1 Introduction „It is impossible to avoid all faults“ „Of cause it remains our task to avoid faults if possible“ Sir Karl R. Popper
Today, the term reliability is part of our everyday language, especially when speaking about the functionality of a product. A very reliable product is a product that fulfils its function at all times and under all operating conditions. The technical definition for reliability differs only slightly by expanding this common definition by probability: reliability is the probability that a product does not fail under given functional und environmental conditions during a defined period of time (VDI guidelines 4001). The term probability takes into consideration, that various failure events can be caused by coincidental, stochastic distributed causes and that the probability can only be described quantitatively. Thus, reliability includes the failure behaviour of a product and is therefore an important criterion for product evaluation. Due to this, evaluating the reliability of a product goes beyond the pure evaluation of a product’s functional attributes. According to customers interviewed on the significance of product attributes, reliability ranks in first place as the most significant attribute, see Figure 1.1. Only costs are sometimes considered to play a more important role. Reliability, however, remains in first or second place. Because reliability is such an important topic for new products, however it does not maintain the highest priority in current development. Reliability Fuel Consumption Price Design Standart Equipment Repair-/Maintanence Costs Resale Value Service Network Delivery Time Prestige Good Price by Trade-in
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Assessment Scale from 1 (very important) to 4 (unimportant)
1.9 2.1 2.1
2.1 2.5 2.6
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Figure 1.1. Car purchase criteria (DAT-Report 2007) B. Bertsche, Reliability in Automotive and Mechanical Engineering. VDI-Buch, doi: 10.1007/978-3-540-34282-3_1, © Springer-Verlag Berlin Heidelberg 2008
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1 Introduction
Amount of callbacks
Surveys show that customers desire reliable products. How does product development reflect this desire in reality? Understandably, companies protect themselves with statements concerning their product reliability. No one wants to be confronted with a lack of reliability in their product. Often, these kinds of statements are kept under strict secrecy. An interesting statistic can be found at the German Federal Bureau of Motor Vehicles and Drivers (Kraftfahrt-Bundesamt) in regards to the number of callbacks due to critical safety defects in the automotive industry: in the last ten years the amount of callbacks has tripled (55 in 1998 to 167 in 2006), see Figure 1.2. The related costs have risen by the factor of eight! It is also well known, that guarantee and warranty costs can be in the range of a company’s profit (in some cases even higher) and thus make up 8 to 12 percent of their turnover. The important triangle in product development of cost, time and quality is thus no longer in equilibrium. Cost reductions on a product, the development process and the shortened development time go hand in hand with reduced reliability. 170 160 150 140 130 120 110 100 90 80 70 60 50 40 30 20 10 0
167 137 123 116 105 86 64
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1998 1999 2000 2001 2002 2003 2004 2005 2006
Figure 1.2. Development of callbacks in automotive industry
Today’s development of modern products is confronted with rising functional requirements, higher complexity, integration of hardware, software and sensor technology and with reduced product and development costs. These, along with other influential factors on the reliability, are shown in Figure 1.3.
1 Introduction Minimization of Failure costs
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Higher Complexity Higher Functionality
Shorter Development Times
System / Product with mechanics / materials, elektronics, sensors und software in macro or microtechnology Reduced Development Costs
Increased Costomer Requirements
Increased Product Liability
Figure 1.3. Factors which influence reliability
Qualitative
To achieve a high customer’s satisfaction, system reliability must be examined during the complete product development cycle from the viewpoint of the customer, who treats reliability as a major topic. In order to achieve this, adequate organizational and subject related measures must be taken. It is advantageous that all departments along the development chain are integrated, since failures can occur in each development stage. Methodological reliability tools, both quantitative and qualitative, already exist in abundance and when necessary, can be corrected for a specific situation. A choice in the methods suitable to the situation along the product life cycle, to adjust them respectively to one another and to implement them consequently, see Figure 1.4, is efficacious.
- Know-How Specifi-Lasten cations heft
time
Planing
Quantitative
- ...
Reliability Target
Conception
- Fuzzy Data - Calculation
-Qualitiy - Field Data Management Collection
- ABC- Analysis - Design Review - FMEA - FTA - ....
Layout
- Audit
- Early Warning
-....
Q
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Field Design Production usage
- Weibull, Exponential... - Testplaning - Boolean Theory - Markov Model - FTA - ....
- Statistical Process Planing - ...
Figure 1.4. Reliability methods in the product life cycle
- Recycling Potential - ....
Recycling
- Field Data Analysis
- Remaining Lifetime
- ......
- ....
