In fact, gears and bearings do not contact directly during rotation. In the middle, the oil film is formed by non-contact rolling and sliding of lubricating oil. This is the lubricating effect of oil. Although they are non-contact rolling and sliding, there are relative rolling friction and sliding friction in the rotation due to machining accuracy and other reasons, thus generating a certain amount of heat. If the heat is not eliminated in the process of rotation, more and more heat will be accumulated, which will eventually lead to high temperature combustion of gears and bearings. Therefore, gears and bearings must be cooled by lubricating oil during rotation. Therefore, on the one hand, lubricating oil plays a role of lubrication, on the other hand, it plays a role of cooling.