Piston stroke control mechanism and variable pump

Abstract

The present application relates to a kind of piston stroke control mechanisms, it is characterized in that: including valve body, piston, the piston cavity is equipped in the valve body, the piston is located in the piston cavity and can be axially moved, the piston cavity includes the upper pressure chamber in the upper end of piston;The piston is equipped with pressure oil port and oil return port, further include the rotating shaft that inserts into upper pressure chamber and inserts into piston, the circumferential wall of the rotating shaft is equipped with with upper pressure chamber always communicating inclined groove;The rotation of the rotating shaft can make that inclined groove is communicated with pressure oil port, oil return port both one of them to thereby piston moves up or down, in the state that pressure oil port and oil return port are all blocked with inclined groove, then, piston stops moving.This application also relates to a kind of variable displacement pump.

Description

Technical Field

[0001] This invention belongs to the field of fluid transmission and control, and particularly relates to a piston stroke control mechanism and a variable pump. Background Technology

[0002] Various valves (such as directional control valves) and variable displacement pumps are widely used in the hydraulic and pneumatic transmission industry. One of the core components of directional control valves and variable displacement pumps is the valve core (also known as piston) stroke control mechanism. For many years, directional control valves have used the thrust generated by energizing an electromagnetic coil to drive the valve core, thereby achieving hydraulic or pneumatic reversal and changing the direction of operation of the hydraulic or pneumatic actuator. Existing electromagnetic directional control valves have the following disadvantages: 1. Electromagnetic valves require continuous electromagnetic force to reverse; the valve core resets when de-energized. However, in some applications, pressure needs to be maintained continuously, requiring the directional control valve to remain in the reversing state. Prolonged energization of the electromagnetic coil generates heat and consumes power, significantly shortening the coil's lifespan. 2. Electromagnetic coils can only drive valve cores with small diameters. Larger diameter valve cores require hydraulic actuation controlled by an electromagnetic valve, known as electro-hydraulic directional control valves. Due to the two-stage switching, the response time of the directional control valve is lengthened. 3. Ordinary electromagnetic valves or electro-hydraulic directional control valves only have open, closed, and reversing states, and cannot control the opening degree, limiting their applicability.

[0003] A Chinese utility model patent, ZL201220164212.X (publication number CN202579037U), entitled "Electric Variable Axial Piston Pump with Limiting Device," discloses a piston stroke control mechanism that includes limiting devices such as limit switches, limit screws, and guide keys. Its features include: an upper flange mounted on the upper end of the variable housing; a motor mount fixed to the upper flange with screws; a pull rod connected to the variable nut via screws; a screw connected to the motor shaft of a servo motor via a tapered pin; an upper limit switch and a lower limit switch mounted on the motor mount of the servo motor; and upper and lower limit screws connected to the variable nut via a limit bracket and a guide key.

[0004] The aforementioned patented motor transmits displacement via a lead screw. Based on the transmission ratio, the motor needs to rotate the lead screw multiple times to advance 1mm. If the motor speed is set too high, it will lose steps due to insufficient torque; a suitable speed will result in very slow variable speed. The variable piston stroke of a variable pump is often around 20mm; using a stepper motor, it might take several seconds to complete the maximum stroke.

[0005] In conclusion, existing piston stroke control mechanisms used in valves and variable pumps require further improvement. Summary of the Invention

[0006] The first technical problem to be solved by the present invention is to provide a piston stroke control mechanism that is simple and reasonable in structure, has good dynamic response characteristics, and is easy to control, in light of the above-mentioned existing technology.

[0007] The technical solution adopted by the present invention to solve the first technical problem mentioned above is as follows: a piston stroke control mechanism, characterized in that: it includes a valve body and a piston, the valve body is provided with a piston chamber, the piston is disposed in the piston chamber and can move axially, the piston chamber includes an upper pressure chamber located at the upper end of the piston; the piston is provided with a pressure oil port and a return oil port, and also includes a rotating shaft extending into the upper pressure chamber and inserted into the piston, the peripheral wall of the rotating shaft is provided with an inclined groove that is always in communication with the upper pressure chamber; the rotation of the rotating shaft can make the inclined groove communicate with one of the pressure oil port and the return oil port, thereby moving the piston up or down, and when both the pressure oil port and the return oil port are blocked from the inclined groove, the piston stops moving.

