Classification and Working Mechanism of Excavator Hydraulic Valve System

Classification and Working Mechanism of Excavator Hydraulic Valve System

Various hydraulic valves is the core control elements for fluid pressure,flow rate,and direction in control systems.Based on their Working areas and system functions,the valves inside an Excavator can be divide into main pump Power control,main control Valve distribution ,actuator protection ,and pilot auxiliary control.Understanding the Working mechanisms and standard part numbers of various Valve is of great significance for the precise procurement and efficient maintenance of construction machinery.

 Main Pump Power Control Valve Group (Power Source Regulation)

These valves are mounted directly on the regulator of the main hydraulic pump (e.g., Komatsu HPV series piston pumps). By controlling the servo piston, they change the swash plate angle and thereby directly determine the main pump’s displacement and power output. Typical assemblies and representative parts include the hydraulic main pump regulator assembly for Komatsu PC56-7 and PC60-7 series (part numbers: 708-3S-03572, 708-3S-03850), the regulator assembly for PC30MR-2 and PC35MR-2 series (part number: 708-1S-03423), the main pump regulator (part number: 708-1L-04310), and the pump pressure regulating valve (part number: 708-2K-03560).

PC Valve: 

Internally equipped with a spring-loaded spool that senses main pump discharge pressure. When the excavator encounters high resistance and system pressure rises sharply, the PC valve senses the high pressure, shifts the spool, and directs high-pressure oil into the large chamber of the servo piston to force the swash plate back, automatically reducing pump displacement. This ensures the pump’s absorbed power does not exceed the engine’s maximum power, preventing engine overload and stalling. If the PC valve spring fatigues or the spool sticks, displacement cannot be reduced under high pressure, causing excessive engine load, severe stalling, or even engine shutdown. Conversely, if the PC valve opens too early, the machine exhibits slow movements and weak digging force.

LS Valve: 

As the core component of a closed-center load-sensing system, the spool of the LS valve is subjected to main pump outlet pressure on one end and load pressure after the main control valve on the other, constantly maintaining a fixed pressure differential between the two. The greater the displacement of the operator’s control lever, the larger the main control valve spool opening, and the LS valve commands the main pump to increase displacement; light lever movement reduces displacement. This enables on-demand oil supply and significantly reduces relief losses during fine operations. If the LS valve sticks, the most obvious symptom is extremely poor fine-control performance—actuators either do not move or suddenly surge, completely losing smooth operation.

EPC Proportional Solenoid Valve:

Receives current (PWM signal) from the electronic controller and converts it into a pilot hydraulic signal that pushes against the springs of the PC or LS valve. Higher input current produces a stronger hydraulic signal, allowing the controller to actively intervene in main pump power and switch between different working modes (e.g., heavy-duty P/H mode, standard S mode, economy E mode). When this valve fails, the machine usually displays controller error codes, becomes locked in a fixed mode and cannot switch modes, or experiences overall slowed machine movements.

Core Valves Inside the Main Control Valve  

The main control valve, such as the Komatsu PC200-8MO main control valve assembly (part number: 723-46-29100), is the central distribution point for the entire machine’s hydraulic oil flow. It integrates multiple control valve sections and safety valves. Related components include the LS select valve (part number: 723-30-60101) and the pressure-compensated valve (part number: 723-46-40601).

Main Relief Valve: 

Serves as the highest safety limit for the entire hydraulic system, restricting the maximum working pressure of the main circuit. It is typically a pilot-operated two-stage relief valve: when main circuit pressure exceeds the setting, the small pilot cone valve opens first; the main spool then loses pressure balance and opens fully, dumping high-pressure oil directly back to the tank. Models with instantaneous boost function introduce pilot oil via a solenoid valve to temporarily raise the set pressure. Common representative parts include the main relief valve for PC60-7, PC70-8, PC120-6, and PC130-7 (part number: 723-20-60101) and the heavy-duty model relief valve (part number: 700-86-58001). If the main spool sticks in the open position or the spring breaks, all machine functions become extremely weak and hydraulic oil temperature rises sharply. Conversely, if the pressure setting is too high, frequent relief under heavy loads converts engine power into heat, also causing rapid system temperature rise.

