Design for Reliability Analisa kegagalan Teknik Mesin USU

8/10/2019 Design for Reliability Analisa kegagalan Teknik Mesin USU

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DESIGN FOR RELIABILITY

Dr. Ir. Muhammad Sabri


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The design strategy used to ensure reliability.

Thefail-safe approach is to identify the weak spot inthe system or component and provide some way tomonitor that weakness. When the weak link fails, it isreplaced, just as the fuse in a household electrical

system is replaced.At the other extreme is what can be termed "the one-

horse shay" approach. The objective is to design allcomponents to have equal life so the system will fallapart at the end of its useful lifetime just as thelegendary one-horse shay did.

Frequently an absolute worst-case approach is used;in it the worst combination of parameters is identifiedand the design is based on the premise that all can gowrong at the same time. This is a very conservativeapproach, and it often leads to overdesign.


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Two major areas of engineering activity determine thereliability of an engineering system.

First, provision for reliability must be established duringthe design concept stage, carried through the detaileddesign development, and maintained during the many

steps in manufacture. Once the system becomes operational, it is imperative that

provision be made for its continued maintenance during itsservice."

The steps in building reliability into a design are shown in

Figure 1. The process starts at the beginning of conceptual design by

clearly laying out the criteria for the success of the design,estimating the required reliability, the duty cycle, andcarefully considering all of the factors that make up theservice environment.


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In the configuration step of embodiment design the physicalarrangement of components can critically affect reliability. In layingout functional block diagrams, consider those areas that stronglyinfluence reliability, and prepare a list of parts in each block.

This is the place to consider various redundancies and to be sure thatphysical arrangement allows good access for maintenance. In theparametric step of embodiment design, select components with highreliability. Build and test both computer and physical prototypes.These should be subjected to the widest range of environmentalconditions. Establish failure modes and estimate the system andsubsystem MTBF. Detail design is the place for the final revision ofspecifications, for building and testing the preproduction prototype,and the preparation of the final production drawings. Once the designis released to the production organization the design organization is

not finished with it. Production models are given furtherenvironmental tests, and these help establish the quality assuranceprogram and the maintenance schedules. When the product is putinto service with customers, there is a steady feedback concerningfield failures and MTBFs that helps in redesign efforts and follow-onproducts.


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CAUSESOFUNRELIABILITY

The malfunctions that an engineering system can

experience can be classified into five general

categories.

Design mistakes:

Among the common design errors are failure to

include all important:

ooperating factors

oincomplete information on loads

o

environmental conditionsoerroneous calculations

opoor selection of materials.


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Manufacturing defects:

Although the design may be free from error,

defects introduced at some stage in

manufacturing may degrade it.

Some common examples are

Poor surface finish or sharp edges (burrs) that

lead to fatigue cracks

Decarburization or quench cracks in heat-treated

steel.


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Elimination of defects in manufacturing

Responsibility of the manufacturing engineeringstaff

Also involved R&D function is sometimesrequired to achieve it.

Manufacturing errors

lack of proper instructions or specifications

insufficient supervision

poor working environmentunrealistic production quota

inadequate training

poor motivation.


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Maintenance:

Most engineering systems are designed on the

assumption they will receive adequatemaintenance at specified periods.

Neglected or is improperly maintenance, service

life will suffer. Since many consumer products do

not receive proper maintenance by their owners,

a good design strategy is to design products that

do not require maintenance.


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Exceeding design limits:

If the operator exceeds the limits of temperature,speed, or another variable for which it was designed,the equipment is likely to fail.

Environmental factors:

Subjecting equipment to environmental conditions forwhich it was not designed, such as rain, highhumidity, and ice, usually greatly shortens its servicelife.

A variety of methods are used in engineering designpractice to improve reliability.

a probability of failure ofPI < 10- 6 for structuralapplications and 10- 4


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MARGINOFSAFETY

The variability in the strength properties of

materials and in loading conditions (stress) leads

to a situation in which the overlapping statistical

distributions can result in failures.

The variability in strength of materials has a

major impact on the probability of failure, so

failure can be reduced with no change in the

mean value if the variability of the strength canbe reduced.


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MARGINOFSAFETY

Redundancy

One of the most effective ways to increase reliability is withredundancy. In parallel redundant designs the samesystem functions are performed at the same time by two ormore components even though the combined outputs arenot required. The existence of parallel paths may result inload sharing so that each component is derated and has itslife increased by a longer-than-normal time.

Another method of increasing redundancy is to haveinoperative or idling standby units that cut in and takeover when an operating unit fails. The standby unit wearsout much more slowly than the operating unit does.

Therefore, the operating strategy often is to alternate unitsbetween full-load and standby service. The standby unitmust be provided with sensors to detect the failure andswitching gear to place it in service. The sensor and/orswitching units frequently are the weak link in a standbyredundant system.


