Hydraulic Systems and Their Components: Structure, Types, and Advantages

What Is a Hydraulic System? How It Works and Why It Matters

A hydraulic system is a power transmission and control system that uses liquid as the working medium. By connecting five basic hydraulic components, it transmits energy and controls motion through the pressure of the liquid. Hydraulic systems are especially suitable for heavy-duty machinery in industries such as manufacturing, construction, agriculture, and aerospace.

The fundamental principle behind hydraulic systems is based on the physical property that liquids are incompressible and capable of transmitting pressure. The system uses incompressible hydraulic oil to transmit pressure from one part of the system to another, enabling precise and powerful motion control.

Advantages of Hydraulic Power Transmission vs Pneumatics

Compared to pneumatic systems, hydraulic systems offer several key benefits:

Higher Power Transmission Capability
Ideal for applications requiring high force and torque.

Smooth and Continuous Speed Control
Allows for gradual, accurate movements without jerking.

Large Force with Precise Control
Enables fine control over heavy loads and machinery.

Basic Working Principle: Pascal's Law

Hydraulic systems operate based on Pascal's Law, which states:

"In a confined fluid, any pressure applied is transmitted equally in all directions."

This means that when pressure is applied to a confined liquid, it is distributed evenly throughout the fluid and against the walls of the container—without any loss—allowing for efficient and controlled power transmission.

Workflow of a Typical Hydraulic System

Electric Motor starts the process by driving the hydraulic pump.

The pump draws hydraulic oil from the reservoir and pressurizes it.

The pressurized oil flows through hydraulic lines to control and actuator components.

Control valves determine the direction and flow rate of the oil.

Actuators (such as hydraulic cylinders or motors) convert hydraulic energy into mechanical power to perform work.

Types of Hydraulic Systems: Open Loop vs Closed Loop

Hydraulic systems can be categorized into two main types: open loop and closed loop systems.
The key difference lies in whether the fluid returns to the reservoir between the control valve and the actuator.

Closed Loop Hydraulic System

In a closed loop system, hydraulic fluid circulates continuously between the pump and the actuator, without returning to the reservoir during each cycle.
This design enables high control precision, fast response, and energy efficiency.

Common Applications of Closed Loop Systems:

Aircraft flight control systems

High-precision CNC machine tools

Servo-controlled injection molding machines

Open Loop Hydraulic System

In an open loop system, the fluid flows from the actuator back to the reservoir, and then from the reservoir to the pump via the directional control valve.
This type of system features simple construction, is easy to maintain, and is well-suited for heavy-duty applications.

Common Applications of Open Loop Systems:

Excavators

Wheel loaders

Hydraulic lifting platforms

Overall, open loop hydraulic systems are suitable for applications that require simple structure and high-power, heavy-duty performance; while closed loop hydraulic systems are ideal for applications that demand high control precision, fast response, and energy efficiency.

What Are Hydraulic Components

A hydraulic system is composed of five fundamental components, including the hydraulic pump, hydraulic cylinder (or actuator), various hydraulic valves, as well as auxiliary elements such as the reservoir and filters.

According to Pascal's principle, hydraulic fluid traveling through pipelines must be pressurized to transmit force efficiently.
Which component creates hydraulic pressure?
The hydraulic pump is not only the component responsible for pressurizing the fluid but also serves as the most fundamental part of the entire hydraulic system.

Since the pressurized fluid acts as the medium for converting between mechanical energy and hydraulic energy, the hydraulic pump functions as the primary power source that enables this energy transformation by increasing the fluid's pressure.

Three Main Types of Hydraulic Pumps

In a hydraulic system, the pump serves as the primary power source, supplying the energy needed to pressurize the fluid and drive the system. There are three common types of hydraulic pumps, each with distinct characteristics, advantages, and drawbacks:

Gear Pump

Structure: Consists of two meshing gears—one driven by a motor, the other idler. As the gears rotate, sealed chambers are formed between the gear teeth, drawing in fluid at the inlet and pushing it toward the outlet.

Advantages:

Simple design

Low manufacturing cost

Compact size and lightweight

Suitable for low to medium pressure applications

Disadvantages:

Lower pressure and efficiency compared to vane and piston pumps

Higher pulsation and noise levels

Vane Pump

Structure: Contains a rotor and several sliding vanes that press against the inner surface of the pump housing. As the rotor spins, the vanes extend due to centrifugal force and springs, creating variable volume chambers to draw in and compress the fluid.

Advantages:

Smooth operation with low pulsation and noise

Higher efficiency than gear pumps

Adjustable displacement and flexible control

Suitable for medium-pressure applications

Disadvantages:

More complex structure

Requires cleaner hydraulic oil for reliable operation

Piston Pump

Structure: Uses multiple pistons that move back and forth inside cylinders. The pistons' reciprocating motion changes the chamber volume, allowing the pump to draw in fluid and discharge it under high pressure.

Advantages:

Capable of withstanding very high pressures

High efficiency and long service life

Excellent for high-precision and high-load applications

Disadvantages:

Complex design and higher manufacturing cost

Demands more maintenance and cleaner operating conditions

Typical Applications:
Aerospace systems, high-pressure industrial equipment, and precision hydraulic machinery

The hydraulic pump is responsible for providing energy and power in a hydraulic system, acting as the driving force behind fluid movement.

In addition, hydraulic valves play a vital role in controlling the behavior of the pressurized fluid. 

Three Main Types of Hydraulic Valves

Hydraulic valves are critical components in any hydraulic system. They regulate the direction, pressure, and flow rate of hydraulic fluid to ensure safe, efficient, and precise operation. The three main types of hydraulic valves are:

Directional Control Valve

Function: Controls the direction of hydraulic fluid flow, thereby determining whether an actuator (such as a cylinder) moves forward, backward, or stops.

Common Configurations:

2-way, 2-position (2/2)

4-way, 3-position (4/3)

Application: Used to switch fluid paths in circuits and activate or deactivate hydraulic functions.

Pressure Control Valve

Function: Maintains or limits the pressure within a hydraulic system to ensure safe and stable operation.

Key Feature: The valve opens or bypasses fluid when the system pressure exceeds a preset level, protecting components from overpressure.

Examples:

Relief valve

Pressure reducing valve

Sequence valve

Safety valve

Flow Control Valve

Function: Regulates the flow rate of hydraulic fluid to control the speed of actuators.

Working Principle: Adjusts the cross-sectional area of the flow path to vary the amount of fluid passing through.

Common Types:

Throttle valve

Speed control valve

Flow divider and combiner valve

Overall, a flow control valve is used to adjust the cross-sectional area of the flow passage, thereby regulating the flow rate of the hydraulic fluid.

 In addition, there are various hydraulic quick couplings available. You can browse this article to learn more about hydraulic components.

Summary

A hydraulic system consists of five essential components that use pressurized fluid to perform tasks and is most commonly used in heavy-duty equipment.
Any system that uses pressurized, incompressible hydraulic fluid to transmit energy is considered a hydraulic system.
According to Pascal's Law, hydraulic systems enable the conversion of mechanical energy into hydraulic energy, which can then be transformed back into mechanical energy to perform useful work.