Created on:2021-06-17 09:36

Function and basic principle of hydraulic pump and motor

Function and basic principle of hydraulic pump and motor

 

1.1 working principle and composition of hydraulic system

Hydraulic technology is a kind of technology which takes liquid as working medium and uses the static pressure of liquid in closed system to realize the transmission of information, motion and power and engineering control. A complete hydraulic system is composed of four types of hydraulic components and working media: energy components (hydraulic pump), executive components (hydraulic cylinder, hydraulic motor and swing hydraulic motor), control components (various hydraulic control valves) and auxiliary components (oil tank, filter and pipe fittings). When the mechanical equipment or device of hydraulic transmission and control works, its hydraulic system takes the hydraulic oil with continuous flow as the working medium. Through the hydraulic pump, the mechanical energy of the prime mover (motor or internal combustion engine) driving the pump is converted into the pressure energy of the liquid, and then through the closed pipeline and control valve, it is sent to the actuator, which is converted into mechanical energy to drive the load and realize the work Make the required movement of the mechanism.

1.2 function and basic principle of hydraulic pump and motor

1.2.1 function and importance

Hydraulic pump is an indispensable energy component of any hydraulic mechanical equipment, whose function is to convert the mechanical energy of prime mover into hydraulic energy, that is, to provide the hydraulic system with a certain pressure and flow of liquid; and hydraulic motor is any hydraulic mechanical equipment or working mechanism that needs rotary movement (such as various industrial production machinery, military equipment rotary working mechanism and various vehicles) The function of the actuator is to convert the hydraulic energy into mechanical energy, and drive the working mechanism connected with it to do work in the form of torque and speed.

The function principle of hydraulic pump and hydraulic motor is opposite to each other, but the structure is similar, and both of them occupy a considerable proportion in hydraulic technology. In the development of all kinds of hydraulic equipment and the design and use of hydraulic system, the correct and reasonable selection, use and maintenance of hydraulic pump and hydraulic motor is of great significance to improve the working quality and reliability of hydraulic system and even the whole hydraulic equipment. Therefore, the design and manufacture personnel, installation and debugging personnel and on-site use and maintenance personnel of hydraulic technology must master the working principle, type structure, technical characteristics and use and maintenance methods of hydraulic pump and hydraulic motor.

1.2.2 basic principle

In the hydraulic system, there are many types of hydraulic pumps and motors (such as gear type, vane type, plunger type, etc.) with different structures, but they are all volumetric, that is, they all work based on the change of one or several seal volumes.

Figure a shows a reversible hydraulic device: it can be used as both a hydraulic pump and a hydraulic motor. Its structure is described as follows: the eccentricity of eccentric cam 1 and 3 is e, and the eccentricity of eccentric cam 2 is e. The rotation centers 01, 02 and 03 of the three cams are connected and driven by the same transmission shaft 4 (rotor). Cams 1 and 3 control the opening or closing of check valves 5 and 7; cam 2 keeps contact with plunger 6 (squeezer), and the three cams are guaranteed to contact with parts 5, 6 and 7 by corresponding springs. The plunger can move back and forth in the hole of the cylinder block (stator) 8, and a sealing working cavity 12 with variable volume is formed between the cylinder block and the plunger. Now take figure a as an example to analyze and discuss the basic working principle of hydraulic pump and hydraulic motor.

(1) Basic principle of hydraulic pump When the device shown in Fig. A is used as a hydraulic pump, the prime mover drives the transmission shaft 4 (rotor) to rotate clockwise as shown in the figure, then the three cams rotate along with the transmission shaft in a clockwise direction. Suppose the pump starts to rotate from the position shown in Fig. a (a), then the plunger 6 moves down, the volume of the sealing working chamber 12 increases, and a vacuum is generated; at the same time, the cam 3 opens the oil suction check valve 7 (instead of the piston) The cam 1 just closes the drain check valve 5). Under the action of atmospheric pressure, the oil in the open oil tank (not shown in the figure) is sucked into the sealing working chamber 12 through the oil inlet a, the oil suction check valve 7 and the oil passage B, which is the oil suction process. When the rotor continues to rotate to the position shown in Fig. a (b), the plunger 6 is compressed and moved upward by the cam 2, the volume of the seal working cavity 12 decreases, and the oil absorbed in the cavity is compressed and the pressure increases to discharge the oil; at the same time, the cam 1 just opens the oil discharge check valve 5 (while the cam 3 just closes the oil suction check valve 7), and the oil is transported through the oil passage C, the oil discharge check valve 5 and the oil discharge port D The oil is discharged to the system. The transmission shaft rotates for one circle, pumping and discharging oil once respectively. When the prime mover drives the transmission shaft to rotate continuously, the hydraulic pump continuously absorbs oil from the oil inlet a and discharges oil to the system from the oil outlet D. If the prime mover drives the transmission shaft or rotor to rotate counterclockwise, the oil flow will be reversed, that is, the pump will absorb oil through port D and discharge oil to the system through port a.

