Using Vector Projection Method to evaluate maintainability

of mechanical system in design review

Lu Chen*, Jianguo Cai

Department of Industrial Engineering and Management, School of Mechanical Engineering, Shanghai Jiao Tong University,

Shanghai 200030, People’s Republic of China

Received 18 November 2002; accepted 15 March 2003

Abstract

Maintainability of a mechanical system is one of the system design parameters that has a great impact in terms of ease of maintenance. In

this article, based on the definition of the terms of maintenance and maintainability, an important tool of Design for Maintenance is developed

as a way to improve maintainability through design. A set of standard and organized guidelines is provided and maintainability factors in

terms of physical design, logistics support and ergonomics are identified. As a specific application of design review, a methodology so called

Vector Projection Method is developed to evaluate the maintainability of the mechanical system. Lastly, an example is discussed.

q 2003 Elsevier Science Ltd. All rights reserved.

Keywords: Maintainability; Design for Maintenance; Guidelines; Factors; Vector Projection Method

1. Introduction

Maintainability is recognized as being highly significant

factor in the economic success of engineering systems and

products. Also, design is the stage at which the eventual

characteristics of future systems and products are deter-

mined. Therefore, it is important that designers should take

maintainability into account during their work. However,

there is much to consider at the design stage; a designer

should be provided with simple and logical measure

qualitatively or quantitatively to evaluate and predict the

maintainability. Prediction facilitates an early assessment of

the maintainability of the design and enables decisions

concerning the compatibility of a proposed design with

specified maintenance requirements or the choice of better

alternatives.

There are a number of excellent specialist papers and text

books in maintainability. The problem for designers is that

they are mainly written from the perspective of the

dedicated maintainability engineer. These papers and

books contain in-depth analytical methods that require

information that is not available at the design stage.

Therefore, they are of limited use to designers.

This paper is written entirely from a design perspective.

In this article, an important tool, Design for Maintenance

(DFMAIN) is firstly introduced in Section 2. Also in Section

2, a set of standard and organized guidelines is provided.

Section 3 introduces some general concepts of design

review. Then, as a method of design review, maintainability

evaluation methods are discussed in detail in Section 4. A

specific methodology so called Vector Projection Method

(VPM) is developed to evaluate the maintainability of the

system. Section 5 presents a stepwise evaluation procedure.

In Section 6, a case demonstration is carried out, while

Section 7 concludes the paper.

2. Design for Maintenance

DFMAIN is concerned with achieving good designs that

consider the general care and maintenance of equipment and

the repair actions that follow a failure.

2.1. Maintenance and maintainability

Traditionally, people think that maintenance is only a

kind of guaranteed technical job made by a few of

technicians, and has nothing with the design and production

of the product. In fact, nowadays, mechanical or electronic

0951-8320/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved.

doi:10.1016/S0951-8320(03)00075-9

Reliability Engineering and System Safety 81 (2003) 147–154

www.elsevier.com/locate/ress

* Corresponding author. Tel.: þ86-21-629-321-15.

E-mail address: chenlu@sjtu.edu.cn (L. Chen).

products are becoming more and more complicated. As a

result, the content and meaning of maintenance are richer

than ever. The Chinese Standardization Department publi-

cation of ‘Definition of Terms for Reliability and Main-

tainability’, GJB451-90 defines maintenance as “all the

activities that are intended to retain or restore the product to

a specified condition”.

With respect to maintainability, the designer has to take a

different view from that of the maintenance manager. Some

definitions lead to mathematical analyses using repair rates

in a similar manner to reliability analyses. Although useful

to the maintenance manager in the analysis of data

accumulated in service, this approach is not very useful in

design. The designer should consider those factors which

are under his/her control.

The US military publication MIL-STD-721 [1] defines

maintainability as follows:

Maintainability: The measure of the ability of an item

to be retained or restored to specific conditions when

maintenance is performed by personnel having speci-

fied skill levels, using prescribed procedures and

resources at each prescribed level of maintenance

and repair.

The maintainability definition has fostered the develop-

ment of many maintainability prediction procedures for

providing an assessment of system maintainability. Rather

than predicting how the system will fail, the effort of

maintainability is to make manufacturing systems and

products that require minimal maintenance and that are easy

and inexpensive to fix when they fail. Consequently,

manufacturers strive to design maintainability into the

products and their manufacturing processes.

2.2. Design for Maintenance

DFMAIN is an important part of product/system design.

