FMEA

TOTAL QUALITY MANAGEMENT

325

Fig 4.8 OEE

4.5 FAILURE MODE AND EFFECT ANALYSIS (FMEA):

A failure mode and effect analysis (FMEA) is a procedure in product development and operations management for analysis of

PDF processed with CutePDF evaluation edition www.CutePDF.com

TOTAL QUALITY MANAGEMENT

326

potential failure modes within a system for classification by the severity and probability of the failures. A successful FMEA activity helps a team to identify potential failure modes based on past experience with similar products or processes, enabling the team to design those failures out of the system with the minimum of effort and resource expenditure, thereby reducing development time and costs. It is widely used in manufacturing industries in various phases of the product life cycle and is now increasingly finding use in the service industry. Failure modes are any errors or defects in a process, design, or item, especially those that affect the customer, and can be potential or actual. Effects analysis refers to study the consequences of those failures.

Failure Mode and Effect Analysis (FMEA), also known as risk analysis, is a preventive measure to systematically display the causes, effects, and possible actions regarding observed failures.

The objective of FMEA is to expect failures and prevent them from occurring. FMEA prioritise failures and attempts to eliminate their causes.

FMEA is an engineering technique used to define, identify and eliminate known and / or potential failures, problems, errors which occur in the system, design, process and service ‘before they reach the customer’.

FMEA is a ‘before-the-event’ action and is done when existing systems / products / processes are changed or redesigned. FMEA is a never-ending process improvement tool.

TOTAL QUALITY MANAGEMENT

327

Fig 4.9 FMEA Cycle

4.5.1 BASIC DEFINITIONS

Failure

"The loss of an intended function of a device under stated conditions."

Failure mode

"The manner by which a failure is observed and it generally describes the way the failure occurs."

Failure effect

Immediate consequences of a failure on operation, function or functionality, or status of some item

Indenture levels

An identifier for item complexity. Complexity increases as levels are closer to one.

TOTAL QUALITY MANAGEMENT

328

Local effect

The Failure effect as it applies to the item under analysis.

Next higher level effect

The Failure effect as it applies at the next higher indenture level.

End effect

The failure effect at the highest indenture level or total system.

Failure cause

Defects in design, process, quality, or part application, which are the underlying cause of the failure or which initiate a process which leads to failure.

Severity

"The consequences of a failure mode. Severity considers the worst potential consequence of a failure, determined by the degree of injury, property damage, or system damage that could ultimately occur."

4.5.2 TYPES OF FMEA:

The several types of FMEA includes,

System FMEA – Analyzes components, subsystem and main system in early stage of design.

Design FMEA – Analyzes the products / parts before they are releases to manufacturing.

TOTAL QUALITY MANAGEMENT

329

Process FMEA – Focuses on manufacturing and assembly processes.

Service FMEA – Analyzes service industry processes before they are released to impact the customer.

Equipment FMEA.

Maintenance FMEA.

Concept FMEA.

Environmental FMEA.

However in practice, all the above types can be broadly categorized into two types they are

4.5.2.1 DESIGN FMEA:

• Design FMEA involves the analysis of the potential failures and services due to component or subsystem unreliability.

• Design FMEA is to establish priorities based on expected failures and severity of those failures.

FMEA can provide an analytical approach, when dealing with potential failure modes and their associated causes. When considering possible failures in a design – like safety, cost, performance, quality and reliability – an engineer can get a lot of information about how to alter the development/manufacturing process, in order to avoid these failures. FMEA provides an easy tool to determine in which risk has the greatest concern, and therefore an action is needed to prevent a problem before it arises.

TOTAL QUALITY MANAGEMENT

330

4.5.2.2 PROCESS FMEA:

• Process FMEA involves a failure analysis of a manufacturing process.

• The process FMEA is used primarily to identify critical areas of control and to emphasize the design and more reliable process.

4.5.3 BENEFITS OF FMEA:

Improve product / process reliability and quality. Increase customer satisfaction. Early identification and elimination of potential product

/ process failure modes. Prioritize product / process deficiencies. Capture engineering / organization knowledge. Document and track the actions taken to reduce risk. Provide focus for improved testing and development. Minimize late changes and associated cost. Act as catalyst for teamwork and idea exchange

between functions.

4.5.4 INPUTS FOR PREPARATION OF FMEA:

People inputs :

The FMEA methodology is a team effort. The FMEA team should have an assembly engineer, manufacturing engineer, materials engineer, quality engineer, service engineer, suppliers and the customer.

Data inputs:

TOTAL QUALITY MANAGEMENT

331

The data inputs needed to prepare FMEA are product and process specifications, reliability data, customer priority data, process variability data, process description, and inspection data.

4.5.5 STAGES OF FMEA (FMEA METHODOLOGY)

The FMEA methodology has four stages. They are:

Stage 1. Specifying Possibilities

(i) Functions (ii) Possible failure modes (iii) Root causes (iv) Effects (v) Detection / prevention

Stage 2. Quantifying Risk

(i) Probability of cause (ii) Severity of effect (iii) Effectiveness of control to prevent cause (iv) Risk priority number (RPN)

Stage 3. Correcting high risk causes

(i) Prioritizing work

(ii) Detailing action

(iii) Assigning action responsibility

(iv) Check points on completion

Stage 4. Re-evaluation of risk

(i) Recalculation of risk number.

TOTAL QUALITY MANAGEMENT

332

4.5.6 FMEA PROCEDURE (Eg of a FMEA Form- Table 4.1)

The basic steps for implementation of a FMEA are outlined below.

Describe the product / process and its function.

Create a block diagram of the product / process:

The below diagram shows the logical relationship of components and establishes a structure around which the FMEA can be developed.

Complete the header of the FMEA form worksheet.

Item, design / process responsibility (i.e., team leader), prepared by, model number / year, key date, core team (i.e., team members name), and revision date. Modify these headings as needed.

List product / process functions.

Identify failure modes:

• A failure mode is defined as the manner in which a component, subsystem, system process, etc., could potentially fail to meet the design purpose.

• Examples of potential failure modes include:

Corrosion, torque, fatigue, deformation, cracking, electrical short or open, and hydrogen embrittlement.

Describe the potential failure effects:

• For each failure mode identified the engineer should determine what the ultimate effect will be.

• A failure effect is defined as the result of a failure mode on the function of the product / process as perceived by the customer.

• Examples of failure effects include :

TOTAL QUALITY MANAGEMENT

333

Injury to the user, impaired operation, poor appearance, odours, noise, and degraded performance.

Establish a numerical ranking for the severity (S) of the effect:

• Severity (S) is the assessment of the seriousness of the failure effect. (Detailed explanations given in the forecoming pages)

• A common industry standard scale uses 1 to represent no effect and 10 to indicate very serious effect.

• This numerical ranking enables the engineer to prioritize the failures and address the real big issues first.

The class column is used to classify any special product characteristics for components, sub-systems, or systems that may require additional process controls.

Identify the potential causes / mechanisms of failure:

• A failure cause is defined as a design weakness that may result in a failure.

• The potential causes for each failure mode should be identified and documented.

• The causes should be listed in technical terms and not in terms of symptoms.

• Examples of potential causes include:

Improper torque applied, improper operating conditions, contamination, erroneous algorithms, improper alignment, excessive loading, and excessive voltage.

TOTAL QUALITY MANAGEMENT

334

Enter the probability factor:

• Occurrence (O) is the chance that one of the specific causes / mechanics will occur. (Detailed explanations given in the fore-coming pages)

• A numerical weight should be assigned to each cause that indicates how likely that cause is (i.e., probability of the cause causing).

• A common industry standard scale uses 1 to represent not likely and 10 to indicate inevitable.

Identify current controls (design or process):

• Current controls (design or process) are the mechanisms that prevent the cause of the failure mode from occurring or which detect the failure before it reaches the customer.

• These controls may be supported through tests, mathematical studies, feasibility reviews, and prototype testing.

Determine the likelihood of detection (D):

• Detection (D) is an assessment of the likelihood that the current controls will detect the cause of the failure mode or the failure mode itself. (Detailed explanations given in the fore coming pages)

• The likelihood of detection is also based on a 1 to 10 scale, with being the certain of detection and 10 being the absolute uncertainty of detection.

