During stamping production, press machines are subjected to extremely high instantaneous forming forces. When material deviations, improper die installation, or feeding errors occur, the forming force may exceed the rated capacity of the press within a very short time. In minor cases, this can lead to production stoppages; in severe cases, it may cause serious damage to critical components such as the crankshaft, connecting rods, or dies.
For this reason, press machine from World group are equipped with a hydraulic overload protection system from the initial design stage, providing reliable and timely safety protection for both the machine and the tooling.
What Is Hydraulic Overload Protection?
The hydraulic overload protection system of a press machine is abbreviated as HOLP, which is usually located at inner side of the fuselage.Simply put, hydraulic overload protection functions like a “safety valve” inside the press. Under normal stamping conditions, the slide moves downward smoothly through a rigid mechanical structure, ensuring stable operation. When the actual forming force increases abnormally and approaches or exceeds the rated tonnage, the hydraulic overload protection system automatically responds within milliseconds, rapidly releasing excess pressure and preventing rigid mechanical locking that could cause damage.
How It Works?
During normal operation, hydraulic pressure within the overload protection system remains within a preset range, allowing rigid force transmission without affecting stamping accuracy. When an abnormal overload occurs, the forming force rises sharply and pressure inside the hydraulic chamber increases simultaneously. Once the pressure reaches the preset limit, the hydraulic valve opens automatically and high-pressure oil is released immediately. The slide then retreats by an extremely small distance, unloading the excessive forming force in time. The machine subsequently triggers an alarm and performs an emergency stop to prevent further mechanical damage. The entire process requires no manual intervention and is characterized by fast response and reliable operation.
Why It Matters?
The hydraulic overload protection system effectively safeguards critical press components by preventing serious failures such as crankshaft bending, connecting rod damage, and frame cracking caused by sudden overloads. In the event of die abnormalities or material issues, the system prioritizes the protection of both the tooling and the machine, minimizing damage to high-value dies and reducing additional investment costs. At the same time, it prevents minor abnormalities from escalating into major equipment failures, significantly reducing unplanned downtime and improving overall equipment utilization. By providing an invisible yet essential layer of safety, the system ensures a more stable, reliable, and worry-free stamping process for both operators and management.
How to Resolve Sticking Using Overload Protection Device?
Press sticking refers to a situation during stamping or forging where the ram (or hydraulic cylinder piston) becomes stuck at the bottom dead center or an abnormally low position due to excessive force, stroke limitations, abnormal hydraulic oil viscosity, or valve core jamming, preventing further movement and causing the hydraulic cylinder piston to fail to return. Hydraulic overload protection devices can relieve pressure automatically or manually to resolve jamming conditions.
1. Automatic Relief
Most modern presses feature automatic hydraulic overload protection. Upon overload detection, the system automatically releases pressure and raises the ram.
2. Manual Relief (Using World JH21 Mechanical Press as Example)
① Press the “Stop” button for the main motor to halt its operation. Release the pressure relief port of the hydraulic overload pump to relieve oil pressure (caution: oil splatter). Then tighten the bolts. At this point, the ram is in overload status, and the indicator red light illuminates.
② Turn the overload protection knob to the “Reset” position. Start the main motor, allowing it to reach rated speed, then immediately shut off the main motor, letting the flywheel coast to a stop.
③ Rotate the selector switch to the “Inch” position. Press the dual-hand button to disengage the upper and lower dies. Continue inch operation to stop the ram at the upper dead center, at which point the pump is driven.
④ After the overload protection device resets and the red indicator light turns off, return the knob to the “Normal” position.
How to Maintain the Hydraulic Overload Protection Device for Press Machines?
In addition to regularly inspecting hydraulic lines, fittings, and hoses, the core maintenance tasks for hydraulic overload protection devices involve periodically observing, replenishing, and replacing hydraulic oil.
