What are the differences between Hydraulic System and Pneumatic System?

Hydraulic and pneumatic systems are widely used in modern industry. This article will briefly introduce hydraulic and pneumatic systems, as well as their characteristics, and their differences from each other.

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CONTENTS

§1. What is the Hydraulic System?

The word "hydraulics" is derived from the Greek word ὑδραυλικός (hydraulikos). Hydraulics is primarily related to fluid mechanics and focuses on the mechanical properties and use of fluids. Hydraulic technology is commonly found in the mechanical and electromechanical industries and can be divided into hydraulic transmission technology and hydraulic control technology. Hydraulic transmission is the use of liquid as the working medium, using the pressure energy of the liquid to transmit power. Hydraulic control is to use a liquid with a certain pressure as the control signal. Hydraulic control usually includes hydraulic open-loop control and hydraulic closed-loop control. Hydraulic closed-loop control is also known as hydraulic servo control, which constitutes a hydraulic servo system, usually including electrical hydraulic servo system (electro-hydraulic servo system) and mechanical hydraulic servo system (machine-hydraulic servo system, or machine-hydraulic servo mechanism). The use of hydraulic technology and the composition of the control system is called hydraulic control system.

The use of hydraulics goes back a long way in history, but it was thanks to the 17th century French scientist and philosopher Blaise Pascal and the 18th century Swiss physicist Daniel Bernoulli that hydraulics were able to be used on a large scale in modern industrial systems. Pascal's law or Pascal's principle of hydrostatics states that when water fills a closed container, the pressure at any point is transmitted to all sides of the container. Hydraulic systems use Pascal's principle for pressurization: If a hydraulic system has two pistons, large and small, and a small thrust is applied to the smaller piston, a larger thrust is generated on the larger piston through the transfer of pressure in the fluid. Bernoulli's theorem states that the energy in the fluid is caused by height, motion and pressure, and that if there is no loss due to friction and no work is done, the sum of energy remains constant.

A complete hydraulic system consists of five parts: the liquid medium, the drive unit, the energy unit, the actuator, and the control and regulation unit. The liquid medium is usually hydraulic fluid. The drive unit is usually an electric motor that provides the energy source for the hydraulic system and outputs mechanical energy through the rotary motion. The role of the energy unit (hydraulic pump) is mainly to convert the mechanical energy input from the drive into hydraulic energy of the liquid medium. The role of the actuator (such as hydraulic cylinder, and hydraulic motor) is to convert hydraulic energy into mechanical energy to obtain the required linear reciprocating or rotary motion. The control and regulation unit is able to regulate the flow and direction of flow of the hydraulic medium. The hydraulic circuit system allows the hydraulic system to achieve complex and diverse functions. The principle of operation of hydraulic circuits is similar to that of electrical circuits: the discrete components are connected to control the position of fluid flow (as in the case of thermodynamic systems) and to control fluid pressure (as in the case of hydraulic amplifiers).

Since the invention of the world's first hydraulic press by Joseph Bramah in England in the 18th century, hydraulic systems have been widely used in various fields. Hydraulics are capable of performing heavy mechanical repetitive tasks in areas such as construction, chemical, mining, paper, logging, agriculture, transportation, automotive, mechanical engineering, metalworking, etc. due to their high power transfer, ease of transmission and configuration. In heavy-duty installations, hydraulic systems can deliver very high power, allowing operators to lift heavy objects and perform precise repetitive tasks with ease. Hydraulic systems also act as actuating units for artificial intelligence and are common in a variety of digitally intelligent work robots.

§2. What is the Pneumatic System?

The word "pneumatics" is derived from the Greek πνεῦμα (pneuma). Similar to hydraulics, pneumatics is related to fluid mechanics, but pneumatics focuses on the mechanical properties and use of gases. Likewise, pneumatics is an engineering technology that uses the compressibility of air as a power source and working medium for energy transmission or signal transmission. As "inexpensive automation technology", pneumatics is widely used in modern industrial production and is an important means to achieve various production control and automatic control. Especially, the low cost and simple structure of pneumatic automation device has been widely promoted and applied, and has a very important position in the automation of industrial enterprises. Moreover, pneumatic as a clean and environmental protection automation technology has considerable significance for global environment and resource protection. (Click on "What is the Pneumatic System?" for a brief introduction to pneumatic systems.)

