International Journal of Mechanical And Production Engineering, ISSN: 2320-2092,
Volume- 2, Issue- 4, April-2014
IMPLEMENTATION OF SIX SIGMA TECHNIQUES IN LOAD CONSCIOUS REGULATING VALVE ASSEMBLY LINE 1
SARAVANAMUTHU M, 2OMKUMAR M, 3SENTHIL KUMAR V, 4KRISHNAMOORTHY S
1-4 Dept. of Manufacturing Engg, College of Engineering, Guindy, Chennai Email:
[email protected],
[email protected],
[email protected],
[email protected]
Abstract— Automobile Industry in the world has grown exponentially in the recent decades. Various business models are adopted for the growth of the Industry. Among this Six Sigma is a problem solving approach which focuses on the variation reduction in any manufacturing or service industry. Six Sigma uses structured problem solving approach as DMAIC ( Define, Measure, Analyse, Improve and Control ). The project work is carried out in Load Conscious Regulating Valve assembly which is used in Automobile braking system to avoid rear wheel locking. In this project LCRV valve assembly line is chosen based on Pareto Analysis. Various Six Sigma tools such as Repeatability and Reproducibility Study, Component Search Method are used to identify the root cause for the problem. Based on the root cause analysis improvement actions are taken to reduce the defect. In this project the material of the pressing tool plays a significant role. Various tool materials are chosen and trials conducted for better performance. Latest coatings like Physical Vapor Deposition and Chemical Vapor deposition are explored. PVD coating using TiAlN is coated as the countermeasure. The results are validated through statistical methods. Through Six Sigma techniques the defect in the LCRV assembly line is reduced. Keywords: Component Search, DMAIC, Physical Vapor Deposition, Repeatability & Reproduceability, Six Sigma
I.
granularity. Instead, they wanted to measure the defects per million opportunities. Motorola developed this new standard and created the methodology and needed cultural change associated with it.
INTRODUCTION
Six Sigma at many organizations simply means a measure of quality that strives for near perfection. Six Sigma is a disciplined, data driven approach and methodology for eliminating defects (driving towards six standard deviations between the mean and the nearest specification limit) in any recess from manufacturing to transactional and from product to service. The statistical representation of six sigma describes quantitatively how a process is performing. To achieve six sigma, a process must not produce more than 3.4 defects per million opportunities. The fundamental objective of six sigma methodology is the implementation of measurement based strategy that focuses on process improvement and variation reduction through the six sigma tools and techniques. The roots of Six Sigma as a measurement standard can be traced back to Carl Frederic Gauss who introduced the concept of Normal curve. Six Sigma as a measurement standard in product variation can be traced back to the 1920,s when Walter Shewart showed that three sigma from the mean is the point where a process requires correction. Many measurement standards later came on the scene but credit for coining the term “Six Sigma “goes to Motorola Engineer named Bill Smith. Six Sigma was first developed by Mr. Bill Smith at Motorola with the strong commitment from Mr. Bob Galvin, the CEO of Motorola in 1986. The initiative helped Motorola win the first Malcom Balridge award in 1988. In the early and mid1980 s with Chairman Bob Galvin at the helm, Motorola engineers decided that the traditional quality levels measuring defects in thousands of opportunities didn’t provide enough
II.
LOAD CONSCIOUS REGULATING VALVE ASSEMBLY
Load Conscious Regulating Valve (LCRV) is a rear brake pressure regulating system, which responds to change in weight and weight distribution in the vehicle. It has a valve connected the vehicle’s axle by pivotally connected linkages, a bell crank, a sensing spring and piston actuating lever. LCRV system includes the braking force regulator with shut off device, which is applied to a rear brake system of a large sized vehicle with relatively large load and regulates the brake force of the rear wheel according to the loading condition of the vehicle. The LCRV system applies the hydraulic pressure generated from a master cylinder to the rear wheel brake circuit according to the loading condition of the vehicle. The LCRV Valve has a hydraulic unit as shown in The LCRV system prevents the rear wheel from locking and generate the smooth braking force by transmitting the appropriate amount of hydraulic pressure to the rear wheel by sensing an unloaded state, loaded state and weight transfer state. III.
CASE STUDY
The project is selected is based on the need to reduce the rejection in Valve assembly process. The product is selected based on Pareto Analysis. There is a huge amount of rejection 4% in LCRV assembly process.
