Systemic analysis of the caulking assembly [602387]

Systemic analysis of the caulking assembly
process
Claudiu Rodean1*, Livia Dana Beju2 and Paul Dan Brindasu2
1 Continental Automotive Systems SRL , 8 Salzburg Street, Sibiu, Romania
2”Lucian Blaga” University of Sibiu, Faculty of Engineering, 4 Emil Cioran Street, Sibiu, Romania
Abstract. The present paper highlights the importance of a caulking
process which is nowadays less studied in comparison with the growing of
his usage in the automotive industry. Due to the fact that the caulking
operation is used in domains with high importance such as shock absorbers
and brake systems there comes the demand of this paper to detail the
parameters which characterize the process, viewed as input data and output
data, and the requirements asked for the final product. The paper presents
the actual measurement methods used for analysis the performance of the
caulking assembly. All this parameters leads to an analysis algorithm of
performance establi shed for the caulking process which it is used later in the
paper for an experimental research. The study is a basis from which it will
be able to go to further researches in order to optimize the following
processing.
1 Introduction
Nowadays technologies require fast and efficient processing which ensure high quality
and low costs in addition with the process safety . Among the operations that ensure these
needs it’s included the caulking process. Caulking operation is an advanced proc ess used to
obtain permanent assemblies in various fields such as: automotive, aeronautics, medical or
energy.

Fig. 1 . Presentation of the caulking area.

* Corresponding author: [anonimizat]
Part
1
Part
2
Caulking
area

Caulking is the deformation operation which consists of tightening press of a piece of
material around another to avoid their relative movement (Fig 1). Part 2 is placed on Part 1
and a caulking tool strikes Part 1 in order to deform an amount of material in such a way, that
the resulting assembly becomes non-removable.
Examples of domains where the caulking process is used are the brake systems (Fig. 2)
and the shock absorbers (Fig. 3).

Fig. 2. Caulk ed assembly used at brake systems

Fig. 3. Caulk ed assembly used at shock absorbers systems

In the automotive industry there are certain products that require special plastic
deformation. In this area is framed the caulking operation which is started to be used on a
larger scale operation because of the advantages it presents:
– The operation can be performed on CNC equipment .
– No material loss occurs .
– Replaces more complex operations , which have a higher cost .
– Operation allows the obtaining of accurate geometrical parameters of the assembly .
– The simulation using finite element method allows the optimisation of the process.
– It is a low cost operation .
Although at present, the caulking process begins to be used more and more, the technical
literature do not analyze this process very much.
Caulk ed assembly
Caulked assembly
Caulked assembly

Sin and Lee (1 ) evaluated qualitatively a caulking through numerical analysis and
inspecting a plastically deformed shape. Following the caulking analysis, the structural
responses were considered. In addition, three design variables related to the manufacturing
process were defined, and the effects of design variables with respect to pull -out strength,
caulking depth, and maximum stress were obtained. Finally, the optimum desi gn maximizing
the pull -out strength was suggested.
Park (2) studied an inner tie -rod (ITR) end -assembly module which is widely used in the
steering systems of automobiles. The ITR module is composed of a ball -stud, ball -seat, and
socket and manufactured by a caulking process. To ensure that the final ITR module is
qualified for application, the caulking process of the ITR module was optimized to satisfy
certain requirements, including the allowable displacement, pull -out force, tilting angle, and
torque of the ball -stud. Accordingly, this paper investigates the deformation characteristics
of both the caulking and pull -out processes using a finite element method analysis and
clarifies the effect of the forming parameters on the allowable displacement, pull -out force,
and tilting angle. The finite element method analysis results are also compared with
experiments to validate the finite element method analysis.
Beck ( 3) study the heating tubes from the pressurized water reactor. The tightness is tested
for the repaired tubes done using a roll -expanded plug.
Calcaletty (4) makes an analysis of the metrological leak detection method based on the
application of mass spectrometry with tracer gas (generally helium).
Gorash (5) presents a numerical study involving t he deformation of contact faces in the
metal -to-metal seal in a typical pressure relief valve. The study method is the fluid pressure
penetration technique.

