VDA Band 05.2 Englisch

Verband der
Automobilindustrie
Quality Management 5.2

in the Automotive Industry
Capability of Measurement
Processes for the Torque
Inspection on Bolted Joints
st
1 edition 2013
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Capability of Measurement
Processes for the Torque
Inspection on Bolted Joints
st
1 edition 2013
Verband der Automobilindustrie e.V. (VDA)


ISSN 0943-9412
Printed: 03/ 2013
English edtition: 2013/09
© 2013 by

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VDA-QMC Internetportal am 21.10.2013 um 11:51 3

Preface
Various standards and guidelines specify requirements for estimating and

considering the uncertainty in measurement. In this regard, companies face
various questions in implementing and certifying their quality management
system.
The VDA Volume 5 [1] explains how to meet these various requirements for
reproducible measurable characteristics.
The requirement for repeatably measurable characteristics is not met by

static torques since the original condition is changed by tightening the
bolted joint. The same applies to measurement methods where bolted joints
are loosened and re-tightened.
Additional difficulties in evaluating the capability of measurement processes

of static torques, are caused by the fact that the torque tolerance defined in
tightening specifications for the dynamic assembly torques is often
empirically not suited as the tolerance of the test value. This problem
becomes even more obvious in case of torque-angle controlled tightening
specifications. Due to the change of the reference quantity from torque to
the applied angle, there is no suitable reference value available to evaluate
the capability of the measurement processes.
These differences were the motivation to discuss the evaluation of the

measurement process capability of torque inspection procedures for bolted
joints in this VDA volume.

4 VDA-QMC Internetportal am 21.10.2013 um 11:51

We thank the participating organisations and their employees for their con-
tributions in the compilation of this document.
The following companies were involved in drawing up the volume:
Adam Opel AG, Rüsselsheim

Benteler Automobiltechnik GmbH
BMW Group, Munich
Daimler AG, Sindelfingen
Johnson Controls
Knorr-Bremse Systeme für Nutzfahrzeuge GmbH
Schaeffler Technologies AG & Co. KG
Robert Bosch GmbH, Stuttgart
Volkswagen AG, Wolfsburg
Our thanks also go to all who have given us encouragement and assistance
in generating and improving the document.
Berlin, February 2013
VERBAND DER AUTOMOBILINDUSTRIE E. V. (VDA)


Table of Contents
Preface 4
1 Introduction 8
2 Terms and Definitions 9

2.1 Torque Terminology 9
2.2 Test Limits 12
2.3 Differentiation between Tool and Test Equipment 12
3.1 General 13
3.2 Preload and Torque 15
4 Measurement Procedures 19
4.1 Static Torque 19
4.1.1 Peak Value Measurement 20
4.1.2 Torque after Applying a Defined Static Torque Angle 20
4.2 Loosening and Re-tightening (Back to Mark) 21
4.3 Minimum Torque (Inspection by Attribute) 22
4.4 Loosening 22
5 Capability of Measurement Processes for Torques after

Completing the Bolting Process 23
5.1 Influences Causing the Uncertainty of Results Gained in
Torque Measurements 23
5.2 How to Handle Measurement Process Capability 29
6 Ongoing Check of Measurement Process Capability 34
7 Notes about Small and High Torques 35

7.1 Small Torques 35
7.2 High Torques 35


8 Example for Analyzing Uncertainty Components 36
8.1 General Notes about the Analysis of Influence Components 36
8.2 Employee Influence Affecting the Uncertainty from Angle in
the Peak Value Method 37
9 Bibliography 40
10 Further Reading 42
11 Index of Formula Symbols 44
12 Index 46


1 Introduction
Bolted joints are of high importance in the automotive industry since they
are frequently applied (more than 100 million threaded fasteners are
tightened in German automotive companies per working day).
It has to be destinguished between the actual production process and an
inspection succeeding the production process of a bolted joint. Bolted joints
are assembled by using suitable tools selected according to tightening
specifications. Some of these tools include integrated functions in order to
monitor the process. However, these monitoring functions are not a topic of
this VDA volume. VDA Volume 5.2 is only about the inspection succeeding
the production process.
The most frequently applied type of inspection is based on determining
torques in order to use them as an auxiliary quantity for the actually relevant
preload. Due to the complex relationship between preload and torque (see
Chapter 4), the quality of the “torque” characteristic is not comparable to the
quality of characteristics that can be measured directly. Moreover, multiple
measurements of this characteristic at the same object which are actually
common practice in order to quantify the capability of a measurement
process are not possible at bolted joints. By tightening and loosening the
joint, the condition of the test object changes irreversibly. Thus it is not
possible to use the VDA Volume 5 [1] approach for bolted joints.
The following pragmatic recommendation of how to handle the capability of
measurement processes in case of torque inspections on bolted joints is
based on practical experience. The purpose of this recommendation is to
reduce the required efforts to an adequate degree.The examples described
in this VDA volume help to illustrate this topic. Their notations might differ
from manufacturer specifications.


2 Terms and Definitions
2.1 Torque Terminology

Tightening specifications
Neutral definition for tightening parameters, e.g. nominal torque, snug
torque, tolerance specifications, tightening methods, preload, assembly or-
der, insert direction, application of force, etc.
Example: torque controlled tightening 10 Nm ± 1.5 Nm
10 Nm (nominal torque)
±1.5 Nm (specification limits)
Example: torque-angle controlled tightening 50 Nm ± 7.5 Nm + 90 deg ± 5
deg
50 Nm (snug torque)
± 7.5 Nm (specification limits)
90 deg (additional rotation angle)
± 5 deg (specification limits)
Torque inspection by attribute
Torque inspection by attribute is performed by applying a test torque (typi-
cally ≤ nominal torque) in tightening direction. The bolted assembly is as-
sessed OK if the fastener does not rotate. This inspection indicates only that
the bolted joint was tightened (see breakaway torque during further tighten-
ing or static torque).
Final torque during further tightening MWE
The final torque during further tightening (see Figure 1) is the torque applied
to the driven fastener after the joint has been initially installed.
Snug torque Mα0 (also: zero-reference-angle torque)
Reference torque to trigger angle metering.
Snug torque during further tightening including a defined angle Mα0W
Reference torque to trigger angle metering during further tightening.
Actual torque Mactual
Measured torque value including the uncertainty in measurement.
Loosen/untighten
Rotating the driven fastener against tightening direction.


