Mean time between failures, written MTBF, is the average number of operating hours a machine runs between one failure and the next. The formula is simple: MTBF = total operating time divided by number of failures. Operating time means the hours the machine was actually running or available to run, not the hours on the calendar.
The most common mistake is using scheduled hours instead of operating hours. If an oven was scheduled for 600 hours but spent 20 of them broken down, it operated for 580 hours. Downtime does not count as time between failures because the machine was not working during it. Subtract the downtime first, then divide.
MTBF is an average. A machine with an MTBF of 140 hours will not fail exactly every 140 hours. It may run 300 hours one stretch and 40 the next. The number tells you the typical gap, which is enough to compare months, compare machines, and plan maintenance.
Standard Procedure
- 1Add up the scheduled production hours for the period.
- 2Subtract all unplanned downtime recorded in the log to get operating time.
- 3Count the failures using the agreed definition.
- 4Divide operating time by the failure count and record the MTBF with the period it covers.
Lesson Summary
- ✓Use operating hours, not calendar or scheduled hours, in the numerator.
- ✓Always state the period and the failure count alongside the MTBF so others can judge it.