BS5760-2:1994 - 可靠性评估指南

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"BS5760-2是英国标准协会(BSI)发布的一份关于系统、设备和部件可靠性的评定指南，旨在提供一个评估可靠性的标准方法。这份标准最初发表于1981年，第二版更新于1994年10月15日。BS5760-2属于质量管理和统计标准政策委员会的管辖范围，其目的是确保在设计、开发和生产过程中对系统的可靠性进行有效的评估和管理。" BS5760-2标准详细阐述了如何对系统、设备及部件的可靠性进行评估，这涉及到在不同阶段如设计、制造、测试和使用过程中的可靠性预测、分析和验证。可靠性是衡量产品在规定条件和规定时间内完成预定功能的能力，它是产品质量的重要组成部分，特别是在关键应用领域，如航空航天、医疗设备、军事系统等。该标准提供了以下几个核心知识点： 1. **可靠性定义与概念**：BS5760-2首先明确了可靠性定义，包括可靠性指标（如MTBF，平均无故障时间）和可靠性度量，以及与之相关的概念，如失效率、可靠度函数等。 2. **可靠性计划**：标准指导用户如何制定可靠性计划，包括确定可靠性目标、选择合适的评估方法和工具、制定测试策略等。 3. **可靠性预测**：BS5760-2介绍如何根据组件或系统的工程数据进行可靠性预测，通过故障模式和效应分析(FMEA)、故障树分析(FTA)等方法估计系统的可靠性。 4. **可靠性分析**：涵盖了可靠性数据分析的各个方面，如寿命分布分析、故障模式机理分析，以及如何处理不完整数据或观察到的故障模式。 5. **可靠性验证**：指导进行可靠性试验，验证产品在实际操作环境下的可靠性，包括加速寿命试验、耐久性试验等。 6. **可靠性改进**：标准也关注如何根据评估结果改进产品设计，减少故障发生，提高可靠性。 7. **文档管理**：BS5760-2强调了在整个可靠性工程过程中保持有效记录的重要性，以便追踪和审计。 8. **合规性与法规要求**：标准可能还会涉及行业特定的法规和标准要求，确保产品符合相关法规的同时满足可靠性要求。 9. **风险管理**：可靠性评定过程中会涉及到风险识别、评估和控制，以降低产品失效的风险。 BS5760-2标准的使用者应确保使用最新版本，因为标准会随着技术进步和行业实践的发展而更新。通过遵循这份指南，企业和工程师能够建立一套系统的可靠性评估流程，从而提升产品的质量和市场竞争力。

BS5760-2:1994

2.7 Accuracy of assessment and

prediction

2.7.1 It is important to appreciate that reliability

predictions are subject to uncertainty. This arises

from a number of factors, including the following.

a) There is an inherent uncertainty in

transferring failure (or success) data to different

applications, environments, etc.

b) It may be unclear what constitutes a failure in

various situations (see 2.6).

c) The effect of human actions and interpretation

may be uncertain, for example, in manufacturing

quality control, maintenance, diagnosis, etc.

d) Predictions, by their nature, are based upon

historical data. Differences in technology,

changes in design team personnel and the

changing specification of the product all

introduce possible sources of error into

predictions.

e) The rate of occurrence of failure may not be

constant with respect to time, particularly for

those components that may be said to wear out or

for those that undergo frequent repair.

f) Prediction methods often have to be

oversimplified in order to make problems

tractable.

2.7.2 No matter what reliability may be forecast by

the assessments, the actual reliability achieved will

depend largely on the content of the reliability

programme for the product and the effectiveness

with which it is implemented. Management

commitment to achieving a reliable product will be

a governing factor and this may in turn be

influenced not only by the resources available but by

any warranties, incentives or penalties that may be

invoked.

2.7.3 Reliability assessments are an aid to good

engineering and consideration should be given to

the following measures.

a) Well proven engineering practices should be

applied and complex, latest technology solutions

addressed with great care.

b) Previous designs and processes should be

reviewed to determine the capability of the new

product and also to identify any problems that

can be avoided.

c) Statistical analysis should always be tempered

by engineering judgement. Parallel redundancy,

for example, can be introduced to improve the

overall probability of successful operation of a

system, but it may not be effective if the same

inherent fault exists within the parallel items.

Licensed Copy: samuel sze, May 03, 2002, Uncontrolled Copy, (c) BSI

BS5760-2:1994

Section 3. Assessment throughout the product life

cycle

3.1 General

Reliability assessment is not a simple task that is

limited to a single phase of a project. It is a

continuing process that should normally extend

throughout all project phases.

The generalized life cycle of a product is described in

BS5760-1, and for the purposes of this Part of

BS5760 is deemed to consist of the following

principal phases:

a) definition (including feasibility);

b) design and development;

c) production;

d) installation and commissioning;

e) function and maintenance.