Breakaway torque in loosening direction MLH
The breakaway torque in loosening direction (see Figure 1) is the torque re-
quired to overcome the static friction when loosening the threaded fastener
after its initial installation. The breakaway torque corresponds to the loosen-
ing torque when the static friction is equal to the dynamic friction.
Loosening torque MLG
The loosening torque (see Figure 1) is the torque required to rotate the
driven fastener of a bolted joint against its tightening direction.
Figure 1: Schematic torque-angle diagram in loosening and tightening direction of

an installed bolted joint.
Breakaway torque during further tightening MWH

The breakaway torque during further tightening (see Figure 1) is the torque
required to overcome the static friction to rotate the fastener in tightening di-
rection after initial installation of the joint. The breakaway torque MWH corre-
sponds to the static torque when the static friction is equal to the dynamic
friction. The difference between the nominal torque and the breakaway
torque during further tightening is quantified by the tightening factor. This
factor is influenced by the type of assembly tools (wrench or nut runner), re-
silience of the joint, coefficient of friction depending on the surface finishes,
heat transfer in the tightening process, frictional forces and other influence
factors. The empirical tightening factor amounts to about 0.85 to 1.3 (see
VDI 2230 [2], see Figure 2).


60 60
Brakeaway torque during
further tightening
50 50
Static torque
Static torque
40 40
Torque [Nm]
Torque [Nm]
30 30
Tightening to 50 Nm
Tightening to 35 Nm
20 20
Pause of 60 s Pause of 60 s
10 10
0 0
20 40 60 80 100 120 140 30 50 70 90 110 130 150 170
Angle [°] Angle [°]
Figure 2: Examples of different breakaway behaviors under different surface

conditions
Bolted joint
The bolted joint comprises of all components which are mated in the joint.
Nominal torque Mnom
Default value for the torque in the case of torque controlled tightening (also
assembly or tightening torque MA).
Further Tightening
Rotating the driven fastener of the pre-loaded joint in tightening direction.
Static torque MWG
Torque required to rotate the entire driven fastener (head and thread) of the
bolted joint in tightening direction after initial installation of the joint (see
Figure 1).


2.2 Test Limits
Experience has shown that the maximum and minimum tolerance values of
tightening specifications for torque controlled tightening (see Chapter 5) are
frequently not applicable as test limits for the static torque. In practice there
are different approaches for determining test limits in use. They are based
on experience, general default values, back calculation based on recorded
values or on a combination of these approaches (e.g. general default values
for preproduction and start of the series, back calculation during series pro-
duction). The target of these specifications is to reliably detect errors and to
avoid false alarms, i.e. assessing a condition as NOK though it is OK.
The defined test limits can differ for the same bolted joint due to different
boundary conditions (e.g. production in an air-conditioned/not air-
conditioned environment, manual/automated production).
In some cases, it might be necessary to revise the defined test limits due to
changes in the influence factors or on the basis of actual measurement re-
sults. In this respect the test limits for static torques do not have the same
significance as test limits for directly measured characteristics.
2.3 Differentiation between Tool and Test Equipment
A tool according to this guideline is defined as a device which applies a

torque to a threaded fastener (e.g. to a bolt) in the context of an assembly
process. The evaluation of the capability of a tool is according to a machine
capability study.
Test equipment according to this guideline is defined as a device which

applies a torque to a threaded fastener (e.g. to a bolt) for inspection pur-
poses. The evaluation of the capability of the torque inspection processes is
part of this VDA volume.


3 Mechanical Relationships in the Bolted Joint
3.1 General
Bolted joints are a combination of a bolt and a nut or a male set screw (bolt)
and a female component (nut). The positive locking effect between the
threads of the bolt and the nut determines the effectiveness of the bolted
joint.
The thread corresponds in unwound condition to an inclined plane (Figure 3).

A rotation of the bolt relative to the nut causes the thread flanks of the bolt
to slide across the thread flanks of the nut and thus generates a linear mo-
tion (operating principle).
P = thread pitch α = pitch angle

Figure 3: Mechanisms of a thread
Figure 4 illustrates a simplified model for a bolted assembly. The bolt can be
regarded as a pull-spring elongating upon applying a torque. The clamped
parts are represented by compression springs compressed as a function of
the applied preload, respectively the acting surface pressure and material
properties.


Figure 4: Simplified model for a bolted assembly
FV Preload (after settling)

FSA Additional axial bolt deformation force
FPA Force fraction for which clamped part is relieved
FA Axial applied load
FKR Remnant clamping force
FS Bolt force
fS Bolt elongation due to preload FV
fP Shortening of clamped part due to FV
fSA Bolt elongation due to applied load
fPA Shortening of the clamped parts
Lenght
Figure 5: Joint elasticity diagram


The effective force components can be graphically illustrated in a joint elas-
ticity diagram. Figure 5 displays a torque controlled tightening in the elastic
range. The red triangle represents the bolt resilience; the blue triangle rep-
resents the resilience of the clamped parts. The elongation of the bolt and
the simultaneous length reduction of the clamped part are displayed as a
function of force F.
3.2 Preload and Torque
The physical quantity generated as a technical target in a bolted joint is pre-

load. The preload itself is subjected to multiple influence factors (Figure 6).
In general, it is not possible to measure the preload in a bolted joint under
series production conditions directly with sufficient accuracy. Therefore,
torque is used as an auxiliary quantity.


16
Figure 6:
Friction Settling Test object defects
Head bearing surface Clamping length ratio Stability

Thread Wax Rolling defect
Friction radius Material
Geometry
Sealing elements
Fit Material
Air gap
Surface Dimensional stability
Flatness Hydrogen-induced
Borehole
cracking
Fastening speed Joint parts
Mixing
Surface coating Coating Shrink hole
Lubricant Contact pressure Surface defect
Countersink Fastening speed Cracks
Target:
preload
torque
Default:
Mixing
Parameters
Stability ratio Rework
Counter support Error detection
Material selection Process safety
Savety Error detection Accuracy
Specifications from drawing Threading Drive style
Environment Reliability
Material flow
Calculation

Fixation Revolutions per minute
Thermal resistance Tightening time
Cleanliness
Dimensioning Feeding system Tightening method