Implementation Of Six Sigma Techniques In Load Conscious Regulating Valve Assembly Line 42
International Journal of Mechanical And Production Engineering, ISSN: 2320-2092,
The defect is called as stroke out i.e. when pressure applied at the rate of 30 Bar the plunger should move within the range of 1.5mm to 2.0mm. If it is not meeting the specification it is defined as stroke out.
V. COMPONENT SEARCH METHOD For defect reduction exercise the response can be considered as ‘ Y ‘ and the suspected variables can be considered as X1, X2 etc. then it can be represented as mathematical form as Y = f(X)
The LCRV Assembly Line has 7 stages in which the last stage is performance testing in which the Valve is 100 % tested for performance.
Where, Y = LCRV not passing the stroke out test ( Response is variable ) X = x1 – Assembly Function x2 – Input Child parts
1. Sleeve Ball peening, 2. Ball seating test, 3. Plunger sub assembly, 4. Sleeve crimping, 5. Main Assembly, 6. Low Pressure testing, 7. Performance testing.
VI. IV.
REPEATABILITY & REPRODUCEABILITY STUDY
D = Difference between median of both Good and Bad. d = Average of the range. Then D/d is calculated Component Search Rule : If D/d < 3 then assembly process is creating the problem. If D/d > 3 then parts in the assembly is creating the problem.
G a g e R & R S t u d y fo r S t ro k e O u t S u m m a ry R ep o r t
0% 10 %
30 %
S t u d y I n fo r m a t i o n
1 00 %
Y es
No 5 .9 % 5 . 9 % o f a ll p r o ce s s v a r ia t io n ca n b e a t t r ib u t e d to t h e m e a s u r e m en t s y s te m . T h e p r o ce s s v a ria t io n is e s tim a te d f r o m t h e p a rt s in t h e s t u d y .
30 %
1 00 %
Y es
No 1 1 .8 % M e a s u r e m en t s y s te m v a r ia t io n eq u als 1 1 . 8 % o f t h e t o le r a n c e .
V a r i a t io n B r e a k d o w n Is t h e re a p ro b le m w it h r ep e at a b ilit y o r re p ro d u c ib ilit y ? % of Pr oc e s s % of T o l e r a n c e
45
N u m b e r o f p a r t s in s t u d y N u m b e r o f o p er at o rs in s t u d y N u m b e r o f re p lic a t e s (R ep lic a t e s : N u m b er o f tim e s ea c h o p e r a t o r m e as u r e d e a ch p a rt )
10 3 3
VII.
C o mm en ts
C a n y o u so r t g o o d p a r t s f r o m b a d ? 0% 10 %
For finding out which child part is contributing to the problem swapping analysis to be done. One Good assembly and one bad assembly is dismantled. The body machining is interchanged from Good to Bad and the response is measured. During this swapping there is no change in response i.e. the good remains good and bad remains bad. Hence it is concluded that the interchanged part is not contributing to the problem. This can be represented as shown in Fig 3.
E x a m in e t h e b ar c h a rt s h o w in g t h e c o m p o n e n t co n t rib u t io n s , a n d u s e t h is in fo r m at io n t o g u i d e im p r o v e m e n t s : - - T e s t -R e t e s t co m p o n e n t ( R e p e a t ab ilit y ): T h e v a ria t io n t h a t o c cu r s w h e n t h e s am e p e r s o n m e as u r e s t h e s a m e it em m u ltip le t im es . T h is a c c o u n t s fo r 9 3 . 2 % o f th e m e a s u re m e n t v a ria t io n . It is 5 .5 % o f t h e t o t a l v a ria t io n in t h e p ro c e s s . - - O p e r at o r c o m p o n e n t (R e p r o d u c ib ilit y ) : T h e v ar iat io n t h a t o c cu r s w h e n d if f er en t p e o p le m e a s u r e t h e s a m e it em . T h is a c c o u n t s fo r 3 6 . 1 % o f t h e m e a s u re m e n t v ar iat io n . It is 2 .1 % o f t h e t o t al v a ria t io n in t h e p r o ce s s .
