Reliability Allocation Technique

Reliability Allocation Technique

F e b r u a r y 2 0 1 1

Reliability Allocation Technique | February 2011

© 2011, HCL Technologies. Reproduction Prohibited. This document is protected under Copyright by the Author, all rights reserved.

TABLE OF CONTENTS

Abstract ............................................................................................. 3

Abbreviations .................................................................................... 4

Market trend/ Challenges .................................................................. 5

Solution ............................................................................................. 7

Best Practices ................................................................................. 11

Common Issues .............................................................................. 12

Conclusion....................................................................................... 13

Reference ........................................................................................ 14

Author Info ....................................................................................... 14



3

Abstract

Every system has a reliability goal that needs to be achieved.

Reliability allocations are used to set the goals for various sub-

system or functional blocks such that the overall system level

reliability can be achieved in an effective way. There are various

methodologies that exist to provide guidelines on the allocation

techniques, which are more theoretical. The allocation technique

described here ensures that the allocation is done rationally with

consideration of factors such as complexity, state of art and duty

cycle of the functional group or sub-system. This approach was

experimented in one of the HCL Project and found the approach to

be more pragmatic than compared to the other allocation

techniques.



4

Abbreviations

Sl. No. Acronyms (Page No.) Full form

1 AGREE (5) Advisory Group

on Reliability of

Electronic

Equipment

2 C (8,9,10) Complexity

3 Co (8,9,10) Cost of Reliability

Enhancement

4 Cr (8,9,10) Criticality

5 D (10,11,12) Duty Cycle

6 R (7,10) Reliability

7 Rt (7) Target Reliability

8 S (8,9,10) State of the art

9 Z (8,9,10) Proportionality

Factor

10 (7,10) Weightage factor



5

Market trend/ Challenges

Generally, as mentioned in the abstract, there are several methods

to do a reliability allocation. These methods are selected based on

the information available about the system on its application.

1. Equal Apportionment

The simplest method of all for allocating reliability is to distribute the

target reliability equally among all sub-systems. For example, if the

target reliability for a system is 0.95 and it has got 3 sub-systems,

then the uniform allocation of reliability to all the components may

require each component to have a reliability of 0.984. While this is

the easy method of allocating the reliability, it is not always the best

method of allocation. The weakness of this method is that

subsystem goals are not assigned in accordance with the degree of

difficulty associated with realization of allocated target reliability.

2. ARINC

The ARINC method assumes that failure rates of the subsystems

are known. These rates can be obtained from either existing failure

data or failure rate prediction standards. This method reduces

subsystem failure rates by equal percentages such that the failure

rate goal is reached. The rationale behind the ARINC method is that

it requires equal effort to reduce failure rates by an equal

percentage of failure rates. Even if the effort to reduce the failure

rate increases non-linearly with the percentage of failure rate, it

leads to the minimum overall effort needed to achieve the failure

rate goal.

3. AGREE

This method takes into consideration subsystem complexity,

mission time, and importance. Equal effective failure consequences

are allocated to all elements. Failure rate allocation of a subsystem

is proportional to its complexity. Failure rate allocation of a

subsystem is inversely proportional to its mission time. Failure rate

allocation of a subsystem is inversely proportional to its importance.

When the importance of all elements is the same, the AGREE

method allocates equal effective failure rates for all elements.

4. Feasibility of Objectives

This method was developed primarily to allocate the reliability of

non-repairable mechanical-electrical systems. Subsystem allocation

factors are computed as a function of numerical ratings of system

intricacy, state of the art, performance time, and environmental

conditions. On the basis of their experience, design engineers

estimate and assign ratings on scales from 1 to 10.

Reliability is not by Chance



6

Challenges

The four methods described earlier are available for the reliability

engineer to do the reliability allocation. The result of allocation

varies based on the method used and is always debatable. The

choice and the subjectiveness in the method selection results in

variation of allocated targets and hence leads to situations where

the sub systems / modules are over / under budgeted with respect

to reliability. Even the most sought Feasibility of objectyive

methodology focus on only four important factors and misses out

other factors which could have a sever impact on module /

subsystem reliability.

If the reliability allocation result is very stiff / easy, it has different

impacts on the product design . The stiff target could puts pressure

on the designers in selection of COTS items, material, process and

increase the cost while the easy target means the design is not

sufficiently challenged and is putting extra allocation on complex

sub systems.

The challenge stated above opens a area of research to devise a

new technique which can be tailored each time based on the

product application and product knowledge. This will help to

minimize the subjectiveness and helps to consider each / every

possible factor which influence the reliability of the module / sub

system.



7

Solution

In the theoretical approach of most sought feasibility of objective

methodology, only four factors such as Intricacy, State of Art,

Operating time and Environment were considered, but in our

approach, we have considered cost and criticality factors which play

a vital role in product development and product safety. This

approach is defined as “Modified Feasibility of Objectives” reliability

allocation technique and can be tailored with additional factors to

refine the allocation depending on the product and its application.

