For equipment such as injection molding machines, press machines, or CNC machine tools that work periodically, the continuous high-pressure output of hydraulic pumps often means unnecessary mechanical wear and energy waste. The hydraulic unloading valve is a control component that exists to solve this problem.
This article will provide you with a step-by-step explanation of the concept, function, types, and applications of hydraulic unloading valves, providing clear technical references for your purchase comparison.
What is a Hydraulic Unloading Valve?
The hydraulic unloading valve is a pressure control device installed in a hydraulic system. When the system pressure reaches the set value, it can redirect all the flow output by the hydraulic pump back to the hydraulic tank at an extremely low pressure.
This allows the hydraulic pump to operate at a nearly zero-pressure state, thus avoiding it from continuously bearing high-pressure loads during non-working periods.
Functions of the Hydraulic Unloading Valve
Reduce the Load Pressure of the Pump
The main function of a hydraulic unloading valve is to redirect the output flow of the pump back to the oil tank when the system reaches a predetermined pressure. This prevents the pump from continuing to output under high pressure, allowing the pump to operate in a state of almost no load. This effectively reduces the pressure that the pump has to bear.
System Overpressure Protection
The hydraulic unloading valve should start unloading promptly before the pressure approaches the safe upper limit. It can quickly release pressure to prevent it from continuing to rise.
This can avoid damage to key components such as hydraulic pumps, pipelines, and actuators caused by overpressure, providing a reliable pressure safety barrier for your hydraulic system.
Energy Conservation
During the stage where a high-pressure oil supply is not required, the hydraulic unloading valve causes the pump to operate in a low-pressure mode.
Compared to continuous high-pressure output from pumps, this can reduce energy consumption. In standby mode, the hydraulic unloading valve can reduce the energy consumption of the hydraulic pump by about 80%.
At the same time, it can also reduce the heat generated by the system due to high pressure, thereby alleviating the workload of the cooling devices.
Automatic Pressure Reset and Stable Maintenance
When the system pressure drops to the set reset threshold, the hydraulic unloading valve will automatically close the bypass channel. At this point, all the flow from the hydraulic pump can be redirected back into the working circuit, quickly restoring the system pressure.
This automatic switching mechanism can make sure that the system pressure remains within the set range and promptly responds to the immediate operational demands of the actuating elements.
How does the Hydraulic Unloading Valve Work？
The hydraulic unloading valve is mainly composed of a valve body, a spool valve core, and a pressure regulating spring. In the hydraulic circuit, it should be used in conjunction with the one-way valve.
The one-way valve can be integrated inside the unloading valve or installed independently on the external pipeline. The one-way valve performs physical isolation during the unloading stage. It separates the hydraulic pump in a low-pressure circulation state from the system’s operating end that remains in a high-pressure state.
When the pressure output by the pump is lower than the set value, the elastic force of the spring will keep the valve core in the closed position. At this time, all the oil output by the hydraulic pump is transported along the working channel to the actuator.
When the system pressure rises and reaches the unloading set pressure, the hydraulic pressure overcomes the spring force and drives the main valve core to move, thereby opening a passage directly from the pump outlet to the oil tank.
At this point, the oil discharged by the hydraulic pump flows back to the oil tank directly through the bypass channel in a low-pressure state. The pump enters an approximately unloaded operating state. At the same time, the one-way valve immediately closes to maintain high pressure at the system execution end.
When the system pressure drops to the preset lower threshold due to work consumption, the spring pushes the valve core back to its original position and closes the bypass channel. The hydraulic pump immediately resumed supplying oil to the working circuit of the system.
Types of Hydraulic Unloading Valves
Direct-Acting Hydraulic Unloading Valve
The structure of the direct-acting hydraulic unloading valve is simple and compact. The internal valve core is directly controlled by the spring and system pressure to open and close.
Since the valve core is directly driven by liquid pressure, the direct-acting hydraulic unloading valve has a fast response speed. You can use it for hydraulic systems with low pressure and medium flow rate.
Pilot-Operated Hydraulic Unloading Valve
The pilot-operated hydraulic unloading valve is the most common type in industrial hydraulic systems. Its structure is more complex than the direct-acting type. It uses a small pilot control element to trigger the opening and closing of the main valve.
