Hydraulic Press

Unloading Circuits of Hydraulic Press

Unloading Circuits of Hydraulic Press

Estimated reading time: 18 minutes

The unloading circuit is when the circuit is not working so that the oil pump is running without load. The advantages are longer pump life, reduced power consumption, reduced system heat generation, and increased efficiency and safe operation.

Question 1: Choice of unloading method

1. Unloading circuit with reversing valve

Figure 1-1 is one of the simplest methods of unloading, generally suitable for systems with small flow rates, for high pressure and large flow rates (>3.5MPa, 40L/min) the circuit will produce shocks.

Unloading Circuits of Hydraulic Press

1-1 Unloading circuit with reversing valve

2. Unloading circuit with a two-way valve

Figure 1-2 shows a manually operated two-way valve to unload the circuit. Fig. 1-3 shows a two-way valve operated by a cam so that when the cylinder reaches the end of its return stroke, the two-way valve is operated by itself and the oil from the pump is connected to the tank.

Unloading Circuits of Hydraulic Press

1-2 Unloading circuit with a manual two-way valve

Unloading Circuits of Hydraulic Press

1-3 Unloading circuit with a motorized two-way valve

3. Unloading circuit using an accumulator

The use of accumulator unloading circuit, commonly used in the clamping device, because the circuit with accumulator, in the long-term clamping work, can use the accumulator to the circuit to pressure and supplement the leakage of the components, and the oil pump only intermittently with load work. In this circuit, there is generally a check valve.

Figure 1-4 uses a hydraulic two-way valve, which is opened when the circuit reaches a certain pressure requirement, allowing the pump to be unloaded. Figure 1-5 shows a circuit using a pressure relay and a solenoid two-way valve. When the pressure in the circuit reaches a certain value, the pressure relay operates the solenoid valve to unload the pump.

Unloading Circuits of Hydraulic Press

1-4 Unloading circuit with a hydraulic two-way valve

Unloading Circuits of Hydraulic Press

1-5 Unloading circuit with pressure relay and solenoid 2-way valve

4. Unloading circuit using a liquid-controlled relief valve

In Figure 1-6, the remote control port of the relief valve is connected to the solenoid two-way valve, which can be widely used in automatic control systems due to the use of solenoid valves, which are used in general machinery and forging machinery. The solenoid valve is controlled by a pressure relay in the circuit, so that when the oil pressure in the circuit reaches a certain pressure, the solenoid two-way valve opens, allowing the oil pump to unload. The one-way valve is designed to maintain the pressure in the circuit when the oil pump is unloaded. In this circuit, the solenoid two-way valve only discharges oil flow through the remote port of the relief valve, which is not a large flow rate, so a small size two-way valve can be used.

Unloading Circuits of Hydraulic Press

1-6 Unloading circuit of a liquid-operated relief valve

Unloading Circuits of Hydraulic Press

1-7 Unloading circuit of a liquid-operated relief valve

The situation in Figure 1-7 is similar to the above, except that a sequence valve is used to operate the hydraulic two-way valve and control the pressure in the circuit.

As the relief valve is fitted with a control line, the volume of the control chamber is increased, which will produce an unstable action, for this reason, a damper is added to the line to improve its performance.

5. The use of composite pump unloading circuit

In the machine tool, rolling steel, and other equipment operation process, the cylinder needs a large amount of oil and high-speed work, two pumps at the same time to the circuit to send oil (such as machine tools workpiece feeding process), as shown in Figure 1-8. However, the cylinder advances to contact the workpiece, so that when the oil pressure rises, the unloading valve opens, then the low-pressure large oil pump no-load operation, only by the high-pressure small oil pump to the circuit oil supply.

Unloading Circuits of Hydraulic Press

1-8 Unloading circuit with compound pump

6. Circuit with pressure compensation variable pump

The circuit shown in Figure 1-9 is a variable oil pump that compensates (controls) the amount of oil according to the output pressure of the pump, which saves the power of the pump. As shown in Figure 1-9, using a reversing valve with neutral blocking (O type), when the valve is in the middle position, the pump delivers only the amount of oil that the valve leaks, thus saving power.

Unloading Circuits of Hydraulic Press

1-9 Circuit with pressure-compensated variable pump

7. Unloading circuit for multi-cylinder systems

As shown in Figure 1-10, a pump to more than two cylinders when supplying oil, the four-way valve and two-way valve connected in action, when the cylinder reversing valve is in the middle position, the pump is no load operation (this circuit is currently used in the multi-way reversing valve circuit).

