In recent years, the press brake has been constantly developed towards larger tonnages.
With the increase of machine tool tonnage and length, the weight of the ram increases.
When the speed of the press brake ram switches and the hydraulic valve is closed, due to the inertia of the fluid and moving parts of the ram, a high peak pressure is formed in the hydraulic system, causing hydraulic shock and ram impact.
The impact is particularly significant for large press brakes, often causing vibration, noise, oil leakage and other phenomena in the system, damaging hydraulic system components, especially the connecting bolts between the ram and the oil cylinder, resulting in serious consequences.
1. Oil Cylinder Structure and Principle
1.1 Structure of Oil Cylinder
A schematic diagram of the hydraulic cylinder structure of a certain press brake is shown in Figure 1.
The hydraulic cylinder mainly consists of guide sleeve, cylinder body, piston rod, adjustment rod, adjustment bolt, cylinder head, snap ring, plain bearing, worm gear and other parts. A sinkhole is set inside the piston rod, with an adjustment rod installed inside it.
The outer circle of the adjustment screw uses a trapezoidal thread to form a threaded pair with the inner hole of the piston rod. The middle is connected to the adjustment rod through a key, and moves synchronously with the adjustment rod.
There is a snap ring on the right side of the adjustment screw to effectively prevent excessive movement of the adjustment screw.
The right end of the adjustment rod is connected to the worm gear through a key, and a plain bearing is used between the worm gear and the cylinder head to effectively avoid friction between the worm gear and the cylinder head during rotation.
The worm gear is driven to rotate, and because the worm gear is fixed on the adjustment rod, the adjustment rod begins to rotate, driving the screw on the adjustment rod to rotate (the screw and the adjustment rod are transmitted by a key).
After the screw rotates, since the nut on the piston rod is fixed, the screw drives the piston rod to move in a straight line. As the screw rotates, the left end of the screw moves in the corresponding direction, and the distance between the left end face of the screw and the limit surface A of the adjustment rod changes.
When the hydraulic cylinder moves, the oil flowing into the oil inlet through the hole on the screw enters the inside of the piston rod, and moves to the left with the left end face of the piston rod until it coincides with the limit surface A of the adjustment screw and the adjustment rod, completing one stroke.
1.2 Design Features of the Hydraulic Cylinder
The basic parameters of the hydraulic cylinder of this press brake are: working pressure of 20 MPa, cylinder diameter of 280 mm, rod diameter of 260 mm, maximum stroke of 250 mm, and adjustable stroke of 0-130 mm. According to relevant formulas, the working thrust of the cylinder is calculated to be 1004 kN.
In the design process of the hydraulic cylinder, it is necessary to check the main stressed parts, the piston rod and the adjustment rod. The piston rod can fully meet the requirements after calculation, but for the adjustment rod, due to the deformation under stress, stress analysis and strain analysis were conducted on it using SolidWorks software in this article.
According to the stress situation, the stress concentration point of the adjustment rod is at the junction of the limit surface and the outer circle, and the design of the adjustment screw can fully meet the requirements of use.
Design features of the hydraulic cylinder:
(1) Install a plain bearing between the worm gear and the right end face of the cylinder head to reduce or eliminate the friction generated by the worm gear due to eccentric load during rotation, making the stroke adjustment more flexible.
(2) Design of the screw. Due to the phenomenon of sticking easily occurring during motion transmission with ordinary threads, the traditional ordinary thread was changed to a trapezoidal thread structure, which is conducive to converting rotational motion into linear motion.
(3) A snap ring is set on the adjustment rod. When the screw is adjusted to the right limit point, it is easy to collide with the cylinder head. The setting of the snap ring effectively reduces and eliminates the possibility of collision.
1.3 Issues to Be Aware of During Use
(1) The hydraulic cylinder uses mechanical limit method to adjust the stroke, and cannot adopt buffering mechanisms such as throttle holes and throttle slots in the structural design, so there is a certain hydraulic directional shock. Therefore, corresponding preventive measures should be taken.
In the use process, a return oil throttling speed control circuit is adopted, and the throttle valve makes the hydraulic cylinder’s return oil cavity form a certain back pressure, so it can withstand a certain negative load and improve the smoothness of the cylinder’s speed.
(2) The requirements for the adjustment rod and the adjustment screw are relatively high, especially the fatigue damage caused by the collision of the limit surface, which may generate stress during use.
