1. Analysis of the stress at each stage of the blanking and deformation process of the hydraulic press
Blanking is the use of die to separate the sheet metal stamping process, it is a general term for blanking, punching and trimming processes. The deformation process starts from the elastic deformation of the material under force and ends with fracture separation.
1) Elastic deformation stage (stage 1). At this stage, the punch contacts the sheet, the load rises sharply, the surface of the sheet bears the tensile force and the compression force, and forms a certain angle collapse, the sheet begins to show a tendency to upturn, the degree of uplift is related to the mold gap, the larger the gap and the more serious the upturning, usually the sheet is constrained by the edge pressing ring.
2) Plastic deformation stage (stage 2). At this stage, the internal stress of the sheet metal reaches the yield limit, and with the increase of the amount of punch pressing, microcracks begin to appear around the sheet.
3) Tear spread stage ((stage 3). At this stage, the microcracks inside the sheet metal expand rapidly, and at the same time, the shear stress exceeds the limit value, so that the punching force reaches the maximum value, and the shear area has been torn.
4) Fracture separation stage (stage 4). Due to the tensile stress, the stress concentration occurs, and the material cracks first appear near the die and punch edges, and the workpiece and sheet metal are separated.
5) Vibration & Noise Phase (Stage 5). The entire system of the machine and the mold vibrates in three directions, so it has a great impact on the surrounding environment and at the same time, it also affects the service life of the mold.
2. The method of punching buffer
The noise reduction of hydraulic press mainly adopts the back pressure method. The principle is that when the sheet material is broken, the reverse hydraulic cylinder provides a reverse force acting on the slider, replacing the load of the sheet material acting on the hydraulic press slider during blanking, which can control and release the hydraulic pressure accumulated in the main hydraulic cylinder and the elastic deformation of the fuselage, so that there is no sudden load loss during the working process of the press, and the impact and vibration generated are eliminated. The main reason for the impact of the hydraulic press in the blanking process is that at the moment of sheet breakage, the high pressure of the main cylinder of the press actuator suddenly disappears, resulting in the rapid release of the liquid compression energy in the cylinder and the elastic deformation energy of the fuselage, resulting in strong shaking of the press and pipeline vibration, making the whole equipment very noisy.
3. Structural design of hydraulic press punching buffer device
3.1. Structural design of ordinary punching buffer device
Place two or four buffer cylinders on the left and right sides of the hydraulic press base, install the adjusting screws at the corresponding position of the slider, and manually adjust the adjusting screws according to the different heights of the mold. The advantage of this adjustment method is its simple structure. However, the disadvantage is that it is inconvenient to adjust the position of the buffer action point, and it needs to be measured and calibrated repeatedly.
3.2. The structural design of the bottom-mounted mechanical synchronous adjustment punching buffer device
One of the two outputs of the worm gear reducer directly drives the worm in the worm gear box to rotate, driving the worm gear to rotate. Because the hollow thread sleeve is matched with the screw thread, the upper end of the screw is provided with a guide flat key to prevent rotation, and the worm gear drives the hollow thread sleeve to rotate, and the screw only moves in the vertical direction; Due to the complex intermediate transmission process of the transmission device, the transmission gap between the chain and the sprocket is large, and the transmission error of the two screws is quite different when the forward and reverse rotation is large. Because the traditional hydraulic press will cause the vibration of the body and the accompanying strong noise when unloading, it will not only affect the performance of the equipment, but also bring serious harm to the health of workers due to strong noise pollution.
These factors not only affect the transmission accuracy, but also it is difficult to ensure that the adjusting screws on both sides of the adjustment device are at the same level. Therefore, at the moment of buffering, there is a local overload phenomenon in the screw, which causes a strong shock vibration of the slider. In addition, due to structural reasons, the device is not suitable for presses that require mold changes or feed from the side, and the scope of application is limited. By increasing the blanking buffer device, the hydraulic press smoothly transitions from the load of the blanking process to the load of the buffer cylinder, and the hydraulic energy accumulated in the main cylinder and the elastic deformation of the fuselage can be quickly released and controlled by using the rapid pressure relief device, which eliminates the overshoot phenomenon caused by the sudden fracture of the plate slider in the process of blanking or trimming of the hydraulic press, and reduces the resulting impact and vibration.
3.3. Structural design of overhead precise mechanical synchronous punching buffer device
Overhead type precision mechanical synchronous blanking buffer device, involves: the whole set of transmission device, buffer stroke synchronous adjustment mechanism and buffer cylinder are all installed at the top of the upper cross beam of the press, and two through-hole installation guide bushes and guide pillars are processed in the left and right directions of the upper cross beam, and the guide bracket fixed on the upper cross beam and the guide flat key embedded in the upper end of the adjusting screw can only move vertically. The lower end of the guide pillar is connected with the bracket on both sides of the slider by the lock nut, and is prevented from rotating by the flat key, and the guide section in the middle of the guide pillar passes through the buffer stroke adjustment screw and the buffer oil cylinder, and the buffer nut is screwed into the end thread. Therefore, when the worm gear drives the hollow threaded sleeve to rotate, the adjusting screws on both sides drive the buffer cylinder to move synchronously in the vertical direction, and the number of rotations is detected by the rotary encoder connected with the worm. In this way, the moving distance of the adjusting screw can be precisely calculated by means of the PLC programming controller and the position of the buffer action point can be adjusted precisely and synchronously within the adjustment stroke range. When the buffer nut collides directly with the quenching plate on the buffer cylinder, stamping and cushioning are realized.
