Estimated reading time: 10 minutes

The sequence valve uses the pressure of the oil circuit to control the sequential action of the oil cylinder or the oil motor to realize the automatic control of the oil circuit system. When the pressure of the inlet oil circuit does not reach the pre-adjusted pressure of the sequence valve, the valve is closed; after reaching the pressure, the valve opens, and the oil flow enters the secondary pressure oil circuit to activate the next-level components. By changing the control mode, unloading mode, and the connection of the secondary oil circuit, the sequence valve can also be used as a back pressure valve, unloading valve, and balance valve.

Question 1: Selection of sequence valve

1. Sequence valve type selection

• Sequence valves can be divided into internal control type (also known as direct control sequence valve) and external control type (also known as hydraulic control sequence valve or remote control sequence valve). The internal control sequence valve can directly use the pressure of the imported oil circuit to control the spool valve. The external control type (hydraulic) sequence valve must be controlled by the oil pressure of the control oil circuit (that is, remote pressure control). The valve is closed. If the secondary pressure oil circuit of the external control type sequence valve is returned to the oil tank, it constitutes an unloading valve.
• There are two types of sequence valves: direct-acting and pilot-operated. The former is used for low-pressure systems and the latter is used for medium-high pressure systems.
• The oil drain port must generally lead back to the oil tank. Therefore, if the secondary pressure oil circuit of the valve is connected to the oil tank, it can be drained internally, otherwise, it must be drained externally. The drain port should be connected back to the tank separately.
• The choice of the spool. The sequence valve is a switching element. The valve opens only when the system pressure reaches the set pressure, so the leakage is small when the valve is closed. The sequence valve of the poppet valve structure has a small leakage; the sequence valve of the spool valve structure should have a certain amount of cover to reduce the leakage.

2. Restrictions on the application of sequence valve

• When a one-way component is installed inside the sequence valve, it is called a one-way sequence valve, which allows the oil flow to pass through in the reverse direction freely, and is not restricted by the sequence valve. More one-way sequence valves are used on the oil path that needs to be reversed. . Used to control the sequential actions of hydraulic cylinders or hydraulic motors, sequence valves or one-way sequence valves can be used to control the sequential actions of hydraulic cylinders or hydraulic motors.
• Used as an ordinary overflow valve, connect the secondary pressure oil circuit of the direct control sequence valve back to the oil tank, which becomes an ordinary overflow valve with a safety function.
• Used for automatic unloading of the hydraulic pump in the accumulator system.
• It is used to prevent the oil cylinder and working mechanism from sliding down due to its weight and is used as a balance valve.
• During the sequence operation, the set pressure of the sequence valve should be at least 0.5 MPa higher than the working pressure of the first-acting actuator to avoid misoperation due to pressure fluctuations.

Question 2: The difference between sequence valve and relief valve

The sequence valve is similar to the overflow valve. To make the actuators accurately realize the sequence action, the pressure regulation deviation is required to be small. For this reason, the stiffness of the pressure regulating spring should be reduced. The difference between it and the overflow valve is that its resistance loss through the valve is close to zero.

1. The difference between an internally controlled direct-acting sequence valve and a direct-acting relief valve

The working principle of the internally controlled direct-acting sequence valve is similar to that of the direct-acting relief valve. The difference is the secondary oil circuit, that is, the outlet pressure oil is not connected back to the tank, so the leaking oil port must be connected back to the tank separately, to reduce the stiffness of the pressure regulating spring, Set up the control plunger. The valve port of the internal control sequence valve is always closed before the pressure of the inlet oil path reaches the set pressure of the valve. After the set pressure is reached, the valve port opens, allowing the pressure oil to enter the secondary oil path to drive another execution element.

2. The difference between the unloading relief valve and external control sequence valve as an unloading valve

The unloading relief valve is mainly used in hydraulic circuits equipped with accumulators. When the animal can press the knife and press the knife to the pressure of the heart, it automatically unloads the hydraulic pump. The unloading overflow valve is equipped with a one-way valve to prevent the pressure oil of the energy storage fan from sinking. At this time, the accumulator supplies oil to the system and maintains pressure. When the oil pressure in the accumulator drops to the full pressure of the unloading overflow valve, the unloading overflow valve is closed and the hydraulic pump resumes charging the accumulator. Section 5.10 hydraulic equipment accumulator speed increase circuit,

because in this circuit, the unloading overflow valve should be selected as the unloading valve, and the general external control sequence valve is selected as the unloading valve, which causes the hydraulic pump outlet pressure High and low, resulting in high circuit power consumption, high oil temperature, and the system cannot work normally.