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The single plunger hydraulic pump has the basic structural principle characteristics of the displacement hydraulic pump.

① There are three parts called stator, rotor and squeezer, which vary with the structure of hydraulic pump.

② There are several sealed spaces which can change periodically. This space is called working cavity. It is generally composed of stator, rotor and squeezer. The working cavity is called oil suction cavity when it has oil suction function and oil discharge cavity when it has oil pressure function. The transition zone between oil suction cavity and oil discharge cavity is sealed by the surface of relevant parts. In order to change the volume of the working cavity, there must be a relative moving squeezer in the parts of the working cavity. The squeezer can make the volume of the working cavity periodically from small to large and continuously absorb the liquid; it can make the volume of the working cavity periodically from large to small and continuously discharge the liquid.

③ The utility model has an oil suction port and an oil discharge port. The two oil ports are respectively connected with the oil suction cavity and the oil discharge cavity. The flow area of the oil suction port of the hydraulic pump should be large enough to avoid cavitation and cavitation due to the high flow rate of the oil in it; the flow rate of the oil discharge port of the pump can be large enough to reduce the size and weight of the pipeline.

④ The input parameters of hydraulic pump are mechanical parameters (torque and speed), and the output parameters are hydraulic parameters (pressure and flow).

The pressure of the oil suction chamber of the hydraulic pump depends on the oil suction height and the pressure loss caused by the resistance of the oil suction pipeline; the pressure of the oil discharge chamber depends on the pressure loss caused by the load and the resistance of the oil discharge pipeline.

The theoretical oil displacement of the hydraulic pump is directly proportional to the volume change (or geometric dimension) of the working chamber and the number of changes (or rotational speed) per unit time, but has nothing to do with the oil displacement pressure and other factors. If the theoretical displacement of the pump can not be changed, it is a constant displacement pump, otherwise it is a variable displacement pump.

⑤ It has a valve mechanism (also known as a valve). The conversion of hydraulic pump from oil suction to oil discharge or from oil discharge to oil suction is called valve distribution. In order to ensure that the hydraulic pump regularly sucks and discharges the liquid, it should have the corresponding flow distribution mechanism to separate the oil suction cavity from the oil discharge cavity, so as to ensure that the pump regularly sucks and discharges the liquid. According to the different structure of the hydraulic pump, there are two kinds of flow distribution: the deterministic flow distribution relies on the hole or groove in the proper position of a part of the pump to realize the flow distribution. Most hydraulic pumps adopt this flow distribution mode, which generally has the reversibility as a hydraulic motor; the valve type flow distribution relies on the check valve to realize the flow distribution (the oil suction and discharge valves are in logic) It is often used in ultra-high pressure piston pump. Because the flow direction of this kind of pump can not be changed sometimes, it loses its reversibility as a hydraulic motor.

For example, as shown in figure a, the flow distribution mode of single plunger hydraulic pump is valve type with check valve (suction valve 7 and pressure valve 5).

⑥ The absolute pressure of the liquid in the tank must be equal to or greater than the atmospheric pressure. In order to ensure the normal oil absorption of the pump, the oil tank must be connected with the atmosphere or use a closed gas filled oil tank.