Unloading Circuits of Hydraulic Press

1-10 Unloading circuit for a multi-cylinder system

Question 2: Selection of unloading valves

Firstly, note the difference between the unloading relief valve and the external control sequence valve for the unloading valve, unloading relief valve is mainly used in the hydraulic circuit equipped with an accumulator, when the accumulator filling pressure reaches the set pressure of the unloading relief valve, it automatically makes the hydraulic pump unloading. The unloading relief valve, fitted with a check valve, is used to prevent the pressure oil in the accumulator from flowing backward when the accumulator supplies oil to the system and maintains the pressure. When the fluid pressure in the accumulator drops to around 85% of the set pressure of the unloading relief valve, the unloading relief valve closes and the hydraulic pump resumes filling the accumulator.

Unloading Circuits of Hydraulic Press

1-11 Pilot-operated unloading relief valves are preferred for hydraulic systems with long unloading times

1 – hydraulic pump  2, 3 – valve

Secondly, hydraulic systems with long periods of unloading should use pilot-operated unloading relief valves. Figure 1-11 (a) shows a hydraulic system that requires long intervals of action and high-speed movement of the actuating element. When the hydraulic cylinder stops moving, the outlet pressure of hydraulic pump 1 is high and low, and it cannot unload continuously, resulting in high system power consumption and high oil temperature. This is due to a component in the circuit or pipeline leakage, external control sequence valve repeated opening and closing caused by. Therefore, Figure 1-11 (b) choose the pilot unloading relief valve to replace the original circuit valve 2 and valve 3, unloading, the plunger to the pilot valve spool to apply an additional thrust to ensure that the pump 1 unloading path is open, even if the circuit leaks to reduce the pressure in the accumulator, it can make pump 1 in a continuous unloading state to meet the system requirements.

Question 3: Design of the unloading circuit

In the system shown in Figure 1-12, the hydraulic pump is quantitative, the neutral function of the three-way reversing valve is Y-type, and the hydraulic cylinder does not act when the three-way reversing valve returns to the neutral position. System unloading is by the pilot-operated relief valve and two-position two-way solenoid valve unloading circuit, this time can be remote control port through the small solenoid valve and oil tank connected when the electromagnet power off, two-position two-way solenoid valve access is cut off, the system works normally; when the electromagnet power on, the two-position two-way solenoid valve is connected, so the pressure on the upper part of the main spool of the relief valve is close to zero, the spool up to pick up the highest position, because the valve The upper part of the spool spring is soft, so the pressure at the pressure oil port is very low at this time, and the relief valve makes the whole system unload at low pressure.

Unloading Circuits of Hydraulic Press

1-12 Pilot Operated relief valve pressure control circuit

Problems:

When the hydraulic system was installed and commissioned, the system experienced violent vibration and noise.

It was found that the vibration and noise were generated by the relief valve. Disassembly and inspection of the relief valve, the valve parts, moving parts with clearance, valve cleanliness, installation, and other aspects are in line with the design requirements. The relief valve was tested on the test bench, the performance parameters were normal, and the failure occurred when the system was installed.

After repeated testing and analysis, found that the unloading circuit, the relief valve remote control port to the two-position two-way solenoid valve between the population of the pipeline length is short, the relief valve does not produce vibration and noise, when the pipeline length is greater than 1m, the relief valve will produce vibration, and abnormal noise.

The problem is caused by increasing the volume of the control chamber of the relief valve (the front chamber of the pilot valve). The larger the volume of the cavity, the more unstable, and the long pipeline is prone to residual – some air, so that the oil in the cavity in the two-position two-way reversing valve pass or break, the pressure fluctuation is larger, causing the quality of the valve (or the main valve) spring system self-excited oscillation and generate noise, such noise is also known as high-frequency whistling sound.

Unloading Circuits of Hydraulic Press

Solution:

When remote pressure regulation or unloading of the relief valve, the shorter and thinner the remote control pipeline, the better, reduce the volume of the cavity or set a fixed damping hole to reduce pressure shock and pressure fluctuations. The fixed damping orifice is a fixed throttling element, which should be installed as close as possible to the remote control port of the relief valve, separating the control chamber of the relief valve from the control pipeline, so that the fluid pressure shock and fluctuation will be quickly attenuated, effectively eliminating the vibration and whistling sound of the relief valve.

