Pressure Control Circuits

I. Pressure Regulation Circuits

The operating pressure of a hydraulic system must match the load it needs to handle. When a hydraulic system uses a fixed-displacement pump, the pump's operating pressure can be adjusted using a relief valve. For variable-displacement pumps, the operating pressure primarily depends on the load, with a safety valve limiting the maximum system pressure to prevent overloads. When a system requires two or more different pressures, multi-stage pressure regulation circuits can be employed. The choice between hydraulic fluid vs transmission fluid can significantly impact how these pressure regulations perform under different operating conditions.

1. Single-stage Pressure Regulation Circuit

Figure 7-1 illustrates a single-stage pressure regulation circuit. The system is supplied by a fixed-displacement pump, with a throttle valve regulating the flow into the hydraulic cylinder to achieve the required piston speed. The pump output flow exceeds the cylinder's requirements, with excess oil returning to the tank through the relief valve. In this configuration, the relief valve remains partially open, maintaining constant pump outlet pressure equal to the relief valve setting.

Adjusting the relief valve changes the pump's supply pressure. The relief valve setting must exceed the sum of the maximum cylinder working pressure and all pressure losses in the circuit. Proper fluid selection, considering hydraulic fluid vs transmission fluid properties, ensures optimal valve operation and pressure stability.

Figure 7-1: Single-stage pressure regulation circuit showing the relationship between pump, relief valve, and hydraulic cylinder

2. Remote Pressure Regulation and Two-stage Pressure Regulation Circuits

Figure 7-2 shows a remote pressure regulation circuit where a remote pressure regulating valve 2 is connected to the remote control port of a pilot-operated main relief valve 1. This allows the pump outlet pressure to be adjusted remotely. The remote pressure regulating valve functions similarly to the pilot valve of the main relief valve, with most oil still flowing through the main relief valve. When comparing hydraulic fluid vs transmission fluid in such systems, the viscosity characteristics and response to pressure changes become particularly important factors.

Figure 7-2: Remote pressure regulation circuit with 1-pilot-operated main relief valve and 2-remote pressure regulating valve

The remote pressure regulating valve has a similar structure and operating principle to the pilot valve in a relief valve. The maximum pressure adjusted by the remote valve must be lower than the setting of the pilot-operated main relief valve; otherwise, the remote valve will have no effect. During remote regulation, the pilot valve in the main relief valve remains closed.

Multi-stage pressure regulation can also be achieved using the remote control port of a pilot-operated main relief valve 1 and a remote pressure regulating valve 2. Many hydraulic systems, particularly those where cylinder extension and retraction require significantly different pressures, benefit from two-stage pressure regulation as shown in Figure 7-3 to reduce power loss and fluid heating. The thermal properties considered in hydraulic fluid vs transmission fluid comparisons directly affect the efficiency of such systems.

When the piston moves to the right, the load is large, and pressure is regulated by the high-pressure relief valve 1. When the piston moves to the left, the load is small, and pressure is regulated by the low-pressure relief valve 2. When the piston reaches the end of its leftward stroke, all pump flow returns to the tank through the low-pressure relief valve, reducing power loss during the return stroke. Municipal solid waste treatment equipment hydraulic systems are typical applications of this basic circuit.

II. Pressure Reduction Circuits

In hydraulic systems where a single pump supplies multiple actuators, the main circuit pressure is regulated by a relief valve. When a branch circuit requires a lower pressure than the relief valve setting or needs more stable pressure, a pressure reduction circuit is employed. These circuits are particularly sensitive to fluid characteristics, making the hydraulic fluid vs transmission fluid comparison relevant for optimal performance.

Figure 7-4 shows a pressure reduction circuit commonly used in clamping mechanisms. A pressure reducing valve is connected in series in the line to the clamping cylinder, providing lower, stable clamping force. The outlet pressure of the reducing valve can be adjusted from 0.5MPa up to the relief valve setting.

The reducing valve maintains stable outlet pressure even when system pressure fluctuates or the load changes. The check valve in Figure 7-4 maintains pressure temporarily when main circuit pressure drops below the reducing valve setting (e.g., during rapid cylinder movement in the main circuit), preserving clamping force. This pressure stability is influenced by fluid properties, making the hydraulic fluid vs transmission fluid decision important in such applications.

For safety, clamping circuits often use spring-centered 4-way solenoid valves with positioning or fail-safe clamping circuits to prevent workpiece release during electrical failures. Control and lubrication circuits, which typically operate at lower pressures than the main circuit, also utilize pressure reduction circuits.

Figure 7-4: Pressure reduction circuit showing reducing valve and check valve configuration

Pressure reduction circuits find applications in various industrial systems where different actuators require different pressure levels. Common examples include machine tool clamping systems, robotic grippers, and material handling equipment. In each case, the performance and reliability of these circuits depend on proper component selection, system design, and appropriate fluid choice when evaluating hydraulic fluid vs transmission fluid options for specific operating conditions.

III. Unloading Circuits

When actuators in a hydraulic system stop working temporarily (e.g., during workpiece measurement or loading/unloading), the hydraulic pump should be unloaded to run under no-load conditions. This reduces power loss, minimizes fluid heating, extends pump life, and avoids frequent motor starts/stops. Larger hydraulic pumps should ideally start with the motor under light load in an unloaded state. The thermal characteristics considered in hydraulic fluid vs transmission fluid comparisons are particularly relevant in these circuits, as reduced heating directly impacts fluid performance over time.

1. Unloading Circuits Using Main Directional Valves

Main directional valve unloading uses the center position of a 3-position valve to connect the pump to the tank. The valve must use center configurations like M-type, H-type, or K-type. Figure 7-5 shows an unloading circuit using an M-type center 4-way directional valve. This simple design can cause shock at high pressures and flows, making it suitable for low-pressure, low-flow applications.

