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There is a whole array of maintenance indicators that will be used according to the maturity of our company. More generally, we will use only 3 or 4.

## Introduction

There is a whole array of maintenance indicators, mostly the acronym starting with MT (mean Time‘s initial). In the same way that the term TPM is of Anglo-Saxon origin, the whole neology of the TPM is Anglo-Saxon1. In this panoply of terms we find the following examples:

• MTBD : MeanTime Between Defects
• MTBE : MeanTime Between Errors
• MTBF : MeanTime Before Failure
• MTBF : MeanTime Between Failures
• MTBM : Mean TimeBetween Maintenances
• MTBO : MeanTime Between Overhauls
• MTBR : MeanTime Between Removals
• MTTF : Mean Time To Failure
• MTTM: Means Time To Maintenance
• MTTN: Mean Time To Notification
• MTTR : Mean Time To Repair
• MTUR: Mean Time to Unscheduled Removal
• MTBCF: Mean Time Between Critical Failures
• MTBUR: Mean Time Between Unscheduled Removals
• MTTFF : Mean Time To First Failure
• MTTUR: Mean Time To Unscheduled Removal

## The Maintenance indices

### The MTBF

The MTBF, Mean Time Between Failures, for average time to failure or even average time of operation, is an important parameter in the calculation of reliability. It measures the average time of operation of an equipment, in other words the average time between two failures. Of course, at the most this one is great, at the most our equipment is reliable and produces in the best of ways.

Usually it is calculated in the following way:

MTBF = operating time/number of failures

In a continuous improvement process the MTBF must take into account the micro-stops:

MTBF = operating time/(number of failures + Μ-stops)

or

MTBF = (Good running time + Μ time)/nb of failures

It is understood that the main problem in the measure of this indicator is in the definition of a breakdown.

Example 1

Case of an industrial company wishing to calculate an internal MTBF. In it there is a whole panel of automated or semi-automatic machine. Should we consider it a failure:

• A simple adjustment intervention was enough to solve the problem?
• If we had to change that parts not directly functional (retaining screw that broke…)?
• If the stop is only 10 minutes long?
• What if an on-line operator was able to repair it himself?

Example 2

Case of an industrial company but wishing to calculate a MTBF with regard to the products it supplies. Is it a failure if the product fails because:

• The client misused it?
• The product has deteriorated during transport?

The examples are endless. It is advisable to find a definition that suits us:

• Adapted to our context.
• That don’t make it wrong.
• Easy to identify.
• That everyone agrees with.

### The MTTR

Mean Time To Repair is a performance indicator for maintenance services. It allows to define with a good probability a repair time under defined conditions. The MTTR does not take into account the micro-stops.

MTTR = failure Time/Nb of failures

It will be held that the downtime takes into account the times of repair and the Times ” annexes “:

 Repair time Time Schedules Diagnosis: Problem localization, problem type… Intervention to solve the problem including time to go to the store to look for coins. Validation tests and reroute time to a normal operating state. Identification by the people of the line that a problem has arisen. Time to call maintenance. Time for the arrival of the maintenance technician.

It is understood that the MTTR directly influences the availability but not the reliability.

## The maintenance indicators

### Maintainability

Maintainability defines the probability of the duration of a repair and is the most common reflection of the level of simplicity that the maintenance of an equipment. It is calculated on the basis of the MTTR:

maintainability = 1/MTTR

On the other hand, at the most we get closer to 1, the more our maintainability is good.

### The availability

Availability expresses the likelihood of equipment being ” available “, i.e. to be able to operate in a normal state (according to the NF-X-60-010 standard). It is calculated in the following way:

availability = MTBF/(MTBF + MTTR)

An availability of 0.5, indicates that 50% of the time my equipment is in good working order.

### Repair Rate

It reflects the number of repairs performed by the maintenance per unit of time. It is calculated in the following way:

Repair rate = 1/MTTR

## Source

1-C. Lasse (2009)-English-French dictionary for the Maintenance Professional

2 – Institute of Electrical and Electronics Engineers (1990) – IEEE Standard Computer Dictionary: A compilation of IEEE standards glossaries

H. Procaccia, E. Ferton, M. Procaccia (2011) – Reliability and maintenance of repairable and non-repairable industrial equipment.