Excavator Hydraulic System Explained
Every time an excavator swings its boom and curls its bucket on a jobsite, a precision hydraulic system is silently working behind the scenes. Many people think excavators are just "chunks of iron," but what truly makes them agile as a human arm is the "blood" hidden beneath the steel shell — hydraulic oil — and the entire hydraulic circuit that drives its flow.
Today we'll break down this system to see how it works, why it enables a machine weighing tens of tons to perform precise operations, and how to maintain it in everyday use.
1. Why Can't Excavators Function Without Hydraulics?
Construction equipment chooses hydraulic drive over mechanical drive for a simple reason: power, flexibility, and controllability.
A 20-ton-class excavator's bucket digging force can exceed 150 kN. If pure mechanical gear transmission were used, the structure would be extraordinarily bulky and unable to achieve multi-degree-of-freedom movement. Hydraulic systems transmit pressure through oil, delivering tremendous force in a relatively compact package while enabling stepless speed regulation and precise control.
Pascal's principle is the theoretical foundation of hydraulic systems: pressure applied to a confined fluid is transmitted equally in all directions. In simple terms, a small force applied to a small piston, transmitted through the oil to a large piston, becomes a large force. This is why an excavator operator can simply push a control lever and lift a boom weighing tens of tons.
2. The Four Core Components of a Hydraulic System
### 1. Hydraulic Pump — The System's "Heart"
The hydraulic pump converts the engine's mechanical energy into hydraulic energy. The most common types are gear pumps, piston pumps, and vane pumps. Modern excavators predominantly use swashplate-type variable displacement piston pumps, which automatically adjust displacement based on load — saving fuel while delivering power.
In June 2026, Danfoss released a new hydraulic pump claiming to increase equipment uptime by 50%. Behind this is a dual breakthrough in variable displacement control technology and materials engineering — a more efficient pump means less energy waste and longer continuous operating hours.
### 2. Hydraulic Cylinders — The "Executors" of Force
Hydraulic cylinders convert hydraulic energy back into mechanical energy. Excavators typically have three sets — boom cylinders, arm cylinders, and bucket cylinders — each controlling the movement of one joint. Double-acting hydraulic cylinders achieve reciprocating piston movement by alternately supplying oil to both sides.
Cylinder bore and stroke determine output force and range of motion. Take the Komatsu PC200-8 as an example: its boom cylinder has a bore of approximately 140 mm and a stroke of approximately 1,200 mm. At a rated pressure of 34.3 MPa, it can produce a thrust exceeding 500 kN.
### 3. Multi-Way Valve — The System's "Brain"
The multi-way valve (main control valve) is responsible for directing oil flow, determining which hydraulic cylinder moves and how fast. The operator sends low-pressure control signals through the pilot lever, and the multi-way valve opens or closes the corresponding oil passages accordingly.
High-end models such as the Sany SY75C use a Load Sensing system, where the multi-way valve automatically adjusts flow distribution based on actual load. This prevents interference during compound movements, resulting in smoother operation.
### 4. Hydraulic Oil Tank and Lines — The "Circulatory System"
The oil tank not only stores hydraulic fluid but also serves functions including heat dissipation, sediment settling, and air bubble separation. The piping system consists of hard pipes, hoses, and various fittings, with operating pressures typically ranging from 25 to 35 MPa. High-pressure hoses are consumables with a service life of generally 2,000 to 3,000 hours and require regular inspection and replacement.
3. How Does the Hydraulic System "Move"?
Operator pushes the pilot lever → pilot pump generates low-pressure control oil → control oil pushes the multi-way valve spool to shift → high-pressure oil from the main pump flows through the multi-way valve into the corresponding hydraulic cylinder → hydraulic cylinder piston moves → boom/arm/bucket actuates.
The entire process from lever input to mechanical response takes less than 0.5 seconds. High-end models have a pilot pressure of approximately 3–5 MPa, while the main circuit pressure can reach 34.3 MPa — an amplification of roughly 7 to 10 times. This is the "small effort, big result" principle of hydraulic systems.
4. Electro-Hydraulic Hybrid: The Direction of Hydraulic System Evolution
Traditional hydraulic systems rely entirely on the engine to drive the hydraulic pump — when the engine stops, hydraulics stop. But with the wave of electrification, electro-hydraulic hybrid systems are emerging as a new trend.
Volvo Construction Equipment won the Volvo Group Technology Award back in 2021 for its groundbreaking electro-hydraulic system. The core concept is to use an electric motor to assist or replace the engine in driving the hydraulic pump — running purely on electricity under low-load conditions and engaging the engine only under high load. Field testing has demonstrated fuel consumption reductions of over 15%.
On the domestic front, companies such as Sany Heavy Industry and XCMG Group are also actively developing electric excavator product lines. In the first half of 2026, the market penetration rate of fully electric excavators has exceeded 8%, concentrated primarily in the 6–15 ton small and medium class. As battery energy density improves and fast-charging technology matures, electrification of machines rated 20 tons and above is also accelerating.
5. Daily Maintenance: Keys to Extending Hydraulic System Life
Hydraulic system failures are the number one cause of excavator downtime, but the vast majority can be prevented through proper maintenance.
Regularly replace hydraulic oil and filters: Hydraulic oil should generally be changed every 2,000 hours or every six months; return oil filters should be replaced every 500 hours. Using inferior hydraulic oil can cause premature wear on pumps and valves, and repair costs may be tens of times the cost of the oil itself.
Inspect hoses and fittings: High-pressure hoses can burst after aging — a comprehensive inspection every 1,000 hours is recommended. Any signs of bulging, leakage, or outer jacket wear should prompt immediate replacement.
Maintain normal oil temperature: The optimal operating temperature for hydraulic oil is 40–80°C. Excessively high oil temperature accelerates oil oxidation and degrades lubricating performance; excessively low oil temperature reduces fluidity and causes sluggish system response. In summer, pay attention to checking whether the radiator is clogged.
Avoid prolonged off-center loading: Continuously operating a specific set of hydraulic cylinders at full load accelerates wear on the pump and valves in that circuit. Plan work content to allow balanced use of all systems.
6. Selection Reference: Hydraulic Configurations for Different Operating Conditions
Excavators of different tonnages and operating conditions vary greatly in hydraulic system configuration:
- Small machines (under 6 tons): Typically use gear pumps with fixed-displacement systems — simple structure, low cost, suitable for light-duty work
- Medium machines (6–30 tons): Mainstream configuration is variable displacement piston pumps with load sensing systems, balancing efficiency and controllability
- Large machines (30 tons and above): Dual-pump or multi-pump systems equipped with electronically controlled positive flow or negative flow control, pursuing high flow rates and precision operation
When selecting equipment, the hydraulic system configuration directly impacts service life and operational efficiency. We recommend choosing based on actual operating conditions rather than simply pursuing the highest specifications. For hydraulic system specifications and pricing on specific models, please contact our sales team.
*EquipNode — Your global construction equipment procurement partner. For more equipment information or to request a quote, visit equipnode.com or contact our sales team.*