Failure mapping through combination of process mapping, EPCA, FTA, and FMEA techniques Ualison Rebula de Oliveira Universidade Federal Fluminense, Brazil Henrique Martins Rocha (
[email protected]) Production Department, Rio de Janeiro State University – UERJ, Brazil Fernando Augusto Silva Marins Production Department, Sao Paulo State University – UNESP, Brazil
Abstract The increasing need to improve products and services quality, as well as customers’ satisfaction, has intensified the use of methods and techniques for process failure mitigation/elimination. This paper presents the integration of some of these methods in a wheel-tire assembly plant to identify the key operation failure elements: process mapping, expert critical process analysis (ECPA), fault tree analysis (FTA), and process failure mode and effects analysis (FMEA). This paper provided a general overview of each method/technique and demonstrated the use of them in a practical application, showing how those methods and techniques can be used together. Keywords: Process mapping, Expert critical process analysis, Fault tree analysis
Introduction As technology is advancing, several changes are perceived, such as consolidation of quality management systems, increasing rate of products and services customisation, lean manufacturing, etc. However, besides this overall development, failures keep on occurring in production processes, bringing problems to the operations, what requires constant prevention and control investments. Rausand and Oien (1996) define failures as being the events that determine the inability of an item to perform a required function or the termination of its ability to do that. The sources of failures can come from within the organization or from the environment where the organization belongs. Many factors can cause failures, such as mechanism deterioration, component failures, environmental conditions, or any combination of these factors (Blache and Shrisvastava, 1994). According to Almeida et al. (2006), due to the increasing need to improve products and services quality, as well as customers’ satisfaction, the use of methods and techniques for mitigation/elimination of process failures has been intensified. Those methods have as purpose to increase the odds of products and services perform their functions as expected. And, therefore, use a combination of methods and techniques can improve the performance 1
of the operations management. The question that comes up is how to bring together various techniques for prevention of the failures. This research presents the integration of some of these methods in a wheel-tire assembly plant: process mapping, expert critical process analysis (ECPA), fault tree analysis (FTA), and process failure mode and effects analysis (FMEA). These methods were used together to minimize/eliminate failures, and providing the basis steps that can be applied for problem identification into a PDCA cycle, i.e.: observation, analysis, and action plan. Theoretical background The combined use of methods for failure analysis is not new: Mahanti and Antoni (2005) highlighted the use of process mapping with FMEA; Sharma et al. (2007) proposed the use of RCA (Root Cause Analysis), NHPPP (Non-homogeneous Poisson Point Process), and FMEA; Shahin (2004) used the integration between FMEA and Kano model; Fernandes and Rebelato (2008) used QFD (Quality Function Deployment) with FMEA; Xie et al. (2000) suggested the use of Birnbaum's measure and Vesely-Fussel's measure and FTA methodology; and Rath (2008) combined Process Mapping with FMEA and FTA. The methods used into the present research are briefly described in this section. Process Mapping is defined as a technique of plotting a sector, department, or organization process, for an in-deep understanding (Cheung and Bal, 1998). Once processes are understood, it is possible to implement changes in the way the organization manages their processes, so that its strategic goals can be achieved (Tseng et al., 1999). According to Grover and Kettinge (1995), the main Process Mapping techniques are: flow sheet; map flow sheet; Integrated Computer Aided Manufacturing Definition (IDEF) – with its branches IDEF0, IDEF1, IDEF1x, IDEF2, IDEF3, IDEF4, IDEF5, and IDEF6 (Tseng et al., 1999) –; Unified Modelling Language (UML) Systematic Diagram; Service Blueprint; and Service Map. EPCA is a technique used to determine the essential steps of the process (Oliveira et al., 2010). Processes are segmented as critical and non-critical. Critical processes are those in which a failure impacts the whole system, jeopardizing organization results; non-critical processes are those which will hinder some process step; however the final objectives can still be achieved (Slack et al., 2007). Only critical processes would have developed their failure mapping. Lin and Wang (1997) define FTA as a graphical representation, associated to a particular system failure (effect), called ‘top event’, and the basic events (causes) called primary events or failure roots. According to Jung et al. (2005), the top event is broken down into a logical tree, showing the causes of the event by the use of the logic operators (gates) “and” and “or”. The FTA provides a logical breakdown of the failures, making possible to visualize the correlations between a primary cause, and an intermediary cause, with the top event (Long et al., 2000). FMEA can be described as a systematic methodology which makes possible to identify potential failures in the system, project, and/or process, aiming at eliminating or minimizing associated risks before the failure occurrence (Teng et al., 2006; Yang et al., 2006). According to Shahin (2004), FMEA is used to identify all potential failure modes, and determine the effect of each one on the system (product or process) by a deductive reasoning. 2
