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Implementing the Reliability Strategy

Prof. Charlton S. Inao

Unit IX

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The importance of reliability Electrical, electronic and Mechanical equipment is used in a number of fields — in industry for the control of processes, in computers, in medical electronics, atomic energy, in weapon systems, defence equipments, communications, navigation at sea and in the air, and in many other fields. It is essential that this equipment should operate reliably under all the conditions in which it is used. In the air navigation, military and atomic energy fields, for instance, failure could result in a dangerous situation. Very complicated systems, involving large numbers of separate units, such as avionic and aerospace electronic systems are coming into use more and more. These systems are extremely complex and use a large number of component parts. As each individual part is liable to failure, the overall reliability will decrease unless the reliability of each component part can be improved.

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Mechanical reliability The well-reported failures, such as the Space Shuttle Challenger, Chernobyl nuclear accidents, and the Bhopal gas escape, emphasize vividly the necessity for mechanical reliability. Buildings, bridges, transit systems. railways, automotive systems, robots, offshore structures, oil pipe lines and tanks, steam turbine plates, roller bearings, etc., all have their particular modes of failure affecting their reliability. There are a number of common modes of mechanical failures, which are worth listing, e.g. with structures: (1)Corrosion failures(2) Fatigue failures (3) Wear failures (4) Fretting failures (5) Creep failures (6) Impact failures These may be considered the main failure modes, but there are of course many

others, such as ductile rupture, thermal shock, galling, brinelling, spalling, radiation damage, etc.

A ‘failure’ is any inability of a part or equipment to carry out its specified function.

Reliability Engineering

• Reliability engineering is an engineering field that deals with the study, evaluation, and life-cycle management of reliability: the ability of a system or component to perform its required functions under stated conditions for a specified period of time

• Reliability engineering is a sub-discipline within systems engineering. Reliability is often measured as probability of failure, frequency of failures, or in terms of availability, a probability derived from reliability and maintainability. Maintainability and maintenance are often important parts of reliability engineering.

Well-publicized system failures such as those listed below may have also contributed to more serious consideration of reliability in product design• Space Shuttle Challenger Disaster: This debacle occurred in 1986, in which all crew members

lost their lives. The main reason for this disaster was design defects.

• Chernobyl Nuclear Reactor Explosion:This disaster also occurred in 1986, in the former Soviet Union,

in which 31 lives were lost. This debacle was also the result of design defects.

• Point Pleasant Bridge Disaster:This bridge located on the West Virginia/ Ohio border

collapsed in 1967. The disaster resulted in the loss of 46 lives and its basic cause was the metal fatigue of a critical eye bar.

RELIABILITY SPECIALIZED AND APPLICATION AREAS

• Mechanical reliability This is concerned with the reliability of mechanicalitems. Many textbooks and other publications have

appeared on this topic.Example: Critical mechanical component assessment Shaft strength Selection of flexible couplings and transmission brakes Gear life assessment; screening of belt drives Assessment of bearing life, load ratings of slider bearings and shaft

sealing devices Bolt loading and lubrication systems

• Software reliability.This is an important emerging area of reliability as the

use of computers is increasing at an alarming rate. • Human reliability.In the past, many times systems have failed not due to

technical faults but due to human error. The first book on the topic appeared in 1986

• Reliability optimization.This is concerned with the reliability optimization of

engineering systems• Reliability growth.This is basically concerned with monitoring reliability

growth of engineering systems during their design and development

• Structural reliability.This is concerned with the reliability of

engineering structures, in particular civil engineering

• Power system reliability.This is a well-developed area and is basically

concerned with the application of reliability principles to conventional power system related problems. Many books on the subject have appeared over the years including a vast number of other publications

• Robot reliability and safety.This is an emerging new area of the applicationof basic reliability and safety principles to robot

associated problems.• Life cycle costing.This is an important subject that is directly related

to reliability. In particular, when estimating the ownership cost of the product, the knowledge regarding its failure rate is essential.

• Maintainability.This is closely coupled to reliability and is

concerned with the maintaining aspect of the product.

