optimisation of shock absorber parameters using failure mode and effect analysis and taguchi method
TRANSCRIPT
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OPTIMISATION OF SHOCK ABSORBER PARAMETERS USING
FAILURE MODE AND EFFECT ANALYSIS AND TAGUCHI
METHOD
A.Mariajayaprakash
Assistant Professor and Head, Department of Mechanical Engineering,
Rajiv Gandhi College of Engineering and Technology,Puducherry, India;
e-mail : [email protected]
Dr.T. SenthilVelan
Professor and Head , Department of Mechanical Engineering,
Pondicherry Engineering College, Puducherry, India;e-mail: [email protected]
K.P.Vivekananthan
Deputy General Manager (manufacturing),TENNECO, Puducherry, Indiae-mail: [email protected]
ABSTRACT
The various process parameters affecting the quality characteristics of the shock absorber
during the process were identified by using the Ishikawa diagram and FMEA. The
identified process parameters are: Welding process parameters (squeeze, heat control,wheel speed and air pressure), tamper sealing process parameters(load, hydraulic pressure
, air pressure and fixture height),Washing process parameters(total alkalinity,
temperature, PH value of rinsing water and timing),Painting processparameters(flowability, coating thickness, pointage and temperature).Out of above
mentioned four process parameters welding and tamper sealing process parameters were
optimized by using Taguchis method in the earlier work. In this paper, the remainingprocess parameters namely washing and painting process parameters are optimized by
using Taguchis method.
Key words: Taguchis method, Ishikawa diagram, ANOVA, FMEA
INTERNATIONAL JOURNAL OF MECHANICAL ENGINEERING
AND TECHNOLOGY (IJMET)
ISSN 0976 6340 (Print)ISSN 0976 6359 (Online)
Volume 3, Issue 2, May-August (2012), pp. 328-345
IAEME: www.iaeme.com/ijmet.html
Journal Impact Factor (2011):1.2083 (Calculated by GISI)www.jifactor.com
IJMET
I A E M E
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LIST OF SYMBOLS
r Number of tests in a trial
yi Response value of observation in the ith test
DOE Design of experiments Degrees of freedom
A Degrees of freedom for factor A
kA Number of levels for factor A
required Total degrees of freedom required
LN Total degrees of freedom of the available orthogonal array
N Number of trials
ANOVA Analysis of varianceM Overall mean percentage defects
VFactor Variance of factor
SS Sum of squareV Expected amount of variation
P Percent contribution
Mean
Level of riske Degrees of freedom for the error
Ve Error variance
F(, 1, e) F ratio required at the level of riskeff Effective number of replications
n Total number of experiments
CI Confidence interval
T Average values of defects at different levels
1.0 INTRODUCTION
Shock absorber is one of the major component used in automobiles. It is used to absorb
vibrations when the vehicle is moving. Otherwise that vibrations affect the vehicle andthe rider. The defects which are occurred during the process affect the quality of the
shock absorber. In order to minimize the defects and improve the quality, the following
tools are employed. First, Cause and effect diagram or Ishikawa diagram is used foridentifying the parameters that may affect the quality of the shock absorber during the
process. Then, Failure Mode Effect Analysis tool is applied to find out the most
significant process parameters that may affect the quality of the shock absorber. Finally,
Taguchi method is used to optimize the process parameters. Taguchi method is anefficient and powerful tool that can reduce the experimental trails necessary to determine
the optimal conditions. It has been widely used in many different fields (Yung-Tien Liu
et al., 2010).
Tsai et al. (2008) used Taguchi design experiments to identify the optimal abrasive jet
polishing parameters when applied to the polishing of electrical discharged machinedSKD 61 mold steel specimens. This paper shows that ANOVA provides an indication of
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the significance of the individual control factors. Mahapatra and Vedansh Chaturvedi
(2009) proved that Taguchis experimental design is a simple and systematic way ofanalyzing a complex process with less experimental design.