Tightening method Cycle time
Construction Production Torque tool

Influence factors affecting the preload created during torque tightening
M nominal torque
nom
M head friction torque
K
M thread friction torque
G
M pitch torque
St
Mnom = MK + MG + MSt
Figure 7: Basic torque partitioning
The nominal torque Mnom applied to create preload is composed of the head
friction torque MK, the thread friction torque MG and the pitch torque MSt that
actually generates preload (see Figure 7). The formula specifying the rela-
tion between the applied nominal torque Mnom and the achieved assembly
preload FM for metric screw threads according to ISO 68-1 [3] is as follows
(compare VDI 2230 [2]):
 D 
M nom  FM  0.16  P  0.58  d 2  μG  Km  μK 
 2 
where Mnom: nominal torque (in Nm)

F M: assembly preload (in N or kN)
P: thread pitch (in mm)
d2: pitch diameter (in mm)
µG: coefficient of friction in the thread (non-dimensional)
DKm: effective diameter for the friction torque at the bolt head or nut
bearing area (in mm)
µK: coefficient of friction in the head bearing area (non-
dimensional)


The coefficient of friction in the bolt head or nut bearing area µK and of the
friction in the thread µG in the formula are a measure for the lubrication con-
dition of the surfaces in contact moved during assembly.
As Figure 7 illustrates, the major part of the tightening torque is absorbed by

the friction between the head and the head bearing surface and by the fric-
tion in the thread. Finally, only 8 to 16% of the tightening torque develops
the preload holding the components together. Thus, it is not possible to
conclude on the achieved preload based on torque only.
In order to assess the achieved assembly preload in a bolted joint by means

of the tightening torque, the coefficients of friction are kept constant within
defined limits (see VDA 235-101 [4]) by coating threaded fasteners with
specific lubricants (e.g. top coats). However, the assembly preload of a
bolted joint cannot be predicted in the form of a specific value, but by a
range. This range is in practice additionally influenced by e.g. altered sur-
face conditions of the components (clean, oily) and environmental influ-
ences (humidity, temperature, etc.).
The tightening factor A expresses the relation between the minimum and
maximum assembly preload:
FM max
A 
FM min
For torque-controlled tightening, the tightening factor A is in the size of ap-
prox. 2.


4 Measurement Methodes/principles
The target of a series inspection of bolted joints after initial installation is to

detect relevant deviations in the planning and production process respec-
tively to optimize the process. Potential deviations are e.g.:
 errors in production planning
- definition or purchase of the wrong tool
- missing counter bracket or dolly
- wrong production parameters
 operator/process errors in production
- wrong handling of the tool
- usage of the wrong tool
- false programming of the equipment in terms of monitoring and
production parameters
- creep sensitive material on the interfaces
 machine/tool errors
- friction shunt (tool in contact with surroundings)
- tool failure
- excessive wear of efficiency related components
 component defects
- leading to settling effects
- due to creep sensitive material on the interfaces
- due to a defective threaded fastener
Generally, there are torque related procedures and elongation related pro-
cedures applicable for the inspection of bolted joints in series production.
Since the capability of test procedures based on elongation detection (e.g.
time of flight measurement of ultrasonic waves, measurement of changes in
length before and after bolt plastic deformation) can be assessed according
to VDA Volume 5 [1], the following chapters cover only torque related test
procedures.
4.1 Static Torque
The theoretical static torque for a static torque angle of 0 deg cannot be
measured directly. The methods applied in practice provide an approximate
value leading to a similar result compared to the theoretical static torque in
case the friction conditions remain the same. Chapter 5 provides more de-
tailed information about the occurring influence factors. The next two sec-
tions present the two methods that are most common in practice. However,
there are further procedures whose evaluation is based on mathematical


models specific to the respective manufacturer. It is important not to dam-
age the connection during further tightening.
4.1.1 Peak Value Measurement
Peak value measurements record the

maximum torque in the form of a
measured value. In case of significant
breakaway (static friction > dynamic
friction), the peak value corresponds
to a torque between 0 Nm and MWH
(from 0° deg to α1), to the breakaway
torque MWH (from α1 to α2) or to the fi-
nal torque during the further tightening
process MWE (from α2). Which torque it
corresponds to, depends on the static
torque angle. Without breakaway Figure 8: Peak value measurement
(static friction ≈ dynamic friction) the
peak value corresponds to the final
torque during the further tightening
process (see Figure 8).
4.1.2 Torque after Applying a Defined Static Torque Angle
The torque angle curve provides a

measured value based on a snug
torque and a defined static torque an-
gle (see Figure 9). The static torque
angle must be determined in a way
that it is outside the breakaway curve,
but occurs after the start of the further
tightening process (see Figure 9 and
Figure 10). The breakaway curve
helps to specify the static torque an-
gle and the snug torque. Figure 9: Further tightening process
by a defined tightening angle
100
Start of rotaion
of the thread
Torque [Nm]
80
Start of rotaion Further tightening
60 of the head process
40
Elastic torsion of bolt

20
Figure 10: Two-step breakaway of
0 the bolt in the further
0 5 10 15 20
Angle [°] tightening process

4.2 Loosening and Re-tightening (Back to Mark)
This procedure is about loosening and re-tightening by a predefined angle

(see Figure 11). The initial position is marked on the bolt head and on the
bearing area.
Figure 11: Principle of loosening and re-tightening
The Back to Mark procedure consists of the following three steps:
1. Put a mark on the assembled bolt head or nut of the bolted joint
to be checked. This mark is to indicate the position of the
head/nut in reference to the clamped component.
2. Loosen the fastener by a predefined angle, e.g. by 30 deg. The

preload should not be reduced to “0”.
3. Re-tighten the bolted joint again until the mark of the driven fas-
tener (bolt head/nut) is in the initial position again. The recorded
maximum torque represents the torque of the available bolted
joint.
The correct application of this procedure helps to avoid the further tighten-
ing process and thus the increase of the current torque and preload values.
However, the available friction conditions change by loosening the bolted
joint, e.g. the loosening process carries off oil. This is a disadvantage be-
cause a change of the conditions might distort the test results.
Note: This procedure is generally not used in series production control.