Question 3: The difference between the sequence valve function symbol and the overflow valve and the pressure reducing valve 

The difference between the sequence valve function symbol and the overflow valve and the pressure reducing valve is shown in the following.

Pressure switch

The pressure relay is an electro-hydraulic control element that converts the oil pressure signal into an electric signal. When the oil pressure reaches the set pressure of the pressure relay, an electrical signal is sent out to control the action of electrical components such as electromagnets, electromagnetic clutches, relays, etc., so that the oil circuit is depressurized and reversed, and the actuator realizes sequential action or turns off the motor. , Make the system stop working, play a role of safety protection, and so on.

The pressure relay is composed of a pressure-displacement conversion component and a micro switch. According to the structure of the pressure-displacement conversion component, the pressure relay has plunger type, spring tube type, diaphragm type, and bellows type. According to the function of sending electrical signals, pressure relays have a single contact type, double contact type, and other types.

Selection of pressure relay

Type selection

Plunger pressure relays are the most commonly used; spring tube pressure relays have a large working pressure adjustment range, small on-off pressure difference, and high repeat accuracy; diaphragm pressure relays have small displacements, so they have a fast response and high repeat accuracy, but they are not suitable for use It is suitable for high-pressure systems and is easily affected by pressure fluctuations; the bellows-type pressure relay has a displacement amplification effect on the lever, and the displacement of the core rod is small, so the repeatability is high, but the bellows-type is not suitable for high-pressure occasions.

Restrictions on the application of pressure relay

• It is used for automatic pressure relief or reverse movement of the coil circuit (controlled by solenoid valve) when the pressure reaches the set value.
• It is used to start or stop the hydraulic pump or hydraulic motor when the system pressure is greater than the set pressure.
• Used for sequential action of the solenoid valve, under a specified pressure.
• Used to stop machines, safety devices, or pressure alarm signals or signals.
• Used to start the booster pump in a hydraulic press.
• Used to start the time relay.
• It is used to stop its auxiliary device when the pressure of the main oil circuit drops.
• PF type pressure relay can be used for differential pressure control devices between two high and low pressure.

Reduced sensitivity

The sensitivity of the pressure relay is reduced and the reason for the damage of the microswitch

• When the valve core or push rod is clamped radially, the friction force increases. This resistance is opposite to the movement direction of the valve core and the pushrod. One direction helps to adjust the spring force to increase the oil pressure, and the other direction helps the oil pressure to overcome the spring force, so that the oil pressure is reduced, and the pressure is reduced. The sensitivity of the relay is reduced.
• During use, due to the deformation of the microswitch bracket or the looseness of the zero-position adjustable part, the minimum idle stroke of the microswitch that has been adjusted or guaranteed after assembly will increase and the sensitivity will be reduced.
• If the drain cavity of the pressure relay is not directly connected to the fuel tank, the sensitivity will be reduced due to the excessive back pressure of the drain port.
• The micro switch part of the differential pressure relay and the drain cavity are separated by a rubber membrane, so when the oil inlet cavity and the brain cavity are reversed, the pressure oil will break through the rubber diaphragm and enter the microswitch part, thereby damaging the microswitch.

Solution

Since the pressure regulating spring cavity and the brain cavity is connected, there is no sealing device at the adjustment screw, so when the drain pressure is too high, external leakage occurs at the adjustment screw, so the drain cavity must be directly connected to the oil tank.

Design of pressure relay circuit

• In the throttle speed regulating circuit with dead stop iron positioning, the installation position of the pressure relay should be on the same side as the flow control valve and close to the hydraulic cylinder.
• The pressure in the oil inlet cavity of the oil inlet throttle speed regulating circuit changes with the load. When the working part hits the dead stop and stops moving, its pressure rises to the relief valve setting pressure, and this pressure is used as the control sequence action Command signal.
• In the oil return throttle speed control circuit, the pressure of the oil return chamber changes with the load. After the working parts hit the dead stop, the pressure drops to zero, so take this zero pressure to send a signal.

The positioning design of the pressure relay in the speed control loop is shown in the following.