Design changes during production are very costly, but if

DFMAIN is implemented early in the design stage, a great

number of benefits would be realized including:

† Longer life of systems and products.

† Lower operation costs, by performing at high efficiency.

† Lower unscheduled downtime, by preventing failures.

† Lower scheduled downtime, by decreasing the time

required to perform a particular maintenance task.

Maintainability is one of the system design parameters

that has a great impact in terms of ease of maintenance.

System failure is inevitable no matter how reliably this is

built-in, so its ability to be quickly restored is therefore the

most important. To fully realize part or all of these benefits,

some maintainability guidelines are presented next.

2.3. DFMAIN guidelines

A general rule for DFMAIN is to reduce the possibility of

damage to the product or equipment during maintenance

and servicing or better yet to eliminate the need for

maintenance. To assist in establishing a solid foundation for

implementing DFMAIN in the early stage of design, a set of

standard and organized guidelines is provided. Use of these

guidelines can help improve maintainability and enhance

product quality [2].

B Keep the functional and physical characteristics as

simple as possible. Complexity of design has a direct

bearing on production and maintenance cost. To reduce

the number of components and assemblies including

redundant components is helpful in lowering the skill

of technicians and the requirement of the equipment

and maintenance tools.

B There is adequate access for visual and manipulative

tasks, including the assembly of parts and any required

tooling during assembly, inspection, repair, or

replacement.

B Use standardized parts to simplify the maintenance

work, especially for those parts that are mostly like to

fail. Because standard parts are easy to find and be

replaced.

B Select modular design so that subassemblies could be

tested and maintained at that level and not at the final

assembly.

B Design for ease of assembly and disassembly with a

minimum number of parts. A product that is easily

assembled and disassembled is also easily maintainable.

B Provide easy diagnosability. Diagnosability could be

assured by provide functional sharing, monitoring

parameters for failure including alarms, build-in test

equipment facility and indication signal for failure

including fault isolation. Test points are available for

needed test pertaining to maintenance action.

B Provide identification to eliminate accident in main-

tenance. Critical components should be identified; test

points and arrows should be well marked.

B Minimize weight and awkwardness in handling of parts

that must be removed. Provide safety guards to prevent

contact with moving parts, high temperature, high

voltage lines or gaseous leakage.

B Provide appropriate manuals for maintenance instruc-

tions and procedures.

3. Design review

The design review may be defined as: [3]

The quantitative and qualitative examination of a

proposed design to ensure that it is safe and has optimum

performance with respect to maintainability, reliability

L. Chen, J. Cai / Reliability Engineering and System Safety 81 (2003) 147–154148

and those performance variables needed to specify the

equipment.

The review is much more than the scrutiny of design

work in the manner that an examiner checks a student

exercise. It should help the designer and enrich design

activity. It is an integral part of design activity and not a

‘bolt-on’ extra. The specific benefits associated with a good

design review methodology include:

† Added assurance that the voice of the customer has been

heard correctly

† Reductions in design cost and time-to-market

† Reduced likelihood of program delay due to unexpected

problems

† Improved overall design integrity

† Prevention of problems and associated downstream costs

† Increased standardization

† Improved customer satisfaction

† Increased program structure and control.

Given these benefits, it is no surprise that the design

review methodology is widely used and endorsed in

commercial industry. In fact, 89% of the companies

surveyed in a recent benchmarking study of 72 leading

companies in seven basic industries reported using design

review as a design assurance tool [4].

So, as we can see, the design review is one of the most

important ways of achieving good maintainability. It should

contribute to those problem areas such as maintainability

and reliability which may not have been fully taken into

account during the search for cost-effective feasible

solutions.

Usually, there are different kinds of review work at

different levels of design, such as design specification

review, system review, functional unit evaluation, com-

ponent analysis. A comprehensive design review is

summarized in Table 1. Evaluation is a normal part of

design activity, which is to evaluate the status of emerging

designs against customer-driven criteria prior to proceeding

forward to the next phase of a project.

4. Maintainability evaluation

As a specific application of functional unit evaluation in

design review activity, maintainability evaluation is dis-

cussed in this section. In this way alternatives can be

systematically reviewed from the perspective of maintain-

ability view to make them robust and suitable for further

design work.

4.1. Literature review

Various attempts have been made by researchers in

developing procedures for evaluation of the maintainability.