An ideal caulking assembly must fulfil the following requirements:
– Precise geometrical parameters and high surface quality of the deformed volume of
material.
– An adequate stress state along the entire caulking length.
– The assembling must have very good tightness.
– Assembly must withstand to fatigue .
– Low cost of the operation.
During a caulking assembly process , several problems may occu r:
– The assembly does not acquire the imposed geometrical parameters .
– Sealing cracks appear along the caulking zone .
An assembly process can have several reasons which can cause th ese problem s:
– The c aulking force is out of range .
– The active area of the caulking tool is damaged .
– The device which holds the units is damaged .
– The press machine which drives the caulking tool can be damaged also etc.
The paper proposes to develop a method for the study of the performance s of caulking
assemblies by considering the entire caulking process as a Input -Output data system .

2. Caulking operation viewed as a system
Caulking process is a typical one which has input data such as geometrical and technological
parameters and output data like geometrical parameters after caulking and values to be
measured (Fig. 4).

Fig. 4 . Caulking operation – system ic perspective

Due to the fact that there is a big list of requirements which it is asked by different
customers , we intend to establish a list of technological demands which the caulking
process must respect:
– Continuous deformation along the entire profile .
– Relative position between the parts and the caulking tool .
– Tool geometry .
– Caulking force .

In order to have a better understanding of the caulking process , a picture with all the
geometrical parameters (before the caulking deformation process) is presented in figure 5.

Fig. 5 . Geometrical parameters of the caulking process (before deformation)

The geometric al parameters of the two parts, defined before the caulking process is
done (the “Input data”) are:
Input data CAULKING
PROCESS Output data

– H – the height of the material which is deformed during the caulking process.
– ØDp1 – the diameter of the part 1, which will be deformed i n the caulking aria.
– ØDp2 – diameter of the part 2, which is going to be assembled in the caulking aria.
– ØDc – tool diameter.
– r– constructive parameter of part 1 .
– A1 – clearance angle of the tool.
– A2 – rank angle of the tool .
All this parameters are independent one from each other and in this way they have a big
influence on the caulking process.
Together with the geometric parameter s, the caulking process demands also a list of
technological parameters :
– V- The speed (velocity) of the caulking process .
– T – Tool pressing time .
– F – Caulking force .
– The accuracy of the devices: perpendicularity; linearity etc .
After the caulking process is performed , there are several output parameters which
occurs.
The geometrical parameters can be seen in the following picture (Fig 6):

Fig. 6. Geometrical parameters after the caulking process

The output geometrical parameters are:
– Ø Dac – the actual diameter of the caulked material .
– Hc – the height of the caulked material which covers the second part.
Together with the output geometrical parameters , the caulking process includes also a list
of output technological parameters:
– Sp_gas – the speed of the decreasing pressure inside the caulked assembly – gas conditions .
– Sp_liq – the speed of the decreasing pressure inside the caulked assembly – liquid
conditions .
– Bp – the limit of the burst pressure .

Table 1. Caulking parameters
Input Parameters Output parameters
Name Measuring unit Name Measuring unit
H [mm] Ø Dac [mm]
ØDp1 [mm] Hc [mm]
ØDp2 [mm] Sp_gas Mbar*l / s
ØDc [mm] Sp_liq Bar/s
r [mm] Bp Bar
A1 [°]
A2 [°]
V [m/s]
T [s]
F [KN]

The definition of all the caulking parameters ( table 1) allows us to perform a systematic
analysis of the process.
3. The f unction al analysis of the caulked assembly
Starting from the requirements presented before , we aim to establish the assembly functions
(table 2) and then to determine which ones are more important . This analysis allows us to
design the research in order to optimize the caulked assembly.

Table 2 . The function of the caulked assembly
Requirement Function
Precise geometrical parameters and high surface quality
of the deformed volume of material Precision
An adequate stress state along the entire caulking length Resistance
Assembly must withstand to fatigue. Reliability
The assembling must have very good tightness Tightness
Low cost of the operation Cost

The ranking of the functions is evaluated taking into consideration the importance of
each one in the life -cycle of the final assembled product (Product lifecycle methodology ).
The adopted method for this evaluation is “the triple cross method” (Fig 7).

