1 RELIABILITY ABILITY OF THE PRODUCT TO PERFORM REQUIRED FUNCTION UNDER STATED CONDITIONS FOR A STATED PERIOD OF TIME (ISO – 8402 – 1986) IT IS THE CHANCE THAT A PRODUCT WILL WORK FOR THE REQUIRED TIME. WE NEED TO - TO QUANTIFY RELIABILITY IN TERMS OF PROBABILITY - DEFINE PRODUCT PERFORMANCE - DEFINE ENVIRONMENT - DEFINE OPERATING TIME BETWEEN FAILURES
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RELIABILITY
ABILITY OF THE PRODUCT TO PERFORM REQUIRED FUNCTION UNDER STATED CONDITIONS FOR A STATED PERIOD OF TIME (ISO – 8402 – 1986)
IT IS THE CHANCE THAT A PRODUCT WILL WORK FOR THE REQUIRED TIME.
WE NEED TO
- TO QUANTIFY RELIABILITY IN TERMS OF PROBABILITY
- DEFINE PRODUCT PERFORMANCE
- DEFINE ENVIRONMENT
- DEFINE OPERATING TIME BETWEEN FAILURES
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RELIABILITY PROGRAM:
• SETTING OVERALL RELIABILITY GOALS
• APPORTIONMENT OF RELIABILITY GOALS
• STRESS ANALYSIS
• IDENTIFICATION OF CRITICAL PARTS
• FMEA
• PREDICTION OF RELIABILITY
• DESIGN REVIEW
• SELECTION OF SUPPLIERS
• CONTROL OF RELIABILITY DURING MANUFACTURING
• RELIABILITY TESTING
• FAILURE REPORTING AND CORRECTIVE ACTION SYSTEM
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APPLICATION:
COMPLICATED PRODUCTS E.G. ELECTRONIC / SPACE SCIENCES
RELIABILITY FIGURES OF MERIT:
1) MEAN TIME BETWEEN FAILURES (MTBF)
MEAN TIME BETWEEN SUCCESSIVE FAILURES OF A REPAIRABLE PRODUCT
2) FAILURE RATE – NO OF FAILURES PER UNIT TIME
3) MEAN TIME TO FAILURES (MTTF) – MEAN TIME TO FAILURE OF A NON-REPAIRABLE PRODUCT
4) MEAN LIFE – MEAN VALUE OF MAJOR OVERHAUL WEAR – OUT TIME
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5) MTFF – MEAN TIME TO FIRST FAILURE OF REPAIRABLE PRODUCT
6) MTBM – MEAN TIME BETWEEN MAINTENANCE
7) LONGEVITY – WEAROUT TIME FOR A PRODUCT
8) AVAILABILITY – OPERATING TIME EXPRESSED AS A PERCENTAGE
OF OPERATING AND REPAIR TIME
9) SYSTEM EFFECTIVENESS – EXTENT TO WHICH A PRODUCT
ACHIEVES REQUIREMENTS OF USER
10) PROBABILITY OF SUCCESS – RELIABILITY (USED FOR ONE SHOT
OR NON-TIME-ORIENTED PRODUCTS)
11) REPAIRS / 100 NUMBER OF REPAIRS PER 100 OPERATING HOURS
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DERATING
FMECA C = CRITICALITY
SUMMARY OF TESTS:
PERFORMANCE
ENVIRONMENTAL
STRESS
RELIABILITY
TO CLEARLY IDENTIFY THE ROOT CAUSE AND CONSEQUENTIAL FAILURE OF OTHER PARTS
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6.7 EARLY WARNING OF NEW-PRODUCT PROBLEMS:
PROBLEMS CAUSED BY DESIGN RESULTS IN DEVELOPMENT OF VARIOUS
FORMS OF EARLY WARNING ON EXPECTED FAILURES. FOLLOWING
TABLE SHOWS NEW PRODUCT DEVELOPMENT STAGES AND
APPROPRIATE TESTS FOR EACH STAGE.
PHASES OF NEW PRODUCT DEVELOPMENT
TESTS
CONCEPT AND FEASIBILITY STUDY CONCEPT REVIEW
PROTOTYPE DESIGN DESIGN REVIEW, RELIABILITY AND MAINTAINABILITY PREDICTION, FAILURE MODE, EFFECT AND CRITICALITY ANALYSIS, SAFETY ANALYSIS, VALUE ENGINEERING
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PHASES OF NEW PRODUCT DEVELOPMENT
TESTS
PROTOTYPE CONSTRUCTION PROTOTYPE TESTS,
ENVIRONMENTAL TESTS,
OVERSTRESSING
PRE PRODUCTION PILOT PRODUCTION LOTS,
EVALUATION OF TOLERANCES
EARLY FULL-SCALE PRODUCTION,
MARKETING AND USE
EMPLOYEES AS TEST PANELS,
SPECIAL PROVISION FOR PROMPT
FEEDBACK
ALL PHASES FAILURE ANALYSIS, DATA
COLLECTION AND ANALYSIS
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6.8 FAILURE MODE EFFECT ANALYSIS:
FAILURE MODE AND EFFECT ANALYSIS (FMEA) IS A PREDICTION OF
FAILURE OF DESIGN STAGE. THIS COMBINES THE TECHNOLOGY,
KNOWLEDGE AND EXPERIENCE OF A DESIGNER TO IDENTIFY
FORESEEABLE FAILURE MODES OF PRODUCT OR PROCESS AND TO
PLAN FOR ITS ELIMINATION.
FMEA, THEREFORE CAN BE DEFINED AS – “A GROUP OF ACTIVITIES
INTENDED TO RECOGNIZE AND EVALUATE THE POTENTIAL FAILURE OF A
PRODUCT OR PROCESS AND ITS EFFECTS. IDENTIFY ACTIONS THAT
COULD ELIMINATE OR REDUCE THE CHANCE OF POTENTIAL FAILURES.
THE ENTIRE PROCESS IS TO BE DOCUMENTED”.
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FMEA IS A TEM-EFFORT. MOST COMMONLY IT IS USED FOR DESIGN AND
SOME TIMES FOR PROCESS.
THE DEVELOPMENT OF A DESIGN FMEA CONSIDERS THREE ASPECTS TO
PREDICT A FAILURE AND ITS SEVERITY.
1. SEVERITY OF THE POTENTIAL MODE (S)
2. OCCURRENCE (O) – IT IS THE LIKELIHOOD THAT A SPECIFIC
CAUSE WILL OCCUR
3. DETECTION (D) AN ASSESSMENT OF THE ABILITY OF VARIOUS
ACTIONS TO CONTROL THE DESIGN TO DETECT A POTENTIAL DESIGN
WEAKNESS.
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THE TYPICAL FAILURE MODES COULD BE:
CRACKED, DEFORMED, LOOSENED, LEAKING, STICKING, SHORT