How to calculate a mttf

Written by christopher donahue
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Mean Time To Failure (sometimes seen as MTBF or Mean Time Between Failures) is a method of determining the reliability of a piece of equipment. Ideally, this will be drawn from field experience or testing techniques similar to actual use conditions. MTTF documents the operational time of equipment between failures. It is a method to predict the percentage of time the equipment will be available to produce. MTTF is also a first step in determining which components are most prone to failure, allowing the owner to stock appropriate spare parts and the equipment's engineers to focus on areas for reliability improvement. MTTF should be an integral part of a buyer's determination of the Cost of Ownership (CoO) when choosing a piece of equipment. In this article, the examples will be a one month history of a plate stamping machine.

Skill level:

Things you need

  • Accurate history of equipment documenting condition and giving time
  • Spreadsheet (Excel, Lotus, etc.)

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  1. 1

    Enter the date and time of each instance where the piece of equipment changes state in Column A and B of the spreadsheet.

    Ex: 1/1/09 01:30

                                            1/1/09       01:45
                                            1/1/09       09:17
                                            1/1/09       12:25
                                            1/1/09       12:50
  2. 2

    Use the equipment history log to enter the equipment status for each of those instances. This will be used to determine if the equipment was Available, Unavailable or Idle in Columns C and D. Most industries do not count equipment set-up time or routine maintenance time as Available time. They will count Idle time as Available time.

    Ex: 1/1/09 01:30 Machine Prep Unavailable

    1/1/09 01:45 Start Operation Available

    1/1/09 09:17 Failure Unavailable

    1/1/09 12:25 Repaired Available

    1/1/09 12:50 Start Operation Available

  3. 3

    Filter the resulting data for the full month, tagging on Column C. This will result in a string of failures and their times together.

    Ex: 1/1/09 09:17 Failure Unavailable

                                         1/14/09  17:20 Failure  Unavailable
                                         1/15/09  07:40 Failure  Unavailable
                                         1/28/09  11:12 Failure  Unavailable
  4. 4

    Calculate the time between each failure logged. In this case with four failures, there will be three intervals. Obviously, a larger window of time will give more refined results.

    Ex: 1/1/09 09:17 to 1/14/09 17:20 gives 328 hours and 3 minutes (19,683 minutes)

      1/14/09 17:20 to 1/15/09  07:40 gives 14 hours and 20 minutes (860 minutes)
      1/15/09 07:40 to 1/28/09 11:12  gives 323 hours and 32 minutes (19,412 minutes)
  5. 5

    Sum the intervals and divide by the number of intervals to calculate the mean.

    Ex: (19,683 + 860 + 19,412) / 3 gives 13,318 minutes. MTTF is 221.9 hours.

Tips and warnings

  • Adding a column with each failure identifying the part which failed and/or the operation step where failure occurred will be valuable in keeping adequate stocks of the correct spare parts or targeting which parts may need upgrades.
  • MTTF is very useful in setting up a Preventive Maintenance (PM) schedule to catch failures before they happen.

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