Process analysis and reliability evaluation.

Karaulova, Tatyana ; Preis, Irina ; Pribytkova, Marina 等

1. INTRODUCTION

Reliability is the probability that a system will operate successfully for a specified period of time, under specified conditions, when used for the manner and purpose for which it was intended (MIL-HDBK-338B, 1998).

Reliability is a crucial element to business success; therefore the reliability analysis and its methods are developed as a main tool to facilitate companies to achieve their goals. For Estonia the sphere of reliability engineering is rather new, however it is successfully developed for decades in US and Europe. For instance, in 1996-2000, the National Technology Agency of Finland (Tekes) financed a Competitive Reliability technology programme with the aim of increasing the international competitiveness of Finnish industry by developing techniques for more convincing control of production systems' reliability. (Kenneth Holmberg, 2001). Present study considers the process of real retractors' production line existing at AS Norma Estonia. A retractor is the main part of the car seatbelt mechanism. The whole process of retractors' assembling is long and consists of many consecutive operations. The layout of the line is illustrated in the Figure 1 . All operations of components assembling are manual and all the testing operations are automated. The line is divided into 13 working stations responsible for certain type of process operation. All the stations are jointed into a conveyor.

The aim was to create model of the production process with follow-on analysis of the production line reliability applying various RA methods.

2. PROCESS ANALYSIS

System analysis starts with development of process model using IDEF0 methodology.

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This model of every activity then decomposed into more detailed diagrams with further representation in form of Node Tree Diagram (NTD).

Figure 2 shows the conceptual IDEF0 model structure. As the process analysis done structure has been analysed and simulation completed, the reliability analysis is ready to be carried out.

3. RELIABILITY ANALYSIS

Realistically, it is impossible to avoid all feasible failures of a system or a product on the design stage, so one of the goals of reliability engineering is to recognize the most expected failures and then to identify appropriate actions to mitigate the effects of those failures (Lendvau, 2004 ). As shown in Figure 3, for the system safety analysis the qualitative and quantitative methods were used. All they are interrelated and help to understand the logical structure of failure modes of a system. The reliability, R(t), of the component is the probability of a component surviving to a time t and is expressed as

R(t) = number surviving at instant t/number at time t = (1)

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Failure rate--expected rate of occurrence of failure or the number of failures in a specified time period. This parameter is wide used in reliability analysis.

The probability of system failure, or system unreliability, corresponds to the probability of successful system maintenance, or system maintainability (Salvendy, 2001).

3.1 Failure Mode and Effect Analysis (FMEA)

A typical failure modes and effects analysis incorporates some method to evaluate the risk associated with the potential problems identified through the analysis. The most common used method, Risk Priority Numbers, is described next. The risk priority number provides a qualitative numerical estimate of the design risk. This number is then used to rank order the various concerns and failure modes associated with a given design as previously identified in the FMEA. RPN is defined as the product of three independently assessed factors: Severity (S), Occurrence (O) and Detection (D).

RPN = (S) x (O) x (D) (2)

3.2 Fault Tree Analysis (FTA)

The FTA technique is a method for block diagramming constituent lower level elements. It determines, in a logical way, which failure modes at one level produce critical failures at a higher level in the system (MIL-HDBK-338B, 1998). The technique is useful in safety analysis where the discipline of block diagramming helps prevent an oversight in the basic FMEA.

FMEA and FTA have three main differences: boundaries of the analysis, direction of analysis, and presentation of the analysis process and results. FMEA deals with single point failures, is built bottom-up, and is presented as a rule in the form of tables. FTA analyzes combinations of failures, is built top-down, and is visually presented as a logic diagram. By taking into account combinations of failures, FTA avoids the obvious shortcomings of FMEA. (Bluvband et al., 2005).

In figure 4 is shown FTA of the production line by structure introduced in figure 3.

One of the most powerful tools of the results analysis in FTA is a Cut Set. A Cut Set is any group of fault tree initiators which, if all occur, will cause the TOP event to occur. In addition a minimal cut set definition exists. A minimal cut set is a least group of fault tree initiators which, if all occur, will cause the TOP event to occur (Clemens, 2002).

3.3 Reliability Block Diagram (RBD)

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RBD performs the system reliability analyses using block diagrams to show network relationships. The structure of the reliability block diagram defines the logical interaction of failures within a system that are required to sustain system operation. Each module from IDEF0 model (A11, A12, ... A37) is defined by a separate block Figure 5. RBD analysis allows take into consideration duty cycle and failure rate for every block of the system.

Fault trees and reliability block diagrams are both symbolic analytical logic techniques that can be applied to analyze system reliability and related characteristics. Although the symbols and structures of the two diagram types differ, most of the logical constructs in a fault tree diagram (FTD) can also be modelled with a reliability block diagram (RBD).

4. CONCLUSIONS

An integrated modelling method based on system modelling and complemented with reliability evaluation mechanism has the capability to analyse and design manufacturing systems. The tool developed to analyse the production process enables the companies to analyse the processes as a whole and its parts and get efficient prognosis for production process reorganization.

5. REFERENCES

Bluvband, Z.; Polak, R. & Grabov,P. (2005) Bouncing Failure Analysis (BFA): The Unified FTA-FMEA Methodology

Lendvay, M (2004) Dependability Assurance of Industrial Production Processes, Proceedings: Science in Engineering, Economics and Education, Budapest

MIL-HDBK-338B (1998) Military Handbook, Electronic Reliability Design Handbook

Holmberg, Kenneth, (2001)--Competitive Reliability 19962000, Final report, National Technology Agency

Clemens, P. (2002) -, System Safety Scrapbook , 9th Edition Sverdrup Technology, Inc.

Salvendy G., (2001)--Handbook of Industrial Engineering: Technology and Operation Management, A Wiley-Interscience. ISBN 0-471-33057-4