What Is Mean Time Between Failures?

Mean time between failures refers to the average time that a product or system works correctly during two adjacent failure intervals, also known as the mean time between failures. It is an indicator of how long the product or system can work on average, expressed in MTBF. ^[1]

With the wide application of the server, higher requirements are put on the reliability of the server. The so-called "reliability" refers to the ability of a product to perform a specified function under specified conditions and within a specified time; otherwise, the product or a part of it cannot or will fail to perform the specified function is a failure. In summary, the less product failure is high reliability. The ratio of the total number of product failures to the total number of life units is called "Failure rate", which is often expressed as . A WD Caviar RE2 7200 RPM hard drive that can be used in servers with MTBF up to 1.2 million hours and a 5-year warranty. 1.2 million hours is about 137 years, which is not to say that each of these hard disks can work for 137 years without failure. It can be known from MTBF = 1 / that = 1 / MTBF = 1/137 years, that is, the average annual failure rate of the hard disk is about 0.7%. Within one year, 7 of the average 1000 hard disks will only fail. When the product's life follows an exponential distribution, the inverse of its failure rate is called Mean Time Between Failures (MTBF).

There is a repairable product in the process of use, a total of N ₀ failures, after each failure after repair and continue to use as new, the working time is T1, T2, T3, T4, T5. ..... T0: Then the average time between failures of the product, that is, the average life is Q:

Q = MTBF =

Usually, we can see this MTBF value in the manual or packaging of the product, such as 8000 hours and 20,000 hours. Then, how is the MTBF value calculated? Assume a computer has an MTBF of 30,000 hours. What about detecting this computer running continuously for 30,000 hours? The answer is no. If that is the case, we have so many products that will not be tested for decades. In fact, regarding the calculation method of the MTBF value, the most commonly used authoritative standards are MIL-HDBK-217, GJB / Z299B and Bellcore, which are used for military products and civilian products, respectively. Among them, MIL-HDBK-217 was proposed by the U.S. Department of Defense Reliability Analysis Center and Rome Laboratory and became an industry standard, which is specially used for the calculation of MTBF value of military products. GJB / Z299B is our military standard; Proposed and became the industry standard for the calculation of the MTBF value of commercial electronic products.

The main consideration in MTBF calculation is the failure rate of each device in the product. However, the failure rate of the device will be very different under different environments and different conditions of use. For example, the reliability value of the same product in different environments, such as in the laboratory and offshore platform, is definitely different ; For another example, the failure rate of a capacitor with a rated voltage of 16V under the actual voltage of 25V and 5V is definitely different. Therefore, when calculating the reliability index, the above-mentioned multiple factors must be considered. All of the above factors can hardly be calculated manually, but with the help of software such as MTBFcal software and its huge parameter database, we can easily get the MTBF value.

Reliability is a criterion that initially determines the probability that a system will operate effectively within a specific operating time. Reliability measurement requires the system to maintain normal operation for a certain period of time ^[2]

Difference between MTTF, MTBF and MTTR

Reliability is a criterion that initially determines the probability that a system will operate effectively within a specific operating time. Reliability measurement requires the system to maintain normal operation for a certain period of time.

At present, one of the most widely used parameters to measure reliability is MTTF (mean time to failure, mean time to failure), which is defined as "expected values" such as random variables and error times. However, MTTF is often misunderstood as "the shortest life cycle guaranteed". The length of the MTTF is usually related to the product in the life cycle, which does not include aging failure.

MTTR (mean time to restoration) is derived from mean time to repair (IEC61508). The purpose is to clearly define the concept of time in terms. MTTR is the expected value of the recovery time of a random variable. It includes the time required to confirm the failure and the time required for maintenance. The MTTR must also include the time to obtain the parts, the response time of the maintenance team, the time to record all tasks, and the time to put the equipment back into use.

MTBF (Mean time between failures, mean time between failures) is defined as the average time required for failure or maintenance, including the time to failure and the time to detect and maintain equipment. For a simple maintainable component, MTBF = MTTF + MTTR. Because MTTR is usually much smaller than MTTF, MTBF is approximately equal to MTTF and is usually replaced by MTTF. MTBF is used for maintainable and unmaintainable systems.

1) The key countermeasures for high-frequency fault parts and the basis for technical transformation of parts life extension.