[0008] The inclined groove has a certain length and is arranged along the circumference of the rotation axis to form a spiral groove.

[0009] As an improvement, a thrust mechanism is also included to allow the piston to move upward. The thrust mechanism makes it easier to create a pressure difference between the upper and lower ends of the piston. After the upper pressure chamber is depressurized by oil discharge, the piston can move upward more effectively under the action of the thrust mechanism. When the upper pressure chamber is pressurized by oil intake, the force exerted on the piston by the upper pressure chamber is greater than the force exerted on the piston by the thrust mechanism, allowing the piston to move downward more effectively.

[0010] Alternatively, the aforementioned thrust mechanism is a spring acting on the lower end of the piston. When the pressure port is connected to the inclined groove, the force exerted on the piston by the upper pressure chamber is greater than the force exerted on the piston by the spring, causing the piston to move downwards. When the return port is connected to the inclined groove, the force exerted on the piston by the upper pressure chamber is less than the force exerted on the piston by the spring, causing the piston to move upwards. The upward or downward movement of the piston can block both the pressure port and the return port from the inclined groove. Using a spring to provide upward thrust to the piston has the advantages of simple structure and low cost.

[0011] Preferably, the thrust mechanism is a lower pressure chamber located at the lower end of the piston, connected to the oil inlet. This thrust mechanism facilitates oil pressure supply, and the oil inlet is inherently located on the valve body, making this design more rational. Using oil pressure to provide upward thrust to the piston offers the advantage of high precision control, maintaining high control accuracy even after prolonged use. The pressure oil in the upper and lower pressure chambers can be supplied by the same pressure oil circuit, but divided into two separate lines entering the upper and lower pressure chambers from outside the piston. Alternatively, they can be supplied by different pressure oil circuits. The key is to create a pressure difference when oil flows through both the upper and lower pressure chambers, allowing control of the piston's upward or downward movement by altering this pressure difference.

[0012] Further improvements include a connecting channel within the piston that links the pressure port and the lower pressure chamber. The area of ​​the upper pressure chamber's pressure oil acting on the piston's upper force-bearing surface is larger than the area of ​​the lower pressure chamber's pressure oil acting on the piston's lower force-bearing surface. This structure allows the oil pressure in both the upper and lower pressure chambers to be supplied by the same port, simplifying the internal channels of the valve body. After the pressure oil from the lower pressure chamber enters the upper pressure chamber through the connecting channel, the pressures in the upper and lower pressure chambers become equal. Because of the difference in the area of ​​the force-bearing surfaces, a pressure difference is easily generated at both ends of the piston, thereby controlling the piston's movement. Using hydraulic pressure to provide upward thrust to the piston has the advantage of high precision control. Even after long-term use, it can still maintain high control precision. Moreover, the hydraulic pressure can easily achieve balance at both ends of the piston, effectively ensuring that the piston is kept in a certain set position. That is, when the connecting flow channel and the return port are blocked from the inclined groove, the pressurized oil no longer enters the upper pressure chamber. At the same time, the upper pressure chamber is not connected to the return port. The upper pressure chamber neither receives oil nor leaks oil. The piston has moved up to the limit position, and the hydraulic oil in the upper pressure chamber can no longer be compressed. Therefore, the forces on the upper and lower ends of the piston are balanced, keeping the piston in the set position.

[0013] Specifically, based on the area difference of the aforementioned force-bearing surfaces, when the pressure oil port is connected to the inclined groove, the pressure oil enters the upper pressure chamber. The force exerted by the upper pressure chamber on the piston is greater than the force exerted by the lower pressure chamber on the piston, causing the piston to move downward. When the return oil port is connected to the inclined groove, the pressure oil is discharged from the upper pressure chamber, the oil pressure decreases, and the force exerted by the upper pressure chamber on the piston is less than the force exerted by the lower pressure chamber on the piston, causing the piston to move upward. The upward or downward movement of the piston can block both the pressure oil port and the return oil port from the inclined groove.

