Working principle and structure of axial piston pump
Working principle and structure of axial piston pump
1. Axial piston pump
In order to form the reciprocating motion conditions of the plunger, the axial piston pump has an inclined structure, so the axial piston pump is divided into swash plate (straight shaft type) and inclined shaft (swing cylinder type) according to its different inclined structure.
Figure r shows the working principle of the pump. The pump body is composed of cylinder block 1, oil distribution plate 2, plunger 3 and swashplate 4. Several plungers are evenly distributed in the cylinder body along the circumference. The swashplate axis is inclined at an angle with the cylinder axis. The plunger is pressed on the swashplate by mechanical device or under the action of low-pressure oil (spring in the figure), and the oil distribution plate 2 and swashplate 4 are fixed. When the prime mover rotates the cylinder block through the transmission shaft, the plunger is forced to move back and forth in the cylinder due to the action of the swashplate, and oil is absorbed and pressurized through the oil distribution window of the oil distribution plate.
As shown in Figure R, when the plunger moves to the lower semicircle range (π ~ 2 π), the plunger will gradually extend out of the cylinder liner, and the sealing working volume at the bottom of the plunger will increase to absorb oil through the oil suction window of the oil distribution disc; At 0 ~ π, the plunger is pushed into the cylinder block by the swashplate to gradually reduce the sealing volume and press the oil through the oil pressure window of the oil distribution plate. Every time the cylinder rotates, each plunger completes oil suction and pressure once.
Changing the inclination angle of the swashplate can change the length of the plunger stroke, that is, change the displacement of the hydraulic pump; Changing the direction of swashplate inclination can change the direction of oil suction and pressure, so as to make the pump a two-way variable displacement pump.
The width 1 of the sealing area between the oil suction window and the oil pressure window on the oil distribution plate shall be slightly greater than the width L1 of the oil passage hole at the bottom of the plunger cylinder block. However, the difference shall not be too large, otherwise oil trapping will occur. Generally, small triangular grooves are opened at both ends of the two oil distribution windows to reduce impact and noise.
2. General structure of swashplate axial piston pump
Figure s shows the structural diagram of a swashplate axial piston pump, which is composed of the main structure on the right and the variable adjustment mechanism on the left.
The main part is composed of a cylinder block 10, a plunger 17, a swashplate 4 and a port plate 13 installed in the intermediate pump body 16. The cylinder block 10 rotates driven by the transmission shaft L2. The plunger 17 is installed in each axial plug hole of the cylinder block. The plunger head is matched with the sliding shoe 1 by spherical surface, and the outside is riveted, so that the plunger and the sliding shoe will not fall off, and the matching spherical surface can move relatively; When the return disc 2 and the plunger sliding shoe rotate together, the plunger is pushed to move axially with the help of the swashplate 4 during oil drainage; The spring 11 pushes the return disc 2 through the steel ball 8, so that when absorbing oil, the sliding shoe 1 is tightly pressed on the surface of the swashplate 4 by the return device composed of the return disc, steel ball and spring, so that the pump can have white suction capacity. There is an oil chamber at the contact part between the sliding shoe 1 and the swashplate 4, which is connected with the working chamber in the cylinder block 10 through the small hole in the middle of the plunger 17, so that after the pressure oil enters the oil chamber, an oil film is formed between the contact surface between the sliding shoe and the swashplate, which plays the role of hydrostatic support, greatly reducing the force of the sliding shoe on the swashplate and reducing the wear. The drive shaft 12 drives the cylinder block 10 to rotate through the spline at its left end, and the plunger moves back and forth in the cylinder block while rotating with the cylinder block. The sealing working volume at the bottom of the plunger in the cylinder block is communicated with the inlet and outlet of the pump through the port plate 13. With the rotation of the transmission shaft, the hydraulic pump continuously absorbs and discharges oil.
The cylinder block 10 is supported on the intermediate pump body through the large bearing 9, so that the radial component of the swashplate 4 acting on the cylinder block through the plunger can be borne by the large bearing, so that the transmission shaft L2 is not affected by the bending moment, and the stress state of the cylinder block is improved, so as to ensure better contact between the end face of the cylinder block and the port plate.
The variable adjustment mechanism on the left is used to adjust the output flow. The variable axial piston pump is equipped with a special variable adjustment mechanism, which can be used to change the inclination of the swashplate to adjust the flow of the pump.
The variable control forms of axial piston pump generally include manual control and hydraulic servo control. Figure s shows the manual variable control mechanism. Its working principle is: rotate the hand wheel 7 to rotate the screw 6, which can make the variable piston 5 move up and down, so as to drive the swashplate 4 inserted at the lower end of the variable piston 5 to swing around the circular arc surface on the shell, so as to change the tilt angle of the swashplate 4, so as to achieve the purpose of manually controlling the output flow.
