Figure 1.6 Possible realization of the load and the strength of an item.
1.5.2 Systems Approach to Reliability
By the systems approach, all our information about the operational loads and the strength of an item is incorporated in its probability distribution function
Quantitative results are based on information about the reliability of the components. Such information comes from statistical data on past experience with the same or similar components, laboratory testing, or from expert judgments. This approach has similarities to actuarial assessments, and the systems approach to reliability is, therefore, sometimes referred to as an actuarial approach. This book is concerned with the systems approach to reliability.
System Models
In reliability studies of technical systems, we always have to work with models of the systems. These models may be graphical (networks of different types) or mathematical. A mathematical model is necessary in order to be able to bring in data and use mathematical and statistical methods to estimate reliability parameters. For such models, two conflicting interests always apply:
1 The model should be sufficiently simple to be handled by available mathematical and statistical methods.
2 The model should be sufficiently “realistic” such that the deducted results are of practical relevance.
We should always bear in mind that we are working with an idealized, simplified model of the system. Furthermore, the results we derive are, strictly speaking, valid only for the model, and are accordingly only “correct” to the extent that the model is realistic.
Figure 1.7 The system reliability analysis process.
The modeling situation is illustrated in Figure 1.7. Before we start developing a model, we should clearly understand what type of decision the results from our analysis should provide input to, and also the required format of the input to the decision. To estimate the system reliability from a model, we need input data. The data will usually come from generic data sources, as discussed in Chapter 16. The generic data may not be fully relevant for our system and may have to be adjusted by expert judgment. This is especially the case when we are introducing new technology. Some data may also come from the specific system. When establishing the system model, we have to consider the type, amount, and quality of the available input data. It has limited value to establish a very detailed model of the system if we cannot find the required input data.
1.6 Reliability Engineering
Engineering deals with the design, building, and use of technical items. Reliability engineering is an engineering discipline that provides support to the engineering process. To be successful, reliability engineering must be integrated in the engineering process and the reliability engineer(s) must take full part in the engineering team.
An item development project is split into a number of phases. The number and the title of these phases vary from industry to industry and also between companies in the same industry. A typical set of phases is shown in Figure 1.8.
Figure 1.8 The phases of a system development project (example).
The phases in Figure 1.8 are arranged as a time axis, but iterations are usually required, for example, to make a redesign after a defect has been revealed in a later phase. Each phase is usually divided into stages, and many manufacturers have procedures describing in detail which reliability analyses to carry out in each stage together with procedures for the data flow.
Reliability engineering has its most important role in the three first phases in Figure 1.8, but should be integrated in all phases.
1.6.1 Roles of the Reliability Engineer
The objective of reliability engineering is to identify, analyze, and mitigate failures and operational problems during all phases of an item's life cycle. The reliability engineer has an important role in all these phases. Below, the roles of the reliability engineer are listed briefly in the design and development phases and in the operational phase.
Roles in Design and Development
A reliability engineer has her most important role in the specification, design, and development phases of a new item. In these phases, the reliability engineer helps the development team to
1 Identify potential failures of suggested component and module concepts such that failures may be designed out.
2 Quantify the reliability of suggested system concepts.
3 Provide input to decisions about modularization, stacking, and system layout.
4 Make tradeoffs between factors such as cost, functions, performance, reliability, time to market, safety, and security.
5 Identify weaknesses of the system design such that they can be corrected before the system goes to manufacturing or to the customers.
6 Clarify benefits and drawbacks related to redundancy of components and modules.
7 Identify causes and effects of possible failure modes.
8 Compare the LCC of design alternatives.
9 Evaluate the cost of suggested warranty policies.
10 Calculate the reliability of system options as input to choice between these.
11 Plan and perform reliability acceptance or qualification testing (e.g. in a TQP framework).
Roles in Normal Operation
The main role of the reliability engineer in normal operation is to track items causing abnormally high maintenance cost and production losses or service outages, then find ways to reduce these