The structure and working principle of pneumatic system is similar to that of hydraulic system. The pneumatic system uses compressed air (or compressed inert gas) as the power source and working medium, and uses the air pressure generated by the action of impact or rotation to drive the mechanical unit to complete the telescoping or rotating action. The air in the atmosphere can be sucked by the air compressor into the storage tank, as the compressor reduces the volume of air to increase its pressure, so the compressed air in the storage tank will be like a spring with elasticity. Through the pneumatic control element to control the flow direction of compressed air, it can drive the pneumatic system of the actuators of the rotation and retraction and other mechanical movements. Since the amount of air inhaled from the atmosphere by the pneumatic system is the same as that discharged into the atmosphere, no chemical reaction will be produced in the whole process, and no components of polluted air will be consumed.

The history of pneumatic applications is very long, as early as B.C., the Egyptians began to use bellows to generate compressed air for combustion. Starting from the industrial revolution in the 18th century, the German physicist Otto von Guericke invented the vacuum pump, and pneumatic technology was gradually applied in various industries, but most pneumatic components were transformed or evolved from hydraulic components and were large in size. From the twentieth century, pneumatic technology combined with the electronic technology, and self-contained system, began to integrate, miniaturization and modularization. And pneumatic components are becoming smaller and smaller, more and more accurate, cheaper and cheaper. Pneumatics has changed from small handheld devices to large machines with multiple components providing different functions, and is widely used in automation. From the twenty-first century, pneumatics is combined with computer, electrical, sensing and communication technologies, and pneumatics becomes intelligent and informative. Pneumatic servo-positioning technology can make the cylinder achieve automatic positioning at any point under low speed movement. Intelligent valve island technology is very effective for solving the problem of decentralized and centralized control of the whole automatic production line. (Click on "What are the Basic Pneumatic Components?" for a brief introduction to basic pneumatic components.)

Since the 1960s, with the development of industrial mechanization and automation, pneumatic transmission has become an independent field of expertise, the development of pneumatic transmission components has exceeded the speed of hydraulic components. Pneumatic technology, as a low-cost way of industrial automation, is more and more widely used in various industrial fields. Since pneumatic systems are very environmentally friendly and do not emit any toxic or hazardous substances, their safety and reliability is better than hydraulic, electronic and electrical systems, so pneumatic systems are also widely used in food, medicine, light industry, textiles, printing, precision testing and other fields.

§3. Hydraulic System vs Pneumatic System

1. Working Medium

- Hydraulic systems use incompressible hydraulic fluid as the working medium, while pneumatic systems uses compressible air as the working medium.

Hydraulic systems use hydraulic fluid as the working medium, which is stored in a closed hydraulic oil tank. Pneumatic systems use air as the working medium (inert gas is used in only a few cases). The air for pneumatic systems is drawn from the atmosphere by an air compressor and stored in a storage tank. Since atmospheric air contains dirt, water vapor and other contaminants, pneumatic systems need to be equipped with filters and air dryers to keep the compressed air clean and dry, thus improving the reliability and service life of pneumatic components and pneumatic systems. Since the air has low viscosity and weak lubrication, it needs to be equipped with a separate lubrication device. hydraulic fluid is always circulated in the hydraulic system, so no filtering device is required. When the hydraulic fluid stops flowing, the slightest movement of the load releases the pressure on the load, while the pneumatic system needs to release the pressure on the load by discharging compressed air. And because hydraulic fluid is viscous and highly lubricious, there is no need for any lubrication device. Hydraulic fluid also has a cooling function.

2. Working Efficiency

- Hydraulic systems have higher energy transfer efficiency than pneumatic systems.

Since hydraulic fluid is incompressible, the spring action is minimal and does not absorb any provided energy, and energy is not lost due to compression during the process of power transmission, so its output force or torque is larger. As the air is more compressible, the spring action is large, and is affected by the change of external load, so its output force or torque is smaller, and the structure size should not be too large. Therefore, the hydraulic system usually works at a pressure of 6.9 to 34.5 MPa; the pneumatic system works at a pressure of 0.3 to 1 MPa.

- Pneumatic systems have higher maneuverability than hydraulic systems.

Pneumatic components move quickly, react quickly and adjust easily, and can use pneumatic signals to achieve automatic control. However, the pneumatic signal transmission speed in pneumatic components is slower than electronic and light speed, so pneumatic signal transmission is not suitable for complex circuits with high frequency. Because of the air viscosity is very small, in the transmission of friction is small, energy loss is very small, heat loss is very small, so suitable for centralized supply and long-distance transport, and can release energy in a short period of time to obtain intermittent movement of high-speed response. The action speed of pneumatic cylinder is generally 50~500mm/s. Pneumatic system can also use vacuum generator and vacuum cup for precision operation, such as lifting and moving large pieces of glass or eggs and other precision objects.

