1 Introduction

Today, the term reliability is part of our everyday language, especially when speaking about the functionality of a product. A very reliable prod-uct 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 prob-ability that a product does not fail under given functional und environ-mental 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 at-tributes, 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 reli-ability is such an important topic for new products, however it does not maintain the highest priority in current development.

Assessment Scale from 1 (very important)

to 4 (unimportant)1.61.6

1.61.7

1.92.12.12.1

2.6

0 0.5 1 1.5 2 2.5 3 3.5 4

ReliabilityFuel Consumption

PriceDesign

Standart EquipmentRepair-/Maintanence Costs

Resale ValueService Network

Delivery TimePrestige

Good Price by Trade-in

1.3

2.5

Figure 1.1. Car purchase criteria (DAT-Report 2007)

„It is impossible to avoid all faults“

„Of cause it remains our task to avoid faults if possible“

Sir Karl R. Popper

B. Bertsche, Reliability in Automotive and Mechanical Engineering. VDI-Buch, doi: 10.1007/978-3-540-34282-3_1, © Springer-Verlag Berlin Heidelberg 2008

2 1 Introduction

Surveys show that customers desire reliable products. How does prod-uct 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 sta-tistic 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 turn-over. 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.

Am

ount

of c

allb

acks

0102030405060708090

100110120130

1998 1999 2000 2001 2002 2003 2004 2005 2006

140

5564 72

86

105116

137123

167150160170

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, soft-ware 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 3

System / Productwith mechanics / materials,

elektronics, sensors und softwarein macro or microtechnology

HigherFunctionality

IncreasedProduct LiabilityIncreased

Costomer Requirements

ReducedDevelopment Costs

Minimization of Failure costs Complexity

Higher

Shorter Development Times

Figure 1.3. Factors which influence reliability To achieve a high customer’s satisfaction, system reliability must be ex-

amined during the complete product development cycle from the view-point 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. Meth-odological 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.

Planing Design Production FieldusageConception Recycling

ReliabilityTarget

Lasten -

heft

- Fuzzy Data

- Know-How- ...

time

- ABC- Analysis - Design Review- FMEA- FTA- ....

- Weibull, Exponential...- Testplaning- Boolean Theory- Markov Model

- ....

-QualitiyManagement

- Audit-....

- Statistical ProcessPlaning

- ...

- Field DataCollection

- Early Warning

- ....

- Field Data Analysis

- ......

- RecyclingPotential

- ....

- RemainingLifetime

- ....

Qua

litat

ive

Qua

ntita

tive

Specifi-cations

Calculation

- FTA

-

Q

Layout

Figure 1.4. Reliability methods in the product life cycle

4 1 Introduction

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.

Cos

ts p

er F

ailu

re

1.00

10.00

100.00

0.10

Failure Prevention Failure DetectionChance to Act Need to React

Design FieldProduction 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 appro-priate 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 5

System Reliability Assurance

Constructive:Optimal construction prozesswith sophisticated construction techniques and -methods

• genaues und voll-ständiges Lasten-heft

• gesicherte Berechnung mit genau erfaßten Last-kollektiven

• bewährte Konstruktions-richtlinien

• frühzeitige und um-fassende Erprobung•

• exact and complete specifications

•

•

•

• ......

Analytical:Determination and/or reliability prediction by reliability techniques and afterwardsoptimizationTarget:

• calculation of the predictable reliability

•

•

• ..... methods:

- Boole - Markoff - FTA - ....

quantitative• Systematische Untersuchung der Auswirkungen von

• Ausfallartenanalyse• ..... Methoden: - FMEA/FMECA - FTA - Ereignisablauf- analyse - Checklisten

• systematical analysis of effects of faults and failures

•

• ..... methods: - FMEA/FMECA - FTA - event sequence analysis - checklists - ....

qualitative

- reliability prediction- detection of weaknesses- realization of comparative trial

assured calculation with exact collected load collectives

established construction guidelines

early and broad tesing

failure rate

probabalistic reliability analysis

failure analysis

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 funda-mentals and methods described in the previous chapter.

The most well-known qualitative reliability method is the FMEA (Fail-ure Mode and Effects Analysis). The essential contents, according to the current standard in the automotive industry (VDA 4.2), are shown in Chap-ter 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 gen-eral 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 "confi-dence range" will be explained in detail.

There is little collected and edited information pertaining the failure be-haviour of mechanical components. However, the knowledge of the failure

6 1 Introduction

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 ma-chine 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 ma-chine 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 treat-ment is compulsory. For this, a reliability safety program has been devel-oped. 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 so-lutions can be found at the end of chapter 11.