The working principle and troubleshooting methods of the hydraulic overload protection system for mechanical presses

The old-fashioned overload protection devices in mechanical presses, such as the collapse block type, have gradually been phased out because they cannot accurately transmit signals and effectively protect the machine bed, and their replacement is very troublesome. Instead, it is equipped with a hydraulic overload protection system that features rapid unloading and quick recovery.

The composition and working principle of the system

The hydraulic overload protection system is generally composed of components such as pneumatic pumps, unloading valves, pressure relays, solenoid valves, and pressure gauges. 1. Air source 2. Oil mist lubricator 3. 4. Solenoid valve 5. 14. Pressure reducing valve 6 Pneumatic pump 7. Silencer 8. Oil tank 9. Oil filter 10. Stop valve 11,12. Check valve for oil inlet and outlet 13. Check valve 15. Unloading valve 16. Pressure relay 17. Pressure gauge and buffer valve 18. Hydraulic cylinder

When the mold height is improperly adjusted or double material occurs, the press will be overloaded. At this point, the pressure inside the hydraulic cylinder rises sharply. The pressure in the high-pressure chamber of the unloading valve is greater than that in the back pressure chamber, pushing the piston down to open the return oil port, and the hydraulic oil is quickly discharged back to the oil tank. Under the action of the balancer, the slider moves upward by 20mm, protecting the press and the die from damage. At the same time, the pressure relay and the limit switch act, cutting off the clutch control circuit, and the press stops. When solenoid valves 3 and 4 close the air source and quickly expel the air in the back pressure chamber of the unloading valve, the "Normal" indicator light goes out and the "unloading" indicator light comes on. The press cannot be started.

After the press fault is eliminated, turn the hydraulic protection switch of the control station to the "reset" position, start the "inching" stroke, and raise the slider to the top dead center. At this point, solenoid valves 3 and 4 are connected to the air source. Compressed air enters the lower chamber of the unloading valve, causing the piston to rise and return to its original position, thus closing the return oil channel of the hydraulic cylinder. At the same time, compressed air enters the pneumatic pump, driving the pneumatic piston to reciprocate, which in turn drives the hydraulic plunger to reciprocate. Thus, the low-pressure oil is drawn into the oil tank and output to the hydraulic cylinder through a check valve, gradually increasing to the specified pressure. At this point, the hydraulic pad resets, the pressure relay and the limit switch of the unloading valve act, the "unloading" indicator light of the hydraulic protection goes out, the "Normal" indicator light is on, and the hydraulic overload protection system returns to normal. Turn the hydraulic protection work selection switch to the "normal" position, and the press can operate normally. Each time the hydraulic overload protection system is restored, it only takes about 3 minutes, which can meet the requirements of general high-efficiency stamping production.

2 Common Faults and Troubleshooting Methods

Due to the frequent reciprocating movements of the press slider and the severe vibration during operation, the hydraulic and pneumatic components that make up the system may frequently experience some malfunctions, causing the press to stop. At the same time, the materials such as sealing rings used in hydraulic and pneumatic components will wear out, age or fail over long-term use, leading to malfunctions. Based on our factory's many years of experience in manufacturing and debugging hydraulic overload protection systems, this article will introduce some common faults and their troubleshooting methods.

2.1 Pneumatic Pump

The pneumatic pump is a functional component that provides the specified pressure for the hydraulic overload protection system, consisting of a pneumatic part and a hydraulic part. When analyzing faults, first listen to the sound of the pneumatic pump. If it is a normal, regular and rhythmic reversing sound without an increase in pressure, it can basically be determined that there is no fault in the pneumatic part, and the focus of the analysis should be placed on the hydraulic part. If the sound is chaotic and rhythmic, or if the air source of the pneumatic pump is connected but does not operate, or if there is obvious air leakage, it can basically be determined that the fault lies in the pneumatic part.

The hydraulic part of the pneumatic pump mainly consists of plungers and seals, check valves for oil inlet and outlet, oil suction pipes and oil filters, cylinder bodies, etc. Common faults that are prone to occur include

(1) Poor sealing or loosening of the oil suction pipe causes air to be drawn into the cylinder and cannot be expelled, resulting in the pneumatic pump idling and not discharging oil. Troubleshooting method: Remove the oil suction pipe, reapply glue and tighten it, and fill the cylinder with oil.

(2) The oil filter is clogged with dirt. It should be disassembled and cleaned thoroughly.

(3) The seal at the plunger is worn or aged and fails. It should be disassembled and replaced.

(4) Dirt entangles or blocks the oil inlet and outlet check valves, preventing the steel balls from returning to their original positions, or the spring seats become loose. It should be disassembled, cleaned and tightened.

The pneumatic part of the pneumatic pump is composed of a directional control valve, a pilot directional control valve, a valve body, a valve cover, a cylinder, a piston, a gas guide pipe, and fastening bolts, etc. The pilot directional control valve is equivalent to the limit switch at the upper and lower positions of the piston, controlling the switching of the directional control valve. The faults that are prone to occur in the pneumatic part include:

(1) The seal at the pilot reversing valve failed. Since the main directional control valve changes direction by utilizing the pressure difference at both ends of the directional control plunger, if the seal fails, the directional control valve cannot change direction, and the pneumatic pump will not operate or leak air.

