Basics of press brake hydraulic system
Composition of hydraulic system
Hydraulic pump. The mechanical energy input by the prime mover is converted into the pressure energy of the liquid, which is used as a system energy supply device.
Hydraulic cylinder (or motor). The fluid pressure energy is converted into mechanical energy and work is performed on the load.
Various hydraulic control valves are used to control the direction, pressure and flow of the fluid to ensure that the actuator completes the intended work task.
The fuel tank, oil pipe, oil filter, pressure gauge, cooler, water separator, oil mister, muffler, pipe fittings, pipe joints and various signal converters, etc., create the necessary conditions to ensure the normal operation of the system.
Hydraulic oil or compressed air, as a carrier to transfer motion and power.
The function of the fuel tank:
- Hold the reserved oil.
- Heat dissipation.
- Separate the air from the oil.
- Precipitation of pollutants.
- Separating condensate
Structure of the fuel tank:
Fuel tank size (volume) – V = 3～5q for fixed equipment; V≈1q for walking equipment.
The unit of V is liter and the unit of q is liter/minute.
When designing the fuel tank, there should be 10 ~ 15% space in the upper part of the fuel tank, mainly considering factors such as liquid level changes, foam, etc.. The effective volume of the fuel tank is 6-12 times the total flow of the hydraulic system’s oil pump.
The oil temperature is recommended to be 30-50 ° C, the highest is not higher than 65℃, and the lowest is not lower than 15℃.
The bulkhead should be designed in the fuel tank.
The distance between the oil suction area and the oil return area should be as far as possible.
It is very important for the perfect function, operational reliability, life and economy of the hydraulic system.
- Transmission of energy from the hydraulic pump to the hydraulic motor or cylinder
- Lubrication of moving parts
- Protect oil-immersed metal surfaces
- Remove dust, impurities, water, air, etc.
The important concept of oil
- High cleanliness = high reliability
- New oil is dirty oil
- Oil use time: 2000-4000h
Viscosity standard: The viscosity value is always related to a certain temperature.
The viscosity value decreases as the temperature increases and increases as the pressure of the hydraulic oil increases.
The viscosity standard is ISO standard at 40 ℃, which can be divided into #10, #22, #32, #46, #68, #100 hydraulic oil.
Oil pollution degree standards: International ISO-4406 and American NAS-1638
At the NAS9 level, the hydraulic system generally does not fail. When the pollution level drops to NAS10 ~ 11 level, the hydraulic system will occasionally fail. When the pollution degree of the oil drops below the NAS12 level, faults often occur. At this time, the hydraulic oil must be circulated and filtered.
Commonly used hydraulic valves
According to function, it can be divided into:
- direction valve
- flow valve
- pressure valve
According to the installation method, it can be divided into:
- plate valve
- stack valve
- two-way cartridge valve
- threaded cartridge valve
According to the control method, it can be divided into:
- pneumatic operated valve
- hydraulic valve
- motor valve
- solenoid valve
- proportional valve
- proportional servo valve
- servo valve
The basic function of the directional valve is to realize the communication and cut-off between two different hydraulic circuits, or to control the direction of starting, stopping and movement of the actuator (cylinder or motor) as required.
Classification of directional control valves
Divided by control method:
- Solenoid valve
- Manual Directional Valve
- Hydraulic directional valve
- Motorized directional valve
- Pneumatically actuated valve
Divided by installation method:
- Disc valve
- In-line valve
- Threaded Cartridge Valve
The most important function of the relief valve is to limit the system pressure, thereby protecting various components and pipelines, and preventing the danger of overload and burst.
This valve is therefore also called a pressure valve or a safety valve.
When the system pressure reaches the set pressure value, the relief valve starts to act as a pressure limiter. The originally closed valve is now opened, and the excess flow flows back to the tank through the valve port.
When working in this way, the relief valve is installed on the bypass.
It should be paid attention that the power relief loss of flow Q with pressure P passing through the relief valve is P×Q/612.
The lost energy is transmitted to the hydraulic system, which causes the temperature of the hydraulic oil to rise.
