QUALITY MANAGEMENTBy: Asrat MekonnenAksum university

College of Engineering and Technology

Department of Mechanical Engineering

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CHAPTER FIVE

RELIABILITY STUDY AND ANALYSIS

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“The RELIABILITY of an item/system is the probability that the item/system performs a specified function under specified operational and environmental conditions at and throughout a specified time.” Quantitatively, reliability is the probability of success.

Reliability - “A collection of planned activities (established through formal and informal management systems) that are effectively working together to prevent loss of system function.”

This second definition is a managed approach to maintain the reliability of system functions. Both definitions refer to the system and maintaining the functionality of the system.

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Introduction

Focuses on eliminating maintenance requirements.

Utilizes technology analysis to achieve reliability and maintenance task improvements.

Improves the uptime and productive capacity of critical equipment using formalized problem-solving techniques

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What is Reliability Engineering?

1. The adaptation and application of concepts such as TPM and RCM.

2. Minimize the use and costs of reactive maintenance, maximize the benefits of PM and PdM, and achieve increasing levels of integrated asset management.

3. The ability to lead or technically support multidisciplinary teams.

4. During design, advises other engineers on reliability (prediction) for their systems and tactics to improve reliability such as redundancy, Failure Mode and Effects Analysis, etc. 5

Important Aspects of Reliability Engineering

6. During design, participates in trade-off studies among performance, cost, and reliability. Reliability estimates are a key input to Life Cycle Costing (LCC)

7. During development, continues to update reliability predictions and prepares reliability test plans.

8. During pre-production, verifies reliability of subsystems and entire system through various types of testing.

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…………..cont’d

Maintenance Prevention Optimizes equipment life-cycle costs by using life-cycle costing (LCC) analysis

Employs proactive techniques to extend machinery life: Failed- part analysis (FPA) ; Root-Cause Failure Analysis (RCFA)

Adapts Total Productive Maintenance (TPM), Reliability Centered Maintenance (RCM) and other concepts to the operating environment

Provides specifications of new/rebuilt equipment Develops plans and analysis of capital equipment (and other assets) replacement

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What do reliability Engineers do?

This evaluates system design configurations based on subsystem and component reliabilities.This modeling facilitates tradeoffs between;

Reliability (e.g. redundancy) Resources (e.g. cost, weight, power)

A product or system’s reliability is a function of the reliabilities of its component parts and how the parts are arranged.

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Reliability assessment modeling

We have five basic system model types Series Parallel Mixed series and parallel Complex Standby

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Modeling ………….cont’d

Series

Parallel

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………cont’d

A B

A

B

Mixed series and parallel

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…………..cont’d

1 2

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4

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If all parts must function for the product or system to operate (series arrangement), then the system reliability is the product of the component part reliabilities.

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Reliability calculation

Example 1 If two component parts are required and each has a reliability of 0.90, what is the reliability of the system?

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……………cont’d

0.90 0.90 0.90 x 0.90 = 0.81Components in series

Failure of some components in a system is more critical than others, for instance, the brakes on a car. To increase the reliability of individual parts and (thus the system as a whole), redundant parts can be built in to back up the failure. In such case these components are said to operate in parallel. And the reliability of the system is;

OR

Where, R1 is the reliability of the original

component, R2 the reliability of the backup

component, and Rs is the system reliability.

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…………cont’d

Example 2

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Rapid prototyping……………cont’d

0.95 + 0.90(1 - 0.95) = 0.995

Components in parallel

0.95

0.90 R2

R1

Reliability can also be expressed as the length of time a product or service is in operation before it fails, called the mean time between failures (MTBF). In this case, it is concerned with the distribution of failures over time, or the failure rate. The MTBF is the reciprocal of the failure rate ().

For example, if your laptop battery fails four times in 20 hours of operation, its failure rate would be

4/20 = 0.20 MTBF = 1/0.20 = 5 hours.

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Rapid prototyping……………cont’d