The tractor hydraulic pump is a device that converts mechanical energy into liquid pressure energy and provides pressurized liquid for the tractor hydraulic system. It belongs to a positive displacement pump, and its basic principle is the volume change of the closed oil chamber. When the volume of the closed oil chamber in the pump expands, it will absorb oil, and if it shrinks, it will squeeze out and force the oil to overcome the resistance and output to the outside.

The volume of oil discharged by the hydraulic pump for one revolution is displacement. Quantitative pumps have a fixed displacement, while variable displacement pumps are variable. The oil delivery volume of the hydraulic pump per unit time is the flow rate, and the flow rate is the product of the displacement and the speed of the hydraulic pump. The product of the pressure difference between the inlet and outlet of the hydraulic pump and the flow rate is the hydraulic power output by the hydraulic pump.

Hydraulic pumps can be divided into gear pumps, rotor pumps, sliding vane pumps, and plunger pumps according to their structure.

For the quantitative pump. Commonly used external gear pumps consist of a pair of involute cylindrical spur gears. The tooth top of the gear and the inner cavity of the housing, as well as the end surface of the gear and the side wall of the housing, serve as radial and axial sealing surfaces respectively, so that a closed oil chamber is formed between the teeth. The meshing point M separates the meshing teeth into left and right oil chambers. When the gear rotates in the direction shown in the figure, the teeth of the right chamber withdraw from meshing, and the volume expands to become an oil suction chamber; the left chamber enters meshing, and the volume shrinks to become an oil pressure chamber.

In order to reduce the leakage of the end face of the high-pressure gear pump, the front and rear bushings are axially floating. The high-pressure oil on the oil outlet channel is led to the back of the bushing, and the bushing is pressed against the end face of the gear. The pressing force is directly proportional to the oil outlet pressure of the hydraulic pump, which can reduce the end-face leakage caused by the pressure rise. External gear pumps are simple in structure, low in cost, small in size, and light in weight, and are the most widely used medium and high-pressure hydraulic pumps on tractors and agricultural machinery.