co., Ltd. 12 60

8 T3DT60H TONE Co., Ltd. 12 60

9 CEM50N3X12D Tohnichi Mfg. Co., Ltd. 10 50

Square Reso-

drive lution

/ mm / N・m CW CCW
6 ATECH2F100BN Snap-on Inc. 10 0.1 2 3

7 GEK-060-R3 Kyoto Tool Co., Ltd. 10 0.05 3 3

8 T3DT60H TONE Co., Ltd. 10 0.01 3 3

9 CEM50N3X12D Tohnichi Mfg. Co., Ltd. 10 0.05 1
2)

1
2)

*1) Some of definitions of accuracy are unclear.

*2) ±1 % + 1 digit.

*3) "0.005" in the case of less than 10 N・m.

Type Manufacturer
Accuracy*

1)

 / ±%

Effective

length

mm

237

150

Type Manufacturer

Meas. range

N・m

160

317

217

length

N・m mm
Type Manufacturer

Meas. range Effective

347

150

165

 / ±%

254

Type Manufacturer
Accuracy*

1)

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ACTA IMEKO | www.imeko.org December 2020 | Volume 9 | Number 5 | 191 

 
Figure 2: The torque wrench tester (TWT) used as a 

reference standard 

Figure 3: The reference torque wrench (RTW) used for 

the calibration of the torque wrench tester (TWT) 

 
Figure 4: The calibration of the TWT using the RTW as 

a reference standard 

The reference [7] and [8] describe the detailed 

procedures of calibrations and uncertainty 

evaluations for the RTW and TWT, respectively. 

The uncertainty of the calibration for the TWT 

includes the uncertainty contribution due to the 

indicating error in the case of the display with the 

unit of torque. Then, the indicated values of the 

TWT were not corrected in the testing of HTWs 

afterward. The maximum values of indicating errors 

were 0.12 % for CW and 0.13 % for CCW. The 

normative requirement that “The maximum 

measurement error of the torque measurement 

device shall not exceed 1/4 of the claimed maximum 

permissible relative deviation of the torque tool at 

each target value.” in ISO 6789-1:2017, was 

satisfied as the maximum permissible relative 

deviation is 4 % in this experiment [1]. 

2.2. Experimental procedure 
In the testing of HTWs, the effective lever lengths 

set out in the TWT were adjusted according to the 

specification of each HTW. Besides, both the 

horizontal positions of HTW and TWT were 

adjusted by using just a level.  

The testing was conducted according to the 

procedure written in ISO 6789-1:2017. Here, the 

testing was separated to CW and CCW. One pre-

loading was carried out by the maximum torque of 

that testing beforehand. Five times at 6 N·m, five 

times at 12 N·m, and five times at 20 N·m were 

loaded out for the Group I. Five times at 12 N·m, 

five times at 30 N·m, and five times at 50 N·m were 

loaded out for the Group II. The torque steps were a 

bit different from the requirement of 

ISO 6789-1:2017 (at the lowest specified torque 

value, at approx. 60 %, and finally at 100 % of the 

maximum torque). Therefore, the experimental 

results cannot be directly compared to the maximum 

permissible relative deviation according to the ISO 

document. The indications of the TWT was read out 

when the readings of HTWs strictly coincided with 

the target values. The another consecutive five times 

loading was conducted after complete unloading to 

zero when overloading accidentally occurred. 

Besides, the author conducted an extra testing at 

the maximum step of CCW torque, changing the 

effective length of the lever to 5 mm longer than 

setting average length. 

3. RESULTS AND DISCUSSION 

3.1. Relative deviation 
Figures 5 and 6 show relative deviations of the 

TWT indications from the indication values of 

DTWs for Group I and II, respectively. The symbols 

of DTW-A to DTW -E, and DTW-K to DTW-N are 

just random. They are not related to the order in 

Table 1. 

It was found that almost all DTWs tested had a 

small repeatability with simple five measurements 

at each torque step. There was a little more 

extensive repeatability at the +6 N·m step of DTW-

A and at the +12 N·m step of DTW-L. 

