Mean time to repair

Mean time to repair (MTTR)

Mean time to repair (MTTR) is the average time required to troubleshoot and repair failed equipment and return it to normal operating conditions. It is a basic technical measure of the maintainability of equipment and repairable parts. Maintenance time is defined as the time between the start of the incident and the moment the system is returned to production (i.e. how long the equipment is out of production). This includes notification time, diagnostic time, fix time, wait time (cool down), reassembly, alignment, calibration, test time, back to production, etc. It generally does not take into account lead-time for parts. Mean time to repair ultimately reflects how well an organization can respond to a problem and repair it.

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How is MTTR calculated?

Expressed mathematically, it is the total maintenance time divided by the total number of maintenance actions over a specific period.

Over the lifetime of an asset, each failure will vary depending on the severity of the issue. Some issues will require a simple parts swap, while others could take days to diagnose and repair. The frequency v.s. repair time plot follows the log-normal distribution. We will have a large number of repairs that are quick to repair, and a small number that take much longer.

Why is MTTR important?

For mission critical equipment, mean time to repair can have a dramatic effect on the organization’s bottom line. Taking too long to repair equipment can mean product scrap, missed orders and soured business relationships. To limit the impact of mean time to repair, organizations have their own maintenance teams, hold spare parts onsite or run parallel production lines.

What can MTTR tell you?

Prediction of the number of hours that a system or component will be unavailable whilst undergoing maintenance is of vital importance in reliability and availability studies. Mean time to repair yields a lot of information that can help reliability engineers make informed decisions such as repair or replace, hire, optimize maintenance schedules, store parts onsite or switch parts strategy. For example, as the system ages, it may take longer to repair systems. MTTR will trend upwards prompting the repair versus replace decision.

You can also use mean time to repair to predict performance or the life cycle cost of new systems. Equipment manufacturers are now using a modular design philosophy so parts or sub assemblies can be swapped out quickly and easily. Consider being faced with a purchasing decision that involves 2 similar systems – one has a higher MTTR because repairable items are difficult to remove due to their location. The additional time and costs to maintain should be factored into the life of the system to simplify the purchasing decision. Manufacturers also use MTTR to justify redesigning or improving systems.

For an accurate calculation of MTTR, we must make the following assumptions:

  • One technician performs all tasks sequentially.
  • Appropriately trained personnel perform the maintenance.

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