Takata and Saito [5] have proposed a structure of the facility

model for a computer-assisted life cycle maintenance

system. It provides a flexible representation scheme for

technical information as well as for the physical structure of

the facility, and could be used to do deterioration evaluation

of the facility. Vujosevic and Raskar [6] have developed

procedures for identification of disassembly sequence,

animation of human technicians while carrying out the

disassembly sequence. Based on these, the maintainability

of the systems is evaluated. Wani and Gandhi [7] developed

Table 1

Design review procedure

Activity Purpose Timing

Review of the design

specification

To ensure that the significance of

all the points contained within the

design specification is understood

Prior to the commencement of any

design activity

System level design (1) To identify critical areas of

the design that may affect plant

availability and to comment on the

advisability of pursuing projects with a

high risk content

Prior to the start of functional

unit design

(2) To examine equipment groups to

maximize uniformity and suitability and to

maximize the reliability systems formed by

manufacturing and process consideration

After the completion of the first

functional unit design

Functional unit evaluation To evaluate quantitatively critical items of

functional unit and to undertake qualitative

reviews of functional unit generally

After the completion of the first

detail design

Component analysis To check that certain important sets

of components will not give rise

to, say, maintainability problems in service

After the completion of the first

detail design

L. Chen, J. Cai / Reliability Engineering and System Safety 81 (2003) 147–154 149

a procedure based on a digraph and matrix method to

evaluate the maintainability index of mechanical systems.

The department of highway in Chaoyang, Liaoning

province, China [8] has evaluated the automobile

maintainability base on the respective calculation of

maintenance time and maintenance fee.

In all these methods, the maintainability characteristics

of the system affecting the maintainability have not been

fully identified. Here, a methodology so called VPM is

proposed to evaluate the maintainability of the system

which is a multi-objective assessment. The concepts of

fuzzy set theory and multiple-criteria decision analysis

are widely used to solve this kind of problem in which a

source of vagueness is involved. A fuzzy decision-

making method integrates various linguistic or, in other

word, qualitative assessment and weights to evaluate

different alternatives. Details on this technique could be

found in Refs. [14,15]. Chen [16] used this method to

solve the distribution center location selection problem

under fuzzy environment. Karasak and Tolga [17]

proposed a fuzzy decision algorithm to select the most

suitable advanced manufacturing system alternative from

a set of mutually exclusive alternatives. Ammar and

Wright [18] even applied this method in social science,

such as performance measurement, namely to evaluate

the state government performance and to survey client

satisfaction.

Even though fuzzy set theory is proved to be very

effective in multi-objective evaluation, it has certain

limitations. For example, when comparing two alterna-

tives, only comparative goodness can be obtained.

Therefore, in this paper, a methodology so called VPM

is developed based on fuzzy set theory. With this method,

a more absolute evaluation result could be obtained. And

at the same time, the algorithm complexity is not

increased.

4.2. Evaluation factors

In maintainability evaluation, assessment criteria are

defined in terms that are suitable for concept evaluation.

For example, a repair time would be inappropriate because

the repair time could not be calculated form the

information available at the concept stage. Similarly,

mean time to failure or mean failure rate calculations

cannot be made. Instead, criteria should be used that refer

to such factors as: simplicity and elegance of the design;

minimum number of parts; suitability for modular

construction, etc.

Maintainability evaluation factors Vj in terms of design

factors, logistics support, ergonomic factors are defined in a

hierarchical structure as in Table 2.

Under different circumstances, some or all of these

factors are chosen to be the evaluation criterion. It depends

on which of them are more important for a certain

evaluation task.

4.3. Weight calculation

As we all know that not all the factors are equally

important, in other words, weights must be assigned to these

factors when evaluation work is done.

Here we use Analytical Hierarchy Process (AHP)

method to calculate the weight. AHP is a decision-aiding

method developed by Saaty [9]. It is one of the extensively

used multicriteria decision making methods. It aims at

quantifying relative priorities for a given set of alternatives

on a ratio scale. Numerous applications of the AHP have

been made since its development and it has been applied to

many types of problems [10–13].

In this case, the factors are in one level, so the calculation

is compared simple, the following steps are developed for

applying the AHP:

1. A pair-wise comparison matrix (size n £ n) is constructed

for each factor by using the relative scale measurement

shown in Table 3. The pair-wise comparisons are done in

terms of which factor dominates the other.

2. There are n ðn 2 1Þ judgements required to develop the

matrix in step 1, reciprocals are automatically assigned in

each pair-wise comparison.