Procedures and techniques The research has been performed at a commercial vehicle manufacturing plant in the southern Rio de Janeiro state, Brazil. Several authors (Doran; Hill, 2009; Salerno, Camargo, and Lemos, 2008) studied this plant, due to its Modular Consortium model, in which the suppliers act directly on the final product assembly line, dividing physical space and responsibilities. A case study has been performed in a supplier that offers the wheel-tire set, and feeding the vehicle assembly line with more than 64,000 assembled units per quarter (as of second quarter 2011). The main objective of the study was to identify the operation key failure elements, so that countermeasures could be implemented to minimize (or eliminated) pitfalls. That has been done through the integration of Mapping Process with EPCA, FTA, and FMEA, showing how those methods and techniques can be used together. The integration mechanism of the four techniques is shown at Figure 1.
-
1 PROCESS MAPPING Definition of the process to be mapped, the mapping technique to be used and accomplishment of the mapping
1
2
EPCA Expert Critical Process Analysis
3
Detailing Level
2
FTA Failure Decomposition (top event) and its origins
P-1
3 P-3
P-9
4
P-8
P-10
FMEA Analysis of primary and intermediary failures effects
P-4 P-2
P-14
P-12
P-11
P-13
4
FMEA
+ Figure 1 – Process Mapping, EPCA, FTA, and FMEA combination
The study has started by the wheel-tire assembly process mapping, checking how process parts are physically linked and related. The map flow sheet and the process flow sheet developed can be seen at Figures 2 and 3. 3
Office
STATIO N 3
STATION 7
STA TION 1
Traditional pneumatic tire assembly process
STA TION 4
STATION 5
S TATION 6
STATION 2
S TATION 2 S TATION 6
STATION 3
STATION 4
S TATION 5
STATIO N 1
Tubeless tire assembly process
RING RIVETING
TIRE ASSEMBLER
BLOWER
BLOWER
BALANCE MACHI NE
FLIPPER
CONVEYOR
MATERIAL HANDLING CART
Figure 2 – Tire assembly map flow sheet
Start Mo ve ti re
Mou nt va lve
Lu bricate ti re
Move tire
Mou nt wheel -tire
Infl ate ti re No
Ye s
P ressu re ok?
Ba lan ce whe el-ti re No
Move wh eel -tire assy to stock Yes Ba lancin g o k? Stock whee l-tire a ssy
End
Figure 3 – Process flow sheet
4
Next it was applied the EPCA to identify critical processes on the mapped processes. Upon having a complete understanding about the whole system by the process mapping previously performed, interviews with the unit manager, supervisors, and direct manpower have been performed in order to establish the critical processes to be analyzed by FTA and FMEA. The non-critical processes were not studied, what would have made the research more complex and time-consuming. There was two assembly lines related to tubeless tires and traditional tires, each one with its own processes, and the first one has been selected, due to its higher volume (about 80% of the vehicles leave the factory with tubeless tires) and criticality. By means the interviews, results showed be converging, pointing the most critical process as being the tire-wheel balance, due to the fact that tire imbalance cannot be detected by naked eyes, as the occurrence of empty tire, that can be easily detected with a simple inspection. The next step includes failure analysis, being the most indicated tools the FTA and FMEA (Rath, 2008), since they provide guidelines for systems improvements through the discovery of aspects related to failures. The FTA has been used to identify the most relevant faults on the tire balancing process, with its respective intermediate causes/effects and initial causes (primary failures). Problems such as balancing machine with component worn out, tire weight out of specification, inappropriate set ups, failure in the machine display (or mistakenly reading), glue quality, and manpower faults have been pointed out as potential root causes, as it can be seen at Figure 4. FTA provides inputs to develop a FMEA, establishing relationships between cause and effect in advance. Finally, the last step of the process starts: the FMEA has been performed, detailing failure modes, effects, causes and recommended corrective/preventive actions for tire balancing, based on a scale from 1 to 10 for: a) the possibility of occurrence of a certain failure mode and/or a certain cause; b) severity of the impact of a certain failure mode on the process; and c) possibility of detection of a certain failure mode and/or cause. The scale has been established based on the following standards: • Occurrence: Improbable (1); Very low (2 to 3); Moderate (4 to 6); High (7 to 8); and Alarming (9 to 10). • Detection: High (1); Moderate (2 to 3); Low (4 to 6); Very low (7 to 8); and Improbable (9 to 10). • Severity: Noticeable (1); Low relevance (2 to 3); Moderate (4 to 6); Severe (7 to 8); and Very severe (9 to 10).
