Design for Reliability

OBJECTIVES● Define and explain reliability and reliability

concepts;● Identify measures of reliability; and● Use reliability tools

OUTLINE1. Introduction

a. What is reliabilityb. Reliability concepts

2. Measures of Reliabilitya. Failure Rateb. MTTFc. MTBF

3. Steps in Designing for Reliability

4. Reliability Toolsa. Block Diagramb. P-diagramc. QFDd. FMEAe. Design Verification

Planf. Reliability

Demonstration Matrix

DESIGN FOR RELIABILITY• Developed by MPD Cohort Team

(team members: Julie Earle, Dave Herczeg, and Jim Van Glider)

• Developed in 2002

DESIGN FOR RELIABILITYWhy design for reliability?● it can make or break the long-term success

of a product:o too high reliability = too expensive producto too low reliability = high warranty and repair costs

(market share will be lost)● aka design durability, design for robustness,

design for useful life

RELIABILITYGeneral Definition

● the ability of a product to perform as expected over time

Formal Definition

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

RELIABILITYFurther Definitions

● elimination or avoidance of failure modes

● the probability that a product will perform its intended function:o under customer operating conditionso for a specified lifeo in a manner that meets or exceeds customer

expectations

● a reliable product is robust and mistake-free

RELIABILITY

TYPES OF FAILURES

1. Functional failureo failure that occurs at the start of product

life due to manufacturing or material detects

2. Reliability failureo failure after some period of use

RELIABILITY

TYPES OF RELIABILITY

1. Inherent reliabilityo predicted by product design.

2. Achieved reliabilityo observed during use.

RELIABILITY

Reliability Failure Modes● Two Types of Failure Mode

1. hard (when something breaks)2. soft (when the performance degrades)

● Two Root Causes1. lack of robustness2. mistakes

MEASURES OF RELIABILITY

Failure Rate (λ)● number of failures per unit time

“Infant mortality period”

MEASURES OF RELIABILITY

Mean Time To Failure (MTTF)● average time of operation to first failure

Mean Time Between Failures (MTBF)● average time between product failures

STEPS IN DFR1.Develop a Reliability Plan

2.Analyze Noise Factors

3.Tests for Reliability

4.Track Failures and Determine Corrective Actions

STEPS IN DFR

1.Develop a Reliability Plan

A reliability plan helps ensure that product reliability is optimized within the cost and performance constraints of a program and customer requirements.

STEPS IN DFR2. Analyze Noise Factors

a.Change the design conceptb.Make basic current design assumptions insensitive to the noises – design out failure

c.Insert a compensation deviced.Disguise the effect - Send the error state/noise

where it will do less harm

STEPS IN DFR

3. Test for Reliability•How robust are the products?

–Test to Bogey: assessing performance at a predetermined time, cycle or number of miles. It estimates the proportion of failures at a particular time. pass/fail–Test to Failure: shows when a component or system can no longer perform at a specified level–Degradation Testing: focuses on the key stresses associated with real world uses – for example - increasing the tire load to create a tire failure

•How can you shorten the reliability test time for new designs?

–Key Life Test/Accelerated Test

STEPS IN DFR

4.Track Failures and Determine Corrective Actions

This process involves:•Data collection and selection

•Set up databases for tracking failures•Warranty, Early Warranty, Things Gone Wrong

•Analyzing trends•Performing closed loop analysis/corrective action

•Calculating observed reliability parameters•Assessing reliability growth.

RELIABILITY TOOLS

Block DiagramThree Categories:

1. Series

RS = R1 R2 ... Rn

1 2 n

RELIABILITY TOOLS


2. Parallel o redundant

RS = 1 - (1 - R1) (1 - R2)... (1 - Rn)

1

2

n

RELIABILITY TOOLS


3. Complex o combination of series and parallel

Example of a complex block diagram:

RELIABILITY TOOLS

Convert to equivalent series system

A B

C

C

D

RA RB RCRD

RC

A B C’ D

RA RB RD

RC’ = 1 – (1-RC)(1-RC)

Example:

RELIABILITY TOOLS

Exponential Distribution Model• Note that the mean time between failure

(MTBF) in the infant mortality phase can be modeled by a negative exponential distribution. The exponential distribution therefore can be used to

determine various probabilities of interest specially on the analysis of equipment failures as well as product failures since they usually occur in this pattern.

RELIABILITY TOOLS• Let T represent length of service, the

probability (P) that failure will not occur before time T is given by

P (failure before T) = 1 – e–T/MTBF

Where e = 2.7183… T = Length of service before

failure MTBF = Mean time between

failures • The probability (P) that failure will occur

before time T is given byP (failure before T) = 1 – e–T/MTBF

RELIABILITY TOOLS

Normal Distribution Model• Product failure due to wear-out can

sometimes be modeled by normal distribution

z = T – Mean wear-out time Standard Dev of wear-out

time

RELIABILITY TOOLS

P-Diagram

RELIABILITY TOOLS

QFD

RELIABILITY TOOLS

Failure Mode andEffectsAnalysis(FMEA)

RELIABILITY TOOLS

Design Verification Plan

RELIABILITY TOOLS

Design Verification Plan● Key Life Testing

● Accelerated test designed to detect a major component failure mode

● Used to improve quality by designing reliability into the product

● Weibull Testing● Used to estimate the life of all products in the

population by fitting a statistical distribution to life date from a representative sample of units

RELIABILITY TOOLS

Reliability Demonstration Matrix• Demonstrates whether a product has met

a certain reliability requirement with a specific confidence

• Usually performed at the system level and is typical set up as a pass/fail system

Design for Reliability

Documents

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