Figure t shows a hydraulic servo variable control mechanism, which is composed of housing 5 and servo piston 4. Its basic working principle is as follows: the pressure oil output by the pump enters the lower chamber D of the variable mechanism housing 5 through the channel through the check valve a, and the hydraulic pressure acts on the lower end of the variable piston 4. When the pull rod connected with the servo valve spool 1 does not move (as shown in the figure), the upper cavity g of the variable piston 4 is closed, the variable piston does not move, and the swashplate 3 is in a corresponding position. When the pull rod moves downward, push the valve core 1 to move downward together, and the pressure oil in chamber D enters the upper chamber g through channel E. Since the effective area of the upper end of the variable piston is greater than that of the lower end, and the downward hydraulic pressure is greater than the upward hydraulic pressure, the variable piston 4 also moves downward until the oil port of channel e is closed. The displacement of the variable piston is equal to the displacement of the pull rod. When the variable piston moves downward, the swashplate 3 is driven to swing through the shaft pin, the inclination angle of the swashplate increases, and the output and inflow of the pump increases accordingly; When the pull rod drives the servo valve spool to move upward, the spool will open the channel f, the upper chamber g will connect the oil tank through the pressure relief channel to relieve the pressure, and the variable piston will move upward until the spool closes the pressure relief channel. Its movement is also equal to the movement of the pull rod. At this time, the swashplate is also driven to swing accordingly, so that the inclination angle decreases and the flow of the pump decreases accordingly.
It can be seen from the above that the servo variable mechanism realizes the variable by operating the hydraulic servo valve and using the pressure oil output by the pump to push the variable piston. Adding a small force to the pull rod can sensitively control the larger piston 4. Therefore, the variable piston 4 is called the servo follow-up piston. The pull rod can be operated manually or mechanically, and the swashplate can be tilted by ± 18 °, so the suction direction of the pump can be changed during operation. Therefore, this pump is called two-way variable hydraulic pump.
3. Basic structure of through shaft axial piston pump
The plunger pump shown in figure s is also known as non-through shaft axial plunger pump. One of its main disadvantages is that large roller bearings are used to bear the huge radial force exerted by swashplate 4 on the cylinder block. The service life of the bearings is low, the speed is not high, the noise is high and the cost is high. Therefore, a through shaft piston pump has been developed in recent years, and its typical structure is shown in Figure U. Its working principle is basically the same as that of non-through shaft type. The main difference is that the main shaft of the through shaft pump is directly supported at both ends as shown in Figure u, so that the radial component of the swashplate acting on the cylinder block through the plunger can be directly borne by the main shaft, so the large bearing at the outer edge of the cylinder block is cancelled, so that the speed of the through shaft pump can be improved.
4. Inclined shaft axial piston pump
(1) Structure of inclined shaft axial piston pump. Figure V shows a typical structure of inclined shaft unidirectional variable displacement pump, which is composed of main shaft, pump shell, bearing, plunger with connecting rod, central shaft, cylinder block, port plate and variable mechanism. The main shaft is driven to rotate by the prime mover, and the cylinder body is driven to rotate by the connecting rod and plunger. Because the cylinder axis intersects with the rotation axis at an angle, when the cylinder rotates, the plunger moves back and forth in the cylinder and absorbs and presses oil through the port plate. The contact surface between the valve plate and the variable housing is made into an arc, and the valve plate is connected with the variable mechanism through a shift pin.
(2) White motion compensation principle of inclined shaft axial piston pump. The pump can automatically compensate and adjust the pressure by increasing the flow when the output pressure decreases through the white action compensation and adjustment mechanism on the right. When the load pressure increases, the pressure oil acts on the upper end of the pilot piston 3 through the nozzle 2 and pushes the guide rod 4 and the control valve core 9. Because this thrust is greater than the force of the adjusting spring, the control valve core 9 will move downward, so that the pressure oil enters the lower cavity of the variable piston 13 through the radial hole of the valve sleeve 10. Because the area of the lower end of the piston is large and the area of the upper end is small, the piston 13 will move upward, Thus, the cylinder block 16 is driven to reduce the swing angle, reduce the flow of the pump and realize the purpose of variable. At the same time, the large and small springs sleeved on the guide rod are also under pressure. The pressure acts on the pilot piston through the guide rod to balance the force at the lower end of the pilot piston with the hydraulic pressure at the upper end. The pressure of the guide rod on the control valve core decreases and the control valve core moves upward until the radial hole of the valve sleeve is closed, so the variable piston is fixed at a certain position. On the contrary, when the load pressure decreases and the pressure transmitted by the regulating spring to the piston through the force control valve core and guide rod is greater than the pressure at the upper end of the pilot piston, the control valve core moves upward under the action of the regulating spring to communicate the control oil in the large chamber of the variable piston with the low-pressure chamber. The pressure at the small end of the variable piston is high and the pressure at the large end is low, The variable piston moves downward under the action of the difference of hydraulic pressure, and the swing angle and flow between the cylinder block and the main shaft increase through the shift pin. At the same time, the pressure of the large and small springs on the pilot piston decreases. Under the action of the above pressure, the fork of the pilot piston pushes the guide rod and the control valve core downward until it is balanced with the force of the adjusting spring. At this time, the variable piston is in a new equilibrium state at a certain position. Therefore, this variable mode is to make the flow act automatically and change accordingly with the change of pressure, which can roughly keep the product of flow and pressure unchanged, that is, constant power variable.
(3) Characteristics of inclined shaft axial piston pump. Compared with swashplate pump, the biggest feature of this kind of pump is the large variable range. This kind of swashplate pump has a large variable range because the radial force on the plunger and cylinder block is small, the pump strength is high, and the allowable inclination angle is large. Generally, the maximum inclination of swash plate pump is about 20 °, while the maximum inclination of swash shaft pump can reach 40 °. On the other hand, because of its complex structure, large overall dimension and mass, and because it changes the flow by swinging the cylinder block, its volume and inertia of the variable mechanism are large, and the response speed of the variable mechanism is low.