4.3 Minimum Torque (Inspection by Attribute)
The procedure is suitable for the following applications:

 process validation, e.g. in case of start of production and small series
production or used as an emergency strategy in case of system failure
 inspection by attribute on a bolted assembly that must not be tightened
any further due to the risk of damage (e.g. upon using thread-forming
screws for plastics, thin sheet metal screws, etc.)
For the minimum torque process a defined torque is applied to the threaded
fastener in tightening direction. The test result is OK if the threaded fastener
does not rotate. The test torque shall be determined to detect errors with
certainty and without rating bolted joints that are OK as NOK.
Compared to VDA Volume 5 [1], Chapter 9, it is not possible to establish the
capability of measurement processes in a minimum torque process. This is
due to the fact that the application of the minimum torque to a bolted joint
that has been fastened by means of insufficient torque results in tightening
of the bolted joint. Thus the actual condition of the bolted joint is changed.
4.4 Loosening
Loosening is not suitable for inspections in series production and serves

analysis purposes in general. The idealized torque-angel curve displayed in
Figure 1 is hard to detect in practice since the declining clamping force and
torque overlap.
Note: The loosening torque is determined e.g. in order to evaluate the thermal
loosening characteristics (see VDA 235-203 [5]).


5 Capability of Measurement Processes for Torques after
Initial Joint Installation
In case of inspections for series production control and conformity assess-

ments it is required that characteristics relating to the tolerance are correctly
and reliably detected as OK (within the specification limits) or NOK (outside
the specification limits). It is important to consider deviations caused by the
measurement process in addition to the deviations of measured values due
to variations of the production process. Deviations caused by the measure-
ment process lead to the uncertainty of measurement results and the test
decision. These deviations shall be known. If this is not totally feasible like
in the case of torque measurements, the deviations should stay within cer-
tain limits and should remain within an adequate relation to the test toler-
ance.
5.1 Influences Causing the Uncertainty of Results Gained in

Torque Measurements
Influences caused by measuring systems, operators, test objects, environ-

ment, etc. usually affect the measurement result as random and/or system-
atic measurement errors. The following section provides some further de-
tails about the main influence components displayed in Figure12.


24
Men Test object / Test / measurement Recording of data/
Figure 12:
production process process evaluation method Environment
Geometry Material Settling / relaxation Speed InstallationObject

combination characteristics Filtering space location
Motion sequence Mathematical
Constructional Inspection at a during measurement
design models Accessibility
static object
Training Conformity Interpolation Visibility
Surface Temperature
finishes Material Location
coating Vibrations
Inspection at Climatic
Qualification Product moving object influences
Tightening variation
method Torque / angle Humidity In production
Physical Variation of rotation process
Tightening Prior to
constitution tolerance operating load Recording
Accuracy of data Location of
Process measurement
Motivation Tolerance variation
Inspection time Torque External
Psychological vibrations In laboratory
constitution Discipline Coefficient of friction After
operating load Torque / time
Test / measurement
Random result torque
Static torque process measurement
deviation Tool setting Size
Peak value Ultrasonic uncertainty
testing Lengths
Measurement Lever arm
Torque after applying a uncertainty Angle
defined static torque angle Static Force
Unrecorded Geometry of
systematic measuring Stability of the

Inspection by measurement equipment measuring instrument
Dynamic attribute error Location of the
Measurement
Loosening torque (Attribute) Break-over means mounting device Type of display

Loosening / measurement torque wrench
Torsion sensor Measuring interval
re-tightening
Design (e.g.) Resolution Simulation
Breakaway torque Stability of the
measurement Mechanical Calibration / mounting device
Digital torque drag indicator adjustment
Loosening wrench
Test / measuring Measuring / Mounting device Measurement

method test equipment standard
Important influences on the uncertainty of measurement results

Recording of data/evaluation method
 Data recording
Depending on manufacturer, measuring instrument and test method,
different combinations of the characteristics torque, angle and time are
recorded and displayed with a different resolution and/or uncertainty in
measurement.
 Mathematical models
Depending on manufacturer, measuring instrument and test method, the
recorded raw data are processed and evaluated differently. Various
interpolations, filterings and smoothings are available for processing
data. Depending on the strategy (e.g. peak value or curve analysis) of
the evaluation, different algorithmic models (e.g. double line, peak value,
gradient change, torque/angle) are available in order to determine
measurement results (see Figure 13). Thus there might be a systematic
difference between the determined measurement results. As a
consequence, it may be necessary to perform comparison measure-
ments, when using different measuring instruments.
1 Intersection of double line

2 Maximum (during breakaway)
3 Gradient change
4 Minimum (during breakaway)
5 Torque/angle
6 End value
Figure 13: Systematic differences due to various methods for determining

measurement results
Test object/production process

 Material coating
The surface coatings of the bearing areas between the clamped parts af-
fect the underhead friction and thread friction. As an example, electro-
plated surface coatings may lead to a distinct loosening torque and anti-
corrosive coatings with high thicknesses (e.g. powder coating) might in-
fluence the settling characteristics.


 Material combination
In total, the clamped materials affect the result of the bolting process.
The material combination influences the joint hardness and the settling
effects during and after the bolting process.
 Surface finishes
The surface finsihes (structure, roughness, flatnes, lacquer thickness,
contamination and changes in these characteristics during the bolting
process) change the friction behavior and affect the measurement result.
 Coefficient of friction
The coefficients of friction are factors connecting preload and nominal
torque. They are significantly influenced by the surface coatings of the
threaded fasteners and clamped parts. Other influence components are
temperature and humidity. Definition and determination of the friction co-
efficients are given in ISO 16047 [6].
 Settling and relaxation characteristics

After completing the bolting process, settling and relaxation effects di-
minish the clamping force. The dimension of these effects depends
amongst others on the following factors:
- process control (fastening speed/stage model in case of programma-
ble nutrunning systems)
- heat treatment in one of the following process steps (e.g. heat trans-
fer by furnaces in the paint shop)
- material combinations (e.g. elastic materials or materials with a ten-
dency to relax the bolted assembly)
- material coating
There is also a difference between cold settling (without operating
load/measurement in the production process) and hot settling (after op-
erating load or due to environmental/production influences).
In inspections, the loss of clamp force while the coefficients of friction
remain the same leads to a reduced static torque, compared to the nom-
inal torque. The settling characteristics must be considered, if required,
in determining the inspection time.