In this paper, test parameters are optimized for minimum wear by using Taguchis
method. Ganapathy et al. (2009) optimized the operating parameters in Jatropha bio
diesel engine by using Taguchi method.In this paper it is proved that signal to noise ratio is used to optimize the various input
parameters of the model. This paper shows that the Taguchi approach based
thermodynamic model has improved the performance parameters slightly. Pishbin et al.(2010) investigated the effects of processing parameters, including suspension
concentration, pH and electric field by using Taguchis DOE approach in electrophoretic
deposition of bio glass suspension. Mojtaba momeni et al. (2010) obtained the optimalcondition for DOS measurement based on Double- loop EPR technique in H2 SO4
containing KSCN solution using the Taguchi method. In this paper, it is concluded that
higher the S/N ratio, greater is the effect on the DOS measurement. Te-Sheng Li et al.(2009) explained how Taguchi method is utilized for optimizing the parameters in
thermal flow techniques for sub-35 nm contact-hole fabrication. In this article, theoptimal thermal flow parameter settings are determined by signal to noise ratio and
ANOVA analysis. Balamurugan Gopalsamy et al. (2009) applied Taguchi method to findoptimum process parameters for end milling while hard machining of hardened steel.
Results obtained from Taguchi method closely match with ANOVA and it is found that
cutting speed is most influencing parameters corresponding to quality characteristics.
Khandoker abul Hossain et al. (2010) illustrated that the cause and effect diagram
provides the solution to reduce the emission of co2. Andrejkovic et al. (2011) explainedthat Ishikawa diagram quickly identify the causes of quality problems. Tlale et al. (2008)
applied the usage of cause and effect diagram in cost drivers for manufacturing process.
Hu-chen liu et al. (2011) reported about the usage of FMEA tool and how it is appliedto identify the failure modes. Arabian-Hoseynabadi et al. (2010) explained that theFMEA tool is applied in wind turbine systems. In this paper, the reliability of the wind
turbine system is improved by applying FMEA tool. Boldrin et al. (2009) identified and
reduced the risks of failure in ITER NB injector using FMEA tool. .
2.0 CAUSE AND EFFECT DIAGRAM
Cause and effect diagram is a tool for identifying the root causes of quality problems. It is
also called as Ishikawa diagram or Fishbone diagram ( Ilie and Ciocoiu , 2010).An
Ishikawa diagram is constructed as shown in Fig.1. By using this diagram the various
process parameters that affect the quality of the shock absorber are identified ( Mohansen and shan, 2005).The identified process parameters are listed below:
1.Washing parameters
2.Welding parameters3.Painting parameters
4.Tamper sealing parameters
5.Assembly parameters6.Lubrication parameter
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7.Inspection and Checking Parameters
8.Dimensioning parameters9.Polishing parameter
10.Testing parameters
DEFECTSIN
SHOCK
ABSORBR
DEFECTSINSHOCK
ABSORBR
painting
DIMENSIONING
INSPECTION/CHECKING POLISHING
Paint notadheringRustcorrosion
Pointagetemperature
FlowabilityCoatingthickness
Cylinder notball sized
Ballsizing
Cylinder notball sized
1.concentricity2.cylinderbendchecking
1.Breakage2.burrs
1.Concentricity morethan specification2.wrong jaws3.worn out jaws
.
Setupnot correct
Poorsurfacefinish
Bad stonecondition
WASHING
Improperfrequency ofchange
Presence ofoil/grease
1.wrong parameters2.improperpreparationof solvent
Fig.1Cause and effect diagram
3. FAILURE MODE AND EFFECT ANALYSIS
After constructing Ishikawa diagram, it is important to perform Failure Mode Effect
Analysis. FMEA is a powerful tool which is used to define, identify and eliminate known
potential failures, problems, errors and so on (hu-chen liu et al, 2011).Today, FMEAtechnique has been applied in many places, such as automobiles, aerospace, military,
electricity and mechanical industries. FMEA is conventionally carried out by a team of
engineers. By using their knowledge and past data, Risk Priority Number(RPN) value isassigned for each failure component.(Zaifang Zhang and Xuening Chu, 2011). RPN is the
product of the occurrence (o), severity(s), and detection (d) of a failure. The three risk
factors are evaluated using 10 points scale. Failure modes with higher RPN values areassumed to be more important and are given higher priorities than those with lower RPN
values (Ying ming wang et al., 2009). In this paper, washing and painting process
parameters are having higher RPN values. Hence these two process parameters areconsidered as most significant process parameters.
