In order to reduce the friction resistance between relative moving parts in the gearbox, reduce the wear of parts, oil is constantly transferred to the friction surface of each moving parts, forming a lubricating film for lubrication. The oil itself contains a certain amount of gum, impurities, moisture and additives. At the same time in the transmission work process, metal debris into the oil oxide generation, so that the oil in the debris gradually increased. If the oil is not filtered clear, directly into the lubricating oil road, the oil will contain sundries into the friction surface of the sports pair, accelerate the wear of parts, reduce the service life of the transmission.