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Tire imbalance
inappropriate machine setup
Failure in balancing checking
weight low adherence
wrong weight location marking
tire weight out of spec
Labour
machine display failure
Lack of checking
machine display mistakenly read
Labour
Labour
glue quality
weight misplacement
Labour
bad weight marking operation setup
Labour
Labour
balancing machine worn out component
Figure 4 – FTA for Tire imbalance
The overall risk (or criticity), identified as RPN (Risk Priority Number) for each item, is determined by multiplying the factors of occurrence – O; severity – S; and detection - D, as it can be seen at Table 1, with priority for mitigating the failures by using countermeasure actions, - corrective and/or preventive. The following parameters have been considered to the RPN: Low (1 to 100); Moderate (101 to 300); and High (301 to 1000). The occurrence of a priority risk value on FMEA recommends immediate implementation of the following actions: qualification/certification of the suppliers of glue and tire (including a Statistic Process Control), preventive maintenance improvement of the machine, and manpower training.
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Table 1 – Actions after FMEA results
Weight out of spec Incorrect weight position marking due to machine deterioration
Tire balancing
Weight detachment
Digital display does not work properly
Function
Failure Mode
Effect
Balancing checking / verification inability
Posterior imbalance
Bad balancing
Balancing results deterioration
Causes
Control
O
D
S
RPN
Blown display fuse Broken display electrical wires
Visual inspection
4
2
5
40
None
1
7
5
35
None
3
7
5
105
View finder glass broken Glue quality problems Inadequate weight contact surfaces Reuse of weights
Visual Inspection
2
1
5
10
None
4
8
7
224
Surface cleaning
4
3
7
84
Discard used parts
2
8
5
80
Out of spec weights
Suppliers certification
3
8
9
216
Weight dimensional non conformity
Suppliers certification
4
5
7
140
Weight quality problems
Suppliers certification
4
9
6
216
Machine worn out
None
7
2
5
70
Display parts damaged
Equipment misuse
Training
4
6
6
144
Lack of lubrication
Periodic and systematic lubrication
2
4
8
64
Lack of maintenance
Preventive maintenance
4
6
7
168
Action
Display testing routine
Suppliers certification
Statistical process control Statistical process control Statistical process control
Intensify training on recurrent problems
Intensify preventive maintenance with shorter intervals
Results discussion and conclusion This paper introduced a combined use of Process Mapping, ECPA, FTA, and FMEA for the understanding of failure root causes on the assembly process of wheel/tire and establishes countermeasures for them. The proposed procedure aimed to provide a clear and objective way for analysing processes and identify its weakness, enabling rapid, efficient and accurate conclusions. The paper provided a general overview of each method/technique and demonstrated the use of them in a real case. After developing the map flow sheet, as the first step of the Process Mapping, the use of ECPA helped analysing some related areas and problems. Just critical areas and process have been selected, i.e.: tubeless tires and balancing. The procedure allowed expanding the 7