This structure can control a huge main oil circuit with a small flow rate. The pilot-operated hydraulic unloading valve can achieve more stable unloading under high-pressure and large flow conditions. It is suitable for the pressure control requirements of large-scale industrial hydraulic systems.
High-Low Pump Hydraulic Unloading Valves
The high-low pump hydraulic unloading valves are specifically designed for circuits driven by a high-flow low-pressure pump and a low-flow high-pressure pump, such as log splitters, garbage compressors, or presses. It is controlled separately according to the system pressure.
During the low-pressure stage, the valve is in a closed position. At this point, the output oil from the high-flow pump and the low-flow pump converge and jointly supply oil to the system to achieve rapid movement.
When the system pressure rises to the preset unloading pressure, the valve automatically opens. The output of the high-flow pump is directly bypassed back to the oil tank. At this time, the system only continues to provide high-pressure and low-flow output from the small flow pump.
High-low pump hydraulic unloading valves achieve reasonable power matching by automatically unloading large pump loads. This greatly reduces energy consumption under high-pressure conditions.
Applications of The Hydraulic Unloading Valve
Industrial Manufacturing
In the hydraulic fixture system of CNC router machines or machining centers, once the workpiece is clamped, the hydraulic cylinder is mainly in a static state of maintaining pressure, which requires almost no flow replenishment.
At this point, the hydraulic unloading valve allows the hydraulic pump to unload and idle. This prevents the system from generating excessive heat due to high-pressure overflows during long periods of cutting operations.
This can help you save energy. More importantly, it maintains the stability of the hydraulic oil temperature and prevents deformation due to heat from affecting the machining accuracy of the machine tool.
Engineering Machinery
In the hydraulic circuit of excavators, bulldozers and other equipment, when the actuators such as the boom and boom stop moving and maintain their position, the hydraulic unloading valve opens for unloading.
This avoids long-term high-pressure output of the hydraulic pump, reduces energy consumption and component wear during equipment operation.
Agricultural Machinery
In agricultural machinery such as harvesters, rice transplanters, and tractors, hydraulic unloading valves are used in scenarios such as positioning after lifting agricultural tools and stopping the drive of hydraulic motors.
For example, after the cutter head of the harvester is raised to the correct position, the hydraulic unloading valve starts to unload, reducing the load on the engine driven by the engine.
During the suspension of agricultural tools on the tractor, the hydraulic unloading valve keeps the pump in an unloaded state, reducing fuel consumption. It can well meet your intermittent operation requirements of agricultural work.
Material Handling Equipment
In forklifts, stackers, and conveyors, when the fork is lifted to the designated height and held, and the conveyor stops material transportation, the hydraulic unloading valve opens to unload, allowing the hydraulic pump to operate at low load.
In the hydraulic circuit of the stacker crane, the hydraulic unloading valve can unload promptly after the stacker crane is positioned and stopped. It keeps the stable operation of your equipment during high-frequency start-stop operations.
Hydraulic Accumulator System
Accumulators are usually equipped in hydraulic systems such as hydraulic clamping, emergency braking, and high-pressure water supply. The hydraulic unloading valve can unload the hydraulic pump after the accumulator has been pressurized to the set pressure. Then, the accumulator will continuously supply oil to the system or maintain the pressure.
When the accumulator pressure drops below the reset threshold, the hydraulic unloading valve closes and the pump resumes pressurizing the accumulator.
This is commonly used in equipment that requires intermittent high flow or long-term static pressure maintenance. The hydraulic unloading valve can prevent energy waste and high oil temperature caused by the pump continuing to overflow under high pressure after the accumulator is filled.
Differences between Hydraulic Unloading Valves and Hydraulic Relief Valves
Different Purposes
The main purpose of the hydraulic unloading valve is to achieve energy-saving operation of the pump. When the system does not require high-pressure oil, the hydraulic unloading valve opens a low-pressure bypass channel to allow all the oil output by the pump to flow back to the oil tank. This helps to reduce energy consumption and minimize component wear.
The main purpose of a hydraulic relief valve is to maintain the stability of system pressure. When the system pressure exceeds the set value, the hydraulic relief valve only overflows excess oil beyond the working demand back to the oil tank. It always keeps the pressure supply of the working circuit. It can maintain the stability of the system pressure and protect against overpressure.
Different Oil Flow Directions and Pump Loads
When the hydraulic unloading valve operates, it will completely change the flow direction of the oil. All the flow generated by the pump will flow back to the oil tank at a low pressure through the bypass channel. At this point, the pump is operating in a nearly unloaded state, while the working circuit temporarily stops supplying oil.
When the overflow valve is working, the oil flows to the working circuit to supply oil to the actuator. Only when the system pressure exceeds the limit will some of the excess oil flow back through the overflow channel. The pump is always operating under pressure and continuously provides pressure support to the system.
Differences in Applicable Working Conditions
The hydraulic unloading valves are typically used in conditions where the system is in standby mode, the actuator is stopped, or the accumulator is fully charged and does not require continuous output of high pressure.
The hydraulic relief valve is suitable for stable pressure control during the normal operation of the system. It offsets system pressure fluctuations through continuous overflow regulation which keeps the pressure stable within the set range.
Different Pressure Control Characteristics
The pressure control of the hydraulic unloading valve is manifested as a switching characteristic. When the system pressure reaches the unloading threshold, the hydraulic unloading valve quickly opens. When the pressure drops to a lower reset threshold, the valve quickly closes. The entire switching process responds quickly and there is no intermediate transitional state.
The pressure control of the hydraulic relief valve is manifested as a regulating characteristic. The valve opening will be dynamically adjusted according to the real-time changes in system pressure. It balances system pressure by changing the size of the overflow channel. The hydraulic relief valve can achieve smooth pressure control and prevent sudden pressure changes.
Hydraulic Unloading Relief Valve
The hydraulic unloading relief valve is a composite hydraulic control valve that combines unloading and overflow functions.
When the system needs to be unloaded in standby mode, the valve opens the full flow bypass channel to allow the hydraulic pump to operate without load. When the system enters a working state and requires stable pressure, the valve switches to overflow mode. It maintains the system pressure at the set value through dynamic overflow regulation.
You can use the hydraulic unloading relief valve for hydraulic systems that require frequent stand-by operation and stable pressure during operation, such as injection molding machines, press machines and hydraulic power units.
FAQs
Does the Hydraulic Unloading Valve Require an Electrical Control Signal to Operate?
Not necessarily. The hydraulic unloading valve can be of a mechanical type, which automatically switches based on internal springs and oil pressure.
Some hydraulic unloading valves can be integrated with electromagnetic directional valves. You can control the on-off of the electromagnet by sending electrical signals, thereby achieving remote control of the unloading process.
What are the Reasons for a Hydraulic Unloading Valve not to Unload?
It could be that the valve core is stuck, there is dirt blocking inside the valve, or the inlet circuit connection is not smooth.
You can check if there are any foreign objects inside the hydraulic unloading valve and clean them. Make sure the control oil passage is not blocked or damaged, and also check whether the springs and seals are in good condition.
What are the Unloading Pressure and Reset Pressure of the Hydraulic Unloading Valve?
The unloading pressure determines when the pump begins to unload. The unloading pressure is generally set to be higher than the maximum working pressure required by the system’s actuating elements.
The reset pressure determines when the pump will be reloaded. The reset pressure is usually 80%-90% of the unloading pressure.
What are the Precautions for the Installation and Pipeline Connection of Hydraulic Unloading Valves?
When installing, you need to place the hydraulic unloading valve near the outlet of the hydraulic pump. This can shorten the transmission path of the pressure signal and avoid response delay.
The pipeline connection should have the oil flow direction consistent with the valve body identification. The pipeline diameter should not be less than the valve interface specification to reduce throttling losses.
Final Thoughts
The hydraulic unloading valves not only automatically adjust the pump output path based on pressure, but also play an important role in improving system efficiency and protecting equipment.
As a professional supplier of hydraulic components, Target is committed to providing you with high-quality hydraulic unloading valves. We also provide full technical support from system circuit analysis, selection to installation and debugging guidance. If you have any further questions about the hydraulic unloading valve, please feel free to contact us at any time！
