I. Introduction The overflow valve is a pressure control element. Its main function is twofold:
1 By means of overflowing the excess oil flow to the tank to maintain the pressure of the hydraulic system constant, it acts as a regulator. For example in a variable pump system.
2 Only when the pressure exceeds a certain preset pressure value, open the overflow, so that the system pressure will no longer rise, to prevent the system pressure overload, and play a role in safety protection. For example in a variable displacement pump hydraulic system. The role of safety valve.
Commonly used overflow valves are of direct-acting type and pilot type. Direct-acting type is used for low pressure and pilot type is used for medium-high pressure.
The direct-acting type utilizes the spring force and the hydraulic pressure of the oil inlet to balance the pressure control. Therefore, the spring is hard and has a large adjustment torque and cannot be used for medium and high pressures.
The pilot-operated relief valve can be divided into two parts in structure, the lower part is the main spool valve part, and the upper part is the pilot pressure regulator part. This valve is characterized by the use of the pressure difference between the upper and lower ends of the main spool valve to move the main valve spool for pressure control. With this structure, the use pressure is high and the pressure overshoot is small. Under the same pressure, the adjustment torque of the handle is much lower.
First, the principle of failure (a) pressure rise is very slow, or even a little or not go to this fault phenomenon is: When tightening the regulator screw or handle, from the discharge state to pressure adjustment state, the pressure should follow It rises, but in the event of a fault, the pressure rises very slowly, or even does not go up. Even if it rises, it will lag behind for a long time.
Analysis of the state of pressure regulation shows that from the moment when the pressure relief state changes to the pressure regulation state, the main valve core abuts against the valve cover, and the main valve completely opens the overflow. When the boost pressure is adjusted, the pressure in the upper cavity of the main spool is increased. When the pressure rises to open the pilot pressure regulating valve, the overflow valve enters the pressure regulating and boosting state, and the main spool holds a small opening with the valve seat (or the valve body). , Relief valve The main valve spool drops from the unloading position to the time required for regulating the required opening time, which is the overflow lag time of the overflow valve. During this period of time, the pressure rose slowly.
There are many factors affecting the lag time, which are mainly related to the stroke distance of the main spool of the overflow valve itself and the closing speed of the spool. The closing speed is determined by the size of the pilot flow through the main spool orifice and the diameter of the main spool. The pilot flow is related to the bore diameter of the orifice and the hole length.
(B) Although the pressure may rise but does not rise to the maximum regulation pressure, the failure phenomenon manifests itself in that, even if the pressure is only increased to a certain value, the pressure of the full-pressure regulator handwheel cannot continue to rise, especially when the oil temperature is high. This is especially noticeable.
(3) The failure phenomenon under pressure does not appear to be manifested in the fact that, even if the handwheel is fully loosened and pressed, the system pressure will not come.
(4) Pressure fluctuations This type of failure phenomenon manifests itself in that the pressure fluctuation range of the relief valve fluctuates between 0.2 MPa and 0.3 MPa, respectively. Exceeding this index is called pressure fluctuation.
(5) The vibration and noise are large. The vibration and noise of the impact relief valve are ranked first in the valve components and second only to the pump. Vibration and noise are twin brothers, and they often disappear at the same time.
There is a "liquid-mass-spring" oscillating system for both the main spool and the pilot spool. The causes of vibration and noise are similar to those of "large pressure fluctuations," but only to a greater degree. This phenomenon should be considered from the entire hydraulic system. The noise in the piping system includes the low frequency sound of the pump and the high frequency noise of the relief valve. The high-frequency noise of the relief valve is caused by the instability of the pilot valve, that is, high-frequency oscillation of the pressure in the front chamber of the pilot valve causes mechanical vibration to generate noise, which is generally a harsh whistling sound.