At the same time, the surface of the adjustment rod and the adjustment screw should be surface-nitrided to strengthen their impact resistance and wear resistance. During use, the frequency of use should be recorded, and observations should be made. When necessary, maintenance and disassembly should be carried out.
2. Analysis of the Fracture of Connecting Screws between the Oil Cylinder and Ram
2.1 Short Switching Time
When the ram switches from hold-down to rapid return, the directional valve switches, and the lower chamber is filled with oil. The weight of the ram prevents oil from entering the lower chamber of the cylinder, causing kinetic energy to be converted into pressure energy.
This sudden rise in liquid pressure causes instantaneous pressure several times higher than normal pressure, which forces the piston rod of the oil cylinder to drive the ram to return instantly.
2.2 Difficult Connection between Ram and Piston Rod of Oil Cylinder
If the ram is too heavy, it will be difficult to connect with the piston rod of the oil cylinder. The weight of the ram of a large press brake can reach more than 100 tons, and the connection between the oil cylinder and ram requires double-headed bolts of M120 size or even larger.
Due to previous limitations in domestic processing capabilities, the cost of processing was too high.
First, a transition block is machined, and the transition block is connected to the piston rod of the oil cylinder by screws, and then the transition block is connected to the ram by double-ended bolts.
2.3 Large Impact on Return
The large impact on return poses a safety hazard. When the press brake returns in an instant after hold-down (ram changes from stationary to 60mm/s), it causes shock to the hydraulic system and mechanical system.
The dynamic load factor of the shock is at least three times the static load, and sometimes even greater, which easily leads to oil leakage in the hydraulic system and damage to hydraulic components.
Sometimes, when the ram tilts, one side of the oil cylinder bears the load, and the screw connecting the transition block to the piston rod of the oil cylinder and the double-headed bolt connecting the transition block to the ram will be subjected to greater stress from this repeated impact, combined with the influence of thread processing factors, often causing the screw connecting the transition block to the piston rod of the oil cylinder to fracture or the threads of the double-headed bolt connecting the transition block to the ram to fatigue and fail.
This can cause serious safety hazards. The best way to reduce the impact is to increase the buffer device or lengthen the impact-resistant screws.
3. Research on the Connection between Oil Cylinder and Ram
3.1 Reduction of Screws and Addition of Buffering
To reduce the number of connecting screws and increase buffering devices. In recent years, with the greatly improved domestic processing capabilities and the continuous reduction in processing costs, M100 threads can now be directly machined on the end of the oil cylinder piston rod.
Due to structural limitations, lengthening the impact-resistant screw does not have a significant effect, so increasing the buffering device became the only choice.
This connection not only reduces the number of connecting screws but also eliminates the mounting holes on the previous ram body, enhancing the strength and rigidity of the ram.
3.2 Analysis of Impact Force after Adding Buffering
After adding a buffering device, the impact force on the connection between the ram and the oil cylinder piston rod is very short-lived, and it is difficult to analyze the contact force as it changes over time accurately.
To accurately calculate the impact force, according to the law of energy conservation, the load, deformation, and stress during the impact can be roughly estimated.
Neglecting some other energies that do not change significantly (such as thermal energy generated by the system), the variation of kinetic energy T and potential energy V of the sliding block based on the law of mechanical energy conservation should be equal to the deformation energy U1d absorbed by the connecting bolt and the deformation energy Uhd absorbed by the buffering device.
3.3 In-Depth Analysis of Impact Force
If the connecting bolts and buffering device are regarded as a unified deformation component, analysis of the impact force on the connection between the ram and the oil cylinder piston rod can be conducted.
Neglecting some other energies that do not change significantly (such as thermal energy generated by the system), the load, deformation, and stress during the impact can be roughly estimated based on the law of mechanical energy conservation.
During the impact, the elastic system is under tension, and the ram moves upward. Therefore, the variation of kinetic energy T and potential energy V of the impact system should be equal to the deformation energy Ud of the elastic component.
3.4 Analysis without Buffering Devices
If there is no buffering device, analysis of the impact force on the connecting bolts between the ram and the oil cylinder piston rod can be conducted. Assuming that the four connecting bolts are uniformly loaded when treated as a spring, the static deformation of a single connecting bolt is the same as that with the addition of a buffering device (Δst=1.2×10-4m).
In conclusion, the oil cylinder of the press brake is a very important component, so it is necessary to analyze the reasons for the detachment of the cylinder bottom.