 

DOTE50N4

TTS/100Nm

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ACTA IMEKO | www.imeko.org December 2020 | Volume 9 | Number 5 | 192 

 
Figure 5: Testing results for Group I 

 
Figure 6: Testing results for Group II 

The quantities of relative deviations depended 

on the type of DTWs. One was nearly zero 

deviations, whereas the other was more than 2 % 

deviations. The reason for such different deviations 

probably comes from the different pathways of the 

traceability, different mechanisms of DTWs, or both. 

All relative deviations were within the accuracy, 

declared in the catalogues of manufacturers in either 

case. 

The maximum permissible relative deviation for 

DTWs is 4 %, according to ISO 6789-1:2017. Then, 

the maximum measurement error of TWT should be 

equal or less than 1 %. However, many 

manufacturers insist that the accuracy of DTWs of 

less than 4 %, e.g., 1 %, 2 %, or 3 %. In such cases, 

the maximum measurement error of TWT should 

also be much smaller than 1 %. In the strictest case, 

0.25 % of the measurement error is required. The 

value of error is quite difficult to be achieved, 

especially at the lower range of torque values 

(5 N·m or smaller). 

3.2. Influence of changing the effective length 
At the maximum step of CCW torque (-20 N·m 

for Group I and -50 N·m for Group II), the effective 

lengths (loading points) of the lever were changed 

to 5 mm longer than original effective lengths. The 

relative deviations of average values for five 

measurements are shown in Figs. 7 and 8. 

 
Figure 7: Relative deviations when changing the position 

of loading point to 5 mm outward in the case of Group I 

 
Figure 8: Relative deviations when changing the position 

of loading point to 5 mm outward in the case of Group II 

The uncertainty contribution due to the variation 

of the force loading point should be considered for 

the total uncertainty of HTTs according to ISO 

6789-2:2017 [2] (The concept of the uncertainty of 

“calibration” but not “testing results” are introduced 

in part 2 of this standard). The variation value 

should be obtained by measuring ten times at 10 mm 

on either side of the center of the handhold position 

or the marked loading point for the lower limit 

torque values. 

Although the measurement method in this 

experiment was a bit different from 

ISO 6789-2:2017, we could obtain the tendency of 

this effect. 

It was found that there were quite large 

deviations in some DTWs from Figs. 7 and 8. Only 

5 mm change of the lever length caused more than 

3 % deviations. It might be ascribable to the 

difference of structure inside of each DTW. 

There are some types of structure in DTWs as 

follows, for example: 

(1) Measuring the shear strain at the twisting axis, 
(2) Measuring the bending strain of an elastic hinge 

rod inside, and 

(3) Measuring the bending strain of an elastic hinge 
rod but putting an additional pivot and 

shortened the hinge length. 

-3.0

-2.0

-1.0

0.0

1.0

2.0

3.0

-20 -15 -10 -5 0 5 10 15 20

R
e

la
ti
v
e
 d

e
v
ia

ti
o
n
s
 /
%

Torque / N·m

A
B
C
D
E

-3.0

-2.0

-1.0

0.0

1.0

2.0

3.0

-50 -40 -30 -20 -10 0 10 20 30 40 50

R
e

la
ti
v
e
 d

e
v
ia

ti
o
n
s
 /
%

Torque / N·m

K
L
M
N

-4.0

-3.5

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

A B C D E

R
e
la

ti
v
e
 d

e
v
ia

ti
o
n

w
it
h
 c

h
a
n
g
in

g

th
e
 l
o
a
d
in

g
 p

o
in

t 
/ 
%

Type of digital torque wrench 

-4.0

-3.5

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

K L M N

R
e

la
ti
v
e
 d

e
v
ia

ti
o
n

w
it
h
 c

h
a
n
g
in

g

th
e
 l
o
a
d
in

g
 p

o
in

t 
/ 
%

Type of digital torque wrench 

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ACTA IMEKO | www.imeko.org December 2020 | Volume 9 | Number 5 | 193 

Those differences of structure might affect the 

influence of changing the effective length of the 

lever. 

Finally, the handhold shapes of DTWs possibly 

affect the influence of lever length change. DTWs 

tested in this study have various shapes at the 

handhold grip (loading point) such as rounded, 

inclined, grooved, or protruded ones. 