Unload Valve:

When all control levers are in neutral, the LS pressure differential signal is zero. The unload valve is pushed open by main pump pressure against spring force, allowing the main pump’s minimum standby flow to return to the tank at ultra-low pressure, thereby reducing standby power consumption and oil temperature. If the unload valve sticks closed, high pressure builds in the main circuit during standby, causing a muffled engine sound and rapid oil temperature rise. If it sticks open, cylinder functions on one side of the machine become completely inoperative.

Main Directional Spool:  

Pushed by pilot oil pressure, the linear displacement of the spool changes oil flow direction and orifice area, directly controlling the direction and speed of boom, arm, bucket, swing, and travel movements. Under high-temperature and oxidized oil conditions, viscous varnish easily forms inside the valve body, making the originally smooth directional spool sticky and sluggish. As a result, the spool cannot complete direction change cleanly after receiving an operating command, causing delayed excavator movements or lagging lever response.

Straight Travel Valve and Confluence/Diversion Valves: 

Excavators normally use dual pumps supplying the left and right tracks independently. When the machine is traveling while simultaneously operating a working device (e.g., raising the boom), the straight travel valve senses the combined pilot signals, shifts, and interconnects the dual-pump independent circuits so that one pump supplies both travel motors while the other supplies the working device, preventing the machine from drifting due to unequal flow. If this valve fails, severe drifting occurs during combined operations. Confluence and diversion valves combine the high-pressure oil from the front and rear pumps to supply the same cylinder during high-flow actions such as boom raise or arm dig, and divert flow during fine adjustments.

Actuator Protection and Buffer Valve Group

Installed directly on hydraulic motors, at the base of cylinders, or integrated at the main control valve outlets to absorb extreme external impact forces and protect end actuators.

Overload Valve and Anti-Cavitation (Make-up) Valve:

Mounted at the main valve ports connected to the cylinders, with a set pressure higher than the main relief valve, they limit the maximum pressure of individual actuators (e.g., boom or bucket cylinders). When the main spool is closed and a large rock impacts the bucket, cylinder pressure spikes instantly; the secondary relief valve opens independently to relieve pressure, preventing cylinder rupture or linkage breakage. These valves often integrate an anti-cavitation valve that rapidly draws oil from the return line when a swing motor or descending boom continues moving due to inertia and creates local vacuum in the supply line, preventing hydraulic cavitation. If a secondary relief valve develops internal leakage, the corresponding single function becomes weak or cannot hold position in mid-air.

Counterbalance Valve and Swing Cushion Valve:

The counterbalance valve is usually integrated inside the travel motor. When traveling downhill, gravity causes the machine to accelerate and lowers pressure in the motor inlet chamber; the counterbalance valve senses insufficient pressure and automatically reduces the motor return orifice, creating back pressure that acts as “hydraulic braking” to prevent runaway. If it fails, the machine shakes violently or cannot be braked on downhill slopes. The swing cushion valve absorbs the large hydraulic pressure waves generated at the start and stop of the swing motor, protecting the swing reduction gear and making swing operation smooth.

Boom Holding Valve: 

Essentially a pilot-operated check valve installed in the boom and arm circuits. When the control lever returns to neutral, the holding valve completely locks the return path of the boom cylinder rod end. Even if the main directional spool has minor wear leakage, oil cannot escape. Only when the operator pushes the boom-lower lever does pilot oil force the holding valve open to allow the cylinder to lower. If the holding valve seal fails, the boom or arm will visibly drift downward after the machine is stopped (severe boom drop).

Pilot and Auxiliary Control Valve Group  

Responsible for handling pilot control oil, generating control signals, and providing low-pressure safety interlocks.

Self-Pressure Reducing Valve: 

High-pressure oil from the main pump cannot be used directly by the control levers. The self-pressure reducing valve uses the balance of area difference and spring force to proportionally reduce high-pressure oil, providing a stable low-pressure oil source for the entire pilot control system and solenoid valves. If this valve fails completely, the whole machine loses pilot pressure and all levers produce no response.

Pilot Proportional Control Valve:  

Refers to the control lever and travel pedal assemblies in the cab. Multiple plungers are located under the levers; moving the lever causes a cam to depress the plunger and compress the internal metering spring, opening a small orifice. The deeper the lever is pushed, the higher the output pilot pressure, which pushes the main control valve spool farther. Common faults include oil leakage at the base of the cab levers, extremely heavy lever operation, or sluggish/sticking response in a specific direction.