15 10
0
T ota l Ga g e
R e p e a t a b i li t y
SWAP ANALYSIS
G e n e ra l r u le s u s e d to d e t e rm in e t h e c a p a b ilit y o f t h e s y s t e m : < 1 0 % : a c c e p t ab le 1 0 % - 3 0 % : m a r g in a l > 3 0 % : u n a cc ep t a b le
30
30
CALCULATION OF D/d Ratio
To find out the D/d ratio One Good assembly is taken and rated as 1 and one bad assembly is taken and rated as 5. The Initial values of Good and bad are 1 & 5 respectively. After that both the assemblies are dismantled and assembled and the response is measured and the corresponding values are 1 & 5 respectively. Similarly second disassembly and assembly is done and the response are noted as 1 & 5 respectively. The median and range for both good and bad assembly is calculated.
Before conducting the root cause analysis the measurement system in the process has to be qualified for its repeatability and reproduceability. The measurement system variation has to within 30 % of the total variation. In Performance testing LVDT is used to measure the stroke out distance when the 30 bar pressure is applied to the inlet port. R & R study has to be conducted for Performance testing. R & R study starts with selecting 10 different samples and 3 different operators. Each sample has to be repeated for 3 times. Totally there will be 90 readings. The R & R study calculates the repeatability and reproduceability individually and the total Gauge R & R takes into account both the factors. This R & R study can also be carried out using MiniTab Software Package as shown in Fig. 1 & Fig. 2 The R & R study reveals that the Total Gage R & R is 11.79 % of the total variation. It is well within 30 % limit. Hence the measurement system can be accepted. C a n y o u a d e q u a t e ly a s s e s s p r o c e s s p e r fo r m a n c e ?
Volume- 2, Issue- 4, April-2014
R e p r o du c ib il it y
Fig. 1
The R & R Calculation can also be calculated using manual calculation which is explained in below Table. Both the Software results and manual calculation reveals the same result. G a g e R & R S t u d y fo r S t r o k e O u t V a r ia t i o n R e p o r t
2 .0 0
X b a r C ha r t o f P a r t A v e r a g e s b y O p e ra to r A t le as t 5 0 % s h o u ld b e o u t s id e th e lim it s . (a ct u al : 1 0 0 . 0 % ) B C
A
1 .7 5 1 .5 0 R C h a r t o f T e st - R e t e st R a n g e s b y O p e r a t o r ( R e p e a t a b i l it y ) O p er at o rs an d p a rt s w it h la r g e r r an g e s h a v e le s s c o n s is t e n cy . 0 .0 4 0 .0 2 0 .0 0 R e p r o d u c ib il it y — O p e r a t o r b y P a r t In t e r a c t io n L o o k f o r ab n o r m al p o in t s o r p a t t er n s .
V a r ia t io n B r e a k d o w n % P ro c es s S tD e v (d a ta )
2 .0 0
So urc e
1 .7 5
T o t al Ga g e R e p ea ta b ility R e p ro d u c ib ilit y P a rt -t o - P a r t
0 .0 1 0 0 .0 0 9 0 .0 0 4 0 .1 6 8
5 .8 6 5 .4 6 2 .1 2 9 9 .8 3
1 1 .7 9 1 1 .0 0 4 .2 6 2 0 1 .0 1
P r o c e ss V a r ( d a ta )
0 .1 6 8
1 0 0 .0 0
2 0 1 .3 6
1 .5 0
% T o le ra n c e
T o ler an ce (u p p e r s p e c - lo w e r s p ec ): 0 .5
R e p r o d u c ib il it y — O p e r a t o r M a in E ff e c t s L o o k f o r o p e ra t o rs w it h h ig h e r o r lo w e r a v er a g es . 2 .0 0 1 .7 5 1 .5 0 A
B
C
Fig. 2 Implementation Of Six Sigma Techniques In Load Conscious Regulating Valve Assembly Line 43
International Journal of Mechanical And Production Engineering, ISSN: 2320-2092,
Volume- 2, Issue- 4, April-2014 Table 1
C
Si
Mn
S
P
Cr
Mo
Ni
0.36 /
0.10 /
0.45 /
0.040
0.035
1.00 /
0.20 /
1.30 /
0.44
0.35
0.70
max
max
1.40
0.35
1.70
Fig. 3
VIII.