The inlet pressure P acts on the measurement area A, and the resulting hydraulic pressure is compared with the spring force. When the hydraulic pressure exceeds the setting force of the spring, the valve core compresses the spring and the valve port opens, connecting the path between the valve inlet and outlet.
The flow valve controls the speed of the hydraulic actuator.
This function is achieved by changing the size of the cross-sectional area of the throttle to change the volume flow rate Q of the actuator.
The flow valve can be divided into the throttle valve and speed control valve.
The function of the check valve is to cut off the flow in one direction and allow the flow in the other direction to pass without restriction.
One-way valve sealing elements have spherical, cone-valve, plate-valve forms.
The relatively weak spring force needs to be overcome when the sealing element is opened. These basic principles are directly reflected in the graphic symbols.
Two-way cartridge valve
The two-way cartridge valve is designed as a plug-in structure and is installed in a compact control circuit.
In most cases, the cover plate also functions as a connection block between the main valve and the pilot valve.
By controlling the main valve with a suitable pilot valve, pressure, reversing or throttling functions, or a combination of these functions can be achieved.
The functions include directional control, overflow control, decompression control, and sequence control.
Open-loop proportional valve – electro-hydraulic proportional valve
- proportional relief valve
- proportional pressure reducing valve
- proportional throttle valve
- proportional flow valve
- proportional directional valve
Closed-Loop Proportional Valve – Proportional Servo Valve
- The amplifier integrated proportional servo valve NG6, NG10, NG16, NG25, NG32
- Amplifier external proportional servo valve NG6 ~ NG50
Proportional servo valve
Frequency response: 120Hz
No dead zone (zero cover)
Automatic compensation without balancing valve
If there is no feedback between the output and input of the system, that is, the output of the control system does not have any effect on the control of the system, such a system is called an open-loop control system.
Closed-loop control system:
The closed-loop control system is an automatic control system based on the principle of feedback.
The so-called feedback principle is to control according to the information of the system output change, that is, to compare the deviation between the system behavior (output) and the expected behavior and eliminate the deviation to obtain the expected system performance.
In the feedback control system, there is both a signal forward path from the input to the output and a signal feedback path from the output to the input. The two form a closed loop. Therefore, the feedback control system is also called a closed-loop control system.
The advantages of the open-loop control system are a simple structure and relatively economical. The disadvantage is that the error caused by interference cannot be eliminated.
Compared with the open-loop control system, closed-loop control has a series of advantages.
In the feedback control system, no matter what the reason (external disturbance or internal change of the system), as long as the controlled quantity deviates from the specified value, a corresponding control effect will be generated to eliminate the deviation.
Therefore, it has the ability to suppress interference, is insensitive to changes in component characteristics, and can improve the response characteristics of the system.
However, the introduction of a feedback loop increases the complexity of the system, and improper gain selection can cause system instability.
In order to improve the control accuracy, when the disturbance variable can be measured, the control by disturbance (that is, feedforward control) is often used as a supplement to the feedback control to form a composite control system.
|Open-Loop Proportional Valve||Closed-loop servo valve|
|Frequency response: 15Hz||Frequency response: 120Hz|
|Hysteresis: 5%||Hysteresis: 0.1%|
|Reverse error: 1%||Reverse error: 0.05%|
|Repeat accuracy: 0.1||Repeat accuracy: 0.01|
|Median dead zone||Zero cover|
Principle of the hydraulic system of electro-hydraulic servo press brake
Principle of Electro-Hydraulic Synchronous Press Brake (Take the system below 300 tons as an example)
Start the oil pump motor. According to the required bending force, the proportional pressure valve (4) controls the two-way cartridge valve (2) to adjust the pressure of the hydraulic system to meet the bending force requirements.
The pressure valve (4.1) is a safety valve, which controls the maximum pressure of the system.
Give 1Y1 voltage (20% ~ 30%) to the proportional pressure valve (4), and 1Y2 solenoid valve (6) loses power. When the solenoid valve (5) 4Y3 is energized, it will give a positive voltage to the proportional servo valve.
As the weight of the slider decreases rapidly, the oil is sucked into the upper cavity of the cylinder through the flow valve, and the oil discharged from the oil pump enters the upper cavity of the cylinder through the proportional servo valve (2).
