MTTR (Mean Time To Repair) and MTBF (Mean Time Between Failures) are key maintenance metrics. They help measure machine performance and reliability. Lean Manufacturing and IATF 16949-certified automotive industries use them widely. These metrics support efforts to improve Overall Equipment Effectiveness (OEE).”
MTTR and MTBF are very useful Lean Tools of manufacturing industries. These two key performance indicators are used to review maintenance performance during management review. Mean Time to Repair should be as low as possible, and Mean Time between Failures should be as high as possible.
Mean Time Between Failures (MTBF):
It is the average time between two failures of the same system or plant.
Calculated as:
MTBF = Total Available Time ÷ Number of Failures
- Monitors equipment availability and reliability.
- A higher MTBF indicates better system reliability.
- Organizations aim to maximize MTBF.
- Used specifically for failures in repairable systems.
MTBF Formula | MTBF Calculation
MTBF = Operational Time / No. of breakdowns or failures.
Where, Operational Time = Available Time – Breakdowns
Mean Time Between Failures (MTBF) is calculated using an arithmetic mean. To find MTBF, you first choose a specific period, such as a week, a month, six months, or a year. Then, calculate the available operational time during that period and divide it by the number of breakdowns that occurred. This gives the average time between failures, helping assess the reliability of repairable systems.
Example: Calculating MTBF for a Machine
Let’s calculate the Mean Time Between Failures (MTBF) for a machine over one month.
- Machine Available Time: The machine operates for 2 shifts of 9 hours for 22 working days.
Available Time = 2 × 9 × 22 = 396 hours - Number of Breakdowns in the Month: 3
- First: 1.5 hours
- Second: 2 hours
- Third: 2.5 hours
- Total Breakdown Time = 6 hours
- Actual Operational Time = 396 − 6 = 390 hours
- MTBF = Actual Operational Time ÷ Number of Breakdowns
MTBF = 390 ÷ 3 = 130 hours
So, the average time between failures is 130 hours.
Mean Time To Repair (MTTR)
MTTR is the average time required to repair a machine or equipment after a failure. It is calculated by dividing the total repair or breakdown time by the number of repairs.
Formula:
MTTR = Total Repair Time ÷ Number of Breakdowns
MTTR Formula | MTTR Calculation
Mean Time to Repair Formula:
Formula MTTR = Breakdowns or Repairs or Downtime hours of machine / No. of breakdowns
Example: Calculating MTTR for a Machine
Let’s calculate the Mean Time To Repair (MTTR) for a machine over one month.
- Number of Breakdowns: 3
- Repair Times:
- Breakdown 1: 1 hour
- Breakdown 2: 1.5 hours
- Breakdown 3: 2 hours
- Total Repair Time = 1 + 1.5 + 2 = 4.5 hours
- MTTR = Total Repair Time ÷ Number of Breakdowns
MTTR = 4.5 ÷ 4 = 1.5 hours
So, the average time to repair the machine after a failure is 1.5 hours.
How to calculate Mean Time between Failure (MTBF) and Mean Time to Repair (MTTR) in Excel:
Download the MTTR and MTBF Calculation Excel Sheet.
Update the breakdown details date-wise.
Update the monthly available time as per working hours and working days in a month.
After updating these data, MTBF and MTTR will be calculated automatically.
How to improve the Mean Time to Repair?
For improving MTTR, we have to reduce breakdown time and breakdown numbers. So, for reducing MTTR, there are some steps to follow:
- Standardize, review and update the Maintenance procedure. Follow the Work Instructions and Checklists as defined in the procedure.
- When a breakdown occurs, do an effective breakdown analysis to find out the actual root cause. Take corrective action, implement effectiveness monitoring and do horizontal deployment. There must not be any repetitive breakdown.
- Maintain the critical spare parts as defined in the critical spare parts list and review this list at least monthly.
- Predictive maintenance and preventive maintenance are effective in reducing the MTTR. So, follow the maintenance plan practically.
You may also like to read about:
How to implement the Machine maintenance procedure in a manufacturing industry?
How to implement the Tool maintenance procedure in a manufacturing industry?