Matt Barber
MTBF, MTTR and MTTF are the standard reliability metrics in maintenance. They're often quoted together, and often calculated from data nobody quite trusts.
Each one measures something different.
MTBF: how long it runs between failures
MTBF (mean time between failures) is the average time a repairable asset runs before it fails.
MTBF = total operating time ÷ number of failures
If a filler runs for 1,000 hours in a quarter and fails five times, its MTBF is 200 hours. Higher is better: the asset is more reliable.
MTTR: how long it takes to fix
MTTR (mean time to repair) is the average time it takes to get a failed asset running again.
MTTR = total repair time ÷ number of repairs
If those five failures took 15 hours to fix in total, MTTR is three hours. Lower is better: you recover faster.
Be clear what's included. Some sites measure only hands-on repair time. Others include waiting for a technician and spare parts. The second is what production actually feels, so it's usually more useful.
MTTF: how long it lasts
MTTF (mean time to failure) is the average time a non-repairable item lasts before it fails and is replaced. Think bearings, seals, sensors or light sources.
MTTF = total operating time ÷ number of items that failed
It helps you plan replacement intervals and spare parts stock.
Side by side
| MTBF | MTTR | MTTF | |
|---|---|---|---|
| Measures | Reliability of repairable assets | Speed of recovery | Life of non-repairable items |
| Better when | Higher | Lower | Higher |
| Typical use | Machines and critical equipment | Any repair | Components, parts, consumables |
| Helps you | Target reliability improvements | Improve response and spares | Plan replacements and spares |
How they relate to availability
MTBF and MTTR combine into a simple measure of availability:
Availability = MTBF ÷ (MTBF + MTTR)
With an MTBF of 200 hours and an MTTR of three hours, availability is about 98.5%. Improve either side and availability goes up. That's the same availability that feeds OEE. More in What is OEE?
Where the data comes from
All of them depend on knowing exactly how long each stop lasted and why it happened. If downtime is written on a sheet at the end of the shift, short stops vanish and the reasons are guesses.
MES/MOM records each stop from the machine as it happens, with the reason and the order attached. That gives MTBF and MTTR a reliable basis, and gives maintenance planning the data it needs, whether in MOM or an EAM. More in CMMS vs EAM vs MES and maintenance strategies.
Do your maintenance and production teams calculate MTBF from the same downtime data?