Pilot Safety Lock Valve and Shuttle Valve:

The pilot safety lock valve is linked to the left-side safety lever in the cab. Raising the lever cuts off all pilot oil circuits, ensuring the machine has no movement even if control levers are accidentally touched. Shuttle valves, as hydraulic logic valves, are distributed in the load-sensing network or brake-release circuits; they automatically compare pressures from multiple lines and always transmit the highest pressure oil signal to the control end.

Valve Repair and Replacement Standards and YIKAN KOMSU Quality Assurance System

Main control valve blocks and precision control valve groups are continuously exposed to high-temperature, high-pressure oil flow. Early faults often appear as hardened internal seals, weakened spring force, and minor internal leakage at relief valve seats. These hidden internal leaks cause pressure loss and generate additional heat, which accelerates seal aging, forming a thermodynamic vicious cycle of “leakage → heat generation → accelerated aging → more severe leakage.” During repair work, fault locations must be accurately diagnosed based on equipment structure and principles; blind major disassembly or trial-and-error parts replacement must be avoided. Before installing new valves, appearance and performance inspections must be carried out, and original part numbers must be verified. Arbitrary substitution or incorrect use is strictly prohibited.

YIKAN KOMSU is an integrated industrial and trading enterprise with more than 20 years of experience in hydraulic systems, hydraulic piping, and core components. It specializes in R&D, production, and global trade, providing global customers with end-to-end hydraulic solutions ranging from component selection, custom valve block design and manufacturing, tank and piping fabrication, to system assembly and testing. In response to the extremely tight fit tolerances required between hydraulic valve spools and valve bores, the company has established a strict independent quality control system:

Material analysis and micron-level machining inspection: Core production processes include not only spectral chemical composition analysis of raw materials but also high-precision CMM measurement of valve body bore diameter and positional accuracy after CNC machining.

Clean-room assembly and full-function bench testing: Hydraulic valve assembly is performed in a clean room that meets the highest cleanliness control standards. Every finished product must pass cracking pressure, internal leakage, and durability tests before shipment. Flexible production lines enable rapid delivery and full product life cycle traceability.

As an important hydraulic component manufacturer in the global construction machinery aftermarket, YIKAN KOMSU will participate in bauma China in Shanghai (booth number: E2-149). At the exhibition, the company will display physical products covering the full Komatsu series of hydraulic main control valve assemblies, main pump regulator assemblies, main relief valves, overload valves, self-pressure reducing valves, and high-precision main directional spool components. On-site engineering and technical teams will provide part number cross-checking, tolerance technical exchanges, and one-stop business negotiation services for global buyers.

  III. Frequently Asked Questions (FAQ)  

What valve fault usually causes sudden surging and loss of smoothness during fine operations on an excavator? 

This is typically caused by sticking of the LS valve mounted on the main pump regulator. The LS valve senses the pressure differential between main pump discharge pressure and load pressure after the main control valve to adjust displacement in real time. Once the spool sticks, the system cannot smoothly regulate flow according to small lever movements, resulting in loss of fine-control performance.

  If only a single function is weak (e.g., only bucket curl is weak or the bucket drops while stopped in mid-air) while all other functions are normal, which component should be checked first?  

Focus on the overload valve or boom holding valve at the corresponding port of the main control valve. If all machine functions are weak and oil temperature rises sharply, the main relief valve is usually at fault. If only a single actuator is weak or cannot hold position, it indicates internal leakage in the secondary relief valve protecting that cylinder circuit, or failure of the corresponding holding valve’s locking function.

 Why does an excavator sometimes exhibit occasional sticking of a certain function after prolonged high-temperature operation, which can be forced through by increasing throttle?  

This is an early signal of gelatinous varnish precipitated inside the main control valve block due to long-term overheating and oxidation of the hydraulic oil. The sticky varnish makes the originally smooth main directional spool adhesive and restricted. If the oil circuit is not cleaned, oil is not replaced, and the valve group is not maintained in time, the spool will eventually be unable to complete direction change after receiving the pilot signal, leading to severe movement delay.