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4. TAGUCHIS METHOD
Taguchi method is an efficient tool for acquiring optimal process parameters. In thismethod, number of experiments are reduced. In this method two important tools are used.
i) orthogonal array and ii) Signal to noise ratio ( Cevdet Gologlu and Nazim Sakarya,
2008).
First, experiment parameter factors and their levels are selected. In this study, fourpainting process parameters are used as control factors and each parameter is designed to
have three levels ( kilickap, 2010).The selected process parameters, ranges and their
levels are shown in Table 1.
Table 1 Process parameters with their ranges and values at three levels
Parameter
designationProcess parameters Range Level 1 Level 2 Level 3
A Flowability ( sec) 15-25 15 20 25
B Coatingthickness(microns)
20-50 20 40 50
C pointage 6-12 6 8 12
D Temperature ( C) 20-50 2 35 50
4.1. Selection of Orthogonal array
An orthogonal array for process design is applied on the knowledge of control factors and levels.The number of experiments can be reduced by using orthogonal array.( yu - tang yen et
al.,2010).The degrees of freedom for the orthogonal array should be greater than or at least equal
to those for the process parameters. In this study, L9 orthogonal array having 8 degrees of
freedom is selected ( Anawa and Olabi, 2008).This orthogonal array has four columns and nine
experiments runs and it is shown in Table 2.
Table 2 L9 orthogonal
array Table 3 Experimental L9 array
Run A B C D
1 1 1 1 1
2 1 2 2 2
3 1 3 3 3
4 2 1 2 3
5 2 2 3 1
6 2 3 1 27 3 1 3 2
8 3 2 1 3
9 3 3 2 1
Trial
no.A B C D
Flow
ability
( sec)
Coating
thickness
(microns)
pointageTempe-
rature (0C)
1 15 20 6 20
2 15 40 8 35
3 15 50 12 50
4 20 20 8 505 20 40 12 20
6 20 50 6 35
7 25 20 12 35
8 25 40 6 50
9 25 50 8 20
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4.2. Conducting the Experiment
After selecting the orthogonal array, the factors at different levels are assigned for eachtrial. The assigned experimental array is shown in Table 3.The experiments are conducted
using single randomization technique. Each experiment is repeated three times for the
same set off parameters.(Ross,1988 ; Hazura Mohamed et.al.,2008).In each trial, the
defects occurred are recorded and it is shown in Table 4. In this work, the defects thatoccur during the welding, washing, painting, and tamper sealing process are considered.
The percentage of the defects for each repetition was calculated by using the given
formula, and then the average of the defects were determined for each trial condition andit was shown in Table 5.
No. of defects occurring due to Painting processPercentage of defects =
Total no. of defects occurring in the process
(welding, washing, painting and tamper sealing)
Table 4 Defects and number of defects occurred during the experiments
Defects No. of defects
Trial
1Trial 2 Trial 3
paint not
adheringrusting
not
drying
not
cleaning4 4 4
paint not
adhering1 2 1
paint not
adhering
manual
coating
powder
deposit
over
heating4 4 4
powder deposit not drying 2 2 2
rusting not cleaning 2 2 2
paint not comingpowder
deposit2 2 2
paint not coming rustingover
heating2 4 4
paint not coming not drying 3 2 2
21 24 23
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Table 5 Painting defects values and signal to noise (S/N) ratios against trail numbers
4.3. S/N Ratio
Taguchi technique utilises the signal to noise ratio(S/N) approach to measure the qualitycharacteristic deviating from the desired value. S/N ratio is used as an objective function
for optimizing parameters. (Gunawan Setia Prihandana et.al., 2009).Control factors are
easily adjustable and it is set by the manufacturer. These factors are most important indetermining the quality characteristics. Noise factors are difficult, impossible, or
expensive to control (weather, temperature, humidity etc.). The S/N ratio is the ratio of
mean (signal) to the standard deviation (noise). There are several S/N ratios availabledepending on the type of characteristics
( Mahapatra et.al., 2008 ; Sanjit Moshat et al., 2010).Usually three types of S/N ratio
analysis are applicable:
1) smaller is better
n
= -10 log 1/n yi2
i=1
2) nominal is the bestn
= -10 log 1/n 2/2
i=1
3) higher is better.
n
= -10 log 1/n 1/yi2
i=1
= S/N ratio, yi = value of quality characteristic at ith setting, = Mean,n= total number of trial runs at ith setting, = standard deviation.