3. Calculate the eigenvectors, the eigenvalue, consistency

index. Judgement consistency can be checked by the

consistency ratio (CR), given by

CR ¼CI

RIðnÞ

where CI is the consistency index given by CI ¼

ðlmax 2 nÞðn 2 1Þ; RIðnÞ is the random consistency

Table 2

Maintainability evaluation factors Vj

Physical design Logistics support Ergonomics

Simplicity Test equipment Fault and operation

indicators

Accessibility Assembly/disassembly

tool or maintenance tool

Skills of maintenance

personnel

Assembly/

disassembly

Maintenance environment

Standardization Documentation Other ergonomics factors

Modularization

Test points layout

Table 3

Pair-wise comparison scale for AHP preferences

Numerical rating Verbal judgments of preferences

9 Extremely preferred

7 Very strongly preferred

5 Strongly preferred

3 Moderately preferred

1 Equally preferred

2, 4, 6, 8 are in the middle scale.

L. Chen, J. Cai / Reliability Engineering and System Safety 81 (2003) 147–154150

index (refer to Table 4) for matrices of size n and lmax is

the principal eigenvalue of the matrix. The CR value is

acceptable, if it does not exceed 0.10. If it is more, the

judgement matrix is inconsistent. To obtain a consistent

matrix, judgements should be retrieved and improved.

For example, if we consider the following five factors:

simplicity, assembly/disassembly, standardization, tools,

and skills of maintenance personal, the factor set V ¼

{V1;V2;V3;V4;V5}: A pair-wise comparison matrix (size

5 £ 5) is constructed as described in step1 and 2 as follows:

V1 V2 V3 V4 V5

V1

V2

V3

V4

V5

1

0:5

0:33

0:33

0:25

���������������

2

1

0:33

0:33

0:25

3

3

1

0:5

0:33

3

3

2

1

0:5

4

3

3

2

1

���������������The eigenvector of this matrix is (0.33, 0.28, 0.16, 0.13,

0.10)T, which is also the weight vector W : While the

consistency is checked by calculating CR with the

expression given in step 3. Here CR ¼ 0.067.

4.4. Vector Projection Method

The relationship between maintainability and the 13

variables (see Table 1) discussed earlier is represented

mathematically as:

M ¼ f ðV1;V2;…;V13Þ ð1Þ

where M is system maintainability and V1;V2;…;V13 are

variables of the system. Here a so called VPM is developed

to analysis this problem which is a multi-objective

assessment.

In this multi-objective assessment, the factor set is

defined as:

V ¼ {V1;V2;…;Vk}; k ¼ 1; 2;…; 13 ð2Þ

where V1;V2;…;Vk are some or all of the 13 variables of the

system. The alternative set is:

P ¼ {P1;P2;…;Pm}; ð3Þ

aij (i ¼ 1; 2;…;m; j ¼ 1; 2;…; k) is the factor value of

alternative Pi to factor Vj: Matrix A ¼ ðaijÞm£k is developed

as the attribute matrix of alternative set P to factor set V :

In general, there are four types of factors: profit type

which means the bigger the factor value the better; cost

type which means the less the factor value the better; fix type

which means the factor has a fixed optimized value; scope

type which means the factor has a range of optimized value.

For example, if we use system component number to

describe the attribute of simplicity, then it is a profit type

factor.

One thing must be considered, that is different value has

different unit. In order to ensure the fairness of the

evaluation, all the value must be processed on a unitary

scale. To different type of the factor, different processing

ways are developed. Only the factors of profit type and cost

type are covered in this paper, so just two processing

method are discussed.

To profit type factor:

bij ¼aij 2 amin

j

amaxj 2 amin

j

i ¼ 1; 2;…;m ð4Þ

To cost type factor

bij ¼amax

j 2 aij

amaxj 2 amin

j

i ¼ 1; 2;…;m ð5Þ

where amaxj ; amin

j is the maximum and minimum value,

respectively, of the factor Vj:

After the value translation, we get a new attribute matrix

B ¼ ðbijÞm£k: Apparently, the bigger the bij; the better, and to

an ideal alternative Pp; its attribute value is

bpj ¼ max{bijli ¼ 1; 2;…;m} ¼ 1 j ¼ 1; 2;…; k ð6Þ

The weights of these factors could be obtained by using

AHP method:

W ¼ {W1;W2;…;Wk} k ¼ 1; 2;…; 13 ð7Þ

Then we can get a weighted unified attribute matrix, which

is written as

C ¼

V1 V2 · · · Vk

P1

P2

..

.

