Reliability Maintenance Engineering 1 - 5 Measuring Reliability

Reliability Engineering

Fred Schenkelbergfms@fmsreliability.com

MEASURING RELIABILITY TO IMPROVE SAFETY AND AVAILABILITY

Day 1 Session 5

Objectives

• Reliability and the bathtub curve• Calculating PDF plot• What to do if you don’t have data at failure

mode level• Measuring reliability of Line Replaceable Units• Developing the next generation reliability

growth analysis

Early Life Failures

Decreasing hazard rate

Typical failure causes• Manufacturing defect• Shipping damage• Installation damage

Useful Life

Constant hazard rate

Typical failure causes

• Overstress• Random events

Note: rarely occurs in real life – may be useful if change in hazard rate is small enough

Wear out

Increasing hazard rate

Typical failure causes

• Material wear• Rust/Oxidation• Creep/Crack growth• Embrittlement

Discussion & Questions

Probability Distribution Plot

• Like a histogram• Construction notes

• Continuous distributions

• Discrete distributions

How to use PDF plot

• Probability plots

• How many will survive

• How many will fail

Discussion & Questions

Data and Level

• Failure mechanism level

• Subunit level (LRU)

• Unit Level

• System level

Plots and levels

• CDF with system and subunit information

• Continuous Distributions

• Discrete Distributions

time

Prob

abili

ty o

f Fai

lure

(log

sca

le)

Repairable Equipment

• MCF example

• How to interpret– Concave– Convex– Straight

Time

Cum

ulati

ve F

ailu

res

Discussion & Questions

Reliability Growth

• CDF reflecting changes

• Look for change in slope

• Look for change in scale

time

Prob

abili

ty o

f Fai

lure

(log

sca

le)

or

Cumulative Failures over time

• During development or improvement projects

• Plot total failures over time (more later with Duane and related plots)

Discussion & Questions

Summary

• Reliability and the bathtub curve

• Calculating PDF plot• What to do if you don’t

have data at failure mode level

• Measuring reliability of Line Replaceable Units

• Developing the next generation reliability growth analysis

Measuring Reliability to improveSafety and Availability