The oil in the lower chamber of the cylinder is returned to the tank through the solenoid valve 5 (A-P) and the proportional servo valve (2) (B → T).
The fast down speed of the slider can be obtained by adjusting the control voltage of the proportional servo valve 4Y5 to control the opening of the proportional servo valve to obtain different speeds.
The proportional pressure valve (4) 1Y1 is energized, the electromagnetic reversing valve (6) 1Y2 is energized, the filling valve is closed, the solenoid valve (5) 4Y3 is de-energized, and the pressure oil discharged from the oil pump is passed through the proportional servo valve (2). Enter the upper cavity of the cylinder (no rod cavity).
The oil in the lower chamber of the cylinder passes through the backpressure valve (4) and the proportional servo valve (2) and returns to the oil tank, and the slider is pressed down.
By adjusting the control voltage 4Y5 of the proportional servo valve to control the opening of the proportional servo valve, different working speeds are obtained.
The safety valve (3) prevents the pressure in the lower cavity of the oil cylinder from being too high, and the set pressure is 10% higher than the system pressure.
The setting pressure of the back pressure valve (4) is generally the equilibrium pressure plus (30 ~ 50) bar.
When the ram reaches the bottom dead point, the proportional servo valve 2 (4Y5) is 0V to cut off the path of the upper and lower chambers of the cylinder, and the slider stops at the bottom dead point.
After the pressure-maintenance of the press brake machine is completed, the proportional pressure valve maintains the pressure, and the system gives the proportional servo valve 2 (4Y5) a certain negative voltage, so that the proportional valve is slightly opened (return direction).
At the same time, the ram will also go up a small amount, and the up amount is set by the unloading distance parameter.
The time taken for the unloading process is set by the decompression speed parameter. The pressure in the upper cavity of the cylinder is unloaded through the proportional servo valve (2).
When the solenoid valve (6) 1Y2 loses power, a certain voltage is applied to the proportional pressure valve (4), the solenoid valve (5) 4Y3 loses power, and the proportional servo valve (4Y5) has a negative voltage. The pressure oil is passed from the pump block through 2 synchronization blocks. The hydraulic oil from the upper proportional servo valve (2) and the electromagnetic reversing valve (5) (P-A) to the lower chamber of the cylinder (with a rod chamber), and the upper chamber of the cylinder (without a rod chamber) are returned to the tank through the filling valve. The ram returns quickly.
The return speed can be obtained by adjusting the control voltage of the proportional servo valve 4Y5 to control the opening of the proportional servo valve (2) to obtain different speeds.
The compensation of the workbench is completed by controlling the proportional pressure reducing valve (10) 1Y3. The pressure oil enters the compensation cylinder through the proportional pressure reducing valve (10), and the pressure of the proportional pressure reducing valve is adjusted by adjusting the voltage of the proportional pressure reducing valve (10). To make the table convex, and compensate the deformation of the table during bending.
Troubleshooting of hydraulic system of electro-hydraulic servo press brake
System without pressure
- Check whether the plug on the proportional pressure valve (04) is loose, whether there is a corresponding electrical signal in 1YI, and whether the safety valve (4.1) is loose;
- Check whether the spool of the two-way cartridge valve (02) is stuck and whether the fluid resistance (09) installed on the spool is blocked. Whether the spool of the proportional pressure valve (04) is stuck;
- Open the fuel tank cover and check the oil return condition of the oil return port if the set pressure cannot be reached. If there is no oil return or the oil return flow rate is not urgent, the oil pump is damaged and the oil pump needs to be replaced.
- First check whether the pressure of back pressure valve and safety valve is lowered;
- Stop the ramat the upper starting point and remove the proportional servo valve on the synchronizing block, and observe whether there is any oil overflow from the A port of the proportional servo valve on the valve block. If the oil overflows, the sync block is leaking. Otherwise, there is a leak in the cylinder. Or reverse the left and right sync blocks. If the sliding phenomenon does not follow the sync blocks, then there is a leak in the cylinder.
- Clean the back pressure valve spool. If the problem cannot be solved, clean the poppet valve and safety valve;
- The ram slides in one section and does not slide in other sections. This is because the cylinder is not well sealed in one section.