NOTEA further phase, the disposal phase, is important in the

life cycle of some products but is omitted from this standard since

reliability assessment is rarely implemented during disposal.

Assessment contributes to the specific objectives of

each project phase in turn, with the ultimate

purpose of ensuring that the defined reliability

requirements are met when the product is used.

Basic forms of assessment should begin during the

earliest stages of conception and the results should

be used to shape the fundamental definition and

design of the product. The assessments should then

be repeated and extended as design becomes more

detailed and refined; they should be continued as

the product passes through development and

production, installation and commissioning and

eventually enters into use.

A wide range of assessment techniques is available

and it is important to select a method that is

appropriate to the individual product and to the

particular phase involved.

Table 1 lists a range of some commonly used

assessment techniques and indicates the particular

phases of the product life cycle when it is normally

most appropriate to apply each technique.

The full and successful application of a particular

method of assessment often depends on the

availability of appropriate data, which may include,

for example, historic data collected from similar

equipment already in use, or data generated during

development and production testing. Nevertheless

it can often be useful to carry out a simplified

assessment at an early stage of the project even

though only limited data are available. For example,

it may prove helpful to use the reliability block

diagram (RBD) technique initially at system and

subsystem level even during the earliest phases of a

project. The analysis may reveal basic problems or

suggest alternative designs that it is valuable to

recognize at this early stage. Subsequently, as more

information becomes available, the diagram can be

extended to lower levels of detail at, for example,

unit and module level.

3.2 Definition phase

The main purpose of the definition phase of a project

is to permit studies to be made to establish precisely

what the future product is required to do and what

form it should take. This clarification involves

identifying and quantifying the essential

performance characteristics and deciding which of

various possible design alternatives will provide the

optimum solution. The selected option is then

defined as a set of formal requirements which

become the basis of a contract specification.

Reliability is a fundamental performance

characteristic which should be included in these

early considerations. Although this standard

primarily addresses the assessment of reliability, it

is important to appreciate the interdependence of

reliability with other characteristics such as

maintainability, availability, testability and safety.

For example, inadequate reliability can seriously

detract from the effective operation of a product and

can add greatly to running costs. If maintainability

is also poor the running costs will be even higher

and system availability will also be undermined.

The measures necessary to achieve acceptable

reliability may add to the cost of developing and

producing a product. These additional costs may be

amply justified by improved effectiveness and

reduced running costs in use. It is important that

those responsible for the fundamental decisions

made during the definition phase should be aware of

these relationships.

Licensed Copy: samuel sze, May 03, 2002, Uncontrolled Copy, (c) BSI

BS5760-2:1994

In order to carry out these studies and assessments,

it is usually necessary to establish the intended life

profile for the product, i.e. to describe how it will be

used throughout the various phases of its life. The

life profile is often expressed as a series of typical

duty cycles and should take account of the various

processes and environments that the product will

encounter such as handling, packaging, storage,

operations and maintenance. It is usual to identify

and describe the most important phase from the

viewpoint of the user, usually the operations phase,

in more detail and to define individual reliability

requirements for such key periods of use. Although

this primary phase is normally of overriding

importance, it is always advisable to be aware of the

possible impact of the subsidiary phases on

reliability.

Quantitative requirements for reliability should be

expressed in terms of an appropriate parameter

such as probability of successful operation or

maximum permissible failure rate. It will be

necessary to make assumptions about the

relationship between the occurrence of failures and

time and to select a suitable statistical model to

represent this relationship (see Annex A and

Annex B). Thus, for electronic devices, it may be

justifiable to apply the constant failure rate model

(see A.3) whereas for mechanical systems

experiencing rough loading and high wear, only an

instantaneous failure rate would be meaningful.

It may be valuable at this stage to construct RBDs

(see section 6) and to use the overall system

estimate or requirement to apportion reliability

values to the lower level blocks. Basic

maintainability and availability block diagrams

may also be constructed.

Reliability predictions may be carried out based on

comparison with systems using similar technology.

Alternatively, the parts count or parts stress

methods may be used where detailed design data

are available, subject to the limitations inherent in

these methods (see section 7). Estimates of

reliability and maintainability will also be used in

assessing the range and scales of spares that will be

needed to support the various product options under

consideration, again with the object of minimizing

costs of supporting the product when it enters

service.

Fault modes, effects and criticality analysis

(FMECA, see section 8) and fault tree analysis

(FTA, see section 9) can also be implemented at

block level and may be used not only to estimate

reliability but also to assess safety and to establish

the critical items list, i.e. to identify those items

whose reliable performance is critical to successful

operation of the product.

Finally, it is again stressed that with the conclusion

of the definition phase, the essential form and

function of the product will have been decided.

