Reliability Engineering

Fred Schenkelbergfms@fmsreliability.com

RELIABILITY TECHNIQUES TO IMPROVE PERFORMANCE

Day 2 Session 2

Objectives

• Examining opportunities and reliability improvement techniques for robust design– RCM, FMEA, RCA, 6 step and FRETT

• Developing corrective actions and determining effectiveness

• Implementing the growth curve technique• Fault and failure forecasting

FMEA

• Organized risk analysis– Severity– Occurrence– Detection

• Use in Design process

RCA

• Root Cause Analysis– Determine fundamentals– Initiating event– Contributing events

• Use in design process

6 Steps+1

FRETT

Cause categories• design errors• material defects• fabrication and processing

errors• assembly and installation

deficiencies• maintenance-related or

procedural errors• unintended operating

conditions• operator error

Failure mode sets

• Force

• Reactive environment

• Temperature

• Time

Discussion & Questions

Corrective Action

• Plan Do Check Act process

• Experiments and measures

• Verify results

• Document results

Plan

Do

Check

Act

An Example

• Define problem

• Plan response

• Do experiment

• Monitor results

• Implement fix

Monitor Effectiveness

• What do you measure?

• What can you measure?

• Does absence of failure mean it’s fixed?– What is expected failure

rate?– What is probability of

failure?

Monitoring

• How to determine monitoring plan?

• Sample size?

• Duration?

• Focus on failure mechanism

Discussion & Questions

Example

Repair rate over time

• Trend plots– MCF – cumulative

failures over time– Inter arrival times– Reciprocals of inter

arrival times

• Duane Plot

• Sample Data• 1500 hour test• With each failure RCA

and improvements implemented

Observed failure times5, 40, 43, 175, 389, 712, 747, 795, 1299, 1478 hrs

Cumulative failure vs. time (MCF)

• Failure count vs time

• Straight line (roughly) means steady failure rate.

• Curve down – improvement over time

• Curve up – increasing problems over time

NIST Engineering Statistics Handbook 8.2.2.3

Inter arrival failure times

• Plot waiting time between failures

• Trend up – improvement

• Trend down – degradation

• Straight – no change over time

Reciprocal Inter arrival times

• Plot failure rate estimates since last failure

• Trend up – • degradation• Trend down –

improvement• Straight – no change

over time

Duane Plot

Time Cum MTBF

5 5

40 20

43 14.3

175 43.75

389 77.8

712 118.67

747 106.7

795 99.4

1299 144.3

1478 147.8

Discussion & Questions

Predictions & Forecasts

Risks

• Might just be wrong

• May miss failure mechanisms due to masking

• Unable to predict everything

Benefits

• Awareness of probability of failure

• Awareness of what to expect to fail

• Maintenance and logistics planning

Predictions & Forecasts

• Empirical models– Extension of field data– Extensions of

experiments– Extensions of vendor

data

• First principle models– Difficult to create– Does it apply in this

situation

Cautions

• Check assumptions• Check sensitivity

• Verify models and fits• Be conservative

The further from reality the more risk of being

wrong exists

Monitor and Adjust

• All models are wrong, some are useful

• Continue to refine and challenge any model

• RCA with focus on failure mechanisms is key to success

Discussion & Questions

Summary

• Examining opportunities and reliability improvement techniques for robust design– RCM, FMEA, RCA, 6 step and

FRETT• Developing corrective

actions and determining effectiveness

• Implementing the growth curve technique

• Fault and failure forecasting

Reliability techniques to improve performance