The bending machine is a widely used bending mechanical device that has long been hydraulically operated. As an essential equipment in sheet metal processing, the bending machine plays an irreplaceable role in determining the quality, processing efficiency, and accuracy of the products.
Typically, the bending machine is an upper piston-type press, consisting of a frame, slide, hydraulic system, front support frame, back gauge, die, electrical system, and other components.
The vertical downward pressure is generated by two parallel moving hydraulic cylinders to drive the die on the bending beam for bending operations. The hydraulic control system serves as the brain of the bending machine, primarily controlling the synchronous operation during the bending process and the positioning of the hydraulic cylinders when the machine is operating at full load. Let's take a universal bending machine as an example to analyze the hydraulic system.
Hydraulic System
Each bending action of the upper bending beam typically involves the following stages in the motion cycle:
1. Pump Start
The motor rotates in the direction indicated by the pump arrow, clockwise, driving the axial piston pump to operate. The oil enters the valve plate and the electromagnetic relief valve through the pipeline and returns to the oil tank. When valve 19 is closed, the oil in the lower chamber of cylinder 20 keeps the slide in a fixed position.
2. Downward Motion
The rapid downward motion of the bending machine is generated by the self-weight of the bending beam and its accessories, as well as the pressure of the hydraulic oil. During this process, the rodless chamber of the hydraulic cylinder is filled with oil through the filling valve, creating back pressure, and the oil flows back rapidly. The rapid approach motion starts from the top dead center and goes through a brief deceleration phase. When the slide is close to a certain distance from the bending sheet, the speed slows down.
When solenoids 9 (YV1), 24 (YV6), 13 (YV4), and 17 (YV5) are activated, the slide rapidly descends. The descent speed is adjusted by valve 18, and the oil in the lower chamber of cylinder 20 flows into the tank through valves 19, 18, and 17. The oil in the upper chamber of cylinder 20 is injected through valve 21.
When the slide reaches the limit switch, solenoids 9 (YV1), 8 (YV2), 11 (YV3), 13 (YV4), and 24 (YV6) are activated, and the slide enters the working speed. If the slide is not synchronized, it is automatically corrected by valve 15. The descent position of the slide is limited by the mechanical stop block in the cylinder.
3. Bending
The bending stage starts with pressure building in the rodless chamber. The bending speed is limited by the oil supply from the pump and can be adjusted through the proportional valve and directional valve. The directional valve also controls the synchronous operation of the bending beam and the positioning of the bottom dead center. The bending force is limited by the proportional relief valve, which limits the pump pressure. The corresponding values for speed, synchronization, positioning, and pressure are all provided by the numerical controller.
Control of solenoids 9 (YV1), 8 (YV2), 11 (YV3), 13 (YV4), and 24 (YV6) is achieved through foot switches or buttons. The duration of their activation determines the incremental distance during the descent of the slide. The descent speed of the slide is adjusted by valve 16. The upward movement of the slide is controlled by solenoids 11 (YV3) and 24 (YV6), with the duration of their activation determining the incremental distance during the upward movement of the slide.
3. Pressure Relief
Pressure relief in the rodless chamber begins either upon reaching the bottom dead center or after a brief holding time, allowing sufficient time for material forming and further improving dimensional accuracy of the workpiece. Both holding and pressure relief are accomplished by proportional directional valves following the instructions from the numerical controller. To enhance processing efficiency, the pressure relief time should be minimized, but to avoid unloading shocks throughout the system, the pressure relief time should be extended as much as possible. In summary, the pressure relief curve should be as smooth as possible and not too steep. The optimization of the entire process is achieved through proportional directional valves.
4. Main Cylinder Return
The maximum return speed is determined by the pump flow rate and the pressure-bearing area of the hydraulic cylinder's rod chamber, typically approaching the fastest speed.
Synchronized operation is also required during the return phase, starting from pressure relief in the rod chamber and ending at the top dead center.
During the instantaneous phase of the return, solenoid 8 (YV2) is first reset for 2 seconds to achieve pressure relief. Then, solenoids 11 (YV3) and 24 (YV6) are activated, causing the slide to return at a constant speed.
5. Machine Pressure Adjustment
Valve 6, the high-pressure relief valve, and valve 11, the electromagnetic relief valve, ensure the rated force of the machine. Valve 14 regulates the return force of the machine to prevent damage caused by overload. The working pressure in the hydraulic system can be read from pressure gauge 7. Accumulator 10 is used to maintain nitrogen pressure, primarily for operating valves 19/21.
Trends in Hydraulic System Development
The development of hydraulic systems has driven the rapid growth of the machine tool industry. To meet the requirements of high performance, high precision, and automation in hydraulic systems, new hydraulic technologies such as CNC hydraulic technology and electro-hydraulic servo technology are also rapidly advancing. Based on the current research status, the trends in hydraulic system development can be summarized as follows:
1. Integration of Modern Hydraulic Technology with new technologies such as microelectronics, computer control, and sensing technology, forming an automation technology that includes transmission, control, and detection. Hydraulic technology has made significant progress in meeting requirements for high pressure, high speed, high power, durability, and high integration. There have also been many achievements in improving proportional control, servo control, and developing control technologies. Additionally, computer-aided design (CAD) and testing (CAT) of hydraulic components and systems, as well as topics such as computer control, hydromechatronics, fluitronics, reliability, pollution control, and energy consumption control, are areas of focus for hydraulic technology development and research.
2. Virtualization utilizing CAD technology to fully support the entire process of hydraulic system design, from conceptual design, product design, performance design, reliability design, to detailed component design. Computer-aided design (CAD), computer-aided analysis (CAE), computer-aided process planning (CAPP), computer-aided inspection (CAI), computer-aided testing (CAT), and integration with modern management systems are all integrated, establishing computer-integrated manufacturing systems (CIMS) for a breakthrough in design and manufacturing technology.
3. Development of fault prediction and proactive maintenance techniques for hydraulic systems. It is necessary to modernize hydraulic system fault diagnosis, strengthen the development of expert systems, establish complete expert knowledge bases with learning capabilities, and utilize the knowledge in computers and knowledge bases to deduce the causes of failures and propose maintenance solutions and preventive measures. Further development of general-purpose software tools for hydraulic system fault diagnosis expert systems and the development of self-compensating systems for hydraulic systems, including self-adjustment and self-correction, are directions of effort in the hydraulic industry.
4. Integration of digital electronic technology and hydraulic technology. By installing electronic control devices inside servo valves or changing the valve structure, a wide range of digital products have been developed. Valve performance is controlled by software, allowing for easy design changes and the implementation of digital compensation and other functions by modifying the program.
5. Miniaturization. With the advancement of hydraulic technology and increased competition, the technology of miniature servo valves is gaining more attention due to its advantages of small size, light weight, and high power density. Research focuses on increasing pressure advantages, applying advanced materials and composite materials to reduce weight, and developing casting processes. The extensive use of casting runners in valve bodies and integrated blocks can optimize internal flow and achieve component miniaturization.
6. Greenization to reduce energy consumption, leakage control, and pollution control. The development of technologies to reduce internal losses and throttling losses, as well as the use of leak-free components such as no-pipe connections and new sealing technologies, should be emphasized. The development of pollution-resistant technologies and new pollution detection methods for online measurement of contamination should also be pursued. The use of rapidly biodegradable pressure fluids, such as vegetable oil-based and synthetic grease-based transmission media, will be widely applied to reduce the environmental hazards of oil leakage. Additionally, efforts should be made to reduce noise and vibration and achieve leak-free systems.