(2) Basic principle of hydraulic motor when the device shown in figure a is used as hydraulic motor, the transmission shaft is no longer driven by prime mover, but connected with the working mechanism. The pressure oil is input from the oil inlet a as shown in Fig. a (a). The pressure oil enters into the working chamber 12 of the motor through the oil inlet check valve 7 and the flow channel B, and generates a hydraulic force on the upper end of the plunger 6 to push the plunger. Due to the existence of the eccentricity e of the cam 2, the force will form a torque on the rotation center 02 of the cam 2, making the cams and the transmission shaft 4 rotate clockwise, After the cam 2 rotates to the position shown in Fig. a (b), it still rotates clockwise to make the plunger 6 move up, and the oil that has been done in the working chamber 12 is discharged to the oil tank (not shown in the figure) through the flow channel C, one-way oil drain valve 5 and oil drain port D. due to the proper phase of cam 1 and 3, the oil drain valve 5 of the hydraulic pressure motor is closed when the oil is fed, and the oil inlet valve 7 is closed when the oil is drained, so as to realize the oil distribution Flow. If the pressure oil is continuously input from the oil inlet a of the hydraulic motor, the motor can drive the working mechanism connected with its transmission shaft to realize continuous clockwise rotary movement, and the used oil is continuously discharged from the oil drain valve 5. Similar to the situation of hydraulic pump, if the direction of the input oil is reversed, that is, the oil is fed from port D and discharged from port a, then the rotation direction of the transmission shaft or rotor will also be reversed, that is, it will rotate counterclockwise.

The plunger type hydraulic motor has the basic structural principle characteristics of the displacement type hydraulic motor.

① Like the hydraulic pump, it also has three parts called stator, rotor and squeezer, which vary with the structure of the hydraulic motor.

② Like the hydraulic pump, it also has several sealed and periodically changeable working cavities, which are generally composed of stator, rotor and squeezer. The working cavity connected with the high-pressure oil is called oil inlet cavity or high-pressure cavity, and the working cavity leading to the oil tank is called oil discharge cavity or low-pressure cavity. The transition area between the oil suction cavity and the oil discharge cavity is sealed by the surface of the relevant parts. In order to change the volume of the working cavity, there must be a relative moving squeezer in the parts of the working cavity. Under the action of pressure oil, the extruder stretches out so that the volume of the working chamber changes from small to large periodically. Under the action of swashplate and other parts, the extruder retracts so that the volume of the working chamber changes from large to small periodically and the low-pressure liquid is discharged continuously.

③ Like the hydraulic pump, the hydraulic motor also has oil inlet and oil outlet, but the oil inlet and oil outlet of the motor are respectively connected with the high pressure chamber and the low pressure chamber. Because the pressure of the low pressure chamber of the hydraulic motor is slightly higher than the atmospheric pressure, different from the hydraulic pump, the size of the oil inlet and the oil outlet of the motor can be the same. The rotation direction of the hydraulic motor can be changed by changing or exchanging the oil inlet and outlet of the hydraulic motor.

④ The input parameters of hydraulic motor are hydraulic parameters (pressure and flow), and the output parameters are mechanical parameters (torque and speed).

The pressure of the oil inlet chamber of the hydraulic motor depends on the pressure loss caused by the input oil pressure and the resistance of the oil inlet pipe, while the pressure of the oil outlet chamber depends on the pressure loss caused by the resistance of the oil outlet pipe.

The theoretical oil displacement of the hydraulic motor is related to the volume change (or geometric dimension) of the working chamber, but not to the oil inlet pressure and other factors. If the theoretical oil displacement of the motor can not be changed, it is a quantitative motor, otherwise it is a variable motor.

The output speed of the hydraulic motor depends on the input flow and displacement of the motor; the output torque depends on the displacement of the motor and the pressure difference between the inlet and outlet.

⑤ Like the hydraulic pump, the hydraulic motor also has a flow distribution mechanism, and its function is basically the same as that of the hydraulic pump. But because the motor needs to rotate forward and backward, the structure of the flow distribution mechanism of the hydraulic motor should be symmetrical. The flow distribution mode of hydraulic motor varies with the structure of the motor. Generally, there are two kinds of flow distribution mode: definite type and valve type. For example, as shown in figure a, the flow distribution mode of plunger type hydraulic motor is valve type with one-way valve.

To sum up, the hydraulic pump and the hydraulic motor are two different energy conversion devices. In principle, the positive displacement hydraulic pump can be used as the hydraulic motor, that is, to input pressure oil into the hydraulic pump and force its transmission shaft to rotate, it becomes the hydraulic motor. But in fact, although the same type of pump and motor are similar in structure, many types of hydraulic pump and motor can not be used in reverse in practice due to the differences in use purpose, performance requirements and structural symmetry.

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