An Introduction to Reliability & Maintainability Engineering

Course Objectives

At the completion of this course, each student• should have an understanding of the basic models and

methods of reliability engineering and their application to complex systems,

• should have a foundation for future study in reliability engineering and have access to the technical literature in reliability.

Course Overview

• L 1 Intruduction

• L-2 Systems Reliability

• L-3 Maintainability

Some Definitions

Reliability is defined to be the probability that a component or system will perform a required function for a given period of time when used under stated operating conditions - R(t).

Maintainability is defined to be the probability that a failed component or systemwill be restored or repaired to a specified condition within a period of time when maintenance is performed in accordance with prescribed procedures - M(t).

Availability is defined as the probability that a component or system is performingits required function at a given point in time when used under stated operating conditions - A(t).

Why Study Reliability?

(a) the increased complexity and sophistication of systems,

(b) public awareness and insistence on product quality,

(c) new laws and regulations concerning product liability,

(d) government contractual requirements to meet reliability andmaintainability performance specifications,

(e) profit considerations resulting from the high cost of failures,their repairs, and warranty programs.

?

Complexity and Reliability

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1.00 0.99 0.98 0.97 0.96 0.95 0.94 0.93 0.92 0.91 0.90

Component Reliability

Sys

tem

Rel

iab

ility N=2

N=5

N=10

N=25

N=50

Government Regulations

• Food and Drug Act

• Flammable Fabrics Act

• Federal Hazardous Substance Act

• National Traffic and Motor Vehicle Safety Act

• Fire Research and safety Act

• Child Protection and Toy Safety Act

• Poisson Prevention Packaging Act

• Occupational Safety and Health Act

• Federal Boat Safety Act

• Consumer Product Safety Act

Gallup Survey

Attribute Average ScorePerformance 9.5Lasts long time (reliability) 9.0Service 8.9Easily Repaired (maintainability) 8.8Warranty 8.4Easy to Use 8.3Appearance 7.7Brand Name 6.3Packaging/Display 5.8Latest Model 5.4

The High Cost of Failures

Beijing - Eighteen factory workers were executed for poor product quality at Chien Bien Refrigerator Factory on the outskirts of the Chinese capital. The managers - 12 men and 6 women - were taken to a rice paddy outside the factory and unceremoniously shot to death as 500 plant workers looked on.

… the managers were blamed for ignoring quality and forcing shoddy work, … the factory’s output of refrigerators had a reputation for failure.

For years, factory workers complained that parts did not meet specification and the product did not function as required.

Customers, who waited up to 5 years for their appliances, were outraged.

Executed included the plant manager, the quality manager, the engineering managers, and their top staff.

Reliability vs Quality

Quality is the amount by which a product satisfiesthe users’ (customers’) requirements. Product quality is in part a function of design and conformance todesign specifications during manufacture.

Reliability is concerned with how long the productcontinues to function once it becomes operational. Therefore reliability can be viewed as the quality ofthe product’s operational performance over time, andas such it extends quality into the time domain.

Reliability Specification

• Define failure - what function is performed?– Identify failure modes

– Unambiguous

– Observable

• Time to failure– Calendar time

– Operating hours

– Number of cycles (on/off, load reversals, missions)

– Vehicle miles - incidents per 1000 vehicles (IPTV)

• State normal conditions– Design loads (weight, voltage, pressure, etc.)

– Environment (temp., humidity, vibration, contaminants, etc.)

– Operating (usage, storage, maintenance, shipment, etc.)

I’m a failure!

Reliability Specification(continued)

• Avoid vagueness– e.g. “as reliable as possible”

• Be realistic– e.g. “will not fail under any operating conditions

• Avoid using only the MTTF (or MTBF)– unless failure rate is constant

• Frame in terms of reliability or design life– a 95 percent reliability at 10,000 operating hours

– a design life of 10,000 operating hours with a 95 percent reliability

R(t)

The Failure Distributionand the MTTF

MTTF = 10

Pr{fails}=.3

MTTF = 10

Pr{fails}=.5

MTTF = 10

Pr{fails}=.7

Probability of Surviving to the MTTF

• Exponential (constant failure rate) Distribution– R(MTTF) = .3678

• Normal Distribution– R(MTTF) = .5

• Weibull with a shape parameter of .5 – R(MTTF) = .24

• Weibull with a shape parameter of 2– R(MTTF) = .455

Example - Reliability Specification

60 wattAvg. lumens 870Avg. life 1000 hours

Which average? - mean, median, mode?

Operating hours or clock time?

What about on/off cycles?

What are the operating conditions?

GE