Trial
no.% of defects in experiment
1 2 3 average S/N ratio
1 4.00 5.00 5.19 4.73 -13.55
2 2.67 2.50 1.30 1.71 -7.00
3 5.33 5.00 5.19 5.18 -14.28
4 1.33 3.75 2.60 2.56 -8.77
5 2.67 2.50 2.60 2.59 -8.26
6 2.67 2.50 2.60 2.59 -8.26
7 2.67 2.50 2.60 2.59 -8.26
8 2.67 3.75 5.19 4.31 -12.06
9 4.00 2.50 2.60 3.03 -9.85
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In this study, in order to minimize the defects, smaller is better S/N ratio is chosen.Smaller is better S/N ratios are computed and the values are recorded in table 5. The
main objective of shock absorber process is to minimize the defects by determining
optimal level of each factor. Since log is monotone decreasing function, it implies that
we should maximize the S/N ratio.(Fang-Jung Shiou and Chih-Cheng Hsu, 2008).Theaverage values of the painting defects and S/N ratios for each parameter at different
levels are calculated and are recorded in Table 6.
Table 6. Average values of painting defects and S/N ratios at different levels
Factors Level 1 Level 2 Level 3
Painting
defects
S/N
ratios
painting
defects
S/N
ratios
painting
defects
S/N
ratios
A 4.02 -11.61 2.58 -8.43 3.16 -10.06
B 3.29 -10.19 2.87 -9.10 3.60 -10.80C 3.73 -11.29 2.58 -8.54 3.45 -10.27
D 3.45 -10.55 2.45 -7.84 3.87 -11.70
The values given in table 6 are plotted in fig 2 and fig 3 .Fig 2 and fig 3 show the average
values of casting defects and S/N ratios for each parameter at different levels. From fig2and fig.3 it is clear that the casting defects are minimum at the parameter levels A2, B2,
C2 and D2 and the S/N ratios are maximum at the same levels of the parameters (A2, B2,
C2 and D2.). Since a higher S/N ratio means a better quality characteristic (minimumdefects), the optimal combination of control factor levels is therefore determined as
A2B2C2D2.
A1 A2 A3 B1 B2 B3 C1 C2 C3 D1 D2 D3
Fig 2 Average values of painting process defects for each parameters
at different levels
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A B C D
1 2 3 1 2 3 1 2 3 1 2 3Fig.3Average values of S/N ratios for each parameter at different levels
4.4. Analysis of Variance (ANOVA)
The purpose of ANOVA is to investigate which process parameters significantly affect
the quality characteristic. The total variation may be decomposed into manycomponents.In this paper, the total variation present in the process is decomposed to the
following components:
1. variation due to factor A2. variation due to factor B
3. variation due to factor C4. variation due to factor D
5. variation due to error
The total variation is calculated using the values given in table 4.
Total variation SST = SSA +SSB +SSC +SSD +SSe
Variation due to error SSe = SST (SSA +SSB +SSC +SSD)
Total degrees of freedom T =(A+B+C+D +e)
e =T (A+B+C+D)
= 26 (2+2+2+2)= 18
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Table. 7 ANOVA for painting defects and S/N ratios
sourcesum of
squares
Degrees of
freedom variance F ratio
Painting
defects
S/N
ratio
Painting
defects
S/N
ratio
Painting
defects
S/N
ratio
Painting
defects
S/N
ratio
A 9.44 14.97 2 2 4.72 7.49 8.90 24.87
B 2.40 4.20 2 2 1.20 2.10 2.26 6.98
C 6.44 11.39 2 2 3.22 5.70 6.07 18.92
D 9.63 23.41 2 2 4.81 11.70 9.07 38.89
error 9.56 0.60 18 2 0.53 0.30 1.00 1.00
total 37.47 54.57 26 8 1.44 6.82
The results of analysis of variance (ANOVA) are shown in table 7. In table 7, it is clearthat the parameters A, C and D significantly affect both mean and variation in the
painting defects.