Fig. 7. Ranking functions; triple cross – method

Each function is compared with another one and receives some points in the following
way:
– If a function is more important than another one it receives 3 points .
– If a function has the same importance with another one it receives 2 points .
– If a function is less important in comparison with another one it receives 1 point.
The ranking shows that the tightness, resistance and the reliability are the most significant
functions for the caulk ed assembly.
4. The actual methods used for the evaluation of the caulked
assemblies
As it was mentioned before, the sealing between the two parts which had suffered the
caulking process is one of the most i mportant requirements. Due to the fact that caulking
process is very often used in brake systems from the automotive field, a simple inadvertence
during the caulking process can lead to important damages to the functionality of the final
product (Fig . 8).

Fig. 8. Possible sealing problems at caulk ed assembly

There are two possible problems if the sealing was not performed in good conditions . A
first one is the possible intrusion of gas or liquid from outside to inside the caulk ed assembly .
The second one is a possible leakage of gas or liquid from inside to outside the caulking
assembly .
In the caulked assembly the tightness is checked to assure that there is no leakage between
the inside of the assembly and the outside.
Intrusion from
outside to inside
Leakage from
inside to outside

Checking out the tightness of a product obtained after a caulking process implies the
measurement of a fluid flow coming out of a defect or entering through it. Leak detection is
a non -destructive test pe rform ed to verify the accordance of products and components with
the prescribed specifications, by using techniques, which do not damage their characteristics
or performances. The leak test is continuously implemented in a production process , to check
the agreement of the goods to the tightness requirements.
Considering the functionality of the final product obtained after caulking process, the
tightness requirements can be checked with liquids or gases . Some methods are presented
next.

4.1 Method for testing the caulked assembly tightness using helium
The testing of a n on-removable assembly is done using a special instrument which is based
on the detection of helium .
The working principle of the helium leaka ge test device is as it follows. The caulkin g
assembly is connected to the test device . Everything that exists inside the assembly is emptied
using a vacuum pump . Helium is flushed towards the caulk ed area to determine tightness . –
Helium can penetrate into very small holes and may reveal the quality of the caulk ed
assembly (Fig. 9).

Fig 9 . Helium leakage method

In normal conditions (proper conditions for the functionality of the final product) the left
side of each curve must be in the right side of the pressure of 1bar.
Due to the fact that the caulking process was not performed in good conditions there was
a leakage of the gas from inside the assembly to outside and in this case the functional curve
(colo ured with blue) was moved to the right side of the pressure o f 1 bar. In these conditions
the assembled product will not be able to perform its normal functionality.
The red curve from the picture shows how the functionality of the product must look
when the caulking process is performed in good conditions.
The pict ure with two curves presents how important is to perform the caulking process in
good conditions. Even if everything seems to be ok from the visual point of view, a simple
pore in the caulking process can lead to clear damage of the final product.
In the f ollowing picture , example s of good and bad tightness can be seen , measure d with
the equipment presented before (Fig. 10).

Fig 10 . Example of a bad sealing behavior

Fig. 10. Tightness evaluation using gas pressure
4.2 Method for testing the caulked assembly tightness using hydraulic
pressure
Another method which can be used to check the tightness of the caulking process is the
hydraulic tightness of the assembly.
This method consists of holding up a certain value of a pressure inside the caulking
assembly and check if there is a pressure drop in a certain amount of time. This type of
tightness test is used for assemblies which are supposed to run at low pressures.
In figure 11, the result of a hydraulic test (done after a caulking process) is presented. The
parameters used for the test were a holding pressure of 22 bars and a holding time of 600
seconds . The graph shows that there was no pressure drop during the entire test.

Fig. 11. Hydraulic tightness testing
Curve for an assembly with bad
tightness
Curve for an assembly with good
tightness

4.3 Method for testing the burst pressure
There are cases when the caulked assembly must stand against big values of pressure. The
best method to test if a caulked unit can meet the expectations of these kinds of big values is
the burst pressure test.
Burst pressure tests are performed on dedicate d test benches which are able to provide
very big values of pressure inside the caulked assembly (test benches can go to values of
1500 bar s – Fig 12).

Fig. 12. Burst pressure test bench

A specific problem with the burst pressure value can be seen in the following picture (Fig
13): The assembly didn’t resist to the testing pressure. In figure 14, the recording of a failed
test is presented. After 20 seconds, the assembly failed.