(2) The spring at the pilot reversing valve has been under pressure for a long time, and its elasticity has decreased, unable to provide the force required for the pilot reversing needle plunger to reset.

When the pilot valve has the above faults, air leakage will occur from the valve cover pilot valve, which is relatively easy to determine. Troubleshooting method: Disassemble for inspection and replace the corresponding parts.

(3) The pilot valve needle plunger is adhered and rusted by moisture and dirt in the compressed air, and thus cannot operate normally.

(4) The main directional control valve is composed of a directional control sleeve, a directional control plunger, a sealing ring, a guide ring, a buffer pad and a compression ring (i.e., a muffler joint). The reversing sleeve and reversing plunger adopt clearance sealing, with the clearance value generally below 0.015mm. Therefore, they are relatively sensitive to impurities and dirt and have high requirements for lubrication. This requires that clean oil mist lubrication must be provided in the air pipeline to ensure its normal operation. During inventory and transportation, its export should be blocked to prevent the entry of contaminants. After long-term storage and use, first inject clean lubricating oil with an oil gun, and then hook the small hole of the plunger with a special tool and gently push and pull it a few times. If it can be pushed gently, it indicates that there is no problem at that location. It is recommended not to disassemble this place easily.

2.2 Unloading valve

The unloading valve is mainly composed of the valve body, piston, buffer pad, lower cover, limit switch, etc. The high-pressure chamber of the valve body is connected to the hydraulic cylinder (or high-pressure pipeline), and the low-pressure chamber is connected to the oil tank. The high-pressure oil is sealed by the mating surface of the 100° conical surface of the valve body and the 100° conical surface of the piston. The back pressure is provided by the compressed air in the lower chamber of the piston. The main fault of the unloading valve is that the pressure oil leaks and cannot maintain pressure reliably, which is mostly caused by impurities and dirt in the oil scratching the 100° conical surface. The method of judgment is as follows: The pneumatic pump sounds normal, oil comes out of the oil outlet and there is pressure, but the system pressure fails to meet the requirements, and the pneumatic pump compensates frequently. The troubleshooting method is as follows: Disassemble the unloading valve, inspect the 100° conical surface. If the damage is minor, it can be ground off with grinding paste; otherwise, replace the unloading valve and clean the dirt inside the system.

2.3 Pressure Relay

Pressure relays are also components that are prone to failure in the system. Its common faults include:

(1) Pressure oil leakage. The pressure relay is mainly composed of a valve body, a plunger, a microswitch and a spring. Under the action of oil pressure, the axial movement of the plunger triggers the microswitch to switch the electrical signal. A gap seal is adopted between the plunger and the valve body. If the manufacturing quality cannot be guaranteed, excessive leakage will inevitably cause a drop in system pressure, resulting in over-frequency compensation of the pneumatic pump and affecting its service life. Judgment method: When the system is under pressure, check whether the oil return port has returned too much oil. If necessary, replace the pressure relay.

(2) The microswitch cannot reset and cannot be interlocked with the electrical system. Due to long-term use, the reset spring of the microswitch gradually loses its elasticity and cannot reset. The solution is to replace the fatigued and failed reed with a copper sheet of the same thickness (or slightly thicker than the original one).

2.4 Solenoid Valve

The main faults of solenoid valves are:

(1) The electromagnetic coil was burned out due to long-term power-on.

(2) The seal inside the valve fails and causes air leakage, or dirt adheres to the valve core and prevents it from operating.

Due to the simple structure of the solenoid valve, for the above faults, it can be disassembled for inspection or replaced with a new solenoid valve.

2.5 Pressure regulating valve

The pressure regulating valve may fail to adjust or have no output due to rusting of internal parts. It should be disassembled and replaced.

3. Precautions for use and maintenance

(1) A water distribution device should be installed before the main pipeline of the workshop or the main cut-off gate of the press, and the accumulated water in the device should be drained before use. Otherwise, it will have an adverse effect on the system, causing rust and failure of various components and reducing their service life. This is particularly important in humid areas.

(2) Regularly check the oil level in the fuel tank and make timely compensation to prevent the pneumatic pump from running dry. Regularly change the hydraulic oil and clean the oil tank and pipelines. Hydraulic oil that meets the requirements should be used. Filtering measures should be taken when refueling. Regularly check if there is any oil leakage in all parts of the system and take timely measures.

(3) When the hydraulic overload protection system malfunctions, do not disassemble it easily. Observe it closely and, if possible, have it repaired by professionals.

(4) When pneumatic pumps are in operation, they require good lubrication. Clean lubricating oil should be frequently checked and replenished.

(5) When conducting maintenance, it is essential to ensure that no iron filings, cotton wool or other contaminants are brought into the system. Before reinstalling components, they should be wiped with a clean cotton cloth; otherwise, they may get clogged and fail to function properly. Do not forcibly reset the component before the cause of the failure is identified. Especially for some imported pumps, valves and gas-liquid safety valves, their pistons, due to magnetization treatment, become magnetic and are prone to attracting iron filings in the oil, causing system malfunctions. They must be disassembled and thoroughly cleaned to eliminate the problem.