As the oil from the remote control port of the relief valve is throttled back to the tank, it will increase the pressure of the oil in the control cavity and so the system discharge pressure will be increased accordingly. To prevent the system unloading pressure from increasing excessively, the damping hole of the fixed throttling element should not be too small, as long as it can eliminate vibration and noise. Moreover, if the holes are too small, they will be easily blocked and the system will not be able to unload. In practice, larger and longer damping holes have proven to be more effective in controlling fluid stability than shorter and thinner damping holes.

Question 4: Problems in the design of the unloading circuit

1. Hydraulic clamping system workpiece loosening problem

As shown in Figure 1-13 (a) for the machine tool-workpiece clamping hydraulic system, when the sequence valve 1 closed, the hydraulic pump can not immediately fill the accumulator, resulting in the workpiece is often loose or even fall off phenomenon, this can be used Figure 1-13 (b) system, the work of manual reversing valve set to the left position, the pump to the accumulator and hydraulic cylinder oil supply, and push the piston right. When in contact with the workpiece, the system pressure rises until the pressure relay sends a signal to energize solenoid valve 2 and unload the pump through valve 3, the accumulator maintains the system pressure and replenishes the system leakage. When the pressure drops to the lower limit of the pressure relay, the relay de-energizes the solenoid valve and the hydraulic pump continues to supply oil to the system and the accumulator. This example illustrates the design of an accumulator to maintain system pressure when unloading the circuit, – must make the hydraulic circuit design composition reasonable. When the workpiece is filled with oil, the workpiece is often loose or even fallen off, then the system is shown in Figure 1-13 (b) can be used, the manual reversing valve is set to the left position, the pump supplies oil to the accumulator and the hydraulic cylinder, and pushes the piston to the right. When in contact with the workpiece, the system pressure rises until the pressure relay sends a signal to energize solenoid valve 2 and unload the pump through valve 3, the accumulator maintains the system pressure and replenishes the system leakage. When the pressure drops to the lower limit of the pressure relay, the relay de-energizes the solenoid valve and the hydraulic pump continues to supply oil to the system and the accumulator. This example shows that when designing an unloading circuit with an accumulator to maintain system pressure, it is important to make the hydraulic circuit design reasonable.

Unloading Circuits of Hydraulic Press

1-13 Workpiece clamping hydraulic system

a. hydraulic system for clamping workpieces on machine tools  1-sequence valve 2-hydrodynamic valve 3-relief valve

b. Improved hydraulic system for clamping workpieces on machine tools

1-pressure relay 2-solenoid valves 3- relief valves

2. Poor unloading of a two-position two-way valve unloading circuit

Unloading Circuits of Hydraulic Press

1-14 Unloading circuit with a two-position two-way valve

1 – hydraulic pump  2 – two-position two-way solenoid valve

3 – relief valve

Unloading Circuits of Hydraulic Press

1-15 Unloading circuit using a small two-position two-way valve to control a pilot-operated relief valve

1-hydraulic pump 2-relief valve

3-two-position, two-way solenoid valve

Unloading Circuits of Hydraulic Press

1-16 Unloading circuit with externally controlled relief valve

Figure 1-14 shows the unloading circuit using a two-position two-way valve. The hydraulic pump is a high-pressure, high-flow quantitative pump, the pump outlet is connected to a two-position two-way solenoid valve when the system is working, the two-position two-way valve is energized, cutting off the hydraulic pump outlet to the tank channel, the hydraulic pump output pressure oil into the system, the actuating element begins to work. When the actuating element stops working, the solenoid is de-energized and the oil output from the pump is directed to the tank via the two-position two-way valve to unload the system. However, it was found that when the two-position two-way valve was de-energized, the system could not be completely unloaded and caused the system to heat up.

The reason for the system not being fully unloaded is that the specification of the two-position two-way valve does not match the output flow of the hydraulic pump. Generally speaking, the flow rate through the two-position two-way valve should be equal to the output flow rate of the hydraulic pump when unloading, and not less than the output flow rate of the hydraulic pump. If the two-position two-way valve specifications can not be hydraulic pump output flow all back to the tank, will inevitably make the pump outlet pressure increased, through the two-position two-way valve hydraulic pressure difference increases, so that the two-position two-way valve act as a throttle, so the hydraulic pump can not completely unload, and cause the system to heat up.