5. RESULTS AND DISCUSSIONS
5.1. Percent contribution
Percent contribution is the function of the sum of squares of each significant item.Percent contribution to the total sum of square can be used to evaluate the importance of
a change in the process parameter on these quality characteristics
(Lakshminarayan and Balasubramanian, 2008; Balamurugan Gopalsamy et al.,2009). It is
calculated using the formulae given below:
Percent contribution (P) = (SSA/ SST) *100SSA = SSA (e) (A)
VA = VA+ V error
where VA is the expected amount of variation due solely to factor A given below:
VA = SSA /A
VA = SSA /A
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Table 8 ANOVA for painting defects, including percent contribution
source
Sum
ofsquares
Degrees
offreedom
Variance F ratioExpected
SS'
Percent
contribution(P)
A 9.44 2 4.72 8.90 6.94 18.52
B 2.40 2 1.20Pooled(2.6)
C 6.44 2 3.22 6.07 6.44 17.19
D 9.63 2 4.81 9.07 9.63 25.69
error
(pooled)9.56 18 0.53 1.00 14.46 38.60
total 37.47 26 1.44 37.47 100.00
Table 9 S/N ratios for painting defects, including percent contribution
sourcesum of
squares
Degrees
offreedom
Variance F ratioExpected
SS'
Percent
contribution(P)
A 14.97 2 7.49 24.87 12.72 23.31
B 4.20
C 11.39 2 5.70 18.92 9.68 17.73
D 23.41 2 11.70 38.89 19.89 36.44
error(pooled) 0.6 2 0.30 1.00 12.29 22.52
Total 54.57 8 6.82 54.57 100.00
The percentage of contribution of flowability, , pointage, and temperature is shown in
table 8.
5.2. Estimating the meanFrom table 8, it is clear that factor B has the least effect on the quality characteristic. In
order to prevent over estimation, factor B is not considered and the estimation of meanfor painting defects is calculated by the following equation: (Te sheng Li et al., 2009)
=T+ (A2-T) + (C2-T) + (D2-T)
where T is the average values of painting defects at different levels.The mean for a selected trial condition for parameters at (A2,B2,C2,D2) is 1.11.
5.2.1. Confidence Interval around the estimated mean
An important step in Taguchis optimization technique is to conduct confirmation
experiments for validating the predicted values. Thus a 95% confidence interval (CI) for
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the predicted mean of optimum quality characteristic on a confirmation test is estimated
using the following two equations : (Horng Wen Wu and Hui Wen Gu, 2010).CI3 = [ F( , 1, e) Ve (1/eff +1/r)]1/2
eff = N/ (1+ total DOF associated in the estimate of mean )
where is the level of risk, Ve is the error variance, e is the degrees of freedom for the error,
eff is the effective number of replications and r is number of test trials.
Using the values in Table 5, the CI was calculated as follows
eff = N/ 1+ (total DOF associated in the estimate of mean )
= 27/ (1+2)= 9
= 1- confidence limits ( 95%)= 0.05
F ratio = (1, 0.05, 18)
= 4.41 (tabulated)Confidence Interval CI3 = 1.02
The 95% confidence interval of the predicted optimum of the shock absorber defect is:0.09
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Table 10 Optimum parameters under economic considerations
Parameter
designationParameter
Optimum
levels
Optimum
value
A Flowability (sec) 2 20B
Coating thickness(microns)
2 40
C pointage 2 8
D Temperature (oC) 2 35
In the same manner calculations are carried out and optimum values are found for
washing process.
