Precision bearings have ● high load capacity ● low friction ● smooth and quiet rotation ● high precision ● high rigidity and other requirements. These functions must be sustained at the same time. These functions are called bearing durability (grease life, sound life, fatigue life, etc.), that is, life in a broad sense. But durability will affect its sustainability due to various damage and deterioration. In addition, there are other damages such as cracking and seizure that will affect the life of the bearing, but these damages should be distinguished from the life of the bearing. The reasons for these damages include not only the wrong bearing selection, but also the surrounding design of the bearing and the use and installation of the bearing. On the other hand, the repeated stress caused by the load will cause damage to the bearing material itself due to rolling fatigue. Even if the bearing is used correctly, it will eventually reach the life of the bearing in the narrow sense. Generally speaking, once there are signs of metal fatigue, it can be immediately judged that the bearing can no longer be used, and by this time the life of the bearing will come to an end. The life of the bearing can be expressed in terms of the total number of revolutions since the start of the bearing or the total running time of the bearing at a constant speed. Even under the same load conditions, the life of the bearing varies according to the size and model of the bearing used. When selecting bearings, the bearing life should be recalculated. The fatigue life (narrow life) of a bearing is originally different for each bearing, and it is usually described by the statistical results of a group of the same bearings under the same conditions. Therefore, when selecting a bearing, the calculated life of the bearing cannot be understood as the average life of all bearings, but can only be understood as the expected life expectancy that most of the bearing can reach. Therefore, we define the rating life and the basic load rating Cr or Ca as follows. ● Basic rating life The total number of revolutions that a group of the same bearings can run under the same conditions, and 90% of them can rotate without material damage due to rolling fatigue. ● The basic dynamic load rating (Cr or Ca) can reach the bearing load in the direction and size of the bearing life of 1 million revolutions. The basic rating life of the bearing is calculated according to formula 2.1 and 2.2. (Formula 2.1) (Formula 2.2) L: Basic rating life (revolution) Lh: Basic rating life (hours) C: Basic dynamic load rating (N) (Cr for radial bearings and Ca for thrust bearings) P : Bearing load (equivalent dynamic load) (N) (Pr for radial load and Pa for thrust bearing) p: 3 (ball bearing), 10/3 (roller bearing) N: Speed: (min- 1) The basic dynamic load rating of radial ball bearings in multi-row assembly is calculated by multiplying the dynamic load rating of the single row bearing by the following coefficients. 2 rows with 3 rows with 4 rows with 1.62 2.16 2.64.