Mean time between failures

Free guide to maintenance metrics

What is mean time between failures?

Mean time between failures (MTBF) is the average time between system breakdowns. MTBF is a crucial maintenance metric to measure performance, safety, and equipment design, especially for critical or complex assets, like generators or airplanes. It is also used to determine the reliability of an asset. MTBF is also one half of the formula used to calculate availability, together with mean time to repair (MTTR). The MTBF formula uses only unplanned maintenance and doesn’t account for scheduled maintenance, like inspections, recalibrations, or preventive parts replacements.

MTBF Calculation

To calculate MTBF, divide the total number of operational hours in a period by the number of failures that occurred in that period. MTBF is usually measured in hours.

MTBF = # of operational hours ÷ # of failures

MTBF equals number of operational hours divided by number of failures

For example, an asset may have been operational for 1,000 hours in a year. Over the course of that year, that asset broke down eight times. Therefore, the MTBF for that piece of equipment is 125 hours.

To get an accurate measure of MTBF, you must collect data from the actual performance of the equipment. Each asset operates under different circumstances and is influenced by human factors, such as design, assembly, maintenance, and more. That’s why you should avoid taking basing your maintenance around an MTBF estimate from a manual.

How to use MTBF

MTBF is used to anticipate how likely an asset is to fail within a certain time period or how often a certain type of failure may occur. When paired with other maintenance strategies, like failure codes and root cause analysis, and additional maintenance metrics, like MTTR, it will help you avoid costly breakdowns. Having this information makes it easier to create PMs so reliability can be improved by tackling issues before they cause failure. If a failure does occur, having all the data allows you to improve maintainability.

What does MTBF mean for maintenance?

Failure is a problem and knowing everything about a problem is often the best way to solve it. Measuring MTBF is one way to get more information about a failure and mitigate its impact. Conducting an MTBF analysis helps your maintenance team reduce downtime while saving money and working faster.

Optimizing your maintenance schedule

Calculating an asset’s MTBF provides you with a baseline for maximizing your preventive maintenance schedule. Knowing approximately how often an asset fails allows you to schedule PMs before that point. This gives you a better chance to prevent the failure while doing as little maintenance as possible and maximize your resources. This method is a good step towards condition-based maintenance.

You can also go a step further and measure MTBF for a certain failure. Not only can you target that failure with PMs, but you can investigate why a certain issue is resulting in a lower MTBF. The culprit can be anything from vague task lists to a defective part or inadequate training. Analyzing your work order data can help you pinpoint the root cause of a particularly consistent failure and eliminate it. You can get a toolkit for finding and using work order data here.

Improving your MRO inventory

Inventory management can also be improved by tracking this maintenance metric. By knowing approximately how long you have before a piece of equipment goes down, you can fine-tune your approach to MRO inventory purchasing. For example, you can get a better sense of minimum quantities and lead times to achieve just-in-time delivery and more accurate parts forecasting, resulting in lower costs and quicker repair times.

Making CapEx decisions

MTBF can also help make tough repair or replace decisions a little easier. It’s never easy to give up on a piece of equipment and buy an expensive new machine. However, if all attempts to combat low MTBF are unsuccessful, it might be in your best interest to replace the asset instead of spending time and money repairing it all the time. MTBF can be used to calculate the cost of repair vs. replacement and make a business case for new equipment.

Your business and MTBF

Calculating mean time between failures is one way to start conquering unplanned downtime at your facility. There are dozens of reasons an asset might fail. Taking stock of the symptoms is the first step toward making a diagnosis and curing the issue. This is possible by tracking and analyzing MTBF. Taking measures to improve MTBF and the reliability of your assets can have a huge impact on your organization, from the shop floor to the top floor.

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