The ram is not fast down, fast down is slow and fast down is not synchronized.
- Check whether the plug on the poppet valve on the sync block is loosened and whether there is a corresponding electrical signal. Is the proportional servo valve power on signal given and feedback consistent? If not, it means that the proportional servo valve spool is stuck and must be cleaned.
- Check whether the liquid resistance 6 in the X port on the sync block is blocked, and check whether the filling valve under the sync block is stuck.
- Check if the ramrail or cylinder is too tight.
The ram is in fast down but without no work progress
- In the diagnosis state, check whether there is pressure in the hydraulic system;
- In the diagnosis state, the corresponding electric signals of the proportional servo valve (2), the proportional pressure valve (04) and the electromagnetic directional valve (06) are respectively given.Make the filling valve close and the corresponding opening direction of the proportional servo valve. If the cylinders on both sides cannot be driven, check whether the plug 1Y2 on the electromagnetic reversing valve (06) on the pump block is loose, whether there is a corresponding electrical signal, and whether the valve core is stuck. If a certain cylinder cannot be driven, check whether the liquid resistance (6) in the synchronizing block on the cylinder is blocked, and whether the filling valve under the synchronizing block is stuck.
The ram quickly turns down and enters the middle pause
- Whether the fuel tank liquid level is too low which makes the filling valve inhale air;
- The oil inlet of the filling valve is not sealed and leaks air;
- The filling valve spring is broken.
The ram cannot return, or the return speed is too slow
- In the diagnosis state, check whether there is pressure in the hydraulic system;
- In the diagnosis state, the corresponding electric signals of the proportional servo valve, the proportional pressure valve and the electromagnetic directional valve are simultaneously given. Make the filling valve opens and the corresponding opening direction of the proportional servo valve. For example, the cylinders on both sides cannot return normally and quickly. Then check whether the electromagnetic directional valve on the pump block has the corresponding electric signal and whether the valve core is stuck. If a cylinder cannot return normally and quickly, check whether the fluid resistance in the synchronizing block on the cylinder is blocked. Whether the filling valve under the sync block is stuck;
- Check whether the power-on signal of the proportional servo valve is the same as the feedback. If not, it means that the spool of the proportional servo valve is stuck and needs to be cleaned.
The oil temperature rises too fast, the system pressure is too high when the oil pump is running dry, and the motor is easy to trip
- When the oil pump is running dry, the system pressure is generally about 1 MPa. If the pressure is too high, check whether the liquid resistance (8) of the Y port on the pressure control cover is blocked.
- When the machine’s oil pump is running dry, there is no pressure in the system, but the oil temperature rises rapidly, the pollutants in the oil, oil tank or pipeline will block the filter element, and the oil filter element needs to be replaced;
- Check whether the working distance is too long or the holding time is too long.
- Whether the piping configuration of the hydraulic system of the machine tool is reasonable.
Debugging of the pump-controlled hydraulic system of electro-hydraulic servo press brake
Fully release the safety valve (014) on the top valve group of the cylinder, then enter the DELEM system diagnostic interface, offset the valve by about 40%, the corresponding speed is about 700 revolutions, and the torque setting value is about 80DA. Set each run for 5-10 minutes, then close the safety valve.
When closing the safety valve, a pressure gauge should be prepared to adjust the pressure in the lower chamber to 20mpa. If there is no pressure gauge, fully tighten the safety valve and then loosen it once. After the exhaust is completed, noise may occur in the first few actions, and the return stroke may not occur. The problem of synchronization and slow return is caused by the machine pipe and cylinder air are not completely exhausted. Generally, the machine will operate normally after 5-8 cycles. If the exhaust of the machine tool is completed, and still cannot return, the lower chamber safety valve needs to be released to exhaust according to the above operation. Do not use automatic parameter search repeatedly and forcefully complete the return stroke to avoid damaging the oil pump. During the initial commissioning, the speed of the fast return stroke should be controlled within 100mm/s to avoid damage to the oil pump due to the lack of air discharge and high speed.
Lower cavity safety valve: The factory setting of the lower cavity safety valve is 20MAP, and it is not necessary to adjust it if it is not necessary.