Thus, the principal elements that decide the

potential levels of future reliability, availability and

maintainability will now be fixed. From this stage

onward, the project reliability and maintainability

programme, and the assessments that support it,

will be mainly directed at achieving this potential.

3.3 Design and development phase

During the design and development phase the main

objectives of reliability assessment should be to

assist those responsible for design and development

of a product to ensure that defined reliability

requirements are met. The results of assessment

are analysed and problem areas identified. The

information is fed back to the designer so that

necessary changes can be made.

The assessment techniques initiated in the

definition phase are carried out in greater depth as

the design is refined and more information becomes

available. Fully detailed assessment may be

restricted to those elements which preliminary

analysis shows to be critical from the viewpoints of

safety, reliability and maintainability.

FMECA (see section 8) is used to assess and classify

the criticality of failures that occur. The technique is

also used in maintainability studies to provide

guidance on fault diagnosis and on the design of

built-in test equipment (BITE), test points and

facilities for continuous monitoring. FMEA and

FMECA may also be implemented to resolve

problems associated with the production process

and human failings.

As the phase continues and development prototypes

of the product become available, some prototypes

may be allocated for the purpose of reliability

growth testing and monitoring. This involves long

term testing with appropriate environmental

stresses applied (see section 12). If failures occur,

the causes are analysed and appropriate design

changes introduced. Mathematical modelling

techniques may be applied to assess what further

test resources will be needed in order to achieve the

specified reliability requirements.

Conventional reliability growth testing of this kind

is acceptable for products with relatively low mean

operating times between failures (MTBFs). For

products where the MTBF value is high, the

duration of testing required would be unacceptably

long. In this case, it might be more appropriate to

apply step stress testing (see 12.4.6).

Licensed Copy: samuel sze, May 03, 2002, Uncontrolled Copy, (c) BSI

BS5760-2:1994

The testing of one-shot devices (see section 24) may

also present problems because cost restricts the

quantity that can be tested. In these cases other

assessment techniques have to be used. For

example, greater attention is given to the tolerance

design of electronic and mechanical items, and

products are manufactured with maximum (or

minimum) tolerance values to select worst-case

situations for testing.

As the development phase nears completion, there

may be a requirement for further development

prototypes to be allocated for demonstrations of

reliability, maintainability and testability. Such

demonstrations represent a form of qualification

testing in which the prototypes are tested and

reliability and other characteristics are assessed

under formal conditions. The object of the tests is to

prove, before transition to the production phase is

authorized, that the development samples are

capable of meeting the defined requirements for

reliability and maintainability. The tests do not in

themselves enhance the reliability of a product but

their inclusion as a formal qualification

requirement is intended to encourage management

to allocate appropriate priority to reliability and

maintainability throughout the development

programme. If the product were to fail these tests,

urgent action would be needed to effect the

necessary improvements and the project schedule

would be jeopardized.

3.4 Production phase

The transition to the production phase provides

many opportunities for the reliability of the product

to be degraded below the levels achieved during

development. Assessment is necessary to indicate

whether required reliability levels are being

maintained or whether such degradation is

occurring.

For example, the production version of a product

may incorporate modifications which significantly

change the design and it is important to assess the

effect of these on reliability. Environmental stress

screening (ESS) and conditioning (see section 14)

may be implemented to eliminate early life failures

and monitoring is necessary to establish whether

these measures are effective or whether levels need

to be adjusted. Reliability assessments carried out

in early phases as a basis for spares forecasting and

other logistics support analysis activities need to be

reviewed and revised.

FMEA and FMECA may be continued and can be

useful in establishing whether any reliability

weaknesses detected are inherent in the design or

have been introduced during the manufacturing

process (see section 8). FMEA used for this purpose

is often known as a process FMEA.

Production reliability assurance testing may be

initiated on a batch basis (see section 15) to provide

formal assurance that defined reliability levels are

being achieved on the production line.

3.5 Installation and commissioning

During the installation and commissioning phase it

is important to continue to collect data and to assess

whether any problems that occur relate to

installation difficulties or arise from problems

which should have been eliminated earlier.

Reliability and maintainability qualification tests

may be carried out to demonstrate formally that

requirements have been met. The results may be

used to establish whether the contractor should

receive bonus payments for exceeding defined

reliability requirements or whether penalties for

falling short of them will be invoked.

3.6 Function and maintenance

It is important to continue to assess reliability and

maintainability of the product during the function

and maintenance phase.

Such in-use assessments may again be part of

contractually required formal tests to establish

whether defined reliability, availability and

maintainability requirements have been met.

Assessments should also be used by management to

draw attention to problem areas. Subsequent

detailed investigation may show that belated design

modifications or changes to the production process

are needed. If major modifications are introduced,

their effect on reliability will need to be assessed

since such changes can in themselves introduce new

reliability problems.

Licensed Copy: samuel sze, May 03, 2002, Uncontrolled Copy, (c) BSI

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