[0014] If the oil inlet is directly connected to the lower pressure chamber, unstable pressure at the inlet will cause fluctuations, which will be directly reflected in the lower pressure chamber, leading to pressure increases or decreases. This disrupts the piston's force balance, causing the piston to move axially. Consequently, the upper pressure chamber connects to the connecting flow channel or return port until the piston rebalances, resulting in unstable piston stroke control. To address this issue, a further improvement is made: the oil inlet is connected to the lower pressure chamber via a check valve. With the check valve, even if the inlet pressure fluctuates after pressure balance is achieved, if the inlet pressure decreases, the pressurized oil in the lower pressure chamber will not flow back to the inlet due to the check valve, thus maintaining piston balance.

[0015] Preferably, the lower end of the aforementioned pressure chamber is sealed with a plug, and the one-way valve is disposed within the inner cavity of the plug. This structure allows the one-way valve to be pre-installed inside the plug, and then the plug with the one-way valve can be directly installed to the lower opening of the pressure chamber, making assembly more convenient.

[0016] Preferably, with the inclined groove blocked from both the pressure port and the return port, the pressure port and the return port are located on opposite sides of the inclined groove. This allows the rotating shaft to connect the inclined groove to the pressure port and the connecting flow channel when rotating in one direction, and to connect the inclined groove to the return port when rotating in the opposite direction. This facilitates control and allows the motor to rotate within a small angle range, achieving axial movement of the piston. The circuit controlling the motor rotation is easily digitally controlled. The integrated motor control circuit can control the rotating shaft simply by inputting a signal.

[0017] If the piston moves axially beyond its design range, it may damage the control mechanism. As an improvement, the outer wall of the piston has an outer shoulder, and the inner wall of the piston chamber has an inner shoulder. When the piston moves to its extreme position, the inner shoulder blocks the outer shoulder. The engagement of the inner and outer shoulders constrains the piston's extreme movement position, improving safety.

[0018] Preferably, the valve body is equipped with a motor, the output of which is connected to a rotating shaft, thereby driving the rotating shaft to rotate. The motor can easily drive the rotating shaft, and if a servo motor is used, the motor's rotation angle can be precisely controlled. The motor control response is rapid; the entire control stroke only requires the motor to rotate 90 degrees or even less. The rotation angle can be precisely controlled via an encoder, and the motor's rotation angle can be converted into the piston's up-and-down movement position via the rotating shaft. The current piston position can be read and fed back via the built-in rotary encoder, thereby controlling the piston's stroke and improving the accuracy of the control mechanism. Of course, other mechanisms can also be used to drive the rotating shaft.

[0019] Compared with existing technologies, the advantages of this piston stroke control mechanism are:

[0020] 1. By driving the rotating shaft to rotate, the inclined groove can be connected to either the pressure oil port or the return oil port, thereby changing the oil pressure in the upper pressure chamber. When the pressure oil port is connected to the inclined groove, the oil pressure in the upper pressure chamber increases, thus increasing the downward force of the pressure oil on the piston, allowing the piston to move downward. When the return oil port is connected to the inclined groove, the oil pressure in the upper pressure chamber decreases, thus decreasing the downward force of the pressure oil on the piston, allowing the piston to move upward. This control mechanism only needs to control the rotation of the rotating shaft within a 90-degree or even smaller angle range to control the maximum stroke displacement of the piston. The speed and time are often completed in a few milliseconds, exhibiting excellent dynamic response characteristics. Furthermore, the core components of this control mechanism are the rotating shaft and the piston, with a simple interaction, resulting in low cost and a simple and reasonable structure.

[0021] 2. The rotation of the drive shaft requires only a small force. For example, a servo motor can be used to easily drive the piston to rotate. Even when driving a large-diameter piston, the energy required is very small, making it more energy-efficient.