Fig. 13. Assembly failed at burst pressure test

Fig. 14 . Recording of a failed burst pressure test
It is visible that the caulking
process was not performed in
specific conditions so the assembly
failed at high values of pressure

4.4 Measuring the geometrical parameters of the caulked area
The amount of material which covers one of the parts and in this way create s the caulked
assembly is also very important for the success of the process. This amount of material can
be measured in many ways . The simplest one is the measur ement of the height of the
deformed material (Fig 1 5).

Fig. 15. Height of the deformed material at caulking process

It is highly important that this height of deformed material to be constant on all over the
assembled area. A single zone where the height is smaller can lead directly to problems such
as air leakage or low burst pressure value.

4.5 Comparative analysis of the testing methods for the caulked assembly
By analysing the testing methods presented before, it was possible to underline the input and
the output data for each of them. The synthesis of this information is presented in table 3.
Table 3. Comparative analysis of the measurement methods used for the verificatio n of the caulked
assembly

Measurement
methods Input Parameters Output Parameters
Measurement of
caulked assembly
tightness using
helium – ØDc;
– H;
– T;
– F;
– ØDp1;
– ØDp2. – Hc;
– ØDac;
– Sp gas.
Measurement of
caulked assembly
tightness using
hydraulic pressure – ØDp1;
– ØDp2;
– ØDc;
– H;
– T;
– F – Hc;
– ØDac;
– Sp liq.
Measurement of b urst
pressure limit – ØDp1;
– ØDp2;
– ØDc;
– H;
– A1; – Hc;
– ØDac;
– Bp.

– A2;
– r;
– T;
– S;
– F

The presented methods analyse the tightness and the resistance of the caulked assemblies .
None of the methods analyse the reliability of the caulked assembly. Therefore there is a need
to develop a new test bench which will be able to test this function of the product.
5 Experimental researches
The analysis done before was applied to a n experimental research. The aim of the research
was to test the validity of the analysis made in this work and to develop a coherent
methodology for testing the caulked assemblies. The input parameters are presented in table
4.

Table 4. Input parameter for the experimental research

Name Values Measuring unit
H 2.65 [mm]
ØDp1 25.63 [mm]
ØDp2 25.48 [mm]
ØDc 2.85 [mm]
r 0.82 [mm]
F 76 [KN]

The visual inspection reveals a good shape of the caulked assembl y.
At the testing of the tightness using helium, the speed of the decreasing pressure inside
the caulked assembly – gas conditions. (Sp gas) – result 7.8 * 10E -10 mbar*l/s . As a result,
the assembly tightness function regarding gas pressure is assured.
At the testing of the tightness using – liquid (Sp liq) – the assembly was tested under a
pressure of 20 bar for a time of 20 minutes – without any leakage drop . The assembly
tightness function regarding liquid is assured.
The t est for burst pressure (Bp limit te st) – was done for the burst pressure limit and the
value is ca. 363 bar . The assembly is assured from the burst limit point of view.
The experiment reveals the fact that for the geometrical parameters presented in the table
all the requirements for the fi nal caulked product are respected.
Experimental research revealed the algorithm necessary to fully evaluate a caulked
assembl y.

6 Methodology for testing the caulk ed assemblies
The functional analysis done before revealed that tightness, resistance and reliability are the
most significant functions for the caulked assembly. Actual testing methods are focused on
tightness and on resistance. The logical order of doing these tests is presented in figure 16.
First the gas pressure test is done. If the assembly pass the test, then the liquid pressure test
is done. If the assembly passes this test too, than the burst pressure test is done.

Fig 1 6. Algorithm of testing caulked assembl ies

The study shows that the reliability function is not analysed. There is a need of developing
a new stand for reliability testing.
7 Conclusion
The paper highlights the caulking process as a systemic process. The input data and the output
data of the process were presented together with the methods of analysing the performance
of the caulking assembly.
The actual knowledge and test equipment allowed us to perform an experimental research
which reveals the validity of the measurements methods.
The performed analyse and the presented algorithm allows a complete and complex
assessment of the performances regarding the caulking process.
In the fut ure we propose to study the optimization of the input parameters in order to
obtain precise assemblies for different materials and also for different dimensions of the parts.
This methodology can be used to achieve optimization of the process in order to p rocess
parts of various materials and also with different dimensions.
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