2) Calculate the life cycle of parts and prepare the best maintenance plan.

3) The selection and improvement of the inspection objects and items, and the inspection standards.

4) It is used to guide the internal and external maintenance work distribution. According to the evaluation of the equipment repair capability in the company, the risks of the maintenance quality and equipment efficiency of the internally undertaken work are determined by the different types of equipment and types of operations, as an important reference for maintenance outsourcing.

5) Set the standard of spare parts. Each reserve item and basic inventory of machinery and electrical parts should be judged according to the MTBF record analysis to make the inventory level reach the most economical situation.

6) As a reference basis for selecting the improvement points of maintenance technology methods. In order to increase the operating rate of the equipment, it is necessary to shorten the long-term maintenance work and engineering adjustment and switching time related to equipment shutdown. Therefore, it is necessary to inspect the maintenance operation method, and the basic conditions of the inspection items and the selection of priorities need to be based on the MTBF analysis record sheet.

7) It is used for the research of equipment object setting estimated operating time standard, and its maintenance operation selection and maintenance time standard. The setting of the estimated time standard for the maintenance plan and the selection of the maintenance work must take into account factors such as the equipment maintenance repetitive cycle or the difference between the standard time value and the actual maintenance time and the characteristics of the corresponding maintenance work. Therefore, an MTBF analysis table is very necessary.

8) Reference for drawing arrangement and re-selection of key equipment or parts. MTBF's analysis record sheet records the information about equipment parts transformation projects or friction degradation, as well as equipment drawing modification or preliminary production. Through frequent analysis analysis and importance ranking management, engineering drawing management can be made easier.

9) Set, revise and determine the responsibility of equipment maintenance business.

10) Provide technical information on equipment reliability and maintainability design. The most important maintenance technology is based on the MTBF analysis table to collect technical information about the reliability and maintainability design of the equipment for the reference of the design department when designing the equipment.

1) It is difficult for the maintenance department to understand the relationship between maintenance activities and product quality. For example, in terms of fault repair, functional repair is generally the focus, and the quality improvement of the product achieved by the maintenance operation is not confirmed. However, how to combine product quality and maintenance activities is very important. The MTBF analysis table can be used as an important reference.

2) PM (production maintenance) focuses on predictive maintenance centered on equipment diagnostic technology, but the specific subject results are difficult to grasp; from the MTBF analysis table, it is very useful to find the subject of equipment diagnostic technology development. Effect approach.

3) Production of equipment training materials: Training personnel who are familiar with the equipment and strong ability is an important topic of PM. However, the general training is mostly based on books sold in the market as a template, and does not fully integrate the problem situation of the company and the unit, The background question is even more different. Therefore, if MTBF analysis can be used to train the structure, function, weak links and precautions of the company's production equipment, it is a more specific approach.

4) The grasp and research data of equipment life cycle cost (LifeCycleCost, LCC): MTBF analysis table is a list compiled based on equipment for a long period of time. Therefore, grasping the equipment life cycle cost data according to the maintenance operations, costs, spare parts and losses that occur in the equipment is an important basis for equipment lifetime management. In summary, MTBF analysis is not only one of the methods of maintenance records, but its maintenance records can also be used as the source of maintenance activities, management, and technical activity guides. It is of great value and significance ^[3]

Step 1: Determine the equipment object to be analyzed-usually select the key equipment to record first, or you can record according to a certain type of equipment group or a certain key part of the equipment.

Step 2: Collect fault data-analyze the fault data of equipment in the past 3 to 5 years or at least 30 times.

Step 3: Draw the fault mapDraw the overall schematic diagram of the equipment, and use the data in step 2 to mark the fault location.

Step 4: Prepare MTBF analysis form.

The content of the analysis table is based on the data that can be recorded in one year.

Record the content of step 3 in order of site type and date of occurrence.

Whenever possible, use graphical methods or color categories and marks to increase legibility.

Continue recording until "When the sudden equipment failure is zero".

Step 5: Failure analysis and countermeasure inspection.

The MTBF analysis table is used to analyze the cause of the failure and test the countermeasures (common analysis methods include Plato, feature factor diagram (fishbone diagram), fault tree method, etc.).

The countermeasures should be easy to understand and practical.