Measuring/Test equipment
 Measuring equipment geometry

A measuring system includes the measuring equipment and necessary
measurement aids required to perform a measurement. Measurement
aids are extensions, counter support, adapters and guidance
(stabilizers). Their application might create further sources of errors, e.g.
clearance, tilting and loss of efficiency.
180
160
Torque [Nm]
140
120
100
80
60
Without extension
40
With extension
20
0
0 2 4 6 8 10 12
Angle [°]
Figure 14: Measurement of the same bolted joint with and without extension.
Static torque angle of 10° deg starting from 65 Nm (50% of the
nominal torque)
Measurement process
 Work movement during the
measurement
The temporal and spatial work movement
during a manual measurement (speed,
conformity, tilting) may affect the mea-
surement in different ways. As an example,
the coefficient of friction can depend on the
sliding speed at thecontact zones with
thread and underhead (see Figure 15).
Figure 15: Further tightening with
1 RPM and 5 RPM on
cathodic dip-paint coating


 Inspection at a moving or movable object
Inspections at moving (e.g. part placed on an assembly line) or move-
able (e.g. engine in the hanger, rubber-cushioned) objects might distort
the measurement result.
 Inspection at a static object

Test object does not move, without relative movement
 Inspection time
Describes the inspection time after the production process is completed.
The following factors influence the selected inspection time: accessibility,
application of screws with an adhesive coating (according to e.g. DIN
267-27 [7] the bolted assembly or, if necessary, the inspection of the
torque must be completed within 5 minutes), settling and relaxation
characteristics, condition of the test object (prior to/after operating load),
temperature of the part.


5.2 How to Handle Measurement Process Capability
1 Meet require-
7 ments for D&D
2
no No subsequent
7 Inspection
required? inspection
yes
3 Select test
procedure
yes 4 Select
Inspection by attribute test
attribute? equipment
no
5 Select meas-
uring equipment
6 Set require-
ments for meas-
urement process
capability
7 Perform
measurement
8 Analyze
measurement data
9 yes
Ppk ≥ target Capability of mea-
value? surement process
no
10 Analyze un-
certainty compo-
nents
Improve product- Improve inspec- Improve product,

ion process tion process change test limits
if needed
Figure 16: Process flow chart


Due to the changes of the test object caused by the inspection process, the
repeated measurements required in order to quantify uncertainty compo-
nents, as described in VDA Volume 5 [1], are not possible in torque testing.
Measurements can only be taken from different test objects. Therefore the
influences of the production and the measurement process cannot be clear-
ly separated. As a consequence, the process given in VDA Volume 5 [1]
has to be adapted and the capability ratio QMP is not applied. The basic
steps for preparing capability analyses and establishing the capability of the
measurement process in torque measurements (see Figure 16) are ex-
plained in the following.
In general, the capability of measurement processes should be determined
as early as possible in order that the results are available at the start of pro-
duction. As long as the capability of the measurement process has not been
established, it is necessary to take the respective risk-oriented (A/B classifi-
cation, experience from comparable processes) backup measures, e.g.
higher subgroup inspection or 100% inspection.
Step 1: Meet input requirements for design and development
The input requirements for design and development include the geometry of
the bolted assembly, the used materials and surface coatings, the tightening
specifications and the classification of the bolted joint (see VDI 2862 [8]).
Step 2: Inspection required?
Based on VDI 2862 [8], an inspection can be required, e.g. in terms of pro-
cess control, for special characteristics or for A- and B-classified bolted
joints.
Step 3: Select test procedure
The test procedure (see Chapter 4) to be applied is selected according to
the requirements and boundary conditions. A measuring inspection is pref-
erable.
In order to gain comparable results and to limit the efforts for additional
analyses, it is recommended to use the same test procedures for the same
inspection tasks, even at different locations.
Step 4: Select attribute test equipment
Mechanical click wrenches are typically used as attribute test equipment.
Meeting the requirements of DIN EN ISO 6789 [9] serves as a criterion for
establishing capability.


Step 5: Select measuring equipment
Based on the specified test procedure a suitable measuring equipment or a
suitable measuring system is selected. It is important to consider the evalu-
ation method, resolution, recorded characteristics, e.g. angle, time, torque,
expanded measurement uncertainty of the measuring equipment etc. in se-
lecting the equipment since these factors might differ depending on the
manufacturer of the test equipment (see Chapter 4).
In order to establish the capability of the measuring equipment or measuring
system, the procedure described in VDA Volume 5 [1] is used. The capabil-
ity of the measuring equipment or measuring system is usually established
if the resolution is ≤ 5% (see Note 1) of the test tolerance TOLtest (see
U MS
Chapter 2.2) and the capability ratio QMS  2   100% calculated
TOLtest
from the expanded measurement uncertainty UMS does not exceed the limit
QMS_max. A percentage of 10% is recommended to be used as the capability
ratio limit for measuring systems QMS_max since a coverage probability of
99,73% is common practice in screwdriving technology (see VDI 2647 [10]).
In case an appropriate calibration certificate is available for the measuring
equipment (see e.g. VDI/VDE 2646 [11]), the capability ratio QMS may be
calculated from the expanded measurement uncertainty given in the certifi-
cate.
Example: According to the information given in the calibration certificate,
the expanded measurement uncertainty (including the coverage
factor k=2) for the torque measurement amounts to UMS = 1 Nm
in case of a torque of 100 Nm.
Test limits of 100 Nm ±15 Nm, i.e. TOLtest = 30 Nm leads to
QMS ≈ 6.7 %.
Note 1: A lower resolution is permissible in justified individual cases.
Note 2: Further specifications of the limit were renounced. The proposed limit
serves as a guide value that cannot be generalized in any case. In
individual cases, the limit must be agreed upon between supplier and
customer. If the proposed limit is unrealistic, individual agreements must
be made depending on the respective characteristic and its specifications
(wide or narrow/very narrow tolerances). It is important always to take into
account the entire measurement process. In order to determine the limit, it
is necessary to consider economic and technical requirements. For this
reasons, the limit should be as wide as possible and as narrow as
necessary.