Backpressure valve adjustment: first observe the static back pressure of the machine (usually 4-5MPA), and then add 3-4MPA as the dynamic back pressure of the machine based on the static backpressure. It can be adjusted according to the actual working conditions of the machine.
Slide the ram to the bottom
Enter the DELEI diagnosis interface, offset the two valves by 20%, set the pressure valve (torque) DA value to about 80DA, and then open the quick-release valve, the ram will slowly drop until it is pressed to the lower mold.
The adjustment pressures of the backpressure valves on both sides should be basically the same. Excessive errors will cause problems such as asynchronous work.
When sliding the ram to the end, be sure to apply torque, otherwise the ram will fall quickly and hit the mold or hit the bottom of the cylinder, which will cause an inestimable danger.
Advantages of electro-hydraulic servo bending machine
- Significant energy savings, improved efficiency, and energy savings of 70%
- Use pump control instead of conventional valve control to eliminate throttling losses;
- The precise distribution of the required oil quantity is optimized by dynamically adjusting the speed of the servo motor;
- Less useless power consumption: when flow or pressure is not needed, the motor can be turned off
- Positive impact on environment and use costs
- Reduced power consumption and correspondingly reduced C02 emissions
- Reduced installation capacity: the servo motor can be significantly overloaded in a short time, and the actual installation power is only 50% of the theoretical installation power
- Reduce the fuel tank volume by 50% and reduce the use of hydraulic oil
- Low thermal equilibrium temperature, no cooling device required, and long life of hydraulic components
- Noise reduction: The noise is significantly reduced under idle, fast down, pressure holding, and returning conditions, improving the working environment
- Improved safety and economy
- Servo motor brakes faster than ordinary motors, and pressure and flow are cut off quickly in emergency situations
- Oil particle sensitivity is reduced, from NS7 (proportional servo valve) to NS9 (plunger pump), temperature sensitivity is reduced, proportional servo operating temperature is 20 ℃ -50 ℃, servo motor 10 ℃ -80 ℃, plunger Pump20 ℃ -90 ℃
- Excellent speed control performance
- Speed matching is very high. The same valve group is equipped with three pumps of 6, 8, and 10, which can cover 30-300 tons of bending machine.
- Maximum speed of fast down and return up to 200mm/s under certain conditions
- Arbitrary speed can be set arbitrarily within 0-20mm/s
- Outstanding position control performance
- Repeat positioning accuracy 0.005mm, high-precision bending
- Outstanding track following performance: high synchronization accuracy, within 0.020mm during industrial advance
- Overload protection performance: for different specifications of machine tools, the system maximum torque control to prevent human factors from causing system overload
Hydraulic system of torsion bar synchro press brake
Start the oil pump motor. Depending on the required bending force, the remote control valve (10) or proportional pressure valve controls the two-way cartridge valve (90) to adjust the pressure of the hydraulic system to meet the bending force requirements.
Power on Y2 and Y3, and power off on Y1. Due to the weight of the ram falling rapidly, the oil is sucked into the upper cavity of the oil cylinder through the filling valve. In addition, the oil is passed to the upper cavity of the oil cylinder through the No. 40 electromagnetic directional valve (P-A) and the check valve No. 30. The oil in the lower cavity of the oil cylinder passes through the 100-way one-way throttle valve to the 50-way poppet valve, and then returns to the oil tank through the 40 electromagnetic directional valve (B-T). The fast down speed of the slider can be controlled by adjusting No. 100 one-way throttle valve.
Power on Y2 and Y4, and power off on Y1 and Y3. The normally closed filling valve (hydraulic check valve) controls the pressure relief of the oil port, and the filling valve is closed. The pressure oil discharged from the oil pump passes the No. 40 solenoid valve (P → A) and No. 30 check valve to the upper cavity of the cylinder. The oil in the lower cavity of the oil cylinder is returned to the tank through the No. 60 poppet valve, the No. 70 throttle valve, and the No. 40 solenoid valve (B-T). The co-feed speed can be adjusted by the No. 70 throttle valve, and the M2 port is the lower cavity pressure measurement port.
