The neutral position function refers to the interconnection of ports (P, T, A, and B) when the directional control valve’s spool is in its central or original position. It directly determines the state of the actuators during downtime (such as locking or floating) and whether the hydraulic pump is unloaded. In the design of construction machinery, selecting the appropriate neutral position function based on specific operating conditions is critical to ensuring energy efficiency, starting stability, and operational safety.

1. O-type neutral position directional valve

Where P represents the inlet, T represents the return port, and A and B represent the working ports.

Neutral position: The inlet (P), return port (T), and working ports (A and B) are all blocked, and the oil inside the system is in a stagnant, non-flowing state.

Core Characteristics Analysis

(1) High-Precision Position Locking

Because the working ports A and B are completely closed, the actuators (such as hydraulic cylinders or motors) can be tightly locked in any position. Even under external load interference, the mechanism remains perfectly still.

(2) Smooth Start-up, but Large Braking Shock

During startup: The return oil chamber is filled with trapped oil, forming a natural oil resistance that effectively buffers the sudden pressure increase at startup, making the transition smoother.

During braking: Due to the instantaneous cutoff of the oil circuit, the energy generated by the inertia cannot be released through the oil circuit, easily causing a large hydraulic shock (water hammer effect) within the system.

(3) Excellent Reversing Accuracy

The locked state ensures that the actuators have no extra displacement at the moment of reversal, making it very suitable for equipment with extremely high requirements for motion accuracy.

(4) No System Unloading

This is the most significant drawback of the O-type function. In the neutral position, the pressure oil discharged by the pump cannot flow back to the oil tank, causing the system pressure to always remain at the maximum value set by the relief valve. This will result in significant energy loss and cause the oil temperature to rise rapidly.

2. H-type neutral position directional valve

Neutral position: The inlet (P), return (T), and working ports (A, B) are all interconnected. The system is in a fully depressurized state in the neutral position.

Core Characteristics Analysis

(1) High-efficiency pump unloading

Since the P port and T port are directly connected, the hydraulic pump output oil can flow directly back to the tank at extremely low pressure. This significantly reduces system power loss, heat generation, and extends pump life.

(2) Actuator in a “floating” state

The A, B ports are connected to the return port T, meaning the actuator (hydraulic cylinder or motor) is not hydraulically locked.

Advantages: The working mechanism can be easily moved by hand or other external force, making it ideal for situations requiring manual alignment or follow-up.

Disadvantages: Automatic positioning cannot be achieved at any position.

(3) Smooth braking, but with a shock during startup

During braking: When the valve core returns to the neutral position, due to the interconnection of A, B, and T, the inertial oil of the actuator can be directly discharged back to the tank without causing a drastic pressure surge. Therefore, braking is smoother than O-type braking.

At startup: Because the oil circuit is empty and without pressure (back pressure) in the neutral position, the oil chamber is instantly filled with oil when reversing, lacking buffer resistance, which easily causes reversing impact.

(4) “Unbalanced” displacement of single-rod cylinders

For single-piston rod hydraulic cylinders, because the effective pressure-bearing areas at both ends of the piston are unequal, even with weak pressure in the neutral position, the piston may slowly drift due to area difference or its own weight, making it impossible to achieve complete stillness.

3. M-type neutral position reversing valve

In the neutral position: the oil inlet (P) and the oil return (T) are connected to form a loop, while the working oil ports (A, B) are completely closed off.

Core Characteristics Analysis

(1) High-efficiency pump unloading:

The discharged oil flows directly back to the tank at low pressure through the P→T circuit. This eliminates the need for the hydraulic pump to overcome high pressure when not in operation, significantly reducing system power consumption and preventing excessively rapid oil temperature rise.

(2) Actuator position retention:

Because ports A and B are closed, the oil inside the actuator (such as the hydraulic cylinder) is locked, allowing the mechanism to remain stationary.

Limitations: Similar to the O-type, due to the locked oil circuit, it is incompatible with hand-cranked devices or mechanisms requiring external force for forced displacement.

(3) Smooth start-up, impactful braking:

Start-up: At the moment of reversal, the working oil chamber is filled with oil, providing back pressure resistance, effectively reducing acceleration during start-up, resulting in a very smooth transition from rest to motion.

Braking: When switching to the neutral position, because ports A and B are instantly closed, the kinetic energy generated by inertia cannot be released, leading to a significant hydraulic shock in the system.

(4) Typical application scenarios:

This is the most commonly used unloading circuit function. It perfectly solves the contradictory requirement of “both allowing the pump to rest (unloading) and keeping the cylinder stationary (locking)”.

4. Y-type neutral position reversing valve

Mid-position: The inlet (P) is closed, while the working ports (A, B) and return port (T) are interconnected.

Core Characteristics Analysis

(1) Free Floating of the Actuator:

Because the working oil ports A and B are directly connected to the return oil port T, the pressure in both chambers of the actuator (hydraulic cylinder or motor) is equal and connected to the oil tank.

Application: The mechanism is in a “floating” state and can move freely with external force. This makes it very suitable for working conditions that require manual positioning, external traction, or movement following the shape of the mold.

(2) Moderate Dynamic Performance

Starting: During reversing startup, because the oil in the working chamber has partially flowed back to the oil tank in the middle position, there is a lack of sufficient back pressure resistance, thus generating a certain reversing impact. Its impact intensity is between O-type (minimum) and H-type (maximum).

Braking: During braking, ports A and B are not closed but connected to the return oil, which helps to release inertial pressure. Its braking stability is also between O-type and H-type, showing a relatively balanced performance.

(3) System Without Unloading:

Consistent with the function of O-type, the oil inlet P is completely sealed in the middle position. This means that the hydraulic pump cannot unload the hydraulic fluid, and the system pressure will remain at the level set by the relief valve.

Impact: Prolonged operation at the neutral position will lead to significant energy loss and high system heat generation.