Pm

Pp

w1b11

w1b21

· · ·

w1bm1

w1

26666666664

w2b12

w2b22

· · ·

w2bm2

w2

· · ·

· · ·

· · ·

· · ·

· · ·

wkb1k

wkb2k

· · ·

wkbmk

wk

37777777775

ð8Þ

In this matrix, each row vector can be treated as an

alternative, and the last row vector is an ideal alternative.

Then there is an angle ai between each alternative and

the ideal alternative, the cosine of this angle is

ri ¼ cos ai ¼Pi £ Pp

kPik £ kPpk

¼

Xn

j¼1wjbij £ wjffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiXn

j¼1½wjbij�

2q ffiffiffiffiffiffiffiffiffiffiffiffiffiffiXn

j¼1½wij�

2q ;

i ¼ 1; 2;…;m; j ¼ 1; 2;…; k

ð9Þ

Table 4

Average random consistency (RI)

Size of matrix 1 2 3 4 5 6 7 8 9 10

Random

consistency

0 0 0.58 0.9 1.12 1.24 1.32 1.41 1.45 1.49

L. Chen, J. Cai / Reliability Engineering and System Safety 81 (2003) 147–154 151

the module of each alternative is

di ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiXn

j¼1

½wjbij�2

vuut i ¼ 1; 2;…;m; j ¼ 1; 2;…; k ð10Þ

the projection of each row vector on the ideal alternative

vector is

Ti ¼ di £ ri i ¼ 1; 2;…;m ð11Þ

where Ti represents the consistency of each alternative and

the ideal alternative, as shown in Fig. 1. Apparently the

bigger the value of Ti; the better the maintainability of the

system. While the ideal alternative has a value of Tp equal to 1.

5. Steps for maintainability evaluation

With the above methods, the evaluation procedure can be

described as follows:

Step 1

Consider the given system and its various design alterna-

tives P ¼ {P1;P2;…;Pm}: Study functions, structure

details, and design features from maintenance point of

view. Identify the maintainability evaluation factors V ¼

{V1;V2;…;Vk} ðk # 13Þ in the given situation. This step

could be much easier by choosing from thirteen evaluation

factors introduced in Section 4.2;

Step 2

For each alternative Pi; i ¼ 1; 2;…;m; assign a value aij for

each factor Vj; j ¼ 1; 2;…; k; k # 13; according to the

system attribute. For those factors that can not be

quantitatively represented, The values can be assigned on

appropriate scale e.g. 0–4 based on the system design

features for the attribute and using MIL-HDBK-472 [19],

which includes design check lists and scoring criteria for

physical design factors, design dictates-facilities and design

dictates-maintenance skills. The attribute takes value 4, if

the system features favor maintainability to the maximum

extent. Then the attribute matrix A ¼ ðaijÞm£k is developed;

Step 3

Identify the type of each factor Vj; j ¼ 1; 2;…; k; k # 13; out

of altogether four different types, namely profit type, cost

type, fix type and scope type. Then using Eqs. (4) and (5) to

translate attribute matrix A ¼ ðaijÞm£k to matrix B ¼

ðbijÞm£k;

Step 4

Use AHP method to calculate the weight of these k factors

expressed as W ¼ {W1;W2;…;Wk}; k ¼ 1; 2;…; 13;

Step 5

Derive the weighted unified attribute matrix C as described

in Eq. (8) Section 4.4;

Step 6

Calculate ri; di; Ti for each alternative Pi; i ¼ 1; 2;…;m:

Refer to Eqs. (9)–(11) Section 4.4;

Step 7

Compare the maintainability of each alternative based on

the Step 6, and identify the best alternative form the

maintenance point of view.

6. Case study

An example of mechanisms of valve-driving system is

considered for illustrating the earlier procedure. The two

design alternatives of the valve-driving system are: rocker

mechanisms and jib mechanisms under hydraulic pressure

shown in Fig. 2. First of all it is necessary to study the

system details as step 1, Section 5. After identifying

the critical component from maintenance point of view,

the identification of maintainability factors is carried out,

which are: design simplicity, assembly/disassembly, stan-

dardization, tools and skills of maintenance personnel.

In this case, some factors like design simplicity and

assembly/disassembly attribute are described in system

components and assembly/disassembly time obtained from

the experiment. Value of other factors like standardization,

tool and skills of maintenance personnel, cannot be obtained

directly from experiment. These factors are therefore

assigned a value with reference to MIL-HDBK-472 [19].

Here only the scoring criteria for maintenance tools is given

in Table 5 for a better understanding.

The value of all these five factors are shown in Table 6

[20].

