SISTEM PEMELIHARAANBermawi P. Iskandar

KK Sitem Manufaktur, FTI ITB

Products:[Support-Characteristics-Application]

BASIC CONCEPTS OF RELIABILITY

The National Aeronautics and Space Administration (NASA)[Smith (1977)] defines reliability as,

The probability of a device performing adequately for the period of time intended under the operating condition encountered.

British standards institution(1986) specifies reliability as, the probability that an item will perform a required function under stated conditions for a stated period of time.

BASIC CONCEPTS OF RELIABILITY

From the above two-definition, it is obvious that reliability of a device (an item or system) is the ability of the system to satisfy its intended function in probabilistic sense.

BASIC CONCEPTS OF RELIABILITY

Suppose that a device is required to operate satisfactorily under designed condition for a period given by (0,t).

The device is said to have a high reliability if it performs its required function without failure during (0,t) with a sufficiently high probability.

Otherwise, the device is said to have a low reliability and the device is deemed unreliable.

Simple system & Complex system The device can be either:

a very simple system consisting of a single component or

a very complex system involving numerous components or subsystems. The components of the complex system may or may not interact with each other.

Simple system & Complex system If the system is a single component system, its

reliability depends only on the reliability of the component.

In contrast, the reliability of a multi-component system depends not only on each component reliability but also on the relationship between components.

In this case the configuration of the system affects the reliability of the system.

Reliability is: A characteristic that describes how good a

device is. Must be planned for, designed in both in

terms of the initial product and in maintenance of that product.

Failure is: The degradation of the performance of a device

(process) outside of a specified value AND non-performance or inability to perform its function for a given time period within specified conditions.

Defect: imperfection Deficiency: lack of conformance to specs Fault: Cause of failure Malfunction: unsatisfactory performance

Failure Measures Real life failure: fact of life, define normal

operation, anticipate worst, try to design out. Failure rate typically = #failures/unit time =

failures/million hours for devices.

Unreliability is: A measure of the potential for failure of a device (or

process.) Leads to high cost, wasted time, inconvenience, poor

reputation, unsafe operation, … - 9/9/99 more than 10,000 stepladders recalled by

Home Depot – steps too short, improperly attached (RIDGID ladders, Louisville Ladder Co, Louisville KY)

Infant Mortality

<-Useful Life ->

Wearout

Joints,Welds,Contamination,Misuse

Corrosion,Cracking,Wear, Crazing,Shorts

Screening,Design, Burn-in

--Design,PreventiveMaintenance,Replacement,Repair

Time ->

Failu

re R

ate

Failu

re R

ate

Time->

Friction,Fatigue,Erosion,Corrosion,Cracking,Lack of PM

PM,Replacement

Misassembly

<- Useful Life ->

Failu

re R

ate

Time ->

Debug errors,Spec. Errors,Special Cases

Failu

re R

ate

Time ->

QI

QIPM

Lawsuit Lawsuit

NEED FOR HIGH RELIABILITY For large complex expensive systems the lack of adequate

reliability can cause severe economic losses and/or social consequence.

Breakdown of a numerically controlled machine tool, in an automated production line or cell can result in the loss of production; increased production time; increased production cost, customer dissatisfaction to name a few.

In many cases (e.g. aircraft, spacecraft, chemical plant or nuclear reactor), unreliability affects personnel safety. Failure in operation can cause a dangerous situations e.g. Ieak of poisonous gas in a chemical plant can kill people; the effect is more devastating in the case of a leak in a nuclear reactor.

NEED FOR HIGH RELIABILITY If an anti-aircraft missile fails to work when required

the nation can suffer serious property and human loss as well as loss to national prestige.

Considering the consequences of unreliability, the need to have highly reliable systems is paramount.

For example, U.S. Air Force, through Reliability & Maintainability 2OOO,has established the reliability requirement that all new systems have twice the reliability than the system being replaced. [Piotrowski ( 1987 )]

Approaches to ensure high reliability Using redundant components in a system is

one way of achieving it. An integrated quality control also assures

better reliability by ensuring that systems meet the design specification.

Maintenance is also an effective way to control reliability, since it can prevent excessive deterioration of the system.

Approaches to ensure high reliability However, all these methods are costly and in

some cases difficult to achieve, especially for complex systems.

This implies that there is a trade off between system reliability and the cost of assuring the reliability.

The optimal trade off involves the use of quantitative method which is discussed in the next section.