mem-3&4
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good lectureTRANSCRIPT
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Recap
1. Classify Maintenance
2. Define its Types
Todays Discussion
1. Class Participation Assessment
2. Calculating Cost of Breakdown & Preventive Maintenance
3. Life Cycle Prediction of Assets
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Life Cycle Prediction of Assets
1. MTBF
2. MTTR
3. MTTF
4. FIT
5. Availability
6. Failure Rate
7. Bath Tub Curve
ways of providing a numeric value to quantify a failure rate
numeric value can be expressed using any measure of time mostly hours
MTBF
provide the amount of failures per million hours for a product
units for an MTBF value are hours/fail. unit for a failure rate is fails/hour.
MTBF is equal to the inverse of failure rate. For example, a product with an MTBF of 3.5 million
hours, used 24 hours per day:
• MTBF = 1 / failure rate
• failure rate = 1 / MTBF = 1 / 3,500,000 hours
• failure rate = 0.000000286 failures / hour
• failure rate = 0.000286 failures / 1000 hours
• failure rate = 0.0286% / 1000 hours - and since there are 8,760 hours in a year
• failure rate = 0.25% / year
Note
3.5 million hours is 400 years. Do we expect that any of these products will actually operate for 400
years? No! Long before 400 years of use, a wear-out mode will become dominant and the
population of products will leave the normal life period of the bathtub and start up the wear-out
curve. But during the normal life period, the "constant" failure rate will be 0.25% per year, which can
also be expressed as an MTBF of 3.5 million hours.
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MTTR
time needed to repair a failed hardware module
MTTF
mean time expected until the first failure of a piece of equipment
basic measure of reliability for non-repairable systems
FIT
number of expected failures per one billion hours of operation for a device
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Availability
defined as the quality or state of being available
mostly expressed as a percentage
Using this diagram, the availability is calculated as follows:
MTBF / (MTBF + MTTR
related expected downtime is ( 1 – Availability% ) * 1 year.
example, a 500 GB with a MTBF specification of 750,000 hours
24 hours replacement contract and
4 hours to restore the backup
availability would then be 750,000 / ( 750,000 + 28 ) = 0.9999626680%
resulting in a yearly downtime of ( 1 – 0.9999626680) * ( 365 days * 24 hours * 60
minutes ) = 19,6 min.
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