(6) Pressure drop, large pressure deviation This failure phenomenon is manifested as follows: a certain pressure is pre-adjusted, but in use, it gradually decreases (sometimes rises) to another pressure value, and then slowly increases pressure. , restore the regulator value, this phenomenon cycle or repeat. This phenomenon can be observed through a pressure gauge.
(7) Opening and closing characteristics The poor opening and closing characteristics refer to the opening and closing characteristics of the overflow valve. The opening characteristic is expressed as a percentage of the opening pressure and the rated pressure; the closing characteristic is expressed as a percentage of the closing pressure and the rated pressure. The higher the percentage, the greater the ability of the relief valve to maintain constant pressure in the hydraulic system at different overflows. The opening pressure and closing pressure during the bench test are both measured by decreasing or increasing to 1% of the rated flow through the relief valve. They are an indicator of static characteristics. It is a mandatory item.
For the relief valve, the difference in opening and closing characteristics depends not only on the design parameters but also on the quality of the machining, in particular on the machining quality of the pilot needle valve. The conditions of use (such as back pressure, degree of fluid cleanliness, etc.) also have a certain influence on the opening and closing characteristics. The specific description is as follows:
Good needle valve and valve seats have good opening and closing characteristics. For Y-valve, when the set pressure is 6.3Mpa, the closing pressure is >5.5Mpa, otherwise the closing pressure is very low.
2 The influence of the back pressure of the outlet valve of the overflow valve: high back pressure, poor opening and closing characteristics, and the general outlet back pressure should not exceed 0.1 Mpa. When high, it is possible to use the relief valve to return oil separately, which can reduce the back pressure and increase the stability.
3 The influence of the main valve hydraulic power: The main valve has large liquid power and poor opening and closing characteristics. To reduce the main valve hydraulic power, appropriate measures can be taken to increase the diameter of the main valve and change the inlet and outlet oil direction.
The influence of the 4-needle valve hydraulic force: not very obvious. The cone valve angle α is small and the hydraulic power is also small; however, when 2α<280, the needle valve is stuck.
The 5-needle valve regulates the rigidity of the spring and has better opening and closing characteristics.
1 By means of overflowing the excess oil flow to the tank to maintain the pressure of the hydraulic system constant, it acts as a regulator. For example in a variable pump system.
2 Only when the pressure exceeds a certain preset pressure value, open the overflow, so that the system pressure will no longer rise, to prevent the system pressure overload, and play a role in safety protection. For example in a variable displacement pump hydraulic system. The role of safety valve.
Commonly used overflow valves are of direct-acting type and pilot type. Direct-acting type is used for low pressure and pilot type is used for medium-high pressure.
The direct-acting type utilizes the spring force and the hydraulic pressure of the oil inlet to balance the pressure control. Therefore, the spring is hard and has a large adjustment torque and cannot be used for medium and high pressures.
The pilot-operated relief valve can be divided into two parts in structure, the lower part is the main spool valve part, and the upper part is the pilot pressure regulator part. This valve is characterized by the use of the pressure difference between the upper and lower ends of the main spool valve to move the main valve spool for pressure control. With this structure, the use pressure is high and the pressure overshoot is small. Under the same pressure, the adjustment torque of the handle is much lower.
First, the principle of failure (a) pressure rise is very slow, or even a little or not go to this fault phenomenon is: When tightening the regulator screw or handle, from the discharge state to pressure adjustment state, the pressure should follow It rises, but in the event of a fault, the pressure rises very slowly, or even does not go up. Even if it rises, it will lag behind for a long time.
Analysis of the state of pressure regulation shows that from the moment when the pressure relief state changes to the pressure regulation state, the main valve core abuts against the valve cover, and the main valve completely opens the overflow. When the boost pressure is adjusted, the pressure in the upper cavity of the main spool is increased. When the pressure rises to open the pilot pressure regulating valve, the overflow valve enters the pressure regulating and boosting state, and the main spool holds a small opening with the valve seat (or the valve body). , Relief valve The main valve spool drops from the unloading position to the time required for regulating the required opening time, which is the overflow lag time of the overflow valve. During this period of time, the pressure rose slowly.
