Measuring failure metrics is an integral part of asset management. In this article we will look at how to calculate MTBF, MTTF and MTTR and how all these metrics can be used in your maintenance management strategy or how they can be used to improve your service quality.
Mean Time Between Failures or MTBF, often referred to as MTBF, is one of the values that indicate the reliability of a component, product or system.
MTBF is a maintenance metric,meaning the time between failures of a repairable system.
Breakdowns are one of the most frequent sources of equipment unavailability and influence its proper functioning from both a technical and a user point of view.
What interests a company is the effective production time between breakdowns.
Performance metrics are essential for any organisation whose operations depend on the equipment. Only by tracking what is likely to fail can an organisation maximise availability and minimise downtime, and the organisation can develop a maintenance strategy (curative or preventive) based on its data.
1 - Calculation of the mean time between failures (MTBF)
The time lapses from one failure to the next can be calculated using the sum of the running time (uptime) divided by the number of failures.
MTBF = Total uptime / # of Breakdowns
For example, out of an expected running time of ten hours, it ran for nine hours and failed for one hour, spread over three times.
So MTBF = 9 hours / 3 = 3 hours.
As you can see from the example above, the repair time is not included in the MTBF calculation.
In addition to the design conditions mentioned above, there are other common factors that tend to influence the MTBF.
One of the main factors is human intervention. For example, a low MTBF could indicate poor handling of the asset by its operators or poorly executed repair work in the past.
2 - Why is MTBF helpful?
MTBF is an important marker of reliability engineering and has its roots in the aviation industry, where an aircraft failure can result in fatalities.
Even everyday decisions such as whether to buy a particular brand of car or computer are affected by the buyer's desire for a product with a higher MTBF than the next brand has to offer.
For critical assets such as aircraft, safety equipment and generators, MTBF is an important indicator of expected performance. Therefore, manufacturers use it as a quantifiable reliability measure and as an essential tool in the design and production stages of many products. It is commonly used today in the design of mechanical and electronic systems; the safe operation of factories; product procurement, etc.
Although MTBF does not consider planned maintenance; it can still be applied to things like calculating the frequency of inspections for preventive maintenance cases.
Although it is known that an asset is likely to operate for a number of hours before the next failure, the introduction of preventive actions can help to keep this failure to a minimum and extend the operating life of the asset.
3 - What is the difference between MTBF, MTTR and MTTF?
The Mean Time To Repair (MTTR) refers to the time it takes to repair a system and restore it to full functionality.
The MTTR starts at the beginning of the repair cycle and continues until the repair is completed.
This includes the repair time, the test period and the return to normal operating condition.
To calculate the MTTR, divide the total maintenance time by the total number of maintenance actions in a given period.
Imagine an equipment that breaks down three times in one working day. The time spent repairing each of these breakdowns totals two hours. In this case, the MTTR would be 2 hours / 3 = 40 minutes.
Maintenance managers are well advised of the usefulness of MTTR, but taking too long to repair a system or piece of equipment is not desirable, as it can have a very negative impact on business results.
This is particularly the case for processes that are particularly sensitive to failure. This often results in production downtime; missed deadlines; lost revenue, etc.
What is the mean time to failure MTTF?
Mean Time To Failure (MTTF) is a very basic measure of reliability used for non-repairable systems. It represents the expected duration of operation of a component until it fails.
MTTF is what we commonly call the lifetime of any product or device. Its value is calculated by looking at a large number of items of the same type over an extended period of time and seeing what their mean time to failure is.
MTTF is the primary indicator of equipment reliability; with the intent of maximizing asset life.
A short MTTF means more frequent downtime and interruptions.
We will calculate the MTTF like this:
For three identical pieces of equipment until they all fail.
The first one broke down after 10 hours, the second at 12 hours and the third at 14 hours. In this case, the MTTF would be
(10+ 12+ 14) / 3 = 12 hours.
This leads us to conclude that this type of equipment will have to be replaced, on average, every 12 hours.
The only sure way to increase the MTTF is to look for better quality components made from more durable materials.
In conclusion, maintenance managers are there to ensure maximum equipment availability and uptime, as well as safe and efficient operations.
Understanding and reading the data created by the use of failure metrics (MTBF, MTTR, MTTF) helps maintenance professionals in the continuous improvement of their maintenance programmes and practices.
Based on these findings, they can develop better asset management strategies and improve their overall maintenance processes to avoid failure and damage caused by machine breakdowns.
To know more about maintenance strategies: