Classification of Hydraulic Systems
A comprehensive guide to understanding the various classifications and applications of hydraulic systems in modern engineering
Hydraulic systems play a crucial role in numerous industrial applications, utilizing the power of pressurized fluid to transmit force and motion. Understanding the classification of these systems is essential for engineers, technicians, and anyone working with hydraulic equipment. Where can hydraulic systems be found? They are ubiquitous in manufacturing, construction, aerospace, automotive, and many other industries, powering everything from heavy machinery to precision instruments.
This detailed guide explores the primary classification systems for hydraulic systems, examining their working principles, components, advantages, and typical applications. By understanding these classifications, professionals can make informed decisions about system design, selection, and maintenance.
1. Classification by Working Characteristics
From the perspective of working characteristics, hydraulic systems can be divided into two main categories: transmission systems and control systems. Where can hydraulic systems be found that exemplify these characteristics? In nearly every industry that relies on mechanical power, from simple hydraulic presses to complex aerospace control systems.
Transmission Systems
Transmission systems primarily focus on transmitting power, with information transmission being secondary. In hydraulic technology, these are referred to as hydraulic transmission systems.
The main function of these systems is to transfer energy from a power source to various actuators, enabling mechanical work. Where can hydraulic systems be found that prioritize power transmission? In applications such as industrial presses, construction equipment, and material handling machinery where moving heavy loads is the primary requirement.
Control Systems
Control systems prioritize information transmission, with power transmission being secondary. In hydraulic technology, these are known as hydraulic control systems.
These systems are designed to precisely control the position, velocity, or force of actuators. Where can hydraulic systems be found that emphasize control? In precision machinery, robotics, and aerospace applications where accurate movement and positioning are critical.
It should be noted that transmission systems and control systems are often integrated in specific structures. With the rapid development of science and technology, the requirements for modern machinery and equipment are increasingly high, and both power transmission and control indicators are often important. For example, in weapons, aerospace equipment, and numerically controlled machining equipment, the power required for their work depends on fluid systems, which may appear to be transmission systems, but they also have high control requirements, resembling control systems. Therefore, the above classification method is not absolute. Where can hydraulic systems be found that blur these lines? In advanced manufacturing systems, military equipment, and sophisticated industrial robots that require both power and precision.
2. Classification by System Feedback Devices
Based on whether a system uses feedback devices, hydraulic systems can be further divided into two categories: open-loop systems and closed-loop systems. This classification is particularly important for understanding system accuracy and performance characteristics. Where can hydraulic systems be found utilizing these different approaches? Open-loop systems are common in simpler applications, while closed-loop systems are prevalent in precision equipment where consistent performance is required.
Open-Loop Systems
An open-loop system is one that does not use feedback devices, as shown in Figure 1-25. Its output characteristics depend entirely on the characteristics of individual components and their combination in the system.
When external disturbances affect the system, the output of the actuator generally deviates from the original set value, producing a certain error. Due to the poor anti-disturbance ability of open-loop systems, the quality of control is greatly affected by changes in working conditions (such as oil temperature, load, etc.), so the accuracy of the system output is low, and in severe cases, it may even fail to achieve the intended goal.
Where can hydraulic systems be found that use open-loop designs? They are commonly used in applications where high precision is not required, such as simple hydraulic presses, agricultural machinery, and basic material handling equipment where cost considerations outweigh the need for precise control.
Figure 1-25: Open-Loop System
Closed-Loop Systems
Figure 1-26: Closed-Loop System
A closed-loop system is one that uses a feedback device, as shown in Figure 1-26. The feedback device samples the output state, generates a feedback signal proportional to the output, and compares it with the input signal. If there is a difference between the feedback signal and the input signal, it automatically corrects the output to match the input requirements.
Due to their strong anti-disturbance ability, closed-loop systems are less affected by changes in working conditions. They can form relatively precise control systems using ordinary components and achieve higher accuracy in system output.