There are many factors affecting the lag time, which are mainly related to the stroke distance of the main spool of the overflow valve itself and the closing speed of the spool. The closing speed is determined by the size of the pilot flow through the main spool orifice and the diameter of the main spool. The pilot flow is related to the bore diameter of the orifice and the hole length.
(B) Although the pressure may rise but does not rise to the maximum regulation pressure, the failure phenomenon manifests itself in that, even if the pressure is only increased to a certain value, the pressure of the full-pressure regulator handwheel cannot continue to rise, especially when the oil temperature is high. This is especially noticeable.
(3) The failure phenomenon under pressure does not appear to be manifested in the fact that, even if the handwheel is fully loosened and pressed, the system pressure will not come.
(4) Pressure fluctuations This type of failure phenomenon manifests itself in that the pressure fluctuation range of the relief valve fluctuates between 0.2 MPa and 0.3 MPa, respectively. Exceeding this index is called pressure fluctuation.
(5) The vibration and noise are large. The vibration and noise of the impact relief valve are ranked first in the valve components and second only to the pump. Vibration and noise are twin brothers, and they often disappear at the same time.
There is a "liquid-mass-spring" oscillating system for both the main spool and the pilot spool. The causes of vibration and noise are similar to those of "large pressure fluctuations," but only to a greater degree. This phenomenon should be considered from the entire hydraulic system. The noise in the piping system includes the low frequency sound of the pump and the high frequency noise of the relief valve. The high-frequency noise of the relief valve is caused by the instability of the pilot valve, that is, high-frequency oscillation of the pressure in the front chamber of the pilot valve causes mechanical vibration to generate noise, which is generally a harsh whistling sound.
(6) Pressure drop, large pressure deviation This failure phenomenon is manifested as follows: a certain pressure is pre-adjusted, but in use, it gradually decreases (sometimes rises) to another pressure value, and then slowly increases pressure. , restore the regulator value, this phenomenon cycle or repeat. This phenomenon can be observed through a pressure gauge.
(7) Opening and closing characteristics The poor opening and closing characteristics refer to the opening and closing characteristics of the overflow valve. The opening characteristic is expressed as a percentage of the opening pressure and the rated pressure; the closing characteristic is expressed as a percentage of the closing pressure and the rated pressure. The higher the percentage, the greater the ability of the relief valve to maintain constant pressure in the hydraulic system at different overflows. The opening pressure and closing pressure during the bench test are both measured by decreasing or increasing to 1% of the rated flow through the relief valve. They are an indicator of static characteristics. It is a mandatory item.
For the relief valve, the difference in opening and closing characteristics depends not only on the design parameters but also on the quality of the machining, in particular on the machining quality of the pilot needle valve. The conditions of use (such as back pressure, degree of fluid cleanliness, etc.) also have a certain influence on the opening and closing characteristics. The specific description is as follows:
Good needle valve and valve seats have good opening and closing characteristics. For Y-valve, when the set pressure is 6.3Mpa, the closing pressure is >5.5Mpa, otherwise the closing pressure is very low.
2 The influence of the back pressure of the outlet valve of the overflow valve: high back pressure, poor opening and closing characteristics, and the general outlet back pressure should not exceed 0.1 Mpa. When high, it is possible to use the relief valve to return oil separately, which can reduce the back pressure and increase the stability.
3 The influence of the main valve hydraulic power: The main valve has large liquid power and poor opening and closing characteristics. To reduce the main valve hydraulic power, appropriate measures can be taken to increase the diameter of the main valve and change the inlet and outlet oil direction.
The influence of the 4-needle valve hydraulic force: not very obvious. The cone valve angle α is small and the hydraulic power is also small; however, when 2α<280, the needle valve is stuck.
The 5-needle valve regulates the rigidity of the spring and has better opening and closing characteristics.
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