In order to gain comparable results and to limit the efforts for additional
analyses, it is recommended to use the same measuring equipment and
measuring aids for the same inspection tasks, even at different locations.
However, this does not apply a comparison of measuring equipment and
measuring aids has already been performed.
Step 6: Set requirements for measurement process capability

By using suitable methods for risk evaluation and by taking the correspond-
ing preventive measures before starting the application in series production
it is ensured that potential weaknesses (see Chapter 5.1) in the measure-
ment process are detected in time and can be eliminated.
In addition, the first measurements are taken at an early stage (e.g. proto-
type construction, pre-series) by using the measuring equipment and means
(including extensions, counter support, etc.) planned to be applied in series
production. If required, the relevant setting and evaluation parameters are
determined during these first measurements. The achieved results are
plausibilized e.g. by considering the following aspects.
 comparison between the measured values and the specified test
limits
 variation of the measured values
 comparison between the measured values and the shut off values
or setting value of the tools
 comparison between the tightening curves of the inspection and the
torque-angel curves of the tightening process
Based on the results, further corrective action may be identified and execut-
ed, where appropriate.
In case a test procedure with operator influence is selected (e.g. static
torque angle when using the peak value method) it is recommended to con-
duct inspections in prototype construction, pre-series or of comparable bolt-
ed assemblies in series production. These inspections help to evaluate the
employee influence provided that it is unknown or has not been assessed
yet (see Chapter 8.2).
Equally, it is recommended to inspect new or modified test procedures or
measuring and test equipment by means of a test for significant differences
provided that they are unknown or have not been assessed yet.


Step 7: Perform measurement
It is preferable to take the measurements from products manufactured un-
der series production conditions (series production part, facility, environ-
ment and inspector) and to use series production measuring equipment and
means in order to perform the measurements. For reasons of statistical ac-
curacy, 50 parts are typically required.
Note: If values outside the specified test limits or further abnormalities occur while
recording measured data, the respective corrective action must be taken.
Step 8: Analyze measurement data

The measurement results are checked for plausibility and evaluated by
means of statistical methods. The plausibility check is e.g. based on the
consideration of the aspects mentioned in step 6.
Note: It might be reasonable to combine different bolted joints of the same joint,
e.g. if they cannot be distinguished (example: wheel bolting).
Step 9: Ppk ≥ target value

If the capability of the production process is established based on the typical
50 measured values mentioned above by means of a suitable measuring
system, a separate measurement process capability analysis will not be re-
quired anymore.
This proceeding takes into account that the process evaluation includes the
variation of the measurement process. However, it also considers the fact
that an exact separation of influence components affecting the production
process or the measurement process is not feasible for the reasons already
mentioned.
In back-calculations from available measurement data taken from bolted
assemblies assessed to be OK, the target value for the capability index de-
pends on the number N of the available measured values. In case of a high
number N, a target value of 1.33 is recommended.
Note: It is assumed that the specified coefficients of friction are met.
Step 10: Analyze uncertainty components

In case the target value for the capability index is not met, this might be due
to the variation of the measurement or production process. The relevant in-
fluence components and suitable corrective actions are determined by
means of an analysis.


6 Ongoing Check of Measurement Process Capability
The ongoing check of measuring system capability is based on the monitor-

ing of measuring equipment at specified inspection intervals and by taking
samples of a defined size.
An ongoing check of measurement process capability is not required in

case of processes whose capability has been established. Only changes in
the measurement process might require the verification of measurement
process capability.
By selecting staff with the required qualification for this task, it is ensured
that the human factor only slightly affects the measurement process.


7 Notes about Small and High Torques
7.1 Small Torques

Experience in the automotive industry shows, that it is almost impossible to
establish the capability of a measurement process in case of torques of e.g.
less than 5 Nm. This is due to the available measuring equipment and the
values of the influence components. In this case, alternative process valida-
tions that are not based on torque testing are applied (e.g. inspection by at-
tribute, process monitoring, monitoring of production parameters).
7.2 High Torques

In case of high torques, disadvantageous ergonomic conditions (e.g. bad
accessibility of the joint/overhead work) may result in a higher uncertainty
budget.


8 Example for Analyzing Uncertainty Components
8.1 General Notes about the Analysis of Influence Components
Comparability of op- This procedure can be used in order to determine Experiment

erators (operator in- the measurement uncertainty of the static torque according
fluence) using test angle (see Chapter 8.2). In the peak value method, to type-2
objects uAV the measurement uncertainty of static torque angle study [1]
leads to an uncertainty of the static torque. The un-
certainty depends on the joint hardness. Due to the
changes of the test object in the inspection, meas-
urements can only be taken from different test ob-
jects.
Determine uAV using the method of ANOVA.
Repeatability on test Due to the changes of the test object in the inspec- Experiment
objects without op- tion, torque measurements can only be taken from according
erator influence uEVO different test objects. Thus the influences of the to type-2
production and the measurement process cannot be study [1]
clearly separated. However, if the requirements for
Reproducibility of a stable production process are met, significant dif-
the equal measuring ferences due to the specified influence components
systems can be detected.
Reproducibility over
time An inspection of the specified influence components
is restricted to the analysis in each individual case.
Form deviation / sur-
face finish / material
property of the test
object
Uncertainty from
temperature
Uncertainty from
other influence
components


8.2 Employee Influence Affecting the Uncertainty from Angle in the
Peak Value Method
In case of a peak value measurement, the measurement result without

breakaway (see Chapter 4.1.1) is the highest value measured in the tighten-
ing process. This value depends on the static torque angle and in manual
measurements it also depends on the operator.
Example 1: Two trained inspectors, nominal value of 20 Nm, peak value

method
25 measurements of the static torque angle per inspector (taken from differ-
ent test objects) led to the following results:
inspector 1: mean of the static torque angle < α WP1 > = 2.79° deg
standard deviation of the static
torque angle s WP1 = 0.51° deg
inspector 2: mean of the static torque angle < α WP2 > = 2.72° deg,
standard deviation of the static

torque angle s WP2 = 0.35° deg
The tightening curve leads to m ≈ 1.3 Nm/° deg

These results help to calculate the components uEVO (repeatability on test
objects) and uAV (comparability of operators) for the torque resulting from
the uncertainty of the static torque angle:
uEVO  m 
sWP1  sWP2   α WP1    α WP2 
uAV  m  *
2 d2
uEVO ≈ 0.6 Nm uAV ≈ 0.1 Nm
*
where d2 = 1.41 (2 inspectors)
Key:
Wstatic torque angle
uw measurement uncertainty of the
static torque angle from
s WP1  s WP2  and  αWP1    αWP2 
2
uDW measurement uncertainty of static
torque due to uw
m slope of the torque-angle curve
during further tightening
Figure 17: Torque-angle curve in the peak value measurement


Example 2: Five untrained inspectors, Mnom = 55 Nm, peak value method
Each one of 5 inspectors measures the static torque and the corresponding
static torque angle 10 times. The evaluation of a total of 50 measured
values leads to a Ppk value of 0.7 and a Pp value of 0.85 (see Figure 18)
based on the test limits of 50 Nm ± 15%. The standard deviation amounts to
s = 2.92 Nm and considers the influences of the production and measure-
ment process.
Figure 18: Evaluation result of the 50 measured values taken by 5 inspectors
Use the 10 measurements per inspector in order to estimate the measure-

ment uncertainty from the inspectors‘ influence on the mean and standard
deviation of the static torque angle as follows.
inspector inspector inspector inspector inspector

1 2 3 4 5
mean of the static 3.88° deg 4.45° deg 7.85° deg 7.73° deg 4.18° deg
torque angle
<WPi >
standard devia- 0.58° deg 0.86° deg 1.02° deg 1.76° deg 0.99° deg
tion of the static
torque angle sWPi
The tightening curve leads to m ≈ 1.1 Nm/° deg.