- During daily operation, check the oil level in the overload protection reservoir before each startup. Add hydraulic fluid as needed to ensure the level remains between the upper and lower limit marks.
- Perform a complete hydraulic oil change annually. If oil degradation is suspected (e.g., emulsification, sedimentation, abnormal odor or color), consider replacing the oil earlier.
Hydraulic Oil Selection
Select industry-standard Anti-wear Hydraulic Oil 32# or 46# (adjust based on machine load, season, etc.). Ensure reliable sourcing and cleanliness compliance (e.g., meeting ISO 4406 cleanliness standards).
Filling Method (using World JH21 mechanical press as an example)
- Unscrew the oil plug on top of the overload pump
- Fill oil to the level indicator mark
- Screw the plug back on
Hydraulic Oil Replacement Procedure
- Disconnect the press’s main power supply and shut off the air source.
- Use the pressure relief valve to fully release system pressure to zero.
- Completely drain the old oil from the reservoir.
- Clean the interior of the reservoir, the filter element, and the main oil passages.
- Fill with new oil to the level indicator mark.
- Use an air-bleed tool to circulate the oil and purge air from the system.
Hydraulic Overload Protection Troubleshooting
1. Hydraulic Overload Pump Does Not Operate
Causes:
If the hydraulic overload pump does not operate, the failure is usually related to the air supply, hydraulic components, or electrical control system. Common causes include insufficient or disconnected air pressure, blockage in the air line, broken spring inside the air valve, rust or seizure of the hydraulic overload pump, or failure of the solenoid valve.
Solution:
To resolve this issue, first check the air supply and adjust the air pressure to the specified value. Inspect the air lines and repair or replace any blocked or damaged pipes. Replace broken valve springs if necessary. If the pump is rusted, clean it thoroughly and reassemble it. In addition, check the solenoid valve and repair or replace it as required.
2. Hydraulic Overload Pump Continues Pumping Oil
Causes:
Continuous oil pumping is usually caused by insufficient hydraulic oil, air trapped in the pump, oil leakage in the system, or improper adjustment of the temperature compensation valve.
Solution:
Corrective actions include replenishing hydraulic oil to the specified level, bleeding air from the hydraulic system and replacing the oil if required, repairing any oil leakage points, and adjusting the temperature compensation valve to the correct setting.
3. Unloading Occurs Below the Rated Load
Causes:
If unloading occurs before the rated load is reached, the most common reason is insufficient hydraulic pressure in the system.
Solution:
The solution is to check the system pressure using a pressure gauge and increase the hydraulic pressure to the specified value so that unloading occurs only at the rated load.
4. No Unloading at Rated Load
Causes:
When the system reaches the rated load but does not unload, the failure is usually related to malfunction of the pressure regulating valve or the temperature compensation valve. Typical causes include failure of the pressure regulating valve, resulting in excessively high air pressure, or failure of the temperature compensation valve.
Solution:
To eliminate this fault, remove and inspect the pressure regulating valve and replace it if necessary. The temperature compensation valve should also be replaced to ensure proper overload protection under varying operating conditions.
5. Overload Indicator Lamp Does Not Light Up
Causes:
If the overload indicator lamp does not illuminate during an overload condition, the problem is generally related to the limit switch or the indicator circuit. The most common cause is malfunction of the upper limit switch of the air-oil pump or a damaged indicator lamp.
Solution:
The corrective action is to inspect the upper limit switch and replace it if it is faulty. Replace the indicator lamp to restore proper overload indication.
Final Word
World Group believes that a high-quality press should not only be functional but also deliver long-term, stable, and reliable performance.
The hydraulic overload protection system is not intended to replace proper operating procedures; rather, it serves as the final line of defense to safeguard equipment when unexpected incidents occur. In long-term, high-intensity stamping production, true value is reflected not in a single smooth operation, but in the machine’s ability to protect itself at critical moments. World Group hydraulic overload protection system is engineered for safety and designed for long-term, stable operation.