This example shows that such an unloading circuit is only suitable for hydraulic systems with a small flow rate, i.e. usually for hydraulic pumps with a flow rate of less than 63L/min. High pressure and high flow systems should use other forms of unloading circuits, such as the one shown in Figure 1-15, which uses a small two-position two-way valve to control a pilot-operated relief valve.

When the two-position two-way solenoid valve 3 is energized, the remote control port of relief valve 2 leads to the tank and the pressure oil output from the hydraulic pump 1 opens the relief valve 2 at very low pressure, and all flows back to the tank to achieve unloading. The size of the unloading pressure depends on the strength of the main valve spring of the relief valve and is generally 0.2 to 0.4 MPa. valve 3 only needs to control the oil flowing out of the oil circuit through the relief valve 2, thus allowing the use of a smaller valve specification and allowing remote control. When the solenoid valve is disconnected, this unloading circuit also has a far more moderate ramp-up process than the circuit shown in Figure 1-14.

However, when valve 3 is connected to the remote control port of valve 2, the volume of the control chamber of valve 2 increases, which may produce instability in the work, and for this reason, a damping device is often set up on the connecting oil circuit. The unloading circuit using externally controlled relief valves is also of this type, as shown in Figure 1-16. It unloads the hydraulic pump and at the same time ensures that the hydraulic cylinders are fed quickly and that the work is fed sequentially. In general, the unloading circuit needs to make the unloading pressure as low as possible (for this reason, the components in the circuit should be as few as possible, the pipeline should also be as short as possible), but also to make the system in the working state and the unloading state when switching between each other as smooth as possible, small impact, low noise.

3. The problem of incomplete unloading

Unloading Circuits of Hydraulic Press

1-17 Oil pump unloading circuit with accumulator pressure retention

1-hydraulic pump 2-hydraulic sequence valve 3-check valve 4-accumulator 5-relief valve 6-two-position, two-way, liquid-operated directional valve

As shown in Figure 1-17(a), when the pressure of the accumulator 4 rises to the pressure set by the unloading valve (liquid-controlled sequence valve) 2, valve 2 opens, hydraulic pump 1 unloads, check valve 3 closes, and the system maintains pressure (pressure maintenance); when the system pressure is lower than the pressure set by valve 2, valve 2 closes, and pump 1 re-supplies pressure oil to the system. The relief valve 5 acts as a safety valve at this time. The main fault of this circuit is that the unloading is not complete and there is a loss of power.

The reason for this – failure: when the pressure rises, the liquid-controlled sequence valve 2 is only partially opened as is the relief valve so that the hydraulic pump 1 is unloaded, thus causing a loss of power.

Solution:

Firstly, Use the small hydraulic sequence valve 2 as a pilot valve to control the opening of the main relief valve 5, as shown in Figure 1-17(b), to ensure that valve 5 is fully open when unloading.

Secondly, The circuit shown in Fig. 1-17(c) is used to open the two-position three-way liquid-operated reversing valve 2 first with the pressure of the accumulator (system), and then to make the two-position two-way liquid-operated reversing valve 6 open fully, thus ensuring that the main relief valve 5 opens fully, thus allowing the pump 1 to be fully unloaded.

4. Unloading instability

Unloading Circuits of Hydraulic Press

1-18 Unloading circuit with pressure relay and solenoid relief valve

1-hydraulic pump 2-check valve 3,3′- Pressure relay 4-accumulator 5-solenoid relief valve

The accumulator circuit in Figure 1-18 uses a pressure relay 3 to control the unloading or working of the oil pump. The main fault that occurs in this circuit is that the system pressure fluctuates back and forth around the regulated pressure value of pressure relay 3 (return interval) and the oil pump frequently appears, which can greatly shorten the service life of the oil pump.

The solution is to use a differential pressure control with dual pressure relays as shown in Figure 1-18(b). The pressure relays 3 and 3′ are adjusted to the high and low-pressure adjustment values, the unloading of the oil pump is controlled by the high-pressure adjustment value, while the re-working of the oil pump is controlled by the low-pressure adjustment value so that when the oil pump is unloaded, the accumulator continues to discharge oil until the pressure gradually decreases to below the low-pressure adjustment value when the oil pump re-working. There is an interval in between, thus avoiding the phenomenon of frequent switching.

1 thoughts on “Unloading Circuits of Hydraulic Press

  1. Avatar of Maksud Sayidov Maksud Sayidov says:

    Great articles

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