Approach

As we are now aware, Reliability allocation is the process of

allocation of system reliability target to sub-system or functional

groups according to rational factors so that the target reliability

requirement or goal into subsystem and component requirements or

goals. The balancing act of allocation is done as per the below

process:

Identification of all the functions associated with the

system.

Grouping of system units according to its function which

should be traceable to system function

Identification of applicable rational factors that should

be considered for apportionment

Assignment of relative grades to each functional group

under respective factor considered for apportionment.

Allocation of apportioned reliability to each functional

group with weightage factor.

Rational Factors Considered

The relationship between apportioned reliability of ith functional

group (Ri) and target reliability (Rt) is defined with weightage factor

(ωi)

R i = (Rt)ω

i

Also weightage factor (ωi) can be expressed with proportionality

factor (Zi)

ωi = Zi / ΣZi

Proportionality factor (Zi) in turn bears relationship with various

rational factors considered for apportionment. In following paragraph

such relationships has been defined considering the factors

mentioned. The factors can be increased / decreased based on

product and its application.

Reliability Allocation is the

Delicate Art of Balancing the

Budget



8

Complexity (C)

Higher the complexity of functional group, more difficult it would be

to attain the target reliability. Therefore the functional group having

relatively higher complexity should be allocated lower reliability

target. The following guidelines have been adhered to arrive at the

relative grade for complexity

- Multiple functional relationships with the

other groups

- Architectural complexity with higher number

of components

Criticality (Cr)

Functionally critical sub-systems should be allocated higher

reliability target and thus Zi is proportional to criticality. The following

guidelines have been adhered to arrive at the relative grade for

Criticality

- The failure effect of the functional group on

system

- Frequency of failure

State of Art (S)

It is expected that state-of-art system should have higher reliability.

The functional group with high novelty should be apportioned higher

reliability target. Relative consideration of novelty of technology

used is considered to arrive at the relative grade for Start of Art

Cost (Co)

Higher the reliability enhancement cost, lower should be the

apportioned target. This relationship defines the practical approach

towards attaining higher reliability targets.

Duty Cycle (D)

The functional groups with higher duty cycles should be apportioned

higher reliability target. It provides assurance under continuous

operation of functional group the desired level of reliability is

maintains. Relative operational time of the functional group w.r.t the

total system operating time is considered to arrive at the relative

grade for Duty Cycle

Thus proportionality factor can be expressed as

Z i = C* Co /(S * Cr * D)



9

Table 1 provides the guidelines for selecting a relative grade factor

Table 1 – Guideline on selection of relative grade factor

Factors

Scale C Cr S Co D

10

High High Novelty-High High High 9

8

7

Medium Medium Novelty-Medium Medium Medium

6

5

4

3

Low Low Novelty-Low Low Low 2

1



10

Table 2 shows the reliability apportionment done for a project

(Indigo - Life Science), where the system reliability requirement was

0.99907 for its 10 hr operation.

Table 2 – Guideline on selection of relative grade factor

Functional

Group (C) (Cr) (S) (Co) (D) Zi ωi Ri

PCB 9 10 8 6 10 0.0675 0.02482 0.99998

Valve 5 5 5 9 2 0.9000 0.33093 0.99969

Sensors 5 6 5 7 10 0.1166 0.04289 0.99996

Backup Ckt 3 6 2 3 2 0.3750 0.13788 0.99987

Display 6 8 4 5 10 0.0937 0.03447 0.99997

Heater 3 3 3 7 2 1.1666 0.42898 0.99960



11

Best Practices

However the industry and the reliability practitioners are still aligned

to the feasibility of objective methodology against other methgods

mentiuoned, the successful use of the proposed allocation method

provides a unique approach to consider the cost, criticality and

other factors of the product for reliability allocation. This approach

has helped the design engineers to focus on improving the reliability

of the functional group with consideration of Cost constraint and the

Safety of the product in the project refered in the paper.

The proposed methodology was successfully used for different

projects.

The process of allocation of relative grades should to be carried out

as a team exercise, comprising of experienced members from the

each of the functional group identified.

Step with care and great tact

And remember that Life‟s a

Great Balancing Act

Just never forget to be

dexterous and deft

And never mix up your right

foot with your left.

- Dr. Suess, Oh, the

places You‟ll Go.



12

Common Issues

The major challenge in the propsed solution is the subjectivity in

selecting the factors and then rating them as per the method

proposed. This subjectiveness varies with the experience of the

reliability practitioner. For example, it may be possible for different

practitioners to select different factors (apart from the factors

suggested here) and they can grade them differently based on the

individual / team knowledge. However despite this challenge, this

method deemed to be best suited to address the challenges for

doing the reliability allocation.

Common Issues are not so

common

Commom Issues are not SO

Common



13

Conclusion

Reliability Allocation is always a tricky task which needs “balancing

act” to allocate reliability targets to the components or sub-system

and also ensure that the reliability requirements are met as a

system, without compromising on performance or cost or safety of

the product.

