When introducing a machine or system, we select it by considering various points such as its performance and safety. Among the selection points, the product life is also included. No matter how good the performance and safety of a machine or system is, if it breaks down quickly, it cannot be used with peace of mind. Especially in the case of products intended for risk assessment, such as safety-related products, service life is a very important item.
MTTF (mean time to failure) is an indicator of product life. If you know the MTTF of a product, you can know the guideline for the life of the product. Here, we will introduce the idea of what MTTF is.
What is MTTF, the index of product life?
MTTF stands for "Mean Time to Failure" and is called mean time to failure in Japan. Since MTTF is used for products that cannot be repaired, it has the same meaning as the so-called life expectancy. In other words, it is an index that shows how long it will take to break. A MTTF of 10 years means that the product will fail after 10 years of operation on average.
The MTTF of a product can be obtained by examining the time from operation to failure of multiple identical products and calculating the average. For example, if product A1, product A2, and product A3 with the same specifications as product A are 100 hours, 110 hours, and 105 hours from operation to failure, the MTTF of product A is
MTTF = (100 + 110 + 105)/3 = 105 hours
becomes.
In general, it can be said that the higher the MTTF value, the more stable the product can be used for a long period of time.
MTBF is used for repairable products
The above MTTF is a metric used for products that cannot be repaired. However, there are many products that can be repaired, and there are indicators used for them. That is MTBF.
MTBF is an acronym for "Mean Time Between Failure" and is called the mean time between failures in Japan. As the term “failure interval” suggests, it is the time between the start of operation and the next failure. If the MTBF is 10 years, it can be said that the product will fail on average once in 10 years of operation.
A product's MTBF is calculated by dividing the product's total operating hours by the total number of failures. For example, in the case of product A, which fails after 100 hours of operation, after 150 hours of repair, and after 130 hours of repair, its MTBF is
MTBF = (100 + 150 + 140)/3 = 130 hours/case
becomes.
What is the common problem of MTTF and MTBF, the divergence between the theoretical value and actual feeling?
When examining the MTTF and MTBF of a product, in order to obtain the most accurate data, it is necessary to monitor one product until it fails (until it becomes unusable in the case of MTBF). However, it is realistically impossible to monitor for several years or decades just for that purpose. Therefore, it is usually calculated by running several identical products at the same time and summing the operating time of each.
For example, when looking at MTBF. If 1,000 identical products are operated simultaneously and monitored for 100 hours, they can be considered to have operated for a total of 100,000 hours. If 10 units fail during that time, the MTBF will be 10,000 hours/case. If you operate only one unit and monitor it for 100,000 hours, if it fails 10 times, the MTBF is the same 10,000 hours/case. Thus, both methods result in the same MTBF.
However, since the concept of wear and aging exists when actually operating a product, the theoretical value calculated from the total operating time of 1,000 units is not necessarily correct. For this reason, there is a discrepancy between the theoretical value and the perceived failure interval when the product is actually introduced.
In addition, there is a word "MTTFd" that is very similar to MTTF. ISO13849-1 "Safety of machinery - Safety-related parts of control systems" is one of the parameters that satisfies "PL (Performance Level)" in the scale of functional safety. represents the average time it takes. Next time, I will introduce this PL and MTTFd.