Mean Time Between Failure
What is mean time between failure?
Mean time between failure (MTBF) is a metric that defines the average time that equipment is operating between breakdowns or stoppages.
MTBF as a maintenance metric indicates the duration that an equipment operates without disturbances. This intuitively relates to the availability of the equipment. Availability, also known as uptime, is one of the key indicators of overall equipment effectiveness and is always a focus area for improving productivity. An equipment’s total uptime can be expressed in terms of the MTBF together with another metric, the MTR (mean time to repair).
It is important to note that MTBF is applicable only to repairable items. Manufacturing processes can use it to plan for contingencies that require the repair of key equipment. Being aware of this data provides insights for making sound decisions for the plant.
Differentiating MTBF from MTTF
Another metric that relates time with failure is the mean time to failure (MTTF). MTTF, like MTBF, is a measure of time. It quantifies the time before an equipment totally breaks down. The difference of MTTF from MTBF is that it is only applicable to items that are not repairable. The assumption is that once the MTTF is reached, the item has reached its maximum hours of service.
How to calculate MTBF
The MTBF is calculated by taking the total time a piece of equipment is running (i.e. uptime) and dividing it by the number of breakdowns that occurred over the same period.
MTBF = Total uptime / # of breakdowns
Take for example a mechanical mixer designed to operate for 10 hours per day. Suppose the mixer breaks down after normally operating for 5 days. The MTBF for this case is 50 hours as calculated below.
MTBF = (10 hours per day * 5 days) / 1 breakdown = 50 hours
The calculation will require more steps when accounting for longer periods of time with increased occurrences of breakdowns.
Let’s say that the same mechanical mixer, running for 10 hours per day breaks down twice in the span of 10 days. The first breakdown occurred 25 hours from the start time, and took 3 hours to repair. The second breakdown occurred 50 hours from the start time and took 4 hours to repair before the mixer was operating normally.
The succession of events is illustrated by the timeline below:
Based on the timeline, we can more easily account for the total operating time of the machine. For the given example, the total uptime period is the sum of 25, 22, and 46 hours.
Given the total uptime and number of breakdowns, the calculation is taken by the plugging the values into the MTBF formula as follows:
MTBF = (25 hours + 22 hours + 46 hours) / 2 breakdowns = 93 hours / 2 breakdowns = 46.5 hours
How to improve MTBF
Increasing the MTBF increases the uptime of equipment. Keeping track of the MTBF for each piece of equipment, especially those that require continuous operation, allows the maintenance team to efficiently schedule maintenance activities.
The first step to improving MTBF is to make sure that the data being collected is accurate. Developments of tools such as various maintenance software can ensure that data is being recorded correctly and accurately.
The next step to gathering data is using it to proactively perform preventive maintenance. Investing the time to perform maintenance checks and activities such as checking for proper lubrication, aligning calibration systems, and cleaning pays off by significantly reducing the number of major stoppages due to breakdowns.
Relating MTBF to Availability
As previously mentioned, availability is expressed in terms of the metrics MTBF and MTR. Think of it as calculating the availability based on the actual time that the machine is operating—excluding the time it takes for the machine to recover from breakdowns.
This is quantified by the following equation:
Availability = MTBF / (MTBF + MTR)
Availability is one of the three factors contributing to the overall equipment effectiveness of a plant. The above equation signifies the relationship of improving the MTBF and other metrics related to improving the plant’s performance.