[0022] 3. When the piston moves to the set position, the pressure oil port and the return oil port are blocked from the inclined groove, so that the upper pressure chamber neither enters nor exits oil. This allows the upper and lower ends of the piston to be balanced, keeping the piston in the set position without the need for continuous power supply. This avoids the situation where traditional solenoid valves require the solenoid coil to be constantly energized to maintain the piston in the set position.

[0023] 4. The aforementioned intermittent forward or reverse rotation of the rotating shaft is a decomposed action to fully demonstrate the axial movement process and principle of the piston, facilitating a thorough understanding of the invention by those skilled in the art. Of course, in practical applications, based on a pre-designed program and given parameters, the rotation of the rotating shaft can be continuous, and the displacement of the piston can also be continuous. This allows for precise control of the piston's displacement by controlling the angle of rotation of the rotating shaft.

[0024] The second technical problem to be solved by the present invention is to provide a variable pump with a simple and reasonable structure, good dynamic response characteristics, and the ability to accurately control the swing angle of the variable pump head, in light of the above-mentioned existing technology.

[0025] The technical solution adopted by the present invention to solve the second technical problem mentioned above is: a variable pump, including a variable head, characterized in that: it further includes the aforementioned piston stroke control mechanism, wherein the variable head is oscillatingly connected to the piston, so that the axial movement of the piston drives the variable head to oscillate.

[0026] Preferably, the variable displacement head and the piston are connected by a ball joint structure. The ball joint structure allows the axial movement of the piston to more easily drive the variable displacement head to swing.

[0027] Compared with existing technologies, the advantages of this variable displacement pump are as follows: This variable displacement pump controls the swing angle of the variable displacement head by controlling the axial movement of the piston. The aforementioned piston stroke control mechanism has excellent dynamic response characteristics, and the piston displacement can be precisely controlled by controlling the rotation angle of the rotating shaft. Therefore, the swing control of this variable displacement pump also has excellent dynamic response characteristics and can precisely control the swing angle of the variable displacement head. Of course, the aforementioned piston stroke control mechanism can be used in other fields or products, such as variable displacement piston pumps, variable displacement gear pumps, variable displacement vane pumps, hydraulic valves, hydraulic cylinders, and hydraulic transformers. Attached Figure Description

[0028] Figure 1 This is a three-dimensional structural diagram of an embodiment of the variable pump of the present invention. Figure 1 ;

[0029] Figure 2 This is a three-dimensional structural diagram of an embodiment of the variable pump of the present invention. Figure 2 ;

[0030] Figure 3This is a cross-sectional view of an embodiment of the variable pump including a piston stroke control mechanism of the present invention (the inclined groove is connected to the oil return port);

[0031] Figure 4 for Figure 3 Sectional view along axis AA;

[0032] Figure 5 for Figure 3 BB-direction sectional view;

[0033] Figure 6 This is a cross-sectional view of a variable pump embodiment of the present invention, in which the inclined groove, pressure port, and return port are all blocked.

[0034] Figure 7 This is a cross-sectional view of an embodiment of the variable pump including a piston stroke control mechanism of the present invention (the inclined groove is connected to the pressure oil port);

[0035] Figure 8 for Figure 7 CC-direction sectional view;

[0036] Figure 9 for Figure 7 DD section view;

[0037] Figure 10 This is a three-dimensional structural diagram of the rotating shaft of the present invention;

[0038] Figure 11 This is a three-dimensional structural diagram of the piston of the present invention. Figure 1 ;

[0039] Figure 12 This is a three-dimensional structural diagram of the piston of the present invention. Figure 2 . Detailed Implementation

[0040] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0041] like Figures 1-12 The figure shows a preferred embodiment of the variable pump of the present invention employing a piston stroke control mechanism.