Based on these results, the uncertainty components uEVO (repeatability on
test object) and uAV (comparability of operators) for the torque resulting from
the uncertainty of the static torque angle are calculated.
uEVO  m 
 sWPi  u AV  m 
 α WPi max   α WPi min
5 d2 *
uEVO ≈ 1.1 Nm uAV ≈ 1.8 Nm
*
where d2 =2.48 (5 inspectors)
The standard deviation s = 2.92 is calculated as follows.
uEVO  u AV  srest
2 2 2
s=
sRest includes all influences from the production and measurement process
that have not been considered in detail. uEVO =1.1 Nm and uAV = 1.8 Nm
lead to
srest = 2.02 Nm
If the uncertainty components uAV and uEVO can be lowered to the level
reached in Example 1 by providing suitable training to the inspectors, the
calculated standard deviation sopt following the optimization of the inspection
process amounts to
0.62  0.12  srest ≈ 2.11 Nm

2
sopt =
This improves the process performance indices:

Pp ≈ 1.18
Ppk ≈ 0.97
However, the target value of Ppk ≥ 1.33 is not met yet. Further analyses and
improvements of the measurement process will be required. Otherwise it
has to be checked whether test tolerances can be adapted.


9 Bibliography
[1] VDA - Verband der Automobilindustrie e.V.

VDA Volume 5: 2011-07: Quality Management in the Automotive Industry –
Capability of Measurement Processes – Capability of Measuring Systems;
Capability of Measurement Processes, Expanded Measurement Uncertain-
ty; Conformity Assessment.
Verband der Automobilindustrie e.V., Frankfurt 2011.
[2] VDI - Verein Deutscher Ingenieure

VDI 2230-Blatt 1: 2003-02: Systematic calculation of high duty bolted joints -
Joints with one cylindrical bolt.
Verein Deutscher Ingenieure, Düsseldorf 2003
[3] DIN – Deutsches Institut für Normung

ISO 68-1: 1998: ISO general purpose screw threads -- Basic profile -- Part
1: Metric screw threads
Beuth Verlag, Berlin, 1999.

VDA 235-101: 2009-11: Reibungszahleinstellung von mechanischen Ver-
bindungselementen mit metrischem Gewinde.

VDA 235-203: 2005-08: Verschraubungsverhalten/Reibungszahlen – Praxis
und montageorientierte Prüfung.
[6] DIN - Deutsches Institut für Normung

ISO 16047:2005 + Amd 1:2012: Fasteners -- Torque/clamp force testing

DIN 267-27: 2009-09: Fasteners - Steel screws, bolts and studs with adhe-
sive coating, Technical specifications.
[8] VDI/VDE - Verein Deutscher Ingenieure, Verband der Elektrotechnik,

Elektronik und Informationstechnik
VDI/VDE 2862-Blatt1: 2012-04: Minimum restrictions for application of fas-
tening systems and tools in the automotive industry
Verein Deutscher Ingenieure, Düsseldorf 2012.


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
[10] VDI/VDE - Verein Deutscher Ingenieure, Verband der Elektrotech-

nik, Elektronik und Informationstechnik
VDI/VDE 2647: 2013-02: Type test of nutrunning tools - Torque and
torque/angle checking
[11] VDI/VDE - Verein Deutscher Ingenieure, Verband der Elektrotech-

nik, Elektronik und Informationstechnik
VDI/VDE 2646: 2006-02: Torque measuring devices - Minimum require-
ments in calibrations

ISO/IEC Guide 98-3: 2008: Guide to the expression of uncertainty in meas-
urement
International Organization for Standardization, Geneva, 2008.

ISO/TS 14253-1:: 1999-03: Geometrical product specifications (GPS). In-
spection by measurement of workpieces and measuring equipment. Part 1:
Decision rules for proving conformance or non-conformance with specifica-
tions.

ISO 3534-1 to 3534-3: Statistics – Vocabulary and symbols.


10 Further Reading
DIN - Deutsches Institut für Normung

DIN 267-28: 2009-09: Fasteners - Steel screws, bolts and studs with locking
coating, Technical specifications.

DIN 32878: 1993-06: Angular displacement measuring systems with abso-
lute and incremental encoders; concepts, requirements, testing.

DIN 51309: 2005-12: Materials testing machines - Calibration of static
torque measuring devices.

ISO 898-1:2009: Mechanical properties of fasteners made of carbon steel
and alloy steel - Part 1: Bolts, screws and studs with specified property
classes - Coarse thread and fine pitch thread.
ISO - International Organization of Standardization

ISO 6544: 1981: Hand-held pneumatic assembly tools for installing thread-
ed fasteners; reaction torque and torque impulse measurements.
VDI/VDE - Verein Deutscher Ingenieure, Verband der Elektrotechnik,

VDI/VDE 2645-2 (Entwurf): Fähigkeitsuntersuchung von Maschinen der
Schraubtechnik - Maschinenfähigkeit MFU.

VDI/VDE 2648-Blatt1: 2009-10: Transducers and measuring systems for
measurement of angle - Instructions for traceable calibration - Direct meas-
ure angle measuring systems.


VDI/VDE 2648-Blatt2: 2007-03: Transducers and measuring systems for
measurement of angle - Instructions for traceable calibration - Indirect
measure angle measuring systems.