Table 11 Process parameters with their ranges and values at three levels
Parameterdesignation
Process parameters Range Level 1 Level 2 Level 3
A Total Alkalinity (g/lit.) 40-80 40 70 80
B Temperature(oC) 45-75 45 60 75
C Rinsing water (PH) 5.5 -8.5 5.5 6.5 8.5
D Timing (min) 0-25 5 15 25
Table 12 Defects and number of defects occurred during the experiments
Defects defects number
oil presence over heatedmore time
taken3 2 3
oil presence over heated 1 2 1
oil presencepowder
deposited2 2 2
oil presence more time taken 1 2 1
oil presence 1 1 1
oil presence 1 1 1
over heated 1 1 1
over heated more time taken 1 1 2
over heated oil presenceMore time
taken1 3 2
12 15 14
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Table 13 washing defects values and signal to noise (S/N) ratios
against trail numbers
Trial
no.% of defects in experiment
1 2 3 average S/N ratio
1 4.00 2.50 3.90 3.47 -10.96
2 1.33 2.50 1.30 1.71 -5.10
3 2.67 2.50 2.60 2.59 -8.27
4 1.33 2.50 1.30 1.71 -5.10
5 1.33 1.25 1.30 1.29 -2.24
6 1.33 1.25 1.30 1.29 -2.24
7 1.33 1.25 1.30 1.29 -2.24
8 1.33 1.25 2.60 1.73 -5.279 1.33 3.75 2.60 2.56 -8.77
Table 14 Average values of washing process defects and S/N ratios
at different levels
Factors Level 1 Level 2 Level 3
washing
defects
S/N
ratios
washing
defectsS/N ratios
washing
defects
S/N
ratios
A 2.59 -8.11 1.43 -3.19 1.86 -5.43
B 2.16 -6.10 1.58 -4.20 2.15 -6.43
C 2.16 -6.16 2.00 -6.32 1.73 -4.25
D 2.44 -7.32 1.43 -3.19 2.00 -6.21
Table 15 ANOVA for washing process defects and S/N ratios
sourcesum of
squares
Degrees of
freedomVariance F ratio
washing
defects
S/N
ratio
washing
defects
S/N
ratio
washing
defects
S/N
ratio
washing
defects
S/N
ratio
A 6.14 36.25 2 2 3.07 18.13 7.51 112.84
B 1.98 8.54 2 2 0.99 4.27 2.42 26.58
C 0.87 7.85 2 2 0.44 3.93 1.07 24.45
D 4.59 27.30 2 2 2.30 13.65 5.62 84.97
error 7.36 0.32 18 2 0.41 0.16 1.00 1.00
total 20.95 108.23 26 8 13.53
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Table 16 ANOVA for washing process defects, including percent contribution
sourcesum of
squares
Degrees
offreedom
Variance F ratioExpected
SS'
Percent
contribution(P)
A 6.14 2 3.07 7.51 5.32 25.40
B 1.98 2 0.99 pooled(2.42)
C 0.87 2 0.44 pooled(1.06)
D 4.59 2 2.30 5.62 3.78 18.02
error
(pooled)7.36 18 0.41 1.00 11.85 56.58
total 20.95 26 20.95 100.00
Table 17 S/N ratios for washing process defects, including percent contribution
sourcesum of
squares
Degreesof
freedom
Variance F ratioExpected
SS'
Percent
contribution(P)
A 36.25 2.00 18.13 112.84 35.93 33.20
B 8.54 2.00 4.27 26.58 8.22 7.59
C 7.85
D 27.30 2.00 13.65 84.97 26.98 24.93
error
(pooled) 0.32 2.00 0.16 1.00 37.10 34.28
Total 108.23 8.00 13.53 108.23 100.00
The mean for a selected trial condition for parameters at (A2, B2, C2, D3) is 0.90.
The 95% confidence interval of the predicted optimum of the shock absorber defect is:
0.13< 0.90 < 1.67.
Table 18 Optimum parameters under economic considerations
Parameter
designation ParameterOptimum
levels
Optimum
value
A Total Alkalinity (g/lit.) 2 70
B Temperature(oC) 2 60
C Rinsing water(PH) 2 7.5
D Timing(min) 3 10
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6. Conclusion
In this study, Taguchi method has been employed for optimizing the process parametersof painting and washing during shock absorber manufacturing and the following
conclusions are drawn:
* It is proved that, the quality of shock absorber is improved by Taguchis method at the
lowest possible cost.* Ishikawa diagram or cause and effect diagram is very effective to sort out all the
possible causes affecting the shock absorber quality.
* The most significant parameters affecting the quality of the shock absorber areidentified by using the FMEA tool.
* The parameter pointage is significantly affect the painting process and parameter timing
is significantlyaffect the washing process.
* The optimum levels of flowability, coating thickness, pointage, and temperature in
painting process and total alkalinity, temperature, rinsing water, and timing in washingprocess are estimated.
* The predicted range of optimum painting defects is 0.09
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