RAMS parameters step by step. What MTBF is?


When we carry out RAMS studies for our clients, it is very common that we will observe the difficulty in differentiating several parameters or basic objectives of reliability, maintenance and availability (RAM objectives) that characterize products, systems or facilities.

In this way, using a graphical approach and in a very simple way, we present here the most relevant values in RAM studies, as well as optimization and improvement strategies. Next, we expose the parameters that we will define, explain and understand in this article:

Most important RAM indicators

MTTF: Mean Time to Failure
MDT: Mean Down Time
MUT: Mean Up Time
MTBF: Mean Time Between Failure
λ: Failure rate
MTTR: Mean Time to Repair

As seen in the previous graphic, we have a system that, over time, (marked in blue) is working correctly until an error occurs (ray marked in red). When the error occurs, the system will be down for a while (marked in yellow) and, after a while -assuming that will be once it has been repaired-, it will come into service again (marked in blue).

"In service" state (marked in blue) will remain until, after a while, a failure will appear again (red lightning).

What means parameter MTTF?

The MTTF parameter stands for Mean Time to Failure and it is the time the system is active, fulfilling the functionalities for which it has been designed. In repairable systems (where the system can go out of service and start up once it has been repaired), MTTF and MUT (Mean Up Time) are exactly the same.

It is very common to mistake MTTF and MTBF. As can be easily seen in the previous graphic, the difference between both parameters is MDT time (marked in yellow), that is, the time the system is out of service. In this way, we can say that MTBF = MTTF + MDT.

As we observe, MTTF is really the parameter that tells us that, once the system has been put into service (for the first time or after corrective maintenance, for example), how long we have planned to keep it in service.

What means parameter MTBF?

In the field of RAMS Engineering, MTBF (Mean Time Between Failures) parameter or objective is the most used, requested and extended. MTBF (Mean Time Between Failures) indicates what is the forecast of frequency failure of the system. That is, will this answer the following question: how often is it expected that a system failure will appear under review? The MTBF is totally linked to the reliability of the equipment, the product or the installation.That is, quantitative parameter of reliability(reliability, "R") is MTBF.Moreover, failure rateλ is 1/MTBF. 

What means parameter MDT?

MDT (Mean Down Time) parameter is the time when a system will not work properly if an error occurs. Going into detail, MDT time is divided by the time until the equipment failure is detected -and therefore, from which we can order its repair-, plus the time it takes to repair, the so-called MTTR (Mean Time to Repair).

It is important to make a special mention of "Failure Detection Time". During that time, risk is considered to be extremely critical since there is a system that does not work properly while it is believed to be in service and functioning properly. Therefore, this time must be minimised to the maximum, and it is intolerable in many applications that this time will be different for a few seconds or milliseconds.

What means parameter MTTR?

The Mean Time to Repair (MTTR) is the average time taken to repair the equipment, the product or the installation under analysis. Thus, MTTR will indicate the capacity or goodness of repair. The aim is making this as small as possible. As shown in the graph, the MTTR is divided into the diagnosis of the particular error, the possible replacement of equipment, the adjustment of the equipment or the system itself when introducing a new equipment and, finally, putting it into service again. The Mean Time To Repair (MTTR) is totally linked to the maintainability of the equipment, the product or the installation.That is, the quantitative parameter of maintainability (maintainability, "M") is MTTR.

Why are we always talking about "Mean Time"?

As you can see, when talking about objectives or RAM values, it is referred as "Average Time of", in all cases. The explanation of this particularity is that these parameters are calculated on statistical analysis basis. Also, there are projections or probabilities of what may happen in the future. In addition, these projections -in their simplest analysis process- are used to carry out in an exponential model that will result in a constant failure rate, throughout the service life of the product.

At Leedeo Engineering we are specialists in RAMS analysis and in calculating and determining the RAMS parameters / objectives of products, systems or facilities, as well as their optimization. Trust on us to develop the RAMS activities of your projects. Contact >>

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