Review risk priority number (RPN):

• The risk priority number (RPN) is defined as the product of the severity (S), occurrence (O), and detection (D) rankings.

TOTAL QUALITY MANAGEMENT

335

That is, RPN = Severity × Occurrence × Detection

or RPN = (S) × (O) × (D)

• The RPN is used to prioritize items that require additional quality planning or action.

Determine recommended action(s):

• Determine recommended action(s) to address potential failures that have a high RPN.

• These actions may include specific inspection, testing, de-rating, selection of parts and materials, redesign of the items, monitoring mechanics, and performing preventive maintenance.

Assign responsibility and a target completion date for these actions. This makes responsibility clear-cut and facilitates tracking.

Indicate actions taken:

• After these actions have been taken, re-assess the severity, occurrence and detection and review the revised RPN’s.

Update the FMEA as the design or process changes, the assessment changes or new information becomes known.

336    

Failu

re M

ode

Effe

ct A

naly

sis

(D

esig

n / P

roce

ss F

MEA

)

FMEA

Num

ber _

____

____

____

____

____

____

____

____

__ P

age_

____

____

____

____

____

___o

f___

____

____

____

____

____

_

Item

___

____

____

____

____

____

____

____

____

____

_Des

ign

/ Pro

cess

Res

pons

ibili

ty _

____

____

____

____

____

____

____

__

Prep

ared

By_

____

____

____

____

____

____

____

_Mod

el N

umbe

r / Y

ear _

____

____

____

___

Key

Dat

e __

____

____

____

___

FMEA

Dat

e (O

rig) _

____

____

____

___

(Rev

)___

____

____

____

____

____

Cor

e Te

am _

____

____

____

____

____

____

____

____

____

____

____

____

____

____

____

____

____

____

____

____

____

____

___

Func

tion 

Failu

re

mod

e Ef

fect

s  

S (s

ever

ity

ratin

g) 

Cau

se(s

)  

O

(occ

urre

nce

ratin

g) 

Cur

rent

co

ntro

ls 

D

(det

ectio

n ra

ting) 

CR

IT

(crit

ical

ch

arac

teris

tic 

RPN

(r

isk

prio

rity

num

ber)

Rec

omm

ende

d ac

tions 

Res

pons

ibili

ty

and

targ

et

com

plet

ion

date

Act

ion

take

n

Tabl

e 4.

1FM

EA W

orks

heet

TOTA

L Q

UA

LITY

MA

NA

GEM

ENT

337

Func

tion

Failu

re

mod

e Ef

fect

s S

(sev

erity

ra

ting)

Ca

use(

s)

O (o

ccur

ren

ce ra

ting)

Curre

nt

cont

rols

D (d

etec

tion

ratin

g)

CRIT

(c

ritica

l ch

arac

teris

tic

RPN

(risk

pr

iorit

y nu

mbe

r)

Reco

mm

ende

d ac

tions

Resp

onsib

ility

and

targ

et

com

plet

ion

date

Ac

tion

take

n

Fill tu

b Hi

gh

level

sens

or

neve

r trip

s

Liquid

sp

ills

on

custo

mer

floor

8 lev

el se

nsor

fai

led

level

sens

or

disco

nnec

ted

2 Fil

l tim

eout

base

d on

time

to fill

to

low

level

sens

or

5 N

80

Perfo

rm co

st an

alysis

of

addin

g ad

dition

al se

nsor

ha

lfway

be

twee

n low

an

d high

lev

el se

nsor

s

XYZ

15/06

/2011

Ta

ble

4.2

FMEA

Exa

mpl

e

338   

Step 1: Occurrence

In this step it is necessary to look at the cause of a failure mode and how many times it occurs. This can be done by looking at similar products or processes and the failure modes that have been documented for them. A failure cause is looked upon as a design weakness. All the potential causes for a failure mode should be identified and documented. Again this should be in technical terms. Examples of causes are: erroneous algorithms, excessive voltage or improper operating conditions. A failure mode is given an occurrence ranking (O), again 1–10. Actions need to be determined if the occurrence is high (meaning > 4 for non-safety failure modes and > 1 when the severity-number from step 1 is 9 or 10). This step is called the detailed development section of the FMEA process. Occurrence also can be defined as %. If a non-safety issue happened less than 1%, we can give 1 to it. It is based on your product and customer specification.

