Prof G.R.C.Nair

FMEA

Prof GRC Nair

What is FMEA?

Prof G.R.C.Nair

• Failure • Modes• Effects• Analysis

Prof G.R.C.Nair

Reasons to do FMEA• Define and guide a logical design

process • Identify, quantify, and reduce design

risk • Provide a traceable document for

design and development • Justify design activities • Provide a means for continuous

product improvement

Prof G.R.C.Nair

Scope of FMEA• The FMEA is a team function

• All team members must participate• Multi-disciplinary expertise and input is beneficial • Input from all engineering fields is desirable

• Representatives from all areas (not just technical disciplines) are generally included as team members • The DFMEA will be refined and evolve with the

product • Numerous revisions are required to obtain the full

benefit of the DFMEA• The DFMEA should include all systems, sub-systems,

and components in the product design

Availability

• Measure of Availability

MTRMTBFMTBFA

MTBF = mean time between failures

MTR = mean time to repair

MTTF = mean time to fail

MTBF or MTTF= (T1+T2+--+--+TN)/NMTR = (t1+t2+t3+---+---+tN)/ N

Prof G.R.C.Nair

Availability is also called UTRUTR = Up time ratioDTR = down time ratioUTR + DTR = 1

System availability can be increased by • Increasing MTBF• Decreasing MTR• Or any favourable combination

Prof G.R.C.Nair

Reliability• Generally defined as the ability of a product to perform as expected, over a period of time

• Formally defined as the probability that a product, piece of equipment, or system performs its intended function adequately for a stated period of time, under specified operating conditions.

Prof G.R.C.Nair

Types of Reliability

• Inherent reliability – predicted by product design.

• Achieved reliability – observed during use.

Prof G.R.C.Nair

Cumulative Failure Curve

Prof G.R.C.Nair

Average Failure Rate

Prof G.R.C.Nair

Bath Tub Curve

“Infant mortality period”/ Debug period

Prof G.R.C.Nair

To take care of ‘Infant Mortality phase’, consider debugging team, Warrantee,free service, free replacement, pre testing for that period before release to market etc

Prof G.R.C.Nair

Reliability Measurement • Failure rate () – number of

failures of break downs per unit time.

• Alternative measures•Mean time to fail (MTTF)•Mean time between failures(MTBF)MTBF orMTTF

Prof G.R.C.Nair

Reliability Function

1

R(t)

R(t)

Time t

Prof G.R.C.Nair

Reliability Function

• Reliability for a period of ‘t’ r(t) = e-t

• Probability of failure within time ‘t’

f(t) = 1 – e-t Prof G.R.C.Nair

Exercises• MTBF = 500 hrs • Find Also find r (50, 400, 800,

1000,2000)

Prof G.R.C.Nair

System Reliability

Depends on ,• Component Reliability• Number of Components• The Configuration

Prof G.R.C.Nair

Series Configuration

R = r1 * r2 * r3 *... rn

1 2 n

Prof G.R.C.Nair

Parallel Configuration

R = 1 – [(1 - r1) (1 - r2)…… (1 – rn)]

1

2

n

Prof G.R.C.Nair

Optimum Reliability

Reliability

costProf G.R.C.Nair

Prof G.R.C.Nair

Frequency Ranking

Severity Rankings

Prof G.R.C.Nair

• RPN (Risk Priority Number) 1 to 1000• Higher the number higher the risk• RPN= SxOxD each 1 to 10 increasing• S= Severity of failure - 1 to 10• O= Prob of Occurrence- 1 to 10• D= Difficulty in identifying 1 to 10

Prof G.R.C.Nair

Prof G.R.C.Nair

Classification of “Risk”

PossibilityOf Occurrence

Severity of Outcome

High Risk

Moderate Risk

Low Risk