3. Environmental Protection

- Pneumatic systems are more environmentally friendly than hydraulic systems.

The hydraulic fluid in the hydraulic system must be sent back to the hydraulic oil tank after use to avoid polluting the working environment, so the hydraulic system needs to be equipped with an oil return device. The air in the pneumatic system does not undergo any chemical reaction, and the pneumatic system neither consumes any components in the air nor produces any toxic or harmful substances. Therefore, the air in the pneumatic system can be directly discharged into the atmosphere after use, and there is no need to set up oil return devices like the hydraulic system.

4. Safety

- Pneumatic systems are safer than hydraulic systems.

The hydraulic fluid in a hydraulic system has a lot of hydraulic energy, and uncontrolled hydraulic energy can cause great harm to the staff. In order to avoid the hydraulic energy from causing physical damage to the staff, hydraulic systems usually require additional pressure relief devices to release the unnecessary hydraulic energy, as well as additional isolation devices to avoid uncontrolled hydraulic energy from triggering the actuators. Moreover, hydraulic fluid is a combustible material, and leakage will not only pollute the working environment, but also cause electrical fires. When operating, staff need to wear not only goggles, protective clothing and other protective devices. During installation and maintenance operations, it is also necessary to ensure that the pressure of the hydraulic fluid is zero.

The pneumatic components in the pneumatic system do not require electricity, so if there is a leak in the pneumatic system, the leaking air will not cause an electrical fire. The only thing to worry about is the loss of compressed air pressure. Since a reduction in compressed air pressure can affect the normal operation of a pneumatic system, the leak needs to be sealed as soon as possible. Because of the small danger of air, the pneumatic system is well adapted to the working environment and is very suitable for high temperature, flammable, explosive, dusty, strong magnetic, strong radiation, vibration and other harsh conditions.

5. Service Life

- Pneumatic systems have a longer service life than hydraulic systems.

Since pneumatic systems do not use electricity or hydraulic fluid, they can continue to operate even after a power failure as long as there is sufficient pressure in the air reservoir. The air in a pneumatic system is compressible, so it can absorb shocks and perform cushioning functions. Therefore, pneumatic components are self-protecting and less prone to shock damage. Less maintenance is required in the long run. It is important to note that the service life of pneumatic systems is greatly affected by the cleanliness of the air source and frequency of use. Pneumatic systems usually have a long service life and require little maintenance because they are more resilient to shocks and overloads.

Hydraulic systems rely on internal combustion engines or electric motors to provide a source of energy and therefore cannot continue to operate after a power failure. Since hydraulic fluid is not compressible and cannot absorb shocks, it needs to be equipped with a cushioning device to reduce the impact of the load on the hydraulic components. Because the efficiency of the internal combustion engine and electric motor is limited by the load and temperature, the hydraulic system has a weak overload capability and requires regular maintenance.

6. Costs

- The overall cost of a pneumatic system is lower than that of a hydraulic system.

Pneumatic systems operate at lower pressures, so pneumatic components can be made of thinner and lighter materials (e.g. aluminum and engineering plastics). Pneumatic components are highly modular, and a pneumatic system can be easily designed using standard cylinders and other components and operated with simple on/off controls. Due to the simple structure, low cost and long life of pneumatic components, easy standardization, series and generalization, the installation and maintenance cost of pneumatic system is therefore low. Pneumatic system has a large exhaust noise, mufflers should be equipped when high-speed exhausting. The working pressure of hydraulic system is very high, so the hydraulic components are usually made of expensive steel and cast iron, and the installation and maintenance costs are higher than pneumatic system.

The working medium of pneumatic systems is the inexhaustible free air in the atmosphere. The exhaust of pneumatic systems is simple to handle and does not pollute the environment. The working medium of hydraulic system is hydraulic fluid, and in order to improve the service life of hydraulic components, many additives (such as anti-oxidants, rust inhibitors, anti-wear agents, viscosity index improvers, etc.) are added to hydraulic fluids to optimize the performance of hydraulic fluids (such as thermal stability, viscosity temperature, low temperature performance, anti-wear performance, viscosity, etc.). And the better the quality of hydraulic fluid, the higher the price. Hydraulic fluids, being organic, deteriorate in their properties over time and therefore require additional costs for maintenance and replacement the hydraulic fluid.

Hydraulic systems need to be equipped with expensive and complex electrical drive control systems to ensure that motors do not burn out due to overload and overheating. Pneumatic systems using air to control torque, force and speed , which usually require simple pressure or flow control valves rather than an electrical drive controls system. However, pneumatic systems have a very low energy conversion rate, so their electrical power consumption is relatively high.