In high-power hydraulic systems such as hydraulic presses, bending machines, and straightening machines, it is generally necessary to set up pressure relief circuits. Due to their high working pressure, when the upper chamber of the main cylinder is filled with pressure oil for pressing, stretching, or bending, the high-pressure oil carries a significant amount of energy. In addition to driving the piston of the cylinder downward to perform work, it also causes different degrees of elastic deformation in the cylinder frame, working cylinder itself, hydraulic components, pipes, and fittings, accumulating a large amount of energy. After the pressing or holding process is completed, when the cylinder moves upward and the upper chamber of the cylinder is connected to the return oil, the compressed hydraulic energy stored in the upper chamber and the elastic deformation energy accumulated in the frame and other parts are suddenly released, causing the frame system to quickly rebound. This results in intense vibration (jitter) and loud noise, similar to the sound of a gun firing. This phenomenon is known as "gunfire."
The reason for this phenomenon is insufficient understanding of energy release in the design of high-pressure and high-flow hydraulic systems, which leads to the absence of effective and reasonable pressure relief measures. The harm caused by "gunfire" is extremely serious and mainly manifests in the following points:
1) Strong vibration and loud noise during the vertical cylinder's upward (return) stroke.
2) Vibration leads to loose connections, causing severe oil leakage in the equipment.
3) Vibration causes hydraulic components and pipes to rupture and pressure gauges to be damaged.
4) The system may not be able to continue working, and it may even cause accidents involving personal safety equipment.
The key to eliminating the "gunfire" phenomenon lies in first allowing controlled pressure relief in the cylinder, gradually releasing the energy, and then changing direction (performing the return stroke). There are several specific methods for pressure relief, which are introduced below.
1) Using a small solenoid valve for pressure relief
Pressure relief controlled by a small solenoid valve and unloading valve
During the downward stroke of the cylinder, the small solenoid valve 1 is not powered. After the main cylinder completes the pressing process, before the three-way four-port solenoid valve 2 starts to change direction, the valve 1 is temporarily turned on for about 2-3 seconds using a time relay. When the pressure in the upper chamber of the cylinder drops close to the predetermined value or zero, the solenoid valve 2 is switched on. By changing direction when there is almost no pressure, the cylinder moves upward, thereby eliminating the "gunfire" phenomenon.
2) Using an unloading valve for pressure relief control
During the downward stroke of the cylinder for workpiece pressing, 2YA is powered. When the directional valve 1 is in the middle position, the hydraulic press enters the holding process after pressing, and the high-pressure oil in the upper chamber of the main cylinder keeps the pilot-operated check valve 2 open. After the holding process is completed, the time relay sends a signal, and 1YA is powered. Valve 1 switches to the right working position, and the pressure oil flows back to the oil tank through valve 2. The pressure in front of valve 2 opens the relief valve spool in the pilot-operated check valve 3, and the relief oil flows back to the replenishing oil tank through the damping hole until the pressure in the upper chamber of the main cylinder gradually decreases to the piston return pressure set by valve 2. Then, valve 2 closes. At the same time, the pressure in front of valve 2 opens the main valve of valve 3, allowing pressure oil to enter the lower chamber of the main cylinder. A large amount of return oil from the upper chamber flows back to the replenishing oil tank through valve 3, allowing the piston to return.
3. Pressure relief using a dedicated throttle valve
The lower chamber of the main cylinder is the pressing chamber (working chamber). When 2YA is powered, pressure oil enters the lower chamber of the main cylinder for pressing through the three-way four-port solenoid valve 1 and the pilot-operated check valve 2. After the pressing force reaches the required tonnage, the pressure gauge 3 sends a signal, and 2YA is powered off for holding. During pressure relief, the operator slowly opens the dedicated throttle valve 4 to gradually return the high-pressure oil to the oil tank. When the pressure value shown on the pressure gauge 5 drops to 3-5 MPa, 1YA is powered on, and a large amount of low-pressure oil flows back to the oil tank through valve 2 and valve 1.
4. Pressure relief using a manual unloading directional valve (special)
The manual unloading directional valve consists of three parts: the left end is a large-diameter check valve, serving as the replenishing valve; the middle part is the directional valve for changing direction, and the right end is the directional valve for unloading the pump, with the oil port connected to the pump.
Structure and working principle of a closed-loop unloading valve
When the handle is in the middle position, both chambers of the cylinder are sealed. Passages 5 and 6 are connected, and the pump unloads. When the handle moves to the left, the spool moves to the right, disconnecting passages 5 and 6, and the pump operates. The pressure oil is discharged through channel 7 via passage 2, and the piston descends. When the work is completed and the manual handle changes direction, the spool moves to the left. Due to the closure of the check valve under the pressure of the cylinder, the movement of the spool stops when the top 9 of the spool touches the valve lever of the check valve. At this time, passage 2 is connected to the small hole 8 in the spool, allowing the cylinder to relieve pressure through passages 2, 7, 8, and 4. The speed of pressure relief is determined by the size of the small hole 8. Passages 5 and 6 are connected, and the pump unloads. Only when the cylinder is fully relieved of pressure, the spool can continue to move to the left and open the check valve. Passages 2 and 4 are connected, while passages 1 and 3 are connected, and passages 5 and 6 are disconnected. This initiates the return stroke without the occurrence of the "gunfire" phenomenon.
5. Pressure relief using an unloading directional valve during the closed-loop return stroke
Pressure relief directional valve in a closed-loop
When the piston is pressurized and descending, a is the pressure side, and b is the suction side, so the lower slide valve opens while the upper slide valve closes. The unloading valve is not in operation. After the work is completed, the pump changes direction, and b becomes the pressure side. The hydraulic pressure overcomes the spring force of the slide valve, moving it to the left, and all passages are connected. As a result, the upper chamber of the cylinder relieves pressure through a, 1, 4, and throttle valve 7, while the pump's oil supply helps with pressure relief. The speed of pressure relief is adjusted by throttle valve 7. At this time, the pump's oil supply flows back to the oil tank through h, 2, 3, 5, and 6 for unloading, maintaining only low pressure to balance the spring force of the upper slide valve. When the pressure in the upper chamber is lower than the spring force of the lower slide valve, the lower slide valve gradually closes. The closing speed can be adjusted by throttle valve 8 to ensure complete pressure relief. The closure of the lower slide valve cuts off the unloading passage of the pump, and the pressure in the lower chamber of the cylinder rises, causing the piston to rise during the return stroke.
6. Pressure relief using an electromagnetic directional valve with K-type spool for energy dissipation
Energy dissipation with K-type spool in a pressure relief circuit
When 1YA is powered on, pressure oil enters the upper chamber of the main cylinder for pressing through the three-way four-port solenoid valve 1. When the pressure reaches the predetermined value, the pressure gauge 2 sends a signal, and after a delay through a time relay, the pressure is held. After the holding process is completed, 1YA is powered off, and under the control of the damper on valve 1, the valve switches to the middle position after a delay. The high-pressure oil is gradually released from small to large as the K-type spool moves, and when the spool reaches the fully middle position, most of the energy of the high-pressure oil has been released, greatly reducing the occurrence of "gunfire." To ensure reliable direction change, a back pressure valve should be added at point a.