basic FTA events (which are conceptually non-expandable) as FMEA items. It also helped developing the FMEA, since the use of FTA makes easier to establish failure cause and effects. Based on analyses and results, countermeasures have been established to mitigate failures and/or their effects on the operation, such as supplier certification and statistical control process applied for the balancing of the manufacturing process. It can be concluded that the proposed procedure is feasible, providing a sequence of steps for understanding processes, identifying critical points, determining failures and their effects, and it prioritises the corrective and preventive actions, by means a process with minimization or elimination of the failures. The main contribution of the procedure is to offer a management process for the problems that can be replicated by companies involved into different business areas, in fact the procedure is like a framework to help them analysing failures occurring into their work processes. References Almeida, D. A.; Leal, F.; Pinho, A. F. & Fagundes, L. D. (2006), “Knowledge management in the analysis of failures: failures by mapping information system”, Revista Produção, Vol. 15, No. 1, pp. 171-188. (in Portuguese) Blache, M. K. & Shrivastava, B. A. (1994), “Defining failure of manufacturing & equipment”. In: Proceeding Annual Reliability and Maintainability Symposium, pp. 69-75. Cheung, Y. & Bal, J. (1998), “Process analysis techniques and tools for business improvements”, Business Process Management Journal, Vol. 4, No. 4, p.274-290. Doran, D. & Hill, A. (2009), “A review of modular strategies and architecture within manufacturing operations”, Proceedings of the Institution of Mechanical Engineers, Part D: Journal of Automobile Engineering, Professional Engineering Publishing, Vol. 223, No. 1, pp.65-75. Fernandes, J. M. R. & Rebelato, M. G. (2006). Proposal for a method to integrate QFD and FMEA. Revista Gestão e Produção, Vol.13, nº 2, pp. 245-259. (in Portuguese) Grover, V & Kettinger, W. J. (1995), Business process change - Reengineering concepts, methods and technologies, Idea Group Publishing, Harrisburg. Jung, W. S.; Yang, J. & Ha, J. (2005), “Development of measures to estimate truncation error in fault tree analysis”, Reliability Engineering and System Safety, Vol. 90, pp. 30–36. Lin, C. & Wang, J. J. (1997), “Hybrid fault tree analysis using fuzzy sets”, Reliability Engineering and System Safety, Vol. 58, pp. 205-213. Long, W.; Sato, Y. & Horigome, M. (2000), “Quantification of sequential failure logic for fault tree analysis”, Reliability Engineering and System Safety, Vol. 67, pp. 269–274. Mahanti, R. & Antony, J. (2005). Confluence of six sigma, simulation and software development. Managerial Auditing Journal, Vol. 20, nº 7, pp. 739-762. Oliveira, U.R.; Paiva, E. J. & Almeida, D. A. (2010). Integrated methodology for mapping failures: a proposal for joint use of process mapping techniques with the FTA, FMEA and critical analysis of experts. Revista Produção, Vol. 20, nº 2, PP. 77-91. (in Portuguese) Rausand, M. & Oien, K. (1996), “The basic concepts of failure analysis”, Reliability Engineering and System Safety, No. 53, pp. 73-83. Rath, F. (2008). Tools for developing a quality management program: proactive tools (process mapping, value stream mapping, fault tree analysis, and failure mode and effects analysis). International Journal Radiation Oncology Biology Physics, Vol. 71, nº. 1, pp. 187–190. Salerno, M. S.; Camargo, O. S. and Lemos, M. B. (2008), “Modularity ten years after: an evaluation of the Brazilian experience”, International Journal of Automotive Technology and Management, Vol. 8, No.4, pp. 373 – 381. Slack, N.; Chambers, S. & Johnston, R. (2007). Operations Management. 5th Edition. New Jersey: Prentice Hall. Shahin, A. (2004). Integration of FMEA and the Kano model: An exploratory examination. International Journal of Quality & Reliability Management, Vol. 21, nº 7, pp. 731-746. Sharma, R. K.; Kumar, D. & Kumar, P. (2007). Modeling and analysing system failure behaviour using RCA, FMEA and NHPPP models. International Journal of Quality & Reliability Management, Vol. 24, nº 5, pp.
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