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

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.

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

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.