Mean time to repair maintenance & MTTR formula

What is mean time to repair?


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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MTTR formula & how is it calculated?

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

Mean time to repair formula

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 vs. 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 mean time to repair 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 mean time to repair calculation 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 repair strategy. For example, as the system ages, it may take longer to repair systems. MTTR will trend upwards prompting the repair versus replace decision. Check out this MTTR example as calculated on the Fiix CMMS reporting dashboard:

mean time to repair example

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 subassemblies 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 MTTR calculation, we must make the following assumptions:

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

How to improve MTTR

Improving your facility’s MTTR metrics means reducing the mean time to repair for critical assets. Two critical ingredients for reducing your MTTR are preventive maintenance and repair efficiency.

Using a preventive maintenance approach will increase asset reliability and availability so equipment runs longer, failures happen less frequently and maintenance breakdowns aren’t as severe. When regular checks are done on assets, problems can be identified early, before they become something worse. Subsequently, MTTR is reduced.

When an asset does break down unexpectedly, having the right tools and processes in place is the key to fast and efficient repairs, leading to lower MTTR. From being able to quickly assign work orders to having the right parts on hand, being as organized as possible means tasks get done on time and get done right, which causes mean time to repair to drop. Maintenance software, such as a CMMS, is one way to increase repair efficiency and reduce MTTR. A CMMS contains all the tools necessary to organize, track and fine-tune maintenance processes and procedures, such as work orders and inventory, so repairs run smoothly and downtime is minimized.

The bottom line

Mean time to repair is a good indicator of an organization’s ability to respond to a problem and repair it. MTTR formula is calculated by dividing total maintenance time by the total number of maintenance actions over a specific period. A solid understanding of mean time to repair for critical assets can have a dramatic effect on the organization’s bottom line, reliability, labour, inventory management and more. Mean time to repair can help facilities predict performance or the life cycle cost of new systems so design, build, purchasing and maintenance decisions are easier and more efficient.

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