The advantages of this structure are: (1) It is suitable for the punching force required for different thickness plates, with low vibration and low noise. (2) The buffer adjusting device and the buffer cylinder are assembled into an integrated structure and placed on the top of the machine body, which is simple in structure and convenient for adjustment, meets the requirements of replacing the mold or feeding press on the side, and expands the application range; (3) Through rigid mechanical transmission, the stability and synchronization of the adjusting screws on both sides of the vertical direction are ensured, and the requirements of high accuracy of the position of the buffer action point are met, and the operation is convenient.
The design and application of the buffer device of the hydraulic press
4.1. Calculation of buffer device parameters
The 2000 t hydraulic press has a mold height of 500 2200 mm, and the table size is 4000 mm and 1900 mm. Among the main punching dies, the size of the side column blanking die is 3200 mm 1170 mm 820 mm, and the size of the large beam blanking die is 3850 mm 860 mm 705mm, and the required punching force is about 11000 kn. In order to ensure that the buffer device does not hinder the use of the blanking die after installation, four buffer cylinders are installed on the left and right sides of the hydraulic press base, and the nominal force of each buffer cylinder is 2500 kN. According to the table size of the hydraulic press and the width of the mold, the maximum outer diameter of the buffer cylinder is 520 mm. Combined with the adjustment amount and the wall thickness of the buffer cylinder during mold installation, the inner diameter of the buffer cylinder shall not be greater than 360 mm.
Calculated with an inner diameter of 360 mm, the pressure area of the buffer cylinder is 0102m2。According to the pressure formula, the pressure of the buffer cylinder can reach 245mpa。Imported seals are selected for sealing to prevent hydraulic oil from leaking outward. According to the height of the mold, the minimum height of the buffer cylinder is 700 mm, the buffer stroke is 50 mm, the adjustable total height is 400 mm, the fine-tuning height is 50 mm, and the total adjustment range of the buffer pressure is 4000 kN to 10000 kn.
4.2. Buffer cylinder structure
The buffer cylinder is composed of a cylinder block, a piston rod, a butterfly spring, a support plate, an adjusting screw, a cushion block, an upper fixing pad, an oil tank, etc.
In order not to hinder the use of other molds, the buffer cylinder is connected with the hydraulic press table through bolts, which has high flexibility and easy installation and disassembly. Each buffer cylinder has its own independent hydraulic oil tank, which circulates in the hydraulic line without the need for power drive, which can reduce energy consumption and failure rate. The piston rod is processed into the form of threads, and it is also equipped with an exhaust device, which can quickly discharge the air in the cylinder block to ensure the stability of pressure and smooth operation. The butterfly spring is installed in the cylinder block and guided by the guide rod, which meets the requirements of no deviation of the butterfly spring under vibration conditions and ensures the stable and reliable jacking action of the piston rod. When the adjustment height is greater than 50mm, different numbers of cushion blocks can be placed on the end face of the adjustment screw to meet the requirements of different height molds. The adjusting screw is arranged in the piston rod, and the adjusting screw and the piston rod are threaded screw structure, and the height fine-tuning function is realized by rotating the adjusting screw. The height micro-adjustment device is equipped with a lock nut, and when the height is adjusted to a suitable height, the lock nut is tightened to prevent loosening. There are positioning slots between each pad to prevent the pad from sliding due to vibration.
4.3. Buffer cylinder hydraulic system
The hydraulic system consists of one-way relief valves, fittings and piping. The one-way relief valve is equipped with a scale and pressure carousel for easy pressure setting. The hydraulic system of the buffer cylinder is installed on the pipe between the tank and the buffer cylinder to provide pressure to the buffer cylinder.
4.4. Application of buffer device
The buffer unit has been installed on a 2000 t hydraulic press and has been successfully used in the production of side columns and girders. There is only a slight noise during production, which greatly reduces the impact and vibration of the equipment during punching. When changing from a side column punching die to a large beam punching die, you only need to remove the two cushion blocks according to the height and adjust the height of the screws to perform punching production.
With the rapid development of modern machinery industry, hydraulic presses, as forging equipment, are also developing in the direction of large-scale, automated, precise and intelligent. At the same time, it needs to meet the development requirements of low-carbon economy and green manufacturing technology. These are all in order to reduce vibration and noise, and the blanking hydraulic press needs to be equipped with a blanking buffer device. The buffer device has a reliable cushioning effect, which effectively reduces the vibration and blanking noise of the hydraulic press, and improves the working environment of workers, which is worth promoting.
Note:This article is compiled from "Mechanical Engineering" in April 2021, and the author is Li Hongxia.