VDI/VDE 2649: 2011-01: Rotary tools for bolted connection - Guideline for
comparative power-measurements of hydraulic impulse tools.
VDI/VDE/DKD - Verein Deutscher Ingenieure, Verband der Elektrotech-

nik, Elektronik und Informationstechnik, Deutscher Kalibrierdienst
VDI/VDE/DKD 2639: 2008-10: Characteristics of torque transducers.


11 Index of Formula Symbols
Symbol Term
 pitch angle
A tightening factor
W static torque angle
WPi static torque angle of operator i
<WPi>min, minimum, maximum mean of the static torque angle of all
<WPi>max operators
µG coefficient of friction in the thread
µK coefficient of friction between bearing surfaces (under nut
or bolt head)
d thread diameter
d2 pitch diameter
*
d2 factor of range method in accordance with MSA
DKm effective diameter for the friction in the bolt head or nut
bearing area/effective friction diameter
F force
FA axial applied load
FKR residual clamping force
FM assembly preload
FM min, FM max minimum, maximum assembly preload
fP shortening of the clamped part caused by preload FV
FPA force fraction to load release
fPA shortening of the clamped parts
FS bolt force
fS bolt elongation due to preload FV
FSA additional axial bolt deformation force
fSA bolt elongation due to applied load
FV preload (after settling)
k coverage factor
1)
L minimum value L (limit specifying the lower limiting value)
m slope/gradient of the torque-angle curve during further
tightening
Mnom nominal torque
MG thread friction torque
MK head friction torque
MLG torque to un-tighten/loosen
definition in accordance with VDA 235-203 [5]
MLH breakaway torque in loosening direction
definition according to VDA 235-203 [5]


MSt pitch torque
MWE final torque during toque audit process
MWG static torque
MWH breakaway torque during further tightening
N number of measured values
P thread pitch
PP potential process performance index
PPK minimum potential process performance index
QMP capability ratio (measurement process)
QMS capability ratio (measuring system)
QMS_max capability ratio limit (measuring system)
s standard deviation
sopt standard deviation following optimization of inspection
process
srest all components of the standard deviation that have not
been considered in detail
sWPi standard deviation of the tightening angle of rotation in
the case of operator i
TOLtest test tolerance (difference between upper and lower test
limit)
1)
U maximum value U (limit defining the upper limiting value)
u(xi) standard uncertainty
u(y) combined standard uncertainty
uw measurement uncertainty of angle for static torque W
uAV comparability of operators (operator influence)
uDW measurement uncertainty from static torque caused by
uw
uEVO repeatability (on test object)
UMP combined standard uncertainty (measurement process)
UMS combined standard uncertainty (measuring system)
1)
The GUM [12] or ISO 14253 [13] uses the formula symbol U for the expanded
measurement uncertainty. However, new standards, such as ISO 3534-2 [14] refer
to the upper specification limit as U. In order to avoid confusions in this document,
the expanded measurement uncertainty is referred to as UMS where the measuring
system is concerned and UMP when it is about the measurement process.


12 Index
actual torque .................................. 10 peak value measurement ......... 20, 38

Back to Mark .................................. 21 preload ... 9, 10, 14, 16, 18, 21, 26, 46
breakaway torque .................... 11, 20 recording of data ............................ 25
capability ratio .................... 31, 32, 47 relaxation characteristics ......... 26, 29
clamping force.......................... 22, 26 repeatability ....................... 37, 38, 40
coefficient of friction ..... 17, 18, 26, 28 resolution ................................. 25, 32
dynamic friction ........................ 11, 20 re-tightening ................................... 21
employee influence .................. 33, 38 settling characteristics.................... 26
evaluation method.................... 25, 32 small torques ................................. 36
expanded measurement uncertainty snug torque .................................... 10
................................................... 32 static friction ............................. 11, 20
final torque ......................... 10, 20, 47 static torque ..... 10, 11, 12, 19, 26, 47
head friction torque ........................ 17 surface coating .................. 25, 26, 31
inspection time ............................... 29 surface finish ............................ 26, 37
inspector ............................ 38, 40, 41 test equipment ....... 13, 28, 31, 33, 35
joint elasticity diagram .................... 16 test limits .......... 12, 13, 32, 33, 34, 40
loosen .......................... 10, 11, 21, 22 thread friction torque ...................... 17
loosening torque ............................ 11 tightening specification.... 5, 9, 10, 12,
material combination ...................... 26 31
mathematical models ..................... 25 tightening torque ............................ 12
measurement process ............. 23, 33 tool ........................................... 13, 19
measurement process capability.... 35 torque controlled tightening ...... 10, 16
measuring equipment .............. 28, 32 torque inspection by attribute ......... 10
measuring equipment geometry..... 28 torque inspection minimum ............ 22
measuring system .................... 32, 34 torque-angle controlled tightening .. 10
nominal torque 10, 11, 12, 17, 26, 27, type-2 study ................................... 37
46 untighten ........................................ 10
ongoing check ................................ 35 verification...................................... 35


Quality management in der Automotive Industry
The current versions of the published VDA volumes about the quality man-
agement in the automotive industry (QAI) are found online under
http://www.vda-qmc.de/en/.
Our publications can be ordered from our website directly.
Reference:

Qualitäts Management Center (QMC)
Behrenstraße 35, 10117 Berlin,
Phone +49 (0) 30 897842 - 235, fax +49 (0) 30 897842 - 605
E-mail: info@vda-qmc.de, Internet: www.vda-qmc.de
Forms:
Henrich Druck + Medien GmbH

Schwanheimer Straße 110, 60528 Frankfurt am Main
Phone +49 (0) 69 96 777-0, fax +49 (0) 69 96 777-111
E-mail: info@henrich.de, Internet: www.henrich.de



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Various standards and guidelines specify requirements for estimating and

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2 Terms and Definitions 9

5 Capability of Measurement Processes for Torques after

6 Ongoing Check of Measurement Process Capability 34

7 Notes about Small and High Torques 35

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11 Index of Formula Symbols 44

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2.1 Torque Terminology

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Figure 1: Schematic torque-angle diagram in loosening and tightening direction of

Breakaway torque during further tightening MWH

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Figure 2: Examples of different breakaway behaviors under different surface

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2.3 Differentiation between Tool and Test Equipment

A tool according to this guideline is defined as a device which applies a

Test equipment according to this guideline is defined as a device which

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The thread corresponds in unwound condition to an inclined plane (Figure 3).

P = thread pitch α = pitch angle

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FV Preload (after settling)

Figure 5: Joint elasticity diagram

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