ROOT CAUSE ANALYSIS
From the component search swapping analysis the sleeve in the assembly is contributing to the problem. During analysis a steel ball is pressed in the sleeve. The height of the ball from the sleeve centre is maintained within 11.03 / 11.07 mm in the Ball pressing machine. The ball pressing machine as shown in Fig. 4 consists of a pressing pneumatic cylinder with 300 N force. A LVDT is fitted on top of the pressing tool so that during pressing it measures the height of the ball from the sleeve. If the height is not within limit then the pressing tool presses once again. During pressing the steel ball creates deformation on the pressing tool. Because of this the pressing tool has a indentation mark. This will make a difference in contact point when compared to the new flat pressing tool. The change in contact point due to indentation creates a error in the measurement system.
Fig. 5
IX.
NEW PRESSING PIN
The pressing pin material is changed to NSOH Tool Steel ( Non shrinking Oil hardening tool steel. ) The Hardness of the pressing tool material is 60 ~ 65 HRc. Which same as Steel ball hardness. The diagram of the pressing pin is shown in Fig.6 and the composition is shown in Table 2. The steel ball is getting damaged due to pressing since the hardness of the pressing pin is higher than the steel ball as shown in Fig. 7. Table 2 C
Si
Mn
S
P
Cr
V
Tungsten
0.85 /
0.40
1.10 /
0.035
0.035
1.40
max
1.35
max
max
0.4 /
0.25
0.4 / 0.6
0.6
max
Fig. 4
The pressing pin is made of EN 24 steel. The hardness of the base material is 28 HRc. The pressing pin is hardened and tempered to 45 ~ 48 HRc. The pressing pin diagram is shown in Fig. 5 and the composition is shown in Table. 1. The indentation is shown in Fig. 5
Fig. 6 X.
Fig. 7
PHYSICAL VAPOUR DEPOSITION
New pressing pin is coated with Physical Vapor deposition technique using TiAlN material on EN 24 material as shown in Fig. 8. The tool is fitted on the machine and the results over more than one month it does not got any indentation on the tool. The variation is reduced because of the new tool. The rejection levels before and after condition are validated using hypothesis testing. Fig. 5 Implementation Of Six Sigma Techniques In Load Conscious Regulating Valve Assembly Line 44
International Journal of Mechanical And Production Engineering, ISSN: 2320-2092,
Volume- 2, Issue- 4, April-2014
ACKNOWLEDGEMENTS I express my sincere thanks to M/s. Brakes India Pvt. Ltd for providing the opportunity, knowledge and support for this project. I thank to my guide Dr. M. OMKUMAR, Associate Professor, Department of Manufacturing Engineering, Anna University for his encouragement, guidance. I thank Dr. G. ARUMAIKKANNU, Head, Department of Manufacturing Engineering, Anna University for giving me the opportunity to do this project.
Fig. 8
The coating adhesion with the base material is under laboratory testing. The coating provides improvement in Toughness value so that the indentation is avoided during impact loading.
REFERENCES [1] Lars Krogstie and Kristian Martinsen 2013, “ Beyond Lean and Six Sigma ; Cross Coloborative Improvement of Tolerance and Process variations – A case study “, The journal of Procedia CIRP, Vol. 7, pp. 610 – 615. [2] Rodica Pamfilie, Andreea Jenica Petcu and Mihai Draghici 2012, “ The importance of Leadership in driving strategic Lean Six Sigma management “, The journal of Procedia Social and Behavioural sciences, Vol. 58, pp. 187-196. [3] Rahani AR and Mohamed Ashraf 2012, “Production flow analysis through Value Stream Mapping : A Lean manufacturing process case “, The journal of Procedia Engineering, Vol. 41, pp. 1727-1734. [4] Adnan Bakri , Abdul Rahman Abdul Rahim , Noordin Mohamed Yusof and Ramli Ahmed 2012 , “ Boosting Lean Production via TPM “, The journal of Procedia Social and Behavioural sciences, Vol. 65, pp. 485-491.
CONCLUSION The project gives a insight knowledge on the subject of Six Sigma methodology to identify the root cause for the problem. The methods like R & R provides a checking methodology for the measurement system verification which is vital since the entire metrology depends upon the measurement instruments. Thorough knowledge on the root cause finding methodology and the latest engineering solutions provide a solution to the complicated problems.
Implementation Of Six Sigma Techniques In Load Conscious Regulating Valve Assembly Line 45
