1. Mean time between failures
Mean Time Between Failures MTBF (MeanTimeBetweenFailures) refers to the average value of the working time of adjacent failures for repairable products. It is an important indicator to measure reliability. The specific values are given in the product standards. According to statistics, the minimum acceptable MTBF of the CNC system should not be lower than 3000h. Statistics show that the MTBF of foreign CNC systems is 5000h ~ 22000h. The evaluation of reliability is mainly to evaluate the non-faulty parameters. The loss of the specified function of the CNC system is called failure. The average trouble-free working time can accurately reflect the normal working time of CNC equipment. It refers to the average value of the working time without fault gap before the next fault occurs after one fault occurs.
After the early wear-out period of the CNC machine tool, the early failure is eliminated and the normal working stage is entered. Its work is basically controlled in the accidental failure stage. It can be considered that the time between failures follows an exponential distribution. The interval estimation of CNC machine tool fault interval time generally takes the confidence interval level of 1-α = 90%, that is, the probability that the true value falls within the estimation interval is 90%. According to the number of failures of CNC machine tools and the time of occurrence, the calculation can be carried out according to the above formula. The longer the MTBF, the higher the reliability and the better the ability to work correctly.
2. Inherent availability
Inherent availability, also known as availability, is the probability that mechanical equipment and parts maintain their specified functions under specified conditions of use, or A for short. Effectiveness is an important index to evaluate the utilization rate of equipment, and it is also an important factor that directly restricts the production capacity of equipment. Effectiveness is a function of time. Generally speaking, effectiveness refers to the instantaneous validity when time t tends to ∞. It is the ratio of the average trouble-free working time of CNC machine tools to the sum of the average trouble-free time and the average repair time Within a certain observation period, the ratio of product working time to non-working time and working time.
3. Average repair time
Mean Repair Time (MeanTimeToRepair), also known as the average after-sales repair time, is the average repair time required to recover the specified performance from the fault to the machine tool, referred to as MTTR. It includes the time necessary to confirm the occurrence of a failure, the time required for repairs, the time to obtain accessories, the response time of the maintenance team, the time to record all tasks, and the time to put the equipment back into use. MTTP is not only related to the design of the product itself, but also closely related to the method of use, maintenance level, and spare parts strategy.
4. Accuracy holding time
Accuracy retention time (kT) is the time for the CNC machine tool to maintain its accuracy within the range specified by the machine tool accuracy standard under the conditions of two shifts and compliance with the rules of use. The observed value is based on the precision retention time of the machine tool with the shortest precision retention time among the extracted prototypes.
Among the above four evaluation indicators, MTBF focuses on the trouble-free nature of CNC lathes and is the most commonly used evaluation indicator; MTTR reflects the maintainability of CNC machine tools, that is, the difficulty of performing maintenance; the inherent availability A comprehensively reflects the failure-free And maintenance, that is, effectiveness; accuracy retention time reflects the durability and reliable life of CNC machine tools.