Tractor pump


A tractor pump is a device that converts mechanical energy into liquid pressure energy to provide pressurized liquid for the hydraulic system of a tractor. It belongs to the positive displacement pump, and its basic principle is the volume change of the closed oil chamber. The volume of the closed oil chamber in the pump is expanded, the oil is absorbed, and the oil is squeezed out and forced to overcome the resistance to export when it is reduced.


The volume of oil discharged by the hydraulic pump for one rotation is the displacement. The displacement of the fixed displacement pump is fixed, while the variable displacement pump is 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, rotary lobe pumps, sliding vane pumps and plunger pumps according to their structure.


The gear pump is a fixed displacement pump. A commonly used external gear pump consists of a pair of involute cylindrical spur gears. The top of the gear tooth and the inner cavity of the shell, as well as the end face of the gear and the side wall of the shell, are used as the radial and axial sealing surfaces respectively, so that a closed oil cavity is formed between the teeth. The meshing point separates the teeth that are being meshed into left and right oil cavities. When the gear rotates, each tooth of the right cavity withdraws from meshing, and the volume expands into an oil-absorbing cavity; The left chamber enters meshing, and the volume is reduced to become an oil pressure chamber.


In order to reduce the leakage of the end face, the front and rear axle sleeves are axially floating. The high-pressure oil on the outlet is directed to the back of the bushing, which is pressed against the end face of the gear. The compression force is proportional to the hydraulic pump outlet pressure, which reduces the leakage of the end face due to the increase in pressure. The external gear pump has simple structure, low cost, small size and light weight, and is the most widely used medium and high pressure hydraulic pump in tractors and agricultural machinery.


The rotary lobe pump consists of an inner rotor with a short cycloidal equidistant curve tooth profile (active) and an outer rotor with circular arc tooth shape (driven). The number of teeth of the inner and outer rotors differs by one tooth, and each rotates around the two centers with an eccentricity of e in the same direction. The meshing point changes position as it rotates, forming each closed oil chamber with variable volume. The right cavity absorbs oil; On the left side, the oil is drained. Each rotation of the inner rotor completes oil absorption and oil discharge once in each oil chamber. Rotary lobe pumps are often used as medium and low pressure hydraulic pumps.


The sliding vane pump is mainly composed of a rotor, a stator and a sliding vane that slides radially in the sliding groove of the stator. The rotor offset forms annular channels of varying widths in the stator and is separated by sliding vanes into closed oil cavities of varying volumes. When the rotor rotates with the sliding vane, the sliding vane is always in contact with the inner wall of the stator under centrifugal force. Each oil chamber changes its volume with the rotor to complete oil absorption and oil discharge. Sliding vane pumps are often used as medium and low pressure hydraulic pumps.


The piston pump relies on the reciprocating motion of the plunger to achieve volume change. The plunger and plunger bore are easy to precision machine, have a good seal, and maintain high volumetric efficiency at high pressures. According to the plunger arrangement, the piston pump can be divided into two types: axial type and radial type.


The axial piston pump cylinder body is driven by the drive shaft to rotate, and the plunger in the cylinder body realizes reciprocating motion by the swash plate. Half of the plunger is filled with oil, and the other half is oiled, and the inlet and outlet oil are collected or distributed by the distribution plate respectively. The displacement can be changed by changing the bevel angle y of the swash plate. There is a relative slide between the plunger end and the swash plate, and a sliding shoe is installed in between. The high-pressure oil in the plunger is led to the bottom, and the hydraulic pressure cushions the sliding shoe to make it statically balanced, and the oil film reduces wear and improves mechanical efficiency. The axial piston pump has a small weight per unit power, but the structure is complex, and the use and maintenance requirements are strict, and it is often used in the closed-center hydraulic system with pressure and flow compensation on high-power tractors.


Each plunger of the radial piston pump is arranged in the same plane perpendicular to the axis, and the plunger is pushed by an eccentric wheel to press the oil. The plunger returns by springing the plunger against the eccentric wheel. Each plunger has a check valve for oil inlet and outlet. When the outlet oil pressure is very low, the eccentric wheel chamber communicates with the inlet channel. The plunger is close to the eccentric wheel, when the outlet oil pressure reaches the working pressure, the pressure is transmitted to the eccentric wheel chamber through the control valve and acts on the bottom of the plunger, the plunger is kept in the position that the bottom hydraulic pressure is balanced with the compression force of the return spring, and the contact with the eccentric wheel is detached, the actual stroke is reduced, and the corresponding displacement is reduced. As a result, this pump can achieve large flow changes in a range where the pressure does not change much, and it is a constant pressure variable pump. This pump is used in constant pressure closed-center hydraulic systems on large and medium power tractors.


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