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A number of companies have proven, even nowadays, that it is possible to achieve very high system reliability by utilizing such methods. The earlier reliability analyses are applied, the greater the profit. The well-known “Rule of Ten” shows this quite distinctly, see Figure 1.5. In looking at the relation between failure costs and product life phase, one concludes that it is necessary to move away from reaction constraint in later phases (e.g. callbacks) and to move towards preventive measures taken in earlier stages. Failure Prevention Chance to Act
Failure Detection Need to React
Costs per Failure
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10.00 0.10
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Production
Field
Figure 1.5. Relation between failure costs and product life phase
The easiest way to determine the reliability of a product is in hindsight, when failures have already been detected. However, this information is used for future reliability design planning. As mentioned earlier, however, the most sufficient and ever more required solution is to determine the expected reliability in the development phase. With the help of an appropriate reliability analysis, it is possible to forecast the product reliability, to identify weak spots and, if needed, comparative tests can be carried out, see Figure 1.6. For the reliability analysis quantitative or qualitative methods can be used. The quantitative methods use terms and procedures from statistics and probability theory. In Chapter 2 the most important fundamental terms of statistics and probability theory are discussed. Furthermore, the most common lifetime distributions will be presented and explained. The Weibull distribution, which is mainly and commonly used in mechanical engineering, will be explained in detail.
1 Introduction
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System Reliability Assurance Constructive:
Optimal construction prozess with sophisticated construction techniques and -methods
Analytical:
Determination and/or reliability prediction by reliability techniques and afterwards optimization Target: - reliability prediction - detection of weaknesses - realization of comparative trial
quantitative ••exact and complete genaues und vollspecifications ständiges Lastenheft • assured calculation •with gesicherte exact collected Berechnung mit load collectives genau erfaßten Lastkollektiven established • • bewährte construction guidelines Konstruktionsrichtlinien and broad • early •tesing frühzeitige und umfassende Erprobung • • ......
• calculation of the predictable reliability • failure rate • probabalistic reliability analysis • ..... methods: - Boole - Markoff - FTA - ....
qualitative • systematical analysis • Systematische ofUntersuchung effects of faults der and failures Auswirkungen von • failure analysis • Ausfallartenanalyse ..... • •..... Methoden: methods: FMEA/FMECA - -FMEA/FMECA FTA - -FTA Ereignisablauf- -event sequence analyse analysis Checklisten - -checklists - ....
Figure 1.6. Securing of system reliability
Chapter 3 illustrates an example of a complete reliability analysis for a simple gear transmission. The described procedure is based on the fundamentals and methods described in the previous chapter. The most well-known qualitative reliability method is the FMEA (Failure Mode and Effects Analysis). The essential contents, according to the current standard in the automotive industry (VDA 4.2), are shown in Chapter 4. The fault tree analysis, described in Chapter 5, can be used either as a qualitative or as a quantitative reliability method. One main focus of this book is the analysis of lifetime tests and damage statistics, which will be dealt with in Chapter 6. With these analyses general valid statements concerning failure behaviour can be made. In order to describe the lifetime distribution the Weibull distribution is used, which is the most common distribution in mechanical engineering. Next to the graphical analyses of failure times, analytical analyses and their theoretical basics will be discussed. The important terms "order statistic" and "confidence range" will be explained in detail. There is little collected and edited information pertaining the failure behaviour of mechanical components. However, the knowledge of the failure
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behaviour of a component is necessary, in order to be able to predict the expected reliability under similar application conditions. With the help of system theory it is also possible to calculate the expected failure behaviour of a system. In Chapter 7 results from a reliability data base for the machine components gear wheels, axles and roller bearings will be presented. In many cases the indicated Weibull parameters can prove to serve as a first orientation. To prove reliabilities before the start of production, it is obligatory to carry out the appropriate tests. Here, the amount of test specimens, the required test period length and the achievable confidence level may be of interest. In Chapter 8 the planning of reliability tests will be described. Each quantitative reliability method portrays a kind of enhanced fatigue strength calculation. The basic principles of a lifetime calculation for machine components are summarized in Chapter 9. The reliability and the availability of systems, which include repairable elements, can be determined by various calculation models. Chapter 10 describes methods in their differing complexity and their assessment for repairable elements. In order to achieve high system reliability, an integrated process treatment is compulsory. For this, a reliability safety program has been developed. This program will be described with its basic elements in Chapter 11. In conclusion, this chapter offers a complete overview on an optimal reliability process. For all the chapters there are problems at the end of each one and the solutions can be found at the end of chapter 11.
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