Reliability allocation by “Modified Feasibility of Objectives” method

helps us to perform this “balancing act” in a pragmatic approach

rather than a theoretical approach. This methodology can be

applied to any system which needs reliability allocation to be

performed for its sub-systems or functional groups.

Further studies can be performed to eliminate the subjectivity in

selecting the relative grade factor.

The authors also suggests the allocation exercise to be rationally

planned and implemented so that it will be feasible to meet overall

system reliability target. Apart from the proposed top down

approach of reliability allocation, one can try the non conventional

bottoms up approach of reliability allocation where reliability data for

similar components exits and can be best used for this exercise.

Conclusions are most of times

“The Beginning”



14

Reference

[1] Reliability Resources in www.relex.com

[2] Reliability Allocation Report – Indigo

[3] Paper by Prof K.B.Mishra on Reliability Allocation Technique.

[4] Reliability and Six Sigma By Dinesh Kumar, U. Dinesh Kumar

Author Info

Prateeck Biswas: Mr. Biswas is a reliability

consultant and has been working in the field of

reliability and quality for more than 25 years,

including about 20 years with aviation industries.

He is heading reliability engineering practice in

HCLT for last 5 years. He holds his post-

graduate degree from Indian Institute of

Technology (IIT), Bombay in Reliability

Engineering. He has been associated with the

leading aviation industries of the country like

Indian Airlines, Hindustan Aeronautics Limited

and Aeronautical Development Agency (ADA).

He has worked as a senior reliability professional

and a led team of engineers to carry out R&S

analysis on civil and military development aircraft

projects during his association with ADA. His

work on aircraft system safety assessment and

lessons learnt in R&S during development of

aircraft has been published in Annual Reliability

and Maintainability Symposium (RAMS), a

renowned international journal in the field of

relibility. His work on measurement uncertainty

has also been published in international journal.

He has worked extensively to propagate

reliability concepts to practicing engineers at

various national workshops and seminars.

But it's not just a game of

finding literary references.

- Dan Simmons

http://www.relex.com/



15

Arunkumar S: Mr. Arunkumar is a reliability

practitioner and has been working in the field of

reliability and quality for more than 12 years He

holds his engineering in electrical and

electronics. He is a Certified Reliability Engineer

and a Certified Quality Engineer from American

Society of Quality. He is also a Certified Green

Belt.

He has been associated with HCL Technologies

Ltd from last 4 years and has worked on various

aerospace, hitech & life sciences programs on

Reliability planning, Reliability testing, Project

Planning, Co-ordination, Data Analysis and

System Safety. and has good exposure to

various standards and tools used in reliability

engineering. Prior to HCL technologies, he has

served as reliability practioner in Honeywell.

Abhay Waghmare: Mr. Abhay Waghmare is a

reliability practitioner and has been working in

the field of reliability and design for more than

10 years. He holds his post graduate degree in

Production Engineering from REC Allahabad

and his bachelors in Mechanical Engineering

from Nagpur University. He is a Certified

Reliability Engineer from American Society of

Quality. He has undergone the two semester

course in Reliability Engineering & Testing

under Prof. Dimitri from Univeristy of Arizona.

He is also a Certified Green Belt from GE.

He has been associated with HCL Technologies

Ltd from last 5 years and has worked on various

aerospace, hitech & life sciences programs on

Reliability planning, Reliability testing, Project

Planning, Co-ordination, Data Analysis and

System Safety. and has good exposure to

various standards and tools used in reliability

engineering. Prior to HCL technologies, he has

served as reliability practioner in GE India.

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About HCL Technologies HCL Technologies is a leading global IT services company, working with clients in the areas that impact and redefine the core of their businesses. Since its inception into the global landscape after its IPO in 1999, HCL focuses on „transformational outsourcing‟, underlined by innovation and value creation, and offers integrated portfolio of services including software-led IT solutions, remote infrastructure management, engineering and R&D services and BPO. HCL leverages its extensive global offshore infrastructure and network of offices in 26 countries to provide holistic, multi-service delivery in key industry verticals including Financial Services, Manufacturing, Consumer Services, Public Services and Healthcare. HCL takes pride in its philosophy of „Employee First‟ which empowers our 72,267 transformers to create a real value for the customers. HCL Technologies, along with its subsidiaries, had consolidated revenues of US$ 3.1 billion (Rs. 14,101 crores), as on 31

st

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HCL is a $5.5 billion leading global technology and IT enterprise comprising two companies listed in India - HCL Technologies and HCL Infosystems. Founded in 1976, HCL is one of India's original IT garage start-ups. A pioneer of modern computing, HCL is a global transformational enterprise today. Its range of offerings includes product engineering, custom & package applications, BPO, IT infrastructure services, IT hardware, systems integration, and distribution of information and communications technology (ICT) products across a wide range of focused industry verticals. The HCL team consists of over 77,000 professionals of diverse nationalities, who operate from 29 countries including over 500 points of presence in India. HCL has partnerships with several leading Global 1000 firms, including leading IT and technology firms For more information, please visit www.hcl.com

Reliability Allocation Technique

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