After the press brake is pressurized, Y1, Y2, Y3, and Y4 are all de-energized, and the pressure oil in the upper cavity of the cylinder passes through the 20th orifice to the 40th electromagnetic directional valve (A → T) to release the pressure. The load removal time is controlled by the time relay.
Y1 is energized while Y2 and Y3 lose power, the pressure oil discharged from the oil pump passes the No. 40 solenoid valve (P-B), through the No. 50 poppet valve, and the No. 100 one-way throttle valve to the lower cavity of the cylinder. At the same time, the pressure oil opens the filling valve (hydraulic check valve). A large amount of oil in the upper cavity of the oil cylinder is returned to the oil tank through the filling valve.
- First check whether the pressure of the lower cavity safety valve of No. 80 is lowered.
- Clean No. 60 poppet valve and No. 50 poppet valve and No. 80 lower cavity safety valve.
- Stop the ramat the top dead center and completely close the 70th throttle valve and the 100th one-way throttle valve to judge whether the 50th and 60th east valve are damaged.
No fast down or slow down
- Check whether the plug of the 50th poppet valve is loosened, and whether the electric signal of the 40th directional valve is normal and whether there is a stuck valve, such as a stuck valve that needs to be cleaned.
- Check whether the 100-way one-way throttle valve is completely released.
- Loosen the lower cavity safety valve No. 80 to determine whether the oil cylinder and guide rail are too tight.
- Check whether the filling valve is stuck.
No work progress at speed change point
- Check whether the travel switch is adjusted in place.
- Check whether the poppet valves 50 and 60 are stuck.
- Check whether the filling valve is stuck. Touch the filling oil return pipe with your hand, and whether there is any oil overflow during the co-feeding stage.
- Check if there is a lot of air when the fuel tank returns.
- Is the system pressure and the pressure in the lower chamber M2 normal?
Cannot return or is slow
- Check whether the hydraulic system is under pressure or has reached the required pressure
- Check whether the electric signal of directional valve No. 40 is normal and whether there is a valve jam.
- Check whether the F port of the filling valve control port is blocked. Whether the filling valve is stuck.
- The problem of slow return trips when the 50 valve is stuck
Attached Table & Diagram
Attached Table 1: Selection of hydraulic pipe diameter
Determining Tube Size for Hydraulic Systems
Proper tube material, type and size for a given application and type of fitting is critical for efficient and trouble-free operation of the fluid system. The selection of proper tubing involves choosing the right tube material, and determining the optimum tube size (O.D. and wall thickness).
Proper sizing of the tube for various parts of a hydraulic system results in an optimum combination of efficient and cost-effective performance.
A tube that is too small causes high fluid velocity, which has many detrimental effects. In pressure lines, it causes high friction losses and turbulence, both resulting in high-pressure drops and heat generation. High heat accelerates wear in moving parts and rapid aging of seals and hoses, all resulting in reduced component life. High heat generation also means wasted energy, and hence, low efficiency.
Too large of a tube increases system cost. Thus, optimum tube sizing is very critical. The following is a simple procedure for sizing the tubes.
Determine Required Flow Diameter
Use Table to determine the recommended flow diameter for the required flow rate and type of line.
The table is based on the following recommended flow velocities:
If you desire to use different velocities than the above, use one of the following formula to determine the required flow diameter.
Appendix: Schematic Diagram Of Electro-Hydraulic Servo Press Brake Hydraulic System
Appendix: Schematic Diagram Of Electro-Hydraulic Servo Press Brake Hydraulic System (400-1200 tons)
Appendix: Schematic Diagram Of Electro-Hydraulic Servo Press Brake Hydraulic System (400-1200 tons)
Appendix: Schematic Diagram Of Electro-Hydraulic Servo Press Brake Hydraulic System (1600-3000 tons)
Appendix: Timing Chart Of Electro-Hydraulic Servo Press Brake
Appendix: Schematic Diagram Of Press Brake Action Sequence
Appendix: Principle Of Pump-Controlled Hydraulic System Of Electro-Hydraulic Servo Press Brake
Appendix: Schematic Diagram Of Hydraulic System For Torsion Bar Synchro Press Brake