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TERMS AND DEFINITIONS• Reliability: This is the probability that an item will

carry out its assigned mission satisfactorily for the stated time period when used under the specified conditions.

• Failure: This is the inability of an item to function within the initially defined guidelines.

• Downtime: This is the time period during which the item is not in a condition to carry out its stated mission.

• Maintainability: This is the probability that a failed item will be repaired to its satisfactory working state.

• Redundancy :This is the existence of more than one means for accomplishing a defined function.

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Active redundancy: This is a type of redundancy when all redundant items are operating simultaneously.Availability: This is the probability that an item is available for application or use when needed.Useful life: This is the length of time an item operates within an acceptable level of failure rate.Mission time: This is the time during which the item is performing its specified operating condition.Human error: This is the failure to perform a given task (or the performance of a forbidden action) that could lead to disruption of scheduled operations or result in damage to property/equipment.Human reliability: This is the probability of completing a job/task successfully by humans at any required stage in the system operation within a defined minimum time limit (if the time requirement is specified).

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MEAN TIME BETWEEN FAILURES (MTBF): The mean exposure time between consecutive failures of a component. This applies to repairable items, and means that if an item fails, say 5 times over a period of use totaling 1000hours, the MTBF would be 1000/5 or 200hours.

MEAN TIME BETWEEN MAINTENANCE (MTBM): The average time between all maintenance events that cause downtime, both preventative and corrective maintenance, and also includes any associated logistics delay time.

MEAN TIME TO FAILURE (MTTF): Mean Time To Failure (MTTF): It is the average time that elapses until a failure occurs. MTTF is commonly found for non repairable items such as fuses or bulbs, etc.

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GENERAL RELIABILITY ANALYSIS RELATED FORMULAS

Evaluating the left-hand side of Equation (6) yields

From Equation (7), we get

The above equation is the general expression for the reliability function. Thus, it can be used to obtain reliability of an item when its times to failure follow any known statistical distribution, for example, exponential, Rayleigh, Weibull, and gamma distributions.

)7...(dt)t()t(Rlnt

0

)8...(e)t(R

t

0

dt)t(

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GENERAL RELIABILITY ANALYSIS RELATED FORMULAS

Mean time to failure: This can be obtained by using any of the following three formulas:

where: MTTF is the item mean time to failure, E(t) is the expected value, s is the Laplace transform variable, R(s) is the Laplace transform for the reliability function, R (t). is the failure rate

)11...(1

)(

)10...(..........)(

)9...()()(

0

0

0

sRLimitMTTF

or

dttRMTTF

or

dtttftEMTTF

s

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Mean time between failure MTBFwhere MTBF stands for mean operating time between failures. MTBF should be confined to the case of repairable items with constant failure rate

GENERAL RELIABILITY ANALYSIS RELATED FORMULAS

1

MTBF

is the failure rate

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Bathtub Hazard Rate Curve• Bathtub hazard rate curve is a well known concept to

represent failure behavior of various engineering items/products because the failure rate of these items changes with time.

• Its name stem from its shape resembling a bathtub as shown in Figure 1.

• Three distinct regions of the curve are identified in the figure:

burn-in region(early failures), useful life region, and wear-out region.

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• These regions denote three phases that a newly manufactured product passes through during its life span.

• During the burn-in region/period, the product hazard rate (i.e., time dependent failure rate) decreases and some of the reasons for the occurrence of failures during this period are poor workmanship, substandard parts and materials, poor quality control, poor manufacturing methods, …….

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incorrect installation and start-up human error, inadequate debugging, incorrect packaging, inadequate processes, and poor handling methods. Other names used for the “burn-in region” are “debugging region,” “infant mortality region,” and “break-in region.”

• During the useful life region, the product hazard rate remains constant and the failures occur randomly or unpredictably. Some of the reasons for their occurrence are undetectable defects, abuse, low safety factors, higher random stress than expected, unavoidable conditions, and human errors.

• During the wear-out region, the product hazard rate increases and some of the reasons for the occurrence of “wear-out region” failures are as follows: Poor maintenance, Wear due to friction, Wear due to aging, Corrosion and creep, Wrong overhaul practices, and Short designed-in life of the product.