4. CONCLUSION 

The author examined the testing of hand torque 

wrenches (HTWs) with electrical indicating devices 

(Type I, Class C defined in ISO 6789-1:2017; called 

“digital torque wrenches: DTW”), manufactured by 

several manufacturers. The torque wrench tester 

(TWT) was used for the reference standard, which 

was calibrated traceable to the national torque 

standard. All relative deviations were found to be 

within the accuracy declared in the catalogues of 

manufacturers. However, there were quite large 

deviations in some DTWs when the effective 

lengths (loading points) of the lever were changed 

to 5 mm longer than original effective lengths. The 

structure problem of DTWs might affect the total 

uncertainty of calibration when evaluated by using 

ISO 6789-2:2017. 

The author is going to examine the testing results 

of DTWs with different TWTs which have different 

pathways of traceability, as the next stage. 

ACKNOWLEDGMENT 

The author appreciates Mr. Sakae Kimura, who 

is the technical staff in NMIJ, for his contribution to 

data acquiring, photographing, and processing. 

REFERENCES 

[1] ISO 6789-1, Assembly tools for screws and nuts - 
Hand torque tools – Part 1: Requirements and 

methods for design conformance testing and quality 

conformance testing: minimum requirements for 

declaration of conformance, International Standard 

Organization, 2017. 

[2] ISO 6789-2, Assembly tools for screws and nuts - 
Hand torque tools – Part 2: Requirements for 

calibration and determination of measurement 

uncertainty, International Standard Organization, 

2017. 

[3] D. Röske, ISO 6789:2003 Calibration results of 
hand torque tools with measurement uncertainty – 

some proposals, Proc. XX IMEKO World 

Congress, Busan, RP. Korea, 9 - 12 September 

2012.  

Online [accessed 20200727]:  

https://www.imeko.org/publications/wc-

2012/IMEKO-WC-2012-TC3-O34.pdf 

[4] A. Brüge, Operating conditions for transfer click-
torque wrenches, Proc. IMEKO 22nd TC3, 15th TC5 

and 3rd TC 22 Int. Conf., Cape Town, RP. South 

Africa, 3 - 6 February 2014.  

Online [accessed 20200727]:  

https://www.imeko.org/publications/tc3-

2014/IMEKO-TC3-2014-017.pdf 

[5] J. O. Bangi, S. M. Maranga, S. P. Nganga and S. M. 
Mutuli, Torque wrench calibration and uncertainty 

of measurement, Proc. IMEKO 22nd TC3, 15th TC5 

and 3rd TC 22 Intr. Conf., Cape Town, RP. South 

Africa, 3 - 6 February 2014.  

Online [accessed 20200727]:  

https://www.imeko.org/publications/tc3-

2014/IMEKO-TC3-2014-030.pdf 

[6] K. M. Khaled and S. M. Osman, Improving the new 
ISO 6789:2017 for setting torque tools – Proposal, 

Measurement, 112, 150-156, 2017. 

[7] K. Ogushi, A. Nishino, K. Maeda and K. Ueda, 
Range Expansion of the Reference Torque Wrench 

Calibration Service to 5 kN m at NMIJ, 

Measurement, 45-5, 1200-1209, 2012. 

[8] K. Ogushi, A. Nishino, K. Maeda and K. Ueda, 
Calibration of a torque wrench tester using a 

reference torque wrench, Proc. SICE Annual 

Conference，411-416, Tokyo, Japan, 2011. 

 
 

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https://www.imeko.org/publications/wc-2012/IMEKO-WC-2012-TC3-O34.pdf
https://www.imeko.org/publications/wc-2012/IMEKO-WC-2012-TC3-O34.pdf
https://www.imeko.org/publications/tc3-2014/IMEKO-TC3-2014-017.pdf
https://www.imeko.org/publications/tc3-2014/IMEKO-TC3-2014-017.pdf
https://www.imeko.org/publications/tc3-2014/IMEKO-TC3-2014-030.pdf
https://www.imeko.org/publications/tc3-2014/IMEKO-TC3-2014-030.pdf