[0042] A variable displacement pump includes a variable displacement head 2 and a piston stroke control mechanism. The variable displacement head 2 is pivotally connected to the piston 3, such that axial movement of the piston 3 drives the variable displacement head 2 to pivot. The variable displacement head 2 and the piston 3 are pivotally connected via a ball joint structure. Other parts of the variable displacement pump refer to the structure of existing variable displacement piston pumps.

[0043] The piston stroke control mechanism in this embodiment includes a valve body 1 and a piston 3. The piston 3 is cylindrical, and a sealing ring is provided between the outer periphery of the piston and the piston cavity. The valve body 1 has a piston cavity, and the piston 3 is located in the piston cavity and can move axially. Ideally, the piston 3 can only move up and down and cannot rotate. If a section of the cross-section of the piston 3 is non-circular, the cross-section of a corresponding section of the piston cavity is also non-circular, thus allowing the piston 3 to move only up and down. The piston 3 has a pressure port P and a return port T. Specifically, the piston 3 divides the piston cavity into an upper pressure cavity 11 and a lower pressure cavity 12. It also includes a thrust mechanism that acts on the piston 3 to allow it to move upward. In this embodiment, the thrust mechanism is the lower pressure cavity 12 located at the lower end of the piston 3, and the lower pressure cavity 12 is connected to the oil inlet 14 of the valve body 1. The outer wall of the piston 3 has an outer shoulder 33, and the inner wall of the piston cavity has an inner shoulder 13. When the piston 3 moves to its extreme position, the inner shoulder 13 blocks the outer shoulder 33.

[0044] It also includes a rotating shaft 5 that extends into the upper pressure chamber 11 and is inserted into the piston 3. A bearing can be installed between the rotating shaft 5 and the inner wall of the valve body 1 to reduce the rotational friction of the rotating shaft 5. The top surface of the piston 3 has an insertion hole for the rotating shaft 5. A motor 7 is installed on the valve body 1. The output end of the motor 7 is inserted into the upper end of the rotating shaft 5 and connected to it by a pin. A universal joint can be installed between the output shaft of the motor 7 and the rotating shaft 5 to eliminate errors and ensure good concentricity, thereby driving the rotating shaft 5 to rotate.

[0045] The circumferential wall of the rotating shaft 5 is provided with an inclined groove 51 that is always connected to the upper pressure chamber 11. The inclined groove 51 refers to an inclined groove structure with a certain length that is inclined along the circumferential wall of the rotating shaft 5, thus having a certain spiral shape. The rotation of the rotating shaft 5 causes the inclined groove 51 to connect with one of the pressure oil port P and the return oil port T, or causes the inclined groove 51 to block both the pressure oil port P and the return oil port T. When the pressure oil port P is connected to the inclined groove 51, the pressure oil enters the upper pressure chamber 11. The force exerted by the pressure oil in the upper pressure chamber 11 on the piston 3 is greater than the force exerted by the pressure oil in the lower pressure chamber 12 on the piston 3, and the piston 3 moves downward. When the return oil port T is connected to the inclined groove 51, the upper pressure chamber 11 begins to discharge oil, the oil pressure decreases, the force exerted by the upper pressure chamber 11 on the piston 3 is less than the force exerted by the lower pressure chamber 12 on the piston 3, and the piston 3 moves upward. The up and down movement of the piston 3 can block both the pressure oil port P and the return oil port T from the inclined groove 51.

[0046] In this embodiment, the pressure port P is connected to the lower pressure chamber 12. The piston 3 has a connecting channel 31 for connecting the pressure port P and the lower pressure chamber 12. The rotation of the rotating shaft 5 can cause the inclined groove 3 to connect to either the pressure port P or the return port T, or to block both the inclined groove 3 from the pressure port P and the return port T. The area of ​​the pressure oil in the upper pressure chamber 11 acting on the upper force-bearing surface 3a of the piston 3 is greater than the area of ​​the pressure oil in the lower pressure chamber 12 acting on the lower force-bearing surface 3b of the piston 3. The axial movement of the piston 3 can block both the pressure port P and the return port T from the inclined groove 51, so that neither oil enters nor exits the upper pressure chamber 11, and the upper and lower pressures of the piston are balanced. When both the pressure port P and the return port T are blocked from the inclined groove 51, the inlet ends of the pressure port P and the return port T are located on both sides of the inclined groove 51.