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Figure 1: Bathtub hazard rate curve.

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Example 1 :

• Assume that a railway engine’s constant failure rate λ is 0.0002 failures per hour. Calculate the engine’s mean time to failure.

Thus, the railway engine’s expected time to failure is 5000 h.

• Assume that the failure rate of an automobile is 0.0004 failures/h. Calculate the automobile reliability for a 15-h mission and mean time to failure.

Using the given data in Equation

h50000002.0

1

λ

1MTTF

994.0

)8...()(

)15)(0004.0(

)(0

ee

etRt

dttt

Example 2 :

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Similarly, inserting the specified data for the automobile failure rate into Equation MTTF, we get

h

dteMTTF

dteMTTF

dttRMTTF

t

t

500,20004.0

1

..

..

)10...(..........)(

0

)0004.0(

0

0

Thus, the reliability and mean time to failure of the automobile are 0.994 and 2,500 h, respectively.

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Definition of Maintainability Maintainability is a measure of the speed with which loss of performance is detected, diagnosed and made good.Maintainability is the probability that a unit or system will be restored to specified conditions within a given period when maintenance action is taken in accordance with prescribed procedures and resources. It is a characteristic of the design and installation of the unit or system.

The ‘availability’ or time an equipment is functioning correctly while in use depends both on reliability and on maintainability.

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Definition of AvailabilityAvailability. Availability is defined as the percentage of time that a system is available to perform its required function(s). It is measured in a variety of ways, but it is principally a function of downtime.Availability can be used to describe a component or system but it is most useful when describing the nature of a system of components working together. Because it is a fraction of time spent in the “available” state, the value can never exceed the bounds of 0 < A < 1. Thus, availability will most often be written as a decimal, as in 0.99999, as a percentage, as in 99.999%,

Availability• AvailabilityThis is the probability that an item is available for

application or use when needed.Maintainability together with reliability

determine the availability of a machinery system. Availability is influenced by the time demand made by preventive and corrective maintenance measures.

Availability(A) is measured by:A= MTBF/MTBF + MTTR

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Quality and reliability The quality of a device is the degree of performance to applicable specification and workmanship standards. What is the difference between Quality and Reliability? Quality means good performance and longevity. Quality of any manufactured product is determined by its design, the materials from which it is made and the processes used in its manufacture.Quality control measures performance and its variations from specimen to specimen by statistical methods to determine whether production satisfies the design requirements.Quality of a product is determined by conformity and reliability.In Reliability it matters how long a product will maintain its original characteristics when in operation.

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Analytical Techniques and Methods in Reliability

Built-in test (BIT) (Testability analysis)

Failure mode and effects analysis (FMEA)

Reliability Hazard analysis

Reliability Block Diagram analysis

Fault tree analysis

Root cause analysis

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Accelerated Testing

Reliability Growth analysis

Weibull analysis

Thermal analysis by Finite Element Analysis (FEA) and / or Measurement

Thermal induced, shock and vibration fatigue analysis by FEA and / or Measurement

Electromagnetic analysis

Statistical interference

Predictive and preventive maintenance: Reliability Centered Maintenance (RCM) analysis

Human error analysis

Operational Hazard analysis

Results are presented during the system design reviews and logistics reviews. Reliability is

just one requirement among many system design requirements. 

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KEY POINTS • Reliability is a measure of uncertainty and therefore estimating reliability means using statistics and probability theory • Reliability is quality over time • Reliability must be designed into a product or service • Most important aspect of reliability is to identify cause of failure and eliminate in design if possible otherwise identify ways of accommodation • Reliability is defined as the ability of an item to perform a required function without failure under stated conditions for a stated period of time • The costs of unreliability can be damaging to a company

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Steps in Implementation

1. Arrange for schedules to be in corporated in relevant work plans

2. Identify the training needs in discussion with relevant personnel

3. Assist personnel to develop required skills for inspections and servicing within scope and authority.

4. Collect data/information with performance indicators5. Recommend improvements to reliability strategy in

accordance with procedures.

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The End