Where can hydraulic systems be found that utilize closed-loop designs? In precision manufacturing equipment, robotics, flight control systems, and other applications requiring high accuracy and consistent performance regardless of external conditions or component wear.
3. Classification by Oil Circulation Method
According to the oil circulation method, hydraulic systems can be divided into two types: open systems and closed systems. This classification is fundamental to understanding system design, maintenance requirements, and performance characteristics. Where can hydraulic systems be found utilizing these different circulation methods? Open systems are common in many industrial applications, while closed systems are prevalent in specialized equipment requiring compact design and energy efficiency.
Open Systems
As shown in Figure 1-27, in an open system, the pump draws oil from the tank, and the discharged oil supplies the hydraulic cylinder or hydraulic motor to drive them to do work, while the return oil from the hydraulic cylinder or hydraulic motor flows back to the tank.
Figure 1-27: Open System
Open systems have a simple structure. The oil tank serves as a storage place for the working medium in an open system, so the oil can be well cooled, air can escape, and impurities can settle in the tank, but this requires a tank with a larger volume.
Open systems rely on operating directional valves to reverse the actuator. Therefore, during reversal (such as when the directional valve in Figure 1-27 is shifted left to the neutral position), a directional valve with a positive遮盖 structure will cause pressure shocks in the hydraulic transmission system.
Where can hydraulic systems be found that use open-loop designs? They are commonly used in applications such as hydraulic presses, injection molding machines, and construction equipment like excavators and loaders, where the simplicity and ease of maintenance outweigh the energy efficiency considerations.
Characteristics of Open Systems:
- Simpler structure and easier maintenance
- Larger oil tank required for cooling and contamination control
- Oil is exposed to air, increasing the risk of contamination and aeration
- Prone to pressure shocks during directional changes
- Inertial energy from loads is dissipated as heat during braking
- Lower cost compared to closed systems
- Easier to inspect and replace fluid
Additionally, in gravity descent systems, when external loads act on the system, the hydraulic motor acts as a pump. To prevent the external load from descending too quickly, it is necessary to install hydraulic components that can generate back pressure in the return line, but this will cause energy consumption and heat the oil (energy-consuming speed limitation). Where can hydraulic systems be found with these characteristics? In applications like elevating platforms, crane booms, and material handling equipment where controlled descent is necessary.
Closed Systems
As shown in Figure 1-28, the inlet and outlet ports of pump a are connected to the inlet and outlet ports of hydraulic motor b through pipes, forming a closed loop. The direction of the hydraulic motor can be reversed by manipulating the variable mechanism of pump a to change the direction of fluid flow. Valves 1-5 together form a bidirectional safety valve to prevent the oil pressure in pipes A and B from exceeding the set value of valve 3.
To compensate for system leakage, a smaller auxiliary pump c is required, whose working pressure is set by relief valve 6, which should be slightly higher than the back pressure required by hydraulic motor b. The flow rate of pump c should be slightly higher than the leakage rate of the system.
Oil discharged from pump c is filtered and补充到系统的低压边 through check valve 1 or 2 (which also serve as make-up valves); excess oil flows back to the tank through valve 6.
Where can hydraulic systems be found that use closed-loop designs? They are commonly used in applications such as metal forming machinery, marine propulsion systems, and high-performance mobile equipment where energy efficiency, compact design, and precise control are important considerations.
Figure 1-28: Closed System
Characteristics of Closed Systems:
- More complex structure compared to open systems
- Smaller oil tank required, resulting in more compact design
- Oil is not exposed to air, reducing contamination risks
- Uses bidirectional variable pumps for speed control and reversal
- Requires auxiliary pump for makeup oil and cooling
- Enables energy recovery during braking and deceleration
- Better performance with inertial loads and frequent direction changes
From the above analysis, closed systems have a more complex structure. In a closed system, one oil source can generally supply oil to only one hydraulic actuator, and bidirectional variable pumps are used for speed regulation and reversal. In addition, since the oil basically circulates in a closed loop, the amount of oil exchanged with the tank is only the leakage of the system, so the oil temperature rises quickly, but the required tank volume is small and the structure is compact.