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MARGINOFSAFETY

Durability

The material selection and design details should

be performed with the objective of producing a

system that is resistant to degradation from such

factors as corrosion, erosion, foreign object

damage, fatigue, and wear." This usually

requires the decision to spend more money on

high-performance materials so as to increase

service life and reduce maintenance costs. Lifecycle costing is the technique used to justify this

type of decision.


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DAMAGETOLERANCE

Crack detection and propagation have taken on

great importance since the development of the

fracture mechanics approach to design .

A damage-tolerant material or structure is one in

which a crack, when it occurs, will be detected

soon enough after its occurrence so that the

probability of encountering loads in excess of the

residual strength is very remote.


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DAMAGETOLERANCE

Figure 2 illustrates some of the concepts of

damage tolerance. The initial population of very

small flaws inherent in the material is shown at

the far left. These are small cracks, inclusions,

porosity, surface pits, and scratches. If they areless than a1 they will not grow appreciably in

service.

Additional defects will be introduced by

manufacturing processes. Those larger than a2will be detected by inspection and eliminated as

scrapped parts.


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DAMAGETOLERANCE

However, some cracks will be present in thecomponents put into service, and they will growto a size a3 that can be detected by thenondestructive evaluation (NDE) techniques that

can be used in service. The allowable designstresses must be so selected that the number offlaws of

size a3 or greater will be small. Moreover, thematerial should be damage-tolerant so that

propagation to the critical crack size acriticalis slow. In conventional fracture mechanics analysis, the

critical crack size is set at the largest crack sizethat might be undetected by the NDE techniqueused in service


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DAMAGETOLERANCE

The value of fracture toughness of the material is

taken as the minimum reasonable value. This is

a safe but overly conservative approach. These

worst-case assumptions can be relaxed and the

analysis based on more realistic conditions byusing probabilistic fracture mechanics (PFM)


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DAMAGETOLERANCE

Distribution of defects in engineering components.


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MINIMIZINGFAILURE

Ease of Inspection

The importance of detecting cracks should beapparent from Figure 2. Ideally it should be possibleto employ visual methods of crack detection, butspecial design features may have to be provided inorder to do so.

In critically stressed structures, special features topermit reliable NDE by ultrasonic's or eddy currenttechniques may be required. If the structure is notcapable of ready inspection, then the stress level mustbe lowered until the initial crack cannot grow to acritical size during the life of the structure. For thatsituation the inspection costs will be low but thestructure will carry a weight penalty because of thelow stress level.


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MINIMIZINGFAILURE

Simplicity

Simplification of components and assemblies

reduces the chance for error and increases the

reliability.

The components that can be adjusted by

operation or maintenance personnel should be

restricted to the absolute minimum. The simpler

the equipment needed to meet the performance

requirements the better the design.


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RELIABILITYOFDESIGN

Specificity

The greater the degree of specificity, the greater theinherent reliability of design.

Whenever possible, be specific with regard to materialcharacteristics, sources of supply, tolerances and

characteristics of the manufacturing process, testsrequired for qualification of materials andcomponents, and procedures for installation,maintenance, and use. Specifying standard itemsincreases reliability. It usually means that thematerials and components have a history of use so

that their reliability is known.Also, replacement items will be readily available.

When it is necessary to use a component with a highfailure rate, the design should especially provide forthe easy replacement of that component.


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RELIABILITYOFDESIGN

Sources of Reliability Data

Data on the reliability of a product clearly is highlyproprietary to its manufacturer.

However, the U.S. defence and space programs havecreated a strong interest in reliability, and this has

resulted in the compilation of a large amount of dataon failure rates and failure modes. The ReliabilityInformation Analysis Centre (RIAC)30, sponsored bythe DOD Defence Information Analysis Centre, hasfor many years collected failure data on electroniccomponents. Extensive reliability data on electronic

components is available online, for a fee, in 217 Plus,the successor to MIL-HDBK-217. Reliability data onnon electronic components is available on compactdisk NPRD-95. Information on European sources ofreliability data can be found in the book by Moss.


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RELIABILITYOFDESIGN

Cost of Reliability

Reliability costs money, but the cost nearly always is less than thecost of unreliability.

The cost of reliability comes from the extra costs associated withdesigning and producing more reliable components, testing forreliability, and training and maintaining a reliability organization.

Figure 3 shows the cost to a manufacturer of increasing the reliabilityof a product.

The costs of design and manufacture increase with product reliability.

Moreover, the slope of the curve increases, and each incrementalincrease in reliability becomes harder to achieve.

The costs of the product after delivery to the customer, chieflywarranty or replacement costs and reputation of the supplier,

decrease with increasing reliability. The summation of these two curves produces the total cost curve,

which has a minimum at an optimum level of reliability.

Other types of analyses establish the optimum schedule for partreplacement to minimize cost.


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RELIABILITYOFDESIGN

Influence of reliability on cost.

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Design for Reliability Analisa kegagalan Teknik Mesin USU