Figure 7-5: Unloading circuit using M-type center 4-way directional valve

Figure 7-6: Unloading circuit with check valve maintaining control pressure

For larger flows, hydraulic or electro-hydraulic directional valves can be used, but a check valve should be installed in the circuit (Figure 7-6) to maintain 0.3~0.5MPa pressure during unloading, ensuring sufficient starting pressure for control circuits. This pressure maintenance is influenced by fluid characteristics, making the hydraulic fluid vs transmission fluid comparison important for system efficiency.

The selection of an appropriate unloading method depends on system requirements, including pressure levels, flow rates, cycle times, and energy efficiency goals. Regardless of the method chosen, proper circuit design must consider fluid characteristics, with careful evaluation of hydraulic fluid vs transmission fluid properties to ensure optimal performance, longevity, and efficiency. Unloading circuits represent an essential aspect of modern hydraulic system design, contributing significantly to energy conservation and system reliability.

IV. Pressure Boosting Circuits

Pressure boosting circuits increase pressure in specific branches of a hydraulic system. They enable achieving higher working pressures using lower-pressure hydraulic pumps, resulting in energy savings. These circuits subject fluids to extreme pressure differentials, making the hydraulic fluid vs transmission fluid comparison critical for system performance and safety.

1. Pressure Boosting Circuits Using Boosters

As shown in Figure 7-10, a booster 4 consists of two parts: a large cylinder a and a small cylinder b, with large and small pistons connected by a single piston rod. When hydraulic oil from pump 1 enters large cylinder a through directional valve 3, pushing the piston to the right, high-pressure oil is output from the small cylinder.

The pressure amplification principle is based on Pascal's law: the pressure acting on the large piston generates force, which is transmitted to the small piston, resulting in higher pressure due to the smaller area. This pressure multiplication effect enables significant pressure increases from standard system pressures.

The performance of these systems is heavily influenced by fluid characteristics. When evaluating hydraulic fluid vs transmission fluid for booster circuits, factors such as viscosity index, shear stability, and pressure-temperature behavior become particularly important to ensure reliable operation and prevent premature component wear.

Figure 7-10: Pressure boosting circuit with 1-hydraulic pump, 2-relief valve, 3-directional valve, 4-booster, 5-working cylinder

Pressure boosting circuits are widely used in applications requiring intermittent high-pressure operation, such as press machines, clamping systems, and heavy lifting equipment. By providing high pressure only where and when needed, these circuits offer significant energy savings compared to systems operating at high pressure continuously. Proper fluid selection, considering the unique demands of boosted pressure environments, ensures reliable operation. The hydraulic fluid vs transmission fluid evaluation must account for the extreme pressure conditions and potential for increased fluid shear in these specialized circuits.

V. Balancing Circuits

To prevent vertical hydraulic cylinders and vertically moving components from descending due to their own weight or accelerating downward uncontrollably, balancing circuits are employed. These circuits incorporate sequence valves in the return line of vertical cylinders to create appropriate resistance that balances the weight. The dynamic characteristics of the fluid, including considerations in the hydraulic fluid vs transmission fluid comparison, play a vital role in maintaining stable motion control.

1. Balancing Circuits Using Check Sequence Valves (Counterbalance Valves)

Figure 7-12 shows a balancing circuit using a check sequence valve. The setting pressure of the check sequence valve should slightly exceed the pressure created in the cylinder's lower chamber by the weight of the moving components. When the cylinder is not operating, the check sequence valve remains closed, preventing the moving components from descending under their own weight.

When pressure is applied to the upper chamber of the cylinder, the lower chamber's back pressure opens the sequence valve once it exceeds the set pressure. With the weight balanced, no overspeed occurs. When hydraulic oil flows through the check valve into the cylinder's lower chamber, the piston rises.

This circuit may experience gradual lowering of the moving components when stopped due to sequence valve leakage, requiring valves with minimal leakage. The back pressure in the return chamber results in significant power loss. Fluid characteristics affect valve performance, with the hydraulic fluid vs transmission fluid comparison focusing on viscosity stability and leakage prevention.

Figure 7-12: Balancing circuit using a check sequence valve

2. Balancing Circuits Using Hydraulically Controlled Check Sequence Valves

Figure 7-13: Balancing circuit using a hydraulically controlled check sequence valve

Figure 7-13 shows a balancing circuit using a hydraulically controlled check sequence valve, suitable for applications where the balanced weight varies, such as crane lifting mechanisms. When the directional valve is shifted to the right position, hydraulic oil flows through the check valve into the cylinder's lower chamber, with return oil from the upper chamber flowing directly to the tank, raising the piston and lifting the load.

When the directional valve shifts to the left position, hydraulic oil enters the cylinder's upper chamber and the control port of the hydraulically controlled sequence valve, opening the valve and allowing return oil from the lower chamber, lowering the piston and the load.

If the load causes the moving components to descend too rapidly, the pressure in the cylinder's upper chamber decreases, causing the hydraulically controlled sequence valve to close partially, increasing resistance and preventing rapid descent. When the directional valve shifts to the neutral position, the upper chamber depressurizes, closing the hydraulically controlled sequence valve and locking the piston in position. Fluid responsiveness is critical in these dynamic applications, making the hydraulic fluid vs transmission fluid evaluation essential for maintaining control stability.

Balancing circuits represent a critical safety feature in hydraulic systems with vertical motion, preventing uncontrolled movement that could cause equipment damage or personal injury. The selection between basic and hydraulically controlled valves depends on load variability, safety requirements, and operational precision needs. In all cases, fluid characteristics significantly impact performance, with careful evaluation of hydraulic fluid vs transmission fluid properties ensuring optimal circuit operation, responsiveness, and longevity.