Since the return oil of hydraulic motor b directly flows into the suction port of pump a, the return oil with back pressure can help the motor drive pump a and put pump a in a pressure supply state, reducing the requirement for the self-priming performance of pump a. In open systems, return oil with back pressure cannot play these positive roles, but instead wastes this usable energy in the throttling heat generation of the back pressure valve.
The braking process of a closed system is achieved by manipulating the variable mechanism of pump a to gradually reduce its displacement to zero. During this process, the inertial force of the external load becomes a driving force, trying to drag hydraulic motor b to move at the original speed, acting as a pump, supplying oil to pump a, making pump a act as a hydraulic motor, and pump a driving the motor to accelerate rotation to generate electricity, which is supplied to other loads in the power grid.
In this way, the inertial motion energy of the external load is converted into oil pressure energy through hydraulic motor b, increasing the oil pressure on the return side (B side) of hydraulic motor b, whose maximum value is limited by valve 3, thereby preventing pressure shocks on the B side and gradually decelerating and braking hydraulic motor b. At the same time, the oil pressure energy on the B side is converted into electrical energy through pump a driving the motor, thereby realizing energy recovery (regenerative braking) during the braking process.
Where can hydraulic systems be found utilizing this energy recovery capability? In applications with frequent braking and reversing such as cranes, excavators, and material handling equipment, where the energy savings can be substantial. When the external load inertia is large and reversing is frequent, this regenerative energy is considerable.
In gravity descent mechanisms, when external loads act on the system, the hydraulic actuator acts as a hydraulic pump, and pump a acts as a motor, driving the generator to generate electricity, which is supplied to other loads in the power grid, thereby preventing the external load from descending too fast (regenerative speed limitation). However, when the hydraulic pump is driven by an internal combustion engine, regenerative braking and regenerative speed limitation cannot be achieved.
In summary, open systems are suitable for small-power mechanisms, internal combustion engine-driven mechanisms (such as forklifts, aerial work vehicles, hydraulic truck cranes, and excavators), and fixed machinery. Closed systems are suitable for the following mechanisms where the hydraulic pump is driven by an electric motor: mechanisms with large external load inertia and frequent reversing (such as rotating and running mechanisms of some cranes, and worktables of planers, broaching machines, and precision surface grinders), gravity descent mechanisms (such as unbalanced lifting and boom swing mechanisms), and gravity descent mechanisms with large external load inertia (such as luffing mechanisms of balanced cranes). Additionally, closed systems are also suitable for mobile machinery requiring a particularly compact structure (such as hydraulic cars, tractors, and mining vehicles), and are also commonly used in pump-controlled motor and pump-controlled cylinder systems such as steering gears and controllable pitch propellers of 10,000-ton ships. Where can hydraulic systems be found that best utilize these different designs? The selection between open and closed systems depends on specific application requirements including power, control precision, energy efficiency, and space constraints.
Summary of Hydraulic System Classifications
Understanding the classification of hydraulic systems is essential for selecting the appropriate design for specific applications. Whether categorized by working characteristics, feedback mechanisms, or oil circulation methods, each type offers distinct advantages and disadvantages. Where can hydraulic systems be found that exemplify each classification? In virtually every industry, from manufacturing and construction to aerospace and marine applications, each utilizing the type of hydraulic system best suited to its specific operational requirements.
The choice between transmission and control systems depends on whether power transfer or precision control is prioritized. Open-loop versus closed-loop systems are selected based on the required accuracy and ability to compensate for disturbances. Open and closed oil circulation systems are chosen based on considerations of energy efficiency, space constraints, and maintenance requirements.
As hydraulic technology continues to evolve, the lines between these classifications sometimes blur, resulting in hybrid systems that combine the best features of different types. Where can hydraulic systems be found that represent these advanced hybrid designs? In cutting-edge industrial machinery, high-performance vehicles, and sophisticated aerospace systems that demand both power and precision.