AF Series Air Filters Working Principle and Application: When the hydraulic system is operating, the oil level in the reservoir fluctuates, rising and falling. When rising, air is expelled from the inside to the outside, and when falling, air is inhaled from the outside to the inside. To purify the oil in the reservoir, an air filter is vertically mounted on the lid of the reservoir, which filters the inhaled air. Additionally, the air filter also serves as the oil filling port, filtering out impurities from the new working oil that is injected. In the liquid oil system, purifying the working oil is a crucial step.