[0047] The oil inlet 14 is connected to the lower pressure chamber 12 via a one-way valve 4. The lower end of the lower pressure chamber 12 is sealed with a plug 6, and the one-way valve 4 is located in the inner cavity of the plug 6.

[0048] Of course, the thrust mechanism can also be a spring acting on the lower end of the piston 3, preferably placed in the lower pressure chamber 12. When the pressure oil port P is connected to the inclined groove 51, the force exerted on the piston 3 by the pressure oil in the upper pressure chamber 11 is greater than the force exerted on the piston 3 by the spring, causing the piston 3 to move downward; when the return oil port T is connected to the inclined groove 51, the force exerted on the piston 3 by the upper pressure chamber 11 is less than the force exerted on the piston 3 by the spring, causing the piston 3 to move upward; the upward or downward movement of the piston 3 can block both the pressure oil port P and the return oil port T from the inclined groove 51. This embodiment is not shown in the drawings.

[0049] The working principle and process of this piston stroke control mechanism are as follows:

[0050] When motor 7 drives rotating shaft 5 to rotate in the positive direction by an angle, such as Figures 3-5 As shown, the inclined groove 51 on the rotating shaft 7 is connected to the oil return port T. The pressurized oil in the upper pressure chamber 11 is discharged through the inclined groove 51 and the oil return port T, thus depressurizing the upper pressure chamber 11. The pressurized oil in the oil inlet 14 flows to the lower pressure chamber 12 through the one-way valve 4, so the piston 3 moves upward under the action of oil pressure. The greater the angle of rotation of the rotating shaft 5 in this direction, the greater the upward distance of the piston 3. When the motor 7 stops working, the piston 3 moves upward to the inclined groove 3, where it is blocked from the pressure oil port P and the oil return port T. Figure 6 As shown, the upper pressure chamber 11 no longer depressurizes and no more pressurized oil enters. The force exerted by the upper pressure chamber 11 on the piston 3 is just balanced with the force exerted by the lower pressure chamber 12 on the piston 3, and the piston 3 remains in this position.

[0051] Conversely, when motor 7 drives rotating shaft 5 to rotate in the opposite direction by an angle, such as... Figures 7-9As shown, the inclined groove 51 on the rotating shaft 7 is connected to the pressure oil port P. The pressure oil from the oil inlet 14 flows down to the lower pressure chamber 12 via the one-way valve 4, and simultaneously flows up to the upper pressure chamber 11 via the connecting flow channel 31 and the inclined groove 51. This results in the pressure in the upper pressure chamber 11 and the lower pressure chamber 12 being equal. However, the area of ​​the pressure oil in the upper pressure chamber 11 acting on the upper force-bearing surface 3a of the piston 3 is greater than the area of ​​the pressure oil in the lower pressure chamber 12 acting on the lower force-bearing surface 3b of the piston 3. Under the condition of equal pressure, differential motion is formed, meaning the piston 3 experiences a greater downward pressure than an upward pressure, causing the piston 3 to move axially downward. The greater the angle at which the rotating shaft 5 rotates in this direction, the greater the downward distance the piston 3 moves. When the motor 7 stops working, the piston 3 moves down to the point where the inclined groove 3 blocks both the pressure oil port P and the return oil port T. The upper pressure chamber 11 will not receive any more pressure oil and will not release pressure. The force exerted on the piston 3 by the pressure oil in the upper pressure chamber 11 is just balanced with the force exerted on the piston 3 by the pressure oil in the lower pressure chamber 12. The piston 3 remains in this position.

[0052] This variable displacement pump achieves its pumping flow rate (i.e., pump displacement) per unit time by changing the swing angle of the variable displacement head 2, which is driven by the axial movement of the piston 3. This is a conventional design for variable displacement pumps.