Rating Meaning

1. No effect

2/3 Low (relatively few failures)

4/5/6 Moderate (occasional failures)

7/8 High (repeated failures)

9/10 Very high (failure is almost inevitable)

Step 2: Severity

Determine all failure modes based on the functional requirements and their effects. Examples of failure modes are: Electrical short-circuiting, corrosion or deformation. A failure

TOTAL QUALITY MANAGEMENT

339

mode in one component can lead to a failure mode in another component, therefore each failure mode should be listed in technical terms and for function. Hereafter the ultimate effect of each failure mode needs to be considered. A failure effect is defined as the result of a failure mode on the function of the system as perceived by the user. In this way it is convenient to write these effects down in terms of what the user might see or experience. Examples of failure effects are: degraded performance, noise or even injury to a user. Each effect is given a severity number (S) from 1 (no danger) to 10 (critical). These numbers help an engineer to prioritize the failure modes and their effects. If the severity of an effect has a number 9 or 10, actions are considered to change the design by eliminating the failure mode, if possible, or protecting the user from the effect. A severity rating of 9 or 10 is generally reserved for those effects which would cause injury to a user or otherwise result in litigation.

Rating Meaning 1 No effect 2 Very minor (only noticed by discriminating customers)

3 Minor (affects very little of the system, noticed by average customer)

4/5/6 Moderate (most customers are annoyed)

7/8 High (causes a loss of primary function; customers are dissatisfied)

9/10 Very high and hazardous (product becomes inoperative; customers angered; the failure may result in unsafe operation and possible injury)

Step 3: Detection

When appropriate actions are determined, it is necessary to test their efficiency. In addition, design verification is needed. The

TOTAL QUALITY MANAGEMENT

340

proper inspection methods need to be chosen. First, an engineer should look at the current controls of the system, that prevent failure modes from occurring or which detect the failure before it reaches the customer. Hereafter one should identify testing, analyzing, monitoring and other techniques that can be or have been used on similar systems to detect failures. From these controls an engineer can learn how likely it is for a failure to be identified or detected. Each combination from the previous 2 steps receives a detection number (D). This ranks the ability of planned tests and inspections to remove defects or detect failure modes in time. The assigned detection number measures the risk that the failure will escape detection. A high detection number indicates that the chances are high that the failure will escape detection, or in other words, that the chances of detection are low.

Rating Meaning 1 Almost certain 2 High 3 Moderate 4/5/6 Moderate - most customers are annoyed7/8 Low 9/10 Very remote to absolute uncertainty

After these three basic steps, risk priority numbers (RPN) are calculated

Risk Priority Numbers

RPN plays an important part in the choice of an action against failure modes. They are threshold values in the evaluation of these actions.

After ranking the severity, occurrence and detection the RPN can be easily calculated by multiplying these three numbers:

TOTAL QUALITY MANAGEMENT

341

RPN = S × O × D

This has to be done for the entire process and/or design. Once this is done it is easy to determine the areas of greatest concern. The failure modes that have the highest RPN should be given the highest priority for corrective action. This means it is not always the failure modes with the highest severity numbers that should be treated first. There could be less severe failures, but occur more often and are less detectable.

After these values are allocated, recommended actions with targets, responsibility and dates of implementation should be noted. These actions can include specific inspection, testing or quality procedures, redesign (such as selection of new components), adding more redundancy and limiting environmental stresses or operating range. Once the actions have been implemented in the design/process, the new RPN should be checked, to confirm the improvements. These tests are often put in graphs, for easy visualization. Whenever a design or a process changes, an FMEA should be updated.

A few logical but important thoughts come in mind:

• Try to eliminate the failure mode (some failures are more preventable than others)

• Minimize the severity of the failure

• Reduce the occurrence of the failure mode

• Improve the detection