Then we get the attribute matrix written as

A ¼30 12 2 3 2

21 8 2 2 4

" #ð12Þ

Fig. 1. Representations of ri; di; Ti:

Fig. 2. Two kinds of mechanisms of valve-driving system.

L. Chen, J. Cai / Reliability Engineering and System Safety 81 (2003) 147–154152

where the first row represents mechanism (a), the second

row represents mechanism (b). This completes step 2.

After data translation to a unified scale by using Eqs. (4)

and (5), matrix A becomes as

B ¼0:45 0:77 0:5 0:75 0:5

0:86 0:85 0:5 0:5 1

" #ð13Þ

Here, we set ðamaxj ; amin

j Þlj¼1 ¼ ð40; 18Þ; ðamaxj ; amin

j Þlj¼2 ¼

ð52; 0Þ from the empirical data of design engineers and some

skilled maintenance technicians; and ðamaxj ; amin

j Þlj¼3 ¼

ð4; 0Þ; ðamaxj ; amin

j Þlj¼4 ¼ ð4; 0Þ; ðamaxj ; amin

j Þlj¼5 ¼ ð4; 0Þ

according to the 0–4 scale adopted by MIL-HDBK-472

[19]. This completes step 3.

Using AHP method, we can get the weight of these five

factors (see calculation steps in Section 4.3), W ¼

{0:33; 0:28; 0:16; 0:13; 0:10}; which completes step 4.

Then the weighted unified attribute matrix C is obtained

as per the step 5 as:

C ¼

0:15 0:22 0:08 0:10 0:05

0:28 0:24 0:08 0:07 0:1

0:33 0:28 0:16 0:13 0:10

2664

3775 ð14Þ

r1; d1; T1 and r2; d2; T2 are obtained from expressions (9)–

(11) as per step 6, and the result of the calculation is shown

in Table 7.

Where in Table 7, d; r; T mean the module of each

alternative, the cosine of the angle between each alternative

and the ideal alternative, and the projection of each

alternative, respectively. From the result we see that

mechanism (b) is better. This result is perfectly identical

with the experiment we made. Alternative (a) is composed

by rocker shaft, rocker shaft spring, rocker and valve. It

takes a long time to disassemble and to conduct mainten-

ance. Instead of rocker mechanism, alternative (b) uses the

jib mechanism, then its maintenance is rather easy

compared with alternative (a).

This procedure provides a convenient method to

determine the best design alternative from maintenance

point of view.

7. Conclusion

DFMAIN is introduced as a way to improve maintain-

ability through design. Maintenance, maintainability of

mechanical system is defined. A number of maintainability

guidelines have been presented. These guidelines are used to

develop a set of maintainability factors. An evaluation

method called VPM is presented in this paper as a specific

application of design review to assess and comprise the

system maintainability. The proposed procedure is useful for

designers and practicing engineers to compare various

alternatives of a system from a maintainability point of view.

Acknowledgements

Sponsored by National Science Foundation of China

(Granted No. 59935120).

References

[1] Military Standardization MIL-STD-721. Definitions of terms for

reliability and maintainability, US Department of Defense. 1966.

[2] Billatos SB, Basaly NA. Green technology and design for the

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[3] Thompson G. Improving maintainability and reliability through

desgin. London: Professional Engineering Publishing Ltd; 1999.

[4] Criscimagna, Ned H. Benchmarking of commercial reliability

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[5] Takata S, Saito D. Facility model for life-cycle maintenance system.

Ann CIRP 1995;44(1):117–211.

[6] Vujosevic R, Raskar R. Simulation, animation and analysis of design

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Table 5

Scoring criteria for maintenance tools

Description of scoring criteria Scoring

No supplementary materials are needed to

perform task

4

No more than two pieces of

supplementary materials is need to perform

task

2

Three or more pieces of supplementary

materials are needed

0

Table 6

The value of the maintainability factors of the two systems

Maintainability factors Mechanism (a) Mechanism (b)

Simplicity (in terms of

system components)

30 21

Assembly/disassembly (in terms of

assembly/disassembly time (min))

12 8

Standardization 2 2

Tools 3 2

Skills 2 4

Table 7

Calculation result

Parameter Mechanism (a) Mechanism (b)

d 0.299 0.399

r 0.969 0.980

T 0.290 0.391

L. Chen, J. Cai / Reliability Engineering and System Safety 81 (2003) 147–154 153

[8] Gong L. Study on the evaluation of automobile maintainability.

Liaoning Transportation Technol 1999;8:46–8.

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