[0053] The aforementioned intermittent forward or reverse rotation of the rotating shaft 5 is a breakdown of the action to fully demonstrate the up-and-down movement process and principle of the piston 3, facilitating a thorough understanding of the invention by those skilled in the art. Of course, in practical applications, based on a pre-designed program and given parameters, the rotation of the rotating shaft 5 can be continuous, and the displacement of the piston 3 can also be continuous. Therefore, the displacement of the piston 3 can be precisely controlled by the angle of rotation of the rotating shaft 5 by the motor 7.

[0054] It should be noted that in the description of this embodiment, the terms "front," "rear," "left," "right," "inner," "outer," "upper," and "lower," etc., indicating orientations or positional relationships, are based on the orientations or positional relationships shown in the accompanying drawings. They are merely for the convenience of describing the invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the invention. The terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication between two elements. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

Claims

1. A piston stroke control mechanism, characterized in that: The device includes a valve body (1) and a piston (3). The valve body (1) has a piston chamber, and the piston (3) is located in the piston chamber and can move axially. The piston chamber includes an upper pressure chamber (11) located at the upper end of the piston (3). The piston (3) has a pressure oil port (P) and a return oil port (T). The device also includes a rotating shaft (5) that extends into the upper pressure chamber (11) and is inserted into the piston (3). The peripheral wall of the rotating shaft (5) has a groove (51) that is always connected to the upper pressure chamber (11). The rotation of the rotating shaft (5) can make the groove (51) connect with one of the pressure oil port (P) and the return oil port (T), thereby moving the piston up or down. When both the pressure oil port (P) and the return oil port (T) are blocked from the groove (51), the piston stops moving. It also includes a thrust mechanism that acts on the piston (3) to make it move upward; the thrust mechanism is a lower pressure chamber (12) located at the lower end of the piston (3), and the lower pressure chamber (12) is connected to the oil inlet (14) of the valve body (1); the piston (3) has a connecting flow channel (31) for connecting the pressure oil port (P) and the lower pressure chamber (12), and the area of ​​the upper force-bearing surface (3a) of the piston (3) acted by the pressure oil in the upper pressure chamber (11) is greater than the area of ​​the lower force-bearing surface (3b) of the piston (3) acted by the pressure oil in the lower pressure chamber (12).

2. The piston stroke control mechanism according to claim 1, characterized in that: When the pressure port (P) is connected to the inclined groove (51), the force exerted by the upper pressure chamber (11) on the piston (3) is greater than the force exerted by the lower pressure chamber (12) on the piston (3), and the piston (3) moves downward; when the return port (T) is connected to the inclined groove (51), the force exerted by the upper pressure chamber (11) on the piston (3) is less than the force exerted by the lower pressure chamber (12) on the piston (3), and the piston (3) moves upward; the upward or downward movement of the piston (3) can cause both the pressure port (P) and the return port (T) to be blocked from the inclined groove (51).

3. The piston stroke control mechanism according to claim 1, characterized in that: The oil inlet (14) is connected to the lower pressure chamber (12) via a one-way valve (4).

4. The piston stroke control mechanism according to claim 1, characterized in that: With the inclined groove (51) blocked from both the pressure oil port (P) and the return oil port (T), the pressure oil port (P) and the return oil port (T) are located on both sides of the inclined groove (51).

5. The piston stroke control mechanism according to claim 1, characterized in that: The piston (3) has an outer shoulder (33) on its outer wall and an inner shoulder (13) on its inner wall. When the piston (3) moves down to its limit position, the inner shoulder (13) blocks the outer shoulder (33).

6. The piston stroke control mechanism according to claim 1, characterized in that: The valve body (1) is equipped with a motor (7), and the output end of the motor (7) is connected to the rotating shaft (5), thereby driving the rotating shaft (5) to rotate.

7. A variable pump, comprising a variable head (2), characterized in that: It also includes a piston stroke control mechanism as described in any one of claims 1 to 6, wherein the variable head (2) is oscillatingly connected to the piston (3), such that the axial movement of the piston (3) drives the variable head (2) to oscillate.

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